Proceedings Volume 7933

Physics and Simulation of Optoelectronic Devices XIX

Bernd Witzigmann, Fritz Henneberger, Yasuhiko Arakawa, et al.
cover
Proceedings Volume 7933

Physics and Simulation of Optoelectronic Devices XIX

Bernd Witzigmann, Fritz Henneberger, Yasuhiko Arakawa, et al.
View the digital version of this volume at SPIE Digital Libarary.

Volume Details

Date Published: 16 February 2011
Contents: 16 Sessions, 60 Papers, 0 Presentations
Conference: SPIE OPTO 2011
Volume Number: 7933

Table of Contents

icon_mobile_dropdown

Table of Contents

All links to SPIE Proceedings will open in the SPIE Digital Library. external link icon
View Session icon_mobile_dropdown
  • Front Matter: Volume 7933
  • Photovoltaics I: Advanced Concepts
  • Semiconductor Lasers and Optical Injection Effects
  • Photovoltaics II: Materials
  • Photovoltaics III: Optical Design
  • Photovoltaics IV: Physics and Simulation
  • Integrated Optoelectronics
  • Quantum Dot and Quantum Dash Devices
  • Physics and Simulation of Light Emitting Diodes
  • Electromagnetic Analysis of Resonators and Gratings
  • Semiconductor Lasers
  • Nonlinear Dynamics and Mode-Locking of Semiconductor Lasers I
  • Nonlinear Dynamics of Semiconductor Lasers II
  • Photodetectors
  • Devices for Quantum Communications: Joint Session with Conference 7948
  • Poster Session
Front Matter: Volume 7933
icon_mobile_dropdown
Front Matter: Volume 7933
This PDF file contains the front matter associated with SPIE Proceedings Volume 7933, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.
Photovoltaics I: Advanced Concepts
icon_mobile_dropdown
Quantum wells in multiple junction photovoltaics
Thomas N. D. Tibbits, Matthew P. Lumb, Alison Dobbin
Triple-junction (3J) solar cells are the world's most efficient photovoltaic conversion devices, hero cells operating >41% under concentration between 300 and 500 suns. The typical 3J approach has a bandgap combination that limits the cell efficiency at approximately 49%. Different combinations of bandgaps can increase the theoretical efficiency to closer to 60%, and use of metamorphic materials has attempted to demonstrate still higher efficiencies. Multiple quantum wells (MQW) can also be used to fabricate materials with different effective bandgaps from the host semiconductor, and can do so without the attendant lattice constant change and dislocations associated with metamorphics. We show that sufficiently high absorption in MQWs increases the efficiency of 3J solar cells without incorporating defects during epitaxy, both in simulations and in practice.
Simulation of nipi photovoltaic devices
M. A. Slocum, D. V. Forbes, C. D. Cress, et al.
The simulation and characterization of multi-period GaAs n-type/intrinsic/p-type/intrinsic (nipi) doping structure solar cells has been demonstrated. The nipi device depends almost exclusively on drift rather than diffusion currents to collect the carriers. This architecture has been proposed to increase the radiation hardness of a device due to a decreased dependence upon diffusion length. This doping superlattice will allow photo generated carriers to be rapidly transported through the junction by drift. Converting them to majority carriers, and subsequently conducted laterally to selective contacts positioned at opposite sides of etched V-groove channels in the device. The result is a parallel connected multiperiod solar cell, which has been evaluated extensively under simulation. The nipi solar cells have been simulated, giving a greater understanding of the physical mechanisms at work in the device. Design variables such as finger spacing, doping concentration, nipi stack thickness, and the doped to intrinsic thickness ratio are varied to optimize the device. These results show the nipi device has great promise for development as a high efficiency solar cell, with the potential to be used in applications where radiation hardness is required, such as satellite power systems or radioisotope batteries.
Semiconductor Lasers and Optical Injection Effects
icon_mobile_dropdown
Optical injection of quantum dash semiconductor lasers at 1550nm for tunable photonic oscillators
M. Pochet, N. A. Naderi, V. Kovanis, et al.
In this manuscript, we will theoretically compute and experimentally investigate the dynamics of an optically injected nanostructure laser as a function of the injection strength and the optical detuning frequency. A model describing the optically-injected semiconductor laser is derived in dimensionless format that accounts for the large nonlinear gain component associated with nanostructure semiconductor lasers through the gain coefficient's derivative with respect to perturbations in the carrier and photon density. The nonlinear gain (carrier) relaxation rate and gain compression coefficient account for the perturbation in the slave laser's photon density, and are theoretically shown to have a strong impact on the level of stability exhibited by the optically-injected laser. The theoretical model is experimentally verified through the optical-injection of a quantum-dash Fabry-Perot laser at an operating wavelength of 1550 nm. The quantum-dash laser's large damping rate and sufficiently small linewidth enhancement factor are observed to inhibit period-doubling and chaotic operation under zero frequency-detuning conditions. Additionally, the optically injected quantum-dash laser is found to exhibit a large period-one operational state as the injection strength and the detuning frequency are varied, resulting in a highly tunable microwave frequency in the coupled system's equilibrium state. The enhanced and undamped relaxation oscillations of the period-one state are discussed as a building block for tunable photonic oscillators in various RF photonics applications. Finally a path towards realizing an optically-injected diode laser with a THz resonance frequency will be reviewed.
Polarization-resolved nonlinear dynamics in long-wavelength single-mode VCSELs subject to orthogonal optical injection
P. Pérez, A. Quirce, L. Pesquera, et al.
In this work we perform an experimental study of the polarization-resolved nonlinear dynamics of a 1550 nm single-mode linearly polarized VCSEL when subject to orthogonal optical injection. We have measured the stability maps that identify the boundaries between regions of different nonlinear dynamics by using the RF-spectrum of the total power. These maps are measured in the plane of the frequency detuning between the injected light and the orthogonal linear polarization of the VCSEL versus injected power. Stability maps are obtained for two different values of the bias current. Analysis of the dynamics is given in terms of the time traces of the total power and of both linearly polarized output signals. The corresponding RF and optical spectra are also measured. Different dynamical regimes including periodic, period doubling, and irregular dynamics are observed for both polarizations. When the frequency detuning is positive polarization switching can be observed in a periodic dynamical regime, including both period one and period two behaviours. For positive frequency detuning, the only polarization that contributes to the dynamics of the total power is the orthogonal polarization. For negative frequency detuning values both linear polarizations contribute to the dynamics of the total power.
Rate equation analysis of dynamic response in strongly injection-locked semiconductor microring lasers
A novel optical injection-locking scheme for modulation bandwidth enhancement is proposed, involving a distributed Bragg reflector master laser monolithically integrated with a strongly injection-locked unidirectional microring laser. Enhanced high-speed performance of the proposed scheme is confirmed in numerical modeling by comparing it with the scheme where weak optical injection is provided by a waveguide directional coupler adjacent to the ring laser.
Influence of the linewidth enhancement factor on the modulation response of a nanostructure-based semiconductor laser operating under external optical feedback
Frédéric Grillot, Nikhil Dubey
The knowledge of the linewidth enhancement factor (αH-factor) is very important to understand the performance of semiconductor lasers. It affects several fundamental aspects such as the linewidth, the laser's behavior under optical feedback, the chirp under direct modulation and the occurrence of the filamentation. The dramatic variation in the (αH-factor that has been reported for quantum dot lasers makes them an interesting subject for optical feedback studies. In the particular case of QD lasers, the carrier density is not clearly clamped at threshold. The lasing wavelength can switch from the ground state to the excited state as the current injection increases meaning that a carrier accumulation occurs in the ES even though lasing in the GS is still occurring. The purpose of the paper is to show that the exploitation of the nonlinear properties arising from quantum nanostructure based semiconductor lasers operating under external optical feedback can lead, under specific conditions, to a bifurcation of the modulation properties. Starting from the generalized rate equations under optical feedback, the laser's modulation response is determined. Under the short external cavity assumption, calculations show that large variations of the (αH-factor can contribute to improve the dynamical properties such as the relaxation frequency as well as the laser's bandwidth. On the contrary, assuming the long external cavity situation, numerical results show that even small reflections in the percent range when combined to significant variations of the (αH-factor alter the laser's modulation response.
Photovoltaics II: Materials
icon_mobile_dropdown
Dilute nitride multi-quantum well multi-junction design: a route to ultra-efficient photovoltaic devices
Gopi Krishna Vijaya, Andenet Alemu, Alex Freundlich
The current high-efficiency triple junction (Al)InGaP (1.9eV)/GaAs(1.42eV)/ Ge(0.66eV) design for a solar cell can be improved upon by the use dilute nitrides to include a sub-cell in the 1eV range. Addition of a small percentage of nitrogen to III-V semiconductor alloys (such as GaAsN) enables us to achieve the required bandgap, however these bulk dilute nitride structures suffer from a reduced minority carrier lifetime, decreasing the overall current output. The route suggested herein is to include dilute nitride multi-quantum wells (with thicknesses much lesser than the minority carrier diffusion length) within the intrinsic region of a GaAs subcell. Modeling has been done for this structure to obtain the confined energies of the electrons and holes, as well as the absorption coefficient and thereby the spectral response of the 4-junction cell. The results show that it is possible to achieve with the appropriate current matching, a conversion efficiency of ~40% under AM0 (1 sun) with up to ~18 mAcm-2 short circuit current.
Carrier dynamics in MOVPE-grown bulk dilute nitride materials for multi-junction solar cells
Yongkun Sin, Stephen LaLumondiere, Toby Garrod, et al.
Dilute nitride materials with a 1eV band-gap lattice matched to GaAs substrates are attractive for high-efficiency multi-junction solar cells. Carrier lifetime measurements are crucial in optimizing material growth and p-i-n field-aided carrier-extraction-device design. One research group has reported carrier lifetimes of MBE-grown bulk InGaNAsSb materials, but there has been no report of carrier lifetime measurements from bulk InGaNAsSb grown by MOVPE. In this study, we report the growth of bulk InGaNAsSb by MOVPE and the first carrier lifetime measurement from MOVPE-grown bulk InGaNAsSb materials with Eg= 1.0 - 1.2eV at 300K. We studied carrier dynamics in MOVPE-grown bulk dilute nitride materials nominally lattice matched to GaAs (100) substrates: 1μm thick In0.035GaN0.025As (Eg= 1.0eV at 300K) and ~0.2μm thick In(0.05-0.07)GaN(0.01-0.02)AsSb(0.02-0.06) layers (Eg= 1.2eV at 300K). Both structures are fully strained. The incorporation of N in InGaNAs leads to degradation in photoluminescence efficiency, but prior studies indicate the addition of Sb in MBE-grown InGaNAsSb improved the PL efficiency. Two-step post-growth thermal annealing processes were optimized to obtain maximum PL efficiencies that yielded a typical blue shift of 50 and 30meV for InGaNAs and InGaNAsSb, respectively. We employed a streak camera to measure carrier lifetimes from both as-grown and thermally annealed samples. Carrier lifetimes of <30psec were obtained from the InGaNAs samples, whereas carrier lifetimes of up to ~150psec were obtained from the InGaNAsSb samples. We discuss possible reasons for short carrier lifetimes measured from MOVPE-grown InGaNAs(Sb) materials.
Up-conversion luminescence enhancement in erbium-doped porous silicon photonic crystals for photovoltaics
Solar cell efficiency can be increased by adding a rear layer that captures unabsorbed low-energy photons and combines their energy to emit higher-energy photons. This concept has been demonstrated for silicon solar cells using erbium-doped phosphors. Here we investigate the possibility of enhancing intra-4f up-conversion processes within band-edge slow light modes in photonic crystals. We discuss the potential efficiency enhancement realizable one-dimensional erbium-doped porous silicon photonic crystals and present preliminary investigations into these interactions in a real structure.
Photovoltaics III: Optical Design
icon_mobile_dropdown
Rigorous optical simulation of light management in crystalline silicon thin film solar cells with rough interface textures
Daniel Lockau, Lin Zschiedrich, Sven Burger, et al.
We apply a hybrid finite element / transfer matrix solver to calculate generation rate spectra of thin film silicon solar cells with textured interfaces. Our focus lies on interfaces with statistical rough textures. Due to limited computational domain size the treatment of such textures requires a Monte Carlo sampling of texture representations to obtain a statistical average of integral target quantities. This contribution discusses our choice of synthetic rough interface generation, the Monte Carlo sampling and the need for an incorporation of the cell's substrate into optical simulation when illumination of the cell happens through the substrate. We present results of the numerical characterization and generation rates for a single junction cell layout.
High efficiency triple-junction solar cells employing biomimetic antireflective structures
Meng-Yih Chiu, Chia-Hua Chang, Feng-Yu Chang, et al.
In this work, we demonstrate a thorough device design, fabrication, characterization, and analysis of biomimetic antireflective structures implemented on a Ga0.5In0.5P/GaAs/Ge triple-junction solar cell. The sub-wavelength structures are fabricated on a silicon nitride passivation layer using polystyrene nanosphere lithography followed by anisotropic etching. The fabricated structures enhance optical transmission in the ultraviolet wavelength range, compared to a conventional single-layer antireflective coating (ARC). The transmission improvement contributes to the enhanced photocurrent, which is also verified by the external quantum efficiency characterization of fabricated solar cells. Under one-sun illumination, the short-circuit current of a cell with a biomimetic structures is enhanced by 24.1% and 2.2% due to much improved optical transmission and current matching, compared to cells without an ARC and with a conventional ARC, respectively. Further optimizations of the biomimetic structures including the periodicity and etching depth are conducted by performing comprehensive calculations based on a rigorous couple-wave analysis method.
Photovoltaics IV: Physics and Simulation
icon_mobile_dropdown
Modeling of radiation induced defects in space solar cells
Robert J. Walters, Scott Messenger, Jeffrey H. Warner, et al.
The radiation response mechanisms operative in space solar cells are described. The effects of electron and proton radiation-induced defects on the cell performance are identified and methods for modeling the radiation response are presented. The space radiation environment is described, and a methodology for modeling the response of a solace cell to exposure to the space radiation environment is presented. It is shown how this model an be used to predict on orbit performance, and examples from space experiments are shown.
Modeling and analysis of multijunction solar cells
María González, Ngai Chan, Nicholas J. Ekins-Daukes, et al.
The modeling of high efficiency, multijunction (MJ) solar cells away from the radiative limit is presented. In the model, we quantify the effect of non-radiative recombination by using radiative efficiency as a figure of merit to extract realistic values of performance under different spectral conditions. This approach represents a deviation from the traditional detailed balance approximation, where losses in the device are assumed to occur purely through radiative recombination. For lattice matched multijunction solar cells, the model predicts efficiency values of 37.1% for AM0 conditions and 52.8% under AM1.5D at 1 sun and 500X, respectively. In addition to the theoretical study, we present an experimental approach to achieving these high efficiencies by implementing a lattice matched triple junction (TJ) solar cell grown on InP substrates. The projected efficiencies of this approach are compared to results for the state of the art inverted-metamorphic (IMM) technology. We account for the effect of metamorphic junctions, essential in IMM technology, by employing reduced radiative efficiencies as derived from recent data. We show that high efficiencies, comparable to current GaAs-based MJ technology, can be accomplished without any relaxed layers for growth on InP, and derive the optimum energy gaps, material alloys, and quantum-well structures necessary to realize them.
Resonant thermo-tunneling design for ultra-efficient nanostructured solar cells
Nanostructured solar cells are touted to lead to super high photo-conversion efficiencies. Nevertheless the inclusion of potential energy fluctuations associated with those structures hinders the smooth vertical transport of photo-generated carriers. We present an innovative energy level engineering design that significantly facilitates the collection of all photo-generated carriers. Using dilute nitride III-V semiconductor quantum wells embedded in a conventional III-V GaAs host, we demonstrate the possibility of achieving a quasi-flat valence band that will ease the smooth transport of holes. The conduction band confinement energies are designed in a way that promotes thermo-tunneling electrons from their potential wells to the conduction band continuum. Energy levels were calculated by including strain and spin-orbit interaction. The calculation of confinement energies was also undertaken. Once confinement energies and potential barrier heights were determined we complemented the theoretical evaluation by calculating carrier escape times via thermionic and tunneling routes at 300 K. Here we demonstrate that an optimized resonant thermo-tunneling design leads to ultra rapid escape. The suggested approach is thus expected to circumvent recombination losses and lead to a substantial carrier collection and efficiency improvements.
Integrated Optoelectronics
icon_mobile_dropdown
Finite-element method simulations of high-Q nanocavities with 1D photonic bandgap
High-Q optical resonances in photonic microcavities are investigated numerically using a time-harmonic finite-element method.
Design of complex large-scale photonic integrated circuits (PICs) based on ring-resonator structures
C. Arellano, S. Mingaleev, E. Sokolov, et al.
The exponentially growing number of components in complex large-scale Photonic Integrated Circuits (PICs) requires the necessity of photonic design tools with system-level abstraction, which are efficient for designs enclosing hundreds of elements. Ring-resonators and derived structures represent one example for large-scale photonics integration. Their characteristics can be parameterized in the frequency-domain and described by scattering matrix (S-matrix) parameters. The S-matrix method allows time efficient numerical simulations, decreasing the simulation time by several orders of magnitude compared to time-domain approaches yielding a better modeling accuracy as the number of PIC elements increases. We present the modeling of optical waveguides within a sophisticated design environment using application examples that contain ring-resonators as fundamental structure. In the models, the two orthogonally polarized guided modes are characterized by their specific index and loss parameters. Systematic variation of circuit parameters, such as coupling factor or refractive index, allows a comfortable design, analysis and optimization of many types of complex integrated photonic structures.
Ultra wideband integrated polarization splitter/combiner using electro-optic effect in GaAs
Mohamed A. Swillam, Mohamed H. Bakr, Xun Li
We propose a novel design of integrated polarization splitter/combiner with ultra wide bandwidth. The proposed design utilizes the electro-optic (Pockels) effect in GaAs for splitting the polarizations. It also exploits the self imaging phenomenon in MMI couplers with a parabolic index distribution in the vertical direction to significantly increase the bandwidth. A stair case index approximation of this index profile is utilized to facilitate the fabrication process. The fabrication of this profile is feasible through the current technology using multiple etching. Our proposed design maintains a variation of less than 0.5 dB in the power coupling over a bandwidth of 400 nm. We also propose a novel approach for design optimization of the proposed structure. This approach is capable of extracting the propagation constants and their gradient with respect to all the design parameters. This allows for using gradient-based optimization The computational time of this optimization procedure is only a fraction of that for other recently proposed approaches.
Analysis of thermoelectric properties of AlInN semiconductor alloys
Jing Zhang, Hua Tong, Juan A. Herbsommer, et al.
The thermoelectric properties of n-type wurtzite AlInN alloys are investigated by simulating the electron and phonon scatterings. The electrical conductivity, Seebeck coefficient and electronic thermal conductivity are obtained by considering all major electron scatterings. The lattice thermal conductivity is simulated by considering phonon scatterings. The simulation provides useful guideline in material optimization for thermoelectricity.
Quantum Dot and Quantum Dash Devices
icon_mobile_dropdown
Modeling and simulation of nanocrystal solids with rate equations
O. Cellek, Matt Law
Colloidal semiconductor nanocrystals and their solid films created a promising field of research during the last decade. We present a rate equation model to simulate unipolar electron transport in DC-biased one dimensional chain of PbSe nanocrystals. Tunneling, cooling, trapping and heating of electrons are modeled with transition rates and parameters. Three lowest orbitals in each quantum dot are taken into account to simulate unipolar transport through nanocrystal solids. Transitions between the orbitals and neighbor quantum dots are modeled using experimental reports in the literature. Numerical solutions of the rate equations for each state results in a balance between all states in the device.
Modeling the temperature performance of monolithic passively mode-locked quantum dot lasers
Mark T. Crowley, David Murrell, Nishant Patel, et al.
This paper examines and models the effect of temperature on the mode-locking stability of monolithic two-section InAs/GaAs quantum dot passively mode-locked lasers. A set of equations based on an analytic net-gain modulation phasor approach is used to model the observed mode-locking stability of these devices over temperature. The equations used rely solely on static device parameters, measured on the actual device itself, namely, the modal gain and loss characteristics and describe the hard limit where mode-locking exists. Employment of the measured gain and loss characteristics of the gain material over temperature, wavelength and current injection in the model provides a physical insight as to why the mode-locking shuts at elevated temperatures. Moreover, the model enables a temperature-dependent prediction of the range of cavity geometries (absorber to gain length ratios) where mode-locking exists. Excellent agreement between the measured and the modeled mode-locking stability over a wide temperature range is achieved for an 8-stack InAs/GaAs mode-locked laser. This is an extremely attractive tool to guide the design of monolithic passively mode-locked lasers for applications requiring broad temperature operation.
Investigation of spectral responsivity of InAs QD-embedded GaAs solar cells
Christopher G. Bailey, David V. Forbes, Ryne P. Raffaelle, et al.
GaAs p-i-n solar cells embedded with varying number of QD layers (0-60) were grown by OMVPE. 1x1 cm2 cells were fabricated and standard solar cell testing was performed. Illuminated AM0 current-voltage characteristics were measured of both a baseline and 10-layer quantum dot (QD) embedded GaAs p-i-n. The QD solar cell (QDSC) gave an short circuit current of 23.1 mA/cm2 increase in of 0.7mA/cm2 above the baseline with no QDs. The QD embedded cell also showed limited loss in open circuit voltage characteristics of 0.99 V compared to 1.04 V of the baseline. Conversion efficiencies were 13.4 and 13.8 for the QDSC and baseline solar cell, respectively. Spectral responsivity measurements revealed equivalent GaAs response in the visible for the baseline, 10x and 20x layer QD samples, while systematically degraded emitter lifetime was found to be responsible for loss in visible responsivities for the 60x QDSC. Sub-GaAs bandgap response gave a systematic increase of 0.25 mA/QD layer. Spectral responsivity modeling was used and found that bulk GaAs emitter and i-region lifetimes degraded from 102 ns to 102 ps, with increasing number of QD layers.
Physics and Simulation of Light Emitting Diodes
icon_mobile_dropdown
Optical properties of individual GaN nanorods for light emitting diodes: influence of geometry, materials, and facets
We present a systematic analysis of the optical properties of GaN nanorods (NRs) for the application in Light Emitting Diodes (LEDs). Our focus is on NR emitters incorporating active layers in the form of quantum-disc or core-shell geometries. We concentrate on the properties of individual NRs, neglecting any coupling with neighbouring NRs or ensemble effects. The distribution of power among guided and radiative modes as well as Purcell enhancement is discussed in detail in the context of different NR geometries, materials and the presence of interfaces.
Transport equation model for the optics of light-emitting diodes with arbitrary surface microstructure
Oskari Heikkilä, Jani Oksanen, Jukka Tulkki
We present a transport equation model for the light emission and transport in light-emitting diodes. Using the model, we compare the brightness and quantum efficiency of LEDs with light extraction through a smooth, light scattering or perfectly transparent planar surface. We show that surface roughened LEDs can perform almost ideally and that the thickness of the active region in LEDs has a large effect on the photon extraction and the overall efficiency.
Numerical study on efficiency droop of blue InGaN light-emitting diodes
Specific designs on the band structures near the active region are investigated numerically by using the APSYS simulation program with the purpose to surmount the efficiency droop in the InGaN blue LEDs. Systematic analyses included the energy band diagrams, radiative and SRH recombination rates, distribution of electrons and holes in the active region, and electron overflow. Simulation results show that, with appropriate designs, the efficiency droop may be effectively reduced due to the increase of hole injection efficiency, the enhancement of blocking capability for electrons, or the uniform carrier distribution of carriers in the active region.
Electromagnetic Analysis of Resonators and Gratings
icon_mobile_dropdown
The pole condition as transparent boundary condition for resonance problems: detection of spurious modes
In many implementations of transparent boundary conditions for resonance problems, spurious modes arise. We have developed a transparent boundary condition based on the pole condition that has one complex tuning parameter. Numerical experiments suggest that the artificial eigenvalues are due to badly converged solutions in the exterior domain and thus are strongly dependent on variations of this parameter while physical solutions are well converged and thus almost invariant. Hence it is possible to differentiate between spurious and physical solutions by doing a sensitivity analysis of the eigenvalues.
Similar structures, different characteristics: optical performances of circular polarizers with single-, double-, and multi-helical metamaterials
ZhenYu Yang, Ming Zhao, PeiXiang Lu
Recently, it was demonstrated by J.K. Gansel et al. that three-dimensional single-helical metamaterials can serve as broadband circular polarizers in the infrared range. Firstly, in this paper, we used the finite difference time domain (FDTD) method to study circular polarizers with double-helical metamaterials. The results show that the operation bands are more than 50% broader than those of the single-helical structures. However, we also notice that the signal-to-noise (S/N) ratios of them are both poor (~10 dB). Secondly, we analyzed the performances of triple-helical structures. It is obvious that the triple-helical circular polarizer has not only a broad operation band, but also a much higher S/N ratio (~35 dB) than single- and double-helixes.
Semiconductor Lasers
icon_mobile_dropdown
Analogue modulation performance of 20 GHz vertical cavity surface emitting lasers for radio over fiber applications
M. J. Crisp, Z. Qureshi, J. D. Ingham, et al.
The analog modulation performance in terms of linearity and modulation of IEEE 802.11g signals with carrier frequencies of up to 20GHz is reported. An oxide aperture Vertical Cavity Surface Emitting Laser (VCSEL) with a low current density and 20GHz 3dB bandwidth is shown to operate at 70°C with an SFDR better than 80dB/Hz2/3 upto 20GHz. This is only a slight reduction in performance over the behavior at 20°C. A novel integrated Electro-Optically Modulated (EOM) VCSEL is shown to offer a bandwidth of 18GHz with better performance in-terms of SFDR. However it currently suffers greater temperature dependence and lower optical output power.
Giant lasing effect in magnetic point contacts and its detection by DC electrical measurements
A. M. Kadigrobov, R. I. Shekhter, S. I. Kulinich, et al.
We propose a new principle for a compact solid-state laser in 1-100 THz regime based on a new mechanism for creating spin-flip processes in ferromagnetic conductors. On the base of this mechanism, a giant lasing effect is predicted. The optical gain is estimated to exceed the optical gain of conventional semiconductor lasers by 4 or 5 orders of magnitude. We propose to use a point contact between ferromagnetic metals in order to create an inverted spin-population of hot electrons in the contact region. While point contact spectroscopy is an established technology the use of magnetic point contacts as a photon source is a new and potentially very useful application. We show that the generated photons conveniently can be detected by measuring the current through the illuminated point contact.
Nonlinear Dynamics and Mode-Locking of Semiconductor Lasers I
icon_mobile_dropdown
Modulation effects in multi-section semiconductor lasers
Nicholas G. Usechak, Matt Grupen, Nader Naderi, et al.
The current modulation of a two-section semiconductor laser is first reviewed analytically using a well-known, closed-form, modulation expression. A system of traveling-intensity equations is then used to investigate spatial effects in these lasers including cavity layout and the role played by cavity length. The numerical simulations verify the accuracy of the analytic expression for short cavities (low frequencies) but identify shortcomings as the cavity length (modulation frequency) is increased. One notable difference is the presence of resonant peaks in the modulation response. Although this effect has been addressed in the past, the arrangement of sections within the laser is shown to play a prominent role in these monolithic devices for what we believe to be the first time. In the course of this investigation the thirteen different ways a two-section semiconductor laser can be current modulated are identified and computationally investigated.
Mode-locking theory for ultra-short few-femtosecond laser pulses
Edward Farnum, J. Nathan Kutz
We propose a new model which is valid for ultra-fast pulse propagation in a mode-locked laser cavity in the few femtosecond to hundreds of attoseconds pulse regime, thus deriving the equivalent of the master mode-locking equation for ultra-short pulses that has dominated mode-locking theory for two decades. The short pulse equation with dissipative gain and loss terms allows for the generation of stable ultra-short optical pulses from initial white-noise,thus providing the first theoretical framework for quantifying the pulse dynamics and stability as pulseswidths approach the attosecond regime.
Four-wave mixing analysis of quantum dot and quantum well lasers
Hung-Hsin Lin, Chih-Hao Lin, Fan-Yi Lin
In this paper, we characterize and compare a quantum dot and a quantum well lasers using the four-wave mixing analysis. The optical and power spectra of the four-wave mixing state in the quantum dot laser are studied both numerically and experimentally. The tendency of the amplitude versus detuning in the quantum dot laser is very similar to those seen in the quantum well laser. The four-wave mixing signals and the power spectra from both lasers are symmetric, while asymmetry in the regenerated signal is found. Compared to the quantum well lasers, the higher resonance peak of the regenerated signal of the quantum dot lasers appears on the opposite side of the detuning in the optical spectra. The intrinsic parameters of the lasers are also obtained by fitting the optical spectra and power spectra obtained experimentally with those derived directly from the rate equations. The measured value of the linewidth enhancement factor has a good agreement with that obtained by the injection locking method.
Characterizing and suppressing multi-pulsing instabilities in mode-locked lasers
Eli Shlizerman, Edwin Ding, Matthew O. Williams, et al.
The onset of multi-pulsing, a ubiquitous phenomenon in laser cavities, imposes a fundamental limit on the maximum energy delivered per pulse. Managing the nonlinear penalties in the cavity becomes crucial for increasing the energy and suppressing the multi-pulsing instability. A Proper Orthogonal Decomposition (POD) allows for the reduction of governing equations of a mode-locked laser onto a low-dimensional space. The resulting reduced system is able to capture correctly the experimentally observed pulse transitions. Analysis of these models is used to explain the the sequence of bifurcations that are responsible for the multi-pulsing instability in the master mode-locking and the waveguide array mode-locking models. As a result, the POD reduction allows for simple and efficient way to characterize and optimize the cavity parameters for achieving maximal energy output.
Noise suppressions in synchronized chaotic lidars
Yi-Huan Liao, Wen-Ting Wu, Fan-Yi Lin
We numerically study and compare the noise suppressions in the chaos lidar (CLIDAR) and the synchronized choas lidar (S-CLIDAR) systems with the optoelectronic feedback (OEF) and the optical feedback (OF) schemes. The S-CLIDAR system with both OEF and OF schemes show better noise immunity than the CLIDAR system in the low SNR region. Compare with the OEF scheme, the S-CLIDAR system with the OF scheme is more sensitive to the phase noise. For the S-CLIDAR system with both schemes, an open-loop configuration under a generalized synchronization condition is desired.
Nonlinear Dynamics of Semiconductor Lasers II
icon_mobile_dropdown
Oscillation frequency shifts observed in vertical cavity surface emitting lasers exposed to magnetic fields
T. Kobayashi, Y. Yamagishi, H. Arai, et al.
As long ago as the 1960s, scientists understood that Diode lasers' oscillation wavelengths showed a significant shift to the shorter wavelength (high frequency) side, when exposed to strong (<4[T]) magnetic fields, at extremely low temperatures (<80[K]). Not surprisingly, then, in preliminary tests, when we exposed Fabry/Perot-type diode lasers oscillating at 780[nm] to weak magnetic fields (<1.4[T]), at room temperature (300[K]), we observed that the oscillation wavelength shifted to the longer (low frequency) wavelength side. In the present work, we used vertical-cavity surface-emitting lasers (VCSEL) to check whether its change into the shorter wavelength side takes place. In discussions of shift mechanisms, we consider how wavelength (frequency) and optical output-power shifts are correlated. Our expanded knowledge base has forced us to use a completely different mechanism to explain how/why our results differ from those obtained in studies conducted in the 1960's. We are now introducing a mechanism that affects a rise in temperature and an increase in the carrier density, affect the characteristic shifts observed in our experiments, when a magnetic field is applied to the laser diodes parallel to the injection current.
New modeling of laterally coupled diode lasers: analysis and comparison with experimental results
In this paper, a theoretical investigation of the coupling phenomena of two laterally coupled diode lasers is presented. The analysis is centered in a new dynamic modeling of laterally coupled diode lasers where the modulation response shows additional resonance that is beyond the normal relaxation oscillation frequency. This additional resonance is attributed to the coupling effect between the two coupled diode lasers. We present results obtained with this new model and we compare them with previous experimental results in order to demonstrate the good agreement between them.
Compression performance of a NOLM including a SOA for pulse shaping of gain switching diode lasers: influence of the alpha factor
In this work we report a NOLM configuration based on a Semiconductor Optical Amplifier (SOA) and a highly nonlinear Microstructured Optical Fiber (MOF) that offers a stable compression ratio and reshaping capabilities for pulses obtained from Gain Switching diode laser sources. The device is perfectly adapted to the particularities of such sources, namely their low power, their asymmetric nature and the presence of wide pedestals, without the need of intermediate stages to pre-process these optical pulses. Experimental and numerical studies are presented where the influence and importance of the dynamic of the Semiconductor Amplifier on the overall pulse compression performance of the NOLM is explored. Results show that the saturation and gain compression are important to understand the system behavior. Along with these effects, the influence of the alpha factor and its dependence on the carrier density under high input conditions is evaluated. Numerical and experimental results show relevant agreement, what sheds light on the device behavior and helps understand the influence of the several physical effects behind it.
Photodetectors
icon_mobile_dropdown
Modeling of afterpulsing in single-photon avalanche diodes
Michele Anti, Alberto Tosi, Fabio Acerbi, et al.
The afterpulsing noise in Single-Photon Avalanche Diodes (SPADs) is modeled and investigated in order to evaluate its impact on SPAD performance, in terms of maximum count rate, signal-to-noise ratio, etc. From measurements fitting, we identified three/four types of defects that we then used to simulate the behavior of the SPAD when operated in different conditions. We show how the presented modeling is a valuable tool for the estimation of the performance of different SPADs and the identification of optimal operating conditions, in terms of temperature, voltage bias, gate width, gate repetition frequency, quenching time, etc.
Determination of internal quantum efficiency of a photodetector through its voltage-current characteristics
Aleksey Mikryukov, Alexander Kovalev, Anatoly A. Liberman, et al.
This paper, for the first time, suggests a method of determining internal quantum efficiency of an opaque p+nn+ photodetector and some of its characteristics based on a comparison between experimental measurements of photodetector's voltage-current characteristics and characteristics calculated with PC1D. For our research we chose a silicon photodetector Hamamatsu 1337. It was necessary that reflection coefficient of the front surface of the photodetector is known. The inverse problem solution consisted in determination of following parameters of the photodiode: intensity of incident radiation (q), background doping value (n), peak value of front doping (N), depth factor (L) and front surface recombination velocity (S). The shape of doping profile was figured in the form of complimentary error function. For "experimental" curves we used dependencies derived from voltage-current characteristics calculated by PC1D with nominal parameters values. Variation range represented almost one order of magnitude. The problem consists of the fact that there is an infinite set of local minimums corresponding to the selected initial points in a 5-dimensional space of variables. Hill climbing algorithm was used to find local minimums. A special algorithm to find the absolute minimum is presented in this article. Search for absolute minimum among many local minimums was done through method of consecutive approximations toward the minimum of mean square deviation. From performed calculations, we established that value of incident radiation (q) (and, as result, interior quantum efficiency of the photodiode) can be determined with 0.014% accuracy.
Devices for Quantum Communications: Joint Session with Conference 7948
icon_mobile_dropdown
A light emitting diode for entangled photons
R. M. Stevenson, C. L. Salter, I. Farrer, et al.
Existing sources of entangled photons require a laser excitation, imposing a practical limit on their potential for large-scale quantum information applications. For the widely used parametric down-conversion sources, zero or multiple photon-pairs are usually emitted due to the probabilistic nature of the non-linear process. This presents an additional fundamental limitation in the form of efficiency and errors. Here we demonstrate the first electrically driven entangled light source, based on a layer of InAs quantum dots embedded in a p-i-n light emitting diode structure, with potential to operate 'on-demand'.
Modeling the evolution of quantum optical states in optoelectronic devices
Teppo Häyrynen, Jani Oksanen, Jukka Tulkki
We have developed a quantum trajectory model for describing the evolution of an optical mode interacting with optoelectronic devices. Our model includes the field-material coupling, the pumping rate of the material, the loss rate of the material, and also the mirror losses of the cavity. We derive a relation between the model parameters and semiconductor material and device properties and apply the model to calculate the photon statistics of semiconductor devices. We show that depending on the material and device parameters the setup can operate as a light emitting diode or as a laser. In addition to the steady state solutions, the model can be applied to calculate the transient phenomena of the density operator of the optical field. It can also be applied to model the single photon detectors coupled to a cavity field.
High Q photonic crystal cavities with tapered air holes
F. S. F. Brossard, S. G. Schirmer, A. R. A. Chalcraft, et al.
The quality factor (Q) of resonators defined in photonic crystal (PhC) membranes is known to be sensitive to the tapering of the air-holes. A small deviation from an ideal structure with perfectly vertical holes can result in orders of magnitude decrease in the Q, hence greatly jeopardizing applications of such devices in the field of cavity quantum electrodynamics (cQED). Tapering breaks the vertical reflection symmetry of the structure, allowing confined TE-like cavity modes to couple to the resonant continuum of lossy TM-like modes. We show, using a combination of finite-difference time-domain (FDTD) modeling and plane wave expansion method (PWE), that the membrane thickness plays a critical role in the mode coupling. By choosing the thickness about a third of the wavelength of the mode, the high Q of a PhC cavity can be restored despite 10 degrees taper. These findings show that high Q resonators relatively insensitive to the tapering are achievable.
Electrically pumped photonic nanowire single-photon source with an efficiency of 89%
Niels Gregersen, Torben Roland Nielsen, Jesper Mørk, et al.
We propose a new electrically-pumped single-photon source design based on a quantum dot in a photonic nanowire. For realistic parameters, the design features an efficiency of 89 % predicted by numerical simulations. Unlike cavity-based designs, our approach allows for broadband spontaneous emission control and has high tolerance towards surface roughness. In the nanowire, a geometrical effect ensures good coupling between the quantum dot and the optical mode, and an inverted tapering section is introduced to adiabatically expand the mode waist and control the far field emission profile while minimizing the relative modal overlap with the metal contacts.
Poster Session
icon_mobile_dropdown
Top transmission grating GaN LED simulations for light extraction improvement
We study the top transmission grating's improvement on GaN LED light extraction efficiency. We use the finite difference time domain (FDTD) method, a computational electromagnetic solution to Maxwell's equations, to measure light extraction efficiency improvements of the various grating structures. Also, since FDTD can freely define materials for any layer or shape, we choose three particular materials to represent our transmission grating: 1) non-lossy p-GaN, 2) lossy indium tin oxide (ITO), and 3) non-lossy ITO (α=0). We define a regular spacing between unit cells in a crystal lattice arrangement by employing the following three parameters: grating cell period (Α), grating cell height (d), and grating cell width (w). The conical grating model and the cylindrical grating model are studied. We also presented in the paper directly comparison with reflection grating results. Both studies show that the top grating has better performance, improving light extraction efficiency by 165%, compared to that of the bottom reflection grating (112%), and top-bottom grating (42%). We also find that when grating cells closely pack together, a transmission grating maximizes light extraction efficiency. This points our research towards a more closely packed structure, such as a 3-fold symmetric photonic crystal structure with triangular symmetry and also smaller feature sizes in the nano-scale, such as the wavelength of light at 460 nm, half-wavelengths, quarter wavelengths, etc.
Influence of pressure on optical absorption of triglycine selenium single crystals
Katarzyna Ozga, Iwan V. Kityk, Jan Owsik, et al.
For the first time we have shown that increasing uniaxial pressure on the triglycine selenium single crystals leads to the occurrence of several spectral maxima below the energy gap. It is principal that after interruption of the applied uniaxial pressure one observes remarkable spectral shifts up to 20 nm of the principal spectral maxima at 280 nm and 340 nm. The effect is caused by the changes of inter-molecular interactions of the van der Waals type in such kinds of crystals and occurrence of in elastic interactions. The effect has irreversible character and after applying of uniaxial pressure several times we see that there occur several strains which may be considered as the remaining infer-molecular stresses. The observed phenomenon may be used for creation of the optoelectronic tensors of the pressure with the forces up to 4 kG. The performed investigations have shown that the spectral broadening is not sensitive to the pressure, however their spectral shifts are substantially sensitive to the applied pressure. The observed effect possess a long time reversibility (up to one month). Additional studies of piezooptical effects have shown its sensitivity to the external cw green laser light at power 300 mW. Using the thermoluminescence we have established that the crystals are very sensitive to the number of trapping levels within the energy gap. Additionally there were performed studies of electrooptical effects for the pulsed 10 ns Nd:YAG laser.
Design and theoretical characteristics of directional coupler-type optical polarization splitters using dielectric periodic multilayers
Kazutaka Baba, Takenao Nakai
Directional coupler-type optical polarization splitters using dielectric periodic multilayers have been proposed and designed. The periodic dielectric multilayer with large birefringence is loaded on one core of the directional coupler as an outer cladding layer. The periodic dielectric multilayer is designed as the effective refractive index for the TE-polarization becomes equal to that of the isotropic glass layer loaded as the outer cladding layer on another core. In addition, we used a phase-front accelerator proposed by Shiina, et al. at an abrupt bend in the core 2 to reduce an insertion loss for the TM-polarization. The insertion losses of the typically designed waveguide optical polarization splitter are less than 0.01 dB for the TM-polarization and 0.21 dB for the TE polarization. The crosstalks are calculated to be <-23 dB for the TE polarization and <-54 dB for the TM-polarization.
Demonstration of arbitrary channel selection utilizing a pulse-injected double phase-locked semiconductor laser
Yu-Shan Juan, Fan-Yi Lin
We demonstrate and characterize arbitrary channel selection utilizing both the double phase-locked and optical injection schemes experimentally. The double phase-locked scheme is realized by both optical injection and electrical modulation to the slave laser (SL) from a pulsed laser. The pulsed laser is generated by the semiconductor laser under optoelectronic feedback, which outputs repetitive pulse train with the repetition frequency controlled by the feedback delay time and feedback strength. When the SL subject to only the optical pulse injection from the pulsed laser, a broadband microwave frequency comb with amplitude variation ±5 dB in a 20 GHz range is generated. By further applying an electrical modulation to form a double phase-locked condition, a main channel can be selected accordingly. The advantages of large channel suppression ratio, system stabilization, and spurious noise reduction are obtained by using the double phase-locked technique. Moreover, by further applying an optical cw injection from a tunable laser, we demonstrate the selection of a secondary channel. A selection range of about 7.2 GHz is achieved by adjusting the cw injection strength. Average channel suppression between the main and secondary channels to the undesired channels with ratios of 41.8 and 25.9 dB are obtained, respectively. The single sideband (SSB) phase noise of -60 dBc/kHz (-90 dBc/Hz estimated) is achieved at offset frequencies of 25 and 200 kHz for the main and secondary channels, respectively. Demonstration of communication between the main and secondary channels is also demonstrated.
Energy enhancement in mode-locked lasers using sinusoidal transmission functions for saturable absorption
Edwin Ding, Eli Shlizerman, J. Nathan Kutz
A generalized master mode-locking model is presented to capture the periodic transmission created by a series of waveplates and polarizer in a mode-locked ring laser cavity, and the equation is referred to as the sinusoidal Ginzburg-Landau equation (SGLE). Numerical comparisons with the full dynamics show that the SGLE is able to capture the essential mode-locking behaviors including the multi-pulsing instability observed in the laser cavity and does not have the drawbacks of the conventional master mode-locking theory. The SGLE model supports high energy pulses that are not predicted by the master mode-locking theory, thus providing a platform for optimizing the laser performance.
Qualitative analysis of ultra-short optical dissipative solitary pulses in the actively mode-locked semiconductor heterolasers with an external fiber cavity
Alexandre S. Shcherbakov, Joaquin Campos Acosta, Pedro Moreno Zarate, et al.
An advanced qualitative characterization of simultaneously existing various low-power trains of ultra-short optical pulses with an internal frequency modulation in a distributed laser system based on semiconductor heterostructure is presented. The scheme represents a hybrid cavity consisting of a single-mode heterolaser operating in the active mode-locking regime and an external long single-mode optical fiber exhibiting square-law dispersion, cubic Kerr nonlinearity, and linear optical losses. In fact, we consider the trains of optical dissipative solitons, which appear within double balance between the second-order dispersion and cubic-law nonlinearity as well as between the active-medium gain and linear optical losses in a hybrid cavity. Moreover, we operate on specially designed modulating signals providing non-conventional composite regimes of simultaneous multi-pulse active mode-locking. As a result, the mode-locking process allows shaping regular trains of picosecond optical pulses excited by multi-pulse independent on each other sequences of periodic modulations. In so doing, we consider the arranged hybrid cavity as a combination of a quasi-linear part responsible for the active mode-locking by itself and a nonlinear part determining the regime of dissipative soliton propagation. Initially, these parts are analyzed individually, and then the primarily obtained data are coordinated with each other. Within this approach, a contribution of the appeared cubically nonlinear Ginzburg-Landau operator is analyzed via exploiting an approximate variational procedure involving the technique of trial functions.
Simulation of hot electron quantum well photovoltaic devices
H. Z. Fardi
The effects of carrier escape from quantum well (QW) and the interaction of hot electrons with crystal lattice are of importance to the physical understanding of QW hot carrier solar cells where the cooling dynamics in photo-excited structures affect the cell efficiency. The absorption of high-energy photons produces electron hole pairs with excess kinetic energy, which are dissipated to the lattice thru phonon scattering. These hot electrons alter the conversion efficiency in photovoltaic solar cells. We have studied the hot electron effect in an AlxGa1-xAs / GaAs structure with quantum wells placed in the intrinsic region similar to the device described in reference . Our results show that hot electrons lead to an increase in short circuit current. The increase in short circuit current is due to carriers escaping from the well without any significant recombination, which may also lead to a higher cell efficiency. These results support the experimental data recently published by others.
Rapidly optimizing optoelectronic devices using full wave 3D simulation software
Igor Bendoym, Nafiseh Pishbin, Eli Lansey, et al.
Fabricating optoelectronic devices can be extremely costly due to the need for using high end fabrication methods such as photo lithography. Therefore, the importance of being able to accurately and rapidly prototype an optoelectronic device cannot be overstated. By using commercially available full wave 3D simulation software (FW3D), rapid prototyping can be achieved. A complete rapid prototyping process would require a discussion on simulation as well as fabrication work, however for this paper we will only focus on the simulation aspect which is rapid optimization. The bulk of our work will be to model and rapidly optimize an optoelectronic device which is currently of interest to many optoelectronic researchers. This structure is labeled as a frequency selective surface. By using two widely known numerical methods, we will demonstrate the modeling and simulation aspects needed for achieving rapid optimization and fully characterizing the optoelectronic performance of this device.
Enhanced efficiency of organic solar cells with silver nanowire electrodes
Ting Xu, Myung-Gyu Kang, Hui-Joon Park, et al.
Surface plamon resonance (SPR) in metallic nanostructures offers a promising way to enhance the power conversion efficiency (PCE) of organic solar cell as it exhibits strongly enhanced electromagnetic fields in the vicinity of the metal, which can lead to increased optical absorption in the organic film. Here we demonstrate the SPR enhanced photo-current and PCE of organic solar cells using periodic Ag nanowires as transparent electrodes as compared to the device with conventional ITO electrode. Photo-currents and external quantum efficiencies (EQE) are enhanced as much as 40 % and 2.5 fold at a wavelength of 570 nm, respectively, resulting in 35% overall increase in power conversion efficiency than the ITO control device. The use of plasmonic transparent Ag nanowire electrode may help to realize low cost and high performance organic solar cells.
Polarization characteristics of plasmonics-based optical waveguides for polarization rotation
In this paper, we propose a plasmonics-based optical polarization rotator. The proposed structure consists of a slotted waveguide and a metal film. The proposed device is designed using skewing phenomena of propagation waves at the slotted waveguide with metal film by 3D-FDTD method. We have analyzed various structures such as a slotted waveguide, a metal-clad optical waveguide, and a metal-clad optical waveguide with buffer layers. A metal film on the waveguide acts to rapidly rotate the optical polarization, because it has characteristics of slow group velocity according to the metal-clad optical waveguide. Therefore, an ultra-small sized polarization rotator can be realized by the plasmonics-based asymmetric cross sections of waveguides. The length of designed polarization rotator is just 5 μm for 80% polarization rotation ratio.
2×2 photonic crystal fiber splitter for 800 nm spectral domain optical coherence tomography
Joo Beom Eom, Eun Jung Min, Myeong Jin Ju, et al.
We report the fabrication and performance of the 2x2 photonic crystal fiber (PCF) splitter that was designed as a single mode splitter at the 800 nm optical band and used as the beam splitter for a spectral domain optical coherence tomography system. The PCF splitter has been made by coupling PCFs to a planar lightwave circuit (PLC) splitter chip. The PLC splitter chip was fabricated to have a single mode property at the 800 nm wavelength band and the PCFs were securely connected to the PLC chip through PCF block arrays having lithographically fabricated V grooves. The core size of the splitter chip was about 5 μmx5 μm and the core-cladding index difference was about 0.15 %. With the implemented PCF PLC splitter, we have obtained a low excess loss of 1.2 dB at 850 nm and a low polarization-dependent loss of 0.19 dB. With the proposed 2x2 PCF splitter, optical coherence tomography images of pearls were successfully obtained.
Super fast physical-random number generation using laser diode frequency noises
Tetsuro Ushiki, Kohei Doi, Shinya Maehara, et al.
Random numbers can be classified as either pseudo- or physical-random in character. Pseudo-random numbers' periodicity renders them inappropriate for use in cryptographic applications, but naturally-generated physical-random numbers have no calculable periodicity, thereby making them ideally-suited to the task. The laser diode naturally produces a wideband "noise" signal that is believed to have tremendous capacity and great promise, for the rapid generation of physical-random numbers for use in cryptographic applications. We measured a laser diode's output, at a fast photo detector and generated physical-random numbers from frequency noises. We then identified and evaluated the binary-number-line's statistical properties. The result shows that physical-random number generation, at speeds as high as 40Gbps, is obtainable, using the laser diode's frequency noise characteristic.
Modeling and optimal designs for dislocation and radiation tolerant single and multijunction solar cells
Crystalline defects (e.g. dislocations or grain boundaries) as well as electron and proton induced defects cause reduction of minority carrier diffusion length which in turn results in degradation of efficiency of solar cells. Hetro-epitaxial or metamorphic III-V devices with low dislocation density have high BOL efficiencies but electron-proton radiation causes degradation in EOL efficiencies. By optimizing the device design (emitter-base thickness, doping) we can obtain highly dislocated metamorphic devices that are radiation resistant. Here we have modeled III-V single and multi junction solar cells using drift and diffusion equations considering experimental III-V material parameters, dislocation density, 1 Mev equivalent electron radiation doses, thicknesses and doping concentration. Thinner device thickness leads to increment in EOL efficiency of high dislocation density solar cells. By optimizing device design we can obtain nearly same EOL efficiencies from high dislocation solar cells than from defect free III-V multijunction solar cells. As example defect free GaAs solar cell after optimization gives 11.2% EOL efficiency (under typical 5x1015cm-2 1 MeV electron fluence) while a GaAs solar cell with high dislocation density (108 cm-2) after optimization gives 10.6% EOL efficiency. The approach provides an additional degree of freedom in the design of high efficiency space cells and could in turn be used to relax the need for thick defect filtering buffer in metamorphic devices.
1550 nm DFB semiconductor lasers with high power and low noise
Yi-Guang Zhao, Anguel Nikolov, Raj Dutt
We present a high power and low noise DFB laser design. The laser has good performance with output power over 200 mW, side-mode suppression ratio over 50 dB, and related intensity noise (RIN) less than -165 dB/Hz in a wide frequency range. We have modeled the performance of Buried Heterostructure (BH) laser and ridge-waveguide laser. Since the BH laser has a build-in index guiding waveguide, the laser mode is very stable, the relaxation oscillations are suppressed, resulting in peak RIN which is much lower than that of the ridge-waveguide laser. In addition, the BH laser has good confinement of the injected carriers in the cavity, the laser threshold current is very low, and the RIN peak is pushed to high frequency at high bias.
Modeling and design particularities for distributed feedback lasers with laterally-coupled ridge-waveguide surface gratings
Antti I. Laakso, Jukka Karinen, Mihail Dumitrescu
Unlike the conventional buried-grating distributed feedback lasers, those using surface gratings can be fabricated in a single growth and processing sweep. With the surface gratings resulting from lateral corrugations of the ridge-waveguide the epitaxial overgrowth is avoided, which simplifies the device fabrication and reduces the device cost. However, the laterally-coupled distributed feedback lasers have multiple particularities arising from the distinct grating interaction with the optical field. These particularities, especially the effects of grating geometry, including the limitations imposed by the fabrication technology, are analyzed in the paper.
Indium-tin-oxide nanowhiskers crystalline silicon photovoltaics combining micro- and nano-scale surface textures
C. H. Chang, M. H. Hsu, W. L. Chang, et al.
In this work, we present a solution that employs combined micro- and nano-scale surface textures to increase light harvesting in the near infrared for crystalline silicon photovoltaics, and discuss the associated antireflection and scattering mechanisms. The combined surface textures are achieved by uniformly depositing a layer of indium-tin-oxide nanowhiskers on passivated, micro-grooved silicon solar cells using electron-beam evaporation. The nanowhiskers facilitate optical transmission in the near-infrared, which is optically equivalent to a stack of two dielectric thin-films with step- and graded- refractive index profiles. The ITO nanowhiskers provide broadband anti-reflective properties (R<5%) in the wavelength range of 350-1100nm. In comparison with conventional Si solar cell, the combined surface texture solar cell shows higher external quantum efficiency (EQE) in the range of 700-1100nm. Moreover, the ITO nano-whisker coating Si solar cell shows a high total efficiency increase of 1.1% (from 16.08% to17.18%). Furthermore, the nano-whiskers also provide strong forward scattering for ultraviolet and visible light, favorable in thin-wafer silicon photovoltaics to increase the optical absorption path.
Design of simultaneous bidirectional CMOS transceiver with a resistor-transconductor hybrid for optical chip-to-chip interconnects
A simultaneous bidirectional CMOS transceiver for full duplex chip-to-chip optical interconnects is proposed, utilizing a resistor-transconductor (R-gm) hybrid. The hybrid separates the inbound signal from the input/output compound signal. The simultaneous bidirectional CMOS transceiver is designed in a 0.18 μm Si-CMOS technology, with power dissipation of 79 mW and 54.4 mW for the transmitter and receiver, respectively. It shows a 3-dB bandwidth of 4.6 GHz for both the transmitter and the receiver with a 3-dB gain of 26.6 dB and 10.6 dB, respectively, in full-duplex mode.