Proceedings Volume 4643

Ultrafast Phenomena in Semiconductors VI

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Proceedings Volume 4643

Ultrafast Phenomena in Semiconductors VI

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Volume Details

Date Published: 11 June 2002
Contents: 8 Sessions, 32 Papers, 0 Presentations
Conference: Symposium on Integrated Optoelectronic Devices 2002
Volume Number: 4643

Table of Contents

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Table of Contents

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  • THz Generation and Spectroscopy I
  • THz Generation and Spectroscopy II
  • Poster Session
  • Ultrafast Carrier Dynamics and Transport I
  • Ultrafast Carrier Dynamics and Transport II
  • Carrier Dynamics in Wide Bandgap Semiconductors
  • Nonlinear Dynamics
  • Ultrafast Lasers and Applications
  • Poster Session
  • THz Generation and Spectroscopy II
THz Generation and Spectroscopy I
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Magnetic-field enhancement of terahertz emission from semiconductor surfaces: a comparison of experiment with semiclassical model
Jie Shan, Carsten Weiss, Richard E. Wallenstein, et al.
The possibility of strong enhancement of terahertz (THz) emission from photogenerated carriers in the surface depletion region of a semiconductor under the influence of external magnetic field has been well documented in the literature. We describe a simple theory to explain the key features of this phenomenon. The model is based on a combination of the Drude-Lorentz approximation for the carrier dynamics with an appropriate solution of the radiation problem. The magnetic-field enhancement of THz emission arises primarily form the increased radiation efficiency of transient currents flowing in the plane of the surface. The model provides quantitative agreement with experiment for the pronounced angular dependence of the enhancement and predicts the correct trend for degree of enhancement in a variety of semiconductors.
Terahertz emission from magnetoplasma oscillations in semiconductors
James N. Heyman, H. Wrage, C. Lind, et al.
Ultrafast terahertz spectroscopy can be used to probe charge and spin dynamics in semiconductors. We have studied THz emission from bulk InAs and GaAs and from GaAs/AlGaAs quantum wells as a function of magnetic field. Ultrashort pulses of THz radiation were produced at semiconductor surfaces by photoexcitation with a femtosecond Ti-Sapphire laser, and we recorded the THz emission spectrum and the integrated THz power as a function of magnetic field and temperature. In bulk samples the emitted radiation is produced by coupled cyclotron-plasma oscillations: we model THz emission from n-GaAs as magneto-plasma oscillations in a 3-D electron gas. THz emission from a modulation-doped parabolic quantum well is described in terms of coupled intersubband-cyclotron motion. A model including both 3-D plasma oscillations and a 2-D electron gas in a surface accumulation layer is required to describe THz emission from InAs in a magnetic field.
Fabrication and characterization of terahertz photonic crystals
Cheng-Chung Chi, Heidi T. Wang, Shi-Shy Pai, et al.
Two dimensional (2D) terahertz (THz) photonic crystals were fabricated by stacking micro-machined high-resistance silicon substrates. The transmission spectra through 1 to 8 stacked silicon substrates with polarization parallel and perpendicular to the grooves were measured. At normal incidence with either polarization the normalized transmission spectrum quickly converges as the number of stacked substrates increases. For as few as three stacked Si substrates, the observed transmission spectra for the first two bands converge reasonably well to the calculated results for an infinite lattice. The agreement is better at frequencies lower than 0.4 THz, while it becomes worse at higher frequencies. At oblique incidence, the wave vector direction inside the photonic crystal depends on both incident angle and frequency. The calculated direction from the 2D energy dispersion determines the experimentally observed transmission spectrum. The agreement between the measured results and calculation is reasonable, though some discrepancies remain unaccountable.
THz Generation and Spectroscopy II
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Optical pump-terahertz probe investigation of carrier relaxation in radiation-damaged silicon-on-sapphire
Frank A. Hegmann, Kristine P.H. Lui
We use an optical pump - terahertz probe technique to study relaxation dynamics of photoexcited carriers in semiconductors. The optical pump pulse (400/800 nm, 100 fs) from an amplified Ti:sapphire laser generates free carriers within the optical penetration depth of the semiconductor surface, and the transmission of the terahertz (THz) probe pulse is monitored as a function of delay time between pump and probe. In particular, we investigate carrier relaxation dynamics in radiation-damaged silicon-on-sapphire (RD-SOS). We measure pump-induced changes in the transmission of the amplitude of the THz pulse, which proves to be a valid technique for these low-mobility samples due to negligible phase shifts in the transmitted THz pulse. Using a simple thin-film Drude model, single-exponential relaxation times of 4 to 6 ps are observed and transient mobilities of about 420 cm2/Vs are obtained for moderately damaged RD-SOS> Picosecond carrier relaxation dynamics in high-implant-dose RD-SOS and sub-picosecond transients in amorphous silicon thin films are also observed.
THz/optical properties of semiconductor quantum wells
We outline an engineering approach to modeling the optical properties of semiconductor quantum wells which are driven by a growth-direction polarized electric field at frequency in the THz range. The approach is based on solving the Schroedinger equation for the electron-hole envelope wavefunction with inclusion of the excitonic effects. Unlike the usual case of a dc applied field when the optical response is a time-independent function, the presence of the THz field requires introduction of a response function with periodicity given by the THz period. Our focus is on the linear, with respect to the optical power, regime while the THz field can be strong and thus must be accounted nonperturbatively.
Poster Session
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THz oscillators based on bulk GaAs crystal
We show that multiple-bonded, orientation-patterned, and single GaAs plates can be used to achieve tunable THz waves with subpicosecond pulse-width and high repetition-rate based on synchronously-pumped optical parametric oscillation.
Ultrafast Carrier Dynamics and Transport I
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Diffusion-controlled picosecond carrier dynamics in ErAS:GaAs superlattice
Martin Griebel, Jurgen H. Smet, Juergen Kuhl, et al.
Equidistant layers of self-assembled ErAs islands embedded in a GaAs matrix are a promising alternative to low-temperature-grown (LTG) GaAs for the fabrication of ultrafast photoconductive switches. Unlike in LTG-GaAs, the electronic properties of this new material can be engineered straightforwardly by varying the period of the ErAs island superlattice. The carrier dynamics in MBE grown ErAs:GaAs has been investigated using photo-current autocorrelation measurements on a single photoconductive switch as a function of the superlattice period L. The electron lifetime can be tuned from 10.6 ps down to 2.3 ps by varying L between 300 and 100 nm. The dependence of the electron lifetime on L is understood in terms of a diffusion model. Two such switches were incorporated in a coplanar waveguide and illuminated by short laser pulses transferred to the switches via single-mode fibers. This setup enables pulse generation, propagation and detection under cryogenic conditions and high magnetic fields. By compensating fiber dispersion, a time resolution of less than 3 ps was achieved, limited only by the waveguide dispersion. This setup is ideally suited for time domain characterization with ultrahigh resolution of mesoscopic devices integrated on the same wafer.
Ultrafast carrier dynamics in thin film nanocrystalline silicon
Susan L. Dexheimer, Keith E. Myers, Qi Wang
Thin film nanocrystalline silicon (nc-Si), a promising material for photovoltaic and optoelectronic applications, is comprised of nanometer-scale crystals of silicon embedded in a matrix of hydrogenated amorphous silicon. The degree of crystallinity of the material can be controlled by varying the deposition conditions, yielding materials that span the transition from the amorphous to the nanocrystalline state, and yielding variable grain size and crystalline fraction. Pump-probe measurements using optical pulses 35 fs in duration in the near-infrared were carried out on a series of nc-Si films of varying composition. Photoexcitation results in an induced absorbance signal with a nonexponential time dependence that is strongly dependent on excitation density. The response can be understood in terms of a multicomponent model that includes distinct contributions from each phase of the heterogeneous material. We observe a 240-fs exponential relaxation process associated with intraband relaxation in the silicon crystallites, a response characteristic of bimolecular recombination in the amorphous silicon matrix, and a long-lived component assigned to grain boundary states.
Time-resolved carrier and phonon dynamics in the near surface depletion region of GaAs (100)
The coupling between photoexcited plasma and coherent LO phonons in n-type GaAs (100) was investigated via time-resolved second harmonic generation. In addition to standard pump-probe setup, a time-delayed second pump was applied to inject excess plasma in the sample. The initial pump impulsively launched coherent LO phonons and the following second pump photoexcited excess plasma in the near surface depletion region, where the excess plasma could interact with the coherent LO phonons. It was found that the coherent LO phonon mode dephased rapidly as excess plasma was injected. Meanwhile, the coherent LO-electron and LO-hole coupling modes could be clearly observed in the Fourier spectra. These coupling modes showed plasma density dependent frequencies. Their coupling frequencies and dephasing process were studied in detail. The experimental results agree with the simulation of carrier and phonon dynamics in the near surface depletion region of GaAs on sub-picosecond time scale.
Cerenkov generation of confined acoustic and optical phonons in quantum wells
Ki Wook Kim, Sergiy Mikhailovich Komirenko, Viatcheslav A. Kochelap, et al.
It is well known for bulk semiconductors that amplification (generation) of a phonon mode can be achieved via the Cerenkov effect when the electron drift velocity exceeds the phonon phase velocity. The following three requirements are necessary for practical use of this effect: high electron mobilities, large electron densities, and strong coupling between electrons and phonons. In this report we show that in quantum well heterostructures these requirements can be met and both confined acoustic and confined optical phonon modes can be efficiently generated (amplified) by the drift of two-dimensional carriers. General formulae for the gain coefficient as a function of the acoustic phonon frequency and structure parameters as well as for the confined phonon increment are derived. Taking into account the electron-acoustic-phonon interaction through the deformation potential as well as the piezoelectric interaction, we found that amplification coefficient can reach hundreds of 1/cm for the AlGaAs-based heterostructures and thousands of 1/cm for the SiGe-based heterostructures in the terahertz phonon frequency range. Amplification takes place in a spectrally separated and relatively narrow amplification bands. We show that the optical phonon increment depends critically on the electron drift velocity. Detailed analysis of the optical phonon increment as a function of phonon wavevector, electron-phonon coupling strength, electron temperature and drift velocity indicates that the electron drift in selectively doped AlAs/GaAs/AlAs and GaSb/InSb/GaAs quantum wells can generate coherent confined optical modes. Finally, we discuss nonlinear mechanisms which stabilize the increase of phonon population and lead to the steady state phonon generation.
Ultrafast Carrier Dynamics and Transport II
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Quantum-kinetic theory of hot luminescence from pulse-excited semiconductors
Karsten Hannewald, Stephan Glutsch, Friedhelm Bechstedt
We present a theory of time-and-energy-resolved photoluminescence (PL) from semiconductors excited by femtosecond laser pulses. Our approach combines quantum kinetics of hot-carrier relaxation and quantum theory of spontaneous emission, under consistent inclusion of Coulomb interaction. Model calculations show the transition from PL at the pump frequency towards excitonic PL within a few picoseconds. For intermediate times, we predict hot luminescence to be a sensitive tool to study the electron-lattice interaction. Finally, we extend the theory to the description of photoluminescence excitation (PLE) experiments, and we reinvestigate from first principles the assumption of equivalence between PLE and absorption spectra.
Femtosecond x-ray studies of the photo-induced structural phase transition in VO2
Andrea Cavalleri, Csaba Toth, Craig W. Siders, et al.
We report on the application of femtosecond x-ray scattering to experimental studies of the photo-induced, structural phaser transition in VO2. The transition between the two crystalline phases of the material occurs, for sufficiently intense excitation, within 500 fs.
Intersubband relaxation time for excitons in the n=2 subbands of GaAs quantum wells
Bipul Pal, Arvind S. Vengurlekar
We resonantly excite the n=2 excitons at 8 K in 13 nm and 17.5 nm GaAs quantum wells using 0.5 ps pulses from a Ti-Sapphire laser and perform four wave mixing (FWM) measurements. The exciton dephasing time is deduced by fitting the FWM spectra to a numerical solution of the optical Bloch equations for the excitonic resonance. The finite pulse width and presence of small inhomogeneous broadening are taken into account. The homogeneous linewidth thus obtained at different low excitation intensities has a linear dependence on the intensity. The zero intensity intercept is essentially governed by the n=2 exciton lifetime due to intersubband relaxation, the pure transverse dephasing contribution being relatively less important. We deduce the exciton lifetime to be 0.9 ps and 2.6 ps for the 13 nm and 17.5 nm QWs, respectively.
Exciton coherence in GaAs quantum wells probed by four wave mixing measurements at high excitation intensities
Bipul Pal, Arvind S. Vengurlekar
We perform two beam time integrated (TI) degenerate four wave mixing (DFWM) measurements on 8~nm GaAs quantum wells (QW). Pulses from a Ti-Sapphire laser resonantly excite only 1s heavy hole excitons. The power density of the leading pulse (I1) is increased, keeping that of the delayed pulse fixed and relatively small. We measure the TI-DFWM signal at different delays as well as the DFWM spectra at a fixed delay, as I1 is increased. We find that the dephasing rate, after an initial increase with I1, becomes independent of I1 for large I1. The spectrally integrated DFWM signal, which probes the coherent polarization of the resonantly excited QWs, initially increases with I1, reaches a peak and then decreases as 1/(root)I1 at large I1. The results are compared with theoretical models of exciton coherence.
Carrier Dynamics in Wide Bandgap Semiconductors
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Coherent acoustic phonons in GaN and GaN/InGaN heterostructures
Christopher J. Stanton, Gary D. Sanders, Rongliang L. Liu, et al.
We discuss the generation and detection of coherent acoustic phonons in GaN/InGaN superlattices, multiple quantum wells, and epilayers via ultrafast laser excitation. We show that the generation of the acoustic phonons is driven by the ultrafast photoexcitation of electron-hole pairs in the InGaN layers. Under typical conditions, a complicated microscopic theory including the effects of strain induced piezoelectric fields and valence band mixing can be mapped onto a simpler problem: a loaded uniform string with a non- uniform loading function. The string model allows one to obtain analytic solutions under a variety of conditions. We find that in the superlattices and multi-quantum wells, the frequency of oscillation is related to the superlattice period, whereas in the epilayers and single quantum wells, the frequency of oscillation is related to the velocity of sound and the wavelength of the probe laser. In epilayers, we show that the coherent phonons are actually localized wavepackets that can be used as a powerful probe of nano- scale structures. Finally, we look at the application of multiple laser pulses a a means to coherently control the dynamics of the acoustic phonons.
Excitonic optical Stark effect in GaN
Chan-Kyung Choi, Yia-Chung Chang, Jack Biu Lam, et al.
We report experimental and theoretical studies of the excitonic optical Stark effect in GaN photoexcited below the excitonic resonances with various polarization configurations and pump detunings, using nondegenerate pump-probe spectroscopy at 10 K. We observed that the Stark effect in GaN is strongly dependent on pump and probe relative linear polarizations. We found that this dependence results from the small spin-orbit splitting in GaN and a mixing of A and B valence bands induced by a linearly polarized pump. Using two different circular polarization configurations, we also observed splitting of degenerate excitons because of different optical Stark shifts. Our experimental results are explained by a simple theoretical model.
Electron scattering mechanisms at polar GaN/AlGaN interfaces
Leonardo Hsu, Wladek Walukiewicz
We present recent results of calculations of charge transfer and electron mobilities in nominally undoped AlGaN/GaN heterostructures. It has previously been proposed that the two-dimensional electron gas (2-DEG) originates from donor- like defects on the surface of the AlGaN barrier. We have made detailed calculations of a model in which these defects are created under thermodynamic equilibrium at the growth temperature and show that the spontaneous and strain-induced piezoelectric fields in the AlGaN barrier enhance the formation of these defects. In calculating the low temperature electron mobility in these structures, we consider all the major scattering mechanisms including acoustic phonons, Coulomb scattering from charged centers, and alloy disorder scattering. The relative importance of the different scattering mechanisms depends strongly on the 2-DEG density. At densities smaller than about 2x1012cm-2, the mobility is limited by Coulomb scattering. At higher densities, alloy disorder scattering becomes the dominant electron scattering process. Finally, we have calculated the ratio of the transport to quantum lifetimes ((tau) t/(tau) q for various AlGaN/GaN heterostructures and find that the value of the ratio cannot be used to infer the nature of the dominant scattering mechanism, as is traditionally assumed.
Ultrafast relaxation of highly photoexcited electrons in AlN
The ultrafast relaxation of photoexcited electrons in AlN has been investigated using ensemble Monte Carlo approach. The electrons are excited using infra-red laser pulses with energies ranging from 800 mev to 1000 meV above the conduction band edge at different excitation levels. The energy relaxation, valley population, the build-up and decay of the hot phonon distributions are examined. The strong polar optical phonon scattering rates coupled with the short lifetimes of A(LO) leads to quick decay of the hot phonon distributions. Additionally, the rapid electron-electron scattering leads to fast thermalization of the carrier distributions.
Dynamic carrier relaxation in InGaN/GaN multiple quantum well structures
Shih-Wei Feng, Yung-Chen Cheng, Yi-Yin Chung, et al.
We report the fast and slow decay lifetimes of multi- component photoluminescence (PL) intensity decays in the time-resolved photoluminescence measurements at the room temperature and a low temperature (12K). The fast decay component was essentially due to carrier dynamics, that is, carrier transport from weakly localized to localized states. Such a carrier transport process results in extremely long PL decay time (up to almost 120 ns) for strongly localized states at the low temperature. At room temperature, because of thermal energy and hence carrier escape from strongly localized states, effective lifetimes becomes shorter.
Nonlinear Dynamics
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Dynamics of stimulated emission and coherent backscattering of light from amplifying random medium
Kam Sing Wong, Tao Sun, George K.L. Wong
We investigated random scattering of light in a disorder gain medium of ZnO powder using the pump-probe technique. Using a probe beam at (lambda) =390nm, the width ((theta) ) of the coherent backscattering peak from the ZnO powder is measured to be ~7.5 degree(s), thus the coherent scattering length l is approximately 1.2(lambda) ((theta) =(lambda) /2(pi) l) which is close to the strong scattering regime. When a pump beam ((lambda) =267nm) exceeds a certain excitation threshold, supernarrow emission peaks (bandwidth less than 1nm) emerged from the ZnO broad photoluminescence background. Concurrently, we also observed enhancement and sharpening of the coherent backscattering cone. Since light from the center of the backscatter cone experience the largest number of scatterings (i.e. longest gain length), this result is thus consistent with the random laser model that the supernarrow peak is due to amplification and stimulated emission of photon in the random gain medium. The time-resolved pump-probe measurement shows that the lifetime of the emission state above the lasing threshold is only a few picoseconds which is consistent with the interpretation that the supernarrow peaks are due to stimulated emission.
Ultrafast Lasers and Applications
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Improvement in the frequency response of 1.3-um buried heterostructure InAsP lasers
In this paper we present results of small signal response measurements of 1.3mm strained InAsP buried heterostructure multiple quantum well lasers obtained using optical and electrical excitation. Direct modulation of the carrier population in the quantum wells with a femtosecond pulse from an Optical Parametric Oscillator yields frequency response traces with modulation bandwidths of ~ 6 GHz at biases of 1.5 and 1.8+ threshold. These results contrast with those obtained with electrical excitation for which modulation bandwidths of ~ 3 GHz are obtained at the same DC bias conditions. Analysis of the modulation traces obtained with optical excitation show that in these lasers, transport processes play a dominant role in the frequency response.
Demonstration of high-spatial resolution in ultrafast scanning tunneling microscopy
Dzmitry A. Yarotski, Giovanni P. Donati, Antoinette J. Taylor
We demonstrate ultrafast dynamical imaging of surfaces using a scanning tunneling microscope with a low-temperature-grown GaAs tip photoexcited by 100-fs, 800-nm pulses. We detect picosecond transients on a coplanar stripline and demonstrate a temporal resolution (full-width at half maximum) of 1.7 ps. By dynamically imaging the stripline, we demonstrate that the local conductivity in the sample is reflected in the transient correlated current and that 20-nm spatial resolution is achievable for a 2 ps transient, correlated signal. We apply this technique of photoconductively-gated ultrafast scanning tunneling microscopy (PG-USTM) to study carrier dynamics in InAs/GaAs self-assembled quantum dot samples (SAQD) at T=300 K. The initial carrier relaxation proceeds via Auger carrier capture from the InAs wetting layer (WL) on a timescale of 1-2 ps, followed by recombination of carriers on a 900 ps timescale. Finally, we demonstrate junction-mixing ultrafast STM (JM-USTM) using picosecond voltage pulses propagating on a patterned metal-on-metal (Ti/Pt). Using JM-USTM we have achieved a spatio/temporal resolution of 2 nm/20 ps.
Realization of phonon laser with femtosecond technology
One of the most desirable properties of phonon system is sound amplification by stimulated emission of phonon radiation, coined as SASER or called phonon laser or acoustic laser, which is the acoustic counterpart of LASER. Phonon stimulated emission, or sound amplification, has been previously observed fro several occasions in extremely low temperatures, however a lasing behavior of the phonon oscillators has never been established. It is also desirable to build a phonon laser operating at room temperature. Here we present an optically pumped nano-sized phonon laser with an output acoustic wavelength of 9.3 nm, operating at room temperature. The nano phonon laser is composed by InGaN/GaN multiple-quantum-wells (MQWs). By using femtosecond ultraviolet pulses as pumping sources, coherent acoustic phonon amplification with large acoustic gain was observed. When the induced acoustic gain is higher than the acoustic loss due to its traveling nature, a clear laser-like threshold behavior was observed, which resembles a pulsed optical laser. This demonstration will open a new way toward nano-ultrasonics.
Autocorrelation imaging of 3D structures using a femtosecond laser: application to imaging of sandstone
Ping Yu, M. Mustata, William Robert Headley, et al.
Optical coherence imaging (OCI) is an autocorrelation imaging technique that uses short-coherence light and holographic recording and reconstruction to perform laser-ranging into translucent media. OCI is a full-frame variant of OCT and shares excellent discrimination against scattered light from heterogeneous media. We present the first use of OCI to image into a heterogeneous translucent media: sandstone. There are two motivations for studying sandstone. First, it is an excellent example of a heterogeneous translucent medium on which to study the effects of holographic reconstruction in the presence of static scattered speckle. Second, it is of intrinsic interest for energy production as an excellent example of an oil or gas reservoir rock. Using Optical Coherence Imaging (OCI) we have imaged several layers of grains in a sandstone sample. Information on grain geometry was obtained as deep as 400 microns into the sample.
First observation of the short-pulsing Q-switching mode in a double-heterostructure laser diode without ion implantation
Sergey N. Vainshtein, Mikhail I. Sverdlov, Larisa Shestak, et al.
A double heterostructure (DH) laser has been developed and tested with the aim of achieving high-power picosecond optical pulses in the near-infrared range for use in advanced laser radars and other applications. The physical idea consists of achieving fast gain control by means of temporal evolution of the electric field in the active region. The gain is controlled by the variation in current due to transformation in the built-in electric field across the active region, provided that a high current density is used for pumping. This transformation broadens the carrier energy distribution in the active region, thus suppressing lasing until the current pulse stops. The resulting carrier accumulation causes an enlargement in the power of the short-pulsing Q-switching mode. One of the most important features of the laser structure is the placement of the electron injector well outside the two hetero-barriers forming the active region. Three transient lasing modes were observed simultaneously in this laser diode, with a maximum difference in wavelength as large as 60 nm. One of them, a 45 W/ 25 ps Q-switching mode which appears near the trailing edge of the current pulse, being spectrally separated from the other two. A significant further increase in the power of the Q-switching mode can be expected from an optimized laser structure with two parasitic modes completely suppressed. The new laser structure produces much more powerful picosecond pulses than are obtainable from gain-switched lasers and allows lasing wavelength control by means of bandgap engineering.
Poster Session
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Fast and accurate wavelength switching in fiber laser
Donghui Zhao, Shenping Li, Lin Zhang, et al.
A simple method for achieving fast wavelength switching in a fiber laser was demonstrated using a Fabry-Perot semiconductor filter and a set of fiber Bragg gratings. A build-up time as short as 1.6microsecond(s) was obtained.
Carrier dynamics in InxGa1-xAs1-yNy by picosecond Raman spectroscopy
Non-equilibrium electron distributions and energy loss rate in a metal-organic chemical vapor deposition-grown InxGa1-xAs1-yNy(x=0.03 and y=0.01) epilayer on GaAs substrate have been studied by picosecond Raman spectroscopy. It is demonstrated that for electron density napproximately equals 1018cm-3, electron distributions can be described very well by Fermi-Dirac distributions with electron temperatures substantially higher than the lattice temperature. From the measurement of electron temperature as a function of the pulse width of excitation laser, the energy loss rate in InxGa1-xAs1-yNy is estimated to be 64 meV/ps. These experimental results are compared with those of GaAs.
Picosecond Raman studies of field-induced transient hole transport in an Al0.3Ga0.7As-based p-i-n nanostructure
Y. Chen, Kong-Thon F. Tsen, Otto F. Sankey, et al.
Electric field-induced transient hole transport in an Al0.3Ga0.7As-based p-i-n nanostructure has been studied by picosecond Raman spectroscopy at T=300K. Our experimental results demonstrate that at T=300K, for a 5-ps excitation laser pulse and a hole density of nhapproximately equals 5x1017cm-3, transient hole drift velocity increases from zero to approximately equals (3+/- 0.7)x106cm/sec when the applied electric field intensity increases from E=0 to 15 kV/cm. The transient hole drift velocity then becomes saturated at approximately equals (8+/- 0.8)x106cm/sec for the applied electric field intensity of E>=25 kV/cm and up to 65 kV/cm.
Time-resolved photoluminescence studies of Al-rich AlGaN alloys
Si-doped n-type AlxGa1MINxN alloys with x up to 0.5 and Mg-doped p-type AlxGa1-xN alloys with x up to 0.27 were grown by metal-organic chemical vapor deposition (MOCVD) on sapphire substrates. For the n-type AlxGa1-xN, we achieved highly conductive alloys for x up to 0.5. An electron concentration as high as 1x1018cm-3 was obtained in Si-doped Al0.5Ga0.5N alloys with an electron mobility of 16 cm$_2)Vs at room temperature, as confirmed by Hall-effect measurements. Our results also revealed that the conductivity of AlxGa1-xN alloys continuously increases with an increase of Si doping level for a fixed value of Al content (X<0.5), the conductivities of AlxGa1-xN alloys decrease with increasing Al content for a given doping level; the critical Si-doping concentration needed to convert insulating AlxGa$1-x)N with high Al contents (X>=0.4) to n- type conductivity is about 1 x 1018cm-3. Time- resolved photoluminescence studies carried out on these materials have shown that Si-doping reduces the effect of carrier localization in AlxGa1-xN alloys and a sharp drop in carrier localization energy occurs when the Si doping concentration increases above 1x1018cm-3, which directly correlates with the observed electrical properties. For the Mg-doped AlxGa1-xN alloys, p-type conduction was achieved for x up to 0.27, as confirmed by variable temperature Hall measurements. Emission lines of band-to-impurity transitions of free electrons with neutral Mg acceptors as well as localized excitons have been observed in the p-type AlxGa1-xN alloys. The Mg acceptor activation energies EA were deduces from photoluminescence spectra and were found to increase with Al content and agreed very well with those obtained by Hall measurements. From the measured activation energy as a function of Al content, EA versus x, the resistivity of Mg-doped AlxGa1-x with high Al contents can be deduced. Our results have also shown that PL measurements provide direct means of obtaining EA especially where this cannot be obtained accurately by electrical methods due to high resistance of p-type AlxGa1-xN with high Al content.
Exciton-polariton propagation in AlGaN/GaN quantum-well waveguides probe by time-resolved photoluminescence
The propagation properties of light in AlGaN/GaN multiple- quantum-well (MQW) waveguides have been studied by time- resolved photoluminescence (PL) spectroscopy. The waveguides were patterned with fixed width of 0.5micrometers but orientations varying from -30 degree(s) to 60 degree(s) relative to the a-axis of GaN by electron-beam lithography and inductively-coupled plasma (ICP) dry etching. The peak position and line-width of the emission peak were found to vary systematically with orientations of the waveguides and followed the six-fold symmetry of the wurtzite structure. This is explained in terms of anisotropy of the exciton/carrier diffusion coefficient along the different crystal orientations of the semiconductor materials. We also observed a remarkable decrease in the PL intensity as well as increase in time delay of the temporal response as the location of the laser excitation spot on the waveguide is varied. These observations can be understood in terms of exciton- polarization propagation in the waveguides. The speed of generated polaritons with energy corresponding to the well transitions in the waveguides was determined from the time delay of the temporal response to be approximately (1.26+/- 0.16 x 107 m/sec. The optical loss in the waveguides was determined to be about 5-8 cm-1 for different excitation intensities. The implications of these results to waveguiding in optical devices based on the III- nitride semiconductors are discussed.
Ultrafast all-optical switch using LT-GaAs based on DFB
Pratap Ramamurthy
Simulation of the pump-probe transmission changes of an ultrafast all-optical switch using corrugated index profile, based on optically induced non-linear changes in the band tail region of low temperature grown GaAs (LT-GaAs) is reported in this paper. The device has corrugated surface over the substrate layer, over which is the layer of LT-GaAs forming a smooth interface with GaAs above. Next is a layer of reflection coating. The pump is supplied to the LT-GaAs layer through the mirror and the GaAs layers. The probe comes laterally to the GaAs layer. In the absence of the pump pulse, the non-existence of a strong reflection by the LT-GaAs layer, because of the subtle difference in the refractive index between GaAs and LT-GaAs layers, gets the probe pulse to be acted on by the corrugations (Distributed Feedback principle (DFB)), thus giving a weak output on the other side. In the presence of a strong pump-pulse, the LT-GaAs layer undergoes optically induced non-linear absorption changes and a marked change in the refractive index of the LT-GaAs layer reflects the probe to give a strong output pulse. The simulation is demonstrated around a wavelength of 1.5micrometer. The reduction in the switching time is as high as 50%. Unprecedented contrast ratio of ~30dB is theoretically visualized during simulation and is attributed to the compounded action of DFB principle in the LT-GaAs layer and wave-guide effect in the GaAs layer. The improvement in the design has an advantage, which aids in the efficient coupling of the probe with any other integrated device and in making use of the non-linear absorption changes effectively.
THz Generation and Spectroscopy II
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Optical phonon-assisted ultrafast relaxation of hot carriers: electric field effects
Vishal Saxena, J. P. Evans
The ultra fast relaxation of hot electrons in III-V semiconductors occurs at a time scale of femto-second compared to the relaxation of thermalised electrons that occurs at a time scale of nanosecond. The understanding of this relaxation dynamics is extremely essential for the performance of the ultra-fast multiple quantum well/quantum cascade lasers. Electrons are considered 'hot' when their kinetic energy is much more compared to that of thermalised electrons at the bottom of the conduction band. These hot electrons tend to keep the 'memory' of their momentum anisotropy, which is primarily due to the anisotropy of the band structure of III-V semiconductors. The ultra-fast relaxation of hot electrons is assisted primarily by the interaction with 'LO-optical phonons' (Enery = 36.4meV for GaAs). Hot electrons loose their momentum and spin anisotropy while interacting with optical phonons. During this relaxation process a small fraction of hot electrons recombine optically with the holes available at the acceptor level and their spectroscopy unveils some of the most interesting details of the carrier-relaxation dynamic and the band structure of material. In this paper we present the experimental and computational results of this ultra-fast relaxation phenomenon and also the effect of externally applied high electric field on the relaxation dynamics of these hot carriers.