Femtosecond studies of nonlinear optical switching in GaAs waveguides using time-domain interferometry
Author(s):
Kristin K. Anderson;
Michael J. LaGasse;
Hermann A. Haus;
James G. Fujimoto
Show Abstract
We describe the application of a new femtosecond measurement technique, time division interferometry,
for investigating the transient nonlinear index in waveguides. This technique performs
an interferometric measurement using a time division multiplexed reference pulse and achieves
high sensitivity with increased immunity to acoustic and thermal parasitics. Using a tunable femtosecond
laser source, direct measurements of the wavelength dependent nonresonant nonlinear
index have been performed in A1GaAs waveguides. In addition, conventional pump and probe
absorption measurements permit the investigation of carrier dynamics, band filling, and two photon
absorption effects. Two photon absorption is found to be a potentially serious limiting effect
for obtaining all optical switching.
Fabrication and performance of GaAs-MQW nonlinear directional coupler
Author(s):
Chih-Li Chuang;
Ruxiang Jin;
Mial E. Warren;
Hyatt M. Gibbs;
Jason P. Sokoloff;
Paul A. Harten;
Nasser Peyghambarian;
John N. Polky;
G. A. Pubanz
Show Abstract
Modified fabrication technique has been developed to improve the performance of GaAs/A1GaAs
MQW nonlinear directional couplers for all-optical picosecond and subpicosecond switching and
modulation.
A nonlinear directional coupler (NLDC) capable of switching data streams and demultiplexing signals
could be an important component in switching networks. Reliable and reproducible performance depends
on the proper technique used to fabricate these devices. Previously, we demonstrated all-optical
switching in GaAs/AlGaAs multiple quantum well (MQW) nonlinear directional couplers using 10 ps
pulses1 where the origin of the nonlinearities was due to photo-excited real carriers. The contrast of the
PS switching was from 1 : 3 to 3 : 1 . We have also observed subpicosecond all-optical modulation in the
same device using 150 fs pulses2 where the origin of such ultrafast modulation is attributed o optical
Stark effect3. The contrast of the fs modulation was from 1 :2.3 to 1 : 1 .2. Recently, we improved the
fabrication procedures. This results in improved performances in both the picosecond and subpicosecond
regimes.
The MQW waveguide structure was designed to sustain a single planar mode for wavelength close to
the absorption edge using a four-layer waveguide model. The effective index method4 was then used to
model the strip-loaded waveguide performance to ensure single mode operation. The sample is grown by
molecular beam epitaxy (MBE) and has 1 .2 am thick guiding region which consists of 60 periods of
alternating 100 GaAs wells/lOO Al023Ga72As barriers. The guiding in the direction perpendicular
to the MQW is provided by the AlGaAs layers above and below the MQW region whereas the
confinement in the horizontal direction is facilitated by the 2 ,am wide ridge etched into the top
AlGaAs layer. The ridge structures are formed by first patterning the sample through contact-print
photolithography using positive photoresist (KTI 820), and then reactive ion etching the top AlGaAs
layer with photoresist mask. The etch process uses pure BC!3 as an etching gas flowing at 25 sccm
with a pressure of 45 mTorr. The power density delivered is 0.43 W/cm2 and the self bias potential is
1 82 V. A Si wafer is laid on the bottom electrode on top of which the sample is placed. After etching,
the couplers are cleaved on both ends to allow light to be end-fire coupled into the guides.
We have since improved the fabrication procedures. First, previous studies show that
photolithography is a problem when the guide separation is only 1 m whereas a guide separation of 4
m is too large for efficient coupling. A mask is thus made that consists only of guides separated by 2
or 3 'am. Second, more care is taken to prevent dust from falling on the sample during the
photolithographic process in our non-clean lab environment. Third, a new cleaving device is assembled
using a phonographic needle and an x-y-z translation stage. With the help of a stereo microscope,
devices as short as 100 1am can be cleaved with high-quality end surfaces. See Fig. 1. These
improvements not only enhance our yield so that nonlinear coupling behavior is observed in almost
every pair of guides but also allows the light to be coupled into each guide with ease, greatly
shortening the alignment time.
Applications of electro-optic polymers to integrated optics
Author(s):
Richard S. Lytel
Show Abstract
Glassy polymers, doped with nonlinear optical moieties, may be coated onto a variety of substrates and other polymers as
guiding layers for optical waveguides. A nonzero electro-optic effect is achieved in these materials by inducing a partial
alignment of the moieties. This paper describes a number of key properties of the materials and exhibits some of the features
of these materials that may lead to practical integrated optical components and modules.
Optical nonlinearity due to real and virtual charge polarizations in DC-biased quantum well structures
Author(s):
Masamichi Yamanishi
Show Abstract
PQV0 klflds of optical nonlinearity caused by real and virtual charge polarizations in quantum well structures are discussed
with some comments on their uses in practical devices. By internal field screening due to photo-excited real charge
polarization, a positive feedback is established at a relatively low optical power level (-4W/cm2), in a device form without
series resistors and optical resonators at room temperature. The feedback mechanism is illustrated based on experimental data.
An extremely fast modulation of optical properties, which is free from the CR time constant and from carrier life time
limitation, would be obtained by internal field screening due to virtual charge polarization. The theoretical considerations and
implications for the ultrafast modulation are shown, and an electro-static ultrafast switch of quantum interference current
through the virtual charge polarization is discussed. An ultrafast response phenomenon based on the virtual transitions is
observed by an off-resonant pump light, with time-resolved optical pulse mixing technique for the measurement of
photocurrent signals, supporting the ultrafast switching capability of the proposed modulation scheme.
Multielectrode quantum well lasers for digital switching
Author(s):
Anthony F. J. Levi;
K. Berthold;
Richard N. Nottenberg;
Douglas R. Dykaar;
Tawee Tanbun-Ek;
Ralph A. Logan
Show Abstract
Multielectrode laser diodes may be used to perform digital amplitude modulation signal
processing. An important advantage of these devices is the ability to control lasing light output
without using conventional high current electrical switches. Under different operating
conditions, multielectrode lasers may also be used to implement digital wavelength switching.
Growth and fabrication of quantum wells, wires, and dots for optical applications
Author(s):
Arthur C. Gossard;
Pierre M. Petroff;
Larry A. Coldren;
Masahiro Tsuchiya
Show Abstract
Although molecular beam and vapor phase epitaxial growth techniques can control layer thicknesses
and uniformity at the atomic monolayer level of precision, control of quantum structure dimensions in
lateral directions parallel to epitaxial layers is more difficult, with lithographic and focused beam
techniques difficult to control below dimensions of order 100 nm. This is larger than the optimum
maximum size for quantum wire and dot applications, which is of order 10 nm, as required for wire and
dot devices to operate in their lowest quantum state/and not to be restricted to cryogenic temperature
operation.
An alternate, but challenging, means of defining small lateral dimensions is the use of the periodicity of
atomic terrace steps on substrate crystals that are slightly misoriented with respect to principal crystal
1 For small misorientation of a crystal surface by a radians, the distance L5 between step edges is
c/a, where c is the spacing between planes of substrate atoms. For a GaAs crystal misoriented two
degrees from a (1 10) direction, L5 = 8 nm and thus is of the order of magnitude needed for quantum wire
devices. Lateral periodicity of a quantum structure can be produced by growth of alternate half atomic
layer coverage depositions in a step flow crystal growth mode. Structures showing this lateral periodicity
have previously been grown both by molecular beam epitaxy and by metal-organic chemical vapor
deposition.2'3 Deviations from a perfectly ordered lateral superlattice with layers perpendicular to the
surface are important and are expected when the starting surface is not smooth with regular steps, when
the crystal grows in an island growth mode, when there are kinks in the step edges, and when each pair of
depositions deviates from an atomic monolayer total deposition.
In recent work, we have measured surface smoothness and step regularity by in situ observations of
the crystal growth surface by measurement of the widths of the specularly reflected beams in reflection
electron diffraction during epitaxial molecular beam growth. From these measurements, an optimum
temperature (of approximately 600 °C for GaAs and AlAs depositions with arsenic rich As4 growth
conditions) has been determined, above which surface roughness on the growth terraces is minimized.4
Smoothing of the surfaces and development of a periodic array of steps is observed during buffer layer
growth by measurement of narrowing of the specularly reflected beam for glancing incidence beams
traveling in a direction normal to the terrace step edges. The regularization of step edge spacing is a result
of the step growth process of crystal growth in which atomic migration to and bonding at step edges is
accompanied by a repulsion which tends to prevent atoms from moving down a step to a lower terrace.5
Vertical superlattices formed in the above manner have been observed by transmission electron
microscope observations of cross sections of GaAs/AlAs structures. Further evidence of the anisotropic
optical properties expected from the vertical superlattices has been found in the anisotropy of
photoluminescence excitation spectra.6 The technique has been used in conjunction with adjacent quantum
well layer structures to produce quantum wire lasers operating in the lowest quantum wire quantum state.
Most recently, the surfaces of GaAs/AlAs vertical superlattices grown on off axis substrates have been
examined directly by atomic force microscopy.7 When the surface of a vertical superlattice is oxidized in
Coulomb effects in optically excited semiconductor quantum dots
Author(s):
Yiping Z. Hu;
Johan Markus Lindberg;
Stephan W. Koch;
Nasser Peyghambarian
Show Abstract
The linear and nonlinear optical properties of semiconductor microcrystallites are analyzed
taking into account the Coulomb interaction between the carriers and the influence of surface
charges. A numerical matrix diagonalization method is used to evaluate the energy eigenvalues and
the corresponding eigenstates. It is predicted that excited two-pair states lead to a pronounced
induced absorption on the high energy side of the one-pair resonance. This prediction is confirmed
by femtosecond and nanosecond experiments in CdSe and CdS quantum dots. Additionally, effects
of traps or impurities and external dc electric fields are discussed.
Femtosecond hole-burning and nonlinear dynamics of quantum-confined semiconductor-doped glasses
Author(s):
Brian Fluegel;
Stephan W. Koch;
Nasser Peyghambarian;
Daniele Hulin;
Arnold Migus;
Manuel Joffre;
Andre Antonetti
Show Abstract
Nonlinear absorption spectra of quantum confined CdSe microcrystallites in glass are measured in
a femtosecond pump-probe experiment at low temperatures. Simultaneous bleaching of the transitions,
and an induced absorption at higher energy are observed. Spectral hole burning is observed as the
pump is tuned through the transition. The system is modelled as spherical crystallites with one or two
electron-hole pairs interacting through the Coulomb potential. A numerical matix diagonalization method
predicts results essentially similar to the experiment and attributes them to state filling of the one-pair
states, and generation of two-pair states.
Quantum wire and quantum dot semiconductor lasers
Author(s):
Kerry J. Vahala;
John A. Lebens;
Charles S. Tsai;
Thomas F. Kuech;
Peter C. Sercel;
Michael E. Hoenk;
Hal A. Zarem
Show Abstract
There is currently great interest in fabrication of structures that are two and three
dimensional analogs of the conventional quantum well. We review here the physics behind
the use of arrays of such lower dimensional structures in semiconductor laser active layers.
Methods which are currently under investigation for producing such structures will be
discussed.
Theory of a semiconductor laser
Author(s):
Murray Sargent III;
Stephan W. Koch;
Weng W. Chow
Show Abstract
This paper summarizes a simple single-mode theory of a semiconductor laser and two kinds of multimode
extensions. The theories are based on an quasi-equilibrium Fermi-Dirac model of a two-band
semiconductor laser gain medium. We include cavity boundary conditions and find the laser single-mode
steady-state oscillation intensity. The question as to when sidemodes can build up leads to consideration
of a theory of multiwave mixing in the semiconductor medium. This theory is also useful in saturation
spectroscopy and phase conjugation using such media, but it does not predict the saturation behavior of
the sidemodes. We mention a third-order multimode theory of the laser that allows for sidemode saturation
and includes the many-body effects of band-gap renormalization and Coulomb enhancement. These
multimode theories assume that the intermode beat frequencies are small compared to the carrier-carrier
scattering rate, an assumption that should be valid for external-mirror semiconductor lasers. Using a
simple model for the beat frequencies comparable to the carrier-carrier scattering rate, we find two-level
inhomogeously broadened sidemode gain and coupling coefficients. Population pulsations and spectral
hole burning play approximately equal roles in this theory.
Vertical cavity surface emitting laser diodes
Author(s):
Kuochou Tai;
L. Yang;
R. J. Fischer;
Tawee Tanbun-Ek;
Ralph A. Logan;
Yan H. Wang;
Minghwei Hong;
Alfred Y. Cho
Show Abstract
Vertical cavity surface emitting lasers (VCSELs) were realized in MBE-grown GaAS/A1GaAS and
MOVPE-grown InGaAsPf.EnP material systems with emission wavelengths near 0.87 and 1.3 p.m.
respectively. The GaAS/A1GaAS VCSELs incorporating epitaxially grown DBR mirrors on both sides of
the cavities were operated at room temperature cw condition with maximum output power greater than 1
mW. The InGaAsP/InP VCSEL, which employed a much simpler cavity structure containing metal and
dielectric mirrors, operated up to 220 K with a threshold current as low as 5 mA at 77K, indicating that
improvements on the cavity design should yield room temperature lasing operation. Single longitudinal
mode emission and circular near- and far-field patterns were observed for the two VCSEL structures.
Nonlinear dynamics of semiconductor laser arrays
Author(s):
Herbert G. Winful
Show Abstract
In this paper we consider the spontaneous emergence of organized behavior and spatio-temporal
chaos in an array of coupled semiconductor lasers. In the absence of coupling, each laser in the array
exhibits a constant, steady state output intensity in response to a constant input pump current. For constant
pump currents above lasing threshold, the individual lasers do not exhibit any interesting instabilities such as
chaos. In the presence of coupling, the constant steady state can lose its stability and non-trivial
spatio-temporal complexity results. The nature of the spatio-temporal dynamics depends on the strength of
the coupling between adjacent elements. For low coupling strengths the stable steady state is one in which
the fields in adjacent elements have nearly equal amplitudes and a phase difference close to 'ii. As the
coupling is increased a Hopf bifurcation to a spatially ordered and temporally periodic output is obtained.
Further increases in coupling strength lead to full blown spatio-temporal chaos.
Picosecond lasing dynamics in quantum well lasers
Author(s):
Yasuhiko Arakawa;
Tetsuomi Sogawa
Show Abstract
It is now well known that the differential gain is enhanced in quantum well (QW) lasers by a factor
of four compared to conventional double-hetero structure (DH) lasers because of step-like density of
states. This enhancement is effective for the ultra-short pulse generation as well as increased modulation
bandwidth [1] In fact, an extremely short pulse as narrow as 1 .3psec is successfully achieved in the QW
lasers as shown in Fig. 1 [2], On the other hand, in order to achieve efficient carrier injection, the QWs in
the active layer should be appropriately coupled. However, this coupling reduces the quantumconfinement
effects, which results in suppressing the enhancement of the differential gain.
In this paper, we investigate the effects of the QW structure parameters, such as coupling effect and
the number of quantum wells, on lasing dynamics in the gain-switched QW lasers by measuring both
pulse forms and spectral dynamics through a streak camera. The results indicates that, in the coupled QW
lasers, the shift of spectra occurs drastically to longer wavelength, which is due to the fact that the
formation of the mini-band in the coupled QWs causes the bulk-like behavior in the picosecond lasing
properties. We also observed strong dependence of both pulse forms and spectral dynamics on the number
of the QWs. This dependence mainly results from the difference in the gain profile at the initial stage of the
pulse formation due to the difference in the carrier concentration per well (cm2). In addition, the overflow
effect of carriers outside of QWs also plays an important role for the QW laser with a smaller number of
QWs. The results are also theoretically discussed.
Amplified spontaneous emission effects in bulk and quantum well semiconductor laser amplifiers
Author(s):
Weng W. Chow;
Richard R. Craig
Show Abstract
We investigated the effects of increasing excitation on the performance
of semiconductor laser amplifiers. Amplified spontaneous emission (ASE) is a
limitation to the power scaling of these devices. A Rigrod analysis is used
to study the effects of ASE on the gain, signal-to-noise ratio and efficiency
for different values of injection current, facet reflectivity and input laser
intensity. Comparisons are made between bulk and quantum well gain media.
Coherent nonlinear optical resonances in II-VI semiconductors
Author(s):
Jorn M. Hvam;
Claus Doernfeld;
Colin R. Paton
Show Abstract
Coherent nonlinear resonances due to extended and localized excitons in CdSe and
CdSeS1_ are investigated by picosecond time resolved degenerate four-wave mixing. Large
nonlinear coefficients (X(31O_9cm2/V2) are found, with coherence times in the picosecond
range. Preliminary results on picosecond optical switching in CdSe are presented, and the
possibility of a purely coherent optical switching with picosecond response time is discussed.
Nonlinear optical properties of nipi superlattices
Author(s):
P. Paul Ruden;
John A. Lehman
Show Abstract
Two issues relevant to the nonlinear optical properties of nipi-type superlattices are discussed. First, an
experimental comparison of photoinduced changes in the absorption coefficients of nipi, hetero-nipi, and heterostructure
superlattices is presented. This shows the characteristic differences between those structures that make use of the modulation
of internal electric fields and those that operate solely on the basis of free carrier effects. It is shown that the modulation of
internal electric fields gives rise to large optical nonlinearities at low optical power levels. Second, a model that describes the
lateral distribution of photoinjected charge carriers in an inhomogeneously excited nipi-type superlattice is discussed. It is
shown that lateral drift dominates over diffusion, and that the characteristic drift length is large.
Three-level optical Stark effect in semiconductors
Author(s):
Dietmar H. Froehlich;
Christian Neumann;
B. Uebbing;
Robert H. Wille
Show Abstract
The three-level optical Stark effect is due to a dynamical coupling of two excited states
by an intense laser field. This effect has been observed on the heavy and light hole excitons
in multiple quantum wells (MQW) and the dipole allowed 2 P exciton in Cu20. In this
contribution new results on the dipole forbidden but quadrupole allowed 1 S exciton in Cu20
are reported. The experimental results are analyzed in terms of a nonlinear susceptibility
which takes into account the dynamical coupling between the excited states to all orders.
Frequency-domain nonlinear optical studies of relaxation mechanisms and the homogeneous line shape of excitons in GaAs/AlGaAs quantum well structures
Author(s):
Hailin Wang;
Jeffrey T. Remillard;
Duncan G. Steel;
J. Oh;
Jagadeesh Pamulapati;
Pallab Bhattacharya
Show Abstract
Abstract
We report high resolution nonlinear laser spectroscopy studies of excitation relaxation
associated with the excitonic optical nonlinearity at room temperature and low temperature in
GaAs/AlGaAs multiple quantum wells. Using a new method of cw frequency domain four wave
mixing, we show that relaxation of the room temperature nonlinear optical response is
characterized by free carrier recombination and ambipolar diffusion. At low temperature, the
excitation relaxation for localized excitons is dominated by phonon assisted tunneling. In addition,
we use four wave mixing methods to eliminate contributions to the excitation line shape from
inhomogeneous broadening. The observed line shape is highly asymmetric and shows the presence
of spectral diffusion due to the phonon assisted tunneling associated with the excitation relaxation.
Multiphoton resonance effects in conjugated organic polymers
Author(s):
Francois Kajzar
Show Abstract
One, two and three photon resonance enhancements in optical hyperpolarizabilities
of conjugated organic molecules are reviewed and discussed. Different experiments
leading to their observation are described and illustrated with definite examples.
These are: harmonic generation, saturation absorption and photoinduced absorption.
Practical importance, especially related to optical switching of such resonances is
also discussed.
1.
Optical properties of strip-loaded polydiacetylene waveguides
Author(s):
William P. Krug;
Edward C. Miao;
Mark W. Beranek;
Kent B. Rochford;
Raymond Zanoni;
George I. Stegeman
Show Abstract
Strip-loading is a convenient and effective method to design and fabricate channel waveguides in multilayer active polymer
structures. Several micron wide strip-loaded poly(4-BCMU) waveguides are observed to be single mode at 1.06 im and
1.32 rim.
Based on the optical nonlinearity for poly(4-BCMU) measured by THG at 1.32 pm, an intensity of 650 MW/cm2 is
required to satisfy the optical phase shift material figure of merit, 4ir, for a directional coupler with 1 cm interaction
length. To estimate the optical power for directional coupler operation, we initially assume no index saturation effects,
negligible one or two photon absorption, waveguide propagation losses less than 1 dB/cm, and unity coupling efficiency.
40 W is the lowest estimated peak pulse power of operation for a device with 6 .tm2 area waveguides. Actual coupling
efficiencies of light incident on our 6 pj2 ea waveguides are between 30 % and 40 %. The propagation losses for our
sixip-Ioaded waveguides are between 4 dB/cm (i'M) and 9dB/cm (TE). The guided wave intensities are at least ten fold
lower than the input intensities. Incident peak pulse powers of at least several hundred watts will be required for 1 cm
long (lirectional coupler device operation. Initial interferometry results suggest that average power dependent thermal
phase shifts dominate peak power dependent electronic phase shifts at duty cycles approaching unity. Polymer
superlattices are considered as advanced nonlinear optical materials.
Application of diffraction gratings in integrated optics of organics and other nonlinear materials
Author(s):
Robert S. Moshrefzadeh;
David A. Ender
Show Abstract
We present a review of some fabrication techniques and
linear and nonlinear applications of diffraction gratings in
integrated optics. The experiments are performed using organic
materials. Recent theoretical results in optimizing the coupling
efficiency of a grating coupler in the presence of saturable
nonlinear index are reported. Finally, we present an experimental
study of dn/dT in polystyrene.
Efficient grating coupling and optical characterization of poly-4BCMU waveguides
Author(s):
Gaetano Assanto;
Qian Gong;
Raymond Zanoni;
George I. Stegeman;
Ryszard Burzynski;
Paras N. Prasad
Show Abstract
We report on the optical characterization of poly-4BCMU slab waveguides, and on the fabrication of efficient
grating guided-wave couplers using this material. Thin film samples were obtained, by spin coating,
from a cyclopentanone solution. Grating couplers were used to measure the film refractive indices and
thicknesses. Waveguide attenuation was obtained from measurements of the variation in scattering with propagation
distance in the visible and near infrared. These parameters allowed us to design, fabricate and test a
grating coupler with a coupling efficiency of 45% at 1 .064 pm.
Atmospheric effects on corona poling of nonlinear optical doped polymer films
Author(s):
Shelly J. Bethke;
Stephen G. Grubb;
Hilary S. Lackritz;
John M. Torkelson
Show Abstract
The effect of poling atmosphere and voltage polarity on the
second harmonic generation (SHG) and surface voltage of
corona poled doped polymer films has been investigated.
Poling is used to orient the guest nonlinear optical (NLO)
dopants into the noncentrosymmetric orientation required to
observe SHG signal. Poly(methyl methacrylate) films doped
with disperse orange 3, an NLO dye, were poled with positive
and negative polarity corona discharges in humid and dry air,
helium and nitrogen atmospheres. The effect of the poling
polarity and the atmosphere on the temporal stability of the
second harmonic signal is discussed.
Observation of nonlinear reflectivity from InSb surface and optical switching application
Author(s):
R. E. Thomas;
Richard C. Jarnagin;
Eugene A. Irene
Show Abstract
We report the observation of external switching of the reflectivity from the InSb surface by a
plasma enhanced absorption mechanism. A HeNe laser and a pulsed CO2 laser were used as
probe beam and switching beam respectively. We also report reflectivity switching initiated as a
result of a thermal effect in the InSb surface.
Influence of photon energy on the photoemission from nonlinear optical materials and devices under different physical conditions
Author(s):
Kamakhya Prasad Ghatak;
Ardhendhu Ghoshal;
Sankar Bhattacharyya
Show Abstract
An Attempt is made to study the photo-emission from bulk specimen, inversion layer, quantum wells, quantum wires (QWs) and quantum dots (QDs) of non-linear optical materials by using the generalized electron energy spectrum and taking n-CdGeAs2 as an example. We have also studied these same dependences in II2 - VI (CdS/CdTe) and HgTe/CdTe superlattices (SLs) under magnetic quantization. It is found that in all the cases the photo-emission exhibits ladder-like dependences with increasing photon energy. The photo-emission is greatest for HgTe/CdTe SL and laest for bulk n-CdGeAs2
Multistability and instability for intracavity acousto-optic modulation output
Author(s):
Ru-Hua Song;
Shixiao Le;
Junbo Wang
Show Abstract
Based on the properties of Bragg's diffraction in the acousto-optic crystal, the outputs for resonator with acousto-optic modulator in cavity were analysed in this paper. The calculation results show the multistability versus input. We also found the multistability being changed with coefficients (as s, 1, k, Tmand R' etc.), and the output being instability and chaos for some coefficients. Using the 1O.6jim CO2 laser and Ge crystal, the multistability was observed, further experiments showed the multistability cna be used in digital communication.