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.

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.

The all-optical nonlinearity of a quantum well waveguide is studied by measuring the intensity dependent transmission through a Fabry-Perot cavity formed around the guide. Values for the nonlinear refractive index coefficient, n2 ,at a wavelength of 1.O6mare obtained for light whose polarisation is either parallel or perpendicular to the quantum well layers. A simple measurement to estimate the two photon absorption coefficient, 13 2, using relatively low optical power levels is also described.

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.

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.

We discuss the possibility of generating, propagating and detecting THz electrical pulses in semiconductor quantum well structures and present preliminary experimental results.

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.

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

We use nanosecond and picosecond optical spectroscopic techniques to isolate and study the photodynamics of the single particle spectrum of 35A CdSe nanoclusters exhibiting three-dimensional quantum confinement.

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.

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.

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.

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 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.

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.

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.

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 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

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.

Numerical solutions of the full semiconductor Bloch equations for femtosecond pump-probe excitation are presented. Transient absorption oscillations, optical Stark effect and ultrafast bleaching and recovery of the exciton are reported. A transient population build-up and decay on the time-scale of the pump-pulse is calculated.

The planar niicrolens (PML) is a 2-D integrated niicrolens array fabricated by the ion-exchange technique. This paper demonstrates light coupling between LD and optcal fiber using the planar inicrolens. In oder to accept the light power from LD effctively, two classes of High NA planar inicrolens are prepared. A coupled planar inicrolens and new planar inicrolens with swelled structure are evaluated. The minimum coulping loss between LD and single mode fiber was - 5.3 dB using planar muicrolens with the swelled stuctre. (Including 0. 71 dB of Fresnel loss)