We report on the first observation of wave mechanical features in free carrier relaxation. The emission of LO phonons by non-equilibrium electrons is studied via ultrasensitive transmission spectroscopy. Electrons are excited in a bulk GaAs sample by energetically narrow 120 fs pump pulses. The time evolution of the energy distribution is probed measuring the induced transmission changes with broadband 25 fs pulses. It is demonstrated that energy is not conserved in scattering events on ultrafast time scales. A phonon replica of the initial electron distribution starts energetically broadened. Within the phonon oscillation cycle of 115 fs, memory effects resulting from quantum interference become operative. The phonon satellite narrows towards the line width given by the photoexcited distribution. This process is repeated for every step in the LO cascade. These phenomena dominate hot carrier dynamics whenever scattering rates are comparable to or even faster than the frequency of the energy quanta exchanged. The influence of the electron-phonon coupling strength is investigated via non-degenerate four-wave-mixing experiments: LO phonon quantum beats are found in the decay of the coherent interband polarization in GaAs. In contrast, the oscillations are overdamped in InP as a result of the strong Frohlich interaction in this more polar compound.

The time-integrated and time-resolved photoluminescence (PL) of Cd_0.3Zn_0.7S_0.06Se_0.94/ZnS_0.06Se_0.94 single quantum wells (QW) have been studied at various temperatures. The time-resolved PL is obtained by sum frequency conversion technique with 300 fs resolution. The PL line shape analysis based on a simple statistical model including both excitons and free-carriers showed that free exciton radiative recombination dominates the PL emission even at room temperature. However, free-carrier recombination contributed significantly to the emission spectrum down to 77 K. The free excitons are stable up to room temperature because of the large exciton binding energy and strong confinement of electrons and holes in these QWs. The free exciton decay time is approximately 300 ps at 130 K and it increases linearly to 1 ns at 295 K. This is the first time that exciton radiative recombination and linear temperature dependence of exciton lifetime is observed up to room temperatures for wide-gap II-VI QWs. This is due to the excellent quality of the samples and the strong confinement of electrons and holes in the QWs studied. This linear increase in exciton lifetime with temperature agrees with the theoretical prediction by considering the conservation of momentum requirement for radiative recombination for excitons in QW.

Femtosecond spectroscopy such as two color pump-probe and four-wave mixing (FWM) on GaAs quantum wells has been performed using two partially synchronized, independently tunable lasers with external jitter compensation. In the two beam and three beam ((omega) ₁, (omega) ₁; (omega) ₂) four wave mixing experiments, heavy-and-light-hold beatings are observed with these two mutually incoherent lasers. FWM signals are observed when (omega) ₂ is completely below the exciton energies, with no spectral overlap with the absorption profile. These off-resonant signals are stronger than the interband continuum signals for equivalent detunings. Femtosecond frequency mixing (2(omega) ₁- (omega) ₂) is also demonstrated when the grating is formed by two different frequencies.

The initial dynamics of resonantly excited secondary emission from quantum wells are dominated by Rayleigh scattering due to excitons localized in the disordered quantum well potential. We show experimentally and theoretically how the temporal shape of the early response is related to details of the 2D disorder potential, and we determine the mean amplitude of the fluctuations and the correlation length. Spectral interferometry performed with a single speckle allows to investigate the phase of the electric field emitted by the ensemble of excitons. We find evidence for a correlation between the degree of localization and the fluctuations of the phase as a function of energy.

Coherent optical spectroscopy in the form of nonlinear transient four-wave mixing (TFWM) and linear resonant Rayleigh scattering (RRS) has been applied to investigate the exciton dynamics of low-dimensional semiconductor heterostructures. The dephasing times of excitons are determined from the decay of the spectrally resolved non- linear signal as a function of the delay between the incident pulses in a two-beam TFWM experiment, and from the real time analysis of single speckles in RRS experiments (pure dephasing). From the density- and temperature- dependence of the dephasing times the exciton-exciton and the exciton-phonon interactions are determined. The degree of coherence of the secondary emission is determined from the speckle analysis.

Subpicosecond carrier lifetimes of arsenic-rich GaAs grown by molecular beam epitaxy at low substrate temperatures have been determined by time-resolved reflectivity. Effect of growth temperature on change of transient reflectivity and antisite defect concentration were also demonstrated. For the first time carrier lifetime as short as 0.13 ps was observed (resolution limited).

Density matrix approach has been employed to study analytically the absorption spectra of small semiconductor quantum dots under strong confinement regime. The results are obtained for a single quantum dot (SQD) as well as for inhomogeneous distribution of quantum dots (IQD) with Gaussian distribution of quantum dot sizes. A numerical analysis has been made for SQD and IQD of CdS crystal with data taken from recent experimental work. A negative change in the absorption coefficient occurs in the shorter pump wavelength side of the spectrum due to the biexcitonic contribution. The wavelength at which crossover from positive to negative values of the change in absorption coefficient occurs is found to depend upon both, the QD size as well as the excitation intensity. The results agree satisfactorily with the experimental observations in small CdS quantum dots.

We present results of the optically excited dynamics in semiconductor quantum wells on short length and time scales. Nonlinear optical experiments are performed with high temporal and high spatial resolution. To interpret the experimental findings calculations are performed on different approximation levels. Two different time regimes are investigated: in the incoherent time regime we study the dynamics of heating, cooling, and the formation of excitons by measuring the temporal behavior of the lateral expansion rate of locally created electron-hole pairs or excitons. A monomolecular exciton formation process is found. The experimental results in this regime are well reproduced by the Boltzmann equation for incoherent exciton densities with phenomenological scattering rates. In addition we have performed a microscopic density matrix analysis for the heating scenario where we have modeled explicitly the initial transformation of coherent excitations into incoherent exciton densities. It is found that the heating due to scattering with acoustic phonons gives reasonable agreement with the observed rates. In the coherent time regime a spatio-temporal beating is observed. This unexpected non-monotonic modulation of the spatial width arises from excitonic wave-packets which modulate the detected lateral profile of the optical nonlinearity in a characteristic way. It is explained by the superposition of various signal components which are detected simultaneously due to the collinearity of our experiment. This effect is illustrated by calculations using a simplified model on the Hartree-Fock level.

Modulators based on interband (IB) light absorption by intersubband (ISB) excitations in undoped quantum well structures (QWs) has the inherent advantage of ultrafast response without thermal dissipation at high bit rates. In this report we present an efficient scheme to achieve ultrafast modulation in the femtosecond regime using IB and ISB light pulses in a step-like type II semiconductor QW. The threshold control-light intensity for 100% modulation in the proposed structure is less than 1 pJ, which is at least an order of magnitude lower than in any excitonic optical switch proposed until now. The peak modulation efficiency in asymmetric QW's at 1 MW/cm² is 40% which is twice than that estimated in symmetric QW's and can be enhanced to 100% at 10 MW/cm². A modulation speed of 500 fs can be achieved without any serious degradation of the IB signal due to thermal dissipation. This is an important step towards the development of novel ultrafast optoelectronic devices based on the pulse shaping techniques.

We report on the dynamics of resonantly excited polariton states in semiconductor quantum microcavities. We show that the spin orientation of cavity polaritons can be coherently manipulated. The measurements of the optical dephasing time and the decay time of the radiant states as a function of the cavity detuning and the lattice temperature give further insights into the relaxation mechanisms of cavity polaritons. The influence of the polariton lifetime on the optical dephasing time is demonstrated. For zero detuning, a quenching of the polariton-acoustic phonon scattering is evidenced at low temperature.

We investigate infrared-active coherent phonons excited in Te, PbTe (un-doped, n-type and p-type), and CdTe by observing the terahertz (THz) emission from these samples. Coherent THz emission corresponding to the longitudinal- optical (LO)-phonon frequency has been observed for all the samples, while no significant THz emission is observed at the transverse-optical (TO)-phonon frequency even at high carrier densities (> 10¹⁸ cm^-3). The absence of THz emission at the TO-phonon frequency strongly contradicts the observation in the transient reflectivity measurements, where the signal oscillations at the TO-phonon frequencies arising from the LO-phonon-plasmon coupling (L_ mode) have been observed at high carrier densities. It is found from a model calculation that for a dipole oscillator on a sample surface the THz emission around the TO-phonon frequency is strongly suppressed due to the large dielectric constant near this frequency.

We present results for two coherent control schemes in semiconductor heterostructures obtained within Boltzmann- Bloch equations. The latter are obtained from Dyson's equation within the Keldysh nonequilibrium Green's function approach and allow a microscopic analysis of coherent control schemes in semiconductors. Here we focus on the coherent control of optical gain and longitudinal optical photon emission rates in intersubband transitions in a three-subband model of a biased asymmetric double well. We show that by coupling the two energetically close subbands with a coherent microwave field, we can induce coherence into the carriers in the double well, thus manipulating intersubband transitions. By adjusting the phase of the microwave field, we can achieve significant modification to the gain spectra for light pulses shorter than the period of the low frequency intersubband polarization. Similarly, LO- phonon induced intersubband transitions can be controlled due to quantum interference between electron-lattice and electron-light interactions.

A number of different methods of generating Terahertz radiation is considered and the efficiencies of these methods are compared theoretically between themselves as well as with Manley-Rowe limits. It is shown that below 1 THz photoconductive antennae and traditional microwave methods hold advantage, while above 10 THz the advantage goes to traditional nonlinear optical methods. In the 1 - 10 THz region there is no clear winner, but the methods using intersubband transitions in quantum wells do possess significant advantage.

We have fabricated vertical and in-plane nano-structures and measured THz signals from these structures. The vertical nano-structures have been fabricated by molecular beam epitaxy in a p-i-n diode. A superlattice structure is located in an i-region of the p-i-n diode. Carriers are photo-excited in the superlattice and accelerated by a built-in potential. The radiation was measured through a free-space electro-optic sampling, and shows rather strong radiation compared to the radiation from the bulk GaAs. The in-plane nano-structures have also fabricated as a photoconductive switch by micro-anodization process. The gap for the photoconductive switch is covered with a transparent insulator which realizes high bias voltage and ultrafast response. We also introduced current block layer under photo-absorbing layer to reduce slow component of current, which are caused by carriers excited deep inside the substrate. The ultrafast response was measured by an electro-optic sampling, and the slow component of the signal has dramatically reduced by this structure.

We have considered forward and backward optical parametric oscillation and amplification, and difference-frequency generation for efficiently generating and amplifying terahertz waves in CdSe, GaSe, periodically-poled LiNbO₃ and LiTaO₃, and diffusion-bonded-stacked GaAs and GaP plates. The advantage of using birefringence in CdSe and GaSe is tunability of the output terahertz frequency. Furthermore, both CdSe and GaSe can be used to achieve the backward parametric oscillation without any cavity. On the other hand, in periodically-poled LiNbO₃ and LiTaO₃, one can take advantage of large diagonal elements of second- order nonlinear susceptibility tensor. In the diffusion- bonded-stacked GaAs and GaP plates, quasi-phase matching can be achieved by alternatively rotating the plates. The advantage of using coherent parametric processes is possibility of efficiently generating and amplifying temporal-coherent and narrow-linewidth terahertz waves. Compared with a noncollinear configuration, by using the parallel wave propagation configurations, the conversion efficiency can be higher because of longer effective interaction length among all the waves.

In this paper, we report the manipulation of coherent exciton in single- and coupled-quantum-wells using phase locked pulses generated by pulse shaping techniques instead of Michelson interferometer. The pulse shaping system with double liquid crystal spatial light modulator, and the Ti³⁺-sapphire laser with pulse width of about 120 fs are used to generate phase-locked pulses. GaAs/AlGaAs quantum-wells (exciton energy: E₀ equals 1.5470 eV) and asymmetric double coupled quantum-well structures (exciton energy: E₁ equals 1.5348 eV, E₂ equals 1.5422 eV) are used in the experiments. Manipulation of coherent exciton are demonstrated by observation of both the reflectivity-change in the pump-probe and the diffracted power in the degenerate-four-wave-mixing (DFWM) measurements using phase- locked twin-pulse (k1) and single-pulse (k2). Coherent destruction in single quantum-well is observed by the rapid decrease of reflectivity-change after the arrival of the second pulse (pulse interval: 500 fs) and good coherent control with 87% of coherent carrier destruction is demonstrated at low excitation power of 0.15 mW. In case of coupled quantum-wells, the modulation of SLM is carefully set to the twin-pulse having the spectrum with a minimum at both exciton wavelengths for destructive condition. After arrival of the second pulse, coherent control of the quantum beats are observed in DFWM signal.

We present a study of the radiative recombination in In_0.15Ga_0.85N/GaN multiple quantum well samples, where the conditions of growth of the InGaN quantum layers were varied in terms of growth temperature and donor doping. The photoluminescence peak position varies strongly (over a range as large as 0.3 eV) with excitation intensity, with donor doping as well as with delay time after pulsed excitation. The peak position is mainly determined by the Stark effect induced by the piezoelectric field. In addition potential fluctuations play an important role, and largely determine the width of the emission. These potential fluctuations may be as large as 0.2 eV in the present samples. Screening effects from donor electrons and excited electron-hole pairs are important, and account for a large part of the spectral shift with donor doping, with excitation intensity and with delay time after pulsed excitation (shifts up to 0.2 eV). We suggest a dominant role of 2D electron- and donor screening in this case, predicting that rather strong localization potentials of short range (of the order 100 angstroms) are present. The possibility that excitons as well as shallow donors are impact ionized by electrons in these rather strong lateral potential fluctuations present at this In composition is discussed in connection with the long decay times observed at all temperatures.

Femtosecond pump-probe measurements were performed in GaN epilayers to study carrier dynamics in the band edge region. Excitonic absorption was found to begin saturating at a pump fluence of 20 (mu) J/cm² which corresponds to an estimated carrier density of 1 X 10¹⁸ cm^-3. At zero delay between pump and probe, induced absorption is observed below the unpumped band gap due to ultrafast bandgap renormalization. After 375 fs, a large induced transparency is observed just below the excitonic resonance which is due to a transient electron-hole plasma. After 1 ps, the absorption has partially recovered to a level associated with excitonic phase-space filling. The absorption then recovers with a characteristic time of approximately 20 ps, a value which increases with increasing excitation density.

The hot electron relaxation dynamics is studied in n-type GaN films grown on sapphire by molecular beam epitaxy. A novel femtosecond pump-probe technique is used in which the electrons are excited by an infrared pump and the carrier dynamics are monitored by a tunable near UV probe. Complex transients, showing bleaching and induced absorption, are observed. The data are fitted by a model in which the LO- phonon emission is the dominant energy relaxation process. The LO-phonon emission time is measured to be 0.2 ps. Above- bandgap pump-probe experiments, in which the electrons are excited by a near ultraviolet (UV) pump from the valence band and probed by a tunable near UV pulse are also performed. They show that the carrier dynamics vary with the probe wavelengths.

Piezoelectric effects on the dynamics of optical transitions in GaN/AlGaN multiple quantum wells (MQWs) have been investigated by picosecond time-resolved photoluminescence (TRPL) measurements. TRPL spectra of the 40 angstroms well MQWs reveal that the excitonic transition is in fact blueshifted at early delay times due to quantum confinement of carriers. The spectral peak position shifts toward lower energies as the delay time increases and becomes redshifted at longer delay times. By comparing experimental and calculation results a low limit of the piezoelectric field strength of about 460 kV/cm in GaN/Al_0.15Ga_0.85N MQWs and the electron and hole wave functions have been obtained. Temporal response of the excitonic transitions of the GaN/AlGaN MQWs depends on the well width. The recombination lifetimes of the 20 angstroms well MQWs decreases monotonously with an increase of emission energy. However, emission energy dependence of the lifetime on 30, 40, 50 angstroms well MQWs which shows a similar behavior as the cw PL spectrum, is quite different from that of 20 angstroms well MQWs. It has been demonstrated that the results described above are due to the presence of the piezoelectric field in the GaN wells of GaN/AlGaN MQWs subject to elastic strain together with screening of the photoexcited carriers and Coulomb interaction.

In this experiment, we propose and demonstrate a novel exciton-exciton scattering process for the case of semiconductor microcavity exciton-polaritons. We show that the scattering rate should be enhanced due to the final state stimulation effect. We have observed the enhancement of the elastic exciton-exciton scattering rate in a semiconductor microcavity. The signature of this stimulated emission is in the enhancement of the scattering rate that is proportional to the final state occupation number. For this reason, we have interpreted the gain to be due to bosonic stimulation. This gain process for exciton- polaritons is distinctly different from that in a conventional photon laser, because of the fermionic nature of the exciton's constituents, the electron and hole.

Data are presented which show eight excitonic density (Rabi) oscillations in an In_0.1Ga_0.9As/GaAs multiple quantum well at 5 K. Our time resolved, two-color pump-probe experimental technique for observing these oscillations is described. The quantum well sample geometry and linear spectrum are shown along with data characterizing the pump and probe pulses. Experimental data is shown to be in excellent agreement with our theoretical calculation of exciton density versus time, verifying the important affect of the Coulomb interaction between carriers in renormalizing the Rabi frequency. The theoretical calculations include the two-fold degenerate light-hole, heavy-hole, and valence bands in the Hartree-Fock form of the semiconductor Block equations, with all variables selected to conform to the experiment.

We present an investigation of coherent mid-infrared pulse measurement system using free-space electro-optic sampling technique. A series of nonlinear materials are investigated for the nonresonant optical rectification and electro-optic sampling for the generation and detection, respectively. A sampling bandwidth up to 40 THz is achieved. For spectroscopic application we present THz field radiation by optically excited coherent phonons in the mid-infrared frequency range. We compare the THz radiation under extremely different excitation conditions. The comparison shows the THz radiation property of the resonantly driven phonons in the surface field layer. Furthermore we investigate the coherent reststrahl band reflectivity in the time domain. Unexpectedly strong oscillations are observed near the longitudinal-optical phonon frequency. This THz waveform is discussed with the reststrahl band dispersion of the reflectivity.

An exciting interactive program is written in Visual C⁺⁺ to visualized ultrafast electron transport dynamics in user defined quantum structures by solving time- dependent Schrodinger equations. The program accepts certain inputs about the quantum structure, the initial electron wave function, the information on electron-phonon or electron-photon interaction, and applied bias, and immediately displays results in animations at they are being calculated. This program provides powerful tool for those studying semiconductor devices at advanced undergraduate or graduate level. It will be also of interest and value to researchers in physics, materials science, and electrical engineering. In this paper, we will show how femtosecond electron transport dynamics can be visualized. Two examples are presented on the electron capture and decapture dynamics due to electron-phonon interactions as well as the ultrafast optoelectronic switching dynamics due to electron-photon interactions in a single semiconductor quantum structure.

We demonstrate experimentally the improvement of the performance of the dual pump wave mixing scheme in semiconductor optical amplifiers, using long amplifier chips and high optical pump powers. The optical amplifiers used in the experiment had a ridge waveguide structure with bulk active layer and antireflective-coated angled facets. Measurements of the conversion efficiency and SBR as a function of wavelength shift are presented for a wavelength shift of more than 40 nm. The above measurements are carried out for three amplifier lengths (500 micrometers , 1000 micrometers , and 1500 micrometers ) and for different levels of the optical power of the two pumps. It will be shown that an increase in the amplifier length from 500 micrometers to 1500 micrometers results to an increase of more than 25 dB for the efficiency and more than 20 dB for the SBR. This improvement combined with the inherent advantages of the dual pump scheme (almost constant SBR and high efficiency for large wavelength shifts) results in a highly performing wavelength converter/phase conjugator, suitable for many applications.

Decay of the longitudinal optical (LO) phonons in wurtzite GaN has been studied by subpicosecond time-resolved Raman spectroscopy. Our experimental results show that, among the various possible decay channels, the LO phonons in wurtzite GaN decay primarily into a large wavevector TO and a large wavevector LA or TA phonons. These experimental results are consistent with the recent theoretical calculations of the phonon dispersion curves for wurtzite GaN.

The electron-LO phonon interaction in Al_xGa_1-xAs has been studied, for the alloy composition ranging from 0.1 to 0.4, by picosecond Raman spectroscopy. We have found that the relative strength of the Frohlich interaction of the two LO phonon modes is a nonlinear function, and increases non- monotonically with the Al composition. The possible origins of these novel results are discussed in term of nonlinear dielectric theory and ordering in the alloy.

Electron ballistic transport and in a InP-based p-i-n nanostructure under the application of an electric field have been studied by time-resolved Raman spectroscopy at T equals 300 K. The time-evolution of electron distribution, electron drift velocity has been directly measured with subpicosecond time resolution. Our experimental results show that, for a photoexcited electron-hole pair density of n equalsV 5 X 10¹⁶ cm^-3, electrons travel quasi- ballistically--electron drift velocity increases linearly with time, during the first 150 fs. After 150 fs it increases sublinearly until reaching the peak value at about 300 fs. The electron drift velocity then decreases to its steady-state value.

Electron transient transport in a GaAs-based p-i-n nanostructure under the application of an electric field has been studied by transient Raman spectroscopy at T equals 80 K. The non-equilibrium electron distribution function and electron drift velocity were measured at an applied electric field intensity of E equals 25 kV/cm and for various injected electron-hole pair densities ranging from 10¹⁷ cm^-3 to 10¹⁹ cm^-3. Our experimental results show that, the electron distribution is very much out of equilibrium for n is congruent to 10¹⁷ cm^-3; however, as the electron-hole pair density increase the electron distribution function approaches that of the equilibrium case, in particular, as the carrier density increases to n is congruent to 10¹⁹ cm^-3, electron distribution becomes very much in equilibrium and electron drift velocity becomes almost zero. These results have been explained by the effects of momentum randomization and screening of effective electron field by the injected electron-hole pairs.

Electron capturing and de-capturing dynamics in a single semiconductor quantum well due to the interaction with longitudinal optical phonons are studied by directly solving the time-dependent Schrodinger equation. Time-dependent wave functions are calculated with an initial electron wave packet impinging on the quantum well, being reflected and transmitted, captured into and de-captured from the quantum well. We have obtained new information on electron capture and de-capture times as function of electron-phonon coupling strength. The red energy shift of ground state energy is also found to be linearly proportional to the electron- phonon interaction strength. These results can not be simply obtained using Fermi Golden Rule.

Time-resolved photoluminescence (PL) measurements are reported for GaN, InGaN, and AlGaN thin films, as well as InGaN/GaN multiple quantum wells (MQWs) with 3-nm-thick InGaN wells and 4.5-nm-thick GaN barriers. All the samples were grown on c-plane sapphire substrates by metalorganic chemical vapor deposition. We observed that the carrier recombination lifetime of the ternary InGaN and AlGaN alloys becomes longer with decreasing emission energy. An anomalous temperature-dependent emission behavior was observed for InGaN-related PL with increasing temperature. The actual temperature dependence of the PL emission was estimated with respect to the bandgap energy determined by photoreflectance spectra. The temperature-induced S-shaped PL shift is explained by a change in the carrier dynamics with increasing temperature due to inhomogeneity and carrier localization in the alloys. We also investigated the influence of Si doping on the optical characteristics of InGaN/GaN MQWs. The 10 K radiative recombination lifetime was observed to decrease from approximately 30 ns to approximately 4 ns with increasing Si doping concentration from < 1 X 10¹⁷ to 3 X 10¹⁹ cm^-3. A reduced Stokes shift between PL and PL excitation spectra, a reduction in a pump-density-induced blueshift, and an increase in the interface quality with increasing Si doping were observed. From these results, we conclude that Si doping results in a decrease in carrier localization caused by potential fluctuations in the MQWs.

The dynamics of coherent electron-photon coupling between extended electronic states and a confined electronic state is studied by directly solving the time-dependent Schrodinger equation. A new type of optoelectronic switching device is proposed. We present, as an example, a single quantum well device that may potentially operate at a bandwidth of 3 THz.