We report on time-resolved optical investigations on the polarization and the spin dynamics in non-magnetic and magnetic self-assembled II VI semiconductor quantum dots. In case of CdSe/ZnSe quantum dots, no transient loss of polarization is found within the time scale of exciton recombination, if one excites the excitons strictly resonant in the quantum dot ground state with a laser pulse linearly polarized along the [110] or [1-10] crystal axes. This indicates a high temporal stability of the exciton state, which is a coherent superposition of spin-up and spin-down exciton states. Even after replacing some Cd atoms in the crystal matrix by magnetic ions Mn²⁺, the polarization is being conserved as long as the average Mn²⁺ concentration is about (formula available in paper)or less, despite the pronounced exchange interaction between the manganese ion spins and the carrier spins. In case of magnetic semiconductor quantum dots with a large concentration of (formula available in paper)ions, the spin spin interaction between charge carriers and manganese ions results in the formation of a quasi-zero dimensional ferromagnetically aligned spin complex, the exciton magnetic polaron. For (formula available in paper)Se quantum dots this transient spin alignment is directly evidence by a transient shift of the emission energy. We deduce a typical time constant of 125 ps at T = 2 K for the dynamical response of the magnetic ion spins.

Potential application of elementary excitation in semiconductor quantum dot to quantum computation is discussed. Construction of scalable hardware and all optical implementation of logical gate that exploits discrete nature of electron-hole states and their well concentrated oscillator strenth for ultrafast gate operation are proposed. Rabi population oscillation of an excitonic two-level system in an isolated single InxGa1-xAs quantum dot is manifested by a quantum wave function interferometry in time domain, demonstrating a long lived coherence of zero-dimensional excitonic states and revealed the coherent population flopping under strong optical field. A phase-sensitive coherent gate operation on single-dot exciton qubit is demonstrated.

We report on the fast spin flip process in quantum wells and quantum dots. The utilization of spin degree of freedom will open the door to novel functional devices. In the GaAs/AlGaAs quantum ells, the spin relaxation time is found to be about several ten picoseconds at room temperature. InGaAs/InP quantum wells whose bandgap correspond to 1.5 micro meters have the fast spin relaxation of several picoseconds. Regarding to GaAs/AlGaAs multiple-quantum wells, we observed that Dyakonov-Perel process governs the spin relaxation at room temperature. All optical switching devices using this fast spin relaxation process were proposed and demonstrated. In the quantum dots, we have found that anitferromagnetic coupling between quantum dots exist at temperatures lower than 50-80K. The electron spin flips within 70-200 ps after the carrier generation. The spin relaxation time under the antiferromagnetic order is extended to 10-12 ns, an order of magnitude longer than that in isolated quantum dots.

We discuss the dependence of nonlinear optical signals on the carrier-envelope offset phase, i.e., the phase between the rapidly oscillating carrier wave and the electric field envelope of a few-cycle pulse. Our results in the regime on non-perturbative resonant nonlinear optics are compared with the know off-resonant perturbative regime. In particular, we introduce camouflage third-harmonic generation, which shows up as a peak at twice the carrier-envelope offset frequency in the radio-frequency spectra. Corresponding experiments on ZnO with intense 5 fs optical pulses are discussed. We speculate on the perspectives of using such effects for determining the carrier-envelope offset phase itself.

We describe theoretical analysis and design of one-dimensional photonic crystal prisms. We find that inside the photonic crystal, for frequencies near the band edges, light propagation direction is extremely sensitive to the variations in wavelength and incident angle. Outside the photonic crystal, propagation direction of transmitted light is highly sensitive to changes in wavelength, but less so to incident angle. For transmitted light outside the photonic crystal, the superprism effect can be used for wavelength dispersion over entire photonic bands, rather than just near the band edges only. This extended-range superprism effect broadens the useful wavelength rage, admits better optical transmission, and exhibits angular dependence on wavelength with greatly reduced non-linearity.

We have studied, theoretically and experimentally, nonlinear propagation of 90 fs laser pulses at 800 nm in bulk fused silica samples. For the first time, we present comparisons between numerically simulated and experimentally measured angle-resolved, far-field spectra for such pulses. They are in good agreement for pulse powers up to, approximately, critical power for self focusing.

We discuss theoretical predictions for ultrafast (mostly femtosecond) linear and nonlinear optical phenomena in metal/insulator and metal/semiconductor nanocomposites. In metal/insulator nanocomposites, we consider spontaneous and coherently-controlled energy concentration. We show that very strong (many orders of magnitude in local-field intensity) ultrafast concentration of energy on nanoscale is possible. In metal/semiconductor quantum dot nanocomposites, we consider a fascinating possibility of quantum generator due to spaser (surface plasmon amplification by stimulated emission of radiation). Various applications are discussed.

We report on photoluminescence (PL) properties of ZnO epitaxial films and single-crystal nanorods grown by low pressure metalorganic vapor phase epitaxy. Time-integrated PL spectra of the films at 10 K clearly exhibited free A and B excitons at 3.376 and 3.382 eV and bound exciton peaks at 3.360, 3.364, and 3.367 eV. With increasing temperature, intensities of the bound exciton peaks drastically decreased and a free exciton peak was dominant above 40 K. Similarly, vertically well-aligned ZnO nanorod arrays also exhibited free exciton peaks at 3.374 and 3.381 eV, which indicates that ZnO nanorods prepared by the catalyst-fee method are of high optical quality. Furthermore, time-resolved PL measurements at a free exciton peak were carried out at room temperature. The decay profiles were of double-exponential form, and the decay time constants of 180 ps and 1.0 ns were obtained using a least-square fit of the data. Excitation power-dependent PL of ZnO epilayers is also discussed.

The ultrafast carrier dynamics in InGaN multiple quantum well (MQW) laser diodes were investigated using a time-resolved bias-lead monitoring techniques. Both pump and probe beams were from the second harmonic generation (SHG) of a tunable 100-fs Ti:Sapphire modelocked laser. From the optical selection rules of TE and TM polarized lights, one can selectively excite and probe different valance subbands to conduction band transitions in the MQW structure with different polarized pump and probe light. Using this technique, ultrafast inter-subband hole relaxation processes were found to dominate the observed carrier dynamics.

Ultrafast photodiode is a key device, which links electronics and photonics technologies, especially in measurement, sensing and communications systems. Uni-traveling-carrier photodiode UTC-PD is unique in that it provides both a large bandwidth and a high saturation output current at 1.55-m wavelength. In this paper, we describe recent progress in UTC-PD technologies and their analog and digital applications in the frequency regions from giga-hertz to tera-hertz. First, operation principle and characteristics of InP/InGaAs UTC-PDs are shown. Time-domain response exhibited 1-ps pulse width, which corresponds to the 3-dB bandwidth of over 300 GHz. To enhance the output power, resonant-operation of the UTC-PD integrated with a matching circuit is proposed. The CW output power exceeds 20 mW at 100 GHz. Photonically-generated electrical signals are applied as stimulus to ultrafast measurement systems such as IC testers and network analyzers. By integrating PDs with planar antennas in monolithic as well as hybrid fashions, millimeter and submillimeter-wave emitters are developed. These photonic emitters are used as signal sources for transmitters in millimeter-wave wireless links, imaging applications, and local oscillators in radio-astronomy receivers. Finally, a novel functional device, in which an electro-absorption modulator is integrated with the PD, is demonstrated as a demultiplexer for over 300-Gb/s optical communications systems.

We have dramatically improved the optical properties of extremely thin QWs required for ISBT devices operating at optical communication wavelengths using novel InGaAs/AlAs/AlAsSb QW structures with 4-7 monolayers (MLs) of AlAs. The intersubband saturation intensity (Is) was reduced to 3fj/μm². This represented an I_s reduction of nearly 3 orders of magnitude relative to the previous samples whether or not such sample featured 1 ML of AlAs interface layer. This paper reviews the recent results of novel InGaAs/AlAs/AlAsSb quantum well properties grown by molecular beam epitaxy, and discusses the linear and nonlinear optical responses of ISBT.

Photonic-crystal distributed-feedback (PCDFB) lasers can potentially operate in a single optical mode that remains coherent over extremely large device areas, e.g., > 1 mm², in spite of the effects of filamentation induced by the linewidth enhancement factor. Two-dimensional diffraction is induced by gratings that are defined on a rectangular lattice for edge emission, or on a hexagonal or square lattice for surface emission. Numerical simulations based on original algorithms reveal that whereas minimizing that product is almost always advantageous in edge-emitting lasers, an optimized surface-emitter should have an intermediate value. We also review recent experimental demonstrations of both 2_nd-order and 1^st-order optically pumped broad-stripe PCDFB lasers with “W” active regions that emit in the mid-IR.

Ultrabroadband THz spectroscopy is employed to observe how many-particle interactions build up in an extreme non-equilibrium electron-hole plasma. The plasma is photogenerated in bulk GaAs via resonant interband absorption of a 10 fs laser pulse. Subsequently, the dynamics of the complex dielectric function throughout the mid-infrared is directly monitored with uncertainty-limited temporal resolution with a single-cycle THz pulse. Field sensitive detection allows us to measure simultaneously real and imaginary part of the complex dielectric function of the plasma in the multi-THz regime. We show that collective phenomena such as Coulomb screening and plasmon scattering exhibit a delayed onset. This observation is explained in terms of the ultrafast formation of dressed quasiparticles. The time scale for this transient behavior is of the order of the inverse plasma frequency. Our findings support recent quantum kinetic calculations of the temporal evolution of the Coulomb interaction after ultrafast excitation of a dense electron-hole plasma.

We have investigated terahertz (THz) emission due to dynamical electron transport in wide miniband GaAs/Al_0.2Ga_0.7As superlattices. By noting that the time-domain THZ emission spectroscopy inherently measures the step-response of the electron system to the bias electric filed, the obtained THz spectra were compared with the high-frequency conductivities predicted for miniband transport. Excellent agreement between theory and experiment strongly supports that the THz gain due to Bloch oscillating electrons persists at least up to 1.7 THz. It was also found that Zener tunneling into the second miniband sets the high-frequency limit of the THz gain for the samples studied here.

Terahertz (THz) absorbance measurements as a function of confirmation for bacteriorhodopsin has been performed in the steady state demonstrating the dependence on protein conformation and mutation. We introduce a new technique for performing low time resolution visible pump/THz probe measurements relevant to biomolecular systems. We will discuss the experimental protocol required for these measurements and how the THz absorbance can be used as a characterization tool for technologically important biomolecules.

The generation, manipulation and relaxation of optical intersubband excitations in n-type GaAs/AlGaAs and p-type SiGe/Si quantum wells are studied by different techniques of ultrafast spectroscopy in the mid-infrared. For electrons in GaAs/AlGaAs quantum wells, femtosecond time-resolved four-wave-mixing studies demonstrate de-phasing times of coherent intersubband polarizations of several hundreds of femtoseconds which are determined by electron-electron scattering. The measured dephasing times fully account for the width of the stationary intersubband absorption line, giving evidence of a predominant homogeneous broadening. Using phase-locked mid-infrared pulses and phase-resolving detection schemes, coherent optical control of such intersubband polarizations is demonstrated. In a second experiment, we study the intersubband relaxation of heavy holes (HH) in p-type SiGe/Si quantum wells. Intersubband scattering from the HH2 back to the HH1 subband occurs with a time constant of 250 fs determined by scattering with optical phonons through the deformation potential interaction.

We observe ultrafast and large nonlinear optical responses of a semiconductor thin film where excitons are weakly confined and their center-of-mass motion is quantized. The observed degenerate four-wave mixing (DFWM) signal of a GaAs double-hetero-structure layer exhibits an anomalous thickness-dependence, and is much enhanced at the thickness of 110nm. The decay time of the response is the order of ps, which depends on the thickness of the layer and on the number of the layer. These phenomena are elucidated by the theory of the nonlocal interaction between excitonic states and the light beyond the long wavelength approximation regime. The enhancement at the thickness is due to the size-resonant enhancement of the internal field with a nano-scale spatial structure, where the second quantized level of the exciton center-of-mass motion mainly contributes to the large response at the thickness. We, furthermore, demonstrate ultrafast response in a reflective type nonlinear polarization rotation switch with the 110nm thick three double-hetero. The signal intensity of 20% for the reflective pulse was obtained at a pump intensity of 10nJ/cm², and the decay time is 1.5ps at low temperatures. These results provide a novel design of a semiconductor structure in order to obtain both large and ultrafast nonlinear response that is suitable for device application.

We have experimentally proven the Cerenkov generation of optical phonons by drifting electrons in a semiconductor. We observe an instability of the polar optical phonons in nanoscale semiconductors that occurs when electrons are accelerated to very high velocities by intense electric fields. The instability is observed when the electron drift velocity is larger than the phase velocity of optical phonons and rather resembles a “sonic-boom” for optical phonons. The effect is demonstrated in p-i-n semiconductor nanostructures by suing subpicosecond Raman spectroscopy. We suggest that the observed phenomena will have enormous impact on the carrier dynamics in nanoscale semiconductor devices.

Bound and unbound biexcitons in a free-standing bulk GaN are investigated by time-integrated and spectrally-resolved four-wave mixing measurements, where the formation of hetero-biexcitons that consist of A and B excitons (XX_AB) as well as A-biexcitons (XX_AA) and their unbound biexciton (XX*_AA) are clearly observed. The FWM spectra and delay-time dependence are explained qualitatively and the interaction between A- and B-excitons gives rise to the phase shifts of the quantum beating and the energy shifts of the spectra, which is considered as the effect of the unbound state of XX_AB (i.e. XX*_AB). The unbound A-biexciton (XX*_AA). Is also observed clearly in spectral and temporal domain and is found to play an important role in FWM signals for all polarizations.

AlN epilayers with high optical qualities have been grown on sapphire substrates by metal organic chemical vapor deposition (MOCVD). Deep ultraviolet (UV) photoluminescence (PL) spectroscopy has been employed to probe the optical quality as well as optical transitions in the grown epilayers. Two PL emission lines associated with the donor bound exciton D0X, or I2 and free exciton (FX) transitions have been observed, from which the binding energy of the donor bound excitons in AlN epilayers was determined to be around 16 meV. Time-resolved PL measurements revealed that the recombination lifetimes of the I2 and free exciton transitions in AlN epilayers were around 80 ps and 50 ps, respectively. The temperature dependencies of the free exciton radiative decay lifetime and emission intensity were investigated, from which a value of about 80 meV for the free exciton binding energy in AlN epilayer was deduced. This value is believed to be the largest free exciton binding energy ever reported in semiconductors, implying excitons in AlN are an extremely robust system that would survive well above room temperature. The PL emission properties of AlN have been compared with those of GaN. It was shown that the optical quality as well as quantum efficiency of AlN epilayers is as good as that of GaN. It was shown that the thermal quenching of PL emission intensity is greatly reduced in AlN over GaN, which suggests that the detrimental effect of impurities and dislocations or non-radiative recombination channels in A1N is much less severe than in GaN. The observed physical properties of AlN may considerably expand future prospects for the application of III nitride materials.

The carrier dynamics in InGaN layers and InGaN multiquantum-well (MQW) structures were studied by employing the degenerate pump and probe, time-resolved photoluminescence (PL), and white-light pump and probe measurements. The results from degenerate pump and probe measurements showed that the internal field existed in undoped in InGaN but then was screened by the electrons supplied by the Si atoms in Si-doped InGaN. The rise time of Pl in InGaN MQW obtained using the upconversion method was very fast, below 1ps, the mechanism of which is due to the carrier-carrier interaction enhanced by the residual electrons. The ΔOD spectra in InGaN MQW observed in white-light pump and probe measurements indicated that the carrier temperature was substantially higher than the lattice temperature even at 40ps after pulsed photo-pumping.

We have used femtosecond time-resolved reflectivity and luminescence downconversion techniques to study carrier relaxation, localization, and recombination in III-nitride semiconductors. Intensity dependent, frequency degenerate pump-probe reflectivity measurements employing near-bandgap excitation provide information about initial carrier localization, subsequent ultrafast heat generation due to nonradiative recombination or trapping in states deep in the bandgap, and photoinduced absorption associated with excitation of carriers from localized states to the bands. These phenomena and their experimental signatures are illustrated for Al_0.25Ga_0.75N and Al_0.4Ga_0.6N samples, in which the photoinduced change in reflectivity ΔR decays faster with decreasing intensity and changes sign, with faster decays for a given intensity in the higher Al content sample. This behavior suggests that in these cases the dynamics are governed by trapping at localized states associated with alloy fluctuations that become deeper and more numerous as the Al content increases. Within this context the sign change and subsequent temporal evolution of ΔR may be indicative of ultrafast heat generation and/or photoinduced absorption, depending upon A1 content. Nondegenerate pump-probe reflectivity experiments designed to separate the electronic contributions of the ΔR decays from the slower thermal components by using a sub-bandgap probe are used to measure carrier lifetime in GaN. Comparison with data obtained from frequency degenerate experiment sunder identical excitation conditions employing a near bandgap probe indicate that in the frequency degenerate case the decay times in ΔR are inflated due to the presence of an additional long-lived component with the same sign as the electronic contribution. The sign and power dependence of this slow decay suggest that it may be associated with screening of a surface electric field by carriers trapped in deep states. In addition, a new technique is presented that employs luminescence downconversion using an ultrashort gating pulse to enable the characterization of UV light emission from III-nitride semiconductors with subpicosecond temporal resolution. This technique also allows one to measure PL rise times and fast components of multiple decays in the subsequent time evolution of the PL intensity. Comparison of luminescence emission intensity and lifetime in GaN and AlGaN with ~0.1 Al content grown homoepitaxially on GaN templates with the same quantities measured in heteroepitaxial layers grown on sapphire indicate significant improvement in the homoepitaxial layers due to reduction in defect density. Fast (<15 ps) initial decays in the AlGaN are attributed to localization in shallow traps associated with alloy fluctuations, with subsequent recombination through gap states.

We review some experimental and theoretical aspects of coherent acoustic phonon generation in piezoelectric semiconductor multiple quantum wells. In order to model more advanced and complicated nano-acoustic devices, a macroscopic continuum theory for the generation and propagation of coherent acoustic phonons in piezoelectric semiconductor heterostructures is presented. The macroscopic approach is applicable in the coherent regime, and can be easily utilized to study coherent acoustic devices based on piezoelectric semiconductor heterosutructures. For each phonon mode, the corresponding coherent acoustic field obeys a loaded string equation. The driven force has contributions from the piezoelectric and deformation potential couplings. We applied the theory to model the generation of coherent longitudinal acoustic phonons in (0001)-oriented InGaN/GaN multiple quantum wells. The numerical results are in good agreement with the experimental ones. By using the macroscopic theory, we also investigated the crystal-orientation effects on the generation of coherent acoustic phonons in wurtzite multiple quantum wells. It was found that coherent transverse acoustic phonons dominate the generation for certain orientation angles.

We have observed the exciton spin relaxation processes in GaAs quantum wells at low temperatures by three-pulse spin-diffracted four-wave mixing measurements. After investigating the merits and demerits of this new method as compared with the pump-probe technique, we discuss the population transfer to the dark states through the measurements of the excitation-power dependence of exciton spin relaxation. Spin-diffracted four-wave mixing method is a powerful tool to investigate exciton spin dynamical processes where the pump-probe and time-resolved photoluminescence measurements have been used so far, since the method can be used in reflection geometry and be applicable to thin films and quantum dots.

Time-resolved photoelectric spectroscopy measurements of photorefractive CdT:V crystals were carried out by using a short light pulse with 9 ns duration from a nitrogen laser 337.1 nm. The light pulse was focused through the semitransparency Au-electrode. The stationary monochromatic illumination of crystals allowed to measure the time-resolved photocurrent, which is caused by the detrapping of electrons photogenerated by the pulse laser excitation. The dependence of intensity of pulse photocurrent at the delay time (formula available in paper), which corresponds to its maximum value, on the energy of additional monochromatic illumination was investigated. In the case, the spectral dependence of pulse photocurrent caused by the detrapping process of electrons in CdTe:V crystals has been measured under the different intensity of the electric field. It was shown that the additional illumination at (formula available in paper)leads to the increasing of photocurrent intensity that is caused by the detrapping processes of electrons from impurity centers and intrinsic defects. Obtained results indicate that CdTe:V crystals are high-sensitive ultrafast photorefractive materials which may be also used for the elaboration of fast photodetectors in the near IR-region.

Time-integrated and spectrally-resolved four-wave mixing (FWM) has been used to study dephasing dynamics of excitons in a free-standing bulk ZnO. Clear FWM signals due to A⟩Γ₅- and BΓ₅-excitions have been observed. We discuss the dephasing dynamics based on the polariton dispersion and four-particle Coulomb correlations.

Transient subpicosecond Raman spectroscopy has been used to interrogate electron transport properties in In_xGa_1-xAs-based semiconductor nanostructure under the application of an electric field. The deduced electron drift velocity has been found to be much larger than either GaAs or InP-based p-i-n nanostructures at comparable field. We attribute this to both the much smaller electron effective mass and the much larger Γ to X (L) energy separations in InxGa_1-xxAs-based semiconductor nanostructures.