Femtosecond laser-induced melting of GaAs probed by optical second-harmonic generation

Primary steps of melting (disordering) of GaAs surface layers after femtosecond pulse excitation have been measured through second-harmonic generation in reflection. A numerical fit of the experimental data yield a characteristic time delay between excitation and lattice melting of about 75 fs.     

Two-photon absorption of optical and x-ray quanta

Two-photon absorption of an optical (laser) and an x-ray quantum is considered within the context of the Born approximation. A simple expression relating the two-photon and one-photon photoionization absorption coefficients is obtained. If the upper state is one of the conduction bands of a solid, the two-photon absorption coefficient is shown to peak at points in the zone for which the curvature of the band is a maximum. Numerical estimates show that with a 100 kW Nd laser, the intensity of the two-photon absorption process is a few percent of that of the one-photon process.     

Compact 10 GHz loopback arrayed-waveguide grating for high-fidelity optical arbitrary waveform generation

We demonstrate a high-performance optical arbitrary waveform shaper based on a single 10 GHz arrayed-waveguide grating with 64 loopback waveguides and integrated amplitude and phase modulators on each waveguide. The design is compact and self-aligning and allows for bidirectional operation. The device's complex transfer function is manipulated and measured over the full 640 GHz passband. To demonstrate optical arbitrary waveform shaping, high-fidelity 15-line shaped waveforms are measured with cross-correlation frequency-resolved optical gating.     

High power optical fibre delivery system for the laser generation of ultrasound

A plastic-clad optical fibre system has been examined as a delivery system of pulsed laser energy for the generation of ultrasound. The onset of significant optical damage caused by the laser has been investigated. By using multimode fibres of 1 m length with 600 μm core size, it was found that laser power densities from the fibre were sufficient to produce ultrasonic waveforms corresponding to both the thermoelastic and plasma generation regimes. Out-of-plane acoustic displacements of >250 pm can be achieved through 2.5 cm thick aluminium test samples, showing that fibre optical delivery systems may be considered in systems using laser-generated ultrasound for non-destructive testing applications.     

Intracavity terahertz-wave generation in a synchronously pumped optical parametric oscillator using quasi-phase-matched GaAs

We generated 1 mW of average output power at 2.8 THz (bandwidth of approximately 300 GHz) in a diffraction-limited beam by placing a 6-mm-long quasi-phase-matched GaAs crystal inside the cavity of a synchronously pumped optical parametric oscillator (OPO). The OPO used type-II-phase-matched periodically poled lithium niobate as a gain medium and was pumped by a mode-locked laser at 1064 nm, with a 7 ps pulse duration, 50 MHz repetition rate, and 10 W average output power. The terahertz radiation was generated by difference frequency mixing between the signal and idler waves of the near-degenerate doubly resonant OPO.     

GHz frequency band soliton generation using integrated ring resonator for WiMAX optical communication

New system of optical soliton signal generation with a high frequency band is presented. Microring resonator can be used to generate soliton signals with high frequency of GHz. These signals can be transmitted via a wireless network system known as WiMAX, which is providing broadband wireless access up to 50 km. The soliton pulses are more stable with less loss during propagation which is good candidate compare to other current optical waves used in optical communication. In this study, soliton pulses with full width at half maximum of 3 MHz and FSR of 85 MHz could be generated using an add/drop filter system which is used to generate high frequency signals, required for wireless network systems. These pulses can be used as carrier signals in order to transmit information codes without significant changes, thus improving transmission quality and delivery of the right information.     

Stimulated and spontaneous optical generation of electron spin coherence in charged GaAs quantum dots

We report on the coherent optical excitation of electron spin polarization in the ground state of charged GaAs quantum dots via an intermediate charged exciton (trion) state. Coherent optical fields are used for the creation and detection of the Raman spin coherence between the spin ground states of the charged quantum dot. The measured spin decoherence time, which is likely limited by the nature of the spin ensemble, approaches 10 ns at zero field. We also show that the Raman spin coherence in the quantum beats is caused not only by the usual stimulated Raman interaction but also by simultaneous spontaneous radiative decay of either excited trion state to a coherent combination of the two spin states.     

Excitation energy dependence of the optical properties of InGaAs/GaAs quantum well heterostructures

Il Nuovo Cimento D - We report an experimental study on a large set of InGaAs/GaAs quantum well structures by means of continuous-wave photoluminescence and photoluminescence excitation. The...     

10 GHz ultra-stable short optical pulse generation via phase-modulation enhanced dual-loop optoelectronic oscillator

We report on a high rate, ultra-low timing jitter, short optical pulse generator based on cascaded amplitude and phase modulation in an optoelectronic oscillator. The radio-frequency supermodes are shown to be greatly suppressed with the dual-loop architecture, and a highly coherent and flat optical frequency comb is generated. Optical pulses of 12.8 ps duration are obtained with 27.5 fs integrated timing jitter from 100 Hz to 10 MHz.     

Influence of built-in electric filed on the energy and emergence angle spreads of transmission-mode GaAs photocathode

In order to research the transport characteristic of photoelectrons in different-structure transmission-mode GaAs photocathodes, the energy and emergence angle spreads of photoelectrons reaching the band-bending region are calculated and the photoemission properties are analyzed. Based on the established atomic configuration models and ionized impurity scattering formulas of the uniform-doping and exponential-doping photocathodes, the trajectories of photoelectrons in different GaAs photocathodes have been calculated. The results show that, the emergence angle spread of the exponential-doping photocathode is more centralized than that of the uniform-doping one. The influence of the built-in electric field on the photoemission is obvious in the short-wave region. The built-in electric field not only increases the quantum efficient, but also improves the resolution of photocathode. This research can be propitious to investigate the photoemission mechanism, and to analyze the effect of the excited photoelectrons on the image intensifier performance.     

Magnetic-field-induced second-harmonic generation in semiconductor GaAs

We show that application of a magnetic field induces optical second-harmonic generation (SHG) in GaAs. This phenomenon arises from field-induced symmetry breaking causing new optical nonlinearities. A series of narrow SHG lines is observed in the spectral range from 1.52 to 1.77 eV that we attribute to Landau-level quantization of the band energy spectrum. The rotational anisotropy of the SHG signal distinctly differs from that of the electric-dipole approximation. Model calculations reveal that nonlinear magneto-optical spatial dispersion that comes together with the electric-dipole term is the dominant mechanism for this nonlinearity.     

Difference frequency generation in GaAs microdisks

Semiconductor-based whispering-gallery-mode microcavities are very promising for nonlinear optics applications, thanks to the high optical quality factors attainable with today's technology. We propose to exploit this advantage to generate cw light through phase-matched difference frequency generation in a triply resonant GaAs microdisk. A proper choice of the microdisk radius and thickness allows one to select the generated wavelength in the band of 2.5-2.9 mum. Besides illustrating the design features, we numerically show that temperature can be effectively used to compensate for wavelength shifts induced on the generated field by fabrication errors.     

UHF generation in sub-micron layers of GaAs

Mathematical modelling of n⁺-n-n⁺ GaAs structures using the macroparticle method and the Monte Carlo procedure for simulating scattering events indicates that at a lattice temperature of 4.2 K ballistic and quasiballistic electron transport can lead to electron plasma instabilities. If the main scattering mechanism is the emission of polar optical phonons, plasma instability leads to UHF current oscillations at frequencies near 460 GHz.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 10–14, December, 1989.     

Temperature dependence of the energy GAP in GaAs

It is shown that electron-phonon self-energy effects are as important as the Debye-Waller screening of the pseudopotential in the temperature dependence of the band gap when the valence band is degenerate.