D. Patel

Dominant role of the piezoelectric field in the pressure behavior of InGaN/GaN quantum wells

We show that the emission characteristics of InGaN/GaN quantum wells under hydrostatic pressure are strongly influenced by the built-in piezoelectric field. The dominant role of the piezoelectric field is established from the dramatic increase of the photoluminescence decay time with pressure and the dependence of the linear pressure coefficient of the photoluminescence peak energy on Si doping in the barriers and excitation intensity. A nonlinear increase of the piezoelectric field with hydrostatic pressure determined from these experiments is explained as being due to a significant dependence of the InGaN piezoelectric constants with strain.

Large area high efficiency broad bandwidth 800 nm dielectric gratings for high energy laser pulse compression.

We have demonstrated broad bandwidth large area (229 mm × 114 mm) multilayer dielectric diffraction gratings for the efficient compression of high energy 800 nm laser pulses at high average power.

Optical properties of semiconductor lasers with hydrostatic pressure

The pressure dependence of the stimulated emission of a 1.3 μm In1−xGaxAsyP1−y (y=0.6) buried heterostructure laser has been measured up to 2.5 GPa. In this pressure range the laser output was tuned over 200 nm. The laser output energy varied linearly with pressure at a rate of (80±3) meV/GPa, corresponding to variation of the direct band gap of the InGaAsP active layer with pressure. From the measured energy shift and from calculations of the fractional volume change in the active layer for the strain conditions of the epilayer, a hydrostatic deformation potential of (−5.7±0.1) eV was obtained.

Effect of indirect minima carrier population on the output characteristics of AlGaInP light-emitting diodes

We show that carrier transfer to the indirect X level in the confining layer is responsible for most of the substantial decrease in the efficiency of AlGaInP light-emitting diodes (LEDs) operating at short wavelengths. Carrier transfer to the confining X level was obtained by reducing the separation between the AlGaInP direct Γ minimum and the X levels by varying the Al composition in the active region and by the application of hydrostatic pressure. Carrier transfer to the confining X level appeared as an additional peak in the electroluminescence (EL) and resulted in a significant decrease of the LED efficiency. A simple model of the EL emission that takes into account carrier population in the X minima was found to be in excellent agreement with the measured EL behavior.

Detuning of the gain and reflectivity spectra and its effect on the output characteristics of vertical cavity surface emitting lasers

We have investigated the effect of mismatch between the reflectivity resonance and the gain spectra of vertical cavity surface emitting lasers. Detuning was caused by hydrostatic pressure and self‐heating. Hydrostatic pressure shifts the gain peak towards shorter wavelengths with respect to the Fabry‐Perot (FP) resonance without modifying the gain spectrum. The threshold current remained unchanged for a positive mismatch of up to 18 nm. It increased four‐fold for a negative mismatch of −13 nm at 0.5 GPa, where lasing disappeared. Increased threshold current and quenching of the emission are a consequence of the decrease of the gain at the FP resonance. A similar effect was observed when the gain peak was red‐shifted with respect to the FP resonance by increasing the injected power. Increased mismatch is accompanied by a decrease of the gain at the emission wavelength which is responsible for the laser output quenching at high injected powers.

Point defects in Sc 2 O 3 thin films by ion beam sputtering

We show that the concentration of oxygen interstitials trapped in Sc2O3 films by ion beam sputtering from metal targets can be controlled by modifying deposition conditions. We have identified point defects in the form of oxygen interstitials that are present in Sc2O3 films, in significantly high concentrations, i.e., ∼10(18)  cm(-3). These results show a correlation between the increase of oxygen interstitials and the increase in stress and optical absorption in the films. Sc2O3 films with the lowest stress and optical absorption loss at 1 μm wavelength were obtained when using a low oxygen partial pressure and low beam voltage.

The intrinsic frequency response of 1.3-/spl mu/m InGaAsN lasers in the range T=10/spl deg/C-80/spl deg/C

Optical modulation response experiments above threshold are carried out in ridge waveguide InGaAs and InGaAsN (N=0.5%) in a temperature span of 10 degC-80 degC. The modulation traces are analyzed with a complete rate equation model that allows extraction of the resonance frequency and damping that are intrinsic to the carrier and photon processes occurring in the laser active region. This analysis enables calculation of the K-factor and its temperature behavior. K-values for InGaAsN lasers are larger and show a more pronounced dependence on temperature than in InGaAs lasers. This behavior is ascribed to a decrease in the effective differential gain with nitrogen content

Nonlinear macroscopic polarization in GaN/AlxGa1−xN quantum wells

We present experimental evidence of the nonlinear behavior of the macroscopic polarization in GaN/AlxGa1−xN quantum wells. This behavior is revealed by determining the barrier-well polarization difference as a function of applied hydrostatic pressure. The polarization difference and corresponding built-in electric field in the wells increase with applied pressure at a much higher rate than expected from the linear model of polarization. This result, universally observed in the quantum well structures with different AlN mole fraction in the barriers, is explained by the nonlinear dependence of the piezoelectric polarization in GaN and AlN on the strain generated by pressure.

Studies of femtosecond laser induced damage of HfO2 thin film in atmospheric and vacuum environments

The single pulse femtosecond laser induced damage threshold (LIDT) of hafnia and silica films is not affected by the ambient gas pressure. In vacuum, the multiple pulse LIDT drops to ~10% (~10%) of its atmospheric value for hafnia (silica). The water vapor content of the ambient gas was found to control the change in the LIDT. The LIDT of bulk fused silica surfaces did now show any dependence on the ambient gas pressure. Hydrocarbons (toluene) did not change the multiple pulse LIDT for Hafnia films

Enhanced characteristics of InGaAsP buried quaternary lasers with pressures up to 1.5 GPa

Pressure dependent measurements of the threshold current, differential quantum efficiency, and lasing wavelength of a ∼1.3 μm bulk InGaAsP semiconductor laser performed in a diamond anvil cell up to 1.5 GPa are reported. Results show a 40% decrease in the threshold current and a simultaneous enhancement in the differential quantum efficiency of approximately 350%. Large wavelength tunability of 140 nm is observed in this pressure range at room temperature. Calculations indicate that a reduction of the Auger recombination rate is likely to be the dominant loss mechanism responsible for the observed changes in the laser threshold current in this pressure range.