S. Colak

Efficient type I blue second‐harmonic generation in periodically segmented KTiOPO4 waveguides

We report type I phase‐matched blue second‐harmonic generation from periodically segmented channel ion‐exchanged waveguides in KTiOPO4 with output wavelengths from 0.38 to 0.48 μm and efficiencies exceeding 50%/W cm2. Evidence is presented suggesting that these efficiencies result from ferroelectric domain reversals induced by waveguide fabrication. Waveguide structure, wavelength, and output power characteristics are presented.

Influence of broadening and high-injection effects on GaAs-AlGaAs quantum well lasers

Gain spectra and gain-current relations are calculated for GaAs-AlGaAs quantum-well lasers using a model which incorporates a phenomenological description of bandgap narrowing due to many-body effects at high injection, transition broadening by a carrier-density-dependent intraband scattering process, and broadening of the density of states function by fluctuations in the well width. The justification for including all these phenomena is made by examining spontaneous emission spectra observed through contact windows on quantum-well lasers. >

Mechanism for efficient blue second‐harmonic generation in periodically segmented waveguides

We present a theoretical investigation of the mechanisms responsible for the recently reported efficient type I blue second‐harmonic generation in periodically segmented KTiOPO4:Rb,Tl waveguides. In these guides, grating‐assisted phase matching of second‐harmonic light between 390 and 480 nm has been achieved. 5‐mm‐long guides give output powers on the order of 1 mW at around 425 nm with fundamental powers in the 50–100 mW range. We show that such high efficiencies can be expected from strongly perturbed guides through terms representing the modulation of phase mismatch and mode size in addition to the nonlinear susceptibility and refractive index modulations.

dc and ac transport in molecular‐beam‐epitaxy‐grown metal/ZnSe/GaAs heterojunction structures

The room‐temperature electrical transport properties of ZnSe epilayers grown above the critical layer thickness on n+‐GaAs substrates by molecular‐beam epitaxy have been studied. dc current versus voltage and small‐signal ac admittance versus voltage and frequency measurements were made on Schottky contacts (Au or Hg). A novel method of analysis is presented which, extending a previous model to include the three‐dimensional effects of current spreading as a function of frequency, allows the ZnSe epilayer resistivity to be obtained. Combining this with the doping concentration obtained from analysis of the C‐V characteristics, the ZnSe mobility may be calculated. Mobilities thus measured compare well with our own and other workers’ published Hall mobility data on similar samples, as well as with theoretical calculations. We believe that our results represent a unique and detailed characterization of an epilayer heterostructure on a highly conducting substrate.

Temperature dependence of threshold current in GaAs/AlGaAs quantum well lasers

We have calculated the threshold current and its temperature (T) dependence in the range 200–400 K for AlGaAs quantum well lasers with 25‐A‐wide GaAs wells using a model which includes lifetime broadening of the transitions and broadening of the density of states function by fluctuations in the well width. The threshold current varies approximately linearly with T and the principal effect of broadening is to increase the threshold current causing a reduction in the fractional change of current with temperature. The apparent value of the parameter T0 is increased to ≊400 K, compared with ≊320 K without broadening. The calculations are compared with experimental data.

Transition rates of Tb3+ in TbP5O14, TbLiP4O12, and TbAl3(BO3)4: An evaluation for laser applications

The properties of Tb3+ in crystals of three stoichiometric Tb compounds, viz., TbP5O14, TbLiP4O12, and TbAl3(BO3)4 have been studied by spectroscopic absorption and emission methods to evaluate these materials as hosts for Tb3+ laser applications. The absorption spectra between 50000 and 200 cm−1 at 300 °K, the emission spectra between 27000 and 14000 cm−1 at 300 °K, and the fluorescence decay characteristics of 5D4 emission at 300 and 5 °K are given. Because of cross relaxation, the 5D3 and 5G6 emissions in all three materials were concentration‐quenched strongly with a similar behavior observed in other hosts. Quenching in the 5D4 states was negligible except for TbAl3(BO3)4 where moderate quenching was observed due to energy transfer to nonradiative traps. Transition rates between various states were calculated by using the Judd–Ofelt approach and a first order wavefunction set, and good fit was obtained between the calculated and experimental parameters. The transition rates were then used to find the...

Anomalous mobility and photo‐Hall effect in ZnSe‐GaAs heterostructures

The Hall and photo‐Hall effect are studied experimentally in pure, unintentionally doped n‐ZnSe epilayers grown by molecular‐beam epitaxy on semi‐insulating GaAs substrates. An anomalous depression of the dark values of the room‐temperature and peak mobility in the n‐ZnSe layer is observed in a substantial fraction of the samples studied, while normal values for the mobility are recovered under illumination with above‐band‐gap light. This is accompanied by an increase in the effective areal electron density. These results point to space‐charge scattering causing the dark mobility reduction. The photo‐Hall effect in ZnSe‐GaAs heterostructures is discussed in terms of simple models considering both misfit dislocations at the heterostructure interface and extended defects in the bulk of the ZnSe epilayer. Additional photo‐Hall experiments have been performed for longer‐wavelength laser lines. In this case, the areal electron concentration decreases strongly under intense illumination, while a large room‐temp...

Influence of the donor depth on the determination of the band discontinuity of isotype heterojunctions by the capacitance‐voltage technique

Capacitance‐voltage measurements have been simulated to find the conduction‐band discontinuity in an n‐type GaAs‐Al0.3Ga0.7As heterojunction. In the calculations the partial ionization of the donor level in the (Al,Ga)As is taken into account. It is found that exact knowledge of the donor depth is a prerequisite in order to infer the conduction‐band discontinuity from capacitance‐voltage measurements. For a dopant concentration of 3×1016 cm−3 on either side of the heterojunction the same apparent carrier profile is obtained with a conduction‐band discontinuity equaling 65% of the total band‐gap difference and zero donor depth as with 75% discontinuity and 100 meV donor depth. It is also shown that compensation influences the apparent carrier profile. Taking literature values for the donor depth in Al0.3Ga0.7As the discontinuity of the conduction band is estimated to be 70% of the total band‐gap difference.

Single longitudinal mode operation of the electron-beam-pumped semiconductor laser

The influence of the external and internal resonator parameters on the laser emission spectral and spatial characteristics has been investigated. Single longitudinal mode operation of the electron-beam-pumped visible semiconductor laser with an external resonator has been observed. A single-mode peak power of 0.8 W and beam divergence of less than 3° were achieved.

Threshold in electron-beam end-pumped II-VI lasers

Electron‐beam end‐pumped lasers from different bulk grown II‐VI compounds have been experimentally studied and compared under similar preparation and excitation conditions. The reasons for the lasing threshold variations are discussed. The first results on e‐beam pumped CdMnTe lasers and end‐pumped CdTe lasers are reported. The order of lowest to highest threshold is found to be from CdSe, ZnCdSe, CdS, CdTe, CdMnTe, and ZnSe. The comparisons between lasing conditions are used to evaluate the contribution of the intrinsic semiconductor parameters to lasing threshold. Experiments with a large number of samples indicate that the influence of intrinsic and extrinsic parameters on lasing threshold are in most cases comparable. Therefore, for most bulk II‐VI lasers, the average threshold pump power density reductions with the elimination of extrinsic factors are expected to be less than several times. These findings are further supported by our threshold and relative slope efficiency measurements on lasers with...