J. Hays

Two‐photon pumped blue lasing in bulk ZnSe and ZnSSe

We have studied laser action of ZnSe and ZnS0.05Se0.95 samples grown by seeded physical vapor phase transport (SPVT) technique in the near resonant two‐photon absorption regime. The two‐photon pumped lasing was observed using a tunable near‐infrared nanosecond laser (830–890 nm). Blue lasing could be observed up to 200 K and the two‐photon pumped lasing threshold was measured to be ∼7 MW/cm2 at 10 K. This work demonstrates the applicability of using near‐infrared diode lasers as pumping sources to excite the ZnSe‐based materials. By comparing the energy position of two‐photon pumped lasing with that of the one‐photon pumped lasing and examining the red shift of the energy position, we suggest that the free‐exciton free‐exciton scattering and resultant band filling are the dominant processes involved in the lasing action of ZnSe and ZnSSe alloy crystals at low temperature.

Picosecond four‐wave‐mixing in GaN epilayers at 532 nm

Pulsed probe degenerate four wave mixing experiments were performed on GaN epilayers using 13 ps pulses at 532 nm. Intensity and time response of the scattering efficiency was studied. Intensity dependence of the observed signal suggests carrier generation by both single and two photon effects. The absolute scattering efficiency was measured and related to pump‐induced nonlinear index change. The nonlinear refractive coefficient found was 1×10−3 cm2/GW which is greater than an order of magnitude larger than the expected value. Time response of the signal was found to be dictated by carrier lifetimes. Double‐exponential decays to trap levels with lifetimes of 100 ps and 1.1 ns are suggested as the dominant recombination processes.

Optical properties of highly strained CdSe/ZnSe quantum wells

We present a study of the optical properties of highly strained CdSe/ZnSe quantum well system. A variety of CdSe/ZnSe samples containing single quantum well or multiple quantum wells grown by molecular beam epitaxy has been studied by using low‐temperature photoluminescence (PL), photoluminescence excitation, and photoreflectance measurements. The strong PL signals associated with excitonic emissions from the samples show that the CdSe/ZnSe heterostructure system is promising in the development of laser diodes and light‐emitting diodes operating in the blue‐green range. Linewidth narrowing of PL spectra with decreasing well width is observed and attributed to alloy formation at the interface due to lateral interdiffusion. The PL signal intensities and the pressure coefficients of interband transitions are also found to depend on the well width, which can be explained in terms of strain relaxation induced misfit dislocations and the critical thickness in the heterostructure system. Our results suggest that...

Bound exciton luminescence in ZnSe under hydrostatic pressure

We report pressure‐dependent photoluminescence (PL) measurements on ZnSe single crystals grown by the seeded physical vapor phase transport technique. Two kinds of ZnSe samples, the as‐grown and Zn‐extracted single crystals were used in this study. The Id1 line is the predominant spectral feature for the as‐grown samples and shifts with pressure at a rate of 7.3 meV/kbar. The I3 emission dominates the PL spectra of the Zn‐extracted samples and has a pressure coefficient of 6.6 meV/kbar, which is inconsistent with the assumption of the I3 line originating from the recombination of the excitons bound to ionized donors.

Room temperature blue lasing of ZnSxSe1−x alloys by photopumping

Photopumped lasing of ZnS0.05Se0.95 alloys grown by seeded physical vapor‐phase transport technique is observed at room temperature for the first time. This is achieved with the excitation photon energy tuned close to the band gap. The longitudinal lasing modes are well resolved. This work demonstrates that ZnSxSe1−x alloys can be used as an alternative to ZnSe in blue light‐emitting device fabrications.

Femtosecond wave mixing experiments in ZnCdSe/ZnSe quantum wells

Extreme high order nonlinear diffracted signals are found in ZnCdSe/ZnSe quantum wells in two beam, self‐diffraction geometry. Diffracted signals of all observed orders are shown to originate predominantly from excitonic resonances. Using four‐wave mixing, an indication of hole‐longitudinal optical phonon scattering between the heavy and the light hole states is found, and exciton‐phonon interaction is studied.

Optical investigations of ZnSe crystals grown by the seeded vapour phase transport technique

The optical properties of ZnSe bulk crystals grown by the seeded physical vapour phase transport technique (SPVT) were studied by low-temperature photoluminescence and room-temperature stimulated emission measurements. Large strain-free ZnSe single crystals with a diameter as large as 5 cm have been grown uniformly without twinning by the SPVT technique. A variety of ZnSe samples, the as-grown and post-growth treated as well as intentionally doped crystals, were prepared in this study. Low-temperature photoluminescence measurements showed that near band edge I1d line and its LO phonon replicas dominated the spectra, and that donor-acceptor pair emission signatures were absent for SPVT as-grown samples; the intensity of the I1d, line decreased considerably and the I3 line became predominant after Zn extraction treatment. The pressure dependence on the bound exciton emission lines suggested that the I3 line might originate from excitons bound to a donor-like deep centre rather than to ionized shallow donors. The room-temperature optically pumped stimulated emission and blue lasing from the as-grown samples demonstrated that SPVT ZnSe single crystals have the quality sufficient to develop low-threshold, high-power output blue lasers.

Stimulated emission and gain measurements in molecular‐beam epitaxially grown CdTe

Stimulated emission has been observed in molecular‐beam epitaxially (MBE) grown films of cadmium telluride (CdTe), using a nanosecond pulsed dye laser as an excitation source. The films were 2.0–2.4 μm thick. Stimulated emission (SE) is seen to arise at several energy positions of the spectra. The different SE peaks exhibited different gain values. The gain was measured using the single beam method, by varying the length of excitation. The bulk CdTe sample studied exhibited stronger saturation effects than those seen in the MBE films.