Luminescence lifetime measurements in diffusion doped Cr:ZnSe

Abstract

Cr-doped ZnSe single crystals were shown to be promising materials for mid-IR laser applications. Roomtemperature continuous wave Cr:ZnSe lasers with optical-to-optical efficiency more than 60%, output power as much as a few watts and extremely broad tuning range exceeding 1100 nm were demonstrated recently. The post-growth diffision doping was reported to be one of the promising methods for preparation of Cr:ZnSe crystals of laser quality [I-21. Diffusion doping provides low optical losses and ability to control chromium doping level by varying the diffusion time and temperature. The luminescence lifetime measurements study of predominantly polycrystalline samples was published in [I]. Here we report on the extensive lifetime measurements in single crystalline Cr:ZnSe grown by PVT and CVT methods and diffusion doped with different chromium concentrations. Undoped ZnSe bulk single crystals with optical losses less than 0.01 cm-' in the spectral range 6om 1200 to 2500 nm were grown by the vertically arranged physical vapor transport (PVT) in closed quartz tube without seeding. Then the crystals were cut in plates with thickness of -3-4 mm. The 0.1-0.3 microns thick chromium Elms were deposited onto their surfaces in a magnetron sputtering system. Diffision was carried out in evacuated fused silica ampoules in the temperature range (800-1000) C during 1-10 days. Nineteen samples with different doping levels were selected for lifetime measurements. The optical quality of samples was chracterized by FOM (figure of merit) defined as a ratio of absorption coefficients at 1770nm (absorption peak) and 2500 nm (emission peak). For the majority of samples FOM was measured to be as high as 150-200 while optical losses were less than 0.1 cm-' at 2500 nm. An average chromium concentration N, was calculated using the absorption cross section value of 1.lx10~'' cm2 for Cr:ZnSe applied in [I] and recently proven by absorption saturation measurements. It varied from 2x10'' to 22~10'~ cm.' in different samples. The decay time measurements were independently perfonned using two set-ups. The first one consisted of the 1.6 pm Raman laser with IO ns pulse duration as a pump source, focusing and collimating optics and the fast InAsSb photodiode coupled to a digital oscilloscope. The second setup used a modulated 1.6 microns laser diode as a pump source, and a InSb detector with 0.5 microseconds time constant. The reabsorption effect was studied using the samples in the form of the sharp wedge. The lifetime measured in the thin wedge differed by as much as 10-15 % as compared to the bulk crystal. The large reabsoption lifetime increase follows from the significant absorption and emission spectrum overlap at room temperature and high index of refraction of ZnSe, and should always be taken into account. The measured lifetime varied from -6 microseconds for N,.=2xl0'* cm to -3 microseconds for N,=22x101' cm (Fig.1). Decreasing of the lifetime was observed also for samples with comparatively high passive losses (low FOM). The Cr:ZnSe samples with average chromium concentration of (819)xIO cm and FOM>150 exhibited emission with lifetime of 44.5 microseconds. These samples were used in laser experiments including diode pumping and demonstrated high laser efficiency.