Towards Mid-IR Waveguide Lasers: Transition Metal Doped ZnS Thin Films

Abstract

Thin films of Cr and Fe-doped ZnS suitable for waveguide mid-IR applications, with well-isolated TM 2+ peaks, and Cr 2+ absorption and fluorescence comparable to bulk crystals were investigated. Raman data indicate that the TM is substitutional.

Vapor deposited Cr-doped ZnS thin films: towards optically pumped mid-infrared waveguide lasers

“…Well-isolated absorption from Cr 2+ ions is observed for low concentrations, and absorption cross-sections appear to be comparable to those of diffusiondoped single crystals. Fluorescence yield and lifetimes are comparable to those obtained with doped single crystals [22,23], and off-resonance attenuation values are acceptably low for these to be potential gain media. Heating the substrate during deposition introduces unwanted absorption and additional scattering due to the larger grains that are formed.…”

“…The absorption peak is slightly broader for the RT deposited film compared to the crystal sample, but the values are very similar, suggesting that a high percentage of the Cr atoms in the film are in the desired substitutional (Cr 2+ ) state. Broadband fluorescence is observed in these films in the vicinity of 2 µm [22,23], demonstrating the potential for stimulated emission. .…”

“…Since the higher order modes have higher losses, the results here represent an upper bound to losses that would be expected in single mode waveguides. The large thicknesses used for analysis of the fluorescence properties [22,23] result in greater surface roughness than would be expected in a film designed for single mode operation at 2 µm, increasing losses. The perpendicular transmission results also suggest that losses (particularly scattering) in the 2µm fluorescence region will be significantly lower than those at 1060 nm.…”

Nonlinear optical response and structural properties of MBE-grown Fe:ZnS films

“…Cr 2+ doped ceramic ZnS has been successfully used for lasers in the 2-4 µm band [8]. Initially, confinement was provided by using femtosecond processing to define the boundaries of a waveguide [9,10] within a bulk sample, but new studies [11,12] suggest that vacuum-deposited thin films may be suitable for fabrication of waveguide or disk lasers. Moreover, MBE-grown Cr:ZnS thin films show promise as saturable absorbers for 1.5 µm erbium lasers [13].…”

“…Fe:ZnS thin films have received less attention [22][23][24], but an isolated absorption (Fe 2+ ) peak was observed at concentrations as high as 2 at.% Fe -much higher than the level where a well-defined peak is observed for Cr 2+ doped ZnS [11]. This observaton, combined with results on Cr-doped films, suggests that vapor deposition of Fe:ZnS may be a route to a fabrication of thin film saturable absorbers for passive Q-switches at longer wavelengths.…”