Eric Andrew Karhu

Band-edge modification and mid-infrared absorption of co-deposited FexZn1-xS thin films

The bandgap of iron-doped ZnS has been reported by others to change significantly under the addition of a few atomic percent of iron, which would have significant implications for solar energy. Here, thin films of FexZn1-xS with x = 0 to 0.24 were made by co-deposition of Fe and ZnS using thermal evaporation. In contrast to results on nanoparticles and electrodeposited materials, all co-deposited films had optical properties consistent with a direct bandgap of ~3-3.5 eV. The absorption peak at 2.7 µm from substitutional Fe2+ in the ZnS films was well isolated up to concentrations of over 2% (~1021cm−3), despite the small crystallite size, suggesting the films may have applications as mid-infrared saturable absorbers. Increasing dopant concentration resulted in band edge softening. Density functional calculations are presented and are consistent with our observations of the Fe:ZnS films, demonstrating spin-polarized midgap states and additional states at the band edge.

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

Compact, affordable mid-IR lasers require the development of gain materials in waveguide form. We report on the high vacuum deposition of Cr:ZnS films with concentration ranging from 1018-1020 dopants/cm3 . At low concentrations, films display well-isolated absorption associated with substitutional Cr2+ ions in the lattice. Spatial modulation of the dopant concentration suppresses the absorption associated with this substitution. Lateral crystallite sizes less than 30 nm are associated with the lowest substrate temperatures (<50 °C) used during deposition, and waveguide losses as low as 8dB/cm are observed. These materials are promising candidates as gain media for fabrication of waveguide mid-IR lasers.

Spectral-luminescent properties of vapor deposited Cr:ZnS thin films and their application as saturable absorbers for 1.5-μm erbium lasers

We report the results of the extensive spectral-luminescent characterization including concentration quenching analysis of the first laser-quality vapor-deposited Cr:ZnS films, and demonstrate their use as saturable absorbers. Comparison of spectral-luminescent properties with bulk Cr:ZnS indicates their high laser quality, opening the way towards industrial mid-IR thin-disk and waveguide lasers. Successful implementation of the grown films in a Q-switched Er,Yb:GdAB solid-state laser emitting at 1.52 μm gives clear experimental evidence of the high quality of the films.

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

Fe-doped ZnS films of high optical quality were fabricated using vacuum vapor deposition. Preferential crystalline orientation and phase purity of the host ZnS films were increased by the addition of small amounts of Fe, and the spectral shape of the 2–4 μm absorption peak is maintained up to the highest concentration tested (9 at.%). However, Raman shifts of the Fe:ZnS films indicate the inclusion of disordered material at high Fe concentrations, and below-gap optical Kerr effect measurements show an increase in χ(3) correlated with this data. Nonlinear optical properties of the films were also measured using the Z-scan method at 532 nm, and saturable absorption of the Fe-based intraband levels at 2.94 μm was confirmed.

Er, Yb:GdAl 3 (BO 3 ) 4 laser passively Q-switched by MBE-grown Cr:ZnS thin films

Q-switched erbium lasers emitting in the 1.5–1.6 μm spectral region are widely used in laser rangefmders and LIBS systems. These applications require compact and low-cost sources of laser pulses with high average output power. Passive Q-switching is one of the most simple and reliable method to achieve the abovementioned requirements. Er, Yb:GdAl<inf>3</inf>(BO<inf>3</inf>)<inf>4</inf> (Er, Yb:GdAB) crystal was shown to be an efficient laser material for the 1.5–1.6 μm spectral range [1]. Recently a passively Q-switched Er, Yb:GdAB laser was demonstrated with Co²⁺:MgAl<inf>2</inf>O<inf>4</inf>, graphene and SWCNT saturable absorbers [2-4]. Here we report Er, Yb:GdAB laser passively Q-switched by using of MBE-grown Cr:ZnS thin films.

MBE-grown Cr:ZnS thin film laser media

We report results of spectroscopic analysis of the first laser-quality MBE-grown Cr:ZnS films. Comparison of spectral-luminescent properties with bulk Cr:ZnS indicates their good laser properties, opening the way towards industrial mid-IR thin-disk and waveguide lasers.

Optical Coatings as Active Media for Mid-Infrared lasers

Thin films of Cr and Fe-doped ZnS with high infrared transparency have been deposited by molecular beam epitaxy and show optical properties suitable for use as active media in mid-infrared laser applications.