All ETDs from UAB

Advisory Committee Chair

Sergey B Mirov

Advisory Committee Members

Renato P Camata

Gary G Gray

Vadimir V Fedorov

Patrik Kung

Mary E Zvanut

Document Type

Dissertation

Date of Award

2020

Degree Name by School

Doctor of Philosophy (PhD) College of Arts and Sciences

Abstract

The middle-infrared (mid-IR) 2-8 μm and long-wavelength-infrared (LWIR) 8-20 μm spectral range enjoys a lot of attention from the scientific, medical, and industrial communities. A number of applications such as spectroscopy, medical diagnosis, surgical procedures, materials processing, remote detection of organic compounds, free-space-communication, and many others are possible. Transition metal (TM) doped II-VI semiconductors are the materials of choice for accessing the mid-IR. The II-VI host materials feature broad infrared transparency, low phonon frequency, and low optical losses. When doped with TM ions, these media exhibit a four-level energy structure, the absence of excited state absorption, and broad absorption and emission bands. Fabrication technology of the host materials is well-established, and they are commercially available. Doping with TM ions can be done during crystal growth, however, growth of large crystals with good optical quality is hard. An alternative method for fabrication is based on post-growth thermal-diffusion-doping (TDD) of II-VI materials with TM ions. This method yields uniformly doped crystals with preassigned dopant concentration while preserving the good optical quality of the host. However, not all combinations of TM and II-VI hosts can be easily doped via TDD. For example, Fe diffusion in ZnSe takes ~30 days yielding uniform doping through 2 mm, in ZnS during the same time only ~100 μm of diffusion is possible. This hinders the fabrication of sufficiently large crystals with uniform doping, and desired concentrations. Rare earth ions (RE) constitute another family of dopants for mid-IR lasing. However, due to their large ionic radii, it is hard to dope RE ions via thermal diffusion. No reports on significant TDD of RE ions in II-VI materials apppear in the literature. The ability to use TDD for a wider range of dopant ions could greatly benefit mid-IR lasers based on II-VI host materials, potentially adding to the impressive list of output characteristics as well as allowing the use of a wider range of optical pump sources. The first major objective of this work is achieving enhancements diffusion of TM and RE ions in II-VI materials through enhancing conditions such as subjecting the host material to gamma radiation, high energy electron beam, high electric field, overpressure of fast diffusing elements, and elevation of temperature and pressure, during the thermal diffusion process. Despite the slow rate of diffusion of many TM and RE ions in II-VI, significant results have been achieved based on the development of fabrication methods of Cr:ZnSe, Cr:ZnS, and Fe:ZnSe. These results include access to a broad spectral range of 1.8-6 μm with more than 60 % efficiency, tunability exceeding 1000 nm, output powers exceeding 140 W in continuous-wave operation, multi-Joule output energies in free-running and gain-switched regimes, short-pulses of < 16 fs in multi-Watt oscillation. All of these results are achieved under optical pumping. Due to the wide band-gap and low phonon frequencies, these materials have the potential for electrical excitation. Achievement of electrical pumping eliminates the need for external pump lasers, potentially reducing the fabrication cost. Electrical excitation can be carried out through recombination of the charge carrier in p-n junction or by impact excitation of hot carriers. The impact excitation mechanism in these materials requires monocrystalline bulk crystals with appropriate conductivity, high enough optically active dopant concentration, low optical losses, and formation of Ohmic contacts. The second major objective of this work is devoted to understanding the physical processes and conditions necessary for the realization of lasing under direct electrical excitation.

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