Spectroscopic characterization of iron-doped II-VI compounds for laser applications.

摘要

The middle Infrared (mid-IR) region of the electromagnetic spectrum between 2 and 15 ?m has many features which are of interest to a variety of fields such as molecular spectroscopy, biomedical applications, industrial process control, oil prospecting, free-space communication and defense-related applications. Because of this, there is a demand for broadly tunable, laser sources operating over this spectral region which can be easily and inexpensively produced. II-VI semiconductor materials doped with transition metals (TM) such as Co 2+, Cr2+, or Fe2+ exhibit highly favorable spectroscopic characteristics for mid-IR laser applications. Among these TM dopants, Fe2+ has absorption and emission which extend the farthest into the longer wavelength portion of the mid-IR. Fe2+:II-VI crystals have been utilized as gain elements in laser systems broadly tunable over the 3-5.5 microm range [1] and as saturable absorbers to Q -switch [2] and mode-lock [3] laser cavities operating over the 2.7-3 microm. TM:II-VI laser gain elements can be fabricated inexpensively by means of post-growth thermal diffusion with large homogeneous dopant concentration and good optical quality[4,5]. The work outlined in this dissertation will focus on the spectroscopic characterization of TM-doped II-VI semiconductors. This work can be categorized into three major thrusts: 1) the development of novel laser materials, 2) improving and extending applications of TM:II-VI crystals as saturable absorbers, and 3) fabrication of laser active bulk crystals.;Because current laser sources based on TM:II-VI materials do not cover the entire mid-IR spectral region, it is necessary to explore novel laser sources to extend available emissions toward longer wavelengths. The first objective of this dissertation is the spectroscopic characterization of novel ternary host crystals doped with Fe2+ ions. Using crystal field engineering, laser materials can be prepared with emissions placed in spectral regions not currently covered by available sources while maintaining absorption which overlaps with available pump sources. Because optimization of these materials requires extensive experimentation, a technique to fabricate and characterize novel crystals in powder form was developed, eliminating the need for the crystal growth. Powders were characterized using Raman, photoluminescence studies, and kinetics of luminescence. The first demonstration of random lasing of Fe:ZnCdTe powder at 6 microm was reported. These results show promise for the development of these TM-doped ternary II-VI compounds as laser gain media operating at 6 microm and longer.;The second major objective was to study the performance of TM:II-VI elements as saturable absorber Q-switches and mode-lockers in flash lamp pumped Er:YAG and Er:Cr:YSGG cavities. Different cavity schemes were arranged to eliminate depolarization losses and improve Q-switching performance in Er:YAG and the first use of Cr:ZnSe to passively Q -switch an Er:Cr:YSGG cavity was demonstrated.;While post-growth thermal diffusion is an effective way to prepare large-scale highly doped TM:II-VI laser elements, the diffusion rate of some ions into II-VI semiconductors is too low to make this method practical for large crystals. The third objective was to improve the rate of thermal diffusion of iron into II-VI semiconductor crystals by means of gamma-irradiation during the diffusion process. When exposed to a dose rate of 44 R/s during the diffusion process, the diffusion coefficient for Fe into ZnSe showed improvement of 60% and the diffusion coefficient of Fe into ZnS showed improvement of 30%.