Optimal Te-doping in GaSe for non-linear applications

Abstract

Centimeter-sized Te-doped GaSe ingots were grown from the charge compositions of GaSe with nominals 0.05, 0.1, 0.5, 1, and 3 mass% Te, which were identified as ε-GaSe:Te (0.01, 0.07, 0.38, 0.67, and 2.07 mass%) single crystals. The evolution of the absorption peaks of the phonon modes E'(2) (≈ 0.584 THz) and E"(2) (1.77 THz) on Te-doping in GaSe:Te crystals was studied by THz time-domain spectroscopy. This study proposes that the evolution of both E'(2) and E''(2) absorption peaks correlates well with the optical quality of Te-doped GaSe crystals, which was confirmed by experimental results on the efficiency of THz generation by optical rectification. Maximal intensity of the absorption peak of the rigid layer mode E'(2) is proposed as a criterion for identification of optimal Te-doping in GaSe crystals.

Doped GaSe crystals for laser frequency conversion

“…Therefore, for sub-mm-thick and mm-thick samples, the best quality crystals cannot be selected, due to the limited absorptivity. We proposed a selection criteria, determined by an absorption measurement outside of the maximal transparency range, by examining the parameters of the exciton and phonon absorption peaks: the larger the peak magnitude and the narrower the spectral bandwidth, the better the quality 49,50 . To determine adequate absorption coefficients and refractive indices by THz-TDS, a criterion was proposed based on the interference pattern shape-form in the recorded spectra 51,52 .…”

“…The closer the match in the atomic size of the substituting atoms, the larger the mixing ratio of solid solution crystals that may be grown with suitable optical quality, as in the following: GaSe 12x S x with x f 0.44 (11 mass. % of S) 51 , Ga 12x In x Se with x 5 0.03 (2.32 mass.%) of In 55 , and GaSe 12x Te x with x 5 0.012 (1 mass.%) 49 . In turn, the higher the permitted limit in the mixing ratio, the wider the variation in the optical and mechanical properties that can be achieved.…”

“…Additional control of THz dispersions was achieved by temperature tuning (Figure 12b). Notably, the phonon absorption structure and peaks in doped GaSe are transformed with increasing dopant concentration 49,51 , which should be reflected in changes of the dispersion properties. As a result, the widely used formulae for estimating dispersion properties of solid solution crystals, from the visible to mid-IR range, calculated from the dispersions data of parent crystals 81 cannot be applied to estimations in the THz range.…”

“…Bearing in mind the low Er solubility 21 , the increase appears to be due largely to the improved optical quality. It was established properly that the maximal transparency window 42,49,55,61,69 and THz range 51 absorption coefficients for all properly doped crystals grown by the modified method, i.e., with a rotating heat field, are 2-3 times lower (f0.03-0.06 cm 21 ) than those for crystals grown by the conventional vertical Bridgman method. These data are confirmed by the increased frequency conversion efficiency.…”

“…Further doping decreases the optical quality, indicating that there exists an optimal doping level. Measurement by the proposed method, i.e., outside of maximal transparency range 49 , allows us to determine the optimal doping concentrations as:2-3 mass.% of S in GaSe 55,61 , 0.5-1 mass.% of In 69 , 0.07-0.38 mass.% of Te 49,61 , 0.01-0.02 mass.% of Al 36,42 , and 0.5 mass.% of Er 12 , by charge composition. In all cases, the optimal doping concentration was again confirmed experimentally by direct comparison of frequency conversion efficiency in pure and doped crystals at a fixed pump intensity.…”