Doped GaSe crystals for laser frequency conversion

In this review, we introduce the current state of the art of the growth technology of pure, lightly doped, and heavily doped (solid solution) nonlinear gallium selenide (GaSe) crystals that are able to generate broadband emission from the near infrared (IR) (0.8 μm) through the mid- and far-IR (terahertz (THz)) ranges and further into the millimeter wave (5.64 mm) range. For the first time, we show that appropriate doping is an efficient method controlling a range of the physical properties of GaSe crystals that are responsible for frequency conversion efficiency and exploitation parameters. After appropriate doping, uniform crystals grown by a modified technology with heat field rotation possess up to 3 times lower absorption coefficient in the main transparency window and THz range. Moreover, doping provides the following benefits: raises by up to 5 times the optical damage threshold; almost eliminates two-photon absorption; allows for dispersion control in the THz range independent of the mid-IR dispersion; and enables crystal processing in arbitrary directions due to the strengthened lattice. Finally, doped GaSe demonstrated better usefulness for processing compared with GaSe grown by the conventional technology and up to 15 times higher frequency conversion efficiency. Doping of gallium selenide (GaSe) is effective for controlling the physical properties that affect its frequency conversion efficiency. Despite its promising properties for nonlinear optical conversion, GaSe has not achieved the wide application that some other nonlinear optical materials enjoy. This is mainly due to the weak interlayer van der Waals bonding, which makes it difficult to grow and process large, single-crystals samples of high optical quality. Scientists in Russia and China review the growth of nonlinear GaSe crystals for laser frequency conversion. In particular, they assess recent progress in using doping to modify the physical properties of GaSe crystals. Doping confers many benefits, including raising the optical damage threshold, effectively eliminating two-photon absorption and enabling dispersion in the terahertz range to be controlled independently of that in the mid-infrared range.