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Langasite Family Midinfrared Nonlinear Optical Oxide Materials: Structure, Property, and Applications 时间:2017-11-06 21:44 来源:未知作者:admin 点击: 次 Abstract:Midinfrared (IR) nonlinear optical (NLO) materials with high performance are vital in important technological applications in many civil and military fields. Very recently, langasite family compounds have attracted much attention due to their wide transparency to mid-IR region and ultrahigh laser damage threshold (LDT). In this brief review, three important compounds—LGS, LGN, and LGT—are investigated and analyzed based on available experimental data. The electrooptical (EO) Q-switch and mid-IR OPO applications are summarized in detail. Finally, promising search directions for new metal oxides that have good mid-IR NLO performances are discussed.
1. Introduction
In 1961, Franken and his colleagues firstly discovered the second-harmonic generation in quartz using a ruby laser beam. And then Bloembergen built a theoretical framework to explain the principle of nonlinear optical parametric generation. Since then, lasers in the human society have been playing an increasingly important role. Nonlinear optical (NLO) materials have a very important significance to convert the laser wavelength into the spectral region and to obtain a high-power laser source. In the past three decades, many useful NLO crystals in near-infrared (IR), visible, and ultraviolet (UV) regions (wavelength from 0.2 to 2 μm) have been developed. In case of the visible region, the KTP (KTiOPO4) crystal has a wide transmission range, high frequency conversion efficiency, good phase-matchable wavelength, high laser damage threshold (LDT), and excellent physical and chemical stability [1]. Regarding UV and deep-UV regions, Chinese scientists have made remarkable contributions. Chen and his collaborators invented the BBO [2], LBO [3], and KBBF [4] crystals, known as the “Chinese card” crystal. In particular, the KBBF crystal successfully achieves the sixth-harmonic generation of 1.064 μm, which is the only source of deep-UV laser by direct SHG output [5]. BiB3O6 (BIBO) crystal possesses large effective nonlinearity, of 3.7 pm/V, and versatile phase-matching properties. Its large angular and spectral acceptance bandwidths and low spatial walk-off at room temperature thus make BIBO highly attractive for frequency conversion in the visible and UV region [6].
Midinfrared (mid-IR) NLO materials are indispensable for obtaining all-solid-state lasers in the spectral range of 3–20 μm by optical parametric oscillation (OPO) or difference frequency generation (DFG) [7]. Tunable narrow band lasers using NLO crystals have very important applications in many advanced areas of science and technology, such as infrared remote sensing [8], biological tissue imaging [9], environmental monitoring [10], and minimally invasive medical surgery [11]. So far, commercially available mid-IR NLO crystals are AgGaS2 (AGS), AgGaSe2 (AGSe), and ZnGeP2 (ZGP). They possess high second-harmonic generation (SHG) coefficients of about 13 pm/V, 33 pm/V, and 75 pm/V, respectively [12]. However, these materials have some disadvantages that hinder their use in mid-IR laser generation. For example, the laser damage threshold (LDT) values of AGS and AGSe are too small (only about 25 MW/cm2 and 11 MW/cm2 (@1.06 μm, 35 ns), resp.) [13] to bear a high-power pumping source. Meanwhile, the strong two-photon adsorption (TPA) in ZGP resulting from its narrow bandgap (2.0 eV) makes it impossible to use an Nd:YAG laser as the pumping source [14]. Therefore, the current rapid developments of mid-IR lasers urgently demand the discovery of new mid-IR NLO materials with a good performance.
In general, optional midinfrared nonlinear optical materials, in addition to having a noncentral symmetric structure, should meet the following conditions [15, 16]: (1) a wider range of transmission bands that can cover important atmospheric transparent windows of 3–5 μm; (2) a large nonlinear optical coefficient, because the power of the output light is proportional to the effective nonlinear optical coefficient; (3) high LDT, which essentially depends on the wide bandgap of the material; (4) moderate birefringence to achieve a phase-matchable condition; (5) good growth habit and high optical quality; and (6) good physical and chemical stabilities for convenient applications.