Nonlinear Optical Crystals: A Complete Survey

Frequency down-conversion of solid-state laser sources to the mid-infrared spectral range using non-oxide nonlinear crystals

The family of metal thiophosphates is an important but long-ignored compound system of the nonlinear optical (NLO) materials with desirable properties for the mid-infrared (mid-IR) coherent light generation. In the present work, the mid-IR NLO capabilities of metal thiophosphate crystals are systematically investigated based on their structure-property relationship. The linear and nonlinear optical properties of these crystals are predicted and analyzed using the first-principles calculations. In particular, several metal thiophosphate compounds are highlighted to exhibit good mid-IR NLO performances, as supported by the primary experimental results. These candidates would greatly promote the development of the mid-IR NLO functional materials.

Metal Thiophosphates with Good Mid-infrared Nonlinear Optical Performances: A First-Principles Prediction and Analysis

The development of frequency-conversion technology in the infrared region is in urgent need of new excellent infrared nonlinear optical (IR NLO) materials. How to achieve a good balance between laser damage threshold (LDT) and NLO coefficient (dij) for new IR NLO candidates is still a challenge. The combination of the highly electropositive alkali metal (Na) and Zn with d(10) electronic configuration into crystal structure affords one new IR NLO material, Na2ZnGe2S6. It exhibits excellent properties including a wide transparent region (0.38-22 μm), large band gap (3.25 eV), and especially a balance between a strong NLO coefficient (30-fold that of KDP) and a high LDT (6-fold that of AgGaS2), indicating a promising application in the IR region. Moreover, novel common-vertex-linked wavelike ∞[GeS3]n chains are interestingly discovered in Na2ZnGe2S6, which rarely exist in the reported thiogermanides containing alkali metals. In addition, calculated SHG density and dipole moment demonstrate that the large NLO response is mainly attributed to the cooperative effects of the [GeS4] and [ZnS4] units.

Na2ZnGe2S6: A New Infrared Nonlinear Optical Material with Good Balance between Large Second-Harmonic Generation Response and High Laser Damage Threshold.

With the development of laser technology and related scientific fields, understanding of the structure–property relationships in nonlinear optical (NLO) crystals is becoming more and more important. In this article, first-principles studies based on density functional theory, and their applications to elucidate the microscopic origins of the linear and NLO properties in NLO crystals, are reviewed. The ab initio approaches have the ability to accurately predict the optical properties in NLO crystals, and the developed analysis tools are vital to investigating their intrinsic mechanism. This microscopic understanding has further guided molecular engineering design for NLO crystals with novel structures and properties. It is anticipated that first-principle material approaches will greatly improve the search efficiency and greatly help experiments to save resources in the exploration of new NLO crystals with good performance.

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A.O Okorogu

Papers

Tunable middle infrared downconversion in GaSe and AgGaS2