HGS Crystals

What Are HGS Crystals?

Fig 1. HGS OPO experimental set-up. [1]

HGS (mercury gallium sulfide, HgGa₂S₄) is a mid-infrared, non-oxide nonlinear optical crystal used for efficient frequency conversion and optical parametric oscillators (OPOs). It combines a wide transparency window into the mid-IR with a large second-order nonlinearity and good laser-damage resistance, making it a strong choice when converting near-IR pump lasers (e.g., 1.064 μm) into tunable mid-IR output.

Although growing large HGS crystals is quite challenging, their high conversion efficiency and broad radiation wavelength tuning range make them a promising and powerful competitor to other mid-IR crystals like AgGaS₂ (AGS), AgGaSe₂, ZnGeP₂ (ZGP), and GaSe. Alfa Chemistry has mastered the growth and fabrication process to deliver high-quality, homogeneous crystals that deliver reliable and repeatable results for your critical systems.

Features

• Exceptionally Wide Transparency Range: From 0.55 μm to over 13 μm, enabling applications deep into the infrared.
• High Laser Damage Threshold: Manufactured with precision to withstand high power densities, ensuring durability and longevity in demanding experimental and industrial setups.
• High Conversion Efficiency in the 1–10 μm region for SHG, DFG and OPO/OPA processes. Measured on a 4 mm element, frequency-doubling efficiencies on the order of ~10% have been reported for CO₂ laser pulses (30 ns, ≈60 MW/cm²), demonstrating HGS's suitability for high-peak-power pulsed operation.
• Good Damage Resistance when polished and coated appropriately, supporting high-fluence, short-pulse applications.

Specifications Product Options ApplicationsFAQs

Product Name	KTA Crystals
Catalog No.	OPTIX-0012
Chemical Formula	HgGa2S4
Crystal Structure	Tetragonal, point group S4, space group S24
Density	4.95 g/cm3
Mohs Hardness	3~3.5
Bandgap	2.34 eV
Negative Uniaxial Crystal	no > ne
Melting Point	880 °C
Angle Tolerance	± 30 arcmin
Perpendicular Tolerance	15 arcmin
Parallelism	< 120 arcsec
Flatness	<λ/6 @ 633 nm
Axial	± 0.5° @ all axes
Nonlinear Optical Properties
NLO Coefficient	d36(1.064μm) = 1.8; d36(AgGaS2) ± 15% = (35.2±5.3) pm/V d31(1.064μm) = 0.6; d36(AgGaS2) ± 15% = (11.7±1.8) pm/V
Damage Threshold	τ=10 ns, λ=1.06 μm (single pulse), 60 MW/cm2 τ=18 ns, λ=1.064 μm, ν=10 Hz, 30 MW/cm2
Linear Optical Properties
Transparency Range	550 ~ 1300 nm
Refractive Index	no=2.6592; ne=2.5979 @0.5495 µm no=2.5796; ne=2.5264 @0.6500 µm no=2.4774; ne=2.4324 @1.0760 µm no=2.4386; ne=2.3979 @3.5400 µm no=2.3690; ne=2.3290 @11.000 µm

Note: The data provided is for reference purposes. We recommend consulting with our technical team to verify suitability for your application.

HGS Crystal Product Options

• Orientations / Cuts: X-cut, Y-cut, Z-cut and custom phase-matching cuts (we can deliver with specified phase-matching angles).
• Surface Quality: Optical polish polishing to λ/6 or better; lapped and polished faces, edge chamfers.
• Coatings: AR coatings optimized for pump (e.g., 1064 nm) and signal/idler bands or broadband mid-IR AR stacks — applied and tested in cleanroom conditions.
• Standard Sizes (mm):

Crystal Type	Size	θ	φ
HGS	5×5×1 mm	43.5°	45°
HGS	6×6×2 mm	55.8°	0°
HGS	8×8×1 mm	43.5°	45°

Second-Harmonic Generation (SHG) for CO and CO₂ lasers

HGS can be used as an efficient frequency doubler for long-wavelength gas lasers, converting CO/CO₂ outputs into shorter mid-IR lines for spectroscopy and sensing.

Optical Parametric Oscillators / Amplifiers (OPO/OPA)

Ideal for tunable mid-IR generation when pumped by near-IR or visible lasers. HGS's nonlinearity and transmission allow compact OPO/OPA designs that cover broad idler ranges.

Difference- and Sum-Frequency Generation (DFG/SFG)

Useful for mixing two laser lines to produce custom mid-IR wavelengths for trace-gas detection, molecular spectroscopy and laboratory sources.

High-Energy Pulsed Conversion

With suitable polishing and coatings, HGS supports short-pulse, high-peak-power applications (e.g., ns/mJ-level systems) where both conversion efficiency and damage threshold matter.

Mid-IR Sources for Sensing & Spectroscopy

Tunable outputs produced with HGS are well suited to gas sensing, environmental monitoring, biomedical diagnostics and industrial process control where molecule-specific absorption bands lie in the mid-IR.

Q: How does HGS compare to AgGaS₂ (AGS) or ZnGeP₂ (ZGP)?

A: HGS offers a broader transparency range than ZGP (which absorbs below ~2 μm) and a higher nonlinear coefficient and damage threshold than AGS. It is often the ideal compromise between performance and practical handling.

Q: What are the handling requirements for HGS crystals?

A: HGS is a soft, hygroscopic material. It should be handled with care, stored in a dry environment, and cleaned only with appropriate solvents (e.g., dry nitrogen gas or anhydrous alcohol). Our crystals are shipped in sealed, desiccated containers.

Q: Can you provide crystals with custom dimensions and coatings?

A: Absolutely. Custom fabrication is our specialty. We can produce crystals with non-standard cuts, sizes, and with AR coatings tailored to your exact operational wavelengths.

Why Choose Our HGS Crystals?

• Specialized Crystal Growth & Preparation: Guaranteed consistent wavefront distortion and minimal scattering losses for superior beam quality.
• Full Inspection & Documentation: Every part is delivered with dimensional inspection, surface-quality certification, and optional transmission and interferometry reports.
• Technical Support: application engineering support for phase-matching, mounting, and damage-threshold considerations; we can help you choose geometry and coatings for optimal conversion efficiency.
• Flexible Lead Times & Small Runs: we support both prototype quantities and production runs; MOQ and lead time depend on specs: request a quote for current availability.

Reference

1. Petrov, Valentin, et al. Nonlinear Optics. Optica Publishing Group, 2013.