Nonlinear crystal HgGa2S4: structure, properties and application in nonlinear optics

3. Nonlinear optical properties:

• Nonlinear coefficient: 26,8 pm/V.
• Tensor nonlinearity components (at λ=1,064 µm):
• d36​=(35,2±5,3) pm/V (1.8× higher than AgGaS2​);
• d31​=(11,7±1,8) pm/V;
• ratio d36​/d31​=+3.
• Effective nonlinearity increases by ~50 % when implementing Type II three‑wave processes (eo‑e) in a crystal cut at φ=0∘.

4. Thermal and damage resistance properties:

• Thermal conductivity: 0,028 W/cm⋅K.
• Laser damage threshold:
• surface: 60 MW/cm2 (τ=10 ns, λ=1,06 µm, single pulse);
• bulk: 30 MW/cm2 (τ=18 ns, λ=1,064 µm, ν=10 Hz);
• general damage resistance: 1 J/cm2.

5. Comparative advantages over AgGaS2​:

• significantly higher second‑order nonlinear susceptibility;
• enhanced resistance to laser radiation (including long‑term operation without damage at increased pulse repetition rates);
• does not oxidise in air;
• lower pump threshold for comparable crystal sizes;
• higher conversion efficiency;
• competitive performance vs. ZnGeP2​, AgGaSe2​, and GaSe crystals despite challenges in growing large‑size crystals.

6. Performance in optical parametric oscillators (OPOs):

• pumping: nanosecond pulses from a Nd:YAG laser (λ=1064 nm, 100 Hz);
• idler pulse duration: ∼5 ns (in the 4 µm region);
• pulse energy: up to 6,1 mJ;
• average power: up to 610 mW;
• potential output energy: 10 mJ (> 1 W) and more than 5 mJ at idler wavelengths > 5 µm;
• tuning range: 3,7–5,7 µm (with continuous tuning);
• SHG efficiency for CO₂ laser: up to 10 % (for a 4 mm‑long element, τ=30 ns, power density 60 MW/cm2).

7. Practical applications:

• mid‑IR optical parametric oscillators (OPO/OPA) pumped near 1 µm (Nd/Yb‑based systems);
• coverage of the mid‑IR range up to ∼12 µm;
• opto‑acoustic spectroscopy (monitoring volatile organic compounds in exhaled breath);
• medical diagnostics (detection of markers for asthma, pneumonia, tuberculosis, lung cancer, diabetes, etc.);
• neurosurgery: parametric frequency converter at 6,45 µm with 1 W output power for precision neuron surgery (removal of layers as thin as 1 µm);
• use in high‑power systems with nanosecond and femtosecond pulses (e.g., synchronously pumped by Yb‑fiber lasers).

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