One uncoated ZnSe face returns 17.0% at 10.6 µm (n = 2.403). Two faces, (1−R)² ≈ 0.689 — about 31% gone at the surfaces. Coat the resonator. Specify the wavelengths: 10.6 µm, dual-band with 633 nm, or LWIR. A coating is a stack design, not a checkbox.
What we will quote
KM grows and cuts the substrate. Coating is quoted to the wavelengths on the print. The common asks from this desk:
- uncoated — spectroscopy, odd bands, or you coat in-house
- AR 10.6 µm, both faces — straight CO2
- dual-band AR 10.6 µm + 633 nm — HeNe through the same lens
- broader mid-IR / LWIR stacks for cameras — not a CO2 V-coat
A 10.6-only V-coat is usually the lowest residual at the laser line and a poor HeNe window. A dual-band stack is a compromise: you keep alignment, you give back a little at 10.6. Say which failure you cannot live with. If the print only says “AR ZnSe,” we will come back and ask 10.6, LWIR, or dual-band.
Coatings are not the crystal
A perfect AR on optical-grade plate does not turn it into laser grade. The coating sits on the surface. Bulk absorption still cooks the optic from the inside. Specify grade and coating as two lines. Published tables put 10.6 µm absorption near 5.0×10−4 cm−1; KM’s datasheet is < 5×10−4 cm−1. That heat, plus dn/dT of +61×10−6 K−1 and k of 20 W m−1 K−1 on the KM sheet (published tables often 18), is the thermal-lens problem the coating cannot fix.
What the 2025 LIDT paper actually measured
Zi, Niu, Jiao and coworkers deposited ZnS/YbF3 10.6 µm AR stacks on CVD ZnSe and on CVD diamond (Coatings 15, 536, 30 Apr 2025). Both coatings transmitted > 98% at 10.6 µm. Continuous-wave LIDT, 1.5 mm spot, 60 s dwell: 11,890 W/cm² on the ZnSe substrate (damage at 210 W) and 15,287 W/cm² on diamond (270 W) — 28.5% higher on diamond, which they attribute to thermal conductivity (~18 W m−1 K−1 for ZnSe vs ~2000 for diamond in their table). Damage on ZnSe was interface melting, cracks, and substrate perforation. That paper is a coating-and-substrate study, not a KM part number. I cite it because it is recent, open, and it says out loud what every CO2 shop already knows: once absorption is in the film or the bulk, the temperature rise decides the threshold, not the brochure residual.
ZnSe Knoop is 105–120. The coating lasts as long as the clean and the handling. Gloves. No dry paper. Spent windows come back to Florida for disposal if you ask — sales@kminnovationusa.com. Coating curves, residual reflection and damage numbers for a KM part are issued on the quotation.
References. KM datasheet n = 2.403. Zi, X., Niu, X., Jiao, H. et al., “Research on Thermal Effect and Laser-Induced Damage Threshold of 10.6 µm Antireflection Coatings Deposited on Diamond and ZnSe Substrates,” Coatings 15, 536 (2025), doi:10.3390/coatings15050536. Published ZnSe handling notes.