The National Institute of Standards and Technology (NIST) is seeking sources capable of supplying six ultralow loss mid-IR optics with hybrid crystalline coatings for use in cavity ring-down spectroscopy at a center wavelength near 4500 nm and temperatures between 180 K and 220 K. The mirrors must have a reflectivity of at least 0.999970 and transmission of 10 ppm or less, with the coating applied to NIST-supplied silicon substrates. This is a combined sources sought notice and notice of intent to award a sole source contract, but NIST will consider a competitive acquisition if capable alternative sources are identified. No budget amount is specified, and no performance location is mentioned. Responses are due by July 23, 2026.