Sapphire plates, as a high-performance optical material, demonstrate excellent optical transmission characteristics across the ultraviolet, visible, and infrared wavelength ranges. Their unique physical and chemical properties render them an ideal choice for operation in harsh environments. This article will analyze the advantages of sapphire plates from the perspectives of material properties, optical performance, and application scenarios.

1. The material properties and preparation process of sapphire

Sapphire (Al₂O₃) is a single crystal form of aluminium oxide, with a Mohs hardness of 9. Its melting point is 2053°C, and its thermal conductivity (approximately 35 W/m·K at 25°C) and coefficient of thermal expansion (5.3×10⁻⁶/K) are comparable to those of metals such as tungsten and molybdenum. This allows sapphire plates to withstand severe thermal shock. Sapphire crystals produced by the Czochralski method and heat exchange method can undergo directional cutting to yield optical-grade plates, with a surface roughness consistently below 0.5 nm.  

2. Analysis of wide-spectrum transmission performance

(1) Ultraviolet-visible light wavelength (200-550nm)

Sapphire glass boasts an impressive transmittance rate of over 85%, particularly in the ultraviolet range of 250-400nm, making it a superior material to fused silica in this regard. For example, when utilizing a 308-nm excimer laser, the absorption coefficient of a sapphire sample measuring 10 mm in thickness is less than 0.1 cm⁻¹.

(2) Mid-infrared light wavelength (3-5μm)

Maintaining a transmittance rate of over 80% is an ability that standard optical glass is unable to deliver. However, within the wavelength of 2.5-3μm, there is an absorption peak caused by lattice vibrations, which needs to be suppressed through doping modification.

(3) Limitations of far-infrared applications

As the wavelength increases above 6 μm, sapphire displays a significant decrease in transmittance, accompanied by the emergence of a prominent absorption band at 7.5 μm. This absorption band is indicative of the phonon resonance of the Al-O bond.

3. Environmental stability performance

Comparative experiments show that under the same conditions:

(1) Resistance to corrosion

After subjecting the sapphire surface to concentrated hydrochloric acid (37%) for 24 hours, a surface erosion depth of merely 0.02μm is measured.

(2) Radiation resistance

Following the irradiation of sapphire with 10⁶ Gy γ-rays, the transmittance of the material at a wavelength of 380nm decreased by less than 2%. This result was significantly better than the decline observed in radiation-sensitive materials such as calcium fluoride.

(3) Mechanical strength

A 4mm sapphire sample can resist quasi-static pressures of 1.5GPa and a dynamic impact test involving a steel ball travelling at 800m/s.

4. Typical application scenarios

(1) High-energy laser system

Sapphire glass is used as the output window for CO2 lasers (10.6 μm) and exhibits a power tolerance of up to 10 kW/cm².

(2) Special environment monitoring

The deep-sea detector features a conical sapphire window. Utilizing a water pressure of 1,100 meters, the optical distortion is successfully minimized to less than 0.1λ, ensuring high-definition imaging of the hydrothermal nozzle.

5. Surface treatment technology

To optimize optical performance, the current main approaches adopted are:

(1) Broadband anti-reflection coating

The design of TiO2/SiO2 multi-layer film systems has been developed to achieve an average reflectivity in the 400-5000 nm wavelength of less than 0.5%.

(2) Hydrophobic self-cleaning coating

The surface of the sapphire has been modified with fluorosilane, which has a contact angle of up to 115°. This significantly reduces the adhesion of sand and dust.

(3) Antistatic treatment

The sapphire surface is deposited with ITO nanowire networks. This allows the surface resistance to be controlled at 10⁶Ω/cm², thus preventing imaging noise caused by charge accumulation.