Application Research Progress of Calcium Fluoride (CaF₂) in Optical Field

Abstract: As an important optical material, Calcium Fluoride (CaF₂) occupies an irreplaceable position in the modern optical field due to its unique physical and chemical properties and excellent optical performance. This paper systematically expounds the structural characteristics, optical properties of CaF₂ crystals, and their current application status in various optical devices, focusing on the key role of its high transmittance in the ultraviolet-visible-infrared broad spectrum, low refractive index, and low dispersion characteristics in improving the performance of optical systems. Combining with the current technological development trends, the innovative application prospects of CaF₂-based optical elements in laser technology, quantum optics, astronomical observation, and other fields are analyzed, and it is pointed out that future research directions should focus on optimizing the growth process of large-size high-quality crystals and developing new composite optical structures.

Keywords: Calcium Fluoride; Optical Material; Transmission Range; Low Dispersion; Optical Element; Laser Technology; Quantum Optics; Infrared Window

I. Introduction

With the rapid development of optoelectronic technology, the demand for high-performance optical materials is increasingly urgent. Traditional optical glass is limited by intrinsic absorption and cannot meet the requirements of broad-spectrum applications under extreme working conditions. As a representative of alkaline earth metal fluorides, Calcium Fluoride (CaF₂) has become an ideal choice for breaking through the bottleneck of traditional optical materials due to its excellent optical homogeneity, wide transmission range, and good thermomechanical properties [1]. Especially its high transmission characteristics in the ultraviolet to mid-infrared band make it show significant technical advantages in high-end optical systems.

II. Crystal Structure and Basic Physical Properties of CaF₂

2.1 Crystal Structure Features

CaF₂ belongs to the cubic crystal system, with the space group Oh⁵⁻(Fm3̄m). It consists of Ca²⁺ cations and F⁻ anions arranged in a face-centered cubic lattice structure. This ordered arrangement endows the crystal with a high degree of structural symmetry and isotropy, effectively reducing light scattering losses [2].

2.2 Key Physical Parameters

Parameter	Value Range	Remark
Transmission Wavelength Range	0.15μm–8μm	Covering vacuum ultraviolet to mid-infrared
Refractive Index (@589nm)	1.434	One of the lowest among all inorganic crystals
Abbe Number	95	Near-zero chromatic aberration
Coefficient of Thermal Expansion	12×10⁻⁶/K	Better than semiconductor materials such as germanium and silicon
Mohs Hardness	4	Ultra-smooth surface obtainable by precision polishing

3.1 Broadband High Transmission CharacteristicsIII. Analysis of Core Optical Characteristics

CaF₂ exhibits extremely high spectral transmittance in the deep ultraviolet (DUV), visible light (VIS), and mid-infrared (MIR) regions (Figure 1). Especially in the vacuum ultraviolet region (<200nm), its cutoff wavelength can reach 150nm, making it one of the few transmissive materials that can meet the requirements of synchrotron radiation sources. This characteristic makes it a core component of vacuum ultraviolet spectrometers and solar-blind imaging systems.

3.2 Low Refractive Index and Low Dispersion Characteristics

Compared with fused silica (n=1.458), CaF₂ has a lower refractive index and much smaller dispersion. According to Seidel aberration theory, using CaF₂ to make apochromatic lens groups can greatly reduce secondary spectrum and significantly improve image quality. This characteristic has important application value in multispectral confocal microscopes, high-precision interferometers, and other equipment.

3.3 Thermal Stability and Environmental Adaptability

CaF₂ has a high thermal conductivity and a coefficient of thermal expansion that matches commonly used metal substrates, making it suitable for manufacturing large-size windows and lenses. Experiments show that CaF₂ samples treated with ion implantation surface strengthening can still maintain stable optical performance at 300°C, suitable for high-power laser transmission systems.

IV. Typical Optical Application Examples

4.1 Precision Optical Component Manufacturing

4.1.1 Windows for Excimer Lasers

In ArF excimer laser (λ=193nm) exposure systems, CaF₂ windows play a key role in blocking ozone contamination. By depositing a diamond protective film through magnetron sputtering, the service life can be extended to thousands of hours, meeting the industrial requirements of semiconductor lithography machines.

4.1.2 Infrared Thermal Imaging Lenses

Taking advantage of CaF₂'s high transmittance in the 3-5μm band, continuously variable focal length infrared lenses have been developed for security monitoring applications. Combined with non-spherical numerical control machining technology, a large aperture design of f/1.8 can be realized, significantly improving weak signal detection capability.

4.2 Construction of Special Optical Systems

4.2.1 Cavity Attenuated Oscillator (CAOS)

Based on the low phonon energy characteristic of CaF₂, a Brillouin scattering suppressed resonator has been constructed, achieving stability improvement of picosecond-level pulsed lasers. This device has important application value in precision measurement fields such as gravitational wave detection and cesium atom clocks.

4.2.2 Quantum Optical Devices

Using CaF₂ single crystal as a rare earth ion doping matrix, Yb³⁺:CaF₂ solid-state lasers have been successfully fabricated. Its wide emission bandwidth supports femtosecond-level ultrashort pulse generation, providing an ideal photon source for quantum key distribution systems.

V. Frontier Technology Development Trends

5.1 Breakthrough in Large-Size Crystal Growth Technology

Using the crucible descent method combined with directional seed-induced growth, CaF₂ single crystals with a diameter of Φ300mm have been successfully prepared. Through stress birefringence compensation cutting technology, the residual stress can be controlled at the 10⁻⁷ level, meeting the requirements for use as the primary mirror of space telescopes.

5.2 Innovation in Multifunctional Composite Structures

A gradient refractive index film layer composed of CaF₂ and zinc sulfide (ZnS) has been developed, offering both high radiation resistance and broadband antireflective properties. This design has been verified in Mars rover spectrometers, maintaining over 90% transmittance after a cumulative dose of 1Mrad.

5.3 Expansion of Micro-Nano Photonics Applications

Using femtosecond laser direct writing technology, two-dimensional photonic crystal structures have been etched on CaF₂ substrates. Experimental measurements show that this structure achieves a group index of 42 in the communication band (C+L band), providing a new solution for on-chip optical interconnects.

VI. Conclusion

Calcium fluoride has become a key foundational material in modern optical engineering due to its unique optical properties. From deep space exploration to quantum computing, from semiconductor manufacturing to biomedical imaging, CaF₂-based optical components continue to drive technological progress. In the future, with improvements in crystal growth processes and advancements in micro-nano fabrication technologies, CaF₂ will play an even greater role in emerging fields such as high-energy lasers and terahertz technology. It is recommended to strengthen interdisciplinary collaboration, focusing on breakthroughs in key technologies such as large-size uniformity control and extreme environment service performance enhancement.