CLBO: An Excellent Nonlinear Crystal for Short-Wave Ultraviolet Applications
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Publish time:
2025-11-13
CLBO: An Excellent Nonlinear Crystal for Short-Wave Ultraviolet Applications
Nonlinear crystals serve as core components in short-wave ultraviolet solid-state lasers and represent a critical focus for advancements in laser frequency conversion applications. Among nonlinear crystals, CLBO (CsLiB6O10) exhibits outstanding performance. However, producing CLBO crystals with high short-wave UV transmittance, robust moisture-resistant processing techniques, and coatings offering high transmittance and high laser-induced damage threshold (LIDT) requires advanced technical expertise. CASTECH’s CLBO crystals possess precisely these attributes. Let us take a closer look at this remarkable crystal.
Important Applications of Short-Wave Ultraviolet Radiation
Short-wave ultraviolet lasers offer advantages such as short wavelength, high resolution, and minimal diffraction effects, granting them broad application prospects in industrial fields. For instance, short-wave UV provides distinct benefits in Raman spectroscopy, as it avoids fluorescence interference issues commonly caused by visible light sources (Figure 1). Additionally, resonance in the UV region addresses challenges such as weak signal intensity often encountered in near-infrared Raman detection [1].
Figure 1. UV Raman spectroscopy avoids fluorescence interference
Furthermore, lasers operating at wavelengths such as 213 nm, 224 nm, and 266 nm are increasingly being applied in areas including wafer inspection, photoluminescence, and biomedical research. The extensive application potential of short-wave UV lasers is expected to drive further advancements in nanomaterials and biochemical analysis [2].
Excellent Performance of CLBO Crystals in Short-Wave UV Laser
Excellent effective nonlinear coefficient in Fourth/Fifth Harmonic Generation
Nonlinear crystals serve as core components in short-wave UV solid-state lasers, making their development highly significant. A major focus in the global laser frequency conversion field has been the continuous development of short-wave UV nonlinear optical crystals that support increasingly shorter phase-matching cutoff wavelengths while maintaining high harmonic conversion efficiency.
Currently, among commercially available short-wave UV nonlinear crystals, CLBO crystals have attracted considerable attention due to their properties and frequency conversion applications.
Wavelength (nm) | NLO Crystal | Phase Matching Angle (deg) | Deff (pm/V) | Angle Tolerance (mrad·cm) | Walk-off Angle (deg) | Spectral Acceptance (nm·cm) | Temperature Acceptance (℃·cm) |
532 + 532 = 266 | CLBO | 61.70 | 0.84 | 0.49 | 1.83 | 0.13 | 8.30 |
BBO | 47.70 | 1.32 | 0.17 | 4.80 | 0.07 | 4.50 | |
1064 + 266 = 213 | CLBO | 68.40 | 0.87 | 0.42 | 1.69 | 0.16 | 4.60 |
BBO | 51.10 | 1.26 | 0.11 | 5.34 | 0.08 | 3.10 |
Table 1. Nonlinear Optical Properties of CLBO and BBO Crystal
Cesium Lithium Borate (chemical formula: CsLiB6O10, abbreviated as CLBO), as an excellent nonlinear crystal for short-wave ultraviolet applications, exhibits advantages including large nonlinear coefficient, high damage threshold, broad spectral range, wide temperature and angular acceptance bandwidths, small walk-off angle, and high conversion efficiency (Table 1). Consequently, it is commonly used for fourth and fifth harmonic generation of 1064 nm lasers to generate 266 nm and 213 nm laser outputs (Figure 2).
Figure 2. CLBO Crystal and Hermetically Sealed Mount
CLBO crystal belongs to the tetragonal system and is a negative uniaxial crystal. The effective nonlinear coefficient (deff) for sum frequency generation is given by the formula:
Type I: deff = d36sinθsin2φ
From this formula, it can be derived that deff reaches its maximum value when φ = 45° [3]. This indicates that in Type I phase matching, the effective nonlinear coefficient for sum frequency generation is maximized at φ = 45°.
Broadband Transmission with High Transparency Down to 190 nm Short-Wave UV
CLBO crystals grown and developed by CASTECH exhibit exceptionally high transmittance across the 190 nm to 1200 nm wavelength range, particularly in the short-wave ultraviolet region. Figure 3 shows a comparison of the transmittance curves between CLBO and BBO crystals (grown by CASTECH) over the 190–1200 nm range. It can be observed that the transmittance of CLBO remains above 80% even around 193 nm, whereas the transmittance of BBO is nearly cut off in this spectral region.
Figure 3. Transmittance curves of CLBO and BBO crystals (polished grade)
Manufacturing Process of CLBO
Moisture-Resistant Processing Technology for Effective Control of CLBO Crystal Cracking
The unique structural "channels" within the CLBO crystal lattice make it highly susceptible to hydration-induced cracking. As illustrated in Figure 4, H2O molecules from the atmosphere can penetrate the crystal along the a- and b-axes, reacting with Cs and B atoms. This reaction breaks the original chemical bonds [4], macroscopically manifesting as crystal hydration and cracking. Consequently, the growth and processing of CLBO are extremely challenging. Inadequate control during these processes can lead to immediate hydration and cracking of the crystal, a phenomenon often referred to as the "suicidal" behavior of CLBO. This critical issue hindered the progress of CLBO research that began in the early 1990s and remains a key factor complicating its industrial-scale production.
Figure 4. Projected crystal structure of CLBO along the a-axis [5]
To address the issue of hydration-induced cracking in CLBO crystals, CASTECH's R&D team has conducted years of dedicated research and successfully developed specialized growth (as shown by the CLBO crystal boule in Figure 5) and processing techniques. These advancements enable the production of CLBO crystals that meet all required specifications (as detailed in Table 2 and Figure 6) while effectively controlling the problem of hydration-induced cracking.
Figure 5. CLBO Crystal Boule
Dimension Tolerance | (W ± 0.1 mm) × (H ± 0.1 mm) × (L + 0.5/-0.1 mm) × (L≧2.5 mm) (W ± 0.1 mm) × (H ± 0. 1 mm ) × (L + 0.1/-0.1 mm) × (L＜2.5 mm) |
Clear Aperture | Central 90% of the diameter |
Surface Quality (Scratch/Dig) | 10/5 to MIL-PRF-13830B |
Flatness | ≦λ/6 @633 nm |
Parallelism | 20 arc sec |
Perpendicularity | ≦15 arc min |
Angle Tolerance | Δθ≦0.25 °, ΔФ≦0.25 ° |
Table 2. Standard Specifications of CLBO Crystals
Figure 6. Instrument-measured Flatness and Surface Finish of a CLBO Crystal
Furthermore, the application of anti-reflection (AR) coatings, protective (P) coatings, or the use of specially designed hermetically sealed moisture-proof holders from CASTECH can further extend the service life of CLBO crystals.
Coatings for Enhanced Transmittance and Damage Resistance in Short-Wave Ultraviolet Region
The coating process for laser crystals used in fourth/fifth harmonic generation applications demands extremely high precision. Particularly for the entrance surface in fifth harmonic generation, the coating must simultaneously exhibit high transmittance and high laser-induced damage threshold (LIDT) for both near-infrared and short-wave ultraviolet wavelengths.
In practical design, to enhance the transmittance of CLBO crystal products and further reduce surface reflection, customers can opt for dual-wavelength or triple-wavelength anti-reflection (AR) coatings. When a dual-wavelength AR coating is selected, the reflectance can be reduced to below 0.2%. Opting for a triple-wavelength AR coating can reduce the reflectance in the specified bands to below 2% (Table 3).
Base Material | AR-Coating | Reflectance |
CLBO | AR-532 nm/266 nm | R<0.2% @532 nm R<0.5% @266 nm |
CLBO | AR-1064 nm/266 nm/213 nm | R＜1% @ 1064 nm R＜2% @ 266 nm R＜2% @ 213 nm |
Table 3. Standard Coating Specifications for Fourth/Fifth Harmonic Generation
Figure 7. Dual-wavelength/Triple-wavelength Anti-reflection Coatings
Regarding the assurance of high damage threshold for CLBO crystals, CASTECH's coating engineers have developed an optimized CLBO coating process through extensive and repeated testing. For instance, in dual-wavelength AR coatings, the damage threshold reaches 2 J/cm² at 266 nm and 5 J/cm² at 532 nm. The CLBO crystals developed and produced by CASTECH, through this specialized high-performance coating process, demonstrate both excellent transmittance and damage resistance. Depending on the specific application scenario and angle of incidence, CASTECH can design customized coating stacks, offering tailored production solutions.
Knowledge is boundless, and exploration never ceases. CASTECH remains committed to advancing the development of optical crystals and laser applications. For detailed information regarding CLBO crystals, please feel free to contact our sales team to jointly explore new developments.
References:
[1] FAN Fengtao. UV Raman Spectroscopic Studies on Catalytic Materials. State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
[2] 27 Categories of Frontier Applications in Laser Technology. Changchun New Industries Optoelectronics Technology Co., Ltd.
[3] ZHANG Kecong, WANG Ximin. Nonlinear Optical Crystal Materials Science. Science Press.
[4] SHEN Dezhong. Research Progress on UV Frequency-Doubling Crystal CLBO and Fabrication of Frequency Converters. Tsinghua University.
[5] SHEN Dezhong. Research Progress on Novel UV Frequency Conversion Crystal CLBO. Tsinghua University.
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