Characterization of Subsurface Damage Layer in CaF2 Crystals and Its Correlation Properties of Photothermal Weak Absorption

doi:10.16553/j.cnki.issn1000-985x.2026.0064

Abstract

Abstract: The surface quality of optical components directly determines their optical performance and service life. Among these factors， the subsurface damage （SSD） layer is a key factor affecting the functionality and reliability of optical components and has become a focal point in their application. This paper takes CaF₂ crystals as the research subject and focuses on the characterization of the subsurface damage layer and its correlation with photothermal weak absorption in the 355 nm wavelength band. By systematically characterizing the thickness of the subsurface damage layer in crystal samples and measuring their photothermal weak absorption rates at 355 nm， the relationship between subsurface damage layer thickness and photothermal weak absorption rate was systematically analyzed. The results indicate that the photothermal weak absorption rate in the 355 nm band is positively correlated with SSD thickness. As the damage layer thickness increase from 51.57 nm to 154.09 nm， the maximum photothermal weak absorption rise from 2.58×10^-6 to 45.16×10^-6， and the average weak absorption increase from 0.43×10^-6 to 7.85×10^-6. This study has elucidated the mechanism by which subsurface damage layers affect the photothermal weak absorption of CaF₂ crystals， providing both experimental evidence and theoretical support for optimizing crystal processing techniques， mitigating losses due to weak absorption， and enhancing the crystals’ laser damage thresholds.

Key words: CaF₂ crystal; subsurface damage layer; photothermal weak absorption; chemical etching method; surface roughness

CLC Number:

O734

YAN Ke, MEI Bingchu, ZHANG Bo, KOU Huamin, JIANG Dapeng, GAO Wenlan, SU Liangbi. Characterization of Subsurface Damage Layer in CaF₂ Crystals and Its Correlation Properties of Photothermal Weak Absorption[J]. Journal of Synthetic Crystals, 2026, 55(8): 1252-1260.

Figures/Tables 10

Fig.1 Evolution of the surface morphology of a CaF2 crystal during chemical etching， as observed by white-light interference

Table 1 Values of the three-dimensional roughness parameters Sa and Sq during the chemical etching of CaF2 crystals

Time/min	Sa/nm	Sq/nm
5	4.031 1	6.584 4
10	5.603 1	11.634 6
15	12.843 0	23.208 8
20	7.793 3	14.607 5
25	5.419 1	10.644 6
30	11.083 4	21.986 4
35	11.243 8	22.095 1
40	11.976 5	22.396 4
45	12.289 1	23.453 2

Fig.2 Contour plots of residual stress distribution in CaF2 crystals under different corrosion durations

Fig.3 Morphology of the stepped regions of CaF2 crystals at different stages of etching

Table 2 Step height， sub-step height， and interval corrosion rates under different corrosion durations

Time/min	Step height/nm	Sub-step height/nm	Interval corrosion rate/（nm·min^-1）
5	53.63	53.63	—
10	72.68	19.05	3.81
15	95.14	22.46	4.49
20	122.62	27.48	5.50
25	154.09	31.47	6.29
30	175.01	20.92	4.18
35	186.54	11.53	2.31
40	197.57	11.03	2.21
45	208.87	11.30	2.26

Fig.4 Plot of step height variation （a） and plot of corrosion rate variation over the interval （b）

Fig.5 Photothermal weak absorption spectra of the four samples group

Table 3 Values for weak absorption and thickness of the subsurface damage layer for the four groups of samples

Sample	Maximum photothermal weak absorption/10^-6	Average photothermal weak absorption/10^-6	Subsurface damage layer thickness/nm
a	2.58	0.43	51.57
b	4.37	0.86	65.86
c	21.41	6.39	106.31
d	45.16	7.85	154.09

Fig.6 Graph showing the variation in pitting corrosion rates for the four groups of samples

Fig.7 Relationship between photothermal weak absorption and the thickness of the subsurface damage layer

References 23

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Characterization of Subsurface Damage Layer in CaF₂ Crystals and Its Correlation Properties of Photothermal Weak Absorption

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