Spectroscopic Analysis of Ho:BaF2 Crystals in the NIR to MIR Spectral Region

doi:10.15541/jim20250349

Abstract

Abstract:

Infrared lasers in the 1-3 μm region are increasingly important for applications in medical treatment, atmospheric monitoring, and high-power laser systems. Holmium ions (Ho³⁺) are particularly attractive because of their multiple emission channels covering near to mid infrared ranges. This work aims to systematically evaluate the structural and spectroscopic properties of Ho:BaF₂ crystals and determine the optimal doping concentrations for efficient multi-band laser operation. High-quality Ho:BaF₂ single crystals with concentrations of 0.5%-3.0% (in atom) were grown using the temperature gradient technique (TGT). Structural characterization was performed, while spectroscopic properties were analyzed via absorption, fluorescence, and lifetime measurements. Judd-Ofelt analysis was further applied to calculate radiative parameters. All samples exhibited cubic structures, with doping segregation ratios close to unity and uniform Ho³⁺ distribution. Spectroscopic evaluation revealed optimal doping concentrations of 2.0% (in atom) for ~1.2 μm (⁵I₆→⁵I₈, spectral quality factor Q=24.29×10^-21 cm²·ms) and ~2.05 μm (⁵I₇→⁵I₈, Q=67.53×10^-21 cm²·ms), and 1.0% (in atom) for ~2.85 μm (⁵I₆→⁵I₇, Q=44.52×10^-21 cm²·ms). BaF₂ host, with its low phonon energy (~346 cm^-1) and anti-clustering characteristics, enabled enhanced emission performance, including a maximum emission cross-section of 3.81×10^-21 cm² at ~2.05 μm. These results outperform traditional hosts such as YAG and CaF₂. Compared to oxide hosts, BaF₂ offers superior lifetime, reduced non-radiative losses, and greater resistance to concentration quenching. The findings indicate that Ho:BaF₂ supports higher effective doping levels, making it particularly promising for high-power and ultrafast laser applications. Ho:BaF₂ crystals demonstrate excellent potential as efficient, multi-wavelength infrared laser gain media.

Key words: BaF₂ crystal, Ho³⁺ doping, mid-infrared laser, spectroscopic characteristic

CLC Number:

TQ174

QIAN Xinyu, WANG Wudi, GUO Junyao, REN Yongchun, DONG Jianshu, WANG Qingguo, TANG Huili, ZHANG Chenbo, XU Xiaodong, DONG Yongjun, HUA Wei, XU Jun. Spectroscopic Analysis of Ho:BaF₂ Crystals in the NIR to MIR Spectral Region[J]. Journal of Inorganic Materials, 2026, 41(5): 595-603.

Figures/Tables 10

References 55

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Sample	Initial concentration/% (in atom)	Actual concentration/% (in atom)	Doping segregation ratio
1	0.5	0.52	1.04
2	1.0	0.91	0.91
3	1.5	1.37	0.91
4	2.0	1.87	0.94
5	3.0	2.73	0.91

Sample	Initial concentration/% (in atom)	Actual concentration/% (in atom)	Doping segregation ratio
1	0.5	0.52	1.04
2	1.0	0.91	0.91
3	1.5	1.37	0.91
4	2.0	1.87	0.94
5	3.0	2.73	0.91

Gain medium	Concentration/% (in atom)	637 nm (⁵I₈→⁵F₅)		1157nm (⁵I₈→⁵I₆)		1950 nm (⁵I₈→⁵I₇)
Gain medium	Concentration/% (in atom)	σ_abs/(×10^-21, cm²)	FWHM/nm	σ_abs/(×10^-21, cm²)	FWHM/nm	σ_abs/(×10^-21, cm²)	FWHM/nm
BaF₂	0.5	4.022	12	1.212	50	2.865	117
	1.0	5.150	12	1.572	49	3.570	112
	1.5	5.986	12	1.851	48	4.140	102
	2.0	6.041	11	1.835	48	4.181	97
	3.0	6.457	11	1.923	49	4.423	95

Gain medium	Concentration/% (in atom)	637 nm (⁵I₈→⁵F₅)		1157nm (⁵I₈→⁵I₆)		1950 nm (⁵I₈→⁵I₇)
Gain medium	Concentration/% (in atom)	σ_abs/(×10^-21, cm²)	FWHM/nm	σ_abs/(×10^-21, cm²)	FWHM/nm	σ_abs/(×10^-21, cm²)	FWHM/nm
BaF₂	0.5	4.022	12	1.212	50	2.865	117
	1.0	5.150	12	1.572	49	3.570	112
	1.5	5.986	12	1.851	48	4.140	102
	2.0	6.041	11	1.835	48	4.181	97
	3.0	6.457	11	1.923	49	4.423	95

Concentration/% (in atom)	Ω₂/(×10^-20, cm²)	Ω₄/(×10^-20, cm²)	Ω₆/(×10^-20, cm²)	Ω₄/Ω₆	RMSΔS/(×10^-21, cm²)
0.5	0.482	1.792	0.904	1.983	0.469
1.0	0.459	2.178	1.088	2.001	0.476
1.5	0.533	2.493	1.251	1.993	0.457
2.0	0.506	2.554	1.303	1.961	0.430
3.0	0.559	2.697	1.391	1.938	0.427

Spectroscopic Analysis of Ho:BaF₂ Crystals in the NIR to MIR Spectral Region

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Transition	Concentration/% (in atom)	FWHM/nm	β/%	τ_rad/ms	σ_em/(×10^-21, cm²)	τ/ms	Q/(×10^-21, cm²·ms)
~1.2 μm (⁵I₆→⁵I₈)	0.5	23.00	80.43	12.01	2.47	8.71	21.51
	1.0	22.90	81.95	10.16	3.01	8.05	24.23
	1.5	31.63	82.94	8.95	2.83	7.49	21.20
	2.0	24.69	83.21	8.64	3.36	7.23	24.29
	3.0	26.03	83.64	8.14	3.64	6.39	23.26
~2.05 μm (⁵I₇→⁵I₈)	0.5	151.26	-	22.83	2.69	15.49	41.67
	1.0	147.85	-	20.21	3.11	16.27	50.60
	1.5	144.99	-	18.36	3.49	17.22	60.10
	2.0	144.45	-	17.88	3.60	18.76	67.53
	3.0	144.23	-	17.07	3.81	15.68	59.74
~2.85 μm (⁵I₆→⁵I₇)	0.5	116.70	19.57	12.01	4.77	8.71	41.55
	1.0	115.09	18.05	10.16	5.53	8.05	44.52
	1.5	114.01	17.06	8.95	5.81	7.49	43.52
	2.0	115.53	16.79	8.64	5.61	7.23	40.56
	3.0	115.04	16.36	8.14	6.09	6.39	38.91

Waveband	Gain medium	τ/ms	Q/(×10^-21, cm²·ms)	Ref.
~1.2 μm	2.0% Ho:BaF₂	7.23	24.29	This work
	YSGG	0.643	6.43	[44]
	Nano glass	7.8	21.06	[45]
	BaF₂ Nano crystals	2.87	17.22	[46]
~2.05 μm	2.0% Ho:BaF₂	17.76	67.53	This work
	YAG	7.8	62.4	[47]
	CaF₂	11.52	46.81	[41]
	CNGG	6.41	40.38	[48]
	GdScO₃	6.13	30.04	[49]
~2.85 μm	1.0% Ho:BaF₂	8.05	44.52	This work
	LuAG	0.13	20.15	[50]
	LLF	1.83	30.61	[51]
	PbF₂	5.40	77.76	[52]