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Quantum Electronics

Absorption saturation in Cr4+-activated silicate crystals and use of these crystals in passive laser switches

Nickolay Zhavoronkov, V. P. Mikhailov, Н. В. Кулешов, B. I. Minkov, A. S. Avtukh

Published Jan 31, 1995• 10 citations
Publisher

Abstract

The absorption saturation in Cr 4+ -activated Mg 2 SiO 4 and Y 2 SiO 5 crystals was studied. The cross sections were calculated for the absorption of 694 nm light by the ground 3 A 2 ( 3 F ) and the excited 3 T 2 ( 3 F ) states. A ruby laser was passively Q-switched by Mg 2 SiO 4 : Cr 4+ and Y 2 SiO 5 : Cr 4+ crystals. The duration and energy of the output pulses were τ = 100 ns and E = 0.24 J.

DOI
10.1070/qe1995v025n01abeh000279

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The absorption saturation in Cr 4+ -activated Mg 2 SiO 4 and Y 2 SiO 5 crystals was studied. The cross sections were calculated for the absorption of 694 nm light by the ground 3 A 2 ( 3 F ) and the excited 3 T 2 ( 3 F ) states. A ruby laser was passively Q-switched by Mg 2 SiO 4 : Cr 4+ and Y 2 SiO 5 : Cr 4+ crystals. The duration and energy of the output pulses were τ = 100 ns and E = 0.24 J.
We describe a systematic, rigorous procedure for the determination of the optical absorption saturation parameters for Cr4+:YAG and Cr4+:forsterite crystals at 1064 nm. A rate-equation approach was used to analyze the cw and pulsed transmission data of several crystals by accounting for the transverse as well as longitudinal variation of the beam intensity, saturation effects, and excited-state absorption. Use of an iterative procedure whereby the cw and pulsed data were simultaneously analyzed led to a considerable reduction in the error for the determination of cross sections. The average value of the absorption cross section sigmaa and the normalized excited-state absorption cross section fp=sigmaesa/sigmaa were determined to be 6.13×10−19 cm2 and 0.45, respectively, for Cr4+:forsterite and 19.6×10−19 cm2 and 0.06, respectively, for Cr4+:YAG. Detailed comparison was also made with previous saturation measurements in the literature. Our results further show that lumped models based on the thin-length approximation should be used with caution in the determination of cross sections, especially when the pump beam is tightly focused inside the absorber.
The optical absorption, temperature dependences of the luminescence intensity and decay time, and the transient absorption spectra of a Cr 4+ : Gd 2 SiO 5 crystal were determined by the picosecond excitation and probing method. An analysis of the results showed that the induced absorption observed in the 600–650 nm range was due to 3 T 2 ( 3 F ) → 3 T 1 ( 3 P ) transitions from the excited state. The lifetimes of the 3 T 2 ( 3 F ) and 3 T 1 ( 3 F ) levels were estimated to be 15 ns and 12 ps, respectively. The cross section for absorption from the first excited state at the wavelength of 616 nm was (1.1 ± 0.6)×10 -18 cm 2 and that for absorption from the ground state at λ = 540 nm was (2.2 ± 1.1)×10 -18 cm 2 .
An experimental study of saturable absorption and excited-state absorption (ESA) in several inorganic saturable absorbers, Cr/sup 4+/:YAG, Cr/sup 4+/:GGG, and Cr/sup 4+/:YSGG, is presented. We provide the theoretical background of absorption characteristics in saturable absorbers that exhibit ESA, with some new results: approximate analytical solutions are proposed for the optical transmission in the case of a slow absorber, and for various light intensity conditions of spatially or temporally Gaussian beams in fast and slow absorbers. Experimentally, partial bleaching of the first excited state itself could be observed in Cr/sup 4+/:YAG at /spl lambda/=1064 nm, yielding the higher excited-state lifetime as /spl tau/*=(0.55/spl plusmn/0.1) ns. The regular transmission bleaching curve was measured in Cr/sup 4+/:GGG, for the first time in this material, yielding /spl sigma//sub ga/=(58/spl plusmn/5)/spl times/10/sup -1/ cm/sup 2/, and /spl sigma/e/sub s/=(13/spl plusmn/2)/spl times/10/sup -19/ cm/sup 2/ at /spl lambda/=1064 nm, ESA spectra were measured for the three materials between /spl sim/700 and 900 nm. All three exhibit crossing between saturable absorption at longer wavelengths and inverse saturable absorption at shorter wavelengths.
�сследовано насыщение поглощения активированных Cr4+ силикатов Mg2SiO4 и Y2SiO5. Расcчитаны сечения поглощения из основного (3A2(3F)) и возбужденного (3T2(3F)) состояний на длине волны 694 нм. Реализован режим модулированной добротности в лазере на рубине с использованием пассивных затворов на кристаллах Mg2SiO4:Cr4+ и Y2SiO5:Cr4+. Длительность импульсов излучения τ = 100 нс, энергия E = 0.24 Дж.
Saturation absorption experiments are conducted on the title compound at 1.064 μm with laser light propagating along both the [001] and [111] crystallographic axes. Rotation of the crystals about these axes during the experiments reveals that the transmission of the polarized 1.064 μm radiation is highly anisotropic in the saturation regime. The details of the anisotropy and saturation curves are explained using a simple model of subsets of optical centers along with a relevant set of rate equations. The ground and excited-state absorption cross sections at 1.064 μm are calculated to be σgs=5.7(±2)×10−18 cm2 and σes=8(±2)×10−19 cm2, respectively, and the optimum crystallographic orientation for utilization in optical devices is discussed.