Degenerate two-photon absorption spectra in Bi2O3:B2O3 glasses
Introduction
Two-photon absorption (TPA) in oxide glasses has been of interest, because of potential applications in non-linear optical devices, such as optical power limiters and regulators to smooth optical transients [1]. In preceding studies [2], [3], [4], [5], we reported that oxide glasses containing heavy metal oxides (HMO), such as PbO or Bi2O3 as a major constituent, possess degenerate TPA at 532 nm with the TPA coefficient, β, ranging from 5 to 21 cm/GW; these βs are larger than those in wide band gap inorganic oxides [6], [7], [8], [9].
Then, from both scientific (e.g., material-design) and practical (e.g., appropriate operation) points of view, we were motivated to examine the wavelength-dispersion of TPA in oxide glasses containing HMO. To explain the dependence of β on the incident photon energy, it should be noted that optical properties including TPA depend upon the electronic structures of glass. According to Liu et al. [10], optical spectra in oxide glass containing HMO as a major constituent show features typical of electronic structure similar to those of crystalline semiconductors: A `two-parabolic band model', which had been applied to represent the electronic structure in semiconductors, worked well to explain the optical spectra in many glasses. Further, by using the model, a simple formula had been proposed to predict the nonlinear index of refraction and degenerate TPA [10] coefficient in semiconductors. Here, we report on measurements of degenerate TPA coefficient in binary Bi2O3–B2O3 glasses at incident photon energy ranging from 1.8 to 2.5 eV. The degenerate TPA spectra observed are reproduced by using a two-parabolic band model with `effective band gap for direct transition'.
Section snippets
Experimental procedure
Binary xBi2O3:(100 − x)B2O3 (x=60, 70 and 80 mol%) glass slabs of 2 mm thick were subjected to TPA measurements. Details of the glass preparation were described in our preceding paper [5]. A commercial lead silicate glass, FDS9, (0.9 mm thick; Hoya Corporation) was also subjected to the measurements for comparison. Prior to the TPA measurement, the optical transmission spectra for the sample glasses were measured with a spectrophotometer at a wavelength ranging from 300 to 800 nm.
Degenerate
Experimental results
Fig. 1 shows transmission spectra of the sample glasses. As seen in Fig. 1, the transmittance decreases for photon energies greater than 2.51 eV for Bi2O3:B2O3 samples and ∼3.3 eV for the FDS9 sample. The decrease of the transmittance in Fig. 1 is due to absorption, which is intrinsic to the glass composition. This absorption shifts to larger energies with decreasing Bi2O3 content. There were no absorption bands for photon energies less than this absorption. It should be noted that the
Two-photon absorption coefficients in Bi2O3:B2O3 glasses
As seen in Fig. 1, the transmission losses at the photon energy from 1.6 to 2.5 eV for Bi2O3:B2O3 glasses and from 1.6 to 3.1 eV for FDS9, respectively, were due to reflections of light at both incident and exit surfaces of the glasses. The losses of ∼30% for Bi2O3:B2O3 glasses and 17% for FDS9, agreed with those inferred from the index of refraction of Bi2O3:B2O3 glasses (n ∼ 2.3 at 583 nm) [13] and FDS9 (nD=1.846). We then recognized that the effect of internal absorption upon the present Z
Conclusion
The degenerate TPA coefficient, β, that we obtained at a fixed photon energy increased with Bi2O3 content, while β became larger at larger photon energy. The degenerate TPA spectra could be reproduced by using a simple formula based on a two-parabolic band model for the electronic structure of the glasses containing heavy metal oxides: an `effective band gap for direct transition' deduced by the degenerate TPA spectrum, would be an alternative factor for representing electronic structure of
Acknowledgments
This work was in part supported by 1998 Grant Program for Scientific Research from the Asahi Glass Foundation, and by Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture (Grant No. 10750500).
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