Optical Activity and Electrooptic Effect in Bismuth Germanium Oxide (Bi_12GeO_20)
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Optical Activity and Electrooptic Effect in Bismuth Germanium Oxide (Bi 12 GeO 20 ) P. V. Lenzo, E. G. Spencer, and A. A. Ballman Bell Telephone Laboratories, Inc. Murray Hill, New Jersey 07971. Received 29 June 1966. The compound commonly called gamma-bismuth trioxide 1 (γ-Bi 2 O 3 ) has been proved by phase diagram work 2 and more re- cently by single crystal studies 3 to be a true compound of bismuth germanium oxide of the mole ratio 6:1 corresponding to the form- ula Bi 12 GeO 20 ∙ Strong optical activity and static and radio frequency electro- optic effects have been observed and measured in single crystals of this material. We have determined that the point group of this crystal is T-23 by correlation of its properties with those of its point group. It possesses a body-centered cubic structure. 1 Its optical activity immediately places the crystal in one of the two enantiomorphous cubic classes, T-23 or 0-432. Since the material has been shown by Ballman 3 to be strongly piezoelectric, this characteristic eliminates the 0-432 class, which, although a noncentrosymmetric class, does not exhibit piezoelectricity. 4 The rotation of the plane of polarization Φ was measured as a function of wavelength λ; and the optical rotatory power or specific rotation ρ was determined from the expression 5 ρ = Φ/d, where d is the optical path length in the crystal. The results at 23°C are shown in Fig. 1. The rotation observed in all sam- ples checked was right-handed or dextro-rotatory and, by con- vention 6 , is considered to be positive. Anomalous rotatory dis- persion is exhibited below 5000 Å, a region of optical absorption with maximum absorption occurring at 4500 Å. 7 The circular birefringence (n ι — n r ), where n ι and n r are the refractive indices, respectively, of the left and right circularly po- larized components, may be obtained from the expression (n ι — n r ) = ρλ 0 /π. The gyration tensor 8 possesses three equal nonzero coefficients, g 11 = g 22 = g 33 , which will be called g. It can be shown that for class T-23 the gyration coefficient is where n is the index of refraction of the material and ρ is in radians/unit length. The index of refraction is 2.55 at 5100 Å. 9
Transcript of Optical Activity and Electrooptic Effect in Bismuth Germanium Oxide (Bi_12GeO_20)
Optical Activity and Electrooptic Effect in Bismuth Germanium Oxide (Bi12GeO20) P. V. Lenzo, E. G. Spencer, and A. A. Ballman
Bell Telephone Laboratories, Inc. Murray Hill, New Jersey 07971. Received 29 June 1966.
The compound commonly called gamma-bismuth trioxide1
(γ-Bi2O3) has been proved by phase diagram work2 and more recently by single crystal studies3 to be a true compound of bismuth germanium oxide of the mole ratio 6:1 corresponding to the formula Bi12GeO20∙
Strong optical activity and static and radio frequency electro-optic effects have been observed and measured in single crystals of this material. We have determined that the point group of this crystal is T-23 by correlation of its properties with those of its point group. It possesses a body-centered cubic structure.1
Its optical activity immediately places the crystal in one of the two enantiomorphous cubic classes, T-23 or 0-432. Since the material has been shown by Ballman3 to be strongly piezoelectric, this characteristic eliminates the 0-432 class, which, although a noncentrosymmetric class, does not exhibit piezoelectricity.4
The rotation of the plane of polarization Φ was measured as a function of wavelength λ; and the optical rotatory power or specific rotation ρ was determined from the expression5 ρ = Φ/d, where d is the optical path length in the crystal. The results at 23°C are shown in Fig. 1. The rotation observed in all samples checked was right-handed or dextro-rotatory and, by convention6, is considered to be positive. Anomalous rotatory dispersion is exhibited below 5000 Å, a region of optical absorption with maximum absorption occurring at 4500 Å.7
The circular birefringence (nι — nr), where nι and nr are the refractive indices, respectively, of the left and right circularly polarized components, may be obtained from the expression (nι — nr) = ρλ 0 /π .
The gyration tensor8 possesses three equal nonzero coefficients, g11 = g22 = g33, which will be called g. It can be shown that for class T-23 the gyration coefficient is
where n is the index of refraction of the material and ρ is in radians/unit length. The index of refraction is 2.55 at 5100 Å.9
Fig. 1. Optical rotatory power as a function of wavelength.
From Eq. (2) g = 28.3 × 10-5 for λ0 = 5100 Å. This value for g compares favorably with the values for a — quartz, (gu = g22 = ±5.82 × 105, g33 = ±12.96 × 10-5 for λ0 = 5100 Å).10 (The signs of Szivessy and Minister have been changed to conform to the more usual rotation convention used here.) An advantage for practical device applications is that Bi12GeO20 is cubic, thus eliminating the critical orientation dependency of optical activity found with quartz, as well as avoiding the added complication of natural linear birefringence encountered with quartz when propagating in directions other than the optic axis.
A comparison with NaC1O3 and NaBrO3, both of which also belong to class T-23, shows that rotatory power in Bi12GeO20 is approximately ten times larger than in either of these materials (ρ = 2.8°/mm for NaBrOs, Ρ = 3.137mm for NaC1O3 for λ = 5893 Å).11
Further experiments have shown that application of an electric field induces linear birefringence (the usual electrooptic effect) as well as a change in optical activity (circular birefringence). Modulation has been observed at frequencies up to 500 Mc/sec. Since applying an electric field may result in superposed changes in both linear birefringence and optical activity, neither effect can be measured directly. Separation of the two phenomena is accomplished by simultaneous measurements of ellipticity and apparent rotation of the resultant elliptically polarized light as a function of the applied field. Calculations using these data give the separated effects. A detailed description of these measurements as a function of optical wavelength will be reported separately. As an example, measurements show that the static linear electrooptic coefficient, n0
3γ41, is 1.6 × 10-6 cm/statvolt at λ = 6660 Å; the change in optical activity is several percent at moderate applied voltages.
References 1. W. C. Schumb and E. S. Bittner, J. Am. Chem. Soc. 65,
1055 (1943). 2. E. M. Levin and R. S. Roth, J. Res. Nat. Bur. Std. 68A,
197 (1964); E. I. Speranskaya and A. A. Arshakuni, Russ. J. Inorg. Chem. 9, 226 (1964).
3. A. A. Ballman, "The Growth and Properties of Piezoelectric Bismuth Germanium Oxide, Bi12GeO20" (accepted for publication in International Journal for Crystal Growth).
4. J. F. Nye, Physical Properties of Crystals (Oxford University Press, Oxford, 1960), p. 124.
5. Reference 4, p. 262. 6. Reference 4, p. 261. 7. R. N. Zitter and G. G. Douglas (private communication), 8. Reference 4, p. 272. 9. H. J. Levinstein (private communication).
10. G. Szivessy and C. Minister, Ann. Phys. 20, 703 (1934). 11. Handbook of Chemistry and Physics—45th Edition (The
Chemical Rubber Co., Cleveland, 1964).
October 1966 / Vol. 5, No. 10 / APPLIED OPTICS 1689
