PHASE MODULATION AND FAR-FIELD SPATIAL PATTERNS DUE TO THE TRANSFORMATIONAL THERMAL-LENS EFFECT

In earlier work we have reported a thermal-lens effect that is opposite in sign from and much stronger than the well-known effect in fluids and isotropic solids: Very near the transformation temperature in crystals undergoing structural phase transitions, certain indices of refraction have temperature derivatives that are large and positive ( congruent-to + 10(-1) K(-1)), in contrast to the negative values of dn/dT that arise from thermal expansion in glasses or fluids. We evaluated the steady-state and time-dependent behavior of these effects in Ba2NaNb5O15 near T(c) = 850 K. In the present paper we extend these studies to examine the far-field spatial pattern produced by self-induced phase modulation. We also utilize the thermal-lens effect to investigate the critical behavior near the tricritical point of Ba2NaNb5O15, and the critical exponent Beta is obtained as Beta-c = 0.28 +/- 0.01 and Beta-a = 0.31 +/- 0.01.