M. DiDomenico

Dielectric and optical properties of melt-grown BaTiO3

Abstract Dielectric constant, spontaneous polarization, optical absorption, refractive index, and birefringence data are reported for melt-grown single-domain crystals of BaTiO 3 . The temperature dependences of the clamped and unclamped dielectric constants and the spontaneous polarization are found to be similar to results previously reported for flux-grown crystals. Differences in dielectric behavior are largely attributable to the higher transition temperatures in melt-grown crystals, i.e. the tetragonal phase is stable between 6–133†C in melt-grown crystals, and between 2–120°C in flux-grown crystals. The optical properties of melt and flux-grown crystals are, however, significantly different. Melt-grown crystals, in contrast to flux-grown crystals, exhibit an exponential Urbach absorption edge with negligible absorption ( −1 ) for photon energies below approximately 3 eV. Refractive indices are somewhat higher in melt-grown crystals and the dispersion, which fits a Sellmeier relation, differs from that previously reported. The birefringence is shown to be a consequence of the quadratic electro-optic effect with the same electro-optic coefficient as measured in the paraelectric phase.

Fundamental optical attenuation limits in the liquid and glassy state with application to fiber optical waveguide materials

Fundamental optical scattering and absorption mechanisms have been identified which limit light transmission in fiber optical waveguide materials. These mechanisms, which are intimately associated with the random structure in the liquid and glassy state, are described and then used as a basis for comparing fiber optical waveguide materials. It is concluded that pure fused silica is a preferred waveguide material, having ultimate total losses of 1.2 dB/km at the Nd : YAG laser wavelength of 1.06 μ, 3.0 dB/km at the GaxAl1−xAs emission wavelength of approximately 0.8 μ, and 4.8 dB/km at the GaP : Zn, O emission wavelength centered at 0.7 μ.

Evidence for tunneling in reverse‐biased III‐V photodetector diodes

Photodiodes made from III‐V group semiconductor alloys have been found to exhibit anomalously high dark currents. We present evidence that tunneling is the dominant source of dark current in many cases. The tunneling current becomes substantial at peak junction electric fields as low as 105 V/cm due to the small direct energy gaps and small effective masses of the materials tested. Tunneling sets limits on the magnitude of the electric field attainable in these materials, and therefore has serious implications on photodetector design and performance.

RELATIONSHIP BETWEEN LINEAR AND QUADRATIC ELECTRO‐OPTIC COEFFICIENTS IN LiNbO3, LiTaO3, AND OTHER OXYGEN‐OCTAHEDRA FERROELECTRICS BASED ON DIRECT MEASUREMENT OF SPONTANEOUS POLARIZATION

The spontaneous polarizations in LiNbO3, LiTaO3, and Ba2NaNb5O15 have been measured directly at room temperature by a field reversal method and found to be Ps (LiNbO3) = 0.71 C/m2, Ps (LiTaO3) = 0.50 C/m2, and Ps (Ba2NaNb5O15) = 0.40 C/m2. Using these values we show that the linear electro‐optic effect in these materials is related fundamentally to a biased quadratic effect associated with each BO6 octahedron. Based on this result, predictions are made of the spontaneous polarization in other oxygen‐octahedra ferroelectrics.

Ferroelectricity in BaM2+F4

Abstract The piezoelectric compounds BaM2+F4 (M = Mg, Co, NiandZn) are shown to be ferroelectric at room temperature. The spontaneous polarizations are: PS(Mg) = 7.7 μC/cm2, PS(Co) = 8 μC/cm2, Ps(Ni) = 6.7 μC/cm2andPS(Zn = 9.7 μC/cm2.

Red Photoluminescent Efficiency and Minority‐Carrier Lifetime of GaP(Zn,O) Pulled from Nonstoichiometric Melts

GaP (Zn, O) crystals pulled from nonstoichiometric melts by the liquid‐encapsulation Czochralski (LEC) technique bridge the gap between the red photoluminescent quantum efficiencies of platelets grown from solution and crystals pulled by LEC from a stoichiometric melt. A linear correlation is shown to exist between the nonradiative shunt‐path component of the total reciprocal minority‐carrier lifetime and the Ga‐vacancy concentration of these different crystals.

CALCULATION OF THE NONLINEAR OPTICAL TENSOR COEFFICIENTS IN OXYGEN‐OCTAHEDRA FERROELECTRICS

We show, using the concept of the polarization potential tensor β which relates energy band shifts to the crystal polarization, that the nonlinear optical (NLO) tensor coefficients δ in oxygen‐octahedra ferroelectrics are proportional to the quantity βPs where Ps is the crystal spontaneous polarization. Because β is found to be nearly the same in all oxygen‐octahedra ferroelectrics, we are able to calculate the δ coefficients from a knowledge of Ps. The polarization‐induced energy band shifts are shown to be nearly the same for static polarizations (giving rise to the clamped electro‐optic effect) and for optical‐frequency electronic polarizations (giving rise to nonlinear optical effects).

Luminescence and doping characteristics of liquid encapsulation Czochralski grown GaP(Zn, O)

Abstract A study of the luminescence and doping characteristics of liquid encapsulation Czochralski (LEC) grown GaP(Zn, O) is presented. Sections from the seed, middle and tail regions of 8 crystals grown from melts containing varying amounts of gallium oxide and zinc were examined with respect to red photoluminescence quantum efficiency, minority carrier lifetime, hole concentration and concentration of paired and unpaired oxygen. The measured minority carrier lifetimes in these crystals were found to be 2–3 orders of magnitude shorter than in crystals grown from solution with comparable zinc and oxygen incorporation. This result indicates that a strong shunt path for electron recombination exists in the LEC crystals, which in turn accounts for their lower red photoluminescence efficiency when compared with the efficiency of solution grown crystals.