W. A. Nordland

Laser oscillation from quantum states in very thin GaAs−Al0.2Ga0.8As multilayer structures

We report optically pumped laser oscillation from multilayer heterostructures consisting of alternating layers of GaAs and Al0.2Ga0.8As. Very thin GaAs layers (50−500 A) exhibit one−dimensional bound states above the band gap of bulk GaAs. The laser oscillation occurs at energies which are slightly below the exciton associated with the lowest energy n=1 bound state.

Laser oscillation with optically pumped very thin GaAs‐AlxGa1−xAs multilayer structures and conventional double heterostructures

Optically pumped laser oscillation from multilayer heterostructures (ML) grown by molecular beam epitaxy (MBE) that consist of many alternating thin layers of GaAs and AlxGa1−xAs has been studied in some detail over the temperature range 7–300 K. These samples which have GaAs layers 100–200 A thick that act as one‐dimensional potential wells for electrons and holes have thresholds for lasing that are generally a factor of 2 or more higher than conventional comparable GaAs‐AlxGa1−xAs double heterostructures (DH) also grown by MBE. The quantum effects due to the GaAs wells of the ML samples are shown to exist under lasing conditions well above room temperature, but there is no evidence to date of the beneficial effects expected from the modified density of states of this type of structure. Also optically pumped conventional DH lasers grown by MBE and liquid phase epitaxy (LPE) have been compared. The DH lasers grown by MBE are found to have lasing thresholds that are consistently about a factor of 2 higher ...

Dependence of Second‐Harmonic‐Generation Coefficients of LiNbO3 on Melt Composition

The three nonlinear coefficients d31, d22, and d33 that describe optical second‐harmonic generation with LiNbO3 have been determined for crystals pulled from melts with Li/Nb atom ratios, (Li/Nb)m, of 0. 852, 0. 946, and 1. 083. The coefficients d22 and d33 were found to be insensitive to the melt composition whereas d31 increased by about 50% as (Li/Nb)m was increased from 0. 852 to 1. 083. These results suggest a marked Curie temperature dependence of the difference between the two Nb–O bond nonlinearities that give rise to the ds for LiNbO3.

RELATIVE SIGNS OF NONLINEAR OPTICAL COEFFICIENTS OF POLAR CRYSTALS

A determination of the relative signs of the nonlinear optical coefficients that give rise to second harmonic generation has been made for each of the following crystals: LiNbO3, LiTaO3, BaTiO3, ZnO, and LiGaO2. It is found that d33 and d31 have the same signs in LiNbO3, LiTaO3, BaTiO3, and LiGaO2. In ZnO, LiGaO2, and LiTaO3, the ratios d33/d31, d33/d32, and d31/d22, respectively, are negative.

Two‐photon absorption of Nd laser radiation in GaAs

The two‐photon absorption coefficient of GaAs at 1.32 μm is found to be 0.033 ± 0.015 cm/MW. This value was obtained from transmission measurements employing a Q‐switched Nd–YAG laser and an improved theoretical analysis taking into account fluctuations and the Gaussian pulse shape. Evidence of thermal self‐focusing was seen in samples with linear absorption coefficients of order 1 cm−1 or larger.

TUNABLE LiNbO3 OPTICAL OSCILLATOR WITH EXTERNAL MIRRORS

With an optical cavity formed by external mirrors, parametric oscillation in LiNbO3 has been achieved from 0.684 μ to 2.355 μ − 70% of the theoretical range of the oscillator. The oscillator, pumped at 0.53 μ, was tuned by rotation of the LiNbO3 crystal. Total output powers of about 50 W were observed with pump powers of 5 × 104 W. The logarithm of the output power is proportional to the reciprocal of the optical length of the cavity.

Phase‐matched second‐harmonic generation in GaAs optical waveguides by focused laser beams

Using a tunable parametric oscillator operating near 2 μm, we have observed second‐harmonic generation in a GaAs double‐heterostructure waveguide in the region of phase matching. The harmonic intensity is strongly peaked when the guide propagation constants of the TE (m=0) fundamental and TM (m=2) harmonic are equal. Away from phase matching, the characteristic effects of strong focusing in the plane of the guide are observed. These are the one‐dimensional analogs of the harmonic generation effects observed using two‐dimensional focused laser beams.

Nonlinear Optical Properties of Lithium Gallium Oxide

The crystal symmetry (point group mm2) and refractive indices of LiGaO2 show that it can be phase matched for second‐harmonic generation (SHG) of laser fundamentals in the near infrared. Furthermore, these data suggest that LiGaO2 might be a useful nonlinear medium for a 1.06‐μ pumped near‐infrared optical parametric oscillator in which the pump, signal, and idler waves all travel along a principal index direction. The magnitudes and relative signs of the nonlinear optical coefficients that determine SHG and optical parametric interactions in LiGaO2 have been measured using a 1.06‐μ fundamental. It is found that these coefficients relative to d36 for KH2PO4 are: d33=±1.45±0.15, d31=±0.17±0.02, and d32=∓0.37±0.04. The small magnitudes of these coefficients would seem to preclude the use of LiGaO2 in efficient nonlinear optical devices. A discussion of a simple experimental method for the determination of the relative signs of the nonlinear optical coefficients of a given crystal is also included.

Nonlinear optical properties of the cuprous halides

The absolute signs and magnitudes of the second‐order nonlinear optical coefficients, d14, of CuCl, CuBr, and CuI have been determined with laser fundamentals at 1.32, 1.06, and 0.95 μm. All three d14s are real, negative, and show no dispersion over the measured spectral range. Except for CuI, the magnitudes of d14 are in agreement with data at 1.06 μm reported earlier by Chemla et al. The negative signs and magnitudes of d14 most likely require important Cu 3d electron contributions for their explanation.

A Preliminary Investigation of Brillouin Scattering in a Liquid Crystal

Brillouin spectra have been observed in the liquid‐crystal cholesteryl‐2‐(2‐ethoxyethoxy) ethyl carbonate. The frequency shift and the relative intensities of the shifted and unshifted components have been measured as a function of temperature, which was varied through the cholesteric to isotropic transition.