Nonlinear optical properties and carrier recombination lifetime of GaN.

Nonlinear optical properties of a selection of gallium nitride samples have been measured using picosecond and nanosecond duration laser pulses at 532 nm. The values of the two-photon absorption coefficient, free carrier absorption cross section, and free carrier refraction cross section are determined along with the recombination lifetime of photogenerated carriers. The effect of hot isostatic pressing on these properties in samples with linear absorption at the band edge due to defects is explored.

Efficient second harmonic generation of a high-power picosecond CO2 laser.

A comparative analysis of AgGaSe2, GaSe, CdGeAs2, and Te for second harmonic generation (SHG) of a picosecond CO2 laser at intensities up to 50 GW/cm2 is presented. We demonstrate external energy conversion efficiency of >20% in AgGaSe2. Conversion efficiency >5% is measured in GaSe and CdGeAs2. Self-focusing and multifilamentation are found to severely limit the SHG process in CdGeAs2 and Te at such high fields. Demonstration of ≥150 MW SH pulses for a 10 µm picosecond pump, in combination with femtosecond CO2 laser development, will open new strong-field applications in the 4.5-5.5 µm range.

United States Air Force Research Laboratory: introduction to the focus issue.

This focus issue on the United States Air Force Research Laboratory (AFRL) spans the latest trends in imaging and detectors, atmospheric characterization, laser sources and propagation, optics and optical assemblies, optical characterization of materials, photonics, optical processing, and machine learning for applications that cover everything from stellar interferometry to studying damage to the plasma membranes of living cells.

Multiwatt-level continuous-wave midwave infrared generation using difference frequency mixing in periodically poled MgO-doped lithium niobate.

Over 3.5 W of continuous-wave power at 3.4 µm was obtained by single-pass difference frequency mixing of 1.064 and 1.55 µm fiber lasers in a 5 cm long periodically poled lithium niobate crystal. Good agreement was obtained between the observed temperature dependence of the generated power and the prediction from focused Gaussian beam theory.

Measuring refractive index using the focal displacement method.

A simple technique is introduced for measuring the refractive index of plane-parallel samples having thickness of the order of a millimeter. The refractive index values are reported for six bulk semiconductors, each index measured at two infrared wavelengths using this method. The values are found to be within a few percent of those in literature for four semiconductors. The other two semiconductors were newly grown ternary alloys (CdMgTe and CdMnTe), for which the refractive index values have not been reported previously at the wavelengths studied here.

Second-harmonic generation of a tunable continuous-wave CO2 laser in orientation-patterned GaAs.

Tunable, mid-infrared radiation was obtained by frequency doubling of a continuous-wave CO(2) laser in orientation-patterned GaAs crystal. Active cooling of the crystal minimized pump-induced thermal effects, allowing generation of output powers exceeding 300 mW across the wavelength range of 4.63-4.78 µm.

Analytical beam propagation model for clipped focused-Gaussian beams using vector diffraction theory.

Vector diffraction theory is applied to the case of focused TEM(00) Gaussian beams passing through a spatially limiting aperture in order to investigate the propagation of these clipped focused-Gaussian beams. Beam distributions at different axial distances show that a traditional M(2) propagation model cannot be used for the propagation of clipped focus-Gaussian beams. Using Luneberg's vector diffraction theory and Fresnel approximations, an analytical model for the on-axis transverse and longitudinal electric fields and intensity distributions is presented including predictions of the maximum obtainable intensity. In addition, an analytical expression is provided for the longitudinal component of the electric field of a TEM(00) mode unperturbed Gaussian beam. Experimental results are also presented and compared to the model's predictions.

Wavelength dependence of two photon and free carrier absorptions in InP.

Nonlinear absorption at 1.064 and 1.535 microm wavelengths by two photon and free carrier absorption processes in undoped and Fe doped InP has been investigated. Using picosecond and nanosecond duration lasers, a self-consistent set of the two photon and free carrier absorption coefficients are experimentally obtained through nonlinear transmission measurements for the first time. Reduced carrier recombination lifetime caused a decrease in nonlinear absorption of nanosecond duration laser pulses in Fe doped samples.

Application of Hertz vector diffraction theory to the diffraction of focused Gaussian beams and calculations of focal parameters.

Hertz vector diffraction theory is applied to a focused TEM00 Gaussian light field passing through a circular aperture. The resulting theoretical vector field model reproduces plane-wave diffractive behavior for severely clipped beams, expected Gaussian beam behavior for unperturbed focused Gaussian beams as well as unique diffracted-Gaussian behavior between the two regimes. The maximum intensity obtainable and the width of the beam in the focal plane are investigated as a function of the clipping ratio between the aperture radius and the beam width in the aperture plane.

Measurement of optical and thermal properties of Hg1-xCdxTe.

Measurements of optical transmission and several thermal properties of Hg(1-x)Cd(x)Te alloys are reported for a few values of the alloy composition parameter x, which was determined by a microprobe technique. The values of the thermal diffusivity, specific heat, and thermal conductivity were measured using the laser-flash method. These results are reported at four discrete temperatures between 90 and 400 K and compared to those of three well-characterized semiconductor materials: Si, InAs, and InSb.

Temperature-dependent refractive index measurements of wafer-shaped InAs and InSb.

An experimental method is introduced to measure the refractive index and its temperature dependence for wafer-shaped infrared materials over a continuous temperature range. Using a combination of Michelson interferometry, Fabry-Perot interferometry, and a temperature-controlled cryostat in a laser micrometer, refractive index values and their temperature coefficients can be measured for any specific temperature within a desired temperature range. Measurements are reported for InAs and InSb for a laser wavelength of 10.59 microm.

Spatial distribution of power coupling in self-pumped photorefractive reflection gratings.

The spatial distribution of the power transfer achieved by contradirectional two-beam coupling using self-pumped photorefractive reflection gratings is investigated in two materials with different photorefractive gain coefficients, LiNbO3:Fe and KNbO3:Fe. Incremental portions of the volume grating are erased optically by inducing thin optical damage planes, reducing the overall two-beam coupling efficiency. By monitoring the effect of local grating disruption, the distribution of power transfer is spatially resolved throughout the crystal, and the results are found to be in agreement with our theoretical predictions.

Use of Michelson and Fabry-Perot interferometry for independent determination of the refractive index and physical thickness of wafers.

We present a method to independently measure the refractive index and the thickness of materials having flat and parallel sides by using a combination of Michelson and Fabry-Perot interferometry techniques. The method has been used to determine refractive-index values in the infrared with uncertainties in the third decimal place and thicknesses accurate to within +/- 5 microm for materials at room and cryogenic temperatures.

Description of light propagation through a circular aperture using nonparaxial vector diffraction theory.

Using nonparaxial vector diffraction theory derived using the Hertz vector formalism, integral expressions for the electric and magnetic field components of light within and beyond an apertured plane are obtained for an incident plane wave. For linearly polarized light incident on a circular aperture, the integrals for the field components and for the Poynting vector are numerically evaluated. By further two-dimensional integration of a Poynting vector component, the total transmission of a circular aperture is determined as a function of the aperture radius to wavelength ratio. The validity of using Kirchhoff boundary conditions in the aperture plane is also examined in detail.

Refractive-index measurements of zinc germanium diphosphide at 300 and 77 K by use of a modified Michelson interferometer.

A method to determine the absolute refractive index of materials available in the shape of flat wafers with parallel sides by using interferometric techniques is presented. With this method, nondestructive, sample-specific measurements can be made. The method is tested by using silicon, germanium and zinc selenide, and measurements for both the ordinary and extraordinary axes of ZnGeP2 for temperatures of 300 and 77 K are reported.

Temperature and pulse-duration dependence of second-harmonic generation in CdGeAs2.

A detailed investigation of frequency doubling of a transversely excited atmospheric CO2 laser operating at 9.55 microm with a CdGeAs2 crystal was carried out. The temperature of the crystal was varied between 80 and 295 K to maximize the frequency-doubled energy. The temporal shape of the generated beam at 4.775 microm was monitored to calculate its peak power. High values of midwave infrared pulse energy (16.65 mJ) and peak power (92 kW) were obtained, which can be of potential use in lidar systems.