Paul M. Lundquist

Second harmonic generation in hexagonal silicon carbide

We report optical second harmonic generation measurements in single crystal α‐SiC of polytype 6H. The angular dependence of second harmonic intensity was consistent with two independent nonvanishing second order susceptibility components, as expected for a crystal with hexagonal symmetry. For the fundamental wavelength of 1.064 μm the magnitudes of the two components were determined to be χzzz(2)=±1.2×10−7 and χzxx(2)=∓1.2×10−8 esu. The corresponding linear electro‐optic coefficient computed from this value is rzzz=±100 pm/V. The wavelength dependence of the nonlinear susceptibility was examined for second harmonic wavelengths between the bandgap (400 nm) and the red (700 nm), and was found to be relatively uniform over this region. The refractory nature of this compound and its large nonlinear optical coefficients make it an attractive candidate for high power nonlinear optical waveguide applications.

Scanning plasmon optical microscope

A new scanning near‐field optical microscopy based on the surface plasmon resonance is presented. Enhanced fields are localized at individual surface irregularities by the scattering of plasmons and the scattered plasmons produce a conical radiation. Variations in the conical radiation intensity caused by interactions between a raster‐scanned probe tip and the enhanced fields are recorded and related to the surface topography, providing an optical image with lateral resolution exceeding the diffraction limit. This technique is compared to complementary atomic force microscopy and scanning tunneling microscopy measurements and potential advantages are discussed.

ULTRAVIOLET SECOND HARMONIC GENERATION IN RADIO-FREQUENCY SPUTTER-DEPOSITED ALUMINUM NITRIDE THIN FILMS

Optical second harmonic generation in radio‐frequency sputter‐deposited AlN thin films has been studied for harmonic wavelengths from ultraviolet to near infrared. The effective second‐order nonlinearity χ(2)(ω) was determined to have a nonresonant background value of ∼5×10−9 esu for second harmonic wavelengths longer than 400 nm, and it increases dramatically as the second harmonic frequency approaches the bandgap of 6.2 eV. This is likely due to resonance of the second harmonic frequency with the critical point transition associated with the direct bandgap of AlN.

Dispersion of second‐order optical nonlinearity in chromophoric self‐assembled films by optical parametric amplification: Experiment and theory

The dispersion of the second‐order optical nonlinearity in chromophoric self‐assembled stilbazolium films has been characterized in detail as a function of fundamental wavelength from 800 to 1600 nm using an optical parametric amplifier‐based measurement system. The second‐harmonic generation (SHG) spectrum exhibits a distinctive two‐photon resonance at ℏω=1.3 eV (960 nm). The maximum in the second‐order susceptibility coincides with a low‐energy chromophore‐centered charge‐transfer one‐photon excitation at 480 nm (2.6 eV). The experimental SHG dispersion values compare favorably with theoretical results computed using a semiempirical sum‐over‐states formalism. These indicate that the χ(2) response is dominated by an excitation along the long axis of the stilbazolium chromophore from the highest occupied to the lowest unoccupied molecular orbital.

Second order optical nonlinearities of radio frequency sputter‐deposited AlN thin films

Polarized second harmonic generation measurements were performed on AlN films deposited on (100) sapphire substrates by the reactive rf sputtering technique. The bulk effective second order nonlinearity observed in these films is typically about 6×10−9 esu at 1.06 μm, several times larger than that of quartz or KTP. The tensorial properties of the nonlinearity are consistent with the crystal symmetry of AlN and the microcrystallinity of these films.

Potassium titanyl phosphate thin films on fused quartz for optical waveguide applications

Thin films of potassium titanyl phosphate (KTiOPO4) for photonic applications have been fabricated on fused quartz substrates by pulsed excimer laser ablation. The textured films were optically uniaxial with the symmetry axis oriented normal to the film surface. The principle component of the second order electric susceptibility was oriented parallel to this symmetry axis and was determined to be about 16 times the magnitude of quartz for the fundamental wavelength of 1.064 μm. This configuration, with the largest nonlinear optical response for polarizations perpendicular to the film surface, is desirable in TM guided wave applications. Planar optical waveguide measurements were performed and the linear waveguide loss was measured. The frequency dependence of χ(2)(ω) was determined and found to be uniform for visible second harmonic wavelengths.

Observation of an optical-modulation effect on second harmonic generation in C60 thin films

Abstract We report optical modulation effects on second harmonic generation (SHG) in C 60 films. It was observed that a large second harmonic response of C 60 films (in a corona poling field of 5.5 kV at a nominal temperature of 140 °C) was almost completely suppressed when the film was illuminated with a second visible-light source (pulsed or steady state). The magnitude of the SHG suppression was measured as a function of the wavelength and intensity of the second source. By scanning the time interval between a “suppression” pulse and the SHG generation pulse we found that the lead time required for the suppression effect is about 250 ps.

Origins of bulk second harmonic generation in C60 thin films

Abstract Electric field induced second harmonic generation in C60 thin films was investigated. The measured bulk second order nonlinear optical response of C60 was identified as arising from nonlinear polarization moments of higher order than the electric dipole moment. The third order nonlinear optical susceptibility χ(3) (-2ω;ω, ω, 0) was determined from these measurements to be 3.6 X 10-11 esu for the fundamental wavelength of 1.06 μm, in good agreement with published data.

Second harmonic generation in hexagonal SiC

We report optical second harmonic generation measurements in single crystal (alpha) -SiC of polytype 6H. The sample was found to be optically uniaxial, consistent with the hexagonal symmetry of the crystal structure. The two independent components of the second order electric susceptibility tensor were determined to be d33 equals 43 pm/V and d31 equals -4.3 pm/V. From this data the principle electro-optic coefficient was computed to be r33 equals 100 pm/V. The corresponding figures of merit for nonlinear optical device operation compare favorably with leading materials such as lithium niobate and KTP, and thus the relatively convenient and inexpensive fabrication of 6H SiC thin films could prove useful for integrated optical devices.

X(2) dispersion and waveguiding measurements in acentric chromophoric self-assembled NLO materials

An attractive and challenging approach to the construction of robust, thin film materials with large second-order optical nonlinearities is the covalent self-assembly of aligned arrays of high-(beta) molecular chromophores into multilayer superlattices. In this paper, we describe the dispersion of the large second harmonic generation (SHG) response in a self-assembled film containing stilbazolium chromophore building blocks. We also describe a new approach to fabricating SHG waveguides with such materials as well as preliminary confirmatory experimental results.