Gary C. Bjorklund

Frequency-modulation spectroscopy: a new method for measuring weak absorptions and dispersions.

A new type of wavelength-modulation laser spectroscopy is accomplished by utilizing an external phase modulator driven at radio frequencies large compared to the width of the spectral feature of interest. The spectral feature is probed by a single frequency-modulated (FM) sideband, and the associated absorption and dispersion are measured by monitoring the resulting radio-frequency beat signal. Experimental results are presented for the measurement of Fabry–Perot resonances, I2 vapor absorption lines, and saturation holes in Na vapor.

Frequency modulation (FM) spectroscopy - Theory of lineshapes and signal-to-noise analysis

Frequency modulation (FM) spectroscopy is a new method of optical heterodyne spectroscopy capable of sensitive and rapid measurement of the absorption or dispersion associated with narrow spectral features. The absorption or dispersion is measured by detecting the heterodyne beat signal that occurs when the FM optical spectrum of the probe wave is distorted by the spectral feature of interest. A short historical perspective and survey of the FM spectroscopy work performed to date is presented. Expressions describing the nature of the beat signal are derived. Theoretical lineshapes for a variety of experimental conditions are given. A signal-to-noise analysis is carried out to determine the ultimate sensitivity limits.

Novel second‐order nonlinear optical polymers via chemical cross‐linking‐induced vitrification under electric field

We present a novel method of preparing highly efficient and stable second‐order nonlinear optical (NLO) polymers via chemical cross‐linking induced vitrification under electric field. In this method a soluble prepolymer is first prepared that contains cross‐linking sites attached to the NLO‐active groups. Upon preparing samples of desired thicknesses, the prepolymer is heated (precured) to enable some chemical cross‐linking and thus to increase the glass transition temperature (Tg) to an optimum for poling. The precured polymer is then heated above its Tg and subjected to a high electric field to obtain the desired alignment of NLO moieties. Subsequent chemical cross‐linking (curing) under electric field continues to advance the Tg and hence leads to in situ vitrification of the polymer that stabilizes the electric‐field‐induced orientation of the NLO moieties. Detailed results of thermal, linear optical, poling kinetics, and NLO properties are described for the polymer system prepared from tetrafunctiona...

Corona poling and real‐time second‐harmonic generation study of a novel covalently functionalized amorphous nonlinear optical polymer

Thin films for optical second‐harmonic generation (SHG) were prepared from a newly designed and synthesized amorphous polymer that incorporated a high density of active nonlinear optical groups (p‐nitroaniline as attached side groups). For alignment of the nonlinear groups a very high electric field was applied by a corona discharge to the polymer films above Tg (125 °C). The subsequent freezing process resulted in a polymer film initially exhibiting a very high second‐order nonlinear coefficient, d33=31 pm/V, measured by the Maker‐fringe technique, plus excellent thermal, mechanical, and optical properties. The dynamics of polar alignment and decay, studied by in situ poling (or depoling) and SHG measurements, indicated a multiple exponential behavior with the average relaxation time somewhat longer than expected from extrapolation of the dielectric relaxations data according to the Williams–Landel–Ferry equation. The frozen‐in SHG behavior at room temperature (∼100 °C below Tg) relaxed after 5 days to a...

Temperature dependence of the third‐order nonlinear optical susceptibilities in polysilanes and polygermanes

Optical third‐harmonic generation was used to measure third‐order nonlinear susceptibilities χ(3) for thin films of planar zigzag polysilanes and polygermanes. A χ(3) of 11.3×10−12 esu was measured, the largest value ever reported for a polymer with transparency throughout the visible. The value of χ(3) was found to vary substantially with thermally induced reversible changes in polymer backbone conformation and also with film thickness.

Crystal growth and characterization of 4‐(N,N‐dimethylamino)‐3‐acetamidonitrobenzene, a new organic material for nonlinear optics

The linear and nonlinear optical properties have been determined for a new organic nonlinear material 4‐(N,N‐dimethylamino)‐3‐acetamidonitrobenzene. Room‐temperature angularly tuned type I phase matching for doubling of 1.064 μm radiation was achieved with a deff =27 pm/V. Favorable crystal growth habits were observed.

Process for optical information storage

Information is stored by a process involving the steps of (1) sensitizing a recording medium comprising a two-photon, four-level material by exposing said medium to flood illumination with ultra-violet or visible light, and then (2) exposing the sensitized medium simultaneously to two beams of coplanar laser irradiation intersecting within said recording medium to create a microscopic interference pattern.

Generation of blue cw coherent radiation by sum frequency mixing in KTiOPO4

The wavelength, angle, and temperature tolerances of a new room‐temperature noncritically phase‐matched frequency mixing process involving type II frequency mixing of 1064 and 809 nm radiation in potassium titanyl phosphate (KTiOPO4) are measured. The angular and temperature bandwidths are found to be unusually wide. Device applications for upconversion of diode pumped neodymium:yttrium aluminum garnet laser sources to the blue spectral region are explored.

A holographic technique for investigating photochemical reactions

A new experimental technique for following the progress of a photochemical reaction is described. The technique involves following the temporal growth in intensity of a hologram produced by the interference of two beams in a medium containing the species whose photochemistry is being investigated. The theory of the formation of a photochemical hologram is described and a simple expression obtained that relates the growth of the hologram to photochemical parameters of interest. The technique is used to study the photodissociation of dimethyl‐s‐tetrazine and hydrogen abstraction by camphorquinone in polyvinylcarbazole matrices.

Generation of blue light by intracavity frequency mixing of the laser and pump radiation of a miniature neodymium:yttrium aluminum garnet laser

Potassium titanyl phosphate (KTiOPO4,KTP) has been used to generate blue 459‐nm radiation by intracavity sum frequency mixing of the circulating 1064‐nm laser radiation of a miniature neodymium:yttrium aluminum garnet laser and the 809‐nm radiation used as the pump source. A blue output power of approximately 1 mW cw was obtained using 275 mW of pump power from an infrared dye laser. Gain‐switched operation leading to high 459‐nm peak powers was demonstrated. Direct rapid modulation of the blue radiation was achieved by modulating the pump. In preliminary experiments a high‐power laser diode was used as the pump source for the generation of blue radiation.