A. Ashkin

Observation of a single-beam gradient force optical trap for dielectric particles

Optical trapping of dielectric particles by a single-beam gradient force trap was demonstrated for the first reported time. This confirms the concept of negative light pressure due to the gradient force. Trapping was observed over the entire range of particle size from 10 μm to ~25 nm in water. Use of the new trap extends the size range of macroscopic particles accessible to optical trapping and manipulation well into the Rayleigh size regime. Application of this trapping principle to atom trapping is considered.

Forces of a single-beam gradient laser trap on a dielectric sphere in the ray optics regime

We calculate the forces of single-beam gradient radiation pressure laser traps, also called optical tweezers, on micron-sized dielectric spheres in the ray optics regime. This serves as a simple model system for describing laser trapping and manipulation of living cells and organelles within cells. The gradient and scattering forces are defined for beams of complex shape in the ray-optics limit. Forces are calculated over the entire cross-section of the sphere using TEM00 and TEM*00 mode input intensity profiles and spheres of varying index of refraction. Strong uniform traps are possible with force variations less than a factor of 2 over the sphere cross-section. For a laser power of 10 mW and a relative index of refraction of 1.2, we compute trapping forces as high as approximately 1.2 x 10(-6) dynes in the weakest (backward) direction of the gradient trap. It is shown that good trapping requires high convergence beams from a high numerical aperture objective. A comparison is given of traps made using bright field or differential interference contrast optics and phase contrast optics.

Optical Levitation by Radiation Pressure

The stable levitation of small transparent glass spheres by the forces of radiation pressure has been demonstrated experimentally in air and vacuum down to pressures ∼1 Torr. A single vertically directed focused TEM00‐mode cw laser beam of ∼250 mW is sufficient to support stably a ∼20‐μ glass sphere. The restoring forces acting on a particle trapped in an optical potential well were probed optically by a second laser beam. At low pressures, effects arising from residual radiometric forces were seen. Possible applications are mentioned.

Intensity discrimination of optical pulses with birefringent fibers

An intensity discriminator for optical pulses can be made with a birefringent fiber. Such a discriminator would be useful for separating the intense subpicosecond pulses formed by solitonlike compression from the weaker uncompressed background. The discriminator utilizes an intensity-dependent state of polarization out of the fiber.

Resonant optical second harmonic generation and mixing

Experimental and theoretical results are described on the enhancement of optical second harmonic generation (SHG) and mixing in KDP by the use of optical resonance. Both resonance of the harmonic and of the fundamental are considered. Large enhancements are possible for resonators with low loss. Using a planoconcave harmonic resonator containing 1.23 cm of KDP, the authors achieved a loss < 4 percent per pass. This resulted in an enhancement of ∼ 500 times the harmonic power internal to the resonator and ∼ 10 times external to the resonator. When resonating, the fundamental enhancements of ∼ 5 were observed. The theory includes the effect of double refraction. This results in a coupling coefficient of the generated harmonic power to the transverse modes of the harmonic resonator. The experimental results are in substantial agreement with the theory.

Observation of optical resonances of dielectric spheres by light scattering

Use of the wavelength and size dependence of light scattering from optically levitated liquid drops is demonstrated as a sensitive means of detecting optical resonances of dielectric spheres. High resolution spectra are presented of the radiation pressure, far- and near-field backscatter, and 90 degrees scatter. Excellent agreement is found between experimental spectra and high resolution Mie calculations of Chylek et al. Strong evidence supporting the van de Hulst dielectric surface-wave model for these resonances is presented. Use of resonances for high precision measurement of sphere size and sphere distortion, index of refraction, temperature, and vapor pressure is discussed.

Phase‐matched three‐wave mixing in silica fiber optical waveguides

We have observed phase‐matched nonlinear mixing in a silica fiber optical waveguide using the dispersion of the guide modes to compensate for bulk dispersion. A tunable dye laser signal is mixed with a fixed‐frequency pump wave to generate output in specific waveguide modes at different frequencies. The technique is useful as a probe of waveguide properties and for the study of the nonlinearities of the medium.

CURIE TEMPERATURE, BIREFRINGENCE, AND PHASE‐MATCHING TEMPERATURE VARIATIONS IN LiNbO3 AS A FUNCTION OF MELT STOICHIOMETRY

The Curie temperature, birefringence, and phase‐matching temperature of LiNbO3 are shown to vary with the stoichiometry of the melt from which the crystals are pulled. The range of melt stoichiometry, indexed by the Li/Nb mole ratio, was varied from 1.20 to 0.80. This Li/Nb variation changes the Curie temperature, refractive index for the extraordinary ray, and the phase‐matching temperature by 120°C,.03, and 320°C respectively. The relevance of these changes are discussed in terms of the susceptibility of LiNbO3 to index inhomogeneities.

Simultaneous determination of refractive index and size of spherical dielectric particles from light scattering data

The diameter and refractive index of micrometer sized spherical dielectric particles are simultaneously deduced using the wavelength dependence of backscattering data from optically levitated particles. The accuracy of the results is set by experimental errors in the determination of the wavelength of backscatter resonance peaks and the ratio of slopes of specified peaks. At present the refractive index and diameter can be deduced with relative errors of 5 x 10(-5). This represents the most accurate determination of absolute size and refractive index yet made by light scattering. A reduction of these errors by an order of magnitude is possible. We assume a priori knowledge of diameter and refractive index with accuracy of 10(-1) and 5 x 10(-3), respectively.

Four-wave mixing in an artificial Kerr medium

Degenerate four-wave mixing experiments have been performed using a liquid suspension of 0.234-microm-diameter latex spheres as the nonlinear medium. The measured effective optical Kerr coefficient, n(2), is 3.6 x 10(-3) (MW/cm(2))(-1). This is ~10(5)x the value for CS(2). Measured grating reflectivity, formation, and decay times are in reasonable agreement with a simple model assuming Rayleigh scattering and Brownian diffusion.