Directional excitation of surface plasmon using multi-mode interference in an aperture.

Plasmonics is a promising technology that can find many applications in nanophotonics and biosensing. Local excitation of surface plasmons with high directionality is required for many of these applications. We demonstrate that by controlling the interference of light in a metal slot with the adjustment of the angle of incidence, it is possible to achieve highly directional surface plasmon excitation. Our numerical analysis of the structure showing a strong directionality of excited surface plasmon is confirmed by near field scanning measurements. The proposed structure can be useful for many applications including excitation of plasmonic waveguides, nanolithography, and optical sensing. To illustrate its usefulness, we experimentally demonstrate that it can be used for highly directional excitation of a dielectric loaded plasmonic waveguide. We also propose a simple structure for surface plasmon interference lithography capable of providing high image contrast using this scheme.

High-power picosecond optical parametric oscillator based on periodically poled lithium niobate.

A high-power picosecond optical parametric oscillator (OPO) based on a 47-mm periodically poled lithium niobate crystal is described. More than 12 W of total average power-almost 8 W of signal power at 1.85 microm and more than 4 W of idler radiation at 2.5 microm -is simultaneously extracted from less than 18 W of average pump power. The OPO is synchronously pumped by 80-ps (FWHM) cw mode-locked pulses at 1.064 microm , and its output is tunable from 1.7 to 2.84microm . Nearly transform-limited signal pulses are obtained following the introduction of two intracavity etalons.

Observation of anti-stokes fluorescence cooling in thulium-doped glass

We report the first observation of anti-Stokes fluorescence cooling in a thulium-doped solid with pump excitation at 1.82 &mgr;m

Autocorrelation measurement of femtosecond laser pulses by use of a ZnSe two-photon detector array.

We demonstrate autocorrelation measurements of 85-fs Ti:sapphire laser pulses, using a 32-pixel ZnSe detector array in a single-shot geometry. The two-photon photoconductor is fabricated by deposition of an array of interdigitated gold fingers on a single-crystal ZnSe substrate.

Femtosecond autocorrelation measurements based on two-photon photoconductivity in ZnSe.

Two-photon photoconductivity in ZnSe is used to record femtosecond autocorrelation functions. This technique requires <100 muW of average power of a typical mode-locked femtosecond Ti:sapphire laser and distinguishes itself by a dynamic range over several decades and great conversion bandwidth, permitting the sensitive correlation of pulses of a few femtoseconds.

Femtosecond Kerr-lens autocorrelation.

An autocorrelation measurement of femtosecond laser pulse duration using the Kerr-lens mechanism is demonstrated. This technique can also be used as a sensitive and absolutely calibratable method for measuring ultrafast optical nonlinearities. A method that uses an electronic spectral-filtering scheme is proposed for determining the frequency chirp of pulses by interferometric autocorrelation.

Eclipsing Z-scan measurement of lambda/10(4) wave-front distortion.

We introduce a simple modification to the Z-scan technique that results in a sensitivity enhancement that permits measurement of nonlinearly induced wave-front distortion of =lambda/10(4). This sensitivity was achieved with 10-Hz repetition-rate pulsed laser sources. Sensitivity to nonlinear absorption is also enhanced by a factor of =3. This method permits characterization of nonlinear thin films without the need for waveguiding.

Phase-controlled transistor action by cascading of second-order nonlinearities in KTP.

We demonstrate a 4.6-to-1 modulation depth imposed on the fluence of an intense 1.06-microm picosecond pulse by varying the relative phase of a weak second-harmonic control pulse under near phase-matched conditions. This transistorlike action is based on quadratic nonlinearities in KTP.

Large nonlinear phase shifts in second-order nonlinear-optical processes.

We show that processes such as second-harmonic generation and subsequent downconversion, and parametric mixing in general, can lead to large field-dependent phase shifts for the input beams under a variety of conditions.

Self-focusing and self-defocusing by cascaded second-order effects in KTP.

We monitor the induced phase change produced by a cascaded chi((2)):chi((2)) process in KTP near the phase-matching angle on a picosecond 1.06-microm-wavelength beam using the Z-scan technique. This nonlinear refraction is observed to change sign as the crystal is rotated through the phase-match angle in accordance with theory. This theory predicts the maximum small-signal effective nonlinear refractive index of n(eff)(2) congruent with +/-2 x 10(-14) cm(2)/W (+/-1 x 10(-11) esu) for an angle detuning of +/-5 degrees from phase match for this 1-mm-thick crystal with a measured d(eff) of 3.1 pm/V. For a fixed phase mismatch, this n(eff)(2) scales linearly with length and as d(eff)(2) however, for the maximum n(eff)(2) the nonlinear phase distortion becomes sublinear with irradiance for phase shifts near pi/4.

Measurement of nondegenerate nonlinearities using a two-color Z scan.

A simple dual-wavelength (two-color) Z-scan geometry is demonstrated for measuring nonlinearities at frequency omega(p) owing to the presence of light at omega(e). This technique gives the nondegenerate two-photon absorption (2PA) coefficient beta(omega(p); omega(e)) and the nondegenerate nonlinear refractive index n(2)(omega(p); omega(e)), i.e., cross-phase modulation. We demonstrate this technique on CS(2) for n(2) and on ZnSe where 2PA and n(2) are present simultaneously.

High-sensitivity, single-beam n(2) measurements.

We present a simple yet highly sensitive single-beam experimental technique for the determination of both the sign and magnitude of n(2). The sample is moved along the z direction of a focused Gaussian beam while the repetitively pulsed laser energy is held fixed. The resultant plot of transmittance through an aperture in the far field yields a dispersion-shaped curve from which n(2) is easily calculated. A transmittance change of 1% corresponds to a phase distortion of approximately lambda/250. We demonstrate this method on several materials using both CO(2) and Nd:YAG laser pulses.

Dynamic reflectivity switching of infrared lasers by an inhomogeneous plasma.

The effect of an inhomogeneous electron-hole plasma in a semiconductor on the reflection properties at the infrared is theoretically investigated. For an exponential density profile, it is shown that enhanced dynamic reflectivity diminishes significantly for relatively shallow plasma depths. These results can be used to design reflection plasma switches for the production of ultrashort infrared-laser pulses.