Image restoration based on phase conjugation in second-order nonlinear materials under reflection configuration.

We have demonstrated the recovery of the distorted images by a phase plate to nearly their original qualities under a refection configuration. This is made possible by utilizing difference-frequency generation in a bulk KTiOPO(4) (KTP) crystal and an AFB-KTP crystal composite. In such a geometry, both input and phase-conjugated waves propagate through the distorted phase plate for a round trip. They are reflected by the object, which can be far away from the imaging system, and captured by the cameras. Therefore, such a configuration is much more practical for eventually deploying our imaging system for realizing practical applications. We have demonstrated the advantages of the imaging system such as polarization insensitivity and broadband capability.

Investigation of anti-Stokes Raman processes at phonon-polariton resonance: from Raman oscillation, frequency upconversion to Raman amplification.

Raman oscillation, frequency upconversion, and Raman amplification can be achieved in a second-order nonlinear medium at the phonon-polariton resonance. By beating two optical fields, a second-order nonlinear polarization is generated inside the medium. Such a polarization induces a spatially uniform nonpropagating electric field at the beat frequency, which in turn mixes with the input optical field at the lower frequency to generate or amplify the anti-Stokes optical field. Raman oscillation can be efficiently reached for the copropagating configuration. In comparison, efficient frequency upconversion and large amplifications are achievable for the counterpropagating configuration. These Raman processes can be used to effectively remove transverse-optical phonons before decaying to lower-frequency phonons, achieve laser cooling, and significantly enhance coherent anti-Stokes Raman scattering. The counterpropagating configuration offers advantages for amplifying extremely weak signals.

Restoration of blurred images due to phase distortion based on polarization-insensitive phase conjugation in second-order nonlinear medium.

Broadband and polarization-insensitive phase conjugation, achieved based on difference-frequency generation in a second-order nonlinear composite consisting of stacked KTiOPO(4) (KTP) plates, was exploited to restore blurred images due to phase distortion as a novel scheme. Due to the quasi-phase matching in the stacked KTP crystals, our result reveals that the image restoration is insensitive to the polarization direction and wavelength of the input beam.

Direct measurement of band edge discontinuity in individual core-shell nanowires by photocurrent spectroscopy.

Group III-V coaxial core-shell semiconducting nanowire heterostructures possess unique advantages over their planar counterparts in logic, photovoltaic, and light-emitting devices. Dimensional confinement of electronic carriers and interface complexity in nanowires are known to produce local electronic potential landscapes along the radial direction that deviate from those along the normal to planar heterojunction interfaces. However, understanding of selected electronic and optoelectronic carrier transport properties and device characteristics remains lacking without a direct measurement of band alignment in individual nanowires. Here, we report on, in the GaAs/AlxGa1-xAs and GaAs/AlAs core-shell nanowire systems, how photocurrent and photoluminescence spectroscopies can be used together to construct a band diagram of an individual heterostructure nanowire with high spectral resolution, enabling quantification of conduction band offsets.

Phase conjugation based on single backward second-order nonlinear parametric process.

We show that backward difference-frequency generation can be exploited to achieve phase conjugation in a second-order nonlinear medium. The backward configuration can be utilized to achieve broadband quasi-phase-matching, compared with the forward counterpart. Our calculation shows that a nonlinear reflectivity of close to 100% is achievable from a laser emitting an output power of ~1 mW. Such an efficient phase conjugator is made feasible by placing the nonlinear medium inside a pump laser cavity. In addition, a Fabry-Perot resonator at the input frequency is used to significantly improve the nonlinear reflectivity.

Efficient generation of far-infrared radiation in the vicinity of polariton resonance of lithium niobate.

We efficiently generated far-infrared radiation at the wavelengths centered at 20.8 µm in the vicinity of one of the polariton resonances of lithium niobate. Such an efficient nonlinear conversion is made possible by exploiting phase matching for difference-frequency generation in lithium niobate. The highest peak power reached 233 W.

Singly resonant optical parametric oscillator based on adhesive-free-bonded periodically inverted KTiOPO4 plates: terahertz generation by mixing a pair of idler waves.

We implemented a singly resonant optical parametric oscillator based on adhesive-free-bonded periodically inverted KTiOPO4 plates. It has major advantages such as walk-off compensation and oscillation at four wavelengths. The threshold of the oscillation was measured to be 8 MW/cm2, which is about a factor of 4 lower than that based in two separate KTiOPO4 crystals. By frequency-mixing the dual-wavelength output in GaP stacks, we generated the terahertz radiation at 2.54 THz. The tuning range of the terahertz output was demonstrated to be 2.19-2.77 THz.

Investigation of terahertz generation from passively Q-switched dual-frequency laser pulses.

When two Nd:YLF crystals share a Cr:YAG crystal functioning as a single passive Q switch, the timing jitter between each pair of dual-frequency pulses generated by the two crystals has been reduced by a factor of 20. Such a reduction in the timing jitter allows us to generate terahertz pulses by focusing such a passively Q-switched laser beam onto a nonlinear crystal. Such a result represents the first step for us to eventually implement a compact terahertz source based on ultracompact microchip lasers.

Terahertz generation based on parametric conversion: from saturation of conversion efficiency to back conversion.

By stacking alternatively rotated gallium phosphide (GaP) plates, the maximum photon conversion efficiency of 40% for the terahertz (THz) generation based on difference-frequency generation has been achieved. The corresponding peak power generated inside the four GaP plates approaches 4 kW. As the number of plates is increased from four to five, the THz output power is significantly decreased, due to back parametric conversion.

Experimental verification of the rainbow trapping effect in adiabatic plasmonic gratings.

We report the experimental observation of a trapped rainbow in adiabatically graded metallic gratings, designed to validate theoretical predictions for this unique plasmonic structure. One-dimensional graded nanogratings were fabricated and their surface dispersion properties tailored by varying the grating groove depth, whose dimensions were confirmed by atomic force microscopy. Tunable plasmonic bandgaps were observed experimentally, and direct optical measurements on graded grating structures show that light of different wavelengths in the 500-700-nm region is "trapped" at different positions along the grating, consistent with computer simulations, thus verifying the "rainbow" trapping effect.

Compact and portable terahertz source by mixing two frequencies generated simultaneously by a single solid-state laser.

We have demonstrated a compact and portable terahertz (THz) source, based on difference-frequency generation in a GaSe crystal. The two input frequencies, required for achieving frequency mixing, are generated by a single Q-switched Nd:YLF laser incorporating two laser resonators. The average power of the THz output reaches 1 µW at 1.64 THz (182 µm) within a linewidth of 65 GHz. Such a THz source can be packaged into a compact and portable system.

Efficient generation of high-frequency terahertz waves from highly lossy second-order nonlinear medium at polariton resonance under transverse-pumping geometry.

Owing to strong coupling between transverse-optical phonons and high-frequency terahertz waves in zinc-blende semiconductors, second-order nonlinear coefficients can be dramatically enhanced within the forbidden band of the polariton resonance. However, linear absorption in this regime is also dramatically increased. We show that transverse-pumping geometry can be exploited for achieving an efficient terahertz generation at the polariton resonance. Our estimates illustrate that pump powers as low as 100 mW are sufficient for causing the significant depletion of the pump beams.

Efficient generation of backward terahertz pulses from multiperiod periodically poled lithium niobate.

Backward terahertz pulses were converted from ultrafast pulses of a laser amplifier within multiperiod periodically poled LiNbO3 (PPLN) wafers. The average output powers of more than 2 microW were obtained from all six domain periods, corresponding to the frequency range of 217 GHz-2.373 THz. Among the six periods, the highest output power of 10.7 microW and the narrowest linewidth of 5.98 GHz were achieved at the periods of 66.5 microm and 150.0 microm, respectively.

"Rainbow" trapping and releasing at telecommunication wavelengths.

The reported "trapped rainbow" storage of THz light in metamaterials and plasmonic graded structures has opened an attractive new method to control electromagnetic radiation. Here, we show how, by incorporating the frequency-dependent dielectric properties of the metal, the graded grating structures developed for "trapped rainbow" storage of THz light in mum level can be scaled to nm level for telecommunication waves for applications in optical communication and various nanophotonic circuits.

Approach for determining the refractive-index profile of graded-index planar waveguides.

We measured the modal indices of a planar waveguide made from GeSeSb glass sandwiched between SiO(2) and air by using the prism-coupling technique. Based on the measured indices of the TE modes, we determined the position of the turning point corresponding to each mode of the waveguide by using the inverse WKB method. Using the modified fitting criterion introduced previously [Appl. Opt. 33, 3227 (1994)], we accurately determined the spatial profile of the refractive index for such a waveguide. Such a graded-index profile is probably caused by compositional variation of the GeSeSb guiding layer.

Ultrawide-bandwidth slow-light system based on THz plasmonic graded metallic grating structures.

We explore a novel mechanism for slowing down THz waves based on metallic grating structures with graded depths, whose dispersion curves and cutoff frequencies are different at different locations. Since the group velocity of spoof surface plasmons at the cutoff frequency is extremely low, THz waves are actually stopped at different positions for different frequencies. The separation between stopped waves can be tuned by changing the grade of the grating depths. This structure offers the advantage of reducing the speed of the light over an ultrawide spectral band, and the ability to operate at various temperatures, but demands a stringent requirement for the temperature stability.

Transition from photocurrent surge to resonant optical rectification for terahertz generation in p-InAs.

It has been demonstrated that the key to complete understanding of the mechanisms for terahertz (THz) generation from a p-type InAs wafer pumped by a subpicosecond Ti:sapphire amplifier lies in the dependences of the THz polarization on the azimuthal angle and polarization of the pump beam. At low enough pump intensities, photocurrent surge is the dominant mechanism for THz generation. However, the THz radiation originating from photocurrent surge is greatly reduced with increased pump intensity. Therefore, at sufficiently high pump intensities resonant optical rectification becomes the dominating mechanism for THz generation. The highest output power is measured to be 57 microW.

Differentiation of CO isotopic variants by frequency tuning a terahertz source.

The transmission spectra of two CO isotopic variants, i.e., (12)CO and (13)CO, are measured with a recently developed widely tunable THz source. The pure rotational transition lines of J=6 --> 7, 10 --> 11, 11 --> 12, 12 --> 13, and 13 --> 14 have been identified in the spectra. The transition frequencies and the rotational constant of each CO isotopic variant are also presented. Experimental results indicate that the difference of the rotational constants between the two isotopic variants can be used to reliably differentiate (12)CO and (13)CO. Compared with the measurements made by using a Fourier transform infrared spectrometer, our tunable THz source has produced approximately the same accuracies for measuring transition frequencies or determining rotational constants.

Resonant terahertz generation from InN thin films.

Highly efficient conversion from ultrafast optical pulses to their terahertz (THz) counterparts has been achieved with InN thin films. An average THz output power as high as 0.931 microW has been obtained for an average pump power of 1 W, corresponding to a normalized conversion efficiency of 190% mm(-2). Based on our measured dependences of the THz output power on pump polarization, incident angle, pump power, and InN film thickness, resonance-enhanced optical rectification is one of the most plausible mechanisms for the THz generation in the InN films.

Enhanced output power for phase-matched second-harmonic generation at 10.6 mum in a ZnGeP(2) crystal.

Second-harmonic generation was phase-matched at the fundamental wavelength of 10.6 mum in an annealed ZnGeP(2) crystal at room temperature. Our results demonstrated that the phase-matching angle was decreased with increasing the pump power. Such a unique dependence resulted in a significant enhancement on the second-harmonic output power. The highest average output power at 5.3 mum was 55 mW for an average pump power of 5.0 W, which corresponded to a conversion efficiency of 1.1%. Due to the laser-induced heating effect, the second-harmonic output power was increased by 65%. Such an efficient conversion was made possible also by using a short-pulse repetition-frequency-excited waveguide and single-longitudinal-mode CO(2) laser as a fundamental beam.