Engineerable femtosecond pulse shaping by second-harmonic generation with Fourier synthetic quasi-phase-matching gratings

We describe a pulse-shaping technique that uses second-harmonic generation with Fourier synthetic quasi-phase-matching gratings. We demonstrate both amplitude and phase tailoring by generating a picosecond squarelike pulse as well as trains of femtosecond pulses with a terahertz-range repetition rate from either a compressed or a chirped pump pulse.     

Achromatic phase matching for tunable second-harmonic generation by use of a grism

Achromatic phase matching (APM) involves dispersing the light entering a nonlinear-optical crystal so that a wide range of wavelengths is simultaneously phase matched. Using an APM arrangement consisting of a grism (a grating on the surface of a prism) and three prisms, optimized to match a second-harmonic crystal phase-matching angle versus wavelength to high order, we efficiently doubled tunable fundamental light near 650nm with a bandwidth of >95 nm by use of a 4-mm type I beta-barium borate crystal. APM uses no moving parts, and unlike previous APM designs, ours avoids lenses and hence is easy to align and insensitive to translational misalignment of the beam.     

Quasi-phase-matched second-harmonic generation by use of a total-internal-reflection phase shift in gallium arsenide and zinc selenide plates

We fabricated a novel quasi-phase-matched frequency converter, using a zigzag optical beam path in a thin, polished parallel plate. Second-harmonic generation experiments demonstrated angle-tuned output at 4.6 to 5.3mum in GaAs and 1.7 to 2.0mum in ZnSe crystals when pulsed infrared laser sources were used.     

Macromolecular structure of cellulose studied by second-harmonic generation imaging microscopy

The macromolecular structure of purified cellulose samples is studied by second-harmonic generation (SHG) imaging microscopy. We show that the SHG contrast in both Valonia and Acetobacter cellulose strongly resembles that of collagen from animal tissues, both in terms of morphology and polarization anisotropy. Polarization analysis shows that microfibrils in each lamella are highly aligned and ordered and change directions by 90 degrees in adjacent lamellae. The angular dependence of the SHG intensity fits well to a cos2 theta distribution, which is characteristic of the electric dipole interaction. Enzymatic degradation of Valonia fibers by cellulase is followed in real time by SHG imaging and results in exponential decay kinetics, showing that SHG imaging microscopy is ideal for monitoring dynamics in biological systems.     

Probing nonequilibrium electron distributions in gold by use of second-harmonic generation

Second-harmonic radiation is generated at a gold surface by use of a laser pulse that is varied in duration from 14 to 29 fs and in intensity from 10(9) to 10(11)W/cm(2) . At laser intensities below 10(10)W/cm(2) , the second-harmonic signal has the expected quadratic dependence on pump-laser intensity; however, at higher intensities, the dependence is supraquadratic. This difference arises because the leading edge of the laser pulse interacts significantly with the gold electrons to create a nonequilibrium, photoexcited distribution. The second-harmonic generation process occurs before electron-electron or electron-phonon collisions can equilibrate the distribution and therefore serves as a probe of the nonequilibrium distribution.     

Waveguide grating coupler with a tailored spectral response based on a computer-generated waveguide hologram

We propose a novel method that, for the first time to our knowledge, makes it possible to tailor the spectral response of an input grating coupler. This method is based on the idea of computer-generated waveguide holography. Grating couplers with different spectral responses have been designed and fabricated, and experimental results are presented that demonstrate the effectiveness of the proposed method.     

Surface structure of cubic ionic crystals studied by optical second-harmonic generation

Polarization and azimuthal dependencies of optical second-harmonic generation at the surface of cubic ionic crystals have been measured on polished surfaces of BaF₂ (111) and (100), CaF₂ (111) and NaCl (111), using a fundamental wavelength of 532 nm. The results are interpreted in terms of available theory, which shows that for transparent cubic media only the dipolar surface contribution depends on the crystal orientation. For BaF₂ (111) and NaCl (111) the crystallineC _3vgeometry could be identified but not for CaF₂ (111). Although the nature of the electronic surface structure giving rise to a dipolar polarizability remains undetermined, the experimental result suggests that it may be due to impurity or defect states in the band gap at the surface.     

Using a grating structure for phase compensation in achieving an efficient round-trip optical parametric process in periodically poled lithium niobate with an incomplete quasi-phase-matching period

An incomplete period of the last quasi-phase-matching (QPM) segment in a periodically poled nonlinear crystal represents a key problem in implementing a nonlinear wavelength-conversion device with round-trip or oscillating waves of interaction. Such a segment at the crystal end implies broken QPM periodicity for the reflected waves of interaction. We demonstrate numerically that with a grating structure at the crystal end for the reflecting signal the wavelength-dependent phase shift at Bragg reflection can compensate for the phase mismatch of the reflected signal, idler, and pump of an optical parametric process. Therefore, by slightly shifting the signal wavelength we can maintain the phase-matching condition with any length of the last QPM segment. It is shown that with a grating structure the conversion efficiency of the round-trip optical parametric process can always be optimized.     

Detection of freely propagating terahertz radiation by use of optical second-harmonic generation

We report the application of electric-field-induced optical second-harmonic generation as a new technique for measuring the field of freely propagating terahertz radiation. Using silicon as the nonlinear medium, we demonstrate subpicosecond time resolution and a sampling signal that varies linearly with the terahertz electric field. This approach, which is attractive for centrosymmetric media, permits a significantly broadened class of materials to be exploited for free-space sampling measurements.     

Measuring the electric charge in cloud droplets by use of second-harmonic generation

We report on the observation of second-harmonic generation (SHG) from electrically charged microdroplets while they are excited with femtosecond laser pulses. The intense SHG emission results from chi(3) coupling between two photons of the incident laser pulse and the electrostatic field induced by the surface charges. For Iinc= 2 x 10(12) W/cm2, we estimate the second-harmonic emission to be ISHG= 2.5 x 10(3) photons per droplet and per pulse. The possibility of using SHG to measure remotely the electric charge of water droplets in thunderclouds is discussed.     

Evaluating cutaneous photoaging by use of multiphoton fluorescence and second-harmonic generation microscopy

The photoaging process of facial skin is investigated by use of multiphoton fluorescence and second-harmonic generation (SHG) microscopy. We obtain the autofluorescence (AF) and SHG images of the superficial dermis from the facial skin of three patients aged 20, 40, and 70 years. The results show that areas of AF increase with age, whereas areas of SHG decrease with age. The results are consistent with the histological findings in which collagen is progressively replaced by elastic fibers. The AF and SHG changes in photoaging are quantified by a SHG to autofluorescence aging index of dermis (SAAID). Our results suggest that SAAID can be a good indicator of the severity of photoaging.     

Effect of thermal blooming in second-harmonic generation in nonlinear crystals with a regular domain structure

Analytical expressions are obtained for the power of the second harmonic in nonlinear crystals having a regular domain structure with allowance for thermal blooming for a Gaussian beam of basic radiation in the quasi-stationary plane-wave approximation. Calculations were carried out in both the approximation of an assigned field of basic radiation and a substantially nonlinear regime of exhaustion of basic radiation. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 2, pp,. 257–260, March–April, 2000.     

Spectral phase correlation of coded femtosecond pulses by second-harmonic generation in thick nonlinear crystals

We demonstrate a novel all-optical scheme for measuring the correlation of spectrally phase-coded ultrashort optical waveforms that uses second-harmonic generation (SHG) in long, periodically poled lithium niobate crystals. The SHG yield can be controlled over a range of ~30 dB, depending on the correlation of the applied phase codes. Such a spectral phase correlator has applications for ultrashort-pulse optical code-division multiple-access networking and could serve as a nonlinear optical but classical analog for certain schemes for coherent quantum control of multiphoton processes.     

Cascaded second-harmonic and sum-frequency generation of a CO2 laser by use of a single quasi-phase-matched GaAs crystal

Cascaded second-harmonic and sum-frequency generation were used to obtain the third harmonic of a CO(2) laser in a single quasi-phase-matched GaAs crystal. Both continuous-wave and pulsed regimes with a single-pass configuration were studied. The continuous case confirmed the good capability of the GaAs stack to bear high average power density. In the pulsed regime a 0.66% peak power conversion efficiency was achieved for the third harmonic when the laser was pumping at 8.2 MW/cm(2) , in fair agreement with theoretical predictions.     

A reflection interferometer with a noninverted response function based on a phase grating

It is shown that the inclusion of a simple phase grating in the optical scheme of a resonator provides the possibility of performing spectral investigations in reflected light with characteristics of a Fabry-Perot interferometer. The response function of the interferometer is examined, and experimental results are presented.     

Engineerable compression of ultrashort pulses by use of second-harmonic generation in chirped-period-poled lithium niobate

We demonstrate the use of an aperiodic quasi-phase-matching (QPM) grating to generate second-harmonic pulses that are stretched or compressed relative to input pulses at the fundamental frequency. We frequency doubled an externally chirped erbium-doped fiber laser generating 17-ps (FWHM) pulses at 1560nm to produce near-transform-limited 110-fs (FWHM) pulses at 780nm by use of a 5-cm-long lithium niobate crystal poled with a QPM grating chirped from an 18.2- to a 19.8-microm period.     

Monitoring the thermally induced structural transitions of collagen by use of second-harmonic generation microscopy

The thermal disruption of collagen I in rat tail tendon is investigated with second-harmonic generation (SHG) microscopy. We investigate its effects on SHG images and intensity in the temperature range 25 degrees-60 degrees C. We find that the SHG signal decreases rapidly starting at 45 degrees C. However, SHG imaging reveals that breakage of collagen fibers is not evident until 57 degrees C and worsens with increasing temperature. At 57 degrees C, structures of both molten and fibrous collagen exist, and the disruption of collagen appears to be complete at 60 degrees C. Our results suggest that, in addition to intensity measurement, SHG imaging is necessary for monitoring details of thermally induced changes in collagen structures in biomedical applications.     

Optical phase conjugation and waveguide coupling by cascading transverse second-harmonic and difference-frequency generation in a vertical cavity

We propose a novel scheme for implementing optical phase conjugation (OPC) by cascading transverse second-harmonic and difference-frequency generation in a vertical cavity. We propose to achieve phase conjugation in the pump and input-output two-waveguide structure. For a symmetric waveguide structure, the structure can be used as an optical phase conjugate mirror. For a realistic structure based on III–V semiconductor materials, a pump power per unit waveguide of 0.74 W m^-1 is required to reach a conversion efficiency of 9.4%. Combined with another phase-conjugate mirror, the reflectivity of the proposed phase-conjugate mirror can be efficiently modulated by control of the pump power. For an asymmetric waveguide structure, a gain is predicted. In addition, by cascading second-harmonic generation (SHG) and parametric oscillation, we propose to use the same structure as a waveguide coupler. There is a threshold to achieve coupling.