Nanocrystal-size-sensitive third-harmonic generation in nanostructured silicon

Third-harmonic generation (THG) in reflection geometry was studied in nanostructured silicon layers grown by electrochemical porosifying of p-type (110) silicon substrates. An order of magnitude enhancement of the THG efficiency compared to crystalline silicon (c-Si) was observed for the samples prepared on a highly doped substrate, whereas for the samples grown on a low-doped substrate the THG was much less efficient than for c-Si. The finding is discussed in terms of fluctuations of the electric field of the pump-laser irradiation in the layers of anisotropically distributed silicon nanocrystals. Received: 26 July 2002 / Revised version: 9 January 2003 / Published online: 3 April 2003 RID="*" ID="*"Corresponding author. Fax: +7-95/939-1566, E-mail: leo@vega.phys.msu.su     

Phase-matched third-harmonic generation in dye solutions

Various dyes are investigated for their capability of generating 3^rd harmonic of Nd-laser light. The feasibility of achieving phase matching is demonstrated by direct measurements of the index of refraction. Third-harmonic conversion of hexamethylindocarbocyanine iodide, diethyloxacarbocyanine iodide and fuchsin red are compared. The highest conversion is obtained for hexamethylindocarbocyanine iodide (80 times higher than in fuchsin red). A further enhancement of the third harmonic by a factor of 70 is expected for the fundamental laser light at 1.15 μm.     

Phase-matched third-harmonic generation in highly germanium-doped fiber

Phase-matched third-harmonic generation is demonstrated in a germanium-doped optical fiber. Green light at 514.4 nm is generated in an LP(03) mode when a pump field at ~1543.3 nm is launched into the fiber in the fundamental LP(01) mode. The phase matching is achieved for a particular combination of the germanium doping concentration and the fiber core diameter.     

Giant birefringence in anisotropically nanostructured silicon

We performed a study of the in-plane birefringence of anisotropically nanostructured Si layers, which exhibit a greater difference in the main value of the anisotropic refractive index than that of natural birefringent crystals. The anisotropy parameters were found to be strongly dependent on the typical size of the Si nanowires used to assemble the layers. This finding opens the possibility of an application of birefringent Si retarders to a wide spectral range for control of the polarization state of light.     

Phase-matched third-harmonic UV generation using low-order modes in a glass micro-fiber

We observe phase-matched third-harmonic generation at 355 nm in a low-order mode of a sub-micron diameter glass fiber. The third-harmonic signal exhibits a sharp resonance for a fiber diameter d = 0.49 ± 0.02 μm, in excellent agreement with the value d = 0.506 μm predicted by theory. The third-harmonic conversion efficiency is 2 × 10⁻⁶, and is limited by the pump power (1 kW) and effective fiber length (100 μm).     

Phase-matched third-harmonic generation by nonlinear phase shift in a hollow fiber

14 W/cm² are introduced into a 1.8-cm hollow fiber in room air. From the observation of the third-harmonic beam quality and spectra, it is clarified that the harmonics are pumped at the leading edge of the fundamental pulse. The model calculation accounting for the nonlinear refractive index change of air shows a quantitative agreement with the experimental results. Received: 19 September 1997/Revised version: 2 December 1997     

Phase-matched third-harmonic generation of Nd: Glass-laser picosecond pulses in a new cyanine-dye solution

The phase-matched direct tripling of picosecond light pulses of a mode-locked Nd: glass laser in a new cyanine dye PMC is studied. The solvents trifluoroethanol (TFE) and hexafluoroisopropanol (HFIP) are applied. The S ₀–S ₁ absorption peak of the dye is around =480 nm and the absorption cross section at the third-harmonic wavelength of ₃=351.3 nm is only ₃1×10^–19 cm². Phase-matching occurred at concentrations of C_PM=0.0874 mol/dm³ in HFIP and 0.1088 mol/dm³ in TFE. A third-harmonic energy conversion efficiency of _E0.01 was achieved at a pump-laser peak intensity of I _0L2.5×10¹¹ W/cm² in a 5 mm long sample of PMC in TFE. The conversion efficiency is limited by destruction of phase-matching due to the intensity-dependent nonlinear refractive index of the dye solutions.     

Characterization of third-harmonic generation from femtosecond laser-excited silicon

Third-harmonic generation (THG) from silicon crystals excited by femtosecond pulses has been studied. THG as a function of probe pulse delay from linear and circularly polarized probe pulses was examined. Both polarizations exhibit a fast, 300 fs drop in THG when the Si is pumped by pulses with fluence above the melting threshold. This drop for both polarizations indicates that electronic effects dominate THG in laser-melted Si since structural effects should principally influence THG from linearly polarized probes, suggesting that care needs to be exercised in interpreting THG signal as an ultrafast probe of crystalline structure in semiconductors.     

Optical third-harmonic generation in coupled microcavities based on porous silicon

The resonance features of the third-harmonic generation have been observed in 1D coupled microcavities consisting of three Bragg reflectors and two identical half-wave layers of mesoporous silicon. The third-harmonic intensity increases by a factor of about 10³ in the resonance of fundamental radiation with each of the modes of coupled microcavities. It has been shown that the resonance positions in the angular spectra of the third-harmonic intensity depend on the coupling between microcavities that is determined by the transmission of the intermediate Bragg reflector. In the framework of the transfer-matrix method with nonlinear sources, it has been shown that the basic mechanism of the enhancement of the third-harmonic generation in coupled microcavities based on porous silicon is the constructive interference of the partial third-harmonic waves that are generated by near-surface layers.     

Phase-matched second-harmonic generation in hybrid polymer-LN waveguides

Here we propose a hybrid polymer-LN waveguide for achieving phase-matched second-harmonic generation (SHG). From the aspect of super-mode theory, the geometric parameters of the hybrid semi-nonlinear waveguide were optimized to utilize both symmetric (even) and antisymmetric (odd) modes of the pump and SHG waves so as to facilitate phase matching with large modal overlap. Phase matching between a fundamental even (TE₀₀-like) mode at 1320 nm and a fundamental odd (TE₀₁-like) mode at 660 nm was found with a calculated modal overlap integral of 0.299, while utilizing the largest nonlinear coefficient d₃₃, and achieving an efficient calculated normalized conversion efficiency of 148% W^-1·cm^-2. Considering the fabrication feasibility of such hybrid waveguide with features including etchless, large dimension, and low structural sensitivity, we believe our findings would provide a useful reference for future on-chip efficient nonlinear conversion devices.     

Phase-matched third harmonic generation in a plasma

Relativistic third-harmonic generation in a plasma is investigated. The growth of a third-harmonic wave is limited by the difference between the phase velocity of the pump and driven waves. This phase velocity mismatch results in a third-harmonic amplitude saturation and oscillation. In order to overcome this saturation, a phase-matching scheme based on a resonant density modulation is described. The limitations of this scheme are analyzed     

Two-photon-resonant third-harmonic generation in cesium vapor

It is shown that two-photon-resonant tripling of the ruby laser frequency in cesium vapor is limited by ionization. The laser induced Stark shift is found to be favorable to the process in that it counteracts saturation of the resonant transition.     

Picosecond third-harmonic light generation in calcite

The third-harmonic generation of picosecond light pulses in a calcite crystal is studied experimentally and theoretically. A passively mode-locked Nd: phosphate glass laser is used in the experiments. The third-order nonlinear susceptibility components and the effective third-order nonlinear susceptibility of type II phase-matched (ooee) thirdharmodic generation are determined.On leave from Shanghai Institute of Optics and Fine Mechanics, Academia Sinica, Shanghai, P.R. China     

Structure sensitivity in third-harmonic generation microscopy

We characterize experimentally the influence of sample structure and beam focusing on signal level in third-harmonic generation (THG) microscopy. In the case of a homogeneous spherical sample, the dependence of the signal on the size of the sphere can be controlled by modifying the Rayleigh length of the excitation beam. More generally, the influence of excitation focusing on the signal depends on sample geometry, allowing one to highlight certain structures within a complex system. We illustrate this point by focusing-based contrast modulation in THG images of Drosophila embryos.     

Resonant frequency-domain interferometry via third-harmonic generation

A method for measuring resonances using a combination of third-harmonic generation and frequency-domain interferometry is described and demonstrated in an index-matched dielectric material. The phase of the third-harmonic spectrum of a pulse generated from a resonant NdAlO(3) thin film and a temporally displaced sapphire substrate pulse was measured by analyzing the spectral interference pattern. The appropriate combination of substrate and film signals was obtained by translating the sample through the laser focus while observing the third-harmonic intensity.     

Phase-matched electric-field-induced second-harmonic generation in a nematic liquid crystal

The phase-matching ability of nematic liquid crystal is demonstrated experimentally. The potential of nematic and smectic crystals as practical harmonic generators is discussed.     

Atmospheric influences in optical third-harmonic generation experiments

Third-harmonic generation in a thin solid plate surrounded by a gas is investigated. We conclude that atmospheric contributions can be suppressed in third-harmonic generation from thin solid plates. A phase-sensitive measurement of the third-order susceptibility of gases is proposed. We calibrated the third-order susceptibility of air to be X⁽³⁾ _a=(3.4±1.5)×10^–18 esu with a phase uncertainty of ±10°.