Third-harmonic Beam Research Articles

Conversion of spin and orbital angular momentum in the third harmonic generation process in the bulk of an isotropic medium

The interaction between spin and orbital components of the angular momentum of an elliptically polarized beam with helicoidal phase profile and arbitrary axially symmetric intensity profile in the third-harmonic generation process is investigated. The third-harmonic beam is generated in an isotropic medium with cubic nonlinearity within undepleted pump approximation. The conservation of the total projections of both the spin angular momentum and orbital angular momentum components in four-wave mixing is demonstrated for interacting waves.

Conical third-harmonic generation of optical vortex through ultrashort laser filamentation in air.

We experimentally generate third-harmonic (TH) vortex beams in air by the filamentation of femtosecond pulses produced in a lab-built Ti:sapphire chirped pulse amplifier. The generated TH beam profile is shown to evolve with increasing pump energy. At a sufficiently high pump energy, we observe a conical TH emission of the fundamental vortex and confirm that the conical radiation follows the conservation law for orbital angular momentum. We further investigate the far-field angularly resolved spectra of the TH wave to analyze the conical emission angle. We theoretically verify that the formation of the conical TH vortex results from the phase-matching between the fundamental and TH waves during the filamentation process.

Walk-off reduction, using an external optical plate and Bessel–Gaussian interaction

To reduce the walk-off angle of the extraordinary third-harmonic ultraviolet wave at 355 nm generated by type II KTiOPO4 and type I β-BaB2O4 optical crystals, and the Gaussian output beam of a Q-switched Nd:YAG laser, a simple theoretical model was developed based on a rotatable BK7 plate of variable thickness. By rotating the plate up to 35° along the beam direction, we reduced the walk-off angle up to ∼ 13%. The same phenomenon is predicted by the model, confirming the performance of the model. It is found that, due to the walk-off effect, the intensity profile of the third-harmonic generation beam is slightly degraded. To compensate for the observed phenomena and further reduce the walk-off, we used a combination of a convex lens and an axicon to transform the beam profile of the interacting fundamental and second-harmonic generation waves to the zero-order Bessel–Gaussian form. As a result, the walk-off is decreased to ∼48.81 mrad, providing ∼30% relative reduction. By using the same BK7 plate rotated up to 35° along the third-harmonic beam direction, the walk-off angle is further reduced to 38.9 mrad. Moreover, it is observed that the beam profile of the emerged Bessel–Gaussian third-harmonic generation beam remains unchanged with no degradation.

Tunable third-harmonic probe for non-degenerate ultrafast pump–probe measurements

In this article, we report a method to achieve a precisely tunable highly stable probe beam generation for performing pump–probe experiment around a given wavelength by tilting a sum frequency generation (SFG) crystal angle. The width of the generated third-harmonic beam is of the order of 2 nm throughout the tunable range. This method of probe beam generation has its application in isolating contributions from closely separated excitation states.

Effect of spatial refractive-index nonuniformities existed in a large-scale rapid growth KDP crystal on third-harmonic conversion

Effect of spatial refractive-index nonuniformities existed in a large-scale rapid growth KDP crystal (a type I frequency doubler at the wavelength of 1053nm) on third-harmonic conversion is studied. Spatial refractive-index nonuniformities due to imperfections in the crystal structure which prohibit the attainment of phase matching at all locations in the crystal are measured with the orthogonal polarization interferometry technique. And a third-harmonic conversion experiment is conducted on a large-scale facility to investigate the influence of refractive-index nonuniformities on conversion efficiency and third-harmonic beam nearfield. According to the experimental results, refractive-index nonuniformities in this large-scale rapid growth KDP doubler do not reduce the third-harmonic conversion efficiency, but affect the third-harmonic beam nearfield.

Efficient third-harmonic generation through tailored IR femtosecond laser pulse filamentation in air

The process of third-harmonic generation during the filamentation of intense IR femtosecond laser pulses in air is investigated experimentally. It is shown that the introduction of a thin plasma string created by another femtosecond pulse, perpendicularly to the filament's path, dramatically reshapes the third-harmonic beam into a Bessel-like far-field distribution, while at the same time significantly enhances, up to 250 times, its conversion efficiency.

Nonlinear optical study of amorphous SiN:H films on the Si(111) substrate

A correlation between the resistivity-voltage dependences of the hydrogenated SiN/Si(111) films and the photo-induced third-harmonic generation (PITHG) at λ = 1064 nm in the reflected light regime was found. The contribution to the PITHG is both due to linear photo-induced surface changes as well as due to the hydrogen dopants. When the fundamental power density increases to higher than 1.2 GW/cm2, there occurs a drastic decrease of the PITHG. Maximal PITHG changes are observed at a pump-probe delay time about 18 ps and the value of the PITHG slightly increases with the enhanced applied dc-electric field during illumination by a third-harmonic laser beam as a photo-inducing one. The deviation from the tripled frequency wavelength to higher wavelengths substantially suppresses the output PITHG signal. The crucial role of the interface trapping levels and silicon-hydrogen charge transfer is shown.

Spectral evolution of angularly resolved third-order harmonic generation by infrared femtosecond laser-pulse filamentation in air

We experimentally investigate the evolution of an angularly resolved spectrum of third harmonic generated by infrared femtosecond laser pulse filamentation in air. We show that at low pump intensity, phase matching between the fundamental and third-harmonic waves dominates the nonlinear optical effect and induces a ring structure of the third-harmonic beam, whereas at high pump intensity, the dispersion properties of air begin to affect the angular spectrum, leading to the formation of a nonlinear $\text{X}$ wave at third harmonic.

Stimulated Raman Scattering of a Modulated Laser Beam in Air:a Perturbation Approach

Stimulated Raman scattering (SRS) of a third-harmonic beam of ahigh-power laser in air is analysed by using a perturbation approach.It is found that intensity modulations of the third-harmonic beam,resulting from the surface ripple of nonlinear crystal, can besignificantly enhanced due to SRS, and the gain of instabilityincreases with the spatial period of modulation. Numerical simulationsbased on the nonlinear coupled-wave equations verify the validity ofthe perturbation approach.

Efficient third-harmonic generation in partly periodically poled KTiOPO_4 crystal

A partly periodically poled KTiOPO(4) (KTP) crystal has been designed to integrate quasi-phase-matched second-harmonic generation (QPM SHG) with sum-frequency generation in one crystal for generating a third-harmonic beam. The highest conversion efficiencies of 45% and 3% have been achieved in our experiments for QPM SHG and third-harmonic generation, respectively, by use of picosecond laser pulses at 1.327 microm . We have also discovered that periodically poled KTP has slightly different indices (n(z)) from bulk KTP.

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

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.

Spectral-spatial measurements of fundamental and third-harmonic light of intense 25-fs laser pulses focused in a gas cell

We present spatially and spectrally resolved measurements of light emerging from the focus of an intense 25-fs laser pulse in gaseous media. The pulses are focused inside a chamber backfilled with either argon or air to nominal vacuum intensities of up to 5×1015 W/cm2, far in excess of that necessary for ionization. We have recorded spectral-spatial maps for various pressures and pulse energies and have compared the results of argon and air. This survey is done in the regime where ∼0.1% of the laser light is converted into the third harmonic, which is of practical importance since it is the largest conversion efficiency measured to date while maintaining such a short pulse duration. These measurements are, to our knowledge, the first of their type and are performed with a laser beam of very high spatial quality. The dynamics of an intense laser focusing into a gas cell is complicated, and these measurements provide a way to compare numerical simulations with an array of experimental information. The experiments also determine the conditions for optimal third-harmonic spatial beam quality in this regime.

Highly efficient frequency tripling of laser radiation in a low-temperature laser-produced gaseous plasma

Relatively efficient (up to 3%) third-harmonic generation of picosecond Nd:YAG laser pulses for wavelength λ = 1.06 μm in a low-density laser plasma is reported for a coherent (i.e., collinear) geometry. The third-harmonic beam produced is nearly diffraction limited and coherent and can be used in other nonlinear-optical experiments. A simple theoretical model is proposed in order to explain this result. In the model, third-harmonic generation is considered to be a process that arises from the scattering of electrons by ions in the presence of the strong laser field.

動脈硬化病変の血管内蛍光診断に関する基礎的研究

The authors evaluated the pathological feature of atheromatous plaques of rabbit model in relation to spectral properties of laser-excited fluorescence of doxycycline (Dc: tetracycline derivatives) that has selectively accumulated to the plaques.We administrated Dc intravenously to the rabbit models and detected the spectra of their resected aorta. Excitation source used id third-harmonic beam (355 nm) of Nd:YAG laser, whose immediated power and beam diameter are approximately 10μW and 100μm respectively. We employed Hematoxylin-Eosin and Elastica-Masson stains for each 5μm section along the spotts after spectral detection.The result shows that the intensity ratio of atheromatous plaques, that calculated as a ratio of the photon count at proper fluorescence of Dc to that at auto-fluorescence of arterial wall, have pathologically a good relation to the extent of their atherosclerotic change. The intensity ratio mainly reflected the lipid deposits that found superficially in the plaques, and approximately fibrous changes contributed to reducing the ratio. On the other side, it is thought that the lipid deposits found deeply in the plaques were related with the high value persistence of intensity ratio, especially in the plaques with severe atherosclerotic changes.We thought Dc is one of hopeful drugs that may give us a new diagnostic parameter of atherosclerosis.

血管内膜内光増感色素のレーザー光励起蛍光分光特性の検討

We studied experimentally the spectral properties of laser-excited Fluorescence of three photosensitisers, namely tetracycline (Tc), hematoporphyrin free base (Hp) and chlorin e6Na (Chl), taken up in human intima. Excitation source used is third-harmonic beam (355nm) of a Nd:YAG laser, whose irradiated power and beam diameter are approximately 10uW and 100um, respectively. Three photosensitisers (concentration, Hp and Chl: 1×10-4mol/l and Tc: 1×10-3 mol/l) were administrated to resected human vessels for 2 hours in vitro.The main result obtained in our study are following;1) Photosensitisers examined are confirmed to be taken up in intima and the fluorescence intensity decreases as the time goes on.2) The fluorscence intensity of three dyes is different with measuring positions of intima and the fluoresence intensity from atheromatous plaque is usually stronger than that from normal region.3) The fluorescence intensity of human vessel decreases when photosensitiser is administrated, especially in the ease of Hp and Chl. This phenomenon could utilized to estimate quantitatively the concentration of dye taken up in each positjons in the vessel.