Third Harmonic Generation In Air Research Articles

Three-step model for third-harmonic generation in air by nanosecond lasers

Understanding the phenomena involved in harmonic generation in plasmas by high-power pulsed lasers is a paramount task for developing new techniques for generation of coherent radiation in ultrashort bursts. Although first experiments on harmonic generation involved nanosecond lasers and inspired further interest in the subject, numerical simulations on harmonic generation are currently mainly oriented toward ultrashort fs lasers. This paper presents a combined theoretical–experimental approach to the generation mechanisms and the properties of third-harmonic (TH) radiation generated by infrared nanosecond laser pulses in air-breakdown plasma. The paper indicates that, at the microscopic level, the generation of TH can be described by a three-step model, which involves breakdown of nitrogen molecules in the air. First, the nitrogen molecules undergo cascade-impact ionization; then, the ionized molecules are quasi-resonantly excited through three-photon absorption; in the third step, the nitrogen molecules de-excite to the fundamental level with associated emission of TH radiation. At the macroscopic level, the three-step model is implemented considering that the breakdown plasma is a conductive nonlinear medium whose third-order susceptibility and complex conductivity depend upon the cubic root of the driving laser intensity. The 2D numerical simulations performed in the frame of this model are in good agreement with the experimental data in terms of TH generation efficiency, collimation, and polarization of TH radiation, indicating the validity of the theoretical model presented here. The model enables realistic calculations with affordable computing power for prediction and control of the TH generation process driven by nanosecond laser pulses. The results are important from the fundamental and practical points of view, thus providing an efficient tool for prediction of nonlinear optical phenomena in laser-produced plasmas and for noncontact diagnosis of harmonic-generating plasmas.

Application of combustion flames for generation of third harmonic and super-hydrophobic coating of glasses by deposited carbon nanoparticle films

We analyzed the third harmonic generation (THG) of 1064 nm, 28 ps radiation in the candle and ethanol flames. The formation of soot nanoparticles in the candle flame led to the increase of third harmonic yield compared to the ethanol flame. 12- and 24-fold enhancement of the third harmonic yield in candle and ethanol flames compared to the THG in air was achieved. The influence of the nonlinear refraction in air and combustion flames on the phase-matching conditions of THG was found responsible for deviation from the cubic I3ω (Iω) dependence. THG conversion efficiencies of Nd:YAG laser radiation were found to be 1 × 10−5, 1.2 × 10−4 and 2.4 × 10−4 in the cases of air, carbon flame and ethanol flame media, respectively. Furthermore, the super-hydrophobic properties of the deposited carbon species from the candle flame were demonstrated.

Enhanced third harmonic generation in air by two-colour ultrashort laser pulses

We report on third harmonic generation in air in a non-filamentation regime using tightly focused, ultrashort laser pulses (1–2 µm wavelength). Enhancement in the third harmonic efficiency is observed from co-propagating laser pulses of two different wavelengths which emanate from the same source—an optical parametric amplifier—and are spatially and temporally overlapped. The third harmonic efficiency for signal wavelength (1.35 µm) is measured to be 4 × 10−3 %; in the presence of idler wavelength (2.09 µm), the corresponding value becomes 1.6 × 10−2 %—a fourfold enhancement in efficiency. The pulse duration of the generated third harmonic is measured to be 37 fs. We examine the possible role of plasma to account for the observed enhancement in third harmonic generation.

On the generation of polarization-dependent supercontinuum and third harmonic in air

Filamentation and supercontinuum (SC) generation occur in transparent media during the propagation of intense femtosecond laser pulses. We report experimental results of polarization-dependent SC generation and third harmonic generation (THG) in air using intense 40 fs, 800 nm pulses under varying focusing conditions. We observe that tight focusing enhances the extent of the SC compared to when there is weak external focusing. Moreover, we observe that when the incident beam is linearly polarized the SC yield is more than that obtained using circularly polarized light of the same energy, but this difference reduces as focusing becomes tighter and depolarization begins to take effect. We have also carried out measurement for THG in air under the same conditions as for SC generation. A THG efficiency of 0.5% is measured for linearly polarized light in air. Although conservation of spin angular momentum precludes THG with circularly polarized light, we do observe THG with circularly polarized light in our experiments because of depolarization effects. We show that THG measurements allow in situ measurements of the extent to which incident light is depolarized.

Optimization of third harmonic generation in air by noncollinear six-wave mixing

The spectral and spatial properties of a third harmonic signal generated via a noncollinear six-wave mixing in air have been significantly improved by using two crossed pump beams of varying intensity ratio. This technique has allowed us to transfer most of the third harmonic pulse energy into a single near-Gaussian output beam, while the total efficiency of such THG increased by nearly 25%.

Filamentation-induced third-harmonic generation in air via plasma-enhanced third-order susceptibility

We study, both experimentally and theoretically, the underlying physics of third-harmonic generation in air by a filamented infrared femtosecond laser pulse propagating through a thin plasma channel. It is shown that the recently observed more than two-order-of-magnitude increase of the efficiency of third-harmonic generation occurs due to the plasma-enhanced third-order susceptibility. An estimate of the effective value of this susceptibility is given.

Noncollinear interaction of femtosecond filaments with enhanced third harmonic generation in air

We demonstrate that the interaction between two intense femtosecond filaments noncollinearly crossed in air results in third harmonic generation of more than two orders enhancement in energy conversion as compared with that from a single filament. The dependences of the third harmonic generation on the noncollinear crossing angles, input intensity ratios, and input pulse polarizations are investigated.

Third-harmonic generation in air by use of femtosecond radiation in tight-focusing conditions

Third-harmonic generation (THG) in air in tight-focusing conditions is presented. Variation of the pulse duration supplied optimal conditions for THG by femtosecond pulses that varied in the range 110-1300 fs. We show that this third-harmonic generation was caused by Kerr-induced phase variations of fundamental and harmonic beams. Various characteristics (laser intensity, focusing conditions, pulse duration, air pressure, etc.) of THG in air were analyzed to optimize this process. The THG conversion efficiency of 795 nm, 300 fs radiation was 1 x 10(-3). The harmonic radiation did not show considerable disruption of its spectral and spatial distribution in tight focusing conditions for intensities as high as 5 x 10(14) W cm(-2).

Supercontinuum emission and enhanced self-guiding of infrared femtosecond filaments sustained by third-harmonic generation in air

The long-range propagation of two-colored femtosecond filaments produced by an infrared (IR) ultrashort pulse exciting third harmonics (TH) in the atmosphere is investigated, both theoretically and experimentally. First, it is shown that the coupling between the pump and TH components is responsible for a wide spectral broadening, extending from ultraviolet (UV) wavelengths (220 nm) to the mid-IR (4.5 microm). Supercontinuum generation takes place continuously as the laser beam propagates, while TH emission occurs with a conversion efficiency as high as 0.5%. Second, the TH pulse is proven to stabilize the IR filament like a saturable quintic nonlinearity through four-wave mixing and cross-phase modulation. Third, the filamentation is accompanied by a conical emission of the beam, which becomes enlarged at UV wavelengths. These properties are revealed by numerical simulations and direct experimental observations performed from the Teramobile laser facility.

Third harmonic beam profile generated in atmospheric air using femtosecond laser pulses

We present, both experimentally and theoretically, the results on third harmonic generation in air during the filamentation of a powerful femtosecond laser pulse. The third harmonic beam pattern shows a central spot on the propagation axis at lower pump energy and is then surrounded by bright third harmonic conical emission at higher pump energy. We argue that the combined action of self-focusing and plasma generation in air plays an important role for the on-axis and off-axis phase-matching conditions along the filament resulting in high conversion efficiency. The experimental results are in good qualitative agreement with numerical simulations.

New adjustment technique for time coincidence of femtosecond laser pulses using third harmonic generation in air and its application to holograph encoding system

The third harmonic generation of light (266 nm) is enhanced, sensitively depending on the time delay between a pair of pulses split from a single 800 nm femtosecond laser pulse, when they are focused and collided in air. This finding offers a convenient and widely applicable technique to detect temporal and spatial overlapping of two femtosecond pulses. This technique has several advantages over the conventional sum frequency generation method using nonlinear optical crystals, since it obviates the need for expensive crystals, free from phase matching, and elimination of temporal walk off. By applying it to “a holographic encoding system using an interference femtosecond laser pulse,” a periodic fringe spacing is minimized to ∼430 nm by extending the colliding angle between two-pulse beams up to ∼160 °C.

Cherenkov-type phase-matched third harmonic generation in air

The third harmonic of Nd:YAG laser has been generated in air via the four-wave mixing. For the excitation of nonlinear media the second (Gaussian beam) and first (Bessel beam of variable cone angle) harmonics of this laser were used. The spatial spectra of the signal were presented and analysed. It was shown that in this case instead of the full phase matching the longitudinal (Cherenkov-type) phase matching takes place. The generation possibility of low divergence (<1 mrad) signal beam has been demonstrated experimentally.

Determination of Third-Order Nonlinear Optical Susceptibilities for Organic Materials by Third-Harmonic Generation

Abstract Third-order optical nonlinearities have been evaluated for organic materials in powder, solution, and film forms, by third-harmonic generation (THG) measurement. Considerations of the effects of THG interference between film sample and substrate, and of the effect of THG in air, are made for an accurate x(3) evaluation. Several types of monomeric and polymeric materials are examined by this method.