Third-harmonic Yield Research Articles

Ellipticity control of terahertz high-harmonic generation in a Dirac semimetal

We report on terahertz high-harmonic generation in a Dirac semimetal as a function of the driving-pulse ellipticity and on a theoretical study of the field-driven intraband kinetics of massless Dirac fermions. Very efficient control of the third-harmonic yield and polarization state is achieved in electron-doped ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ thin films at room temperature. The observed tunability is understood as resulting from terahertz-field-driven intraband kinetics of the Dirac fermions. Our study paves the way for exploiting the nonlinear optical properties of Dirac matter for applications in signal processing and optical communications.

Detection of HCl molecules by resonantly enhanced third-harmonic generation, driven by midinfrared laser pulses

We studied resonantly enhanced third-harmonic generation, induced by midinfrared, spectrally narrow-band, nanosecond laser pulses tuned in the vicinity of rovibrational molecular resonances. As we drive the frequency conversion process simultaneously close to single-, two-, and three-photon resonances, we get strong resonance enhancements of the third-harmonic yield by several orders of magnitude. The experimental setup requires only a single midinfrared laser beamline; the signal wavelength is easily separable from the much longer driving laser wavelength. We performed extended, systematic studies on the third-harmonic yield, spectral line shape and linewidth with laser intensity, frequency, and pressure. We determined a detection limit of ${10}^{15}$ $\mathrm{molecules}/{\mathrm{cm}}^{3}$, which indicates the potential of the approach for realistic applications in trace gas detection.

Optimizing the third harmonic generated from air plasma filaments pumped by femtosecond laser pulses

Third-harmonic (TH) generation inside air plasma filaments provides a simple method to convert near-infrared laser pulses in the UV range. Here, we report on the optimization of the third-harmonic generation during filamentation in the parameter space of focusing geometry, gas pressure, and incident laser pulse energy. It is observed that, in the nonlinear filamentation regime, the TH yield is largely dominated by the geometrical focusing condition and gas pressure. Using ambient air as the medium for filamentation, an optimal numerical aperture (NA) = 0.07 was found for a laser pulse with a peak power close to and above the critical power for self-focusing. For more loose focusing geometry (NA<0.047), reduction of the air pressure can improve the conversion efficiency. In this case, the spatial mode of the third harmonic was also found to be more uniform than in ambient air. With the scanning pinhole and crossing-filaments methods, it is revealed that in the situation of optimized focusing geometry (NA = 0.07) the TH accumulates constructively along the filament and exits at its end. In contrast, for the loose focusing geometry, the destructive interference effect of the TH generated at the leading and trailing edges of the filaments decreases the TH yield efficiency. This study provides a practical guidance for efficient conversion of near-infrared femtosecond pulses to the UV domain.

Four-photon resonantly enhanced six-wave mixing and third-harmonic generation in potassium vapour

Third-harmonic generation in potassium vapour by femtosecond laser pulses has been investigated for various pump wavelengths and beam-focusing conditions. It was shown for the first time that in the case of tight pump focusing, third-harmonic yield can be resonantly enhanced when the pump wavelength is tuned near the frequencies of allowed four-photon potassium transitions. Spatial properties of the registered signal indicate that the main process responsible for the resonantly enhanced third-harmonic generation is a non-degenerate six-wave mixing.

Third-harmonic generation from regularized converging filaments

Third-harmonic generation was investigated along four regularized filaments obtained by sending a 744 nm ultrashort laser pulse through an amplitude mask with four holes. Experiments show that the third-harmonic angular distribution forms a square-like pattern, which is reproduced by three-dimensional nonstationary numerical simulations. The fusion of the initially separated filaments and the formation of a superfilament arrests the third-harmonic yield but enhances self- and cross-phase modulation effects and produces a fourfold increase in the high-frequency wings of the spectra of the fundamental and the third harmonic. This demonstrates the opposite effect of superfilamentation on nonlinear phenomena that require a sufficiently large coherence length or the intense localized response.

Self-optimization of plasmonic nanoantennas in strong femtosecond fields

Plasmonic dimer nanoantennas can significantly boost the electric field strength in the gap region, allowing for a modification of the feed gap geometry by femtosecond laser illumination. Using resonant bowtie antennas to enhance the electric field of a low-fluence femtosecond oscillator, here we experimentally demonstrate highly localized reshaping of the antennas, resulting in a self-optimization of the antenna shape. From high-resolution scanning electron micrographs and two-dimensional energy dispersive x-ray maps, we analyze the near-field enhanced subwavelength ablation at the nanotips and the resulting deposition of ablated materials in the feed gap. The dominant ablation mechanism is attributed to the nonthermal transient unbonding of atoms and electrostatic acceleration of ions. This process is driven by surface plasmon enhanced electron emission, with subsequent acceleration in the vacuum. This ablation is impeded in the presence of an ambient gas. A maximum of sixfold enhancement of the third-harmonic yield is observed during the reshaping process.

Non-linear increase and saturation of third-harmonic yield from supported silver nanostructures excited by IR femtosecond laser pulses

A second-power yield of resonantly enhanced third harmonic and three-photon luminescence of 744 nm femtosecond laser pump pulses, weakly focused onto a layer of silver nanorolls on a silica substrate, was spectrally detected in the fluence range of 4–20 mJ cm−2, saturating at higher fluences. The third-harmonic yield and its saturation were explored in terms of ultrafast carrier dynamics, based on direct three-photon or cascade one- and two-photon transitions balanced by Auger recombination (and the final band-filling effect) which limited the radiative recombination output in the form of the third harmonic and three-photon luminescence.

Enhanced harmonic generation efficiency using focussing devices based on Fresnel diffraction

We propose the use of focusing devices based on Fresnel diffraction (such as a Fresnel zoneplate) as a means to enhance the efficiency of nonlinear-optical processes starting from radiation in the ultraviolet region, where ordinary lenses cannot be used. We present experimental results of a proof-of-concept experiment involving the efficient generation of second-and third-harmonic laser radiation in the visible range. An enhancement of a factor 6 in the third-harmonic yield is obtained by simply limiting the laser beam with a circular aperture. Higher enhancement factors (up to 16) are obtained using a Fresnel zoneplate. The experimental findings are in good agreement with the results of a simple theoretical analysis.

Enhancement of harmonic generation by Fresnel-lensing effects

We present experimental results of enhanced second- and third-harmonic generation efficiency from Fresnel-lensing effects. We obtained enhancement by a factor of 6 in the third-harmonic yield by simply limiting the laser beam with an iris. Higher enhancement factors (up to 16) were obtained with a Fresnel zone plate. The experimental findings are in good agreement with the results of a simple theoretical analysis.

Experimental and theoretical study of third-order harmonic generation in carbon nanotubes

Third-harmonic generation from solid samples of carbon nanotubes has been studied experimentally, using ultrashort pulses generated by a Cr:Forsterite laser, at a wavelength of 1250 nm. The results show an unusual nonperturbative behavior of the third-harmonic yield, for relatively low input laser fields, of ∼1010 W/cm2. This strong nonlinearity of the laser interaction with carbon nanoubes is also confirmed theoretically, in a full quantum-mechanical theory for harmonics generation from a single-walled carbon nanotube.

Enhancement of third-harmonic generation in absorbing media

The efficiency of third-harmonic generation on glass–air interfaces near the focal point of a tightly focused laser beam is investigated for different material samples. It is found that highly absorbing materials give higher harmonic yield. Moreover, due to the systematic change of the third-harmonic yield with the absorption length, an estimation of the third-order susceptibility χ(3) of absorbing samples can be given from absorption coefficient measurements.

Determination of material properties by use of third-harmonic generation microscopy

The fundamental features of third-harmonic generation microscopy are examined both theoretically and experimentally, and the technique is applied to the characterization of layered structures. Measurements and model calculations have been performed of the third-harmonic yield generated from homogeneous layers. Model calculations based on the paraxial approximation show good agreement with the experimental results, despite the conditions of high numerical aperture. The method proposed here allows for the determination of (i) the layer’s third-order susceptibility relative to that of the substrate, (ii) its index of refraction at the third-harmonic frequency relative to that at the fundamental frequency, and (iii) its thickness and for the identification of a gradient region between two adjacent layers.