Broadband and high power terahertz pulse generation beyond excitation bandwidth limitation via chi2 cascaded processes in LiNbO3

Three-photon and four-photon absorption in lithium niobate measured by the Z-scan technique

Open-aperture Z-scan measurements have been carried out to investigate the three-photon (3 PA) and four-photon absorption (4 PA) coefficients at 800 nm and 1030 nm wavelengths, respectively in congruent and stoichiometric lithium niobate (cLN, sLN) with different concentrations of Mg doping. The laser pulse duration at the two wavelengths were 40 and 190 fs. The peak intensity inside the crystals varied between approximately 110 and 550 GW/cm2. The 3 PA and 4 PA coefficients were evaluated using a theoretical model and the results suggest that their minima are at or around the Mg doping level corresponding to the threshold for suppressing photo-refraction for both cLN and sLN. This result can be attributed to the contribution of crystal defects to the 3 PA and 4 PA processes. Furthermore, the 4 PA at 1030 nm exhibited greater nonlinear absorption than the 3 PA at 800 nm under the same intensity level. Possible reasons for this unexpected behavior are discussed. Overall, comparing the 3 PA and 4 PA values of these crystals will enable for selection of the optimum composition of LN crystal for efficient THz generation and for other nonlinear optical processes requiring high pump intensities.

Parameter sensitivities in tilted-pulse-front based terahertz setups and their implications for high-energy terahertz source design and optimization

Despite the popularity and ubiquity of the tilted-pulse-front technique for single-cycle terahertz (THz) pulse generation, there is a deficit of experimental studies comprehensively mapping out the dependence of the performance on key setup parameters. The most critical parameters include the pulse-front tilt, the effective length of the pump pulse propagation within the crystal as well as effective length over which the THz beam interacts with the pump before it spatially walks off. Therefore, we investigate the impact of these parameters on the conversion efficiency and the shape of the THz beam via systematically scanning the 5D parameter space spanned by pump fluence, pulse-front-tilt, crystal-position (2D), and the pump size experimentally. We verify predictions so far only made by theory regarding the optimum interaction lengths and map out the impact of cascading on the THz radiation generation process. Furthermore, distortions imposed on the spatial THz beam profile for larger than optimum interaction lengths are observed. Finally, we identify the most sensitive parameters and, based on our findings, propose a robust optimization strategy for tilted-pulse-front THz setups. These findings are relevant for all THz strong-field applications in high demand of robust high-energy table-top single-cycle THz sources such as THz plasmonics, high-harmonic generation in solids as well as novel particle accelerators and beam manipulators.

Coherent Terahertz Radiation from Multiple Electron Beams Excitation within a Plasmonic Crystal-like structure

Coherent terahertz radiation from multiple electron beams excitation within a plasmonic crystal-like structure (a three-dimensional holes array) which is composed of multiple stacked layers with 3 × 3 subwavelength holes array has been proposed in this paper. It has been found that in the structure the electromagnetic fields in each hole can be coupled with one another to construct a composite mode with strong field intensity. Therefore, the multiple electron beams injection can excite and efficiently interact with such mode. Meanwhile, the coupling among the electron beams is taken place during the interaction so that a very strong coherent terahertz radiation with high electron conversion efficiency can be generated. Furthermore, due to the coupling, the starting current density of this mechanism is much lower than that of traditional electron beam-driven terahertz sources. This multi-beam radiation system may provide a favorable way to combine photonics structure with electronics excitation to generate middle, high power terahertz radiation.

THz generation using a reflective stair-step echelon

We present a novel method for THz generation in lithium niobate using a reflective stair-step echelon structure. The echelon produces a discretely tilted pulse front with less angular dispersion compared to a high groove-density grating. The THz output was characterized using both a 1-lens and 3-lens imaging system to set the tilt angle at room and cryogenic temperatures. Using broadband 800 nm pulses with a pulse energy of 0.95 mJ and a pulse duration of 70 fs (24 nm FWHM bandwidth, 39 fs transform limited width), we produced THz pulses with field strengths as high as 500 kV/cm and pulse energies as high as 3.1 μJ. The highest conversion efficiency we obtained was 0.33%. In addition, we find that the echelon is easily implemented into an experimental setup for quick alignment and optimization.

Contact grating device with Fabry-Perot resonator for effective terahertz light generation

A novel design for a contact grating device with an incorporated Fabry-Perot resonator is proposed for high-power terahertz (THz) light generation. We deposited a multilayer consisting of Ta(2)O(5) and Al(2)O(3) on a magnesium-doped stoichiometric LiNbO(3) substrate and fabricated grating grooves on the outermost layer. The multilayer was designed such that conditions for a Fabry-Perot resonator were satisfied for light diffracted by the grating. Consequently, the fraction of light transmitted into the LiNbO(3) substrate, i.e., the diffraction efficiency, was enhanced by the resonator. The diffraction efficiency of the fabricated device was 71%, which is close to the calculated value of 78% from the optimized design. THz light generation was also demonstrated with the contact grating device. The THz output of 0.41 μJ was obtained using near-infrared pump light of 2.7 mJ.

Achromatic THz wave plate composed of stacked parallel metal plates

We propose a simple achromatic terahertz wave plate composed of stacked parallel metal plates with a hole array. It consists of an ensemble of designed parallel plate waveguides; the high and low propagation speeds of waves in TE and TM waveguide modes with the same group velocity cause a constant phase difference over a wide frequency region. Using that wave plate, we obtained intense single- and multi-cycle THz pulses with circular polarization.

Saturation of the free carrier absorption in ZnTe crystals

This study systematically investigates the influence of free carriers on the generation of THz in ZnTe crystals, over a wide range of pumping fluences. As the pumping fluence is increased (< 6.36 mJ/cm(2)), the concentration of free carriers gradually increases and the THz output power is saturated, as clearly demonstrated by the time delay in the THz temporal waveforms, the changes in the THz spectral weight and the red-shift in the PL spectra. For high pumping fluences (> 6.36 mJ/cm(2)), spectacularly, there is a significant quadratic increase in the THz output power when the pumping fluence is increased, as well as at low pumping fluences of < 0.58 mJ/cm(2), because of the saturation of free carriers.

High-efficiency terahertz pulse generation via optical rectification by suppressing stimulated Raman scattering process

We experimentally demonstrate high-efficiency terahertz pulse generation via optical rectification in LiNbO3. The spectral broadening of an excitation pulse via the stimulated Raman scattering process coincides with high-efficiency terahertz pulse generation, which enhances undesired stretching of the excitation pulse owing to the very high group velocity dispersion in LiNbO3. We avoid this by the bandwidth control of the excitation pulse and achieve the highest reported efficiency of 0.21% for energy conversion into a THz pulse.

Ferroelectric soft mode in a SrTiO3 thin film impulsively driven to the anharmonic regime using intense picosecond terahertz pulses

The ferroelectric soft mode in a SrTiO(3) thin film was impulsively driven to a large amplitude using intense picosecond terahertz pulses. As the terahertz electric field increased, the soft-mode absorption peak exhibited blueshifting and spectral narrowing. A classical anharmonic oscillator model suggests that the induced displacement is comparable to that of the ferroelectric phase transition. The spectral narrowing indicates that the displacement exceeds that induced by any inhomogeneities in the film, demonstrating that the method can be used to explore intrinsic quartic anharmonicity.

Towards generation of mJ-level ultrashort THz pulses by optical rectification

Optical rectification of ultrashort laser pulses in LiNbO3 by tilted-pulse-front excitation is a powerful way to generate near single-cycle terahertz (THz) pulses. Calculations were carried out to optimize the output THz peak electric field strength. The results predict peak electric field strengths on the MV/cm level in the 0.3-1.5 THz frequency range by using optimal pump pulse duration of about 500 fs, optimal crystal length, and cryogenic temperatures for reducing THz absorption in LiNbO3. The THz electric field strength can be increased further to tens of MV/cm by focusing. Using optimal conditions together with the contact grating technique THz pulses with 100 MV/cm focused electric field strength and energies on the tens-of-mJ scale are feasible.

Ladder climbing on the anharmonic intermolecular potential in an amino acid microcrystal via an intense monocycle terahertz pulse

We experimentally demonstrated 20 ladder climbing steps on the anharmonic intermolecular potential in the amino-acid microcrystals with an intense monocycle terahertz pulse. Absorption spectra show the suppression of the peak and enhancement of the low-frequency absorption for the incident electric field amplitude. These results are reproduced by simulations based on coherent transition processes between quantum levels in the anharmonic potential. The appearance of such nonlinearity allows us to control macroscopic motion via a phase-controlled terahertz pulse.