Ioulia B. Zotova

Compact and portable terahertz source by mixing two frequencies generated simultaneously by a single solid-state laser

By mixing two frequencies generated from a single Q-switched Nd:YLF laser in a GaSe crystal, an average terahertz output power reaches 1 μW within a bandwidth of 65 GHz at 1.64 THz.

Terahertz generation based on parametric conversion: from saturation of conversion efficiency to back conversion

By stacking alternatively rotated gallium phosphide (GaP) plates, the maximum photon conversion efficiency of 40% for the terahertz (THz) generation based on difference-frequency generation has been achieved. The corresponding peak power generated inside the four GaP plates approaches 4 kW. As the number of plates is increased from four to five, the THz output power is significantly decreased, due to back parametric conversion.

Power scalability and frequency agility of compact terahertz source based on frequency mixing from solid-state lasers

We investigate power scalability and frequency agility of a terahertz (THz) source by mixing two frequencies generated by solid-state lasers in a nonlinear crystal. They are made possible by introducing two solid-state laser crystals sharing the same Q switch and output coupler with the same laser beams decoupled to each other by a polarizer. Following the optimization, we have improved the THz output power nearly fivefold at 1.64 THz. By replacing one of the neodymium-doped lithium yttrium fluoride crystals with a neodymium-doped yttrium aluminum garnet crystal, we have produced 2.1 μW at 2.98 THz.

Backward second-harmonic generation in submicron-period ion-exchanged KTiOPO4 waveguide

Abstract For the first time, we demonstrated backward second-harmonic generation (SHG) in a submicron-period ion-exchanged KTiOPO 4 (KTP) waveguide. Such a period is the shortest ever achieved in ferroelectric materials. The characteristics of backward quasi-phase-matching at the 6th and 7th orders were investigated. Our analyses on the backward SHG show that the second-harmonic linewidth and conversion efficiency are very sensitive to the grating period error. According to our measurement on the backward SH linewidth, we determined the period error to be ∼4.5×10 −3 μm for our sample, which is close to the value of ∼7×10 −3 μm measured by other techniques.

Singly resonant optical parametric oscillator based on adhesive-free-bonded periodically inverted KTiOPO 4 plates: terahertz generation by mixing a pair of idler waves

We implemented a singly resonant optical parametric oscillator based on adhesive-free-bonded periodically inverted KTiOPO4 plates. It has major advantages such as walk-off compensation and oscillation at four wavelengths. The threshold of the oscillation was measured to be 8 MW/cm2, which is about a factor of 4 lower than that based in two separate KTiOPO4 crystals. By frequency-mixing the dual-wavelength output in GaP stacks, we generated the terahertz radiation at 2.54 THz. The tuning range of the terahertz output was demonstrated to be 2.19-2.77 THz.

Second-order nonlinear optical materials for efficient generation and amplification of temporally-coherent and narrow-linewidth terahertz waves

We have considered forward and backward optical parametric oscillation and amplification, and difference-frequency generation for efficiently generating and amplifying terahertz waves in several second-order nonlinear optical materials. We have used a single crystal of CdSe as an example. We have also investigated GaSe, periodically-poled LiNbO3 and LiTaO3, and diffusion-bonded-stacked GaAs and GaP plates. The advantage of using birefringence in CdSe and GaSe is tunability of the output terahertz frequency. Furthermore, both CdSe and GaSe can be used to achieve the backward parametric oscillation without any cavity. On the other hand, in periodically-poled LiNbO3 and LiTaO3, one can take advantage of large diagonal elements of second-order nonlinear susceptibility tensor. In the diffusion-bonded-stacked GaAs and GaP plates, quasi-phase matching can be achieved by alternatively rotating the plates. We have shown that it is feasible to achieve forward optical parametric oscillation in the THz domain using these plates. The advantage of using coherent parametric processes is possibility of efficiently generating and amplifying temporally-coherent and narrow-linewidth terahertz waves. Compared with a noncollinear configuration, by using the parallel wave propagation configurations, the conversion efficiency can be higher because of longer effective interaction length among all the waves.

Investigation of terahertz generation from passively Q-switched dual-frequency laser pulses.

When two Nd:YLF crystals share a Cr:YAG crystal functioning as a single passive Q switch, the timing jitter between each pair of dual-frequency pulses generated by the two crystals has been reduced by a factor of 20. Such a reduction in the timing jitter allows us to generate terahertz pulses by focusing such a passively Q-switched laser beam onto a nonlinear crystal. Such a result represents the first step for us to eventually implement a compact terahertz source based on ultracompact microchip lasers.

Efficient generation of backward terahertz pulses from multiperiod periodically poled lithium niobate.

Backward terahertz pulses were converted from ultrafast pulses of a laser amplifier within multiperiod periodically poled LiNbO3 (PPLN) wafers. The average output powers of more than 2 microW were obtained from all six domain periods, corresponding to the frequency range of 217 GHz-2.373 THz. Among the six periods, the highest output power of 10.7 microW and the narrowest linewidth of 5.98 GHz were achieved at the periods of 66.5 microm and 150.0 microm, respectively.

THz generation from InN films due to destructive interference between optical rectification and photocurrent surge

We have investigated the characteristics of THz generation including the dependence of the output power and polarization on the incident angle and pump polarization from two series of InN films grown by plasma-assisted molecular beam epitaxy (PAMBE) and metal organic chemical vapor deposition (MOCVD), respectively. Following the analyses of our results, we have attributed the mechanism of the THz generation from these InN samples to the destructive interference between optical rectification and photocurrent surge. Under the average intensity of 176 W cm−2 for the subpicosecond laser pulses at 782 nm, the THz output powers were measured to be as high as 2.4 µW from the 220 nm InN film, with the output frequencies spanning the band from 300 GHz to 2.5 THz.

Spectral measurements of two-photon absorption coefficients for CdSe and GaSe crystals

We have measured the spectra of two-photon absorption coefficients for CdSe and GaSe based on a z-scan technique. By using extremely high peak intensities from subpicosecond laser pulses, we observed novel behavior of effective two-photon absorption for CdSe. We compared our results for both crystals with theory as well as with the previous values measured at three and two wavelengths, respectively. For both CdSe and GaSe crystals our measured values are lower than all those reported previously.