Yuri Avetisyan

Generation of tunable narrow-band surface-emitted terahertz radiation in periodically poled lithium niobate

Generation of tunable narrow-band terahertz (THz) radiation perpendicular to the surface of periodically poled lithium niobate by optical rectification of femtosecond pulses is reported. The generated THz radiation can be tuned by use of different poling periods and different observation angles, limited only by the available bandwidth of the pump pulse. Typical bandwidths were 50-100 GHz, depending on the collection angle and the number of periods involved.

Surface-emitted terahertz-wave difference-frequency generation in two-dimensional periodically poled lithium niobate

We report on the demonstration of surface-emitted terahertz- (THz-) wave difference-frequency generation from two-dimensional (2D) periodically poled lithium niobate (PPLN). The two orthogonal periodic structures individually compensate for both the phase mismatch of the launched lasers and the generated THz wave. Tunable 1.5-1.8 THz wave generation with a bandwidth of 10-GHz was obtained by use of two 2D PPLN crystals. We also confirmed that THz waves were simultaneously generated into two opposite directions, which suggests the possibility of higher THz-wave output power.

Surface-emitted terahertz-wave generation by ridged periodically poled lithium niobate and enhancement by mixing of two terahertz waves

Surface-emitted terahertz- (THz-) wave generation by difference-frequency mixing with ridge-shaped periodically poled lithium niobate (PPLN) was demonstrated. The PPLN had a ridge height of 300 microm, a thickness of 20 microm, and an interaction length of 35 mm. The ridge behaves as a slab waveguide for optical pump beams. The PPLN gives rise to THz waves in opposite directions, perpendicular to the pump-beam direction. Reflecting the THz wave on one side and overlapping it with the THz wave on the other side increased the total THz-wave intensity approximately 2.7 times compared with that without reflection and mixing.

Tuning characteristics of narrowband THz radiation generated via optical rectification in periodically poled lithium niobate

The tuning properties of pulsed narrowband THz radiation generated via optical rectification in periodically poled lithium niobate have been investigated. Using a disk-shaped periodically poled crystal tuning was easily accomplished by rotating the crystal around its axis and observing the generated THz radiation in forward direction. In this way no beam deflection during tuning was observed. The total tuning range extended from 180 GHz up to 830 GHz and was limited by the poling period of 127 microm which determines the maximum THz frequency in forward direction.

Terahertz-wave surface-emitted difference-frequency generation without quasi-phase-matching technique

A scheme of terahertz (THz)-wave surface-emitted difference-frequency generation (SEDFG), which lacks the drawbacks associated with the usage of periodically orientation-inverted structures, is proposed. It is shown that both material birefringence of the bulk LiNbO(3) crystal and modal birefringence of GaAs/AlAs waveguide are sufficient to obtain SEDFG up to a frequency of approximately 3THz. The simplicity of the proposed scheme, along with the fact that there is a much smaller THz-wave decay in nonlinear crystal, makes it a good candidate for the practical realization of efficient THz generation. The use of a GaAs waveguide with an oxidized AlAs layer is proposed for enhanced THz-wave SEDFG in the vicinity of the GaAs polariton resonance at 8THz.

Bandwidth tunable THz wave generation in large-area periodically poled lithium niobate

A new scheme of optical rectification (OR) of femtosecond laser pulses in a periodically poled lithium niobate (PPLN) crystal, which generates high energy and bandwidth tunable multicycle THz pulses, is proposed and demonstrated. We show that the number of the oscillation cycles of the THz electric field and therefore bandwidth of generated THz spectrum can easily and smoothly be tuned from a few tens of GHz to a few THz by changing the pump optical spot size on PPLN crystal. The minimal bandwidth is 17 GHz that is smallest ever of reported in scheme of THz generation by OR at room temperature. Similar to the case of Cherenkov-type OR in single-domain LiNbO₃, the spectrum of THz generation extends from 0.1 THz to 3 THz when laser beam is focused to a size close to half-period of PPLN structure. The energy spectral density of narrowband THz generation is almost independent of the bandwidth and is typically 220 nJ/THz for ~1 W pump power at 1 kHz repetition rate.

Theoretical analysis of laterally emitting terahertz-wave generation by difference-frequency generation in channel waveguides

Theoretical analysis of top- and side-emitting terahertz-wave generation by difference frequency generation in LiNbO/sub 3/ channel waveguides with domain-inverted grating is presented. The analysis is based on Greens function and plane-wave expansion approaches. Mathematical expressions and numerical data for the conversion efficiency and the directionality were obtained. The results are consistent with those using two-dimensional model, but are more accurate and apply for various realistic device configurations. It was shown that higher efficiencies can be accomplished with narrow channel guides, although the radiation is highly divergent.

A new method of terahertz difference frequency generation using periodically poled waveguide

Summary form only given. Several workers have demonstrated second harmonic generation in surface-emitting geometry. In this paper we suggest a method for THz region surface-emitting difference frequency generation (DFG) using periodically poled lithium niobate (PPLN) waveguide. It is shown that the PPLN waveguide is capable of emitting the difference frequency radiation in direction normal to the waveguide surface if the poling period is equal to L=/spl lambda//n/sub g/. Here /spl lambda/ is a wavelength of the emitted submillimeter radiation and n/sub g/ is a mean refractive index of the optical group velocity. In contrast to the optical waves, the PPLN structure with such period can be easily fabricated for THz waves.

Tunable narrowband terahertz generation in lithium niobate crystals using a binary phase mask

We present a simple scheme of narrowband terahertz (THz) generation by optical rectification in the lithium niobate crystal covered by a binary phase mask. It is shown that a single-domain crystal illumination by spatiotemporal shaped fs-laser pulses is equivalent to the formation of a transversally patterned, quasi-phase-matching structure. Decrease of the optical beam size on the mask leads to an increase of the THz-wave linewidth from 17 GHz to a few THz. The frequency of the generation was tuned in the range of 0.4-1.0 THz by building images of the mask in the crystal with various magnifications. Application results of the presented THz source for measuring transmittance of the superconducting NbN thin film in the 4.2-15 K temperature range are also presented.

Generation of narrowband THz radiation by surface-emitted optical rectification in periodically poled lithium niobate

We present theoretical and experimental results of THz-wave generation via optical rectification in periodically poled lithium niobate (PPLN) crystal. The rectified field in both frequency and time domain is calculated taking into account the divergence of the exciting optical beam. The dependence of the central frequency of the narrow-band THz-wave on of direction of field emission is investigated. The surface- emitting geometry (when the THz-wave is observed in direction perpendicular to the direction of the optical pulse propagation) reduces the damping of the THz-wave in the PPLN crystal considerably. In this geometry the measured central frequency is around 1 THz for our PPLN crystal with a poling period of (Lambda) equals 127 micrometers . The frequency is tuned by rotation of the crystal over a range of 0.65 - 1.1 THz. Typical bandwidths of 50 - 100 GHz were observed, depending on the collection angle and the number of periods involved.