A new scheme for generating high-energy terahertz (THz) pulses by optical rectification of tilted pulse front (TPF) femtosecond laser pulses in ZnTe crystal is proposed and analyzed. The TPF laser pulses are originated due to propagation through a multistep phase mask (MSPM) attached to the entrance surface of the nonlinear crystal. Similar to the case of contacting optical grating the necessity of the imaging optics is avoided. In addition, introduction of large amounts of angular dispersion is also eliminated. The operation principle is based on the fact that the MSPM splits a single input beam into many smaller time-delayed “beamlets”, which together form a discretely TPF in the nonlinear crystal. The dimensions of the mask’s steps required for high-energy THz-pulse generation in ZnTe and widely used lithium niobate (LN) crystals are calculated. The optimal number of steps is estimated taking into account individual beamlet’s spatial broadening and problems related to the mask fabrication. The THz field in no pump depletion approximation is analytically calculated using radiating antenna model. The analysis shows that application of ZnTe crystal allows obtaining higher THz-pulse energy than that of LN crystal, especially when long-wavelength pump sources are used. The proposed method is a promising way to develop high-energy, monolithic, and alignment-free THzpulse source.
Millimeter wave passive radiometric imager based on frequency scanning antenna is suggested. As a frequency scanning antenna, the periodically perturbed dielectric image waveguide is considered. In frequency range 30÷38 GHz, the scan of the main lob over 40° has been realized. Parallel multibeam forming is realized by means of the set of narrow bandwidth resonant modulators, each of them controlled by the own modulating frequency.
We present theoretical and experimental results of surface-emitted THz-wave difference frequency generation (DFG) in 2 dimensional (D) periodically poled lithium niobate (PPLN) crystal. The two orthogonal periodic structures compensate the phase mismatch in two mutually perpendicular directions of the optical and THz-wave propagation. The tunable 1.5 - 1.8 THz wave generation with impulse power of 0.1 mW and repetition rate 1 MHz is obtained. The opportunity of the power enhancement using an interference of THz fields generated by the both forward and backward propagating optical waves is investigated. The power and radiation pattern of generated of THz-wave are calculated in far-field approximation. Analysis of THz-wave DFG in 2D PPLN and its correlation with experimental data is presented.
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.
This paper presents a new method for surface-emitted difference frequency generation (DFG) in planar optical waveguides based on non-ferroelectric materials. The main thrust of this paper is based on the application of a slotted grating to the surface of a waveguide cover. The grating has equally spaced slots at distances equivalent tot eh wavelength of anon-linear polarization wave. DFG power emitted in the direction normal to the surface of the step- index planar waveguide is calculated. It is shown that the efficiency of frequency conversion for a surface emitting geometry is greater than that in a common collinear geometry case. This is especially true for the high-absorption wavelength region of nonlinear materials. According to our estimation, surface-emitted DFG will enable the design of compact solid-state THz-wave sources with a few mW output power.
It is shown that the periodically poled lithium niobate slab waveguide with specific period of poling, (lambda) /n<SUB>g</SUB> (where (lambda) is a wavelength of emitted THz-wave, n<SUB>g</SUB> is a refractive index of group velocity of light) is capable to emit the difference frequency generation (DFG) in the direction normal to the surface of a waveguide. The general expression for power of DFG in doubly resonant cavity (resonance both at a pumping and at difference frequencies) has been obtained. According to estimation the power of DFG is 0.33 mW for pumping power 1 W. Thus the double resonant cavity-enhanced surface-emitted DFG enables to design compact solid-state source of THz waves with the output power sufficient for the practical applications.
The far-infrared Cherenkov-type difference frequency generation (CDFG) in a planar optical waveguide is analyzed theoretically. The general expression for the CDFG angular distribution is given. The novel doubly resonant cavity is proposed. The CDFG efficiency enhancement due to resonant both at the optical and FIR frequencies is estimated to be a factor a few thousands.