Ultrabroad Terahertz Spectrum Generation from an Air-Based Filament Plasma.

We have solved the long-standing problem of the mechanism of terahertz (THz) generation by a two-color filament in air and found that both neutrals and plasma contribute to the radiation. We reveal that the contribution from neutrals by four-wave mixing is much weaker and higher in frequency than the distinctive plasma lower-frequency contribution. The former is in the forward direction while the latter is in a cone and reveals an abrupt down-shift to the plasma frequency. Ring-shaped spatial distributions of the THz radiation are shown to be of universal nature and they occur in both collimated and focusing propagation geometries. Experimental measurements of the frequency-angular spectrum generated by 130-fs laser pulses agree with numerical simulations based on a unidirectional pulse propagation model.

Transformation of terahertz spectra emitted from dual-frequency femtosecond pulse interaction in gases.

We demonstrate that the two basic physical mechanisms of terahertz (THz) generation in a femtosecond filament, namely, the free electron photocurrent and the nonlinear polarization of neutrals, can be identified through the spectral analysis of THz radiation. The contribution from the photocurrent peaks at the units of THz, while the neutrals yield the peak at the tens of THz. We suggest the practical implementation of such spectral analysis by varying the initial transform-limited laser pulse duration.

Fibonacci-like photonic structure for femtosecond pulse compression.

The compression of femtosecond laser pulses by linear quasiperiodic and periodic photonic multilayer structures is studied both experimentally and theoretically. We compare the compression performance of a Fibonacci and a periodic structure with similar total thickness and the same number of layers, and find the performance to be higher in the Fibonacci case, as predicted by numerical simulation. This compression enhancement takes place due to the larger group velocity dispersion at a defect resonance of the transmission spectrum of the Fibonacci structure. We demonstrate that the Fibonacci structure with the thickness of only 2.8 microm can compress a phase-modulated laser pulse by up to 30%. The possibility for compression of laser pulses with different characteristics in a single multilayer is explored. The operation of the compressor in the reflection regime has been modeled, and we show numerically that the reflected laser pulse is subjected to real compression: not only does its duration decrease but also its amplitude rises.

Enhancement of sum frequency generation near the photonic band gap edge under the quasiphase matching conditions.

We analyze theoretically and study experimentally the mechanisms of enhancement of the sum frequency and second harmonic generation in a finite one-dimensional photonic band gap structure with second order nonlinearity under Bragg diffraction conditions. It is shown that, near the photonic band gap edge, the efficiency of conversion in sum frequency and second harmonic generation processes can be significantly enhanced if two conditions are fulfilled simultaneously: grating assisted phase matching, or quasiphase matching, and an increase of the electromagnetic field density at the fundamental frequencies near the photonic band edges. The role of each mechanism is discussed.

Compression of femtosecond laser pulses in thin one-dimensional photonic crystals.

We demonstrate experimentally the effect of compression of femtosecond laser pulses in thin (a few micrometers) one-dimensional photonic crystal. We show that the compression effect is reasonably described by the linear dispersion properties of the photonic crystal itself and the quadratic dispersion approximation cannot be efficiently used for the description of interaction of the femtosecond laser pulses with the thin photonic crystal. For given parameters of the femtosecond pulse it leads to the existence of the optimal dimension of the photonic crystal from the point of view of the compression efficiency. Due to the wide spectral width of the femtosecond laser pulses the high-order dispersion effects play an important role in pulse propagation in photonic crystals and as a result the pulse compression occurs for both positive and negative signs of chirp of the incoming femtosecond pulses.

Enhancement of second-harmonic generation with femtosecond laser pulses near the photonic band edge for different polarizations of incident light.

We present experimental evidence of enhancement of second-harmonic generation as a result of an increase of the fundamental-field energy density within a multilayer structure near the photonic band edge.