Cherenkov-type Radiation Research Articles

Induced Electromagnetic Radiation From A Charged Cloud In A Plane Gravitational Wave

For the perturbative model of a plane gravitational wave on a flat background of Minkowski space-time, electromagnetic radiation from a charged cloud in the field of a gravitational wave, detected by a remote observer, was found. It is shown that the charge density in the cloud does not change, and the radiation is generated by currents induced by the gravitational wave. The angular distribution of the radiation is obtained. If the refractive index of the cloud medium is greater than unity, Cherenkov-type radiation is generated.

Frequency-tunable transient Cherenkov radiation from an inhomogeneous medium

We propose a scheme to generate coherent radiation with a tunable frequency spectrum based on the excitation of an inhomogeneous thin layer of a resonant medium by an ultrashort pulse. The Cherenkov-type radiation emitted in a transient stage, i.e., while the excitation wavefront is still propagating over the medium, is shown to contain additional frequency components, which are determined by the spatial frequency components of the medium's density distribution and the excitation geometry. We demonstrate that the spectral content of this transient radiation can be adjusted in wide limits by controlling the spatial density distribution of the resonant medium and the shape of the excitation wavefront. Our theoretical results pave the way towards simple and compact swept-frequency or frequency-tunable optical oscillators as well as sources of the frequency-modulated optical radiation.

Coherently enhanced microwave pulses from midinfrared-driven laser plasmas.

Ultrafast ionization of a gas medium driven by ultrashort midinfrared laser pulses provides a source of bright ultrabroadband radiation whose spectrum spans across the entire microwave band, reaching for the sub-gigahertz range. We combine multiple, mutually complementary detection techniques to provide an accurate polarization-resolved characterization of this broadband output as a function of the gas pressure. At low gas pressures, the lowest-frequency part of this output is found to exhibit a drastic enhancement as this field builds up its coherence, developing a well-resolved emission cone, dominated by a radial radiation energy flux. This behavior of the intensity, coherence, and polarization of the microwave output is shown to be consistent with Cherenkov-type radiation by ponderomotively driven plasma currents.

Cascaded DFG via quasi-phase matching with Cherenkov-type PPLN for highly efficient terahertz generation.

Terahertz (THz) generation in a periodically poled lithium niobate crystal via cascaded difference-frequency generation based on Cherenkov-type quasi-phase matching (QPM) is proposed. Photon conversion efficiency is evaluated based on a promising structure that combines QPM and Cherenkov phase-matching with reduced wave-vector mismatch. Cascading processes contribute to photon conversion efficiency, and THz radiation with maximum photon conversion efficiency of 1154.2% in a 14-order cascaded Stokes process was obtained. Comparing the processes with and without Cherenkov-type radiation, with a 50-MW pump, power was boosted nearly 1.9 times for the former case. These results provide an experimental approach to high-energy THz-wave generation.

Ultrafast nonlinear phenomena in fiber-optic photon-pair generation by ultrashort laser pulses

Effects of ultrafast nonlinear optics are shown to play a significant role in photon-pair generation by spontaneous four-wave mixing (FWM) of broadband photon packets in an optical fiber. When extended to this regime, the models of photon-pair generation can stay consistent with energy and photon-number conservation only when the frequency dependence of nonlinear wave coupling is included, giving rise to a shock-type waveform dynamics and a strong asymmetry in FWM sideband amplification. Resonant energy transfer from soliton transients to dispersive waves, accompanied by Cherenkov-type radiation, is shown to open a channel whereby FWM output photons are lost from entanglement generation pathways.

Cherenkov-type terahertz emission from ultrafast magnetization in a slab of magnetooptic material

We develop a theory of terahertz emission from a slab of a magnetooptic material traversed by a circularly polarized femtosecond laser pulse. The Cherenkov-type radiation is generated by a moving region of ultrafast magnetization optically induced in the slab via the inverse Faraday effect. A prism is used to couple the radiation from the slab to free space. Our theory accounts for transient effects at the slab boundaries and shows their significance for the interpretation of the terahertz waveform emitted from the slab. The theory predicts detectability of the radiation generated by a mJ-level laser pulse in a terbium gallium garnet slab covered with a sapphire prism.

Wide Spectrum Terahertz-Wave Generation From Nonlinear Waveguides

We have obtained efficient terahertz (THz)-wave generation from surface-emitting or waveguide propagation using Cherenkov-type radiation with nonlinear optical (NLO) crystals. This approach presented the following advantages: 1) many crystals can be used as THz-wave emitters; 2) it is not necessary to satisfy the phase-matching condition inside the crystal; and 3) THz-wave generation is not suppressed by the absorption in the crystal. We demonstrated that the THz-wave generation was enhanced 50× by suppressing the phase mismatch with a surfing configuration for bulk lithium niobate (LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) crystals. Using the prism-coupled Cherenkov phase-matching method with organic crystal 4-dimethylamino-N-metyl-4-stilbazolium tosylate, we produced tunable THz radiation within ~0.1-10 THz. There was no evidence of significant absorption in the crystal. To show the advantages of the waveguide emitter, we demonstrated THz generation using a ridge-type LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> waveguide with a thickness of 3.8 μm. We obtained THz-wave generation with an ideal temporal half-cycle pulse and a wide tuning range, which is applied to reflection tomography imaging.

Extremely frequency-widened terahertz wave generation using Cherenkov-type radiation

Terahertz (THz) wave generation based on nonlinear frequency conversion is promising way for realizing a tunable monochromatic bright THz-wave source. Such a development of efficient and wide tunable THz-wave source depends on discovery of novel brilliant nonlinear crystal. Important factors of a nonlinear crystal for THz-wave generation are, 1. High nonlinearity and 2. Good transparency at THz frequency region. Unfortunately, many nonlinear crystals have strong absorption at THz frequency region. The fact limits efficient and wide tunable THz-wave generation. Here, we show that Cherenkov radiation with waveguide structure is an effective strategy for achieving efficient and extremely wide tunable THz-wave source. We fabricated MgO-doped lithium niobate slab waveguide with 3.8 microm of thickness and demonstrated difference frequency generation of THz-wave generation with Cherenkov phase matching. Extremely frequency-widened THz-wave generation, from 0.1 to 7.2 THz, without no structural dips successfully obtained. The tuning frequency range of waveguided Cherenkov radiation source was extremely widened compare to that of injection seeded-Terahertz Parametric Generator. The tuning range obtained in this work for THz-wave generation using lithium niobate crystal was the widest value in our knowledge. The highest THz-wave energy obtained was about 3.2 pJ, and the energy conversion efficiency was about 10(-5) %. The method can be easily applied for many conventional nonlinear crystals, results in realizing simple, reasonable, compact, high efficient and ultra broad band THz-wave sources.

Dynamics of quasi-1D topological soliton in 2D strongly anisotropic crystal

We investigate a 2D strongly anisotropic crystal: 2D system of coupled linear chains of particles with strong intrachain and weak interchain interactions: a generalization of the Frenkel–Kontorova (FK) model giving better approximation to polymer crystals consisting of all moving chains. We study shape and dynamics of topological solitons located mainly on one chain of the crystal. It turns out that at weak interactions between chains there exist exact propagating soliton-like solutions. We have found that the upper boundary of their velocity spectrum is less than unity (the upper boundary of soliton velocity spectrum in the corresponding FK model). The reason of this new restriction is appearance of Cherenkov-type radiation emitted by the kinks moving at higher velocities. In the case of stronger interchain interactions the exact soliton-like solutions are absent. Kinks emit phonons at any velocity. This emission originates from resonances between the kink and phonon modes touched in the second or the third Brillouin zones and so this effect is a pure result of discreteness of the crystal. Deceleration of kink falls exponentially with its velocity (as in the FK model) at medium velocities and is nonzero constant at low velocities (contrary to the FK model where deceleration at low velocities is zero), so at all velocities there is no generally expected direct proportion between deceleration and velocity. Spatial pattern of phonons emitted proves to be in accordance with the conception that kink emits packets of corresponding resonant phonons in direction of their group velocities.

Cone emission from laser-pumped two-level atoms. II. Analytical model studies.

We establish a mechanism, which supports Cherenkov-type radiation, as a source of cone emission. This can be considered as an additional mechanism to the previously discussed four-wave mixing (4WM) and initial encoding and follow-up refraction effects. The radiation source is provided by spontaneous emission of the driven atoms at the frequencies of interest. Due to the medium dispersion, the pump field propagates faster than the phase velocity of the field that is generated at the Rabi sideband at lower frequency. Cherenkov radiation is then possible if the source has a long correlation length, which is indeed the case despite the random nature of spontaneous emission. Coherent superposition of the source for the lower-frequency field is in an off-axis direction given by the Cherenkov radiation condition. Differences between 4WM and Cherenkov-type models are consequently resolved.

Relaxation of kinks in the Ising chain with a transverse field interacting with a three-dimensional phonon field

The influence of phonon fluctuations on the dynamics of kink-like domain-wall excitations in the one-dimensional Ising model with a transverse field (s=1/2) was investigated. It was found that such non-linear excitations behave almost like classical Brownian particles. This is the consequence of the emission and absorption of acoustic phonons excited by domain walls propagating through the vibrating lattice. The effective friction and diffusion constants are also evaluated. It was established that the overall process has the character of Cherenkov-type radiation.

Cone emission from laser-pumped two-level atoms.

In spite of the fact that cone emission was first observed over a decade ago, there is still no generally accepted theory of the effect. Here we give an outline of a unified approach and point out a mechanism that supports Cherenkov-type radiation due to vacuum fluctuations as a possible source of cone emission. This can be considered as an additional mechanism to the previously discussed four-wave mixing, and initial encoding and follow-up refraction effects. We compare our treatment to previously considered models.