On parametric x-ray radiation

The spectral-angular and angular distributions of parametric x-ray radiation generated in the vicinity of the maximum at small angles to the direction of charged-particle motion are studied under the conditions of Laue and Bragg diffraction geometries. Detector parameters at which the dynamical maxima of parametric x-ray radiation can be observed against the background of transition radiation are indicated. The spectral-angular and angular distributions of parametric x-ray radiation are analyzed for the case of backward geometry. The results of the present theoretical calculations are found to be in good agreement with experimental data.     

Borrmann effect in parametric X-ray radiation

The suppression of the photoabsorption in parametric X-ray radiation by relativistic particles in a crystal is predicted. This dynamical effect caused by the contribution of emitting particle transition radiation to parametric X-ray radiation yield is possible in a crystal with a finite thickness only.     

Focusing of parametric X-ray radiation

The possibility of focusing parametric x rays (PXRs) without x-ray optics has been shown. Parametric x rays are emitted by channeled relativistic charged particles moving along a bent crystal. Parametric x rays emitted from the entire length of the bent crystal are focused at two points on the bending axis of the crystal. The Huygens picture of formation of focused PXRs is given. The width of the spectral peak of this radiation has been estimated. Certain properties of focused PXRs are estimated for typical experimental conditions, and possibilities for the application of focused PXRs are discussed. An experiment for the observation of focused PXRs has been proposed. It has been shown that focused PXRs can be observed for a proton energy of several tens of GeVs or higher.     

Tunable source of parametric X-ray radiation

A new design of an X-ray source with a radiation energy controllably varying from 2 to 130 keV and a radiation intensity exceeding >10^?5 photon/e^?/sr is suggested. The source is based on parametric X-ray radiation (PXR) generated by moderately relativistic electrons in an original geometry. In this geometry, kinematic bunching of PXR reflections is used, which makes it possible to increase the intensity of the source. A model to calculate the PXR characteristics that takes into account absorption of radiation, beam parameters, multiple scattering, and energy loss is developed. The monochromaticity of the source lines is estimated.     

Quantum effects for parametric X-ray radiation during channeling: Theory and first experimental observation

The theory of X-ray radiation from relativistic channeled electrons at the Bragg angles—parametric X-ray radiation (PXR) during channeling (PXRC)—is developed while accounting for two quantum effects: the initial population of bound states of transverse motion and the transverse “form-factor” of channeled electrons. An experiment was conducted using a 255 MeV electron beam from a linac at the SAGA Light Source. We have identified a difference in the angular distributions of PXR and PXRC and obtained a fairly good agreement between the theoretical and experimental results.     

Kinematic grouping of reflections of parametric X-ray radiation

A model of parametric X-ray radiation (PXR) of moderately relativistic particles is developed within the kinematic theory taking into account radiation absorption. The model allows calculation of the PXR reflection intensity in a given direction taking into account diffraction of virtual photons on a set of “incidental” low-index crystallographic planes. It is shown that when a charged particle moves along certain crystallographic directions, the effect of kinematic grouping of reflections, at which lines of parametric radiation from various planes practically coincide, is observed.     

High-power terahertz radiation from surface-emitted THz-wave parametric oscillator

We report a pulsed surface-emitted THz-wave parametric oscillator based on two MgO:LiNbO3 crystals pumped by a multi-longitudinal mode Q-switched Nd:YAG laser.Through varying the phase matching angle,the tunable THzwave output from 0.79 THz to 2.84 THz is realized.The maximum THz-wave output was 193.2 nJ/pulse at 1.84 THz as the pump power density was 212.5 MW/cm 2,corresponding to the energy conversion efficiency of 2.42×10-6 and the photon conversion efficiency of about 0.037%.When the pump power density changed from 123 MW/cm 2 to 148 MW/cm 2 and 164 MW/cm 2,the maximum output of the THz-wave moved to the high frequency band.We give a reasonable explanation for this phenomenon.     

Large polarization squeezing in non-degenerate parametric amplification of coherent radiation

Polarization squeezing is shown to occur in non-degenerate parametric amplification of coherent light and the degree of squeezing at interaction time T can be as large as 1 ? e^-2T. This gives 86.4% polarization squeezing for T = 1 and 98.2% for T = 2. One simple case when this occurs is on taking initially plane polarized light having equal amplitudes in the two modes that finally has equal intensities of two circular polarizations. This suggest the experimental settings of parameters to achieve this extent of polarization squeezing in coherent light.     

Eye-safe radiation source based on an optical parametric oscillator

We have studied an optical parametric oscillator (OPO) with an unstable telescopic cavity, placed inside the cavity of an actively Q-switched multimode Nd³⁺:KGW pump laser. We used a KTP crystal as the nonlinear medium for the OPO. We have compared the emission characteristics of OPOs with unstable telescopic and planar cavities. We have established that compared with the planar cavity, the unstable cavity reduces the OPO beam divergence and improves the spatial distribution of the radiation energy in the far wave zone. Based on our investigations, we have designed a compact eye-safe (λ = 1.578 μm) laser source with natural cooling, emitting (for electrical pumping energy 7.3 J) pulses with pulse energy 22 mJ and pulse duration 6 nsec. The FWHM beam divergence for the source is no greater than 3.5 mrad. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 2, pp. 254–259, March–April, 2006.     

Investigation of the production mechanism of parametric X-ray radiation

Parametric x-ray Radiation (PXR) in the energy range from 5 to 20 keV produced by 855 MeV electrons using a single Si crystal was studied at the Mainz Microtron MAMI. The production mechanism was investigated by measuring the angular distributions for several orders of PXR and its dependence from the crystal thickness. A silicon crystal shaped in steps with thicknesses ranging from 100 μm to 600 μm was used. The experiments were carried out using LN₂-cooled PIN photodiodes with an energy resolution of 950 eV. The absolute photon flux was measured for the (111)-, (220)- and (224)-reflection planes and are in very good agreement with a kinematical model. The angular distributions are well reproduced by an ansatz, which incoherently adds to the PXR a portion of diffracted bremsstrahlung and diffracted transition radiation.     

Detection of parametric X-ray radiation from moderately relativistic protons in crystals

The spectral maxima of parametric x-ray radiation that are emitted by moderately relativistic protons interacting with various crystals have been detected. The spectral positions of these maxima depend on the crystal orientation angle and they are consistent with the theoretical values. The measurements were carried out with silicon and graphite crystals on the 5-GeV proton beam extracted from the Nuclotron at the Laboratory of High Energies, JINR.     

The parametric doppler effect upon oblique radiation incidence in quartz glass

Frequency and reflection angle of probe radiation from a refractive-index inhomogeneity induced by an intense pumping pulse in quartz glass and moving with a relativistic velocity are calculated. Conditions under which the normal component of the wave vector of the reflected wave is directed to the opposite or to the same direction as the same component for the incident wave are determined. Comparison with the case of radiation reflection from a relativistic mirror in vacuum is performed. Conditions of the appearance of the Vavilov-Cherenkov radiation from a relativistic refractive-index inhomogeneity induced in the medium by an intense laser pulse are discussed.     

Phase effects during parametric conversion in layer structures

Coupled-wave equations that describe the parametric amplification and generation of a sum frequency in a three-layer structure are studied theoretically in the constant-intensity approximation taking into account the inverse effect of the excited wave on the phase of the pumping wave. For this purpose, the approximation of constant intensity of the fundamental radiation is applied not to the layer structure as a whole but to each separately taken layer. In this case, the complex amplitudes of the interacting waves at the output of each layer are the input values of the corresponding complex amplitudes for the next layer. Analytical expressions obtained in the constant-intensity approximation for the efficiency of conversion to the sum frequency were analyzed numerically for different parameters of the problem. The efficiency of parametric amplification at a high frequency was found to depend on the intensity of the signal wave at a low frequency.     

Critical effects in photon correlation for parametric generation

An exact quantum theory of nondegenerate parametric generation in a cavity is developed with allowance for quantum noise of arbitrary intensity. Critical behavior of the second-order correlation functions which describe photon correlation effects is found in the threshold region in the bistable generation regime. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 7, 502–506 (10 April 1996)     

Diffractive effects in singly resonant continuous-wave parametric oscillators

I /k_P=0.33and0.50. It is found that the ratio of pump depletion to maximal depletion as a function of the ratio of pump power to threshold power agrees with the plane-wave prediction to within 5%, for a wide range of focusing conditions. The observed trends are explained as resulting from intensity- and phase-dependent mechanisms. Received: 19 January 1998/Revised version: 13 March 1998     

Optical detection of terahertz radiation by using nonlinear parametric upconversion

We describe and demonstrate sensitive room-temperature detection of terahertz (THz) radiation by nonlinearly upconverting terahertz to the near-infrared regime, relying on telecommications components. THz radiation at 700 GHz is mixed with pump light at 1550 nm in a bulk GaAs crystal to generate an idler wave at 1555.6 nm, which is separated and detected by using a commercial p-i-n diode. The THz detector operates at room temperature and has an intrinsic THz-to-optical photon conversion efficiency of 0.001%.     

Pump-enhanced optical parametric oscillator generating continuous wave tunable terahertz radiation

We demonstrate a tunable cw terahertz (THz) parametric oscillator based on periodically poled MgO-doped lithium niobate, directly converting the 1030 nm pump wave into the THz regime. The tunability ranges from 1.2 to 2.9 THz at output power levels between 0.3 and 3.9 μW. To overcome the high pump threshold caused by THz absorption in the nonlinear crystal, we employ an enhancement cavity with a finesse of 500 at the pump wavelength. The intracavity pump threshold at 1.4 THz is measured to be 350 W for a crystal length of 2.5 cm.     

The parametric Doppler effect upon oblique incidence of probe radiation

Transformation of frequency and intensity of probe radiation obliquely incident on a moving inhomogeneity induced by an intense laser radiation in a nonlinear medium is analyzed. The dependences of frequency and intensity of radiation reflected from the inhomogeneity have been found for a variant of frequency dispersion corresponding to collision-free plasma. These dependences possess singularities under conditions corresponding to the Vavilov-Cherenkov effect. The initially weak noise components of probe radiation increase in the reflected radiation up to a considerable value.     

Lagrangian approach for analysis of radiation pressure induced parametric instability in micro-resonators

We introduce a new scheme for determining radiation pressure coupling to microresonator devices. It is shown that for the input pump powers there exists a threshold value for instability behavior. Radiation pressure can couple mechanical modes of a cavity to it's optical modes, leading to parametric oscillation instability. Here we present an approximate analysis of such nonlinear effect with Hamilton's least action principle. Our Lagrangian includes no interaction term, but interaction between optical and mechanical modes has been taken into account through integration limit.     

Amplification of radiation due to the four-photon parametric process in potassium vapor

The results of the experimental investigation of the radiation amplification due to the stimulated four-photon parametric process in potassium vapor are reported. Two cells with active medium (an oscillatoramplifier system) are used in the experiments. The dominating role of IR radiation 6S → 5P (3.66 µm) in the amplification of violet radiation 5P → 4S (0.4 µm) generated due to the parametric process at two-photon excitation 4S → 6S is shown. This role is conditioned by optically pumped stimulated emission (OPSE). Optical conditions for the amplification of a seed radiation are determined in accordance with their intensity and the buffer gas (He) pressure. The problem of the image conversion from the IR to a visible one with quantum efficiency above 100% is discussed.