Increasing bandwidth of Cherenkov-type terahertz emitters by free carrier generation.

We found experimentally that Cherenkov-type terahertz radiation produced by optical rectification of ultrashort laser pulses in LiNbO can experience strong spectral broadening in the regime of multiphoton laser absorption. The broadening is attributed to the terahertz emission from a surge current of the optically generated carriers. The effect can be used to improve the bandwidth of optical-to-terahertz converters based on optical rectification.

Generation of sub-MV/cm terahertz fields with large-size Cherenkov-type optical-to-terahertz converters.

It is known that a structure comprising a tens of microns thick, and ∼1 × 1 cm in size, layer of LiNbO attached to a Si prism can serve as an efficient Cherenkov-type converter of tens of microjoules-energy femtosecond laser pulses to broadband terahertz radiation. Here we experimentally demonstrate scaling up the terahertz energy and field strength by extending the width of the converter to several centimeters, expanding appropriately the pump laser beam, and increasing the pump pulse energy to hundreds of microjoules. In particular, chirped Ti:sapphire laser pulses of 450 fs duration and 600 µJ energy were converted to 1.

Scalable optical-to-terahertz converter with a prism-coupled plane-parallel lithium niobate plate.

A nonlinear optical converter of femtosecond laser pulses to terahertz radiation, which combines the tilted-pulse-front pumping and prism coupling techniques, is proposed and experimentally tested. In contrast to the conventional tilted-pulse-front scheme with a prism-shaped LiNbO crystal, the converter consists of a plane-parallel LiNbO plate sandwiched between two dielectric prisms. One prism is used to couple the pump beam into the LiNbO plate, another prism couples the generated terahertz radiation out of the plate.

Experimental observation of optically generated unipolar electromagnetic precursors.

It was recently predicted [Phys. Rev. A95(6), 063817 (2017) 10.

Noncollinear electro-optic sampling detection of terahertz pulses in a LiNbO crystal while avoiding the effect of intrinsic birefringence.

We propose and experimentally prove efficient high-resolution electro-optic sampling measurement of broadband terahertz waveforms in a LiNbO crystal in the configuration with the probe laser beam propagating along the optical axis of the crystal. This configuration allows one to avoid the detrimental effect of strong intrinsic birefringence of LiNbO without any additional optical elements. To achieve velocity matching of the terahertz wave and the probe beam, the terahertz wave is introduced into the crystal through a Si prism at the Cherenkov angle to the probe beam.

Light scattering at a temporal boundary in a Lorentz medium.

Temporal discontinuity in the permittivity of a nondispersive dielectric (temporal boundary) is a conventional model for considering electromagnetic phenomena in dynamic materials and metamaterials. Here we apply a more general model of a Lorentz medium with the rapidly changing density of its structural elements (oscillators) or their resonant frequency to determine the realms of applicability of the conventional temporal boundary model. We demonstrate the dependence of the continuity conditions and the energy relations at a temporal boundary on the nonstationarity mechanism and the ratio between the rate of nonstationarity and the characteristic frequencies in the system.

Electromagnetic wave transformation in an oscillator medium with growing density.

The model of a medium made of oscillators can describe various materials, including artificial ones made of meta-atoms. Here the transformation of an electromagnetic wave in such a medium with rapidly growing oscillator density is studied. The initial polariton is shown to be transformed not only into new polariton modes but also into natural oscillations of the existing and created oscillators.

Long propagating velocity-controlled Einstein's mirror for terahertz light conversion.

We show that Einstein's relativistic mirror with long (hundreds of µm) propagation distance and controllable propagation velocity can be implemented in the form of a dense free carrier front generated by multiphoton absorption of tilted-pulse-front femtosecond laser pulses in a dielectric or semiconductor medium. The velocity control is achieved by varying the pulse front tilt angle. Simulations demonstrate that such fronts can serve as efficient Doppler-type converters of terahertz pulses.

Efficient Cherenkov-type optical-to-terahertz converter with terahertz beam combining.

A nonlinear structure for efficient Cherenkov-type terahertz emission from ultrashort laser pulses is proposed, modeled, and experimentally demonstrated. The structure comprises a thin (a few tens of micrometers thick) layer of lithium niobate sandwiched between two silicon prisms. A focused-to-a-line laser pulse propagates in the layer and generates a Cherenkov wedge of terahertz radiation in the prisms.

Highly efficient Cherenkov-type terahertz generation by 2-μm wavelength ultrashort laser pulses in a prism-coupled LiNbO layer.

Terahertz generation by optical rectification of femtosecond laser pulses propagating in a 40-m thick LiNbO layer attached to an output Si prism has been experimentally investigated for different laser wavelengths from 800 to 2100 nm. For longer wavelengths, the saturation of the optical-to-terahertz conversion efficiency has been observed at higher laser pulse energies, thus enabling higher efficiencies. In particular, record high conversion efficiency of 1.

Tilted-pulse-front excitation of strong quasistatic precursors.

It was recently predicted [M. I. Bakunov, A.

Terahertz emission from gold nanorods irradiated by ultrashort laser pulses of different wavelengths.

Electron photoemission and ponderomotive acceleration by surface enhanced optical fields is considered as a plausible mechanism of terahertz radiation from metallic nanostructures under ultrafast laser excitation. To verify this mechanism, we studied experimentally terahertz emission from an array of gold nanorods illuminated by intense (~10-100 GW/cm) femtosecond pulses of different central wavelengths (600, 720, 800, and 1500 nm). We found for the first time that the order of the dependence of the terahertz fluence on the laser intensity is, unexpectedly, almost the same (~4.

Generalized analysis of terahertz generation by tilted-pulse-front excitation in a LiNbO prism.

It is commonly adopted that the widely used method of terahertz generation by tilted-pulse-front pumping of a lithium niobate crystal is optimized when both the tilted front of the pump optical pulse and the image of the diffraction grating, which introduces the tilt, are parallel to the exit surface of the crystal. We consider more general situations. In particular, we show that a deviation of the grating image plane from parallel to the crystal exit surface leads to deterioration of the terahertz beam quality but does not significantly affect the terahertz energy.

Nonellipsometric electro-optic sampling of terahertz waves in GaAs.

We report efficient high-resolution electro-optic sampling detection of broadband terahertz radiation by measuring direct intensity modulation of a femtosecond fiber laser beam induced by terahertz field in a 1-cm thick GaAs crystal.

[Efficacy and safety of collagen biomaterial local application in complex treatment of the diabetic foot syndrome (final results of the multicenter randomised study)].

Aim: To evaluate the efficacy and safety of collagen biomaterial application during the 4-week follow-up of patients with diabetic foot syndrome.

Material And Methods: 75 patients with diabetic foot (Wagner II (69.3%) and III (30.

Ultimate propulsion of wavelength-sized dielectric particles.

It is shown that a wavelength-sized dielectric particle can form a resonant state with the below-cutoff mode of a waveguide even for rather small values of the refractive index contrast between the particle and the background. The excitation of the resonant state creates the propelling force twice the momentum flow of the incident mode, and the particle is reliably trapped at the waveguide center. Since neither the particle nor the waveguide possesses individually any resonant properties, this operating regime can be applicable for the manipulation of particles of various materials, sizes, and shapes.

High-resolution terahertz spectroscopy with a single tunable frequency comb.

We report an improvement of three orders of magnitude in the spectral resolution of a recently proposed single-comb terahertz spectroscopy [Opt. Lett.39, 5669 (2014)].

Asymmetric Cherenkov radiation for improved terahertz generation in the Si-prism-coupled LiNbO3 layer.

We show that a Cherenkov emission of terahertz waves from a femtosecond optical pulse propagating in a LiNbO(3) crystal can be strongly spatially asymmetric with respect to the direction of the optical pulse propagation. We propose using this phenomenon to improve the spectral characteristics of one of the most efficient optical-to-terahertz converters: a thin LiNbO(3) layer attached to a Si-prism outcoupler.

Reflection and transmission of electromagnetic waves at a temporal boundary: comment.

Recently, Xiao et al. [Opt. Lett.

High-resolution broadband terahertz spectroscopy via electronic heterodyne detection of photonically generated terahertz frequency comb.

We report an alternative approach to the terahertz frequency-comb spectroscopy (TFCS) based on nonlinear mixing of a photonically generated terahertz pulse train with a continuous wave signal from an electronic synthesizer. A superlattice is used as a nonlinear mixer. Unlike the standard TFCS technique, this approach does not require a complex double-laser system but retains the advantages of TFCS-high spectral resolution and wide bandwidth.

Resonant optical gun.

We propose a concept of a structure-a resonant optical gun-to realize an efficient propulsion of dielectric microparticles by light forces. The structure is based on a waveguide in which a reversal of the electromagnetic momentum flow of the incident mode is realized by exciting a whispering gallery resonance in the microparticle. The propelling force can reach the value up to the theoretical maximum of twice the momentum flow of the initial wave.

Terahertz time-domain imaging of hidden defects in wooden artworks: application to a Russian icon painting.

We use terahertz time-domain imaging and time-of-flight tomography to examine subsurface defects in an early-19th-century Russian icon painting. In the transmission geometry, we distinguish between native wood and higher-absorption knotted wood. In reflection, we identify a void in the wood filled with foreign filler material.

Non-ellipsometric detection of terahertz radiation using heterodyne EO sampling in the Cherenkov velocity matching scheme.

A new electro-optic (EO) sampling scheme, which we refer to as "heterodyne EO sampling", for detection of pulsed terahertz (THz) waves is proposed and experimentally demonstrated. In this heterodyne EO sampling scheme, the intensity change in the optical probe pulse induced by a THz field in a nonlinear crystal is measured without any polarization optics. Applied in combination with the non-collinear Cherenkov velocity matching technique, this method allows one to detect pulsed THz waves efficiently and easily using a simpler optical setup as compared to the conventional ellipsometric EO sampling method.

Eddy currents and magnetic moments of planar rings of arbitrary width.

We analyze the response of a metallic ring to an applied time-harmonic and spatially uniform magnetic field. The self-consistent current distribution and magnetic moment are compared with that from an equivalent LR circuit model and the range of parameters (size, frequency and conductivity) for the model applicability is analyzed. The circuit model does not perform quantitatively well in the parameter region with high magnetic moment and low losses that is important for metamaterial design.

Terahertz difference-frequency generation by tilted amplitude front excitation.

To circumvent a velocity mismatch between optical pump and terahertz waves in electro-optic crystals, we propose to use dual-wavelength optical beams tilted with respect to their planes of equal amplitude. The tilt is achieved by transmission of a dual-wavelength laser beam through a diffraction grating placed on the crystal boundary. The proposed technique extends optical rectification of tilted-front femtosecond laser pulses to difference-frequency generation with longer (nanosecond) pulses.