Phase-matched four-wave mixing of isolated waveguide modes of high-intensity femtosecond pulses in a hollow photonic-crystal fiber

Hollow-core photonic-crystal fibers with a special dispersion profile are shown to allow phase-matched nonlinear optical interactions of isolated air-guided modes of high-intensity femtosecond laser pulses confined in the hollow fiber core. We present theoretical and experimental studies of the four-wave mixing of fundamental and second-harmonic pulses of a Cr:forsterite laser with an initial pulse duration of about 50 fs and an intensity on the order of 10¹⁴ W/cm² in waveguide modes of a hollow photonic-crystal fiber with a core diameter of about 13μm.     

Frequency conversion of subnanojoule femtosecond pulses in microstructure fibers

We experimentally demonstrate highly efficient multiplex frequency conversion of unamplified subnanojoule femtosecond pulses of Ti:sapphire laser radiation in fused silica microstructure fibers. Nonlinear optical spectral transformation of femtosecond pulses in an array of fused silica threadlike channels in these microstructure fibers results in the generation of isolated anti-Stokes spectral components within the wavelength range of 400–500 nm. An efficiency of frequency conversion of about 20% is achieved for 800-nm pump pulses with an energy of 0.7 nJ and a pulse duration of 70 fs.     

Pump-depleting four-wave mixing in supercontinuum-generating microstructure fibers

Four-wave mixing processes are shown to provide a high efficiency of non-linear optical frequency conversion and spectral transformation of ultrashort pulses in supercontinuum-generating microstructure fibers. Pump-depleting conversion of 800-nm radiation to the spectral range around 500 nm is achieved by phase-matching the parametric four-wave mixing process for 80-fs Ti:sapphire-laser pulses. The ways to use microstructure fibers for generating frequency-tunable radiation through four-wave mixing with the maximum efficiency of pump-field frequency conversion are discussed. PACS 42.65.Wi; 42.81.Qb     

Spectrally and temporally isolated Raman soliton features in microstructure fibers visualized by cross-correlation frequency-resolved optical gating

We study soliton phenomena accompanying the propagation of femtosecond Cr: forsterite-laser pulses through a microstructure fiber in the regime of efficient anti-Stokes frequency conversion. The dispersion of the fiber is designed in such a way as to minimize the group delay of the 1.25-m pump and the Stokes pulse within the length of soliton pulse compression in the regime of anomalous dispersion. Spectrally and temporally isolated solitonic features, resulting from soliton self-frequency shift, are detected at the output of such a microstructure fiber by means of cross-correlation frequency-resolved optical gating. PACS 42.65.Wi; 42.81.Qb     

Optical detection of attosecond ionization induced by a few-cycle laser field in a transparent dielectric material

We observe an optical signature induced by the modulation of electron density inside a bulk transparent solid that is quasiperiodically ionized on an attosecond time scale by electric field peaks of a focused few-cycle laser pulse. The emitted optical signal resulting from the attosecond ionization dynamics is spatially, temporally and spectrally isolated from concomitant optical responses through the use of a noncollinear pump-probe technique. The method holds promise for developing an attosecond metrology for bulk solids, in which, unlike in the established attosecond metrology of gases and surfaces, direct detection of charged particles is unfeasible.     

Characteristic Attributes of Multiple Cascaded Terahertz Metasurfaces with Magnetically Tunable Subwavelength Resonators

The characteristics of multiple cascaded metasurfaces comprising H‐shaped, magnetostatically controllable, subwavelength terahertz (THz) resonators made of InAs were systematically investigated, using a commercial solver based on the finite‐integration method, for the design of tunable filters. Three configurations of the biasing magnetostatic field were compared with each other as well as with the bias‐free configuration for filtering of normally incident linearly polarized plane waves. A close study of only one metasurface was found sufficient to broadly determine the sensitivity to the direction of the magnetostatic field and the bandwidth of a stopband. Furthermore, the effects of metasurface geometry and biasing field can be considered separately for initial design purposes. All features in the transmittance spectra for the bias‐free configuration that are related to the number of cascaded metasurfaces are also observed when the biasing magnetostatic field is applied. The coupling of adjacent metasurfaces in a cascade is strongly affected by the relative permittivity and the thickness of the spacer between the two metasurfaces. The spectral locations of stopbands scale with respect to the spacer's relative permittivity, the scaling rule being different from a classical one. The stopbands are redshifted when the spacer thickness is increased, with the redshift dependent on the polarization of the incident plane wave. Inter‐metasurface coupling and inter‐resonator coupling on the same metasurface affect the spectral location of a stopband in opposite ways. On‐off type switching can be obtained by changing the orientation of magnetostatic field. The elucidated characteristics are expected to be important for not only filters but also other tunable THz devices.     

Ultrabroadband, coherent light source based on self-channeling of few-cycle pulses in helium

Self-channeling of few-cycle laser pulses in helium at high pressure generates coherent light supercontinua spanning the range of 270-1000 nm, with the highest efficiency demonstrated to date. Our results open the door to the synthesis of powerful light waveforms shaped within the carrier field oscillation cycle and hold promise for the generation of pulses at the single-cycle limit.     

Relativistic Nonlinear Optical Phenomena in the Field of Subterawatt Laser Pulses

Experiments on the generation of high optical harmonics demonstrating that relativistic regimes of interaction of radiation with matter can be implemented in the field of mid-infrared laser pulses with a peak power of 0.3 TW have been reported. The observation of relativistic phenomena at such extraordinary low peak powers of the laser field becomes possible because of the formation of a high-quality space-time mode of the laser field with an exactly specified polarization state. Such a field structure ensures a high intensity of radiation in the focus of the beam and the effective acceleration of electrons by a low-frequency electromagnetic field of a high-contrast laser pulse with an exactly specified polarization at the extremely sharp vacuum-solid interface.