Dusan Lorenc

Highly birefringent silicate glass photonic-crystal fiber with polarization-controlled frequency-shifted output: A promising fiber light source for nonlinear Raman microspectroscopy.

A highly birefringent silicate glass photonic-crystal fiber (PCF) is employed for polarization-controlled nonlinear-optical frequency conversion of femtosecond Cr: forsterite laser pulses with a central wavelength of 1.24 mum to the 530--720-nm wavelength range through soliton dispersion-wave emission. The fiber exhibits a modal birefringence of 1.2.10(-3) at the wavelength of 1.24 mum due to a strong form anisotropy of its core, allowing polarization switching of the central wavelength of its blue-shifted output by 75 nm. Polarization properties and the beam quality of the blue-shifted PCF output are shown to be ideally suited for polarization-sensitive nonlinear Raman microspectroscopy.

TERAHERTZ TIME-DOMAIN SPECTROSCOPY OF SELECTED LAYERED SILICATES

Micaceous layer silicate clay minerals are attractive materials for applications involving non-linear optics because of their low cost and ability to form well ordered, platy aggregates, but such applications require precise knowledge of the dielectric behavior of the clay. The purpose of the present study was to use Terahertz time-domain spectroscopy (THz-TDS) to determine the dielectric properties of certain cleavable layered clay minerals, including muscovite, vermiculite, phlogopite, and biotite. The samples were characterized by X-ray diffraction and Fourier transform infrared spectroscopy as well as chemical analysis by Energy dispersive X-ray spectroscopy. The THz frequency window investigated was the far-infrared region of 3.3 to ∼40.0 cm−1 corresponding to 0.1 and 1.2 THz, respectively. The samples were selected so that the hydrated form of the interlayer cation, e.g. Mg2+ present in the interlayer gallery of vermiculite, could be compared to species such as phlogopite, biotite, and muscovite with the dehydrated form of interlayer cations such as K+ or Na+. The frequency-dependent complex index of refraction of these natural materials was determined to vary between 2.50 and 2.80. The presence of water in the interlayer space of vermiculite was reflected in the detection of increased values of the absorption index in comparison with the muscovite, phlogopite, and biotite.

Linear and nonlinear properties of multicomponent glass photonic crystal fibers

Processes resulting in supercontinuum generation in multicomponent glass photonic crystal fibers are reviewed in this paper. Multicomponent glass photonic crystal fibers are shown to have a broad transmission range, extending up to 4.5 μm in selected cases. Pumping with a 1240-nm femtosecond pulse at very low sub-nJ energies resulted in soliton formation and dispersive wave generation in a multicomponent PCF sample having a double-core square-lattice structure. These processes were described using a phase-matching model derived from the simulated dispersive properties of the fiber. Third-harmonic generation was observed in the radiation modes of a different cobweb sample with the simultaneous formation of a soliton in the NIR.

Hollow-fiber compression of 6 mJ pulses from a continuous-wave diode-pumped single-stage Yb,Na:CaF 2 chirped pulse amplifier

Here, 200 fs 6 mJ pulses from a cw diode-pumped Yb,Na:CaF(2) amplifier are spectrally broadened in an Ar- or Ne-filled hollow-core fiber and recompressed to 20 fs (Ar) and 35 fs (Ne) using a prism pair. The results of spectral broadening and phase measurement are in excellent agreement with numerical modeling based on the generalized nonlinear Schrödinger equation. The longer laser wavelength of 1030 nm permits favorable energy scaling for the hollow-fiber technique compared to ultrafast amplifiers operating at 800 nm.

CEP-stable tunable THz-emission originating from laser-waveform-controlled sub-cycle plasma-electron bursts

We study THz-emission from a plasma driven by an incommensurate-frequency two-colour laser field. A semi-classical transient electron current model is derived from a fully quantum-mechanical description of the emission process in terms of sub-cycle field-ionization followed by continuum-continuum electron transitions. For the experiment, a CEP-locked laser and a near-degenerate optical parametric amplifier are used to produce two-colour pulses that consist of the fundamental and its near-half frequency. By choosing two incommensurate frequencies, the frequency of the CEP-stable THz-emission can be continuously tuned into the mid-IR range. This measured frequency dependence of the THz-emission is found to be consistent with the semi-classical transient electron current model, similar to the Brunel mechanism of harmonic generation.

Generation of high fidelity 62-fs, 7-nJ pulses at 1035 nm from a net normal-dispersion Yb-fiber laser with anomalous dispersion higher-order-mode fiber

We present a mode-locked 24 MHz Yb-doped fiber oscillator with a higher-order mode fiber for dispersion compensation. The oscillator operates in the net normal dispersion regime and generates clean 6 nJ pulses that can be dechirped down to 150 fs.

Nonlinear microstructured fibers for supercontinuum generation

Microstructured fibers with small core are successfully used as a medium for supercontinuum generation. Since light can be confined in a small core a high density of energy in the fiber is obtained and stimulate nonlinear effects. Use of lead multicomponent glass allows increasing nonlinear refractive index in the fiber and shape dispersion properties of the fiber. In this case effective broadening of the spectrum can be obtained with less then 1 m of the fiber. In this paper we present properties of photonic crystal fibers optimized for supercontinuum generation.

Intense Cr:forsterite-laser-based supercontinuum source

Supercontinuum pulses covering the range from 1100 to 1700 nm with energies >1.0  mJ and excellent beam quality are generated via nonlinear spectral broadening of Cr:forsterite (1240 nm, 110 fs) pulses in pressurized molecular nitrogen. Our spectra, which extend over more than half an octave, offer an attractive alternative to intense few-cycle pulse synthesis in the 1-2 μm range and lend themselves as an important add-on to Cr:forsterite laser technologies.

Fluorescence Dynamics of Coumarin C522 as a Function of Micelle Confinement along with Cyclodextrin Supramolecular Complex Formation

Our aim is to doubly confine a molecule of coumarin C522 in a host-guest supramolecular complex with β-cyclodextrin in a reverse sodium dioctyl sulfosuccinate (AOT) micelle using nonpolar n-heptane and polar water solvents. Varying the volumes of coumarin C522 and β-cyclodextrin dissolved in water allows us to control the water-pool diameters of the reverse micelle in n-heptane with values of w=3, 5, 10, 20, and 40, where w is the ratio of water concentration to AOT concentration in n-heptane. To study the fluorescence dynamics of coumarin C522, the spectral steady-state and time-resolved dependences are compared for the two systems coumarin C522(water)/AOT(n-heptane), denoted C522/micelle, and coumarin C522/β-cyclodextrin(water)/AOT(n-heptane), referred to as C522/CD/micelle. The formation of the supramolecular host-guest complex CD-C522 is indicated by a blue shift, but in the micelle, the shift is red. However, the values of the fluorescence maxima at 520 and 515 nm are still way below the value of 535 nm representing bulk water. The interpretation of the red shift is based on two complementary processes. The first one is the confinement of CD and C522 by the micelle water pool and the second is the perturbation of the micelle by CD and C522, resulting in an increase of the water polarity. The fluorescence spectra of the C522/micelle and C522/CD/micelle systems have maxima and shoulders. The shoulder intensities at 440 nm, representing the C522 at n-heptane/AOT interface, decrease as the w values decrease. This intensity shift suggests that the small micelle provides a stronger confinement, and the presence of CD shifts the equilibrium from n-heptane towards the water pool even more. The fluorescence emission maxima of the C522/micelle and C522/CD/micelle systems for all w values clearly differentiate two trends for w=3-5, and w=10-40, suggesting different interaction in the small and large micelles. Moreover, these fluorescence maxima result in 7 and 13 nm differences for w=3 and w=5, respectively, and provide the spectral evidence to differentiate the C522 confinement in the C522/micelle and C522/CD/micelle systems as an effect of the CD molecule, which might be interpreted as a double confinement of C522 in CD within the micelle. The ultrafast decay in the case of w=3 ranges from 9.5 to 16 ps, with an average of 12.6 ps, in the case of the C522/micelle system. For C522/CD/micelle, the ultrafast decay at w=3 ranges from 9 to 14.5 ps, with an average of 11.8 ps. Increasing w values (from 10 to 40) result in a decrease of the ultrafast decay values in both cases to an average value of about 6.5 ps. The ultrafast decays of 12.6 and 11.8 ps for C522/micelle and C522/CD/micelle, respectively, are in the agreement with the observed red shift, supporting a double confinement in the C522/CD/micelle(w=3) system. The dynamics in the small and large micelles clearly show two different trends. Two slopes in the data are observed for w values of 3-5 and 10-40 in the steady-state and time-resolved data. The average ultrafast lifetimes are determined to be 12.6 and 6.5 ps for the small (w=3) and the large (w=40) micelles, respectively. To interpret the experimental solvation dynamics, a simplified model is proposed, and although the model involves a number of parameters, it satisfactory fits the dynamics and provides the gradient of permittivity in the ideal micelle for free water located in the centre (60-80) and for bound water (25-60). An attempt to map the fluorescence dynamics of the doubly confined C522/CD/micelle system is presented for the first time.

Tunability of THz Emission Originating from Sub-Cycle Electron Bursts in a Laser Induced Plasma

Since the demonstration a decade earlier by Cook and Hochstrasser [1] of intense THz emission from air ionized with a two-color (ω + 2ω) laser field, this phenomenon remains in the focus of attention both as a source of intense single-cycle THz pulses and because the underlying mechanism(s) of THz emission are hotly disputed. Experimental evidence points toward both the four-wave mixing mechanism based on stationary or nonstationary third-order susceptibilities [2] and toward the model of micro electric currents in which electrons experience directional drift in an “AC bias” of the broken-symmetry electric field [3]. To date, only commensurate ω + 2ω schemes were in use, which corresponds to a degenerate case and thus complicated the delineation of different emission mechanisms. In this contribution we show that by fixing the carrier envelope phases and detuning the frequencies of two optical fields the timing of sub-cycle optical field ionization in gas can be controlled yielding continuous frequency tuning of the emitted THz radiation. Our results suggest that this particular mechanism of THz emission is closely related to two prominent phenomena, ATI and HHG generation that are also based on quasi-periodic sub-cycle ionization followed by electron acceleration in the E-field.