Symmetry Breaking and Strong Persistent Plasma Currents via Resonant Destabilization of Atoms.

The ionization rate of an atom in a strong optical field can be resonantly enhanced by the presence of long-living atomic levels (so-called Freeman resonances). This process is most prominent in the multiphoton ionization regime, meaning that the ionization event takes many optical cycles. Nevertheless, here, we show that these resonances can lead to rapid subcycle-scale plasma buildup at the resonant values of the intensity in the pump pulse. The fast buildup can break the cycle-to-cycle symmetry of the ionization process, resulting in the generation of persistent macroscopic plasma currents which remain after the end of the pulse. This, in turn, gives rise to a broadband radiation of unusual spectral structure, forming a comb from terahertz to visible. This radiation contains fingerprints of the attosecond electron dynamics in Rydberg states during ionization.

Raman gas self-organizing into deep nano-trap lattice.

Trapping or cooling molecules has rallied a long-standing effort for its impact in exploring new frontiers in physics and in finding new phase of matter for quantum technologies. Here we demonstrate a system for light-trapping molecules and stimulated Raman scattering based on optically self-nanostructured molecular hydrogen in hollow-core photonic crystal fibre. A lattice is formed by a periodic and ultra-deep potential caused by a spatially modulated Raman saturation, where Raman-active molecules are strongly localized in a one-dimensional array of nanometre-wide sections. Only these trapped molecules participate in stimulated Raman scattering, generating high-power forward and backward Stokes continuous-wave laser radiation in the Lamb-Dicke regime with sub-Doppler emission spectrum. The spectrum exhibits a central line with a sub-recoil linewidth as low as ∼14 kHz, more than five orders of magnitude narrower than conventional-Raman pressure-broadened linewidth, and sidebands comprising Mollow triplet, motional sidebands and four-wave mixing.

Nonlinearity of surface-plasmon polaritons in sub-wavelength metal nanowires.

We investigate the nonlinear propagation of surface plasmon polaritons guided on gold nanowires surrounded by silica glass. Based on the Lorentz reciprocity theorem, we derive a formula for the complex nonlinear susceptibility, and study its dependence on waveguide parameters and wavelength for the fundamental mode. Depending on these parameters both positive and negative signs of the real and imaginary parts of the nonlinear coefficient are predicted. This implies that nanowires exhibit the property of saturable absorption or optical limiting as well as positive and negative nonlinear phase shifts. The physical origin of this phenomenon is discussed.

Boosting terahertz generation in laser-field ionized gases using a sawtooth wave shape.

Broadband ultrashort terahertz (THz) pulses can be produced using plasma generation in a noble gas ionized by femtosecond two-color pulses. Here we demonstrate that, by using multiple-frequency laser pulses, one can obtain a waveform which optimizes the free electron trajectories in such a way that they acquire the largest drift velocity. This allows us to increase the THz conversion efficiency to 2%, an unprecedented performance for THz generation in gases. In addition to the analytical study of THz generation using a local current model, we perform comprehensive 3D simulations accounting for propagation effects which confirm this prediction. Our results show that THz conversion via tunnel ionization can be greatly improved with well-designed multicolor pulses.

Multi-meter fiber-delivery and pulse self-compression of milli-Joule femtosecond laser and fiber-aided laser-micromachining.

We report on damage-free fiber-guidance of milli-Joule energy-level and 600-femtosecond laser pulses into hypocycloid core-contour Kagome hollow-core photonic crystal fibers. Up to 10 meter-long fibers were used to successfully deliver Yb-laser pulses in robustly single-mode fashion. Different pulse propagation regimes were demonstrated by simply changing the fiber dispersion and gas. Self-compression to ~50 fs, and intensity-level nearing petawatt/cm(2) were achieved. Finally, free focusing-optics laser-micromachining was also demonstrated on different materials.

Carrier-envelope phase stabilization with sub-10 as residual timing jitter.

We demonstrate carrier-envelope phase (CEP) stabilization of a mode-locked Ti:sapphire oscillator with unprecedented timing jitter of eight attoseconds. The stabilization performance is obtained by a combination of two different stabilization approaches. In a first step the drift of the CEP is stabilized with a conventional feedback loop by means of controlling the oscillator pump power with an acousto-optic modulator (AOM). In a second step we utilize a recently developed feed-forward type stabilization scheme which has a much higher control bandwith. Here an acousto-optic frequency shifter (AOFS) produces the stabilized output in the first diffraction order. Moreover, we present numerical results on the optimization of the length of the photonic crystal fiber, which is used to generate an octave-spanning spectrum, in order to optimize the sensitivity in the f-to-2f interferometers.

Polarization gating and circularly-polarized high harmonic generation using plasmonic enhancement in metal nanostructures.

We investigate possibilities to utilize field enhancement by specifically designed metal nanostructures for the generation of single attosecond pulses using the polarization gating technique. We predict the generation of isolated 59-attosecond-long pulses using 15-fs pump pulses with only a 0.6 TW/cm2 intensity. Our simulations also indicate the possibility to generate previously inaccessible high-harmonics with circular polarization by using an ensemble of vertically and horizontally orientated bow-tie structures. In the numerical simulation we used an extended Lewenstein model, which includes the spatial inhomogeneity in the hot spots and collisions of electrons with the metal surface.

Two-octave supercontinuum generation in a water-filled photonic crystal fiber.

Supercontinuum generation in a water-filled photonic crystal fiber is reported. By only filling the central hollow core of this fiber with water, the fiber properties are changed such that the air cladding provides broadband guiding. Using a pump wavelength of 1200 nm and few-microjoule pump pulses, the generation of supercontinua with two-octave spectral coverage from 410 to 1640 nm is experimentally demonstrated. Numerical simulations confirm these results, revealing a transition from a soliton-induced mechanism to self-phase modulation dominated spectral broadening with increasing pump power. Compared to supercontinua generated in glass core photonic fibers, the liquid core supercontinua show a higher degree of coherence, and the larger mode field area and the higher damage threshold of the water core enable significantly higher pulse energies of the white light pulses, ranging up to 0.39microJ.

Soliton-effect pulse compression in the single-cycle regime in broadband dielectric-coated metallic hollow waveguides.

We study soliton-effect pulse compression in the single-cycle region in dielectric-coated metallic hollow waveguides filled with a noble gas exploiting the broad region of anomalous dispersion in these waveguides. We predict the compression of a 20-fs pulse to a FWHM duration of 1.7 fs with an energy of 6 muJ and study the physical factors determining the limitations on shortest pulses in the single-cycle regime.

Supercontinuum generation in aqueous colloids containing silver nanoparticles.

We numerically study low-threshold supercontinuum generation using the significant enhancement of nonlinearity in aqueous colloids with silver nanoparticles. We predict octave-spanning spectral broadening by femtosecond pulses with an intensity in the range of tens of GW/cm2. The strong frequency dependence of the effective nonlinear coefficient of the composite significantly influences the spectral broadening by self-phase modulation and leads to a large blueshift of the spectra.

High-power, high-coherence supercontinuum generation in dielectric-coated metallic hollow waveguides.

In this paper we theoretically study a novel approach for soliton-induced supercontinuum generation based on the application of metallic dielectric-coated hollow waveguides. The low loss of such waveguides permits the use of smaller diameters with enhanced dispersion control and enables the generation of two-octave-broad spectra with unprecedentedly high spectral peak power densities up to five orders of magnitude larger than in standard PCFs with high coherence. The predicted high coherence of the supercontinuum is related to the coherent seed components formed by the abruptly rising plasma density. We also predict that high-power supercontinua in the vacuum ultraviolet can be generated in such waveguides.

Dispersion control in ultrabroadband dielectric-coated metallic hollow waveguides.

We show that hollow dielectric-coated metallic waveguides exhibit ultrabroadband transmission and controlled anomalous dispersion in the visible, UV, and VUV range even at high gas pressures. Using the transfer-matrix method we predict that the losses can be significantly reduced in such waveguides, which allows the use of small radii in the range of 10-25 microm. The resulting significant enhancement of the waveguide contribution to dispersion facilitates phase-matching for nonlinear processes with higher efficiencies.

Steplike transmission of light through a metal-dielectric multilayer structure due to an intensity-dependent sign of the effective dielectric constant.

We numerically study light propagation through a specially designed nonlinear nanoscale metal-dielectric multilayer structure with a linear effective dielectric constant just below zero. The calculated dependence of the output intensity on the input intensity shows a steplike behavior. It rests upon an intensity-dependent change of the effective dielectric constant from negative (low-transmission state) to positive (high-transmission state) values, corresponding to a transition of the optical properties from metalliclike to dielectriclike. The study of the transient behavior of the structure demonstrates a switching time of around 1 ps.

Coherence of subsequent supercontinuum pulses generated in tapered fibers in the femtosecond regime.

We measure the degree of coherence of supercontinua generated in tapered fibers by subsequent fs pulses. By means of interference experiments we study its dependence on the input pulse duration and power. We also present numerical simulations that allow us to explain the experimental observations which show a decreasing degree of coherence with increasing input power. We attribute this loss of coherence to phase noise due to the cross-phase modulation by several solitons with randomly varying parameters due to quantum noise.

Frequency-selective self-trapping and supercontinuum generation in arrays of coupled nonlinear waveguides.

We study spatiotemporal dynamics of soliton-induced twooctave- broad supercontinuum generated by fs pulses in an array of coupled nonlinear waveguides. We show that after fission of the input pulse into several fundamental solitons, red and blue-shifted nonsolitonic radiation, as well as solitons with lower intensity, spread away in transverse direction, while the most intense spikes self-trap into spatiotemporal discrete solitons.

Focusing of scanning light beams below the diffraction limit without near-field spatial control using a saturable absorber and a negative-refraction material.

We show that a scanning light beam can be focused below the diffraction limit without the control of moving near-field elements using the combination of two main components: a light-controlled saturable absorber, which creates seed evanescent components from the beam, and a layer of negative-refraction material, which amplifies the evanescent waves. Focusing to spots with a FWHM in the range of 0.2-0.3 lambda is predicted. For slightly off-resonant input beams, an intensity-dependent phase shift leads to smaller spots.

Supercontinuum generation in planar rib waveguides enabled by anomalous dispersion.

We predict and numerically study supercontinuum generation extending over almost two octaves in planar rib waveguides, with anomalous dispersion at the input wavelength provided by the waveguide contribution. Such planar nonlinear waveguides generating broadband coherent radiation can be intergrated with other components to constitute a building block in intergrated optical circuits.

Subdiffraction focusing of scanning beams by a negative-refraction layer combined with a nonlinear layer.

We evaluate the possibility to focus scanning light beams below the diffraction limit by using the combination of a nonlinear material with a Kerr-type nonlinearity or two-photon absorption to create seed evanescent components of the beam and a negative-refraction material to enhance them. Superfocusing to spots with a FWHM in the range of 0.2 lambda is theoretically predicted both in the context of the effective-medium theory and by the direct numerical solution of Maxwell equations for an inhomogeneous pho-tonic crystal. The evolution of the transverse spectrum and the dependence of superfocusing on the parameters of the negative-refraction material are also studied. We show that the use of a Kerr-type nonlinear layer for the creation of seed evanescent components yields focused spots with a higher intensity compared with those obtained by the application of a saturable absorber.

Characterization of a nonlinear filter for the front-end of a high contrast double-CPA Ti:sapphire laser.

A nonlinear filter using rotation of the polarization ellipse in air is investigated. Scheme to enhance the temporal contrast is developed for a double-CPA multi-terawatt Ti:sapphire laser. It supports an energy level of millijoule and has a high efficiency. The method allows suppression of the ASE pedestal, pre- and post-pulses by 3 orders of magnitude and also steepens the pulse front. For the physical interpretation of the results, numerical simulation of the filtering is performed.

Superfocusing of light below the diffraction limit by photonic crystals with negative refraction.

Usual optical elements cannot focus a light beam to a spot with diameter smaller than half of the wavelength of the light; however overcoming this limit is of great importance in several applications in high-tech, such as optical lithography or magneto-optical date storage and numerous other fields of science and industry. Here we show that it is possible to focus light to spots below the diffraction limit (superfocusing) by the combination of two main elements: one which creates weak near-field evanescent components of the beam, like a wavelength-scale aperture, and an amplifier of these evanescent fields, like a slab of a photonic crystal with negative refraction.