Measurement of four-photon absorption in GaP and ZnTe semiconductors.

Intensity-dependent effective four-photon absorption (4PA) coefficients in GaP and ZnTe semiconductors were measured by the z-scan method using pump pulses of 1.75 µm wavelength, 135 fs duration, and up to 500 GWcm-2 intensity. A nonlinear pulse propagation model, including linear dispersion and 4PA was used to obtain the 4PA coefficients from measurements. The intensity-dependent effective 4PA coefficients vary from 2.6 × 10-4 to 65 × 10-4 cm5GW-3 in GaP, and from 3.5 × 10-4 to 9.1 × 10-4 cm5GW-3 in ZnTe. The anisotropy in 4PA was shown in GaP. The knowledge of 4PA coefficients is important for the design of semiconductor photonics devices.

Ultrafast dynamics in polycyclic aromatic hydrocarbons: the key case of conical intersections at higher excited states and their role in the photophysics of phenanthrene monomer.

In this study we reveal the detailed photocycle of a phenanthrene monomer. Phenanthrene serves as a popular building block for supramolecular systems and as an archetypal molecule to study the photochemistry of polycyclic aromatic hydrocarbons. By means of femtosecond time-resolved UV-vis transient absorption spectroscopy and molecular modeling, we found that the first bright transition involves the second excited singlet state, which relaxes toward the lowest excited singlet state with a biphasic internal conversion through a conical intersection region: a fast coherent branching followed by an exceptionally slow (∼ps) incoherent internal conversion. We succeeded to pinpoint the complete relaxation pathways and to extract the relevant parameters, e.g., the branching ratio at the conical intersection and internal conversion rates.

High-energy terahertz pulses from semiconductors pumped beyond the three-photon absorption edge.

A new route to efficient generation of THz pulses with high-energy was demonstrated using semiconductor materials pumped at an infrared wavelength sufficiently long to suppress both two- and three-photon absorption and associated free-carrier absorption at THz frequencies. For pumping beyond the three-photon absorption edge, the THz generation efficiency for optical rectification of femtosecond laser pulses with tilted intensity front in ZnTe was shown to increase 3.5 times, as compared to pumping below the absorption edge. The four-photon absorption coefficient of ZnTe was estimated to be ß4=(4±1)×10-5 cm5/GW3. THz pulses with 14 µJ energy were generated with as high as 0.7% efficiency in ZnTe pumped at 1.7 µm. It is shown that scaling the THz pulse energy to the mJ level by increasing the pump spot size and pump pulse energy is feasible.

Probing ultrafast phenomena with radially polarized light.

A new modality for probing ultrafast phenomena that relies on radially or azimuthally polarized probe pulses is presented. First, we describe the principle and then theoretically analyze the signals expected for different types of pump-induced nonlinearities. Last, we experimentally verify the methodology by probing a pump-induced Kerr gate with a time-delayed radially polarized probe pulse. In general, we find excellent agreement between the simulated and measured results.

Ptychographic reconstruction of attosecond pulses.

We demonstrate a new attosecond pulse reconstruction modality which uses an algorithm that is derived from ptychography. In contrast to other methods, energy and delay sampling are not correlated, and as a result, the number of electron spectra to record is considerably smaller. Together with the robust algorithm, this leads to a more precise and fast convergence of the reconstruction.

Ptychographic ultrafast pulse reconstruction.

We demonstrate a new ultrafast pulse reconstruction modality that is somewhat reminiscent of frequency-resolved optical gating but uses a modified setup and a conceptually different reconstruction algorithm that is derived from ptychography. Even though it is a second-order correlation scheme, it shows no time ambiguity. Moreover, the number of spectra to record is considerably smaller than in most other related schemes which, together with a robust algorithm, leads to extremely fast convergence of the reconstruction.

Space-time coupling in femtosecond pulse shaping and its effects on coherent control.

We present a Fourier optical analysis of a typical femtosecond pulse shaping apparatus and derive analytic expressions for the space-time dependence of the emerging waveform after the pulse shaper and in the focal volume of an additional focusing element. For both geometries the results are verified experimentally. Hereafter, we analyze the influence of space-time coupling on nonlinear processes, specifically second harmonic generation, resonant interaction with an atomic three-level system, and resonant excitation of a diatomic molecule.

Amplitude and phase modulation of time-energy entangled two-photon states.

We experimentally demonstrate amplitude and phase modulation of a time-energy entangled two-photon wave function. The entangled photons are produced by spontaneous parametric down-conversion, spectrally dispersed in an prism compressor, modulated in amplitude and/or phase, and detected in coincidence by sum-frequency generation. First, we present a Fourier optical analysis of the optical setup yielding an analytic expression for the resulting field distribution at the exit plane of the shaping apparatus. We then introduce amplitude and/or phase shaping and present results which can only be obtained through a combination of the two. Specifically, we use a shaper-based interferometer to measure the two-photon interference of an almost bandwidth-limited two-photon wave function.

Shaping a femtosecond pulse with a programmable thermo-optically driven phase modulator.

A femtosecond pulse shaping apparatus based on a thermo-optically driven spatial phase modulator is presented. The modulator cell is illuminated by a standard projector and is easily controllable with a computer. In principle the setup allows for two dimensional pulse shaping, however, all initial demonstrations reported here, such as feedback controlled dispersion compensation or pulse train generation, are performed in a one dimensional phase-only shaping geometry.

Strong field transient manipulation of electronic states and bands.

In the present review, laser fields are so strong that they become part of the electronic potential, and sometimes even dominate the Coulomb contribution. This manipulation of atomic potentials and of the associated states and bands finds fascinating applications in gases and solids, both in the bulk and at the surface. We present some recent spectacular examples obtained within the NCCR MUST in Switzerland.

The interaction of photoexcited carbon nanodots with metal ions disclosed down to the femtosecond scale.

Fluorescent carbon nanodots are a novel family of carbon-based nanoscale materials endowed with an outstanding combination of properties that make them very appealing for applications in nanosensing, photonics, solar energy harvesting and photocatalysis. One of the remarkable properties of carbon dots is their strong sensitivity to the local environment, especially to metal ions in solution. These interactions provide a testing ground for their marked photochemical properties, highlighted by many studies, and frequently driven by charge transfer events. Here we combine several optical techniques, down to femtosecond time resolution, to understand the interplay between carbon nanodots and aqueous metal ions such as Cu2+ and Zn2+. We find that copper inhibits the fluorescence of carbon dots through static and diffusional quenching mechanisms, and our measurements allow discriminating between the two. Ultrafast optical methods are then used to address the dynamics of copper-dot complexes, wherein static quenching takes place, and unveil the underlying complexity of their photocycle. We propose an initial increase of electronic charge on the surface of the dot, upon photo-excitation, followed by a partial electron transfer to the nearby ion, with 0.2 ps and 1.9 ps time constants, and finally a very fast (≪1 ps) non-radiative electron-hole recombination which brings the system back to the ground state. Notably, we find that the electron transfer stage is governed by an ultrafast water rearrangement around photo-excited dots, pointing out the key role of solvent interactions in the photo-physics of these systems.

Analysis of replica pulses in femtosecond pulse shaping with pixelated devices.

We present a detailed analysis of commonly encountered waveform distortions in femtosecond pulse shaping with pixelated devices, including the effects of discrete sampling, pixel gaps, smooth pixel boundaries, and nonlinear dispersion of the laser spectrum. Experimental and simulated measurements are used to illustrate the effects. The results suggest strategies for reduction of some classes of distortions.

Self-compensating amplifier design for cw and Q-switched high-power Nd:YAG lasers.

We experimentally demonstrate a self-adaptive compensation of the pump power dependent thermal lens in an Nd:YAG laser through a thin layer of a medium with a negative temperature dependence of the refractive index. The layer is thermally coupled to the laser rod and leads to a strikingly improved beam quality over a large stability range. The scheme allows for a scaling to high powers as well as pulsed-mode operation.

Giant electric field enhancement in split ring resonators featuring nanometer-sized gaps.

Today's pulsed THz sources enable us to excite, probe, and coherently control the vibrational or rotational dynamics of organic and inorganic materials on ultrafast time scales. Driven by standard laser sources THz electric field strengths of up to several MVm(-1) have been reported and in order to reach even higher electric field strengths the use of dedicated electric field enhancement structures has been proposed. Here, we demonstrate resonant electric field enhancement structures, which concentrate the incident electric field in sub-diffraction size volumes and show an electric field enhancement as high as ~14,000 at 50 GHz. These values have been confirmed through a combination of near-field imaging experiments and electromagnetic simulations.

Iterative Fourier transform algorithm for phase-only pulse shaping.

We demonstrate the adaptation of an iterative Fourier transform algorithm for the calculation of theoretical spectral phase functions required for pulse shaping applications. The algorithm is used to determine the phase functions necessary for the generation of different temporal intensity profiles. The performance of the algorithm is compared to two exemplary standard approaches. i.e. a Genetic Algorithm and a combination of a Simplex Downhill and a Simulated Annealing algorithm. It is shown that the iterative Fourier transform algorithm converges much faster than both alternative methods.

Direct Visualization of a Polariton Resonator in the THz Regime.

We report fabrication of a THz phonon-polariton resonator in a single crystal of LiNbO3 using femtosecond laser machining with high energy pulses. Fundamental and overtone resonator modes are excited selectively and monitored through spatiotemporal imaging. The resonator is integrated into a single solid-state platform that can include THz generation, manipulation, readout and other functionalities.

Femtosecond silicon K alpha pulses from laser-produced plasmas.

Ultrashort bursts of silicon K alpha x-ray radiation from femtosecond-laser-produced plasmas have been generated. A cross-correlation measurement employing a laser-triggered ultrafast structural change of a CdTe crystal layer (320 nm) shows a K alpha pulse duration between 200 fs and 640 fs. This result is corroborated by particle in cell simulations combined with a Monte-Carlo electron stopping code and calculations on the structural changes of the crystal lattice.

Optimization of Kalpha bursts for photon energies between 1.7 and 7 keV produced by femtosecond-laser-produced plasmas of different scale length.

The conversion efficiency of a 90 fs high-power laser pulse focused onto a solid target into x-ray Kalpha line emission was measured. By using three different elements as target material (Si, Ti, and Co), interesting candidates for fast x-ray diffraction applications were selected. The Kalpha output was measured with toroidally bent crystal monochromators combined with a GaAsP Schottky diode. Optimization was performed for different laser intensities as well as for different density scale lengths of a preformed plasma. These different scale lengths were realized by prepulses of different intensities and delay times with respect to the main pulse. Whereas the Kalpha yield varied by a factor of 1.8 for different laser intensities, the variation of the density scale length could provide a gain factor up to 4.6 for the Kalpha output.

Programmable common-path vector field synthesizer for femtosecond pulses.

We demonstrate a novel design for a femtosecond vector field synthesizer. Pulse shaping of all four degrees of freedom of the electric field (amplitude, phase, ellipticity, and orientation angle) is achieved with a single 1D double-layer spatial light modulator in a zero-dispersion compressor by modulating the amplitude and phase of the two transverse polarization components in separate halves of the modulator. Being a common-path arrangement, it is interferometrically stable and therefore usable for long-term measurements. The method can be broadly applied in coherent control and nonlinear spectroscopy.

Diffraction-based femtosecond pulse shaping with a two-dimensional spatial light modulator.

A new diffraction-based method is proposed and demonstrated for simultaneous shaping of both the phase and amplitude of femtosecond laser pulses by use of a phase-only two-dimensional spatial light modulator. The method suppresses certain types of temporal replica features ordinarily observed in femtosecond pulse shaping owing to imperfections in modulator devices and allows for multiplexed outputs suitable for use in various applications.