Gouy phase effects on photocurrents in plasmonic nanogaps driven by single-cycle pulses.

The investigation of optical phenomena in the strong-field regime requires few-cycle laser pulses at field strengths exceeding gigavolts per meter (GV/m). Surprisingly, such conditions can be reached by tightly focusing pJ-level pulses with nearly octave spanning optical bandwidth onto plasmonic nanostructures, exploiting the field-enhancement effect. In this situation, the Gouy phase of the focused beam can deviate significantly from the monochromatic scenario. Here, we study the effect of the Gouy phase of a pulse exploited to drive coherent strong-field photocurrents within a plasmonic gap nanoantenna. While the influence of the specific Gouy phase profile in the experiment approaches the monochromatic case closely, this scheme may be utilized to identify more intricate phase profiles at sub-diffraction scale. Our results pave the way for Gouy phase engineering at picojoule (pJ) pulse energy levels, enabling the optimization of strong-field optical phenomena.

High stability white light generation in water at multi-kilohertz repetition rates.

An efficient supercontinuum (SC) generation featuring high spectral intensity across a large bandwidth requires high peak powers of several megawatts from pulsed lasers. Under these conditions and at multi-kilohertz (kHz) repetition rates, the SC generated in most materials is unstable due to thermal effects. In this work, we leverage the superior dispersion properties of water to maximize the spectral width of the SC, while avoiding stability issues due to thermal loading by means of a constant laminar flow of the liquid. This flow is controlled by a differential pressure scheme that allows to precisely adjust the fluid velocity to an optimum value for maximum stability of the SC. This approach is successfully implemented for repetition rates of 50 kHz and 100 kHz and two different pump wavelengths in the visible (VIS) and near infrared (NIR) spectral region with stability of the SC signal only limited by the driving pulses. The resulting water SC spans more than one octave covering the VIS to NIR range. Compared to established materials, such as yttrium aluminum garnet (YAG) and sapphire, the spectral bandwidth is increased by 60% and 40% respectively. Our scheme has the potential to be implemented with other liquids such as bromine or carbon disulfide (CS2), which promise even wider broadening and operation up to the mid-infrared.

Sub-8-fs pulses in the visible to near-infrared by a degenerate optical parametric amplifier.

This work presents a single-stage optical parametric amplifier (OPA) operating at degeneracy (DOPA) and pumped by the third harmonic of a Yb:KGW laser system. This DOPA exploits the broad amplification bandwidth that occurs with type-I phase-matching in ß-barium borate (BBO) when signal and idler overlap in the spectrum. The output pulses span from 590 to 780 nm (1.59-2.10 eV) with 7.75-fs duration after compression. Ultrashort pulses with similar bandwidths in this spectral window complement the existing array of optical parametric amplifiers that cover either the visible or the near-IR spectral regions with sub-10-fs pulses. This source of ultrashort optical pulses will enable the application of sophisticated spectroscopy techniques to the study of electronic coherences and energy migration pathways in biological, chemical, and condensed matter systems.

Sub-picosecond collapse of molecular polaritons to pure molecular transition in plasmonic photoswitch-nanoantennas.

Molecular polaritons are hybrid light-matter states that emerge when a molecular transition strongly interacts with photons in a resonator. At optical frequencies, this interaction unlocks a way to explore and control new chemical phenomena at the nanoscale. Achieving such control at ultrafast timescales, however, is an outstanding challenge, as it requires a deep understanding of the dynamics of the collectively coupled molecular excitation and the light modes. Here, we investigate the dynamics of collective polariton states, realized by coupling molecular photoswitches to optically anisotropic plasmonic nanoantennas. Pump-probe experiments reveal an ultrafast collapse of polaritons to pure molecular transition triggered by femtosecond-pulse excitation at room temperature. Through a synergistic combination of experiments and quantum mechanical modelling, we show that the response of the system is governed by intramolecular dynamics, occurring one order of magnitude faster with respect to the uncoupled excited molecule relaxation to the ground state.

Ultrafast Thermionic Electron Injection Effects on Exciton Formation Dynamics at a van der Waals Semiconductor/Metal Interface.

Inorganic van der Waals bonded semiconductors such as transition metal dichalcogenides are the subject of intense research due to their electronic and optical properties which are promising for next-generation optoelectronic devices. In this context, understanding the carrier dynamics, as well as charge and energy transfer at the interface between metallic contacts and semiconductors, is crucial and yet quite unexplored. Here, we present an experimental study to measure the effect of mutual interaction between thermionically injected and directly excited carriers on the exciton formation dynamics in bulk WS2. By employing a pump-push-probe scheme, where a pump pulse induces thermionic injection of electrons from a gold substrate into the conduction band of the semiconductor, and another delayed push pulse that excites direct transitions in the WS2, we can isolate the two processes experimentally and thus correlate the mutual interaction with its effect on the ultrafast dynamics in WS2. The fast decay time constants extracted from the experiments show a decrease with an increasing ratio between the injected and directly excited charge carriers, thus disclosing the impact of thermionic electron injection on the exciton formation dynamics. Our findings might offer a new vibrant direction for the integration of photonics and electronics, especially in active and photodetection devices, and, more in general, in upcoming all-optical nanotechnologies.

Passively phase-locked Er:fiber source of single-cycle pulses in the near infrared with electro-optic timing modulation for field-resolved electron control.

A single-cycle light source in the near infrared is demonstrated enabling sensitive applications of ultrafast optical field control of electronic transport. The compact Er:fiber system generates passively phase-locked pulses with broadband spectra covering 150 THz to 350 THz at a duration of 4.2 fs and 40 MHz repetition rate. A second output arm is equipped with an electro-optic modulator (EOM) that switches the arrival time of the pulses by 700 ps at arbitrary frequencies up to 20 MHz, enabling timing modulation of the pump pulse without changing the average intensity. As a benchmark demonstration, we investigate the carrier relaxation dynamics in low-temperature-grown InGaAs (LT-InGaAs) using quantum interference currents (QuICs).

Magneto-Optical Activity in Nonmagnetic Hyperbolic Nanoparticles.

Active nanophotonics can be realized by controlling the optical properties of materials with external magnetic fields. Here, we explore the influence of optical anisotropy on the magneto-optical activity in nonmagnetic hyperbolic nanoparticles. We demonstrate that the magneto-optical response is driven by the hyperbolic dispersion via the coupling of metallic-induced electric and dielectric-induced magnetic dipolar optical modes with static magnetic fields. Magnetic circular dichroism experiments confirm the theoretical predictions and reveal tunable magneto-optical activity across the visible and near infrared spectral range.

Dynamics of electron-emission currents in plasmonic gaps induced by strong fields.

The dynamics of ultrafast electron currents triggered by femtosecond laser pulse irradiation of narrow gaps in a plasmonic dimer is studied using quantum mechanical Time-Dependent Density Functional Theory (TDDFT). The electrons are injected into the gap due to the optical field emission from the surfaces of the metal nanoparticles across the junction. Further evolution of the electron currents in the gap is governed by the locally enhanced electric fields. The combination of TDDFT and classical modelling of the electron trajectories allows us to study the quiver motion of the electrons in the gap region as a function of the Carrier Envelope Phase (CEP) of the incident pulse. In particular, we demonstrate the role of the quiver motion in establishing the CEP-sensitive net electric transport between nanoparticles.

Plasmonic mid-infrared third harmonic generation in germanium nanoantennas.

We demonstrate third harmonic generation in plasmonic antennas consisting of highly doped germanium grown on silicon substrates and designed to be resonant in the mid-infrared frequency range that is inaccessible with conventional nonlinear plasmonic materials. Owing to the near-field enhancement, the result is an ultrafast, subdiffraction, coherent light source with a wavelength tunable between 3 and 5 µm, and ideally overlapping with the fingerprint region of molecular vibrations. To observe the nonlinearity in this challenging spectral window, a high-power femtosecond laser system equipped with parametric frequency conversion in combination with an all-reflective confocal microscope setup is employed. We demonstrate spatially resolved maps of the linear scattering cross section and the nonlinear emission of single isolated antenna structures. A clear third-order power dependence as well as mid-infrared emission spectra prove the nonlinear nature of the light emission. Simulations support the observed resonance length of the double-rod antenna and demonstrate that the field enhancement inside the antenna material is responsible for the nonlinear frequency mixing.

Switchable dissociation of excitons bound at strained CdTe/CdS interfaces.

Charge carrier dynamics of semiconductor nano-heterostructures are determined by band alignment and lattice mismatch of the adjacent materials. However, quantum efficiencies for the separation of excited charge carriers at such an interface are hard to predict and cannot yet be easily controlled. In this work we examine nanorods with a severely strained, axial CdTe/CdS interface using femtosecond transient absorption spectroscopy. We show that charge separation is mitigated by equal contributions of valence band distortion and formation of coulomb pairs across the interface. Left undisturbed such localised excitons relax rapidly via non-radiative recombination channels. By adding a competitive hole acceptor that disrupts the coulomb interaction we overcome the synergetic co-localisation of the carriers and realise charge separation. The thus created long-lived state can be exploited for a broad range of applications such as photocatalysis, water splitting, and switchable nanodevices.

Incoherent Pathways of Charge Separation in Organic and Hybrid Solar Cells.

In this work, we investigate the exciton dissociation dynamics occurring at the donor:acceptor interface in organic and hybrid blends employed in the realization of photovoltaic cells. Fundamental differences in the charge separation process are studied with the organic semiconductor polymer poly(3-hexylthiophene) (P3HT) and either [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) or titanium dioxide (TiO2) acting as the acceptor. By using ultrafast broad-band transient absorption spectroscopy with few-fs temporal resolution, we observe that in both cases the incoherent formation of free charges dominates the charge generation process. From the optical response of the polymer and by tracking the excited-state absorption, we extract pivotal similarities in the incoherent energy pathways that follow the impulsive excitation. On time scales shorter than 200 fs, we observe that the two acceptors display similar dynamics in the exciton delocalization. Significant differences arise only on longer time scales with only an impact on the overall photocarrier generation efficiency.

Coherent field transients below 15 THz from phase-matched difference frequency generation in 4H-SiC.

We experimentally demonstrate tunable, phase-matched difference frequency generation covering the spectral region below 15 THz using 4H-SiC as a nonlinear crystal. This material combines a non-centrosymmetric lattice and strong birefringence with broadband transparency at low optical frequencies. Thorough refractive index measurements in the terahertz spectral range allow us to calculate phase-matching conditions for any near-infrared pump laser source. 4H-SiC is also exploited as a detector crystal for electro-optic sampling. The results allow us to estimate the effective second-order nonlinear coefficient.

Optical Activation of Germanium Plasmonic Antennas in the Mid-Infrared.

Impulsive interband excitation with femtosecond near-infrared pulses establishes a plasma response in intrinsic germanium structures fabricated on a silicon substrate. This direct approach activates the plasmonic resonance of the Ge structures and enables their use as optical antennas up to the mid-infrared spectral range. The optical switching lasts for hundreds of picoseconds until charge recombination redshifts the plasma frequency. The full behavior of the structures is modeled by the electrodynamic response established by an electron-hole plasma in a regular array of antennas.

Stimulated Raman scattering microscopy by Nyquist modulation of a two-branch ultrafast fiber source.

A highly stable setup for stimulated Raman scattering (SRS) microscopy is presented. It is based on a two-branch femtosecond Er:fiber laser operating at a 40 MHz repetition rate. One of the outputs is directly modulated at the Nyquist frequency with an integrated electro-optic modulator (EOM). This compact source combines a jitter-free pulse synchronization with a broad tunability and allows for shot-noise limited SRS detection. The performance of the SRS microscope is illustrated with measurements on samples from material science and cell biology.

615 fs pulses with 17 mJ energy generated by an Yb:thin-disk amplifier at 3 kHz repetition rate.

A combination of Er/Yb:fiber and Yb:thin-disk technology produces 615 fs pulses at 1030 nm with an average output power of 72 W. The regenerative amplifier allows variation of the repetition rate between 3 and 5 kHz with pulse energies from 13 to 17 mJ. A broadband and intense seed provided by the compact and versatile fiber front-end minimizes gain narrowing. The resulting sub-ps performance is ideal for nonlinear frequency conversion and pulse compression. Operating in the upper branch of a bifurcated pulse train, the system exhibits exceptional noise performance and stability.

The nature of singlet exciton fission in carotenoid aggregates.

Singlet exciton fission allows the fast and efficient generation of two spin triplet states from one photoexcited singlet. It has the potential to improve organic photovoltaics, enabling efficient coupling to the blue to ultraviolet region of the solar spectrum to capture the energy generally lost as waste heat. However, many questions remain about the underlying fission mechanism. The relation between intermolecular geometry and singlet fission rate and yield is poorly understood and remains one of the most significant barriers to the design of new singlet fission sensitizers. Here we explore the structure-property relationship and examine the mechanism of singlet fission in aggregates of astaxanthin, a small polyene. We isolate five distinct supramolecular structures of astaxanthin generated through self-assembly in solution. Each is capable of undergoing intermolecular singlet fission, with rates of triplet generation and annihilation that can be correlated with intermolecular coupling strength. In contrast with the conventional model of singlet fission in linear molecules, we demonstrate that no intermediate states are involved in the triplet formation: instead, singlet fission occurs directly from the initial 1B(u) photoexcited state on ultrafast time scales. This result demands a re-evaluation of current theories of polyene photophysics and highlights the robustness of carotenoid singlet fission.

Ultrafast intramolecular relaxation and wave-packet motion in a ruthenium-based supramolecular photocatalyst.

The hydrogen-evolving photocatalyst [(tbbpy)2 Ru(tpphz)Pd(Cl)2 ](2+) (tbbpy=4,4'-di-tert-butyl-2,2'-bipyridine, tpphz=tetrapyrido[3,2-a:2',3'-c:3'',2''-h:2''',3'''-j]phenazine) shows excitation-wavelength-dependent catalytic activity, which has been correlated to the localization of the initial excitation within the coordination sphere. In this contribution the excitation-wavelength dependence of the early excited-state relaxation and the occurrence of vibrational coherences are investigated by sub-20 fs transient absorption spectroscopy and DFT/TDDFT calculations. The comparison with the mononuclear precursor [(tbbpy)2 Ru(tpphz)](2+) highlights the influence of the catalytic center on these ultrafast processes. Only in the presence of the second metal center, does the excitation of a (1) MLCT state localized on the central part of the tpphz bridge lead to coherent wave-packet motion in the excited state.

Ultrastable fiber amplifier delivering 145-fs pulses with 6-µJ energy at 10-MHz repetition rate.

A high-power femtosecond Yb:fiber amplifier operating with exceptional noise performance and long-term stability is demonstrated. It generates a 10-MHz train of 145-fs pulses at 1.03 µm with peak powers above 36 MW. The system features a relative amplitude noise of 1.5·10⁻6 Hz(-1/2) at 1 MHz and drifts of the 60-W average power below 0.3% over 72 hours of continuous operation. The passively phase-stable Er:fiber seed system provides ultrabroadband pulses that are synchronized at a repetition rate of 40 MHz. This combination aims at new schemes for sensitive experiments in ultrafast scientific applications.

Nonlinear photoluminescence spectrum of single gold nanostructures.

We investigate the multiphoton photoluminescence characteristics of gold nanoantennas fabricated from single crystals and polycrystalline films. By exciting these nanostructures with ultrashort pulses tunable in the near-infrared range, we observe distinct features in the broadband photoluminescence spectrum. By comparing antennas of different crystallinity and shape, we demonstrate that the nanoscopic geometry of plasmonic devices determines the shape of the emission spectra. Our findings rule out the contribution of the gold band structure in shaping the photoluminescence.

Coherent ultrafast charge transfer in an organic photovoltaic blend.

Blends of conjugated polymers and fullerene derivatives are prototype systems for organic photovoltaic devices. The primary charge-generation mechanism involves a light-induced ultrafast electron transfer from the light-absorbing and electron-donating polymer to the fullerene electron acceptor. Here, we elucidate the initial quantum dynamics of this process. Experimentally, we observed coherent vibrational motion of the fullerene moiety after impulsive optical excitation of the polymer donor. Comparison with first-principle theoretical simulations evidences coherent electron transfer between donor and acceptor and oscillations of the transferred charge with a 25-femtosecond period matching that of the observed vibrational modes. Our results show that coherent vibronic coupling between electronic and nuclear degrees of freedom is of key importance in triggering charge delocalization and transfer in a noncovalently bound reference system.