Pump-probe Setup Research Articles

Frequency-domain two-dimensional infrared spectroscopy using an acousto-optic programmable dispersive filter.

We report on multidimensional spectroscopy in the mid-infrared, performed using a calomel-based acousto-optic programmable dispersive filter. Although the associated spectral resolution is currently not as good as what has been previously reported using time-domain interferometers or other pulse-shaping technologies, the extreme compactness of the calomel crystal allows a straightforward implementation in a pre-existing pump-probe setup. Furthermore, the frequency-domain approach allows easy measurement of 2D slices of the multidimensional spectrum associated with a given pump frequency. We demonstrate our method with the measurement of the mid-infrared spectrum of carboxy-hemoglobin in three spectro-temporal dimensions.

A setup for extreme-ultraviolet ultrafast angle-resolved photoelectron spectroscopy at 50-kHz repetition rate.

Time- and angle-resolved photoelectron spectroscopy (trARPES) is a powerful method to track the ultrafast dynamics of quasiparticles and electronic bands in energy and momentum space. We present a setup for trARPES with 22.3 eV extreme-ultraviolet (XUV) femtosecond pulses at 50-kHz repetition rate, which enables fast data acquisition and access to dynamics across momentum space with high sensitivity. The design and operation of the XUV beamline, pump-probe setup, and ultra-high vacuum endstation are described in detail. By characterizing the effect of space-charge broadening, we determine an ultimate source-limited energy resolution of 60 meV, with typically 80-100 meV obtained at 1-2 × 1010 photons/s probe flux on the sample. The instrument capabilities are demonstrated via both equilibrium and time-resolved ARPES studies of transition-metal dichalcogenides. The 50-kHz repetition rate enables sensitive measurements of quasiparticles at low excitation fluences in semiconducting MoSe2, with an instrumental time resolution of 65 fs. Moreover, photo-induced phase transitions can be driven with the available pump fluence, as shown by charge density wave melting in 1T-TiSe2. The high repetition-rate setup thus provides a versatile platform for sensitive XUV trARPES, from quenching of electronic phases down to the perturbative limit.

A pump–probe scheme with a single chirped pulse to image electron and nuclear dynamics in molecules

A single chirped few-femtosecond pulse can be used to control and image coupled electron-nuclear dynamics. Using full ab initio simulations of the simplest molecule, as a prototype target, we show that for intermediate values of the chirp, interference between sequential and direct contributions enables significant control over ionization yields, even when taking into account the effective decoherence introduced by nuclear motion and the presence of an electronic continuum. For larger values of the chirp, the single chirped pulse reproduces a classical pump–probe setup, with the chirp parameter mapping an effective time delay between the pumping and probing frequencies of the pulse. After demonstrating this numerically, we present a full analytical solution for the two-photon ionization amplitudes that provides an intuitive analogy between the molecular dynamics induced by a single chirped pulse and a traditional pump–probe setup.

Measurement of Ultrafast Vibrational Coherences in Polyatomic Radical Cations with Strong-Field Adiabatic Ionization.

We present a pump-probe method for preparing vibrational coherences in polyatomic radical cations and probing their ultrafast dynamics. By shifting the wavelength of the strong-field ionizing pump pulse from the commonly used 800 nm into the near-infrared (1200-1600 nm), the contribution of adiabatic electron tunneling to the ionization process increases relative to multiphoton absorption. Adiabatic ionization results in predominant population of the ground electronic state of the ion upon electron removal, which effectively prepares a coherent vibrational state ("wave packet") amenable to subsequent excitation. In our experiments, the coherent vibrational dynamics are probed with a weak-field 800 nm pulse and the time-dependent yields of dissociation products measured in a time-of-flight mass spectrometer. We present the measurements on the molecule dimethyl methylphosphonate (DMMP) to illustrate how using 1500 nm pulses for excitation enhances the amplitude of coherent oscillations in ion yields by a factor of 10 as compared to 800 nm pulses. This protocol may be implemented in existing pump-probe setups through the incorporation of an optical parametric amplifier (OPA) for wavelength conversion.

Ultrafast switching in an atomic wire system at surfaces

Abstract

Voigt effect-based wide-field magneto-optical microscope integrated in a pump-probe experimental setup.

In this work, we describe an experimental setup for a spatially resolved pump-probe experiment with an integrated wide-field magneto-optical (MO) microscope. The MO microscope can be used to study ferromagnetic materials with both perpendicular-to-plane and in-plane magnetic anisotropy via polar Kerr and Voigt effects, respectively. The functionality of the Voigt effect-based microscope was tested using an in-plane magnetized ferromagnetic semiconductor (Ga,Mn)As. It was revealed that the presence of mechanical defects in the (Ga,Mn)As epilayer alters significantly the magnetic anisotropy in their proximity. The importance of MO experiments with simultaneous temporal and spatial resolutions was demonstrated using a (Ga,Mn)As sample attached to a piezoelectric transducer, which produces a voltage-controlled strain. We observed a considerably different behavior in different parts of the sample that enabled us to identify sample parts where the epilayer magnetic anisotropy was significantly modified by the presence of the piezoelectric transducer and where it was not. Finally, we discuss the possible applicability of our experimental setup for the research of compensated antiferromagnets, where only MO effects even in magnetic moments are present.

Control of laser-induced nonadiabatic molecular alignment in dissipative media

Laser-induced nonadiabatic alignment in nitrogen and mixtures of nitrogen with Ar gas was investigated using the ultrafast optical Kerr effect in the collinear pump-probe setup. We find that dissipative media produce desired alignment characteristics in distinct manners to decoherence and to population relaxation. The results show that the intense laser alignment can be used as a control tool of molecular dynamics in dissipative environments.

Spatiotemporal dynamics of Raman coherence in hollow-core fibers for a pump-probe setup

We present an experimental and theoretical study of the stimulated Raman emission in hollow-core kagome waveguides in a pump-probe arrangement. We perform an experimental investigation of the power of the Stokes signal from the probe, which is below the stimulated Raman scattering threshold, as a function the pump-probe delay time. The results show the Stokes power to increase with pump-probe delay, reaching a maximum at 10 ns, and to decrease afterward. In view of a coherence decay time of only 0.25 ns, we demonstrate a surprisingly slow reduction of Stokes signal with the characteristic time much longer than the coherence decay time by a factor of up to 40. The numerical investigations explain the observed phenomenon as a result of the spatiotemporal dynamics of the probe pulse and Raman coherence. We show that the increase of the characteristic time can be related to the spatial position of the intense sideband generation event and its dependence on the pump-probe delay.

Charge dynamics in aluminum oxide thin film studied by ultrafast scanning electron microscopy

The excitation dynamics of defects in insulators plays a central role in a variety of fields from Electronics and Photonics to Quantum computing. We report here a time-resolved measurement of electron dynamics in 100 nm film of aluminum oxide on silicon by Ultrafast Scanning Electron Microscopy (USEM). In our pump-probe setup, an UV femtosecond laser excitation pulse and a delayed picosecond electron probe pulse are spatially overlapped on the sample, triggering Secondary Electrons (SE) emission to the detector. The zero of the pump-probe delay and the time resolution were determined by measuring the dynamics of laser-induced SE contrast on silicon. We observed fast dynamics with components ranging from tens of picoseconds to few nanoseconds, that fits within the timescales typical of the UV color center evolution. The surface sensitivity of SE detection gives to the USEM the potential of applying pump-probe investigations to charge dynamics at surfaces and interfaces of current nano-devices. The present work demonstrates this approach on large gap insulator surfaces.

Correction to: Case study on the dynamics of ultrafast laser heating and ablation of gold thin films by ultrafast pump‑probe reflectometry and ellipsometry

To increase the comprehension of ultrafast laser ablation, the ablation process has to be portrayed with sufficient temporal resolution. For example, the temporal modification of the complex refractive index $${\tilde{n}}$$ and the relative reflectance of a sample material after irradiation with ultrafast single-pulsed laser radiation can be measured with a pump-probe setup. This work describes the construction and validation of a pump-probe setup enabling spatially, temporally, and spectroscopically resolved Brewster angle microscopy, reflectometry, ellipsometry, and shadow photography. First pump-probe reflectometry and ellipsometry measurements are performed on gold at $$\lambda _{\mathrm{probe}}= 440\, \hbox {nm}$$ and three fluences of the single-pulsed pump radiation at $$\lambda _{\mathrm{pump}}= 800\,\hbox {nm}$$ generating no, gentle, and strong ablation. The relative reflectance overall increases at no and gentle ablation. At strong ablation, the relative reflectance locally decreases, presumable caused by emitted thermal electrons, ballistic electrons, and ablating material. The refractive index n is slightly decreasing after excitation, while the extinction coefficient k is increasing.

Time-domain fluorescence lifetime imaging by nonlinear fluorescence microscopy constructed of a pump-probe setup with two-wavelength laser pulses.

We propose a time-domain approach for fluorescence lifetime measurements using nonlinear fluorescence microscopy constructed of a pump-probe setup with two-wavelength laser pulses. Nonlinear fluorescence signals generated by fluorescence reduction due to stimulated emission were detectable through a lock-in technique. Changing the time delay between the two-wavelength pulses enables acquisition of a time-resolved nonlinear fluorescence signal, which directly reflects the fluorescence lifetime of the sample and is thus applicable to fluorescence lifetime imaging. We also quantitatively demonstrate that nonlinear fluorescence microscopy possesses better optical resolution than conventional laser-scanning fluorescence microscopy. Experimental trials indicate that straightforward fluorescence lifetime imaging with high optical resolution is readily available.

Frequency-domain modelling of gain in pump-probe experiment by an inhomogeneous medium

Introduction of a gain medium in lossy plasmonic metamaterials reduces and compensates losses or even amplifies an incident light often with nonlinear optical effect. Here, optical gain in a pump-probe experimental setup is effectively calculated in the frequency-domain by approximating a gain material as an inhomogeneous medium. Spatially varying local field amplitudes of the pump and probe beams are included in the model to reproduce the inhomogeneous gain effect, in which population inversion occurs most strongly near the surface and decays along the propagation direction. We demonstrate that transmission spectra calculated by this method agree well with finite-difference time-domain simulation results. This simplified approach of gain modelling offers an easy and reliable way to analyze wave propagation in a gain medium without nonlinear time-domain calculation.

Case study on the dynamics of ultrafast laser heating and ablation of gold thin films by ultrafast pump-probe reflectometry and ellipsometry

To increase the comprehension of ultrafast laser ablation, the ablation process has to be portrayed with sufficient temporal resolution. For example, the temporal modification of the complex refractive index $${\tilde{n}}$$ and the relative reflectance of a sample material after irradiation with ultrafast single-pulsed laser radiation can be measured with a pump-probe setup. This work describes the construction and validation of a pump-probe setup enabling spatially, temporally, and spectroscopically resolved Brewster angle microscopy, reflectometry, ellipsometry, and shadow photography. First pump-probe reflectometry and ellipsometry measurements are performed on gold at $$\lambda _{\mathrm{probe}}= 440\, \hbox {nm}$$ and three fluences of the single-pulsed pump radiation at $$\lambda _{\mathrm{pump}}= 800\,\hbox {nm}$$ generating no, gentle, and strong ablation. The relative reflectance overall increases at no and gentle ablation. At strong ablation, the relative reflectance locally decreases, presumable caused by emitted thermal electrons, ballistic electrons, and ablating material. The refractive index n is slightly decreasing after excitation, while the extinction coefficient k is increasing.

Analysis of shockwave formation in glass welding by ultra-short pulses

Glass welding by ultra-short pulsed lasers is temporally a highly dynamic process. These dynamics have a high impact on the shape and the thermodynamic properties of the molten zone as well as the plasma. In this work we analyze the local distribution of laser absorption in the melt by shockwave observation and the evolution of the subsequent pressure wave generated during the process. The pressure waves were observed using a pump-probe setup with a high-speed video camera. Furthermore we show a dramatic decrease of pressure waves propagation velocity inside the molten zone as an explicit indicator of the fluid state of the molten zone.

Single-color pump-probe setup at the NovoFEL facility for measurements of carrier relaxation dynamics in semiconductors

Single-color pump-probe setup at the NovoFEL facility for measurements of carrier relaxation dynamics in semiconductors

Super-resolving nonlinear fluorescence microscopy with pump–probe setup using repetitive stimulated transition

Super-resolving nonlinear fluorescence microscopy with a pump–probe setup that utilizes repetitive stimulated absorption and stimulated emission caused by two-color beams was developed. The resulting nonlinear fluorescence that undergoes such a repetitive stimulated transition is detectable as a signal via the lock-in technique. The signal is produced by a multiplicative combination of incident beams, which leads to an improvement of the optical resolution. A phenomenological interpretation of the nonlinear fluorescence process that offers estimation of signal properties is provided with rate equations. The proposed method is demonstrated to provide the scalability of the optical resolution.

Broadly tunable ultrafast pump-probe system operating at multi-kHz repetition rate

Femtosecond systems based on ytterbium as active medium are ideal for driving ultrafast optical parametric amplifiers in a broad frequency range. The excellent stability of the source and the repetition rate tunable to up to hundreds of kHz allow for the implementation of an advanced two-color pump probe setup with the capability to achieve excellent signal-to-noise performances with sub-10 fs temporal resolution.

Anisotropic propagation imaging of elastic waves in oriented columnar thin films

We report on the observation of strongly anisotropic surface acoustic wave propagation on nanostructured thin films. Two kinds of tungsten samples were prepared by sputtering on a silicon substrate: a conventional thin film with columns normal to the substrate surface, and an oriented columnar architecture using the glancing angle deposition (GLAD) process. Pseudo-Rayleigh waves (PRWs) were imaged as a function of time in x and y directions for both films thanks to a femtosecond heterodyne pump-probe setup. A strong anisotropic propagation as well as a high velocity reduction of the PRWs were exhibited for the GLAD sample. For the wavevector k/2π = 3 × 105 m−1 the measured group velocities vx and vy equal 2220 m s−1 for the sample prepared with conventional sputtering, whereas a strong anisotropy appears (vx = 1600 m s−1 and vy = 870 m s−1) for the sample prepared with the GLAD process. Using the finite element method, the anisotropy is related to the structural anisotropy of the thin film’s architecture. The drop of PRWs group velocities is mainly assigned to the porous microstructure, especially favored by atomic shadowing effects which appear during the growth of the inclined columns. Such GLAD thin films constitute a new tool for the control of the propagation of surface elastic waves and for the design of new devices with useful properties.

Low-temperature intracenter relaxation times of shallow donors in germanium

The relaxation times of localized states of antimony donors in unstrained and strained germanium uniaxially compressed along the [111] crystallographic direction are measured at cryogenic temperatures. The measurements are carried out in a single-wavelength pump–probe setup using radiation from the Novosibirsk free electron laser (NovoFEL). The relaxation times in unstrained crystals depend on the temperature and excitation photon energy. Measurements in strained crystals are carried out under stress bar S > 300, in which case the ground-state wavefunction is formed by states belonging to a single valley in the germanium conduction band. It is shown that the application of uniaxial strain leads to an increase in the relaxation time, which is explained by a decrease in the number of relaxation channels.

Modification of magnetization orientation in Pt/Co/Pt ultrathin films by femtosecond laser pulses

Ultrathin Pt/Co/Pt films were irradiated with femtosecond laser pulses of sufficiently high energy to create irreversible structural and in consequence magnetic modifications. The effect was studied as a function of an initial structure of the Co film, determined by different temperatures during preparation of the Pt buffer layer, and a Co film thickness, which determines the initial out-of-plane or in-plane magnetization orientation. A creation of many magnetic phases is shown. The out-of-plane magnetization state is found to be created for significantly broader fluence range for the Co film deposited on the Pt buffer layer grown at 750°C in comparison to that grown at room temperature. Reversible magnetization dynamics was also studied in the low fluence regime, using a pump-probe setup. The results were discussed using simulations of temperature dynamics.