Pump-probe Measurements Research Articles

Vibrational wave packet dynamics of H2O+ and H2O by strong-field Fourier transform spectroscopy

The vibrational wave packet dynamics of H2O+ and H2O are investigated by pump–probe measurements using few-cycle intense laser pulses. By a Fourier transform (FT) of the time-dependent yields of the two-body Coulomb explosion pathway, H2O2+ → H+ + OH+, and the parent ion, H2O+, the vibrational excitation processes in the electronic ground states of H2O+ and H2O induced by the intense laser field are retrieved from the phases of the respective vibrational peaks in the FT spectrum. Accurate quantum-dynamics simulations on H2O+ confirm both the peak assignment and the origin of the time-dependent ionization yields of the vibrational wave packet.

Unwrapping a full temporal cycle in time domain thermoreflectance for enhanced measurement sensitivity in thermally insulating materials.

Time delayed pump-probe measurement techniques, such as Time Domain Thermoreflectance (TDTR), have opened up a wealth of opportunities for metrology at ultra-fast timescales and nanometer length scales. For nanoscale thermal transport measurements, typical thermal lifetimes used to measure thermal conductivity and thermal boundary conductance span from sub-picosecond to ∼6 nanoseconds. In this work, we demonstrate a simple rearrangement and validation of a configuration that allows access to the entire 12.5ns time delay available in the standard pulse train. By reconfiguring a traditional TDTR system so that the pump and probe arrive concurrently when the delay stage reaches its midpoint, followed by unwrapping the temporal scan, we obtain a dataset that is bounded only by the oscillator repetition rate. Sensitivity analysis along with conducted measurements shows that great increases in measurement sensitivity are available with this approach, particularly for thin films with low thermal conductivities.

Direct comparison of molecular-beam vs liquid-phase pump-probe and two-dimensional spectroscopy on the example of azulene.

Although azulene's anomalous fluorescence originating from S2 rather than from S1 is a textbook example for the violation of Kasha's rule, an understanding of the underlying processes is still a subject of investigation. Here, we use action-based coherent two-dimensional electronic spectroscopy (2DES) to measure a single Liouville-space response pathway from S0 via S1 to the S2 state of azulene. We directly compare this sequential excitation in the liquid phase detecting S2 fluorescence and in a molecular beam detecting photoionized cations, using the S2 anomalous emission to our advantage. We complement the 2DES study with pump-probe measurements of S1 excitation dynamics, including vibrational relaxation and passage through a conical intersection. A direct comparison of the liquid and gas phase allows us to assess the effect of the solvent and the interplay of intra- and intermolecular energy relaxation.

Self-assembling cobalt(II) porphyrin - fullero [60]pyrrolidine triads. Synthesis and spectral properties in the ground and excited state

Meso‑4-Trifluoromethylphenyl substituted cobalt(II)porphin, CoT(CF3P)P, and its coordination 1: 2 complex (triad) with 1-N-methyl-2-(pyridin-4-yl)-3,4-fullero [60]pyrrolidine, (PyC60)2CoT(CF3P)P, were firstly synthesized and the properties of their molecular solutions were studied. The chemical structure of CoT(CF3P)P and triad was fully confirmed by the UV–vis, IR and 1H NMR spectroscopy and mass spectrometry methods. The triad formation mechanism as the two-step equilibrium characterized by equilibrium constant K1 (5.89 ± 1.50) × 104 M−1 and K2 (7.40 ± 1.66) × 105 M−1 was revealed. Using femtosecond pump-probe spectroscopy measurements, the photoinduced electron transfer in the synthesized triad was described. The excited state lifetime of the precursors, CoTPP, CoT(CF3P)P and the charge separated state lifetime of (PyC60)2CoT(CF3P)P compared with unsubstituted (PyC60)2CoTPP were determined. It was established that the CF3 groups in the porphyrin periphery slightly change the CoTPP/CoT(CF3P)P excited state lifetime and the electron transfer rate constant in the correspondent triads.

Perspectives of Gas Phase Ion Chemistry: Spectroscopy and Modeling

The study of ions in the gas phase has a long history and has involved both chemists and physicists. The interplay of their competences with the use of very sophisticated commercial and/or homemade instrumentations and theoretical models has improved the knowledge of thermodynamics and kinetics of many chemical reactions, even if still many stages of these processes need to be fully understood. The new technologies and the novel free-electron laser facilities based on plasma acceleration open new opportunities to investigate the chemical reactions in some unrevealed fundamental aspects. The synchrotron light source can be put beside the FELs, and by mass spectrometric techniques and spectroscopies coupled with versatile ion sources it is possible to really change the state of the art of the ion chemistry in different areas such as atmospheric and astro chemistry, plasma chemistry, biophysics, and interstellar medium (ISM). In this manuscript we review the works performed by a joint combination of the experimental studies of ion–molecule reactions with synchrotron radiation and theoretical models adapted and developed to the experimental evidence. The review concludes with the perspectives of ion–molecule reactions by using FEL instrumentations as well as pump probe measurements and the initial attempt in the development of more realistic theoretical models for the prospective improvement of our predictive capability.

High output mode-locked laser empowered by defect regulation in 2D Bi2O2Se saturable absorber

Atomically thin Bi2O2Se has emerged as a novel two-dimensional (2D) material with an ultrabroadband nonlinear optical response, high carrier mobility and excellent air stability, showing great potential for the realization of optical modulators. Here, we demonstrate a femtosecond solid-state laser at 1.0 µm with Bi2O2Se nanoplates as a saturable absorber (SA). Upon further defect regulation in 2D Bi2O2Se, the average power of the mode-locked laser is improved from 421 mW to 665 mW, while the pulse width is decreased from 587 fs to 266 fs. Moderate Ar+ plasma treatments are employed to precisely regulate the O and Se defect states in Bi2O2Se nanoplates. Nondegenerate pump-probe measurements show that defect engineering effectively accelerates the trapping rate and defect-assisted Auger recombination rate of photocarriers. The saturation intensity is improved from 3.6 ± 0.2 to 12.8 ± 0.6 MW cm−2 after the optimized defect regulation. The enhanced saturable absorption and ultrafast carrier lifetime endow the high-performance mode-locked laser with both large output power and short pulse duration.

Photocycle of point defects in highly- and weakly-germanium doped silica revealed by transient absorption measurements with femtosecond tunable pump

We report pump-probe transient absorption measurements addressing the photocycle of the Germanium lone pair center (GLPC) point defect with an unprecedented time resolution. The GLPC is a model point defect with a simple and well-understood electronic structure, highly relevant for several applications. Therefore, a full explanation of its photocycle is fundamental to understand the relaxation mechanisms of such molecular-like systems in solid state. The experiment, carried out exciting the sample resonantly with the ultraviolet (UV) GLPC absorption band peaked at 5.1 eV, gave us the possibility to follow the defect excitation-relaxation dynamics from the femto-picosecond to the nanosecond timescale in the UV–visible range. Moreover, the transient absorption signal was studied as a function of the excitation photon energy and comparative experiments were conducted on highly- and weakly-germanium doped silica glasses. The results offer a comprehensive picture of the relaxation dynamics of GLPC and allow observing the interplay between electronic transitions localized on the defect and those related to bandgap transitions, providing a clear evidence that the role of dopant high concentration is not negligible in the earliest dynamics.

Ultrafast opto-magnetic effects induced by nitrogen-vacancy centers in diamond crystals

The current generation of quantum sensing technologies using color centers in diamond crystals is primarily based on the principle that the resonant microwave frequency of the luminescence between quantum levels of the nitrogen-vacancy (NV) center varies with temperature and electric and magnetic fields. This principle enables us to measure, for instance, magnetic and electric fields, as well as local temperature with nanometer resolution in conjunction with a scanning probe microscope (SPM). However, the time resolution of conventional quantum sensing technologies has been limited to microseconds due to the limited luminescence lifetime. Here, we investigate ultrafast opto-magnetic effects in diamond crystals containing NV centers to improve the time resolution of quantum sensing to sub-picosecond time scales. The spin ensemble from diamond NV centers induces an inverse Cotton–Mouton effect (ICME) in the form of a sub-picosecond optical response in a femtosecond pump–probe measurement. The helicity and quadratic power dependence of the ICME can be interpreted as a second-order opto-magnetic effect in which ensembles of NV electron spins act as a source for the ICME. The results provide fundamental guidelines for enabling high-resolution spatial-time quantum sensing technologies when combined with SPM techniques.

Experimental and analytical evaluation of an active barium vapor notch filter functioning at 355 nm.

This work presents an experimental and theoretical study of the 6s21S0→6s6p3P10 (791 nm) and 5s5d3D2→6s5f3F20 (355 nm) transitions within a low vapor pressure barium vapor in the absence of a buffer gas. To the authors' knowledge, this is the first measurement of the latter absorption feature. The study is motivated by the development of an optically controlled atomic vapor notch filter functioning at the third harmonic of the commonly used Nd:YAG laser at 355 nm. The low-pressure environment within the vapor source has enabled a deeper understanding of barium vapor collisional and velocity changing properties with a simple pump-probe spectroscopy measurement. The results demonstrate depletion of the ground state and subsequent population of the 5s5d3D2 level. Most notably, the 5s5d3D2→6s5f3F20 transition displays a non-thermal, cusped absorption curve. An analysis of this line shape in the context of existing analytical collisional kernels is presented. Additionally, a six-level kinetic model of the low-lying energy levels, incorporating spatial diffusion and collisional and radiative transitions, is introduced and compared to the measured level populations for the barium ground state and excited 5s5d3D2 state.

Enhanced optical nonlinearity and ultrafast carrier dynamics of TiO2/CuO nanocomposites

Transition metal oxides (TMOs) are drawing multitudinous interest due to their strong thermochemical stability. In the present work, we proposed a facile strategy to fabricate TiO 2 /CuO nanocomposites. By using the conventional open- and closed-aperture Z-scan technology, the eminent nonlinear optical (NLO) properties of the as-prepared TiO 2 /CuO nanocomposites were demonstrated. In order to deeply investigate the NLO enhancement mechanism, we established a theoretical model to study the photogenerated carriers transfer dynamics at the interface with rate equations and the fouth-order Runge-Kutta algorithm. The simulation results indicated that under pumping irradiation, the carriers transferring from CuO to TiO 2 promoted the nonlinear optical absorption of CuO nanosheets and increased the modulation depth. Experimentally, the transient absorption spectroscopy and broadband time-resolved pump-probe measurements were presented to study the role of the carrier dynamics in the broadband near-infrared region. To further confirm the enhanced NLO response in TiO 2 /CuO nanocomposites, a highly stable dissipative soliton mode-locked Yb-doped fiber laser with TiO 2 /CuO saturable absorber was constructed at 1 μm for the first time. Our work manifested the key effects of the interface in TiO 2 /CuO nanocomposites to boost the NLO features which can be applied in ultrafast photonics applications.

Quasi-Static and Dynamic Photon Bubbles in Cold Atom Clouds

Turbulent radiation flow is ubiquitous in many physical systems where light–matter interaction becomes relevant. Photon bubble instabilities, in particular, have been identified as a possible source of turbulent radiation transport in astrophysical objects such as massive stars and black hole accretion disks. Here, we report on the experimental observation of a photon bubble instability in cold atomic gases, in the presence of multiple scattering of light. Two different regimes are identified, namely, the growth and formation of quasi-static structures of depleted atom density and increased photon number, akin to photon bubbles in astrophysical objects, and the destabilisation of these structures in a second regime of photon bubble turbulence. A two-fluid theory is developed to model the coupled atom–photon gas and to describe both the saturation of the instability in the regime of quasi-static bubbles and the low-frequency turbulent phase associated with the growth and collapse of photon bubbles inside the atomic sample. We also employ statistical dimensionality reduction techniques to describe the low-dimensional nature of the turbulent regime. The experimental results reported here, along with the theoretical model we have developed, may shed light on analogue photon bubble instabilities in astrophysical scenarios. Our findings are consistent with recent analyses based on spatially resolved pump–probe measurements.

Atomic-scale quantum sensing based on the ultrafast coherence of an H2 molecule in an STM cavity.

A scanning tunneling microscope (STM) combined with a pump-probe femtosecond terahertz (THz) laser can enable coherence measurements of single molecules. We report THz pump-probe measurements that demonstrate quantum sensing based on a hydrogen (H2) molecule in the cavity created with an STM tip near a surface. Atomic-scale spatial and femtosecond temporal resolutions were obtained from this quantum coherence. The H2 acts as a two-level system, with its coherent superposition exhibiting extreme sensitivity to the applied electric field and the underlying atomic composition of the copper nitride (Cu2N) monolayer islands grown on a Cu(100) surface. We acquired time-resolved images of THz rectification of H2 over Cu2N islands for variable pump-probe delay times to visualize the heterogeneity of the chemical environment at sub-angstrom scale.

Characterisation of optical phonons within epitaxial Ge2Sb2Te5/InAs(111) structures

Femto-second pump-probe and micro-Raman spectroscopy (RS) measurements have been made to identify optical phonons in Ge2Sb2Te5/InAs(111) and an InAs(111) substrate. A theory of transient stimulated Raman scattering (TSRS) incorporating the Raman tensor predicts which phonon modes may be observed in transient reflectance (R) and anisotropic reflectance (AR) pump-probe measurements, and how their amplitudes depend upon angles φ and θ that describe the orientation of the pump and probe beam electric fields within the sample plane. AR measurements of an InAs(111) substrate revealed the 6.5 THz T2 transverse optical phonon with amplitude proportional to sin(2(θ−φ)), as expected for both TSRS and the specular optical Kerr effect (SOKE), confirming that TSRS and SOKE are equivalent descriptions of the same phenomenon. The AR responses of Ge2Sb2Te5/InAs(111) revealed a single coherent optical phonon (COP) mode at about 3.4 THz with sin(2(θ−φ)) amplitude variation that confirms the T2-like character of the mode and hence the underlying cubic structure of the epilayer. This mode was also observed in the R measurement for one sample, with amplitude independent of φ and θ as predicted by TSRS theory. Both R and AR signals were heavily damped, which is attributed to dephasing of T2x, T2y and T2z modes that become non-degenerate due to structural distortions. RS measurements revealed three modes for Ge2Sb2Te5/InAs(111) and three modes for InAs(111). Taken together the TSRS and RS measurements provide rich information about optical phonons in the phase change material Ge2Sb2Te5 and the InAs(111) surface.

Ultrafast low-pump fluence all-optical modulation based on graphene-metal hybrid metasurfaces

Graphene is an attractive material for all-optical modulation because of its ultrafast optical response and broad spectral coverage. However, all-optical graphene modulators reported so far require high pump fluence due to the ultrashort photo-carrier lifetime and limited absorption in graphene. We present modulator designs based on graphene-metal hybrid plasmonic metasurfaces with highly enhanced light-graphene interaction in the nanoscale hot spots at pump and probe (signal) wavelengths. Based on this design concept, we have demonstrated high-speed all-optical modulators at near and mid-infrared wavelengths (1.56 μm and above 6 μm) with significantly reduced pump fluence (1–2 orders of magnitude) and enhanced optical modulation. Ultrafast near-infrared pump-probe measurement results suggest that the modulators’ response times are ultimately determined by graphene’s ultrafast photocarrier relaxation times on the picosecond scale. The proposed designs hold the promise to address the challenges in the realization of ultrafast all-optical modulators for mid-and far-infrared wavelengths.

Variable repetition frequency asynchronous optical sampling method without a feedback loop

The ultrafast pump–probe measurement represents a key technique to study fs–ps dynamics. The asynchronous optical sampling (ASOPS) method realizes fast and long time-range measurement with high time resolution using different repetition frequency pump–probe light pulses. The frequency difference Δf is an important parameter, as it dictates the measurement time and time resolution. However, usual ASOPS measurements require a complex and precise stabilizer to control Δf or it is difficult to change Δf. In this study, we use two free-running titanium/sapphire pulse lasers to develop a variable repetition frequency ASOPS (VRF-ASOPS) method without a stabilizer or feedback loop, where we can easily alter Δf by changing the cavity length of the probe light laser. To detect the coincidences of the pump–probe light pulses, we cause the instantaneous reflectivity change in a 100 nm platinum film by irradiating the pump light and observe it by the probe light. We use this signal as the trigger signal to directly determine Δf, which enables us to average and convert the measured responses without a stabilizer or feedback loop. Using this VRF-ASOPS system, we obtain pulse echo signals and 100 GHz Brillouin oscillations, which are equivalent to those measured by the mechanical delay line method, confirming the validity of our developed method.

Excitation polarization-independent photo-induced restoration of inversion symmetry in Td-WTe2

Td-WTe2 is a topologically nontrivial material and exhibits a variety of physical properties, such as giant unsaturated magnetoresistance and the unconventional thermoelectric effect, due to its topological nature. It is also known to exhibit ultrafast topological phase transitions that restore its inversion symmetry by intense terahertz and mid-infrared pulses, and these properties demonstrate the possibility of ultrafast control of devices based on topological properties. Recently, a novel photo-induced topological phase transition by using polarization-controlled infrared excitation has been proposed, which is expected to control the material topology by rearranging the atomic orbitals near the Weyl point. To examine this topological phase transition, we experimentally studied the excitation-polarization dependence of the infrared-induced phase dynamics in a thin-layer of Td-WTe2. Time-resolved second harmonic generation (SHG) measurements showed that SHG intensity decreases after the infrared pump regardless of the polarization. Polarization-resolved infrared pump–probe measurements indicated that the polarization-selected excited state relaxes quite rapidly (i.e., within 10–40 fs). Considering these experimental results, we conclude that it is difficult to control the photo-induced phase transition through orbital-selective excitation owing to the rapid loss of carrier distribution created by polarization-selective excitation in thin-layer Td-WTe2 under our experimental condition. These results indicate that the suppression of the electron scattering process is crucial for experimentally realizing the photo-induced phase transition based on the polarization selection rule of the materials.

Nonlinear optical properties and passively Q-switched laser application of a layered molybdenum carbide at 639 nm.

Molybdenum carbide (Mo2C) exhibits enormous potential applications in various optoelectronic and photonic fields due to its remarkably electrical and optical characteristics. Here, we fabricate a high-quality Mo2C film by the radio frequency magnetron sputtering deposition method. The nonlinear optical response and ultrafast dynamics are thoroughly studied based on open-aperture Z-scan and nondegenerate pump-probe experimental measurements. The open-aperture Z-scan experimental result exhibits a modulation depth of 8.5% and a saturation fluence of 0.28 mJ/cm2. Simultaneously, the relaxation time constant is fitted by a biexponential decay function, showing an ultrafast intraband carrier recovery time of 0.58 ps at 530 nm. Consequently, by employing the Mo2C film as a saturable absorber (SA), stable Q-switched Pr:YLF laser pulses with the shortest pulse width of 160 ns are generated at 639 nm. Our experimental results demonstrate excellent nonlinear optical properties of the layered Mo2C in the visible region and will further advance their potential applications in visible nonlinear optics.

Conformer-Specific Dissociation Dynamics in Dimethyl Methylphosphonate Radical Cation.

The dynamics of the dimethyl methylphosphonate (DMMP) radical cation after production by strong field adiabatic ionization have been investigated. Pump-probe experiments using strong field 1300 nm pulses to adiabatically ionize DMMP and a 800 nm non-ionizing probe induce coherent oscillations of the parent ion yield with a period of about 45 fs. The yields of two fragments, PO2C2H7+ and PO2CH4+, oscillate approximately out of phase with the parent ion, but with a slight phase shift relative to each other. We use electronic structure theory and nonadiabatic surface hopping dynamics to understand the underlying dynamics. The results show that while the cation oscillates on the ground state along the P=O bond stretch coordinate, the probe excites population to higher electronic states that can lead to fragments PO2C2H7+ and PO2CH4+. The computational results combined with the experimental observations indicate that the two conformers of DMMP that are populated under experimental conditions exhibit different dynamics after being excited to the higher electronic states of the cation leading to different dissociation products. These results highlight the potential usefulness of these pump-probe measurements as a tool to study conformer-specific dynamics in molecules of biological interest.

Laser-induced THz magnetism of antiferromagnetic CoF2

Excitation, detection, and control of coherent THz magnetic excitation in antiferromagnets are challenging problems that can be addressed using ever shorter laser pulses. We study experimentally excitation of magnetic dynamics at THz frequencies in an antiferromagnetic insulator CoF2 by sub-10 fs laser pulses. Time-resolved pump-probe polarimetric measurements at different temperatures and probe polarizations reveal laser-induced transient circular birefringence oscillating at the frequency of 7.45 THz and present below the Néel temperature. The THz oscillations of circular birefringence are ascribed to oscillations of the magnetic moments of Co2+ ions induced by the laser-driven coherent E g phonon mode via the THz analogue of the transverse piezomagnetic effect. It is also shown that the same pulse launches coherent oscillations of the magnetic linear birefringence at the frequency of 3.4 THz corresponding to the two-magnon mode. Analysis of the probe polarization dependence of the transient magnetic linear birefringence at the frequency of the two-magnon mode enables identifying its symmetry.

Higher-order Topological States in Acoustic Twisted Moiré Superlattices

Twisted moir\'e superlattices (TMSs) are emergent materials with exotic physical properties. Among their properties, higher-order topology is seldom realized and investigated in experiments. Here, we report on the experimental observation of a class of bilayer higher-order topological states in TMSs. We create the physical realization of acoustic TMSs using moir\'e twisting and ultrastrong interlayer couplings in bilayer honeycomb lattices of coupled acoustic cavities. Through such a design we reach an unexplored regime in TMSs that is not available in graphene TMSs and other solid-state TMS systems. We reveal that the ultrastrong interlayer couplings lead to a large acoustic band gap with unique higher-order band topology, which is characterized by unprecedented topological indices and layer-hybridized corner states. The higher-order topological edge and corner states are observed via acoustic pump-probe measurements, which show consistency with theory and simulations. Our study paves the way toward higher-order topological phenomena in TMSs.