Two-color Pump Probe Research Articles

Ultrafast photoinduced carrier transfer dynamics in monolayer MoS2/graphene heterostructure

Two-dimensional molybdenum disulfide (MoS2) has been proved to be a good candidate in photodetectors, and MoS2/graphene (MoS2/G) heterostructure has been widely used to expand the optical response wavelength of MoS2. To clarify the carrier transfer dynamics in the MoS2/G heterostructure, time-resolved transient absorption and two-color pump–probe measurements are performed. By comparing the carrier dynamics in MoS2 and MoS2/G under different pump wavelengths, we find that interfacial excitons are formed in the heterostructure, and fast hot carriers transfer (<200 fs) from graphene to MoS2 are observed. The results indicate that the formed heterostructure with graphene can not only expand the optical response wavelength of MoS2 but also improve the response time of the device in the near-infrared region.

GHz adhesion dynamics of gold nanoparticles down to 40 nm in diameter

We investigate the acoustic vibrations of single gold nanoparticles on a glass substrate using time-resolved optical spectroscopy. Using a two-color pump–probe setup, we measure the transient change of a particle's reflectivity induced by an optical pump pulse. Our measurements identify both the axial mode, which corresponds to an oscillation of the distance between nanoparticle and substrate, and the particle's fundamental breathing mode, an oscillation of particle diameter. We investigate the scaling of the axial mode frequency with the radial mode frequency. Our analysis reveals that the scaling established for larger particles extends down to diameters of 40 nm. This finding strongly suggests that continuum models of contact mechanics hold for contact areas comprising only a few thousand atoms.

Highly anisotropic transient optical response of charge density wave order in ZrTe3

Low dimensionality in CDW systems leads to anisotropic optical properties, in both equilibrium and non-equilibrium conditions. Here we perform polarized two-color pump probe measurements on a quasi-1D material ZrTe$_3$, in order to study the anisotropic transient optical response in the CDW state. Profound in-plane anisotropy is observed with respect to polarization of probe photons. Below $T_\mathrm{CDW}$ both the quasi-particle relaxation signal and amplitude mode (AM) oscillation signal are much larger with $\mathbf{E}_\mathrm{pr}$ nearly parallel to $a$ axis ($\mathbf{E}_\mathrm{pr} \parallel a$) than for $\mathbf{E}_\mathrm{pr}$ parallel to $b$ axis ($\mathbf{E}_\mathrm{pr} \parallel b$). This reveals that $\mathbf{E}_\mathrm{pr} \parallel a$ signal is much more sensitive to the variation of the CDW gap. Interestingly, the lifetime of the AM oscillations observed with $\mathbf{E}_\mathrm{pr} \parallel b$ is longer than $\mathbf{E}_\mathrm{pr} \parallel a$. Moreover, at high pump fluence where the electronic order melts and the AM oscillations vanish for $\mathbf{E}_\mathrm{pr} \parallel a$ , the AM oscillatory response still persists for $\mathbf{E}_\mathrm{pr} \parallel b$. We discuss possible origins that lead to such unusual discrepancy between the two polarizations.

Measurement of recombination mechanisms in mid-infrared W-superlattices

Mid-wave infrared LEDs based on 6.1Å III/V semiconductors have trailed well behind visible LEDs in their wallplug efficiency. One contributing inefficiency is the low internal quantum efficiency (IQE) of emission of such materials, which is attributable to dominating nonradiative Auger and Shockley-Read-Hall recombination. However, recent work within this group has identified a W-superlattice based on 6.1Å III/V semiconductors with a remarkably high IQE of 77% (8%) at 77K (300K). We present in this work two-color pump-probe, differential transmission measurements to measure the ABC recombination coefficients of this superlattice at both 77K and 300K to elucidate physical mechanisms.

Heterostructure MoS2@ZnO nanowires: Preparation, ultrafast nonlinear optical behavior and photoelectric functional application

The semiconductor heterostructure was proposed and prepared by the wide-band gap zinc oxide (ZnO) and narrow-band gap molybdenum disulfide (MoS2) materials. The interlayer exciton pumping efficiency and saturable/reverse saturable nonlinear absorption (NLA) characteristics of MoS2/ZnO heterostructure can be flexibly adjusted, revealing its important potential in exploring new optoelectronic devices. In this work, based on magnetron sputtering and chemical vapor deposition, the MoS2 nanosheets, unmodified ZnO nanowires and MoS2@ZnO heterostructure were successfully synthesized. The depending relationship of the laser intensity, pulse width and wavelength on the NLA of samples has been studied using Z-scan technique. The ultrafast dynamics process was researched using two-color pump probe technology with the excitation wavelength at 325, 380 and 400 nm. The fast and slow components characters of the ultrafast electron transfer process were discussed to analyzing its producing mechanism in detail. The photoelectric functional performances and physical mechanisms are studied in theory and experiment. This paper will provide a theoretical basis and guidance for manipulation and improvement of the nonlinear optical properties in semiconductor micro/nano-material, as well as the corresponding application in optoelectronic devices.

Low Frequency Collinear Pre-Resonant Impulsive Stimulated Raman Microspectroscopy

In this work, we extend low frequency impulsive stimulated Raman microspectroscopy to the pre-electronic resonance regime, using a broadband two-color collinear pump probe scheme which can be readily extended to imaging. We discuss the difficulties unique to this type of measurements in the form of competing resonant two-photon absorption processes and the means to overcome them, and successfully reduce the noise which arises due to those competing processes by eliminating the detected spectral components which do not contribute to the vibrational signature of the sample yet introduce most of the noise. Finally, we demonstrate low frequency spectroscopy of crystalline samples under near-resonant pumping showing both enhancement and spectral modification due to coupling with the electronic degree of freedom.

Vibrational Energy Redistribution between CH Stretching Modes in Alkyl Chain Monolayers Revealed by Time-Resolved Two-Color Pump-Probe Sum Frequency Spectroscopy.

The vibrational dynamics of the various CH stretching modes in a fatty acid Langmuir-Blodgett film was studied using a resonant narrowband infrared (IR) laser pulse for pumping and a broadband femtosecond IR visible pulse pair for detection in a sum frequency spectroscopy setup. The resulting two-dimensional spectra indicate that pumping either the antisymmetric methyl or methylene stretch results in the transfer of energy to the other modes on a time scale faster than 2 ps. This rapid process is followed by energy redistribution to other modes, presumably the bending and internal rotational modes, with a time constant of approximately 85 ps. The formation of gauche defects is not observed within the first 250 ps. The whole spectrum recovers on a time scale of several nanoseconds, indicating dissipation of the excitation energy into the substrate.

Ultrafast Carrier Dynamics of Undoped and Ho3+-Doped α-Bismuth Oxide Microrods

We report the femtosecond two-color pump–probe studies on bismuth oxide microrods. The undoped data show three distinct decays: (1) fastest decay (τ1 ∼ 0.3–0.5 ps) attributed to intraband nonlinear...

Onset of optical emission in femtosecond laser-induced plasmas and its correlation with surface dynamics monitored by pump–probe time-resolved microscopy

Femtosecond time-resolved images acquired by using a two-color pump–probe microscope have been used in combination with optical emission spectroscopy to determine the time scales for the onset of atomic and molecular emission in laser-induced plasmas.

Nonlinear absorption properties and excited-state charge-transfer dynamics of Er doped ZnO films

This work reports on the nonlinear optical properties of Er doped ZnO (EZO) films, measured by the Z-scan technique with different laser parameters (pulse width, wavelengths and energy). The nonlinear absorption mechanism of EZO films is analyzed by a singlet state three and four-level model, respectively. The effect of the direct current sputtering power of EZO films on the nonlinear optical properties was studied. From the results, the samples show the self-focusing effect and two photon absorption (TPA) induced excited state reverse saturable absorption (RSA) behavior. Under the different laser parameter excitation, it has been determined that the TPA induced saturable absorption (SA) or RSA properties for EZO films with 8 W dc sputtering power. Moreover, these findings indicate that free carrier density increases with Er doping and this behavior leads to excited state absorption. In addition, the ultrafast dynamics of EZO films have been investigated using the two-color pump-probe (TCPP) technique with 325, 380 and 400 nm wavelength excitation and probing over a broad range in the visible region. The decay of the positive signal is found to be biexponential, which we have assigned to the pump-induced deep-level (DL) or excited state absorption. We also present experimental results that the few decade picosecond component has been assigned to vibrational relaxation in the excited electronic states, and the slow components represent the decay from singlet excited state to the ground state ( 1.2 ns). Moreover, these findings indicate that the excitation wavelength increases and the detection wave band widens. Our results show that EZO films are a promising candidate in further optoelectronic device applications.

Probing the non-equilibrium transient state in magnetite by a jitter-free two-color X-ray pump and X-ray probe experiment.

We present a general experimental concept for jitter-free pump and probe experiments at free electron lasers. By generating pump and probe pulse from one and the same X-ray pulse using an optical split-and-delay unit, we obtain a temporal resolution that is limited only by the X-ray pulse lengths. In a two-color X-ray pump and X-ray probe experiment with sub 70 fs temporal resolution, we selectively probe the response of orbital and charge degree of freedom in the prototypical functional oxide magnetite after photoexcitation. We find electronic order to be quenched on a time scale of (30 ± 30) fs and hence most likely faster than what is to be expected for any lattice dynamics. Our experimental result hints to the formation of a short lived transient state with decoupled electronic and lattice degree of freedom in magnetite. The excitation and relaxation mechanism for X-ray pumping is discussed within a simple model leading to the conclusion that within the first 10 fs the original photoexcitation decays into low-energy electronic excitations comparable to what is achieved by optical pump pulse excitation. Our findings show on which time scales dynamical decoupling of degrees of freedom in functional oxides can be expected and how to probe this selectively with soft X-ray pulses. Results can be expected to provide crucial information for theories for ultrafast behavior of materials and help to develop concepts for novel switching devices.

Two-Color Pump-Probe Measurement of Photonic Quantum Correlations Mediated by a Single Phonon.

We propose and demonstrate a versatile technique to measure the lifetime of the one-phonon Fock state using two-color pump-probe Raman scattering and spectrally resolved, time-correlated photon counting. Following pulsed laser excitation, the n=1 phonon Fock state is probabilistically prepared by projective measurement of a single Stokes photon. The detection of an anti-Stokes photon generated by a second, time-delayed laser pulse probes the phonon population with subpicosecond time resolution. We observe strongly nonclassical Stokes-anti-Stokes correlations, whose decay maps the single phonon dynamics. Our scheme can be applied to any Raman-active vibrational mode. It can be modified to measure the lifetime of n≥1 Fock states or the phonon quantum coherences through the preparation and detection of two-mode entangled vibrational states.

Ultrafast responses of two V-shaped compounds with a reverse conjugated structural configuration: an investigation of the reason for the enhanced two-photon absorption cross-section

To study the effect of the conjugated structural configuration on the two-photon absorption (TPA) properties of V-shaped compounds, two 1,3,5-triazine-based compounds with the same electron donor (D) and acceptor (A) connected in a reverse-conjugated structural configuration (T02: D- $$\pi$$ -A- $$\pi$$ -D; R02: A- $$\pi$$ -D- $$\pi$$ -A) were systematically investigated using steady-state and transient absorption spectroscopy, open-aperture Z-scan measurements, and two-photon fluorescence measurements. The TPA cross-section of compound R02 connected in a A- $$\pi$$ -D- $$\pi$$ -A-conjugated structural configuration with triphenylamine as the central core was 203 GM, which showed a 2.3-fold enhancement compared with compound T02 connected in a reverse D- $$\pi$$ -A- $$\pi$$ -D-conjugated structural configuration (90 GM, with 1,3,5-triazine as the central core). This result indicates that the conjugated structural configuration plays an important role in the TPA properties. A two-color pump–probe experiment was used to investigate the effect of the conjugated structural configuration on the excited state and intra-molecular charge transfer (ICT) properties of these V-shaped compounds. The formation and relaxation lifetimes of the ICT state were determined. The results indicate that the electron-donating/accepting strength of the central group, which serves as a communal group for two D- $$\pi$$ -A subunits, was confirmed to be a key role to the overall effect of the ICT for V-shaped compounds. These ultrafast dynamic results are in agreement with the TPA properties.

The photoluminescence, ultrafast nonlinear optical properties and carrier dynamics of 1D In-doped ZnO nanostructures: Experiment and mechanism

1D In-doped ZnO (IZO) nanostructures with different morphologies, namely nanowires, nanoseries(serial), nanorods and nanoprisms, have been successfully synthesized by chemical vapour deposition method. The structural, morphology and optical properties of IZO nanowires have been characterized. The ultrafast nonlinear optical and carrier dynamics characteristics of IZO were studied using femtosecond Z-scan, time-resolved photoluminescence (TRPL) and two-color pump-probe (TCPP) technique, respectively. The TRPL showed that the luminescence decay lifetimes of about several 10 ps and nanoseconds time response scale can be attributed to the shallow trap (ST) state, and deep trap (DT) state emissions, respectively. The mechanisms of the TRPL behavior in the different emission band were analyzed. The result of TCPP experiment showed that the bleach recovery with lifetimes on the fast (100–200 ps), and slow (>1 ns) time scale, caused by the shallow donor mixed with donor bound recombination, and donor−acceptor pair recombination (DAP), respectively. The optical transition process in the IZO nanostructures has demonstrated that the energy transfer process from the host electron-hole pairs can efficiently excite the metal ions. Comparing with the results obtained from the TRPL experiment, we found the fast decay process was resulted from the exciton migration and the slow decay to the recombination process of the relaxed excitons. Our results show that IZO nanostructures are a promising candidate in further optoelectronic device applications.

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.

Ultrafast switching of valence and generation of coherent acoustic phonons in semiconducting rare-earth monosulfides

We report on the ultrafast carrier dynamics and generation of coherent acoustic phonons in YbS, a semiconducting rare-earth monochalcogenide, using two-color pump–probe reflectivity. Compared to the carrier relaxation processes and lifetimes of conventional semiconductors, recombination of photoexcited electrons with holes in localized f orbitals is found to take place rapidly with a density-independent time constant of <500 fs in YbS. Such carrier annihilation signifies the unique and ultrafast nature of valence restoration of ytterbium ions after femtosecond photoexcitation switching. Following transfer of the absorbed energy to the lattice, coherent acoustic phonons emerge on the picosecond timescale as a result of the thermal strain in the photoexcited region. By analyzing the electronic and structural dynamics, we obtain the physical properties of YbS including its two-photon absorption and thermooptic coefficients, the period and decay time of the coherent oscillation, and the sound velocity.

Intense-Field Multiple-Detachment of F2¯: Competition with Photodissociation.

The competition of intense-field multiple-detachment with efficient photodissociation of F2¯ is studied as a function of laser peak intensity. The main product channels are disentangled and characterized by 3D coincidence fragment imaging. The presented kinetic energy release spectra, angular distributions, as well as two-color pump-probe measurements allow identification of competing sequential and nonsequential mechanisms. Dissociative detachment, producing two neutral atoms (F + F), is found to be dominated by a sequential mechanism of photodissociation (F¯ + F), followed by detachment of the atomic anion fragment. In contrast, dissociative ionization (F + F+) shows competing contributions of both a sequential two-step mechanism as well as a nonsequential double-detachment of the molecular anion, which are distinguished by the kinetic energy released in the dissociation. Triple-detachment is found to be nonsequential in nature and results in Coulomb explosion (F+ + F+). Furthermore, the measured kinetic energy release for dissociation on the 2Σg+ state provides a direct measurement of the F2¯ dissociation energy, D0 = 1.26 ± 0.03 eV.

Ultrafast vibrational energy flow in water monomers in acetonitrile

Vibrational relaxation of the OH stretching and bending modes of water monomers in acetonitrile is studied by two-color pump–probe experiments in a frequency range from 1400 to 3800cm−1. Measurements with resonant infrared excitation reveal vibrational lifetimes of 6.4±1.0ps of the OH stretching modes and 4.0±0.5ps of the OH bending mode. After OH stretching excitation, the OH bending mode shows an instantaneous response, a hallmark of the anharmonic coupling of stretching and bending modes, and a delayed population buildup by relaxation of the stretching via the bending mode. The relaxation steps are discussed within the framework of current theoretical pictures of water’s vibrational relaxation.

Theoretical Study of Ultrafast Electron Dynamics in Amino Acids

SynopsisPrompt ionization of large biological molecules may induce ultrafast charge migration along the molecular skeleton, preceding any nuclear rearrangement. This phenomenon has been recently observed in the amino acid phenylalanine in a two-color pump probe experiment, where the production of ionic fragments was measured as a function of the time delay between the two pulses and charge fluctuations manifested as sub-4.5 fs oscillations in the quantum yield of a specific doubly charged fragment. We present our latest results in glycine, and compare with previous findings in phenylalanine. We seek to perform a systematic study including larger aminoacids such as tryptophan.

Tracking ultrafast relaxation dynamics of furan by femtosecond photoelectron imaging

Ultrafast internal conversion dynamics of furan has been studied by femtosecond photoelectron imaging (PEI) coupled with photofragmentation (PF) spectroscopy. Photoelectron imaging of single-color multi-photon ionization and two-color pump–probe ionization are obtained and analyzed. Photoelectron bands are assigned to the related states. The time evolution of the photoelectron signal by pump–probe ionization can be well described by a biexponential decay: two rapid relaxation pathways with time constants of ∼15fs and 85 (±11)fs. The rapid relaxation is ascribed to the ultrafast internal conversion (IC) from the S2 state to the vibrationally hot S1 state. The second relaxation process is attributed to the redistributions and depopulation of secondarily populated high vibronic S1 state and the formation of α-carbene and β-carbene by H immigration. Additionally, the transient characteristics of the fragment ions are also measured and discussed as a complementary understanding.