Probe Photon Energy Research Articles

Dynamics of photoexcited carriers in graphene

The nonequilibrium dynamics of carriers and phonons in graphene is investigated by solving the microscopic kinetic equations with the carrier-phonon and carrier-carrier Coulomb scatterings explicitly included. The Fermi distribution of hot carriers are found to be established within 100 fs and the temperatures of electrons in the conduction and valence bands are very close to each other, even when the excitation density and the equilibrium density are comparable, thanks to the strong inter-band Coulomb scattering. Moreover, the temporal evolutions of the differential transmission obtained from our calculations agree with the experiments by Wang et al. [Appl. Phys. Lett. 96, 081917 (2010)] and Hale et al. [Phys. Rev. B 83, 121404 (2011)] very well, with two distinct differential transmission relaxations presented. We show that the fast relaxation is due to the rapid carrier-phonon thermalization and the slow one is mainly because of the slow decay of hot phonons. In addition, it is found that the temperatures of the hot phonons in different branches are different and the temperature of hot carriers can be even lower than that of the hottest phonons. Finally, we show that the slow relaxation rate exhibits a mild valley in the excitation density dependence and is linearly dependent on the probe-photon energy.

Plasmonic crystals for ultrafast nanophotonics: Optical switching of surface plasmon polaritons

We demonstrate that the dispersion of surface plasmon polaritons in a periodically perforated gold film can be efficiently manipulated by femtosecond laser pulses with the wavelengths far from the intrinsic resonances of gold. Using a time- and frequency- resolved pump-probe technique we observe shifting of the plasmon polariton resonances with response times from 200 to 800 fs depending on the probe photon energy, through which we obtain comprehensive insight into the electron dynamics in gold. We show that Wood anomalies in the optical spectra provide pronounced resonances in differential transmission and reflection with magnitudes up to 3% for moderate pump fluences of 0.5 mJ/cm^2.

Ultrafast relaxation of highly excited hot electrons in Si: Roles of theL−Xintervalley scattering

Two-photon photoemission (2PPE) spectroscopy is used to reveal dynamic relaxation of highly excited electrons generated by 3 eV photons in Si. Monochromatic 2PPE at probe-photon energies ranging from 3.2 to 3.5 eV reveals a coherent 2PPE peak from the valence band maximum, and a transiently populated photoemission peak near L1 in the conduction band. Time-resolved 2PPE measurements show that electrons injected into the L valley undergo L-to-X intervalley scattering with a time constant of 180 fs at 293 K. A deformation potential constant of 1.2 eVA−1 is obtained for the scattering. The highly excited electrons are relaxed down around the conduction band minimum to form a quasi-equilibrated hot electron distribution; the highest density is accumulated at 660 fs after excitation. The hot electron distribution is equilibrated with the lattice within 2 ps after excitation. These results are compared to relaxation processes of electrons injected directly into the X valley by photons with energies below 2.2 eV.

Coherent phonon generation in semiconducting single-walled carbon nanotubes using a few-cycle pulse laser

Coherent phonon dynamics in single-walled carbon nanotubes with several chiralities is investigated by 7.1-fs pump–probe experiments. Vibrational wave-packets corresponding to the radial breathing mode (RBM) and G mode can be detected for four chiral systems, (6,4), (6,5), (7,5) and (8,3). Coherent phonon generation of RBMs is in-depth studied by analyzing the probe photon energy dependent amplitude profiles, which indicates that the real and imaginary parts of the third-order susceptibility can both contribute to the modulation of the probed difference absorbance.

Ultrafast modulation of optical transitions in monolayer and multilayer graphene

We study ultrafast modulations of absorption spectra for both monolayer and multilayer graphene, by performing time-resolved transmission measurements with tuning probe photon energy. While reduced absorptions by photo-excited carriers are observed in monolayer graphene irrespective of the probe energy, multilayer graphene shows increased absorption at around 0.6 eV, which is explained by the optical transitions between subband states. Intraband carrier relaxation and electron–hole recombination times are found to be as fast as 0.5 and 10 ps, respectively. Modifications of ultrafast carrier dynamics are also studied with changing temperature and excitation density.

Broadband Light-Induced Absorbance Change in Multilayer Graphene

We report the ultrafast light-induced absorbance change in CVD-grown multilayer graphene. Using femtosecond pump-probe measurements in 1100-1800 nm spectral range, we revealed broadband absorbance change when the probe photon energy was higher than that of the pump photon. The observed phenomenon is interpreted in terms of the Auger recombination and impact ionization playing a significant role in the dynamics of photoexcited carriers in graphene.

Electronic structure in real time: mapping valence electron rearrangements during chemical reactions

The interest in following the evolution of the valence electronic structure of atoms and molecules during chemical reactions on a femtosecond time scale is discussed. By explicitly mapping the occupied part of the electronic structure with femtosecond pump-probe schemes one essentially follows the electrons making the bonds while the bonds change. This holds the key to unprecedented insight into chemical bonding in short-lived intermediates and reveals the coupled motion of electrons and nuclei. Examples from the recent literature on small molecules and anionic clusters in the gas phase and on atoms and molecules on surfaces using lab-based femtosecond laser methods are used to demonstrate the case. They highlight how the evolution of the valence electronic structure can be probed with time-resolved photoelectron spectroscopy with ultraviolet (UV) probe photon energies of up to 6 eV. It is shown how new insight can be gained by extending the probing wavelength into the vacuum-ultraviolet (VUV) region to photon energies of 20 eV and more by accessing the whole occupied valence electronic structure with time-resolved VUV photoelectron spectroscopy. Finally, the importance of soft X-ray free-electron lasers with probe photon energies of several hundred eV and femtosecond pulses and in particular the key role of femtosecond time-resolved soft X-ray emission spectroscopy or resonant inelastic X-ray scattering for mapping the electronic structure during chemical reactions is discussed.

Femtosecond UV-pump/visible-probe measurements of carrier dynamics in stacked graphene films

The transient differential transmission (ΔT/T) spectra of graphene were obtained from 380 (3.3 eV) to 670 nm (1.9 eV). The intraband carrier equilibration by carrier–carrier scattering occurred within 60 fs. The subsequent carrier relaxation process was governed by carrier-optical phonon scattering and had linear dependence on the probe photon energy (Epr); lifetimes ranged from 180 to 90 fs for Epr from 2.1 to 2.8 eV. Negative ΔT/T signals in kinetic curves were discussed and assigned to thermal diffusion and shrinkage of band separation caused by lattice heating.

The photodissociation of NO2 by visible and ultraviolet light

We present velocity map images of the NO, O((3)P(J)) and O((1)S(0)) photofragments from NO(2) excited in the range 7.6 to 9.0 eV. The molecule was initially pumped with a visible photon between 2.82-2.95 eV (440-420 nm), below the first dissociation threshold. A second ultraviolet laser with photon energies between 4.77 and 6.05 eV (260-205 nm) was used to pump high-lying excited states of neutral NO(2) and/or probe neutral photoproducts. Analysis of the kinetic energy release spectra revealed that the NO photofragments were predominantly formed in their ground electronic state with little kinetic energy. The O((3)P(J)) and O((1)S(0)) kinetic energy distributions were also dominated by kinetically 'cold' fragments. We discuss the possible excitation schemes and conclude that the unstable photoexcited states probed in the experiment were Rydberg states coupled to dissociative valence states. We compare our results with recent time-resolved studies using similar excitation and probe photon energies.

Ligth induced bleaching and absorption kinetics in highly excited InN layers

AbstractWe study the carrier recombination in highly excited InN epilayers by means of femtosecond differential transmission technique. The light induced bleaching or absorption is observed by tuning the probe photon energy by 100‐200 meV above or below the bandgap. The induced absorption decays roughly twice faster than the induced bleaching at the absorption edge. We conclude that the induced absorption kinetics show dynamics of the overall excess carrier population, while the bleaching at the absorption edge is related to carrier recombination in the band tails. Lifetime τ of absorption decay is inversely dependent on the total electron density τ=B*·(n0+N) with B*=4×10–10 cm3/s, which agrees well with the results of previous measurements. High B* value cannot be attributed to band‐to‐band recombination and supports the assumption that defect‐assisted Auger is the dominant nonlinear recombination process in InN. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Puzzling spectral structures of molecular vibration observed in ultrafast pump–probe experiment of transparent material

A sub-5-fs ultrafast pump–probe experiment was applied to neat liquid tetrahydrofuran (THF). The observed periodic modulation of transmittance change (Δ T), ∼5 ×10 −3, is assigned to the ground-state vibrational mode of C–H stretching. Although the sample has no stationary absorption in the wavelength region under study, the modulation amplitude shows strong dependence on the probe photon energy. A model is proposed to explain this feature based on the coherent Raman coupling between the fundamental and Stokes-shifted spectral components. The results demonstrate that a special attention is required in distinguishing the ground- and excited-state vibrational signals in ultrafast transient absorption spectroscopy.

Photoinduced Change in the Charge Order Pattern in the Quarter-Filled Organic Conductor(EDO−TTF)2PF6with a Strong Electron-Phonon Interaction

The quasistable state in the photoinduced phase transition for the quasi-one-dimensional quarter-filled organic conductor (EDO-TTF)2PF6 has been examined by ultrafast reflective measurements and time-dependent model calculations incorporating both electron-electron and electron-phonon interactions. The transient optical conductivity spectrum over a wide probe photon-energy range revealed that photoexcitation induced a new type of charge-disproportionate state. Additionally, coherent and incoherent oscillations dependent on probe photon energies were found, as predicted by the calculation.

Mechanism of ultrafast modulation of the refraction index in photoexcitedInxGa1−xAs/AlAsySb1−yquantum well waveguides

Ultrafast index change induced by intersubband excitation in ${\text{In}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{As}/{\text{AlAs}}_{y}{\text{Sb}}_{1\ensuremath{-}y}$ quantum well waveguides was evaluated by measuring the phase shift $\ensuremath{\Delta}\ensuremath{\phi}$ in a wide spectral range of $1.2--1.7\text{ }\ensuremath{\mu}\text{m}$ based on a spectral analysis. We obtained a large enhancement in $\ensuremath{\Delta}\ensuremath{\phi}$ by tuning the probe photon energy to interband absorption edge. The measured induced absorption $\ensuremath{\Delta}A$ also increased with reduction in the wavelength. The spectra of $\ensuremath{\Delta}\ensuremath{\phi}$ and $\ensuremath{\Delta}A$ were well explained by the interband dispersion model under intersubband excitation while the intersubband plasma dispersion previously reported has a negligible contribution.

The photo-induced phase and coherent phonon in the organic conductor(EDO-TTF)2PF6

We have investigated the nature of the photo-induced stateand coherent phonon in the conducting charge transfer complex(EDO-TTF)2PF6 by measuring the ultrafast reflectivity change over a wide photon energy range from 0.069 eV(18 µm) to 2.1 eV (580 nm). The photo-induced spectra just after photo-excitation indicate that thephoto-induced phase is similar to but clearly different from that in a thermally inducedmetal phase though they are the same for general photo-induced phase transition. Thetemporal profiles at each probe photon energy are accompanied by extraordinarily largeamplitude oscillation originating from coherent phonon generation. The periodof the coherent phonon depends discretely on the probe photon energy, whilethe generation efficiency of both the photo-induced phase and coherent photonreaches a maximum when the peak top of a charge transfer band is excited. Theseresults are probably attributable to the strong electron–vibration coupling in thiscomplex.

Ultrafast carrier dynamics in an InAs/InGaAs quantum dots-in-a-well heterostructure

Ultrafast differential transmission spectroscopy is used to explore temperature-dependent carrier dynamics in an InAs/InGaAs quantum dots-in-a-well heterostructure. Electron-hole pairs are optically injected into the three dimensional GaAs barriers, after which we monitor carrier relaxation into the two dimensional InGaAs quantum wells and the zero dimensional InAs quantum dots by tuning the probe photon energy. We find that carrier capture and relaxation are dominated by Auger carrier-carrier scattering at low temperatures, with thermal emission playing an increasing role with temperature. Our experiments provide essential insight into carrier relaxation across multiple spatial dimensions.

Classification of Dynamic Vibronic Couplings in Vibrational Real-Time Spectra of a Thiophene Derivative by Few-Cycle Pulses

Pump-probe spectroscopy was performed with a few cycle pulses of 6.7 fs duration. The electronic transition intensity modulation was induced by molecular vibration in a quinoid thiophene molecule in solution. The real-time vibrational features were analyzed in terms of dependence of vibrational amplitude and phase on probe photon energy. The electronic transition probability is modulated by molecular vibration via vibronic coupling. Changes in the spectral shape and intensity of the time-resolved spectrum were studied by tracking characteristic spectral features including the peak frequency and intensity, spectral bandwidth, and band-integrated intensity. From the analysis the modulation mechanisms were classified into two groups: (1) Condon type and (2) non-Condon type. The features of the wave packet motions were also classified into zeroth-order derivatives due to quasi-pure non-Condon type and first- and second-order derivative types due to the displacement of the potential minimum and the potential curvature change associated with the relevant vibronic transition, respectively.

Acoustic phonon generation and detection inGaAs∕Al0.3Ga0.7Asquantum wells with picosecond laser pulses

Picosecond acoustic-phonon pulse generation and detection is investigated in a sample containing three GaAs/Al0.3Ga0.7As quantum wells of different thickness with an interferometric optical pump and probe technique. The pump photon energy is tuned through the hh1-e1 transitions of each well and the probe photon energy is chosen to allow the detection of the phonon pulses at the sample surface. The phonon pulse shapes are explained with a model that relates the carrier wave functions to the acoustic strain, and the acoustic strain to the detected optical reflectance and phase changes.

Non-degenerate fs pump-probe study on InGaN with multi-wavelength second-harmonic generation

Non-degenerate fs pump-probe experiments in the UV-visible range for ultrafast carrier dynamics study of InGaN with adjustable pump and probe photon energies are implemented with simultaneously multiwavelength second-harmonic generation (SHG) of a 10 fs Ti:sapphire laser. The multi-wavelength SHG is realized with two beta-barium borate crystals of different cutting angles. The full-widths at half-maximum of the SHG pulses are around 150 fs, which are obtained from the cross-correlation measurement with a reverse-biased 280-nm light-emitting diode as the twophoton absorption photo-detector. Such pulses are used to perform nondegenerate pump-probe experiments on an InGaN thin film, in which indium-rich nano-clusters and compositional fluctuations have been identified. Relaxation of carriers from the pump level to the probe one through the scattering-induced local thermalization (<1 ps) and then the carrier-transport-dominating global thermalization (in several ps) processes is observed.

Investigation of nonlinear properties of CdS-doped glasses

The dynamics of photodarkening and transient absorption in commercial CdS-doped filter glass Asahi Y45 has been investigated at room temperature using pump and probe spectroscopy. The fresh spot of the sample was irradiated by pump pulse with the wavelength of 355 nm above the band gap and the probing was done at different wavelengths (1064, 760, and 469 nm). Influence of exposure as well as probe wavelength on dynamics of carriers has been investigated. The long lived photoinduced absorption dynamics corresponding to the higher probe photon energy illustrates the dominant contribution of the trapping states from the glass matrix. A simple model has been presented to understand the photoinduced carrier dynamics. The amplitude of differential absorbance as well as the photodarkening dynamics connected to the interior states are found to be in good agreement with the Drude's free electron model.

Ultrafast light-induced dichroism in silver nanoparticles

Ultrafast light-induced dichroism in silver nanoparticles is studied. Two differently prepared colloids are examined: one sample has a broad distribution of sizes and shapes of silver nanoparticles, while the second sample, which is processed by laser ablation, presents a more uniform size and shape distribution (greater fraction of spherical particles). The experiments are performed with a femtosecond laser in a two-color, polarization-resolved, pump-probe setup. For the pristine colloid the signal amplitudes for the parallel and perpendicular polarizations present different behaviors as a function of the probe photon energy, which is not observed for the laser ablated samples. The temporal response of the samples is also different, with the laser ablated samples presenting faster electronic cooling rates. A model describing the induced dichroism that takes into account the shape of the colloid particles is presented. The effects of the size distribution are also discussed.