Time-resolved Pump-probe Experiments Research Articles

Syntheses, crystal structures and non-linear optical responses of two new heteroselenometallic cluster compounds containing dithiocarbamate ligands

Two novel dialkyldithiocarbamate-containing heteroselenometallic clusters, [NEt4]2[(WSe4)(Et2NCS2Cu)3] 1 and [NEt4]2[(WSe4)(Me2NCS2Cu)4] 2, have been synthesized through reactions of [NEt4]2[WSe4] with CuCl and R2NCS2Na (R = Et or Me) in DMF. The [(WSe4)(Et2NCS2Cu)3]2− anion in 1 comprises three Et2NCS2Cu fragments ligated by a slightly distorted tetrahedral WSe4 moiety. The [(WSe4)(Me2NCS2Cu)4]2− anion structure in 2 possesses a nearly planar WCu4 core and consists of four Me2NCS2Cu fragments co-ordinated across four edges of the tetrahedral [WSe4]2− moiety. The optical limiting effects of the two clusters were examined at a 0.5 Hz repetition rate. The thresholds of two samples were 6.0 and 1.1 J cm−1 for 1 and 2, respectively. The non-linear responses for the two clusters in DMF have also been studied in picosecond time-resolved pump-probe experiments.

Femtosecond Time-Resolved Near-Field Spectroscopy of CdSe Nanocluster Films

By using scanning near-field optical microscopy in the femtosecond time-resolved pump-probe experiments, we have studied the time-resolved near-field spectroscopy of CdSe nanocluster films. The submicrometer space dependence of electron dynamics was observed, which is attributed to the non-uniform distributions of interstitial defects in the films.

Saturation of intraband absorption and electron relaxation time in n-doped InAs/GaAs self-assembled quantum dots

We have observed the saturation of intraband absorption in InAs/GaAs self-assembled quantum dots. The investigated n-doped self-assembled quantum dots exhibit an intraband absorption within the conduction band, which is peaked at an 8 μm wavelength. The saturation of the intraband absorption is achieved with an infrared pump delivered by a pulsed free-electron laser. The saturation of the transition is observed for an intensity around ≈0.6 MW cm−2. The electron relaxation time under intraband excitation is measured by time-resolved pump–probe experiments. An electron relaxation time T1≈3 ps is reported.

Pump–probe diffraction imaging of vibrational wave functions

The theory of pump–probe diffraction experiments shows that probability density distributions of vibrational wave functions are experimentally observable. In the experiment a laser prepares a molecule in a selected vibrational state, in either the same or a different electronic manifold. The diffraction pattern of the molecule in the excited state is the Fourier transform image of the nuclear probability density distribution, as determined by the vibrational eigenfunction of the molecule. This suggests the possibility to directly observe components of molecular vibrational wave functions. Model calculations illustrate the concept on iodine molecules, and sodium dimers. The relevance to time-resolved pump–probe experiments that prepare vibrational wave packets is discussed.

Picosecond IR dynamics: lessons learned

We present some of the lessons we have learnt as we have attempted to do time-resolved IR pump-probe experiments at FELIX, the FEL at Nieuwegein, The Netherlands.

Investigation of mid-infrared intersubband stimulated gain under optical pumping in GaAs/AlGaAs quantum wells

We have investigated the mid-infrared intersubband stimulated emission under optical pumping in GaAs/AlGaAs coupled quantum wells. The quantum wells exhibit four levels bound in the conduction band. The energy between the ground and first excited subband is close to the optical phonon energy enabling population inversion. Intersubband stimulated gain between subbands E3 and E2 is observed around 14 μm wavelength by optically pumping the E1−E3 intersubband transition at 10 μm. The gain measurement is performed by time-resolved pump-probe experiments using a two-color picosecond free-electron laser. The dependence of the intersubband stimulated emission is analyzed as a function of the pump intensity, and the pump and probe wavelengths. We show that very large intersubband stimulated gain can be achieved at liquid nitrogen temperature in a 2 mm thick waveguide. The stimulated gain is resonant with the pump wavelength with a broadening ≈25 meV. The experimental results are explained with a nonperturbative matrix density treatment of a three-level system.

Time-resolved thermoreflectivity of thin gold films and its dependence on film thickness

to \valunit{500}{nm} have been measured by femtosecond time-resolved pump-probe experiments. A conspicuous change of the relaxation behavior was found around \valunit{100}{nm} for pump pulse fluences of \valunit{1}{mJ/cm^{2}} . Thicker films show a nearly exponential decay of the transient linear reflectivity, which turns into a linear decay during the first \rangeunit{5}{7}{ps} for films with thicknesses of \valunit{100}{nm} or less. This observation is evidence of a mean free path of about \valunit{100}{nm} for hot electrons with temperatures around \valunit{1500}{K} .

Time-resolved pump-probe experiments with subwavelength lateral resolution

We demonstrate picosecond time-resolved pump-probe experiments with subwavelength resolution using a reflection near-field scanning optical microscope (NSOM). The sample was a laterally patterned gold nanostructure. Strain-induced reflectivity changes as low as ΔR/R≊10−4 were detected in the near-field.

Time-resolved studies of carriers dynamics in wide band gap materials

Lasers delivering high intensity ultrashort pulses are adequate tools to study the dynamics of irradiation induced modifications in wide band gap materials. We present the results of two complementary time-resolved pump-probe optical experiments. We use a technique of interferometry in the frequency domain to probe the instantaneous modification of the refractive index of the material induced by a pump pulse. The presence of free carriers in the conduction band induces a negative phase shift of the probe pulse which is proportional to the excitation density. Comparison between different oxides reveals an extremely contrasted behaviour: the photoexcited carriers are trapped in 150 fs in SiO 2, while they remain quasi-free for more than 50 ps in MgO and Al 2O 3. These lifetimes are closely connected to the formation of radiation induced defects. In particular, using a conventional time-resolved absorption technique in the UV (5.2–5.6 eV), we have studied the intrinsic defects formation kinetics in SiO 2. Our results show that the formation of self-trapped excitons (STE) is occuring in the ultrashort time of 150 fs, in agreement with the free carrier lifetime measured in the above experiment. At low temperature, the STE recombine radiatively, while at temperature above the luminescence quenching temperature, a small proportion of the STE is converted into permanent defects (O vacancies).

Femtosecond studies of the phase transition in Ti2O3.

We report femtosecond time-resolved pump-probe DECP experiments using a colliding pulse mode-locked laser performed on ${\mathrm{Ti}}_{2}$${\mathrm{O}}_{3}$ as the sample lattice temperature ${\mathit{T}}_{\mathit{L}}$ is raised from 300 K through the ``soft transition'' at 450 K to a temperature of 570 K. We have observed DECP spectra through the transition, with oscillations in reflectivity of a few percent associated with the low frequency ${\mathit{A}}_{1\mathit{g}}$ mode. A thermodynamic relation is found between the low frequency ${\mathit{A}}_{1\mathit{g}}$ equilibrium displacement and the number of excited electrons removed from the valence band. When applied to ${\mathit{T}}_{\mathit{L}}$-dependent equilibrium coordinate data for ${\mathrm{Ti}}_{2}$${\mathrm{O}}_{3}$ obtained in x-ray experiments, the theory allows a determination of the band overlap vs lattice temperature. The band overlap at ${\mathit{T}}_{\mathit{L}}$=621 K is found to be \ensuremath{\sim}0.06 eV. The ${\mathit{A}}_{1\mathit{g}}$ mode frequency ${\ensuremath{\nu}}_{\mathrm{ph}}$, the electronic relaxation rate (1/${\mathrm{\ensuremath{\tau}}}_{\mathrm{el}}$), and the phonon relaxation rate (1/${\mathrm{\ensuremath{\tau}}}_{\mathrm{ph}}$) have all been followed through the transition. ${\ensuremath{\nu}}_{\mathrm{ph}}$ decreases, and shows a partial recovery in agreement with other Raman studies. The behavior of (1/${\mathrm{\ensuremath{\tau}}}_{\mathrm{}\mathrm{el}}$) can be understood as due to an increase in available states for interband electron-phonon scattering as the band crossing takes place. Applying a deformation potential model to the data for (1/${\mathrm{\ensuremath{\tau}}}_{\mathrm{el}}$) before band crossing, with the low frequency ${\mathit{A}}_{1\mathit{g}}$ mode as the dominant scattering mechanism, a value of |D|\ensuremath{\simeq}2.0 eV is obtained for the valence band deformation potential associated with this mode. (1/${\mathrm{\ensuremath{\tau}}}_{\mathrm{ph}}$) does not show a clearcut correlation with bandcrossing due to greater scatter in the data. The temperature dependence is partially explained by the two-phonon decay of the coherent phonon excited in DECP, and may also have a component due to interaction with hot electrons as well as a dephasing contribution. \textcopyright{} 1996 The American Physical Society.

Control of the spectral-oscillation artifact in femtosecond pump–probe spectroscopy

We present time-resolved pump-probe experiments on a semiconductor in which the signal shape, dominated by perturbed free polarization decay, is strongly dependent on the probe pulse's spectral shape. This effect can be used efficiently to reduce large spectral oscillations that prevent the observation of weak signals near a narrow spectral structure.

Time-resolved pump-probe photoelectron spectroscopy of helium using a mode-locked laser synchronized with synchrotron radiation pulses

We report a new experimental setup consisting in the synchronization of 74.9094 MHz pulses from a mode-locked Ar+ laser with 8.32 MHz pulses of vacuum ultraviolet (VUV) synchrotron radiation from the Super-ACO storage ring of the Laboratoire pour l'Utilisation du Rayonnement Electromagnétique (LURE). The capabilities of the setup are demonstrated in a time-resolved pump-probe (VUV + visible) experiment in which free helium atoms are resonantly ionized via the short-lived 1s3p (1P) state. This experiment allowed us to obtain the value of 1.12 ns for the temporal resolution (FWHM) of the experimental setup and to show the relevance of this technique for the investigation of nanosecond dynamics on gas phase species.

Coherence effects in the anisotropy of optical experiments

It is often assumed that the anisotropy in time-resolved fluorescence and pump-probe experiments is always smaller than or at most equal to 2 5 . From resonance Raman experiments it is well known, however, that the anisotropy can range anywhere from 1 to − 1 2 (depolarization ratio between 0 and ∞) depending on the symmetry of the final state in the Raman transition. Since the resonance Raman intensity depends on the short-time vibrational dynamics in the excited state potential, it is to be expected that similar anisotropies will be observed in femtosecond pump-probe experiments. In this article the anisotropies in ultrashort pulse pump-probe experiments will be calculated and the original results, well known from the theory of resonance Raman, are recovered for zero delay times. In time-resolved pump-probe experiments there is a new effect, however, that is not present in resonance Raman scattering: for increasing delay times the anisotropy is expected to decay due to the electronic dephasing of the degenerate electronic states involved. Therefore measurement of the femtosecond anisotropy decay enables one to obtain electronic dephasing times in condensed phases.

Ultrafast recombination processes in lead chalcogenide semiconductors studied via picosecond optical nonlinearities

Picosecond recombination processes of photoexcited electron-hole plasma in lead chalcogenides are directly monitored via time-resolved pump-probe experiments at wavelengths from 3 to 10 mu m. Bleaching of the interband absorption due to band filling gives rise to a large change in the refractive index. The time-resolved change of absorption of PbSe at T=70 K reveals a rapid recombination within 100 ps for excitation densities above 7*1017 cm-3. Comparative measurements of mid-infrared luminescence spectra demonstrate that this behaviour is due to recombination by stimulated emission. At T=300 K, Auger recombination on a time scale of 2 ns is observed below the threshold of stimulated emission. The analysis of the time-resolved data gives an Auger coefficient of 1.1+or-0.3*10-28 cm6 s-1 for a carrier density of 3*1018 cm-3.

Picosecond recombination processes in lead selenide

Picosecond recombination processes of photoexcited electron-hole plasma in PbSe are directly monitored by time-resolved pump-probe experiments in the wavelength range from 3 to 9 μm. At temperatures T≤70 K, a rapid decrease of carrier density within 100 ps is found for excitation densities higher than 7×1017 cm−3. This behavior is due to recombination by stimulated emission, as is evident from comparative measurements of midinfrared luminescence spectra. At T=300 K, Auger recombination on a time scale of 2 ns is observed below the threshold of stimulated emission. The data for a carrier density of 3×1018 cm−3 give an Auger coefficient of c≂10−28 cm6/s.

Theory for displacive excitation of coherent phonons.

We report femtosecond time-resolved pump-probe reflection experiments in semimetals and semiconductors that show large-amplitude oscillations with periods characteristic of lattice vibrations. Only ${\mathit{A}}_{1}$ modes are detected, although modes with other symmetries are observed with comparable intensity in Raman scattering. We present a theory of the excitation process in this class of materials, which we refer to as displacive excitation of coherent phonons (DECP). In DECP, after excitation by a pump pulse, the electronically excited system rapidly comes to quasiequilibrium in a time short compared to nuclear response times. In materials with ${\mathit{A}}_{1}$ vibrational modes, the quasiequilibrium nuclear ${\mathit{A}}_{1}$ coordinates are displaced with no change in lattice symmetry, giving rise to a coherent vibration of ${\mathit{A}}_{1}$ symmetry about the displaced quasiequilibrium coordinates. One important prediction of the DECP mechanism is the excitation of only modes with ${\mathit{A}}_{1}$ symmetry. Furthermore, the oscillations in the reflectivity R are excited with a cos(${\mathrm{\ensuremath{\omega}}}_{0}$t) dependence, where t=0 is the time of arrival of the pump pulse peak, and ${\mathrm{\ensuremath{\omega}}}_{0}$ is the vibrational frequency of the ${\mathit{A}}_{1}$ mode. These predictions agree well with our observations in Bi, Sb, Te, and ${\mathrm{Ti}}_{2}$${\mathrm{O}}_{3}$. The fit of the experimental \ensuremath{\Delta}R(t)/R(0) data to the theory is excellent.

Second harmonic generation in plasmas produced by intense femtosecond laser pulses

Plasmas were produced by tightly focusing femtosecond laser pulses onto the surface of metal films and glass targets. A relatively intense, well-collimated second harmonic could be observed in the specular direction. The conversion efficiency reached 10/sup -4/ for focused intensities around 10/sup 16/ W/cm/sup 2/. Detailed measurements of the angular dependence, the polarization properties, and the second-harmonic frequency spectra were performed. Time-resolved pump-probe experiments revealed interesting information about the dynamics of the plasmas. It is suggested that resonant absorption plays an important role in second-harmonic generation.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Quantum beats in time-resolved two-photon ionization spectra of molecules

An expression for the transition probability of the time-resolved two-photon ionization process via coherently excited resonant intermediate states is derived using the perturbative density matrix formalism. Within this formalism, the phase relaxation due to system/bath interactions can be properly incorporated. Model calculations of time-resolved spectra have been carried out to demonstrate the appearance of quantum beats and dephasing effects in both time-resolved pump-probe and probing-pulse time-resolved experiments.