Results Of Pump-probe Experiments Research Articles

Naked-eye colorimetric anion probing and fluorescent switching features of conjugated Schiff Bases derived from 4-(Trifluoromethyl) benzenesulfonamide

In this work, synthesis, spectroscopic, density functional theory (DFT) and photophysical studies of the three conjugated benzenesulfonamide Schiff bases were reported. We also calculated the energies of the frontier molecular orbitals (FMOs) and UV–Vis spectra of Schiff bases theoretically using the DFT and TD-DFT methods. Additionally, photophysical properties were investigated by UV–Vis and fluorescence spectroscopy techniques. The ground state interactions show that the absorption band has been separated into two branches and bathochromatically tuned to the visible region, and fluorescence intensity decreased in the case of the Schiff base incorporated naphthalene unit. Furthermore, the charge transfer dynamics of the compounds were analyzed using femtosecond transient absorption spectroscopy measurements proving the triplet-triplet transitions via intersystem crossing mechanism. Based on the ultrafast pump-probe experimental results, as well as decay kinetics of the compounds, the Schiff base with naphthalene unit, showed a fast transition to the triplet state (1.2 ps) as compared to Schiff bases including methyl (4 ps) and chloro (2.7 ps) units. However, upon the addition of various anions, the molecule structure gains anion sensing and fluorescence switching capabilities under daylight and UV light. The obtained experimental results pointed out that the modification of Schiff base molecules with methyl, chlorine and naphthalene units affected its photophysical properties as well as excited-state dynamics significantly.

Observation of robust charge transfer under strain engineering in two-dimensional MoS2-WSe2 heterostructures.

Strain is one of the effective ways to modulate the band structure of monolayer transition metal dichalcogenides (TMDCs), which has been reported in theoretical and steady-state spectroscopic studies. However, the strain effects on the charge transfer processes in TMDC heterostructures have not been experimentally addressed thus far. Here, we systematically investigate the strain-mediated transient spectral evolutions corresponding to excitons at band-edge and higher energy states for monolayer MoS2 and monolayer WSe2. It is demonstrated that Γ and K valleys in monolayer WSe2 and monolayer MoS2 present different strain responses, according to the broadband femtosecond pump-probe experimental results. It is further observed that the resulting band offset changes tuned by applied tensile strains in MoS2-WSe2 heterostructures would not affect the band-edge electron transfer profiles, where only monolayer WSe2 is excited. From a flexible optoelectronic applications perspective, the robust charge transfer under strain engineering in TMDC heterostructures is very advantageous.

“Hot electrons in Si lose energy mostly to optical phonons”: Truth or myth?

Theoretical studies of heat generation and diffusion in Si devices generally assume that hot electrons in Si lose their energy mainly to optical phonons. Here, we briefly review the history of this assumption, and using full-band Monte Carlo simulations—with electron-phonon scattering rates calculated using the rigid-ion approximation and both empirical pseudopotentials and Harris potentials—we show that, instead, electrons lose as much as 2/3 of their energy to acoustic phonons. The scattering rates that we have calculated have been used to study hot-electron effects, such as impact ionization and injection into SiO2, and are in rough agreement with those obtained using density functional theory. Moreover, direct subpicosecond pump-probe experimental results, some of them dating back to 1994, are consistent with the predictions of our model. We conclude that the study of heat generation and dissipation in nanometer-scale Si devices may require a substantial revision of the assumptions that have been considered “common wisdom” so far.

Real time monitoring of fs laser annealing on indium tin oxide

In this study, a set of time-resolved pump-probe systems for the real-time monitoring of femtosecond (fs) laser annealing of ITO substrates is established. A pump beam is used as the light source for thermal annealing, and a probe beam is used to monitor the transmittance variation of ITO in real time. Measurement results show that with increasing machining energy, the transmittance variation detected by the probe beam gradually increases. When the laser energy is small, the transmittance changes arithmetically. When the laser energy is sufficient to complete the modification of ITO, the transmittance variation rises suddenly and sharply, indicating that the thermal annealing modification has been completed in the irradiated area. Then, the laser source is shut down to achieve real-time monitoring. Degenerate and non-degenerate pump-probe experimental results indicate that the 400-nm probe beam is more sensitive than the 800-nm probe beam in measuring curve variation because the variation in the ITO transmission spectrum in the 400-nm wave band is greater than that in the 800-nm wave band. Electron backscatter diffraction analysis indicates that the crystallographic directions of ITO after laser annealing are consistent. This shows that the ITO changed from non-crystallized to crystallized after being irradiated by the pump beam, confirming the thermal annealing modification.

LiY0.3Lu0.7F4: Ce3+,Pr3+Mixed Crystal as a Perspective Up-Conversionally Pumped UV Active Medium

Investigation results of effective population of states of 5d-configuration of Ce 3+ ions by energy transfer from Pr 3+ ions in LiY 0.3 Lu 0.7 F 4 (LYLF) crystals are discussed. The real concentrations of Pr 3+ and Ce 3+ ions in LYLF crystals are determined. Such parameters as excited 4f-5d state photoionization cross-section of Pr 3+ ions, ground state cross-section of Ce 3+ ions at 266 nm wavelengths and energy transfer coefficients of energy transfer from Pr 3+ to Ce 3+ ions were estimated. The results of pump-probe experiments on 5d-4f transitions of Ce 3+ ions in LYLF crystals are presented. The optimal parameters for getting maximal gain on 5d-4f transitions of Ce 3+ ions were determined by mathematical modeling.

Spectroscopic studies of resonant coupling of silver optical antenna arrays to a near-surface quantum well

The coupling of radiation emitted on semiconductor inter-band transitions to resonant optical-antenna arrays allows for enhanced light–matter interaction via the Purcell effect. Semiconductor optical gain also potentially allows for loss reduction in metamaterials. Here we extend our previous work on optically pumped individual near-surface InGaAs quantum wells coupled to silver split-ring-resonator arrays to wire and square-antenna arrays. By comparing the transient pump-probe experimental results with the predictions of a simple model, we find that the effective coupling is strongest for the split rings, even though the split rings have the weakest dipole moment. The effect of the latter must thus be overcompensated by a smaller effective mode volume of the split rings. Furthermore, we also present a systematic variation of the pump-pulse energy, which was fixed in our previous experiments.

All-optical modulation using two-photon absorption in silicon core optical fibers

All-optical modulation based on degenerate and non-degenerate two-photon absorption (TPA) is demonstrated within a hydrogenated amorphous silicon core optical fiber. The nonlinear absorption strength is determined by comparing the results of pump-probe experiments with numerical simulations of the coupled propagation equations. Subpicosecond modulation is achieved with an extinction ratio of more than 4 dB at telecommunications wavelengths, indicating the potential for these fibers to find use in high speed signal processing applications.

Sub-picosecond All-optical Switching of Exciton Spin Polarization in Nanocrystal Quantum Dots Toward Terabit Communication

We report all-optical polarization switching based on resonant excitation of spin-polarized e-h pairs in CdSe nanocrystal quantum dots at room temperature. Pump-probe experiments have been performed using the same and the opposite configurations of circular polarizations. Comparing the co- and the cross-circular polarization cases, it is quite evident that selectively-excited spins are flipped to a balanced state within 1 ps and that during this time, the two spin states are equally populated. The all-optical switching results are found to be in close agreement with pump-probe experimental results. The optical switching technique proposed in the present investigation should be extremely useful for future ultrafast terabit communication systems.

Microcavity-coupled optical Stark effect: Application to ultrafast all-optical modulation

We propose a modeling of the microcavity-coupled optical Stark effect that we compare to degenerate pump-probe experimental results obtained at room temperature on a bulk GaAs microcavity. The cavity energy mode is adjusted 15 meV below the edge of the GaAs bandgap. With a pump intensity as low as 0.4 MW/cm2, we could demonstrate an instantaneous reflectivity modulation with a 5:1 contrast ratio. The good agreement between experiment and modeling is promising to design structures well suited to all-optical modulation with low switching intensities.

Intersubband and interminiband transitions in CdS/ZnSe heterostructures

We report on the first investigation of intersubband and interminiband transitions in CdS/ZnSe multiple quantum wells and superlattices. The cubic heterostructures with type II band alignment were grown by molecular beam epitaxy on semi-insulating GaAs (0 0 1) substrates using CdS and ZnSe compound sources. The transmission spectra of n-type doped CdS/ZnSe heterostructures recorded in waveguide geometry show strong absorption lines in the mid-infrared. These absorption structures are attributed to intersubband (interminiband) transitions from the first to the second subband (miniband). The strong absorption is a direct consequence of the different refraction indices of the GaAs substrate and the wide-gap II–VI semiconductor heterostructure. Furthermore, first results of pump-probe experiments performed on lightly doped and undoped multiple quantum wells are presented. The photoinduced absorption was studied in dependence on the laser power. Thereby, the pump intensity was varied over three orders of magnitude.

Influence of fibre dispersion and bit rate on cross-phase-modulation-induceddistortion in amplified optical fibre links

New results of pump-probe experiments and simulations, investigating signal distortion due to cross-phase modulation in dispersion-shifted and dispersion-compensated, standard singlemode fibre non-return-to-zero systems at bit rates up to 10 Gbit/s for different channel spacings are reported. The mechanisms causing increased distortion with bit rate are identified, and the optimum positioning of the dispersion compensating fibre is shown to be dependent on the residual system dispersion.

Observations of subpicosecond dynamics in GaAlAs laser diodes

We present results of pump-probe experiments on GaAlAs laser diodes indicating a 0.9-ps relaxation time associated with the device transmission. Subpicosecond, tunable near infrared pulses obtained by fiber compression were used to carry out the experiments. The data strongly support a model in which a nonequilibrium carrier temperature in the active layer is responsible for the observed signal.