Photoinduced Transmission Change Research Articles

Extracting quantitative dielectric properties from pump-probe spectroscopy

Optical pump-probe spectroscopy is a powerful tool for the study of non-equilibrium electronic dynamics and finds wide applications across a range of fields, from physics and chemistry to material science and biology. However, a shortcoming of conventional pump-probe spectroscopy is that photoinduced changes in transmission, reflection and scattering can simultaneously contribute to the measured differential spectra, leading to ambiguities in assigning the origin of spectral signatures and ruling out quantitative interpretation of the spectra. Ideally, these methods would measure the underlying dielectric function (or the complex refractive index) which would then directly provide quantitative information on the transient excited state dynamics free of these ambiguities. Here we present and test a model independent route to transform differential transmission or reflection spectra, measured via conventional optical pump-probe spectroscopy, to changes in the quantitative transient dielectric function. We benchmark this method against changes in the real refractive index measured using time-resolved Frequency Domain Interferometry in prototypical inorganic and organic semiconductor films. Our methodology can be applied to existing and future pump-probe data sets, allowing for an unambiguous and quantitative characterisation of the transient photoexcited spectra of materials. This in turn will accelerate the adoption of pump-probe spectroscopy as a facile and robust materials characterisation and screening tool.

Ultrafast optical wide field microscopy

We have developed a new imaging method, ultrafast optical wide field microscopy, capable of rapidly acquiring wide field images of nearly any sample in a non-contact manner with high spatial and temporal resolution. Time-resolved images of the photoinduced changes in transmission for a patterned semiconductor thin film and a single silicon nanowire after optical excitation are captured using a two-dimensional smart pixel array detector. These images represent the time-dependent carrier dynamics with high sensitivity, femtosecond time resolution and sub-micrometer spatial resolution.

Unusual observation of fast photodarkening and slow photobleaching in a-GeSe2 thin film

We report an unusual observation of fast photodarkening and slow photobleaching in a-GeSe2 thin film, when subjected to 532nm band gap light illumination. Photobleaching is observed only after complete saturation of photodarkening. A detailed kinetic analysis of photoinduced transmission change reveals that photodarkening is one order faster in comparison to photobleaching. Interestingly, photobleaching has an induction time period and irreversible in nature whereas photodarkening appears to be an instantaneous process with light illumination showing transient characteristics.

Probing Ultrafast Carrier Dynamics in Silicon Nanowires

We present the first ultrafast optical pump-probe spectroscopic measurements, to the best of our knowledge, on silicon nanowires (SiNWs). In this study, we performed femtosecond pump-probe measurements on vapor-liquid-solid-grown SiNWs to investigate the influence of the NW diameter, pump and probe polarizations, and pump fluence on the observed dynamics while tuning the probe wavelength below and above the indirect bandgap in Si. For smaller NW diameters, carriers were found to relax more rapidly into both extended and localized states, indicating that a surface-mediated mechanism governs the observed dynamics. The magnitude of the photoinduced transmission change exhibited strong polarization dependence, showing that optical transitions in these quasi-1D systems are highly polarized along the NW axis. Finally, density-dependent experiments revealed that the relaxation time decreases with increasing photoexcited carrier density for an above bandgap probe; however, no significant density-dependent changes in the relaxation dynamics were observed when probed below the bandgap. In short, our experiments reveal the influence of diameter, polarization, and carrier density on carrier dynamics in SiNWs, shedding light on the phenomena that govern carrier relaxation in these important nanosystems and giving insight on their future use in nanophotonic applications.

Long lived photo excitations in (6, 5) carbon nanotubes

We study the photo-induced transmission change (ΔT/T) upon quasi-continuum laser excitation, for different modulation frequency and excitation intensity in films of (6, 5) enriched CoMoCat single wall nanotubes (SWNTs) embedded in a polymeric matrix. This technique probes long-lived excited species, with lifetime in the 1 ms–1 s domain. The observed line-shape of ΔT/T spectra that can be assigned to a superposition of ground state bleaching and absorption modulation. We speculate that photo-excitation creates charges which get trapped in the sample, acquiring ms lifetime. This causes bleaching of the ground state absorption due to state filling and possibly a tube diameter expansion, with consequent blue shift of the optical transition.

Subpicosecond transmission change in semiconductor–embedded photonic crystal slab: Toward ultrafast optical switching

We have measured the subpicosecond photoinduced transmission change of the photonic crystal slab, in which inorganic–organic layered perovskite semiconductor is embedded for large optical nonlinearity. It is observed that sharp transmission dips shift to the blue during the irradiation of the pump pulse. This result is explained in terms of the change of the polariton dispersion relation due to the excitonic optical Stark effect. Possible application for ultrafast optical switching is discussed.

Dynamical properties of photo-induced spin disorder in doped manganites

We have investigated dynamical properties of the spin system in a ferromagnetic (FM) La 0.7Ca 0.3MnO 3 thin film by means of a nanosecond pump-probe method. The photo-induced transmission change measured at 2.54 eV exhibits significant temperature dependence, which is explained by photo-induced spin disorder in the FM phase. We have found that the observed temperature variation of the relaxation time near the transition temperature is well interpreted in terms of dynamical scaling theory based on a three-dimensional Heisenberg model for the second-order phase transition.

Improvement of picosecond time-resolved, single-shot pump-probe experiments by indirect measurement of pulse energies

A simple and convenient method to improve the signal-to-noise ratio in single-shot pump-probe experiments with picosecond time resolution is described. The pulse energies, which have to be known very accurately to derive exact data of the photoinduced transmission change in the sample, are measured indirectly via the fluorescence of an appropriate dye-solution. Design criteria for such an indirect energy detection unit are given and experimental results are compared to standard detection schemes. With the described method absorbance changes as low as 10 -3 can be measured routinely.

Influence of interference on photoinduced changes in transmission and reflection

The determination of photoinduced changes in the absorption coefficient (Δα) and index of refraction (Δ n) from the measured changes in transmission (Δ T) and reflection (Δ R) of a thin film is discussed. It is shown that on a film with a thickness gradient special optical thicknesses can be found at which Δα is proportional to Δ T, or Δ n is proportional to Δ R. The analysis is illustrated on the decays of photoinduced transmission and reflection in a-Si:H in the picosecond time domain.

Transient photoinduced phenomena in amorphous chalcogenide thin films

The transient photoinduced transmission change under intense band-gap light irradiation was investigated in amorphous chalcogenide thin films. Spectral distribution, temperature dependence, and the relation with the photocurrent of the change were examined. It appears that the change is due to the excitation of electrons trapped in normally unoccupied states near the valence band edge.