Transient Grating Research Articles

Comparison between Grating Imaging and Transient Grating Techniques on Measuring Carrier Diffusion in Semiconductor

ABSTRACTOptical grating technique, where optical gratings are generated via light inference, has been widely used to measure charge carrier and phonon transport in semiconductors. In this paper, compared are three types of transient optical grating techniques: transient grating diffraction, transient grating heterodyne, and grating imaging, by utilizing them to measure carrier diffusion coefficient in a GaAs/AlAs superlattice. Theoretical models are constructed for each technique to extract the carrier diffusion coefficient, and the results from all three techniques are consistent. Our main findings are: (1) the transient transmission change ∆T/T0 obtained from transient grating heterodyne and grating imaging techniques are identical, even these two techniques originate from different detection principles; and (2) by adopting detection of transmission change (heterodyne amplification) instead of pure diffraction, the grating imaging technique (transient grating heterodyne) has overwhelming advantage in signal intensity than the transient grating diffraction, with a signal intensity ratio of 315:1 (157:1).

A tilted pulse-front setup for femtosecond transient grating spectroscopy in highly non-collinear geometries

We demonstrate a tilted pulse-front transient grating (TG) technique that allows to optimally utilize time resolution as well as TG line density while probing under grazing incidence as typically done in extreme ultraviolet (EUV) or soft x-ray (SXR) experiments. Our optical setup adapts the pulse front tilt of the two pulses that create the TG to the grazing incident pulse. We demonstrate the technique using all 800 nm femtosecond laser pulses for TG generation on a vanadium dioxide film. We probe that grating via diffraction of a third 800 nm pulse. The time resolution of 90 fs is an improvement by a factor of 30 compared to our previous experiments on the same system. The scheme paves the way for EUV and SXR probing of optically induced TGs on any material.

Aggregation-Induced Expansion of Poly-(N-isopropyl acrylamide) Solutions Observed Directly by the Transient Grating Imaging Technique.

The anomalous volume expansion of poly-(N-isopropyl acrylamide) (PNIPAM) solutions was observed during the thermally induced polymer phase transition of aqueous solutions having concentrations in the 3–7 wt % range. The process occurred on a millisecond time scale, and a laser temperature-jump time-resolved technique was used to bring about the process. After heating a solution with a pulse laser exploiting light absorption by dyes added to the solution itself, a phase transition was observed to take place, and the temporal changes associated with it were visualized through the transient grating imaging technique, whereby the solution was heated with a stripe pattern. We found several processes occurring on a millisecond time scale, all of which clearly took place after each PNIPAM molecule had collapsed structurally from a coiled to a globular conformation. During the so-called demixing process, the globular polymers aggregated with each other within 10 ms, and suddenly the polymer phase expanded as aggregation progressed further. After this process, the individual globular polymers reverted to their coiled conformation via hydration during the remixing process. We proposed that solution expansion was caused by the mutual entangling of multiple globular PNIPAM molecules, instead each globule polymer was separated.

Tracking intra-molecular electron and vibrational energy redistribution by time and frequency resolved transient grating spectroscopy

Theoretical calculation results show that electron flow occurs from xanthene to phenyl ring within the Rh101+ after its electronic transition from ground state (S0) to first excited state (S1). Time and frequency resolved transient grating (TG) spectroscopy is then conducted to seek the experimental evidence. Through Fourier analysis at different wavelengths of TG signal and their specific value spectra, vibrational enhancement corresponding to electron acceptor chromophore (phenyl ring) is observed. So the ultrafast vibrational energy redistribution process is detected experimentally and the direction of electron flow is confirmed.

Photoinduced Orientation Change of the Dimer Structure of the Pr-I State of Cph1Δ2

Phytochromes are red and far-red light sensor proteins found in many organisms. Photoisomerization of a chromophore triggers subsequent reactions leading to conformational changes that are essential for signaling. Conformational changes of the N-terminal sensor domain of cyanobacterium phytochrome 1 from Synechocystis sp. PCC6803 (Cph1Δ2) were studied in this report primarily by the transient grating (TG) method in the time domain. Although the reaction kinetics monitored by ultraviolet-visible absorption changes were insensitive to variations in pH, it was found for the first time that the diffusion coefficient and molecular volume monitored by the TG method are significantly dependent on pH. This pH sensitivity indicates that the conformational changes of Cph1Δ2 during the reaction are modulated by pH and was explained consistently by two contributions from two isomers (Pr-I and Pr-II) possessing different protonation states of His260. Thus, His260 is responsible for the conformational heterogeneity of Pr, as discussed previously, and this protonation difference leads to different conformational changes of the sensor domain of Cph1 during the reaction. The photoreaction dynamics of the Pr-I state was studied in detail, and a significant diffusion change was observed, which was attributed to a change in the orientation (quaternary structure) of the dimer with a time constant of 400 μs. Furthermore, it was revealed that this reorientation of the dimer was induced by a conformational change in the tongue region. New site-directed mutations to change the dimer-monomer equilibrium were reported.

Beat noise reduction utilizing the transient acoustic-wave response of an optical fiber in Brillouin grating-based optical low coherence reflectometry.

We periodically generated a transient Brillouin grating by using continuous-wave pump light and 50MHz pulse-wave pump light with optical low coherence reflectometry (OLCR). We extracted the Stokes light generated by the decaying part of the grating with an optical switch, and this enabled us to block the pulse-wave pump light from entering the balanced mixer, resulting in a reduction in the noise caused by the beat between the local oscillator light and the pump light. For the first time, to the best of our knowledge, we succeeded in detecting a Stokes light with an OLCR, despite the fact that the states of polarization of the probe and pump light waves were parallel, and this encouraged us to construct a polarization-independent OLCR for diagnosing optical modules and three-dimensional objects.

Reflectivity of Superimposed Bragg Gratings Induced by Longitudinal Mode Instabilities in Fiber Lasers

We report on reflectivity of transient Bragg gratings created in the active medium of a fiber laser with longitudinal mode instability. Despite the long history of laser physics, reflectivities of such gratings were theoretically studied and experimentally measured only recently. Based on the coupled-mode theory, we derived a theoretical model of superimposed Bragg gratings for a special case of mode instability known as spontaneous laser line sweeping (SLLS). SLLS fiber lasers exhibit a periodic wavelength drift over an interval of several nanometers, followed by a quick bounce backward, narrowband, and mostly single-frequency operation as well as pulsed output with pulse lengths on the order of microseconds. The refractive index modulation inside the active fiber is given by the Kramers–Kronig relations. For ytterbium, the refractive index change in the 1060-nm band is directly proportional to the metastable level population with a factor of ${\text{2.1}}\times {\text{10}}^{- 31}\,{\text{m}}^{3}$ . Taking into account realistic values of temporal damping of the gratings, significant reflectivity on the order of units to tens of percent is estimated. The reflectivity can be controlled by setting the SLLS laser parameters. Effect of the resonator length is shown as an example.

Tracking coherent population transfer and thermal population relaxation in condensed system by broad-band transient grating spectroscopy

Broad-band transient grating (BB-TG) spectroscopy was proposed to track both coherent population transfer (CPT) and thermal population relaxation processes in a condensed system of solvated molecules in solution (Rhodamine101 in methanol). A broad band around 1500 cm−1 and a relative narrow band near 2900 cm−1 emerge in TG and transient absorption contour plots when pump and probe pulses overlap in the sample. The experimental results matched well with the vibrational modes of Rhodamine101 that were obtained by theoretical calculation. In addition, it was found that the population of CPT particles can be evaluated quantitatively through the intensity of the TG signal.

Soret forced Rayleigh scattering instrument for simultaneous detection of two-wavelength signals to measure Soret coefficient and thermodiffusion coefficient in ternary mixtures.

We describe an instrument for the measurement of the Soret and thermodiffusion coefficients in ternary systems based on the transient holographic grating technique, which is called Soret forced Rayleigh scattering (SFRS) or thermal diffusion forced Rayleigh scattering (TDFRS). We integrated the SFRS technique and the two-wavelength detection technique, which enabled us to obtain two different signals to determine the two independent Soret coefficients and thermodiffusion coefficients in ternary systems. The instrument has been designed to read the mass transport simultaneously by two-wavelength lasers with wavelengths of λ = 403 nm and λ = 639 nm. The irradiation time of the probing lasers is controlled to reduce the effect of laser absorption to the sample with dye (quinizarin), which is added to convert the interference pattern of the heating laser of λ = 532 nm to the temperature grating. The result of the measurement of binary benchmark mixtures composed of 1,2,3,4-tetrahydronaphthalene (THN), isobutylbenzene (IBB), and n-dodecane (nC12) shows that the simultaneous two-wavelength observation of the Soret effect and the mass diffusion are adequately performed. To evaluate performance in the measurement of ternary systems, we carried out experiments on the ternary benchmark mixtures of THN/IBB/nC12 with the mass fractions of 0.800/0.100/0.100 at a temperature of 298.2 K. The Soret coefficient and thermodiffusion coefficient agreed with the ternary benchmark values within the range of the standard uncertainties (23% for the Soret coefficient of THN and 30% for the thermodiffusion coefficient of THN).

Exciton diffusion in bifluorene single crystals studied by light induced transient grating technique

Exciton diffusion is considered to be of prime importance for controlling the recombination zone in organic light emitting devices. This is particularly relevant for organic lasers based on single crystals, where undesirable exciton-exciton annihilation at high concentrations is inevitable. Here, exciton diffusion is studied in bifluorene single crystals specifically designed for organic laser applications, therefore featuring exceptionally low threshold of light amplification. The non-destructive light induced transient grating (LITG) technique capable of simultaneously evaluating the exciton lifetime and diffusion coefficient along different directions of the crystal was employed for this study. Highly anisotropic singlet exciton diffusion with the diffusion coefficient varying from <0.05 up to 0.96 cm2/s (in a perpendicular direction) and the corresponding diffusion lengths changing from <60 up to 300 nm, respectively, was revealed in ethylene-bridged bifluorene crystals. The LITG results on the directionality of exciton diffusion are anticipated to be essential for devising single crystal based organic lasers.

Characterization of ultrafast free-electron laser pulses using extreme-ultraviolet transient gratings.

The characterization of the time structure of ultrafast photon pulses in the extreme-ultraviolet (EUV) and soft X-ray spectral ranges is of high relevance for a number of scientific applications and photon diagnostics. Such measurements can be performed following different strategies and often require large setups and rather high pulse energies. Here, high-quality measurements carried out by exploiting the transient grating process, i.e. a third-order non-linear process sensitive to the time-overlap between two crossed EUV pulses, is reported. From such measurements it is possible to obtain information on both the second-order intensity autocorrelation function and on the coherence length of the pulses. It was found that the pulse energy density needed to carry out such measurements on solid state samples can be as low as a few mJ cm-2. Furthermore, the possibility to control the arrival time of the crossed pulses independently might permit the development of a number of coherent spectroscopies in the EUV and soft X-ray regime, such as, for example, photon echo and two-dimensional spectroscopy.

Giant optical nonlinearity in DNA lyotropic liquid crystals.

We report the experimental evidence of nonlinear optical response in DNA lyotropic nematic liquid crystals. Pump-probe experiments indicate that the non-linearity is remarkably large. Quantitative assessment of the non-linear optical coefficient by transient optical grating demonstrates that the response is of the same order of the well-known Giant Optical Nonlinearity (GON) of thermotropic nematics. These results represent a further incentive to the current investigation of potential applications of DNA in biophotonics.

Unifying first-principles theoretical predictions and experimental measurements of size effects in thermal transport in SiGe alloys

In this work, we demonstrate the correspondence between first principle calculations and experimental measurements of size effects on thermal transport in SiGe alloys. Transient thermal grating (TTG) is used to measure the effective thermal conductivity. The virtual crystal approximation under the density functional theory (DFT) framework combined with impurity scattering is used to determine the phonon properties for the exact alloy composition of the measured samples. With these properties, classical size effects are calculated for the experimental geometry of reflection mode TTG using the recently-developed variational solution to the phonon Boltzmann transport equation (BTE), which is verified against established Monte Carlo simulations. We find agreement between theoretical predictions and experimental measurements in the reduction of thermal conductivity (as much as $\sim$ 25\% of the bulk value) across grating periods spanning one order of magnitude. This work provides a framework for the tabletop study of size effects on thermal transport.

Spatiotemporal Coherent Control of Thermal Excitations in Solids.

X-ray reflectivity measurements of femtosecond laser-induced transient gratings (TG) are applied to demonstrate the spatiotemporal coherent control of thermally induced surface deformations on ultrafast time scales. Using grazing incidence x-ray diffraction we unambiguously measure the amplitude of transient surface deformations with sub-Å resolution. Understanding the dynamics of femtosecond TG excitations in terms of superposition of acoustic and thermal gratings makes it possible to develop new ways of coherent control in x-ray diffraction experiments. Being the dominant source of TG signal, the long-living thermal grating with spatial period Λ can be canceled by a second, time-delayed TG excitation shifted by Λ/2. The ultimate speed limits of such an ultrafast x-ray shutter are inferred from the detailed analysis of thermal and acoustic dynamics in TG experiments.

Strain and Thermally Induced Magnetic Dynamics and Spin Current in Magnetic Insulators Subject to Transient Optical Grating

We analyze the magnetic dynamics and particularlythe spin current in an open-circuit ferromagnetic insulator irradiated by two intense, phase-locked laser pulses. The interference of the laser beams generates a transient optical grating and a transient spatio-temporal temperature distribution. Both effects lead to elastic and heat waves at the surface and into the bulk of the sample. The strain induced spin current as well as the thermally induced magnonic spin current are evaluated numerically on the basis of micromagnetic simulations using solutions of the heat equation. We observe that the thermo-elastically induced magnonic spin current propagates on a distance larger than the characteristic size of thermal profile, an effect useful for applications in remote detection of spin caloritronics phenomena. Our findings point out that exploiting strain adds a new twist to heat-assisted magnetic switching and spin-current generation for spintronic applications.

Extremely broadband single-shot cross-correlation frequency-resolved optical gating using a transient grating as gate and dispersive element.

A cross-correlation frequency-resolved optical gating (FROG) concept, potentially suitable for characterizing few or sub-cycle pulses in a single shot, is described in which a counter-propagating transient grating is used as both the gate and the dispersive element in a FROG spectrometer. An all-reflective setup, which can operate over the whole transmission range of the nonlinear medium, within the sensitivity range of the matrix sensor, is also proposed, and proof-of-principle experiments for the ultraviolet and visible-to-near-infrared spectral ranges are reported.

Thermal Diffusivity of Methanol to a Pressure of 5 GPa

Thermal diffusivities of methanol have been measured, using transient optical gratings, at 26 °C, up to a pressure of 5 GPa; together with previously reported measurements of thermal effusivities, they are used to estimate values of methanol’s specific heat and thermal conductivity to a pressure of 8 GPa. At lower pressures, the calculated quantities compare favorably with a published equation-of-state and a separate formulation for conductivity. Use of diffusivities in combination with effusivities is of particular interest, as both can be measured with optical techniques applicable to the high-pressure, diamond-anvil cell. Such experiments thus afford an experimental path to the determination of specific heats and thermal conductivities at the high compressions achievable with this experimental tool.

Modeling quasi-ballistic transient thermal transport with spatially sinusoidal heating: A McKelvey-Shockley flux approach

Ballistic phonon effects, arising on length scales comparable to the mean-free-path, result in non-diffusive heat flow and alter the thermal properties of materials. Simple theoretical models that accurately capture non-diffusive transport physics are valuable for experimental analysis, technology design, and providing physical insight. In this work, we utilize and extend the McKelvey-Shockley (McK-S) flux method, a simple and accurate framework, to investigate ballistic effects in transient phonon transport submitted to a spatially sinusoidal heating profile, simulating a transient thermal grating. We begin by extending a previous McK-S formulation to include inelastic scattering, then obtain an analytical solution in the single phonon energy case (gray approximation), and after show how this approach can readily support a full phonon dispersion and mean-free-path distribution. The results agree with experimental data and compare very well to solutions of the phonon Boltzmann transport equation in the diffusive and weakly quasi-ballistic transport regimes. We discuss the role of ballistic and non-equilibrium physics, and show that inelastic scattering is key to retrieving the heat equation solution in the diffusive limit. Overall the McK-S flux method, which takes the form of a diffusion-like equation, proves to be a simple and accurate framework that is applicable from the ballistic to diffusive transport regime.

Concentration dependent carriers dynamics in CsPbBr3 perovskite nanocrystals film with transient grating

The concentration dependence of the carrier dynamics is a key parameter to describe the photo-physical properties of semiconductor films. Here, we investigate the carrier dynamics in the CsPbBr3 perovskite nanocrystal film by employing the transient grating (TG) technique with continuous bias light. The concentration of initial carriers is determined by the average number of photons per nanocrystals induced by pump light (⟨N⟩). The multi-body interaction would appear and accelerate the TG dynamics with ⟨N⟩. When ⟨N⟩ is more than 3.0, the TG dynamics slightly changes, which implies that the Auger recombination would be the highest order multi-body interaction in carrier recombination dynamics. The concentration of non-equilibrium carriers in the film is controlled by the average number of photons per nanocrystals excited by continuous bias light (⟨nne⟩). Increasing ⟨nne⟩ would improve the trapping-detrapping process by filling the trapping state, which would accelerate the carrier diffusion and add the complexity of the mono-molecular recombination mechanism. The results should be useful to further understand the mechanism of carrier dynamics in the CsPbBr3 perovskite nanocrystal film and of great importance for the operation of the corresponding optoelectronic devices.

Impact of carrier localization and diffusion on photoluminescence in highly excited cyan and green InGaN LED structures

Localization of charge carriers is of crucial importance in InGaN light emitting devices since it governs carrier transport and ensures high radiative efficiency. In this work, we observe the dynamics of carrier localization from an untypical redshift of photoluminescence spectra. We investigate two light emitting diode structures grown by MOCVD on c-plane sapphire and emitting at 500nm (cyan) or 530nm (green). For the study, we employ photoluminescence, differential transmission, and light induced transient gratings techniques. We observe non-monotonous dynamics of photoluminescence peak position: a blueshift at short delay times (<10ns) is later replaced by a redshift, which increases both with time and excitation. The redshift values as large as 30meV in the cyan and 20meV in the green structure were observed at 40ns delay. We attribute this redshift to density-dependent and diffusion-driven carrier redistribution between the shallower and deeper localized states. The carrier delocalization with increasing density is confirmed by growing diffusivity of carriers with excitation. We also demonstrate a correlation between the growth of diffusion coefficient and the onset of efficiency droop.