Grating Technique Research Articles

Anisotropy of Thermal Diffusivity in Lead Halide Perovskite Layers Revealed by Thermal Grating Technique

Heat management of optoelectronic devices is of critical significance in lead halide perovskites due to the intrinsically low thermal conductivity of this material. Despite its importance, thermal conductivity remains understudied, particularly in polycrystalline perovskite layers with different halides. Here, we employ a novel method for investigation of thermal properties in perovskite layers, which is based on a light-induced transient diffraction grating technique. We demonstrate the applicability of thermal grating technique by determining in an all-optical way the thermo-optic coefficient, speed of sound, and thermal conductivity in vapor-deposited polycrystalline layers of MAPbX3 (X = Cl, Br, I), MAPbBr2I, and MAPbCl2Br perovskites. We reveal the spatial anisotropy of thermal conductivity, which is noticeably lower in the direction along the layer surface (0.2–0.5 W/(m K)) if compared to that across the layer (0.3–1.1 W/(m K)). Finally, we demonstrate that for both directions the thermal conductivi...

Correlation between thermophoretic behavior and hydrophilicity for various alcohols⋆.

Recent experiments for various amides and sugars showed a clear correlation of the temperature dependence of the Soret coefficient with the hydrophilicity, quantitatively described by the logarithm of the 1-octanol/water partition coefficient logP . This coefficient is a measure for the hydrophilicity/hydrophobicity balance of a solute and is often used to model the transport of a compound in the environment or to screen for potential pharmaceutical compounds. In order to validate whether this concept works also for other water soluble molecules we investigated systematically the thermophoresis of mono- and polyhydric alcohols. As experimental method we use a holographic grating technique called infrared Thermal Diffusion Forced Rayleigh Scattering (IR-TDFRS). Experiments showed that the temperature dependence of the Soret coefficient of polyhydric alcohols also correlates with logP and lies on the same master plot as amides and sugars.

Origin of optical nonlinearity of photo-responsive liquid crystals revealed by transient grating imaging

A large optical nonlinearity has been observed for the photo-responsive liquid crystals under the condition that the nematic phase is close to the isotropic condition. The direct observation of the photo-response of a liquid crystal by the time-resolved transient grating phase imaging technique revealed that the optical nonlinearity was caused by the transiently generated phase formed inside the photo-induced isotropic region. A shock-like flow was observed for the formation of the transiently generated phase. Based on the theoretical calculation, we propose that a flow generated at the disordered/ordered interface induced the reorientation of the liquid crystal molecules, thereby generating a larger polarization and ultimately causing the optical nonlinearity.

Investigation of intrachain exciton diffusion of MEH-PPV in solution with different polarity

Femtosecond time-resolved transient grating technique was adopted to insight into the intrachain exciton diffusion of MEH-PPV in solution with different polarity. Broadband whitelight continuum was introduced as the probe to observe the transient absorption and the femtosecond time-resolved transient grating information simultaneously. The vibrational dephasing behaviors, single exciton relaxation, and population relaxation dynamics of MEH-PPV were systematically investigated. The result shows that the relaxation processes of the sample solution will be accelerated in the solvent with larger polarity.

Antinutritional Factors Reduction from Cassava (Manihot esculenta Crantz) Roots by Grating or Chipping Processing Technique in Mtwara Tanzania

Regular intake of diets high in cyanogenic amount in iodine deficiency areas with limited sulfur supply diets have been associated with the development of hypothyroidism, goiter and cretinism in young and adult populations. This study aimed at the investigation of the suitable processing technique for the production of high-quality cassava flour (HQCF) with acceptable consumption level of cyanide residues from different cassava varieties available at Mmongo village in Mtwara region. Grating technique was used to produce wet and dried cassava grits, whereas chipping technique produced wet and dried cassava chips. The wet and dried cassava products were all produced in a single day and under the same environmental conditions. The results showed that the hydrogen cyanide (HCN) in fresh cassava roots was 62.18–139.28 mg HCN/kg. Upon processing, the cyanide was lower (P<0.05) in wet cassava grits 24.27–76.74 mg HCN/Kg but higher in wet cassava chips 44.81–92.57 mg HCN/kg. After sun drying, the HCN retention was higher in dried cassava chips (10.7–14.2%) and lower in dried cassava grits (7.3–9.1%). Grating, efficiently reduced HCN from bitter cassava roots to the level within the WHO/FAO recommended safe limit (10 mg HCN/kg) as chipping only suite for sweet cassava roots. Therefore, HQCF can be nutritionally strategic cassava product consumed regularly and during food shortage without causing potential health problems even in iodine deficiency areas.

Reduced thermal conductivity of epitaxial GaAs on Si due to symmetry-breaking biaxial strain

Epitaxial growth of III-V semiconductors on Si is a promising route for silicon photonics. Threading dislocations and the residual thermal stress generated during growth are expected to affect the thermal conductivity of the III-V semiconductors, which is crucial for efficient heat dissipation from photonic devices built on this platform. In this work, we combine a non-contact laser-induced transient thermal grating technique with ab initio phonon simulations to investigate the in-plane thermal transport of epitaxial GaAs-based buffer layers on Si, employed in the fabrication of III-V quantum dot lasers. Surprisingly, we find a significant reduction of the in-plane thermal conductivity of GaAs, up to 19%, as a result of a small in-plane biaxial stress of 250 MPa. Using ab initio phonon calculations, we attribute this effect to the enhancement of phonon-phonon scattering caused by the in-plane biaxial stress, which breaks the cubic crystal symmetry of GaAs. Our results indicate the importance of eliminating the residual thermal stress in the epitaxial III-V layers on Si to avoid the reduction of thermal conductivity and facilitate heat dissipation. Additionally, our results showcase potential means of effectively controlling thermal conductivity of solids with external strain/stress.

Extreme radiation resistance in InN

Carrier dynamics were studied in InN, which is a promising material for radiation-resistant optoelectronic devices. InN epilayers with different background electron densities were prepared by metal-organic chemical vapor deposition and irradiated with protons. Application of the light-induced transient grating technique in the sub-picosecond domain allowed to study the carrier lifetime and diffusion coefficient as a function of the total carrier density (intrinsic, photoexcited, and irradiation-induced). The dominance of the direct Auger process in carrier recombination was revealed. The irradiation of InN by protons, such as those encountered in space applications, decreases the carrier lifetime while simultaneously increasing the diffusion coefficient (up to 190 cm2/s at the highest total carrier density studied). As a result, the average carrier diffusion length does not decrease below ∼140 nm, and exceeds the light absorption depth of InN. The lifetime damage factor was determined, and the lifetime dependence on the displacement damage dose was measured. The radiation resistance of InN surpasses that of GaAs, InGaAs, and InGaN. InN-based photovoltaic devices may work for long space missions.

Uncovering Photo-Excited Charge Carrier Dynamics in Hematite (α-Fe2O3) Hidden in the Nanosecond Range by the Heterodyne Transient Grating Technique Combined with the Randomly Interleaved Pulse-Train Method

Photo-excited charge carrier dynamics in hematite (α-Fe2O3), ranging from 10–8 to 10–5 s, was investigated using the heterodyne transient grating technique in combination with the randomly interleaved pulse-train method. We observed four components in this time scale, and each component was successfully assigned by scavengers and the responses of oxygen-abundant and -deficient samples were compared. Particularly, we found components which have not been discussed so far with an aid of the combination method, and they were assigned to recombination arising from trapped electrons. In addition, the effect of an amorphous NiFeOx coating on the dynamics in this time scale was also investigated under the bias condition and demonstrated that the coated sample strongly depended on the applied bias voltage, while the bare sample did not, implying that the Fermi-level pinning effect in hematite was reduced by the amorphous NiFeOx coating.

Binary metal zinc-lead perovskite built-in air ambient: Towards lead-less and stable perovskite materials

Organo-metal halide perovskites are promising new-generation photovoltaic materials owing to their efficient electrical/optical characteristics. Despite having high efficiency, perovskites are toxic due to the presence of lead and instable in air ambient. Therefore, the toxicity and instability call for more efforts in this area. Here, methylammonium-zinc-lead iodide (MAZn0.2Pb0.8I3) is synthesized in air ambient as lead-less and more stable perovskite material for applying as the active layer of solar cells. 1.65 eV optical band gap and 51 nm minority carrier diffusion length (Ldiff) have been obtained with the help of UV–Visible spectroscopy (UV–Vis) and steady-state photocarrier grating (SSPG) technique respectively. Structural, chemical bonding and surface studies are done by X-ray powder diffraction (XRPD), Fourier-transform infrared spectroscopy (FTIR) and Field Emission Scanning Electron Microscope (FESEM) respectively. The sample has been stored in dark air ambient with average relative humidity of 45% at room temperature (27 °C) for 60 days and the properties of 60 days aged sample is investigated and compared with characteristics of 60 days aged MAPbI3. The results confirm MAZn0.2Pb0.8I3 is more stable than MAPbI3.

Time-resolved photoinduced charge dynamics of Cd chalcogenide-based materials

We have performed a comparative analysis of the photocharging dynamics of quantum dots (CdSe–CdS core–shell nanoparticles in toluene) and a bulk CdS single crystal by using time-resolved luminescence and transient grating techniques. The quantum dots were obtained through a colloidal synthesis method in aqueous–organic media with the average sizes of 2 nm and 2.9 nm. It has been shown that the relaxation of the photocharging dynamics in quantum dots is well fitted by the double-exponential decay function with time constants of a few and tens of picoseconds, which indicates that the energy loss of photoinduced charges is mainly the result of electron–phonon interaction.

Distributed Strain Damage Identification Technique for Long-Span Bridges Under Ambient Excitation

Distributed strain measurement, such as long-gauge fiber bragger grating (FBG) techniques, has developed rapidly in the field of structural health monitoring. However, strategies of corresponding damage identification still need to be enhanced. The damage identification technique based on distributed strain measurement is proposed identifying the structural damage under ambient excitation. Damage indices, like the distributed strain energy difference (DSED) and the relative distributed strain energy (RDSE), are derived from the power spectral density of the frequency response function of distributed strain response and further employed by detecting damages in the structure. A numerical analysis is performed on a long-span cable-stayed bridge with several assumed damage scenarios at various degrees in the girder. Damage localization capability and robustness of the proposed damage indices are discussed. In addition, damage quantification utilizing the proposed indices is conducted. The results indicate that the proposed technique can accurately identify the locations and extent of the damage under ambient vibration excitation.

Thermal conductivity reduction in highly doped mesoporous silicon: The effect of nano-crystal formation

The study of heat transfer properties in mesoporous silicon, fabricated from highly doped p-type and n-type silicon wafers, is presented. Measurements were performed by a laser induced transient thermal grating technique, which allowed us to determine the effective (in-plane) thermal conductivity. It is shown that the thermal conductivity undergoes a significant decrease with respect to bulk values mainly due to a reduction of the phonon mean free path of the solid matrix. This reduction can be ascribed to the formation of nano-crystalline domains, which are a consequence of the wet etching fabrication method. Additionally, the in-plane thermal conductivity was analyzed by employing a modified effective medium approach, which includes the phonon mean free path reduction due to the presence of both the nanometric pores and the nano-crystalline domains. The theoretical analysis shows good agreement with our measurements, indicating that the inclusion of phonon mean free path reduction to an effective medium approach is a well-suited method for studying the thermal conductivity of porous silicon.

Impact of Environmental Stresses Onto Transport Properties of Hybrid Perovskite Investigated by Steady State Photocarrier Grating and Steady State Photocurrent Techniques

In this study, the transport properties of high quality double and triple cation perovskite are explored under different environmental stresses by Steady State Photocurrent (SSPC) and Steady State Photocarrier Grating (SSPG) measurements. SSPG allows a direct measurement of the ambipolar diffusion length that is significantly higher for the cesium containing perovskite (1.4 micron compared to 0.57 micron for a double cation compound). This positive effect is shown to be reversibly annihilated as the samples are put under vacuum, and to be accelerated with the increase of the temperature. The degradation of the material properties appears together with the activation of a defect trap level and a rise of the doping level in the material. This behavior is discussed considering the interaction between the iodine vacancies contained in the perovskite and oxygen which is known to lead to trap passivation.

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.

Comparative study of photo-excited charge carrier dynamics of atomic layer deposited and solution-derived hematite films: Dependence of charge carrier kinetics on surface orientations

Charge carrier kinetics of hematite films prepared via atomic layer deposition and solution-derived methods were investigated using heterodyne transient grating technique in aqueous solution, depending on bias voltage, to reveal the effects of surface orientation differences on the charge carrier dynamics. We found that the electron/hole recombination in atomic layer deposited hematite was slower than that in solution-derived hematite, indicating the generated charge carriers in atomic layer deposited hematite can survive longer than that in solution-derived hematite, which is usually beneficial for water oxidation. On the other hand, the trapping dynamics of the long-lived holes to the surface state for water photo-oxidation in the solution-derived hematite was observed at faster time region than that in atomic layer deposited hematite, implying that the diffusion of the holes in atomic layer deposited hematite was slower than that in solution-derived hematite due to the different barrier height and width of the space charge layer which strongly depends on the surface orientations. It means that the low charge carrier mobility rather than the retardation of the charge recombination in the atomic layer deposited hematite is a dominant factor in its low photoelectrochemical performance.

Simple apodization technique for surface-corrugated waveguide gratings

This paper reports a simple technique for suppressing the sidelobes outside the stop band of surface-corrugated waveguide gratings. To ensure the sidelobe suppression, the dc coupling coefficient arising from the dc component of the dielectric perturbation should be as small as possible. In the proposed waveguide grating, the periodic corrugation whose depth is reduced toward the ends of the grating is formed on both sides across the film-cladding boundary of the waveguide. The 2D simulation based on the coupled-mode theory has shown that the sidelobes can be substantially suppressed while maintaining the reflection within the stop band of the uniform grating. The proposed apodization technique is suitable for the sidewall corrugation from the viewpoint of the ease of fabrication.

Gas-pressure chemical vapor transport growth of millimeter-sized c-BAs single crystals with moderate thermal conductivity

We have grown c-BAs single crystals up to 1000 μm size by the chemical vapor transport (CVT) technique using combined As and I2 transport agents with the As:I ratio of 1:3 under gas pressures of up to 35 atm. Raman spectroscopy revealed a very sharp (∼2.4 cm−1) P1 phonon mode and an interesting splitting behavior of P1 from detailed polarization studies. Electron paramagnetic resonance (EPR) experiments revealed no evidence for EPR active growth-related defects under the experimental resolution. Finally, a moderate thermal conductivity value of ∼132 W/m-K was obtained using a transient thermal grating technique. These results suggest that although the high As gas vapor pressure environment in CVT growth can increase the transport rate of c-BAs significantly, it may not be efficient in reducing the defects and enhancing the thermal conductivity in c-BAs significantly.

Diffusion Enhancement in Highly Excited MAPbI3 Perovskite Layers with Additives.

Carrier mobility is one of the crucial parameters determining the electronic device performance. We apply the light-induced transient grating technique to measure independently the carrier diffusion coefficient and lifetime, and to reveal the impact of additives on carrier transport properties in wet-cast CH3NH3PbI3 (MAPbI3) perovskite films. We use the high excitation regime, where diffusion length of carriers is controlled purely by carrier diffusion and not by the lifetime. We demonstrate a four-fold increase in diffusion coefficient due to the reduction of localization center density by additives; however, the density dependence analysis shows the dominance of localization-limited diffusion regime. The presented approach allows us to estimate the limits of technological improvement-carrier diffusion coefficient in wet-cast layers can be expected to be enhanced by up to one order of magnitude.

Error-correct arithmetic for angular displacement measurement with single linear image detector

This paper proposes a grating eccentricity error correction technique for high-precision angular measurement via image processing. We first built a mathematical model of grating eccentricity error, then we established an error-correct arithmetic based on the least square method. We tested the proposed method to find that it yields a standard error deviation of only 6.33″, compared to the uncorrected standard error deviation of 22.33″. This method is based on a single detector, which is inexpensive and performs well. The results presented here may provide a theoretical and technological foundation for further research on small-size, high-resolution photographic rotary encoders.