Light-induced Gratings Research Articles

Electromagnetically induced gratings created by extremely short non-overlapping pulses of light in a three-level resonant medium

In a fixed spectral range, single- and half-cycle electromagnetic pulses have the shortest duration. Half-cycle pulses are promising tools for ultrafast control of quantum systems. Previously, the possibility of using a sequence of single- and half-cycle attosecond pulses to generate and ultrafast control light-induced population difference gratings has been demonstrated. However, such studies have been carried out using different approximations, such as the sudden perturbation theory and the two-level model for the resonant medium. In this paper, based on the numerical solution of constitutive equations for elements of the density matrix and wave equation it is shown that it is possible to generate and control population gratings in a three-level medium without using the approximation of sudden perturbations used in previous studies. It is shown that taking into account the additional level of the medium does not lead to a violation of the effect of generating such gratings. This extends the applicability of previous results.

Light-Induced Diffraction Gratings on Liquid Metamaterial Metasurfaces

Light-Induced Diffraction Gratings on Liquid Metamaterial Metasurfaces

Optimized optical tunable microfiber-Bragg grating

A light-induced tunable microfiber-based Bragg grating (MF-BG) overlaid with a high refractive index (HRI) photoresponsive liquid crystal (PLC) film is proposed. Around a specific overlay HRI, the refractive index (RI) of the fundamental mode can be tailored to an optimal region by adjusting the overlay thickness and selecting the MF-BG diameter. This optimization makes the device have extended tuning range, low insertion loss, and relative high extinction ratio. The maximum wavelength shift of 1.32 nm is achieved in our experiment under 0.5 mW/mm2 Ar+ laser illuminating.

Accurate Morphology-Related Diffraction Behavior of Light-Induced Surface Relief Gratings on Azopolymers

Holographic relief gratings can be fabricated directly on the surface of azobenzene-containing materials (or simply azomaterials) without additional development steps. Despite often being described to have ideal sinusoidal profiles, the developing surface morphology in large amplitude gratings can affect the light distribution of the writing interferogram, causing profile deformations and deviations in the expected diffraction behavior. In this work, we characterize the temporal evolution of the surface relief grating (SRG) morphology fabricated by means of interference lithography on azopolymer films, quantitatively relating the results of a Fourier analysis of their surface profiles to the accurate measurement of efficiencies in the transmitted diffraction orders. A reliable surface structuration dynamics is empirically extracted from the analysis and used to formulate a simple but detailed diffraction model that, within the scalar diffraction theory, exhaustively describes the diffraction behavior of real SRGs without the need of complex rigorous electromagnetic theories. Our results add a deeper insight in the quantitative description of the morphology and the diffraction behavior of SRGs, which can contribute to their adoption for operating high-performance, reconfigurable, compact, and lightweight diffractive optical devices.

Characterization of Polarization Holographic Gratings Obtained on Azopolymer Thin Films by Digital Holographic Microscopy

Polarization diffraction gratings are formed by one-step polarization holographic recording in azopolymer thin films. The evolution of the gratings parameters, such as the modulation of diffraction efficiency and relief depth with regard to different exposure dose is analyzed. Phase-shifting digital holographic microscopy is applied for the measurement of the light-induced polarization diffraction gratings. For the accurate hologram acquisition and reconstruction of the complex amplitude transmitted by the gratings, we performed all-optical (without moving components) phase-shifting implemented within in the imaging system of the digital holographic microscope. The experimental measurement results and theoretical predictions were compared and analyzed.

Modeling of a diffraction pattern from light-induced gratings in a AgCl-Ag waveguide film as a method of evaluation of the grating parameters

Modeling of a diffraction pattern from light-induced gratings in a AgCl-Ag waveguide film as a method of evaluation of the grating parameters

Charge carrier trapping by dislocations in single crystal diamond

Charge carrier trapping in diamond crystals containing well-defined concentrations of dislocations was investigated by several complementary techniques. Samples with dislocation densities ndis between <1 × 107 and ≈1 × 109 cm−2 were grown heteroepitaxially on Ir/YSZ/Si(001). In optical pump–probe experiments, ambipolar diffusion coefficients were determined from the decay of light-induced transient free carrier gratings. Modeling their variation with excitation density yielded trapping cross sections σ of 29 and 10 nm for the dislocations and a stress-field-induced reduction in exciton binding energies from 80 to 73 and 60 meV at ndis = 1 × 108 and 1 × 109 cm−2, respectively. The lifetime measured by induced absorption scaled proportional to 1/ndis with absolute values ranging from 0.1 to 10 ns. In the electrical measurements on two sets of detector slices, electron–hole pairs were excited by α-particles and transport was measured separately for electrons and holes. Both types of carriers showed fast transient current signals. The time constant of the additional slow component exclusively seen for holes was in agreement with the activation energy of boron acceptors. Their concentration of ≈0.5 ppb yielded σ = 1.77 × 10−13 cm2 for charged point traps. Schubweg and carrier lifetime due to deep trapping roughly reproduced the 1/ndis trend. For electrons at 3 V/μm, a value σ = 40 nm was deduced. Cross sections for holes were significantly smaller. Differences in hole trapping between the samples are attributed to charging of dislocations controlled by chemical impurities. Increase in lifetime at high voltages is explained by reduced capture cross sections for hot carriers.

Population gratings produced in a quantum system by a pair of sub-cycle pulses

Possible generation of light-induced harmonic population gratings in a quantum system by two half-cycle pulses is studied using the approximate solution of the Schrödinger equation. The case is considered where the duration of pulses is shorter than the resonance transition periods in the system and the pulses do not overlap in the medium. The grating modulation depth is determined by the pulse electric area.

Crystal Structure Ideality Impact on Bimolecular, Auger, and Diffusion Coefficients in Mixed-Cation CsxMA1–xPbBr3 and CsxFA1–xPbBr3 Perovskites

Nonlinear nonradiative recombination processes are considered as a main obstacle to efficient lead halide perovskite light emitters as light emitting diodes and lasers. In this work, crystal structure ideality, described by the Goldschmidt tolerance factor, impact on Auger and bimolecular recombination rates in spin-coated mixed-cation CsxMA1–xPbBr3 and CsxFA1–xPbBr3 perovskite layers was investigated. The excitation-dependent carrier lifetime and diffusion coefficient were determined using optical pump–probe techniques: time-resolved photoluminescence and light-induced transient grating. Layers with a Goldschmidt tolerance factor closer to unity revealed Auger and bimolecular recombination rates smaller by up to an order of magnitude. The bimolecular coefficient (0.6–5 × 10–10 cm3/s) increased simultaneously with the Auger coefficient (0.05–2 × 10–27 cm6/s) upon increase of x and simultaneous decrease of tolerance factor, due to variation in the localized carrier population and Rashba splitting. A larger...

Spatial Frequency Response of Epoxy-Based Volume Holographic Recording Material.

Holographic volume phase gratings are recorded in an epoxy-based, free-surface, volume holographic recording material. Light-induced gratings are formed by photo-triggered mass migration caused by component diffusion. The material resolution enables a wide range of pattern spacings, to record both transmission and reflection holograms with many different spatial frequencies. An optimum spatial frequency response is found between the low spatial frequency roll-off and the high spatial frequency cut-off. The influence of the energy density of exposure on the spatial frequency response is investigated. Secondary volume holographic gratings (parasitic gratings) are observed in the high frequency range. The possibility of distinguishing the regular grating from the secondary grating is discussed in the form of probe wavelength detuning.

Population Difference Gratings Induced in a Resonant Medium by a Pair of Short Terahertz Nonoverlapping Pulses

We show that, using a pair of short terahertz pulses, it is possible to form light-induced gratings of the population difference, which can be applied to control terahertz radiation. The pulses coherently interact with the medium and do not overlap in it. The examined effect opens up new opportunities in terahertz spectroscopy with the use of such short pulses.

Surface-Relief Gratings in Halogen-Bonded Polymer-Azobenzene Complexes: A Concentration-Dependence Study.

In recent years, supramolecular complexes comprising a poly(4-vinylpyridine) backbone and azobenzene-based halogen bond donors have emerged as a promising class of materials for the inscription of light-induced surface-relief gratings (SRGs). The studies up to date have focused on building supramolecular hierarchies, i.e., optimizing the polymer–azobenzene noncovalent interaction for efficient surface patterning. They have been conducted using systems with relatively low azobenzene content, and little is known about the concentration dependence of SRG formation in halogen-bonded polymer–azobenzene complexes. Herein, we bridge this gap, and study the concentration dependence of SRG formation using two halogen-bond-donating azobenzene derivatives, one functionalized with a tetrafluoroiodophenyl and the other with an iodoethynylphenyl group. Both have been previously identified as efficient molecules in driving the SRG formation. We cover a broad concentration range, starting from 10 mol % azobenzene content and going all the way up to equimolar degree of complexation. The complexes are studied as spin-coated thin films, and analyzed by optical microscopy, atomic force microscopy, and optical diffraction arising during the SRG formation. We obtained diffraction efficiencies as high as 35%, and modulation depths close to 400 nm, which are significantly higher than the values previously reported for halogen-bonded polymer–azobenzene complexes.

Light-induced spatial gratings created by unipolar attosecond pulses coherently interacting with a resonant medium

Recently, the possibility of the creation, erasing and ultrafast control of polarization and population inversion gratings by sequences of few-cycle bipolar pulses interacting with a medium in a resonant and coherent way was predicted. In this case, the overlapping of pulses in the medium is not needed for the creation of gratings. In this paper, we study the possibility of the ultrafast creation and control of spatial periodic gratings in a resonant medium when subcycle unipolar pulses (that is ones containing the constant spectral component of an electric field) propagate in the coherent regime.

Influence of light-induced gratings on acousto-optic interaction of Bessel light beams in uniaxial gyrotropic crystals

The influence of light-induced gratings, caused by cubic optical nonlinearity, on the Bragg diffraction of Bessel light beams by ultrasound in uniaxial gyrotropic crystals was studied theoretically. It was shown that, when Bessel light beams are used, the threshold for a substantial manifestation of optical nonlinearity during Bragg diffraction is significantly reduced. It was established that the efficiency of the acousto-optic interaction increases with increasing light intensity in the anti-Stokes regime of diffraction and decreases monotonically in the Stokes regime. Satisfactory qualitative agreement between the theoretical and experimental dependences of the diffraction efficiency on the light and ultrasound intensities was demonstrated using a uniaxial gyrotropic quartz crystal.

Photo-electrical and transport properties of hydrothermal ZnO

We performed the studies of optical, photoelectric, and transport properties of a hydrothermal bulk n-type ZnO crystal by using the contactless optical techniques: photoluminescence, light-induced transient grating, and differential reflectivity. Optical studies revealed bound exciton and defect-related transitions between the donor states (at ∼60 meV and ∼240 meV below the conduction band) and the deep acceptor states (at 0.52 eV above the valence band). The acceptor state was ascribed to VZn, and its thermal activation energy of 0.43 eV was determined. A low value of carrier diffusion coefficient (∼0.1 cm2/s) at low excitations and temperatures up to 800 K was attributed to impact the recharged deep acceptors. Electron and hole mobilities of 140 and ∼80 cm2/Vs, correspondently, were determined at room temperature. The decrease of carrier lifetime with excitation was ascribed to increasing rate of radiative recombination at low temperatures and nonradiative recombination above the room temperature.

Photomodification of carrier lifetime and diffusivity in AlGaN epitaxial layers

Nonradiative recombination rate and diffusivity of nonequilibrium carriers were modified by intense laser pulses in AlGaN epilayers with Al content ranging from 16 to 71%. The epilayers were examined before and after the photomodification using light-induced transient grating and photoluminescence spectroscopy techniques. The photomodification resulted in (i) enhancement of the nonradiative recombination rate and (ii) large changes of the diffusion coefficient of the nonequilibrium carriers, without imposing any macroscopic structural damage to the epilayers. The photomodification effect on the recombination rate was stronger in the layers with higher Al content indicating the involvement of the Al atoms in this process. The carrier diffusivity exhibited a rapid initial increase as a consequence of the photomodification followed by a slow decline, as the photomodification duration was increased. The enhancement of the diffusion coefficient of up to 2.4 times was accompanied by 13% decrease in the carrier lifetime.

Nonradiative Recombination, Carrier Localization, and Emission Efficiency of AlGaN Epilayers with Different Al Content

The interplay between nonradiative recombination and carrier localization in AlxGa1−xN epilayers (0.11 < x < 0.78) was studied using the photoluminescence (PL) and light-induced transient grating (LITG) techniques. The carrier localization conditions were estimated from the temperature dependence of the PL band peak position. The room-temperature carrier lifetimes estimated by LITG were limited by the competition between the carrier localization and nonradiative recombination. Strong carrier localization was shown to be insufficient to achieve high internal quantum efficiency in AlGaN epilayers.

Photorefractive effect in an azobenzene chromophore side groups polymer

We have analyzed the photorefractive (PR) effect of a polymer composite was reported which combines a novel bi-functional poly(N-vinyl)-3-[p-nitrophenylazo]carbazolyl (PVNPAK) and 2,4,7-trinitro-9-fluorenone (TNF). PVNPAK was synthesized by a post-azo-coupling reaction, with an azo derivative as the electrooptic chromophore and carbazolyl as photoconductive moiety. The asymmetric two-beam coupling gain of 13.9 cm−1 and diffraction efficiency of 1.2% for poled polymer film fabricated using a corona poling are obtained at the wavelength of 647.1 nm, confirming photorefractivity. We interpreted this result as the orientational enhancement, in which the spatial charge field may enhance the modulated orientation of the azobenzen chromophore. It is unexpected that the photorefractive gain of 9.5 cm−1 for the unpoled polymer film also was observed without external field in two-beam coupling (TBC) experiment. This phenomenon is attributed to a light-induced orientational grating when the azobenzene groups are illuminated by polarized light.

Photoluminescence features and carrier dynamics in InGaN heterostructures with wide staircase interlayers and differently shaped quantum wells

We present a comprehensive study of photoexcited carrier dynamics in differently grown InGaN/InGaN multiple quantum well (MQW) structures, modified by insertion of a wide interlayer structure and subsequent growth of differently shaped quantum wells (rectangular, triangular, trapezoidal). This approach of strain management allowed the reduction of dislocation density due to gradually increasing In content in the interlayer and shaping the smooth quantum well/barrier interfaces. A set of c-oriented MQW structures emitting at 470 nm were grown at Vilnius University, Institute of Applied Research, using a closed coupled showerhead type MOCVD reactor. Photoluminescence (PL) spectra of MQW structures were analysed combining continuous wave and pulsed PL measurements. Reactive ion etching of the structures enabled discrimination of PL signals originated in the InGaN interlayer structure, underlying quantum wells, and quantum barriers, thus providing growth-related conditions for enhanced carrier localization in the wells. Time-resolved PL and differential transmission kinetics provided carrier lifetimes and their spectral distribution, being the longest in triangular-shape QWs which exhibited the highest PL intensity. The light-induced transient grating (LITG) technique was used to determine the spatially averaged carrier lifetime in the entire heterostructure, in this way unravelling the electronic quality of the LED internal structure at conditions similar to device performance. LITG decay rates at low and high excitation energy densities revealed increasing with photoexcitation nonradiative recombination rate in the triangular and trapezoidal wells.

光诱导金纳米颗粒光栅及表面等离子激元的激发

光诱导金纳米颗粒光栅及表面等离子激元的激发