Wavelength Gap Research Articles

Aeolian sand sorting and megaripple formation

Sand is blown across beaches and deserts by turbulent winds. This seemingly chaotic process creates two dominant bedforms: decametre-scale dunes and centimetre-scale ripples, but hardly anything in between. By the very same process, grains are constantly sorted. Smaller grains advance faster, while heavier grains trail behind. Here, we argue that, under erosive conditions, sand sorting and structure formation can conspire to create distinct bedforms in the ‘forbidden wavelength gap’ between aeolian ripples and dunes. These so-called megaripples are shown to co-evolve with an unusual, predominantly bimodal grain-size distribution. Combining theory and field measurements, we develop a mechanistic understanding of their formation, shape and migration, as well as their cyclic ageing, renewal and sedimentary memory, in terms of the intermittent wind statistics. Our results demonstrate that megaripples exhibit close similarities to dunes and can indeed be mechanistically characterized as a special type of (‘reptation’) dune. Wind-mediated ripples form on a centimetre scale in sand, and in dunes on a scale spanning tens of metres, but patterns on intermediate scales are rare. A theory now fills the gap by predicting megaripples, which resemble structures seen on Mars.

Giant enhancement of photoluminescence and tertiary emission in a chiral nematic by matching photonic band gap and excitation wavelength

We demonstrate that chiral nematic liquid crystal compositions incorporating fluorophores, wherein the photonic band gap (PBG) of the liquid crystal is matched with the excitation wavelength of the fluorophore results in (a) significant enhancement in the overall photoluminescence of the system and (b) eliciting secondary and tertiary emission such that a single low wavelength illumination can give emissions at multiple and highly desirable Red, Green and Blue wavelengths. The light-trapping strategy of matching the PBG with the excitation wavelength instead of the previously attempted protocol of matching with the emission wavelength is shown to be superior in terms of the magnitude of emission and consequently, the quantum yield of the device. Further, we show that by using an applied electric field, three different emission levels can be achieved, two of them stable even when the field is switched off; the magnitude of the field provides a powerful handle to control the emission level with an overall change of a factor of 2.The observed PL features are explained using radiative energy transfer mechanism and the short-range ordering of the fluorophores, as obtained from X-ray diffraction experiments.

Numerical analysis of a temperature sensor based on the photonic band gap effect in a photonic crystal fiber

In this paper we investigate, by the plane wave expansion method and an analytical model, the temperature effect on the photonic band gap fiber, and we report on a numerical demonstration of a temperature sensor based on the photonic band gap (PBG) shift in a solid core photonic crystal fiber (PCF) infiltrated with a high refractive index oil. The bandwidth and the position of the central wavelength of the band gap are the parameters of interests for our temperature sensing purpose. Simulation results were found to be in excellent agreement with the refractive index scaling law and the highest sensitivity of 3.21 nm/°C was achieved, and it will be even higher than the grating based sensors written in PCFs with similar structure.

Crystal structure, morphology and optical behaviour of sol-gel derived pyrochlore rare earth titanates RE2Ti2O7 (RE=Dy, Sm)

Nanoscaled pyrochlore rare earth titanates RE2Ti2O7 (RE = Dy, Sm) were prepared by soft-chemical sol-gel route followed by thermal treatment. Dysprosium and Samarium precursors with two compositions (0.3 and 0.5 mol) were taken separately to synthesize rare earth titanates. The size and micro-structure of prepared titanates were analyzed through X-ray diffraction (XRD), Fourier transform infra-red (FTIR) spectroscopy and Transmission electron microscopy (TEM). Pyrochlore structure of rare earth titanates (Dy2Ti2O7 and Sm2Ti2O7) was successfully developed as confirmed by XRD. It was found that each reflection peak corresponding to pyrochlore structure shifted slightly towards the lower angle in case of Sm2Ti2O7 due to larger ionic radii of Sm3+ ion as compared to Dy3+ ion. The lattice constant of samarium titanate was found to be larger than that of dysprosium titanate. TEM images showed that the crystallites of prepared titanates with good dispersion were found to be almost spherical in shape with average size in range 33–40 nm. The optical band gap and emission wavelength were determined via UV–visible absorption and photoluminescence spectroscopy. The band gap of prepared titanates was increased on increasing the concentration of rare earth precursors (Dy/Sm), while the emission peak intensity was decreased.

Regulating Photonic Properties of Lamellar Chloroplast and the Environmental Adaption

Iridescent structures in photosynthetic organisms has recently been identified to possess enhanced light harvesting capability. It was proposed that interference nature of periodic photonic nanostructure results in this enhancement, nonetheless, it will inevitably accompany with light capture efficiency reduction in the photonic band gap spectrum range. Enhancement or reduction of light capture efficiency depends on the coupling condition between the electric field spatial variation and the lamellar structures. This work will provide insight about how photonic band gap wavelength of the lamellae can be regulated and the possibility in correlation between this regulation and different strategies from nature for environmental adaptation. Most of the plants with iridescent chloroplasts dwell in understorey environment, low light coherence condition might affect the predicted efficiency enhancement, and some results will be presented. These results will offer new perception about the formation of lamellar structure become a better tactic than accumulate more chlorophyll (or other light absorbing materials) molecules together for obtaining better light harvesting efficiency.

Rapid preparation of TiO2-SiO2 heterostructure photonic crystal in the near-infrared region via a modified electrophoresis-assisted self-assembly process

TiO2-SiO2 heterostructure photonic crystal was rapidly fabricated by a modified electrophoresis-assisted self-assembly process. The effect of voltage on preparation and properties of TiO2-SiO2 heterostructure photonic crystal was discussed. The results of the tests show that TiO2-SiO2 heterostructure photonic crystal with different structures can be rapidly fabricated by controlling the voltage. With the increase of voltage, the TiO2-SiO2 heterostructure photonic crystal is more likely to form an ordered hexagonal array and shows a more bright and uniform color. Meanwhile, the central wavelength of the photonic band gap will be blue shifted in the near-infrared region.

Study of Signal-to-Background Ratio of Surface-Enhanced Raman Scattering: Dependences on Excitation Wavelength and Hot-Spot Gap

Signal-to-background ratio of surface-enhanced Raman scattering (SERS) plays an important role in the analytic applications of SERS. Its dependences on excitation wavelength and gap between metal particles were studied theoretically and experimentally. We show that this ratio is higher at smaller gaps for the same excitation wavelength and is higher for the wavelengths having higher values of the absolute values of the dielectric function of the metal. This study thus results in design guidelines in the fabrication of SERS-active substrates with high signal-to-background ratio.

Influence of formic acid on optical and electrical properties of glycine crystal

A remarkable influence of formic acid on various properties of glycine single crystal has been investigated. The formic acid doped glycine crystals were grown by slow evaporation solution growth technique. The structural parameters of pure and doped glycine crystals were determined using single crystal x-ray diffraction technique. The presence of functional groups and the red shift caused by dopants were confirmed by FT-IR and FT-Raman spectroscopy. The UV–visible studies measures optical transparency, wide optical band gap (6.32 eV) and lower cutoff wavelength in glycine formic acid (GLFA) crystals. The Second Harmonic Generation measurement reveals high SHG efficiency in doped crystals compared with other glycine based NLO crystals. Dielectric studies were carried out and it explores low dielectric constant and dielectric loss for pure and doped samples. Ferro electric measurement was done to find the value of Remnant polarization (95.71 µC cm−2) and coercive field (583.71 kV cm−1) which have been enhanced for GLFA crystal.

Picosecond pulse compression by modulation of intensity envelope in a gas-filled hollow-core fiber

A method of temporally compressing picosecond pulses is proposed. To increase the spectral broadening, two picosecond pulses at different central wavelengths are overlapped spatiotemporally to induce a modulation on the intensity envelope, thus leading to a high temporal intensity gradient. The combined pulse is then coupled into a gas-filled hollow-core fiber (HCF) to broaden the spectrum through nonlinear propagation. After that the pulse can be compressed by chirp compensation. This method is demonstrated numerically with two 1-ps/5-mJ pulses centered at 1053- and 1064-nm, respectively, which are coupled into a 250-μm-inner-diameter, 1-m-long HCF filled with 5-bar neon. After nonlinear propagation, the spectrum of the combined pulse is broadened significantly compared with the sum of the broadened spectra of a single 1-ps/10-mJ pulse centered at 1053- and 1064-nm. Under proper initial conditions, the pulse can be compressed down to ~16-fs. The influences of the energy ratio, time delay and wavelength gap between two input pulses, as well as the energy scaling are also discussed. These results show an alternative way to obtain ultrashort laser pulses from the picosecond laser technology, which can deliver both high peak power and high average power, and thus will benefit relevant researches in high-field laser physics.

Rietveld refinement, micro-structural, optical and thermal parameters of zirconium titanate composites

Pure titania and zirconium titanate composites have been synthesized using sol-gel technique and annealed at 650, 875 and 1100°C for 4h. Acetylacetone was used to control the rate of hydrolysis of precursors. The Rietveld refinement, micro-structural, optical and thermal parameters of prepared composites was investigated. Effect of annealing and concentration of Zr and Ti in composites on their crystal structure, morphology, PL emission wavelength and band gap have been studied. X-ray diffraction and Rietveld analysis exhibit that all prepared samples of composites have orthorhombic single phase of zirconium titanate with space group pbcn. The lower values of profile parameters such as Rp, Rwp, RB, RF, χ2 indicated that the calculated diffraction pattern is in fair agreement with observed pattern. The increase in zirconia concentration decreases the degree of crystallization of ZrTiO4. The photoluminescence spectra show the blue shifting of emission peak at 420nm as the zirconia concentration increases. The optical band gap of zirconium titanate composites also blue shifted (3.24–3.64eV) with decrease in crystallite size of composites.

New route in the synthesis of Tin(II) oxide micro-sheets and its thermal transformation

The micro-sheets tin(II) oxide (SnO) has been synthesized using hydrothermal-alkaline solution at 95 °C (low temperature) under the ambient atmosphere. The as-prepared material showed a romarchite tetragonal structure without impurities. Furthermore, the study of SnO transformation has been carried out by thermal treatment ranging from 400 °C to 700 °C. The result shows that the transformation was a direct oxidation without forming any intermediate phases. The SnO started to oxidize at 400 °C and fully transformed into tin(IV) oxide (SnO2) at 600 °C. The thermal treatment has also changed the morphology and particle size of SnO from micro-sheets-structure to amorphous-flowerlike nanostructure and shifted the absorption spectra resulting in shorter UV wavelength and higher calculated-band gap.

An insight into the synthesis, crystal structure, geometrical modelling of crystal morphology, Hirshfeld surface analysis and characterization ofN-(4-methylbenzyl)benzamide single crystals

A versatile approach for the synthesis ofN-(4-methylbenzyl)benzamide, C15H15NO, using CuI as catalyst has been reported. Single crystals of the synthesized compound were grown using the slow evaporation solution technique. The crystal structure of theN-(4-methylbenzyl)benzamide crystals has been determined by single-crystal X-ray diffraction. The compound crystallizes in an orthorhombic lattice, noncentrosymmetric space groupPna21. The crystal structure is stabilized by intermolecular N—H...O hydrogen bonds and weak C—H...π interactions to form layers parallel to theaaxis. A user-friendly approach based on centre of mass propagation vector theory was used to predict the crystal morphology. The framework developed here utilizes the concept of intermolecular bond strength to discern the crystal morphology. Fourier transform IR, NMR and high-resolution mass spectrometry analytical techniques were used for the identification of functional groups and confirmation of the structure of the title compound. All of the intermolecular interactions present in the crystal structure, including the C—H...π, C—H...O and N—H...O interactions, were investigated and confirmed by molecular Hirshfeld surface analysis. From linear optical spectroscopy, the transmittance, optical band gap and UV cutoff wavelength were determined. The photoluminescence emission spectrum was recorded for a grown crystal. Dielectric measurements were performed at room temperature for various frequencies. The mechanical strength of the (001) plane of the title compound was measured using the Vickers micro-hardness technique. A piezo-coefficient of 15 pC N−1was found along the (001) plane of the title crystals. The thermal stability and melting point were also investigated. In addition, density functional theory simulations were used to calculate the optimized molecular geometry and the UV–vis spectrum, and to determine the highest occupied molecular orbital/lowest unoccupied molecular orbital energy gap. The results show thatN-(4-methylbenzyl)benzamide is a potential candidate for multifunctional optical and piezoelectric crystals.

Growth, structural, thermal, dielectric and optical studies on HBST crystal: A potential THz emitter

The efficient organic nonlinear optical material 4-hydroxy benzaldehyde-N-methyl 4-stilbazolium tosylate (HBST) was grown from methanol by slope nucleation method combined with slow cooling (SNM-SC) for the first time. The optimum growth conditions based on the cooling rate was further investigated. The single crystal X-ray diffraction (XRD) revealed that the chromophores of HBST crystal make an angle of about 33° with respect to the a-axis, which is close to the optimum of Terahertz (THz)-wave generation and electro-optics applications. NMR and FT-IR spectral studies have been performed to ascertain various functional groups present in the sample. Futhermore, the thermal stability and decomposition stages were analyzed through TG-DTA and DSC techniques. The dielectric constant and dielectric loss of HBST crystal have been studied. Critical optical properties like the absorption coefficient, refractive index, cut-off wavelength and band gap energy were calculated. Photoluminescence (PL) exication studies indicated green emission occured at 507nm. All the results of HBST crystal make it a promising candidate in the fields of optoelectronic and the generation of THz.

Nonlinear optical, optical limiting and dielectric properties of organic cyclohexylammonium acetate single crystal

We report the growth and nonlinear optical properties of cyclohexylammonium acetate (CYHAC) crystal. The functional groups and crystal structure were determined using FT-IR analyses and single crystal XRD (293K). UV–Vis–NIR measures the cutoff wavelength (254nm) and optical band gap (5.17eV) of CYHAC crystal. TG-DSC spectrum shows the melting point (161°C) and decomposition of the compound. Activation energy was calculated from dielectric studies as a function of frequency at different temperatures. Laser damage threshold of CYHAC is 5.23GW/cm2. Nonlinear optical parameters were calculated using Z-Scan technique for this material and its optical limiting behavior were also tested, the value of limiting threshold and clamping output are 24.3mW and 7.1mW respectively.

Impact of the wetting layer thickness on the emission wavelength of direct band gap GeSn/Ge quantum dots

The effects of the wetting layer thickness (tWL) on the electronic properties of direct band gap type-I strained dome shaped Ge(1−x)Snx quantum dot (QD) embedded in Ge matrix is numerically studied. The emission wavelength and the energy difference between S and P electron levels have been evaluated as a function of tWL for different QD size and composition with constant height to diameter ratio. The emission wavelength is found to be red shifted by increasing the wetting layer thickness, with smaller size QD being more sensitive to the variation of tWL. Furthermore, the minimum Sn composition required to fit the directness criteria is found to reduce by increasing the wetting layer thickness.

Synthesis, structural, thermal and mechanical properties of 1,10-phenanthrolinium-3-carboxy-4-hydroxy benzene sulphonate crystal: An efficient SHG material with a high laser damage threshold for NLO applications

Nonlinear optical material, 1,10-phenanthrolinium-3-carboxy-4-hydroxy benzene sulphonate (PCHBS) has been synthesized and single crystals grown by slow solvent evaporation solution growth technique in methanol as the solvent. The 1H and 13C NMR spectra were recorded to establish the molecular structure. The single crystal XRD analysis reveals that the title salt crystallizes in monoclinic crystal system with non-centrosymmetric space group, P21. The presence of various functional groups was confirmed by FT-IR spectroscopic studies. The UV–vis-NIR was recorded in the range 200–1200nm to find the optical transmittance window, lower cut off wavelength and optical band gap. The thermal and mechanical properties were estimated by TG/DTA analysis and Vickers microhardness study, respectively. The SHG efficiency of title crystal was measured by employing modified Kurtz and Perry powder technique. Further, the laser damage threshold value has been determined using a Q-switched Nd:YAG laser operating at 1064nm.

Synthesis, crystal structure, growth, optical and third order nonlinear optical studies of 8HQ2C5N single crystal - An efficient third-order nonlinear optical material

A neoteric organic third order nonlinear optical material 8-hydroxyquinolinium 2-chloro-5-nitrobenzoate dihydrate (8HQ2C5N) was grown by slow cooling technique using ethanol: water (1:1) mixed solvent. The calculated low value of average etch pit solidity (4.12 × 103 cm−2) indicated that the title crystal contain less defects. From the single crystal X-ray diffraction data, it was endowed that 8HQ2C5N crystal belongs to the monoclinic system with centrosymmetric space group P21/c and the cell parameters values, a = 9.6546 (4) Ǻ, b = 7.1637(3) Ǻ, c = 24.3606 (12) Ǻ, α = γ = 90°, β = 92.458(2)° and volume = 1683.29(13) Ǻ3. The FT-IR and FT-Raman spectrum were used to affirm the functional group of the title compound. The chemical structure of 8HQ2C5N was scrutinized by 13C and 1H NMR spectral analysis and thermal stability through the differential scanning calorimetry study. Using optical studies the lower cut-off wavelength and optical band gap of 8HQ2C5N were found to be 364 nm and 3.17 eV respectively. Using the single oscillator model suggested by Wemple – Didomenico, the oscillator energy (Eo), the dispersion energy (Ed) and static dielectric constant (εo) were estimated. The third-order susceptibility were determined as Im χ(3) = 2.51 × 10−5 esu and Re χ(3) = 4.46 × 10−7 esu. The theoretical third-order nonlinear optical susceptibility χ(3) was calculated and the results were compared with experimental value. Photoluminescence spectrum of 8HQ2C5N crystal showed the yellow emission. The crystal had the single shot laser damage threshold of 5.562 GW/cm2. Microhardness measurement showed that 8HQ2C5N belongs to a soft material category.

Growth and physical characterization of organic nonlinear optical single crystal: N,N′-diphenylguanidinium formate

A second-harmonic generation (SHG) crystal N,N′-diphenylguanidinium formate (DPGF) with good quality was successfully grown by the slow evaporation method. Single crystal X-ray diffraction study confirms that the DPGF crystallizes in orthorhombic crystal system with space group P21cn. The powder XRD pattern shows the various planes of DPGF crystal. The 1H and 13C NMR spectral studies further confirm the molecular structure of the grown crystal. The various functional groups present in the grown crystal have been identified by Fourier transform infrared spectroscopy. The UV–vis–NIR spectrum was recorded to find the optical transmittance window, lower cut off wavelength and band gap of the DPGF crystal. Photoluminescence spectrum shows broad emission peak observed at 367nm. Thermal stability of the title crystal was evaluated by thermogravimetric (TG) and differential thermal analysis. Vicker's microhardness study was carried out to reveal the mechanical properties of the grown crystal. The second harmonic conversion efficiency of DPGF crystal was 1.8 times than that of KDP crystal. Gaussian 03W program has been used to calculate the molecular orbital and first order hyperpolarizability of the molecule. The calculated first order hyperpolarizability value of DPGF is found to be 16 times greater than that of urea.

Studies on the growth aspects, structural and third-order nonlinear optical properties of Piperidinium 3-carboxy-4-hydroxy benzenesulfonate single crystal

Organic nonlinear optical material Piperidinium 3-carboxy-4-hydroxy benzenesulfonate (PBS) single crystal was successfully grown by solution growth method. Single crystal X-ray diffraction study confirms that, the grown crystal belongs to P21/n space group. The crystalline quality of PBS was ascertained by HRXRD studies. Infrared spectral analysis showed the vibrational behaviour of chemical bonds and presence of its functional groups. TG/DTA studies were used to study the thermal stability and decomposition stages of the compound. UV–visible transmittance studies showed the transparency region, cut-off wavelength and band gap of the grown crystal. Photoluminescence emission study was carried out for the grown crystal to show its electronic properties. By using Nd:YAG laser, the laser damage threshold was estimated for the grown crystal. The third-order nonlinear optical parameters of the grown crystal were estimated by Z-scan technique using He-Ne laser source. The mechanical property of the PBS crystal was studied by using Vicker's microhardness measurement.

Time dependent Bloch mode transmittance in self-assembled random photonic crystal for photonic time delay switching

Light propagation and localization in a random structure with a periodic background is an upcoming paradigm for novel photonic applications. This paper demonstrates the phenomenon of time dependent transmittance of evanescent Bloch modes (EBM) in ZnS random photonic crystal (RPC) which forms the basis for photonic delay switching. The RPC is fabricated by colloidal self-assembly with ZnS nanospheres of size 215 nm. An anomalous reciprocity and time dependent transmission at EBM (mid band gap wavelength) are observed in coherent back scattering and transmission studies respectively. These are explained on the basis of restricted propagation of EBMs through random channels in the periodic background and enhanced field storage inside RPC. The channelized propagation of EBMs is evident from decreasing time delay of transmittance at reduced thicknesses. The proportionality between transmission time delay and incident power confirms photon (field) storage within the RPC. The results indicate that structures with systematically engineered EBM channels can work as wavelength selective delay switch and further provide a short time photon storage system under non-absorbing conditions.