Type Of Optical Transition Research Articles

A metasurface optical modulator using voltage-controlled population of quantum well states

The ability to control the light-matter interaction with an external stimulus is a very active area of research since it creates exciting new opportunities for designing optoelectronic devices. Recently, plasmonic metasurfaces have proven to be suitable candidates for achieving a strong light-matter interaction with various types of optical transitions, including intersubband transitions (ISTs) in semiconductor quantum wells (QWs). For voltage modulation of the light-matter interaction, plasmonic metasurfaces coupled to ISTs offer unique advantages since the parameters determining the strength of the interaction can be independently engineered. In this work, we report a proof-of-concept demonstration of a new approach to voltage-tune the coupling between ISTs in QWs and a plasmonic metasurface. In contrast to previous approaches, the IST strength is here modified via control of the electron populations in QWs located in the near field of the metasurface. By turning on and off the ISTs in the semiconductor QWs, we observe a modulation of the optical response of the IST coupled metasurface due to modulation of the coupled light-matter states. Because of the electrostatic design, our device exhibits an extremely low leakage current of ∼6 pA at a maximum operating bias of +1 V and therefore very low power dissipation. Our approach provides a new direction for designing voltage-tunable metasurface-based optical modulators.

Correlation between microstructures and optical properties of polyaniline/single‐walled carbon nanotubes composites

The polyaniline was doped with sulfonic acid and functionalized with single‐walled carbon nanotubes. The effects of single‐walled carbon nanotubes (SWCNTs) inserting and thermal annealing at T = 393 K, during 24 h are carried out by optical and structural study. Structural characterizations of D‐polyaniline (PANI) or D‐PANI/SWCNTs compounds at both pristine and annealed states were obtained by Fourier Transform infrared analysis and scanning and transmission electron microscopies. Optical parameters including absorption coefficient, optical transition type, energy gaps, skin depth, and extinction coefficient have been also elucidated from optical absorption measurements. Moreover, the characteristics of the localized states introduced either by SWCNTs adding and/or annealing temperature are carried out. By referring to the starting material, the direct (indirect) optical band gap was found to be reduced from 3.91 (3.75) to 2.35 (2.24) eV, after SWCNTs inserting and annealing. Moreover, new localized states having a band width of 390 meV are created at 1.25 eV. The SWCNTS dispersion quality on the organic matrix and the PANI self‐crosslinking lead to the charge transfer and the resulting interpenetrating network can be used as active layers for organic solar cells. POLYM. COMPOS., 40:E821–E831, 2019. © 2018 Society of Plastics Engineers

Nanorod films of bisbenzimidazo[2,1-a:2′,1′-a′]anthra[2,1,9-def:6,5,10-d′e′f′]diisoquinoline-10,21-dione7 (BI-diisoQ) for highly optoelectronic devices

Nanostructure thin films of bisbenzimidazo[2,1-a:2′,1′-a′]anthra[2,1,9-def:6,5,10-d′e′f′]diisoquinoline-10,21-dione7 (BI-diisoQ) were prepared by using vacuum thermal evaporating procedure under vacuum of 2.45 ×10− 5 mbar with thickness of 150 nm. Structural and optical properties for the as-deposited and the annealed BI-diisoQ thin films were carried out in the temperature range of (373–623 K). The X-ray diffraction (XRD) examination of BI-diisoQ confirmed that the annealed films were a mixture of amorphous and crystalline structure, whilst the as-deposited films are utterly amorphous. Scanning Electron Microscopy (SEM) images showed a clear shape of nanorods of BI-diisoQ with diameter of 40 nm at 632 K. The optical characteristics of nanostructure thin films of BI-diisoQ were investigated by measuring transmittance and reflectance spectrum versus the incidence of visible light in the range of 190–2500 nm. Our conception dissected that the optical transition type of BI-diisoQ nanorod films is indirect allowed transition with optical and fundamental energy gaps equal to1.54 eV and 3.54 eV respectively, which decreased to 1.36 eV and 3.45 eV at the annealed temperature of 623 K. The oscillation energy, E o , and the dispersion energy, E d , were investigated by using the ideation of single oscillator model (SOM). Moreover, the non-linear optical susceptibility, χ(3), and non-linear refractive index, n 2 , were found to be temperature annealing dependence. The optical investigation of BI-diisoQ nanorod films indicated that these films have excellent optical characteristics, and thus can be recommended as a potential material for integrated highly optical applications.

Optical dynamics in the Ag/[formula omitted]-Ga2S3 layer system

In this work, thin films of Ga2S3 are deposited onto 150 nm thick transparent Ag substrate by the physical vapor deposition technique under vacuum pressure of 10−5 mbar. The films are studied by the X-ray diffraction and optical spectrophotometry techniques. It is found that the Ag substrate induced the formation of the monoclinic α-Ga2S3 polycrystals. The transparent Ag substrate also changed the preferred optical transition in Ga2S3 from direct to indirect. It also increased the light absorption by 79 and 23 times at incident light energies of 2.01 and 2.48 eV, respectively. In addition, a red shift in all types of optical transitions is observed. Some the extended energy band tails of Ag appears to form interbands in the band gap of Ga2S3. These interbands strongly attenuated the dielectric and optical conduction parameters. Particularly, an enhancement in the dielectric constant values and response to incident electromagnetic field is observed. The Drude-Lorentz modeling of this interface has shown that the free carrier density, drift mobility, plasmon frequency and reduced electron-plasmon frequency in Ga2S3 increases when the Ag substrate replaced the glass or other metals like Yb, Al and Au. The nonlinear optical dynamics of the Ag/Ga2S3 are promising as they indicate the applicability of this interface for optoelectronic applications.

Synthesis, DFT calculations, spectroscopic and photovoltaic of the novel N″, N‴-bis[(4,9-dimethoxy-5-oxo-5H-furo[3,2-g]chromen-6-yl)methylidene] thiocarbonohydrazide (BFCMT) and its photodiode application

Condensation reaction of 6-formylkhellin (1) with thiocarbohydrazide in 2:1 M ratio afforded the novel N″, N‴-bis [(4, 9-dimethoxy-5-oxo-5H-furo [3,2-g]chromen-6-yl) methylidene]thiocarbonohydrazide (BFCMT) and its electronic absorption spectrum was interpreted by TD-DFT calculations. The electronic transition is direct allowed with onset and fundamental energy gaps of 1.06 and 3.36 eV, respectively. The estimated optical constants were applied to evaluate the optical transition type as well as the effective optical parameters. The current density-voltage characteristics of BFCMT/p-Si heterojunction at 300 K in dark and under illumination of 100 mW/cm2 showed rectifying characteristics. The capacitance-voltage characteristic parameters under illumination showed a reduction in the built-in potential and increasing the active carrier concentration. The loaded J–V characteristics of BFCMT/p-Si heterojunction under illumination were investigated and showed a remarkable power conversion efficiency of 0.83% without consideration of the refection correction or losses from the upper electrode absorption.

Magnetic-optical transitions induced by twisted light in quantum dots

It has been theoretically predicted that light carrying orbital angular momentum, or twisted light, can be tuned to have a strong magnetic-field component at optical frequencies. We here consider the interaction of these peculiar fields with a semiconductor quantum dot and show that the magnetic interaction results in new types of optical transitions. In particular, a single pulse of such twisted light can drive light-hole-to-conduction band transitions that are cumbersome to produce using conventional Gaussian beams or even twisted light with dominant electric fields.

Surface-dependent conductivity, transition type, and energy band structure in amorphous indium tin oxide films

Amorphous indium tin oxide (ITO) thin films were deposited on polymethylmethacrylate and polyethyleneterephthalate substrates by radio frequency magnetron sputtering at room temperature. An interesting substrate morphology effect of ITO films on the conductivity, optical transition type and energy band structure was observed. A simplified film system model with a square potential for surface morphology was employed to explain the difference of conductivity. The energy band structures were also calculated based on the theory of amorphous semiconductor. The conclusion demonstrated the width of optical band gap, as well as the relative position of the Fermi level and mobility edge, which can easily be extended to the band structure determination of other transparent conductive films.

Comparison the optical properties for Bi2O3 and NiO ultrathin films deposited on different substrates by DC sputtering technique for transparent electronics

Bismuth and Nickel transparent oxides thin films were grown on glass and flexible polyethylene terephalate (PET) substrates by DC sputtering technique at room temperature 300 K. The structures of Bi2O3 and NiO films were analyzed by X-ray diffraction (XRD) analyses and scanning electron microscopy (SEM). A profilometry is used to measure the thicknesses of the ultrathin films. These values were established by a spectro-ellipsometry technique with a new amorphous dispersion model in the range of 200 nm–2200 nm with one nm increment. In addition, the optical constants of these films were investigated using a double beam UV–Vis–NIR spectrometer and a spectro-ellipsometry and compared using the same wavelength range. They present an excellent agreement. According to the optical transmittance spectra of the films a high average transmittance in the visible region of each sample deposited onto different substrates is measured. Moreover, the absorption coefficients and the optical energy gaps for four types of optical transitions (allowed direct, forbidden direct, allowed indirect, forbidden indirect) of the films were determined and compared with the two different techniques. Finally, the nature of the structure and morphology of Bi2O3 and NiO ultrathin films has been analyzed and compared.

SYNTHESIS AND LUMINESCENCE PROPERTIES OF ZnSe:Al NANOPARTICLES

Colloidal nanocrystals of ZnSe:Al were synthesized in organic polymer matrices. The optical absorption and longwavelength luminescence were studied, the average sizes of nanoparticles were determined, the efficiency of the transition from bulk crystals to nanocrystals was shown, and the types of optical transitions were determined

Electric- and magnetic-field dependence of the electronic and optical properties of phosphorene quantum dots

Recently, black phosphorus quantum dots were fabricated experimentally. Motivated by these experiments, we theoretically investigate the electronic and optical properties of rectangular phosphorene quantum dots (RPQDs) in the presence of an in-plane electric field and a perpendicular magnetic field. The energy spectra and wave functions of RPQDs are obtained numerically using the tight-binding approach. We find edge states within the band gap of the RPQD which are well separated from the bulk states. In an undoped RPQD and for in-plane polarized light, due to the presence of well-defined edge states, we find three types of optical transitions which are between the bulk states, between the edge and bulk states, and between the edge states. The electric and magnetic fields influence the bulk-to-bulk, edge-to-bulk, and edge-to-edge transitions differently due to the different responses of bulk and edge states to these fields.

ZnSe and copper-doped ZnSe nanocrystals (NCs): Optical absorbance and precise determination of energy band gap beside their exact optical transition type and Urbach energy

The using of a reliable and accurate new method (called in literature as derivation of absorption spectrum fitting (DASF)) for evaluation of the optical band gap (Eg) and also the exact nature of charge carriers optical transitions, is investigated in ZnSe and ZnSe:Cu nanocrystals (NCs) synthesized by rapid microwave irradiation. This method can be performed by using the output of UV–Visible spectroscopy. The obtained Eg values are within the range of 2.985–3.261 eV, depending to the microwave irradiation time and Cu dopant percentage (decreasing trend with increasing of irradiation time and Cu content). The DASF-based obtained results for ZnSe and ZnSe:Cu nanoparticles, showed the more precise values of band gap, with the same trend of previously qualitative reported data on the same samples. Also, the direct gap nature of their optical transitions was justified. To perform the method, there is no any need to the concentration of solutions and merely one need the direct absorption or transmission spectra. In other word, DASF technique was employed on ZnSe NCs to confirm its validity and to avoid non-precise reports on optical band gap which can affect on the device optimizations based on these samples. Moreover, using the values of Eg, refractive index and dielectric constant of each sample were obtained at the absorption edge. Also, the width of the tailing states in the gap (Urbach energy: ETail) was estimated and were within the range of 0.049–0.122 eV, which their very small values in compare with Eg imply to the sharp valence and conduction band edges; it means the good crystallinity nature of the produced samples.

Effect of illumination on thermally evaporated iron (III) chloride tetraphenylporphyrin thin organic films

Influence of illumination by a white light of incandescent lamp (150 W) on the optical properties of thermally evaporated iron (III) chloride tetraphenylporphyrin (FeTPPCl) thin films have been performed. The optical constants of As-deposited and illuminated FeTPPCl thin films have been determined in the wavelength range 400–2500 nm using spectrophotometric measurements at nearly normal incidence of light. The type of optical transitions for As-deposited and illuminated FeTPPCl thin films is ascertained from the spectral behavior of their absorption coefficient, α. The real part of the refractive index of As-deposited and illuminated FeTPPCl thin films in the region of optical transparency is analyzed basing on single-oscillator and Drude models. The third order nonlinear susceptibility, χ(3), of As-deposited and illuminated FeTPPCl thin films is obtained from the single-oscillator model and the Miller’s rule.

Synthesis, optical constants, optical dispersion parameters of CuO nanorods

Thin films of CuO nanorods with 300nm thickness were prepared by using non-vacuum technique, spin coating sol–gel technique. Surface morphology of CuO films were analyzed using atomic force microscopy, AFM. Thin films of CuO have nanorods in its morphology structure. Optical constants, such as refractive (n) and absorption (k) indices of the prepared film were determined using spectrophotometric measurements. Hole mobility of CuO nanorods was deduced at a certain characterized frequency, for the first time for CuO, using the well known Drude equation. Optical transition type was found to be direct allowed with energy of 2.78eV. From optical constants analysis for CuO films, the real (ɛ1) and imaginary (ɛ2) dielectric constants, optical conductivity and dissipation factor were also studied. Comparison of the obtained results with the available experimental data for previously published ones are also considered.

The Band Gap of BaPrO3 Studied by Optical and Electrical Methods

We report on measurements of the electrical and optical properties of BaPrO3. The temperature dependences of the electrical conductivity σ and the Seebeck coefficient α of polycrystalline samples were studied over a wide temperature range (300°C–1050°C). At lower temperatures, the observed charge transport can be described as thermally activated hopping of electron‐based small polarons with an activation energy of 0.37 eV. An observed change in temperature dependence of both σ and α around 700°C was observed and interpreted as a transition from extrinsic to intrinsic carrier transport. The intrinsic conduction can be modeled with an apparent electrical band gap of ~2 eV. Optical absorption and emission spectroscopy in the UV–VIS–NIR range revealed a series of characteristic absorption thresholds and the type of optical transitions was identified by combining transmittance and diffuse‐reflectance spectroscopy methods. An absorption edge of indirect type with onset at 0.6 eV is attributed to small polaron effects. The higher lying absorption thresholds of direct origin positioned at around 1.8 and 3.8 eV are correlated with thermal activation parameters from electrical measurements and discussed in terms of the band gap of BaPrO3.

Temperature Dependence of the Optical Band Gap and Optical Parameters of Tetramethyl Ammonium Tetrachlorozincate (TMA)2ZnCl4 single Crystals Around the Normal and Incommensurate Phase Transitions

Single crystals of tetramethylammonium tetrachlorozincate [N(CH3)4]ZnCl4 abbreviated hereafter as (TMA)2ZnCl4 were grown using the slow evaporation technique at 315 K. The X-ray powder diffraction patterns indicated that [N(CH)]ZnCl belongs to the orthorhombic system with Pmcn symmetry at room temperature. The lattice constants are found to be a= 12.360 Å, b= 15.687 Å and c= 8.985 Å. The values were in good agreement with the values in previous studies. Ultraviolet–visible–near-infrared (UV–Vis–NIR) spectral studies were carried out in the temperature range 276–307 K. This range of temperature involves two phase transition temperatures (Ti=296 K) from normal (parent) to incommensurate phase and (T=279 K) from incommensurate to commensurate-ferroelectric phase. The cut off wavelength was found to be 195.016 nm at room temperature. The optical transmittance increases with increasing temperature, and the cut off shifts to higher wavelengths. Analysis reveals that the type of optical transition is the indirect allowed one. The optical energy gap (Eg) has the value of 5.903 eV at room temperature. The value of optical energy gap (Eg) decreases with increasing temperature. The changes in the values of the cut off wavelength and optical energy gap (Eg) with changing the temperature were found to take different rates at the two phases under study, besides anomalous takes place at Ti and Tc. The absorption coefficient (α) as a function of the incident photon energy shows an exponential behavior near the absorption edge which suggests that the Urbach rule is obeyed and indicated the formation of a band tail. Urbach parameters were calculated at different temperatures and the frequencies of effective phonons and electron–phonon interaction constants were determined for various phases.

Light Emitting Properties of Si-Rich-Si3N4 Films Grown By PECVD Method

Bandgap engineering of Si-based materials through the control of the distribution of Si nanoclusters’ (Si-ncs) offered future applications of Si-based nanostructured materials in optoelectronics as low-cost, miniaturized, and CMOS-compatible, light-emitting and photovoltaic devices. In the past, most efforts were concentrated on the development of luminescent Si-ncs embedded in SiO2 host. However, its insulating nature remains a barrier for the production of future electrically pumped LEDs and efficient photovoltaic cells. This detrimental aspect can be coped with a medium of higher conductivity and lower band gap, for instance, Si nitride. Tunable room temperature visible light emission from Si-ncs embedded in Si nitride matrices has been recently demonstrated. Present work deals with the Si-rich Silicon nitride films were grown by PECVD technique on silicon substrates. The film stoichiometry was controlled via variation of NH3/SiH4 ratio from R=0.45 up to 1.0. Thermal treatment was performed at 1100°C for 30min in nitrogen flow to form Si-ncs. To control structural and light emitting properties of the films Raman scattering, X-ray diffraction (XRD), Transmission electron microscopy (TEM) and photoluminescence (PL) methods were used. The evolution of PL spectra with the temperature of measurements from 20 to 300 K was studied also aiming at the determination of the types of optical transitions. The PL spectra were found to be complex and the shape and magnitude of PL spectra depends on silicon nitride stoichiometry. The increase of gas ratio from R=0.63 to R=1 results in the shift of PL peak position from 1.6 eV up to 2.7 eV. Analysis of the temperature dependence of PL spectra revealed the presence of several PL components with the maxima at 2.9-3.0 eV, 2.5-2.7 eV, 1.9-2.2 eV and 1.8-1.9 eV. The peak position of the former three PL components unchanged with the decrease of temperature of the measurements. This allows describing all these components to the defects of silicon nitride host. At the same time, PL band peaked at 1.8-1.9 eV showed high-energy shift with sample cooling and can be ascribed to the exciton recombination inside Si-ncs. The presence of these latter was confirmed by Raman scattering spectra and TEM images. The nature of light emitting defects in silicon nitride, the mechanism of photoluminescence and the way for the optimization of light emitting properties are discussed.

Gamma radiation-induced changes on the structural and optical properties of aluminum phthalocyanine chloride thin films

Thin films of aluminum phthalocyanine chloride (AlPcCl) were prepared by thermal evaporation technique under high vacuum at room temperature. The prepared films were divided into two groups for study; the first was the as – deposited films, the second group was irradiated in gamma cell type 60Co source in air at room temperature with different absorbed doses (20–60kGy). The surface morphology of γ-irradiated film was observed with a higher aggregation compared with as-deposited film with an average particles size of (10–28nm). The optical parameters were obtained using spectrophotometric measurements of the transmittance and reflectance at normal incidence of light in wavelengths range of 200–2500nm. The type of optical transition was found to be an indirect allowed transition, the band gaps decrease with increasing γ-irradiation doses. The calculated dispersion parameters of AlPcCl films decreased with increasing the γ-irradiation dose. The disagreement between the obtained values of ε∞ and εL may be attributed to the lattice vibration and free carrier contribution.

Photovoltaic response of hybrid solar cells with alloyed ZnS–CuInS2 nanorods

CuInS2 and ZnS are miscible so that quaternary ZnS–CuInS2 alloys can be obtained. This opens the possibility to tune optical properties of the material in a wide range via control of the elemental composition. In the present work, ZnS–CuInS2 nanorods were synthesized by means of colloidal chemistry. Their absorption properties were studied in detail, and different types of optical transitions identified. In view of optoelectronic applications, the nanoparticles were examined for their suitability as absorber material in hybrid polymer/nanoparticle solar cells. Therefore, the nanorods were combined with a common low band gap polymer. Cyclic voltammetry and electron spin resonance were used to study the alignment of the energy levels at the heterojunction as well as the possibility of charge transfer. The material combination forms a type II heterojunction, but with the nanoparticles acting as electron donor material. The blends were implemented in hybrid solar cells. Although the photocurrent density and efficiency achieved were relatively low, the system showed a high open-circuit voltage exceeding the value 1V.

A new method for the determination of optical band gap and the nature of optical transitions in semiconductors

A new method (named as DASF: Derivation of absorption spectrum fitting) is proposed for the determination of optical band gap and the nature of optical transitions in semiconductors; this method only requires the measurement of the absorbance spectrum of the sample, avoiding any needs to film thickness or any other parameters. In this approach, starting from absorption spectrum fitting (ASF) procedure and by the first derivation of the absorbance spectrum, the optical band gap and then the type of optical transition can be determined without any presumption about the nature of transition. DASF method was employed on (60−x)V2O5–40TeO2–xAg2O glassy systems (hereafter named as TVAgx), in order to confirm the validity of this new method. For the present glasses, the DASF results were compared with the results of ASF procedure for, confirming a very good agreement between these approaches. These glasses were prepared by using the melt quenching and blowing methods to obtain bulk and film samples, respectively. Results show that the optical band gap variation for TVAgx glasses can be divided into two regions, 0 ≤ x ≤ 20 and 20 ≤ x ≤ 40 mol%. The optical band gap has a maximum value equal to 2.72 eV for x = 40 and the minimum value equal to 2.19 eV for x = 40. Also, some physical quantities such as the width of the band tails (Urbach energy), glass density, molar volume, and optical basicity were reported for the under studied glasses.

Effect of the stoichiometry of Si-rich silicon nitride thin films on their photoluminescence and structural properties

Si-rich Silicon nitride films were grown on silicon substrates by plasma enhanced chemical vapor deposition. The film stoichiometry was controlled via the variation of NH3/SiH4 ratio from 0.45 up to 1.0. Thermal annealing at 1100°C for 30min in the nitrogen flow was applied to form the Si nanocrystals in the films that have been investigated by means of photoluminescence and Raman scattering methods, as well as transmission electron microscopy. Several emission bands have been detected with the peak positions at: 2.8–3.0eV, 2.5–2.7eV, 2.10–2.25eV, and 1.75–1.98eV. The temperature dependences of photoluminescence spectra were studied with the aim to confirm the types of optical transitions and the nature of light emitting defects in silicon nitride. The former three bands were assigned to the defects in silicon nitride, whereas the last one (1.75–1.98eV) was attributed to the exciton recombination inside of Si nanocrystals. The photoluminescence mechanism is discussed.