1D-photonic crystals prepared from the amorphous chalcogenide films

We describe the preparation and optical properties of the 15-layer chalcogenide dielectric mirrors with the first order stop bands in near infrared range. The high refractive index Sb–Se and low refractive index Ge–S layers were deposited on silicon and glass substrates using thermal evaporation method. To centre the stop bands of the prepared chalcogenide mirrors at 1.55 μm, the layer thicknesses, d(Sb–Se) = 117 and d(Ge–S) = 183 nm, were calculated from the quarter wave stack condition. The optical reflectivity measurements revealed the total reflection from the 15-layer chalcogenide mirrors in the range of 1,400–1,600 nm for the unpolarized light with normal incidence. The effect of annealing on the optical properties of the prepared chalcogenide mirrors was studied as well. Using spectral ellipsometry, we examined the angular dependence of the multilayers reflectivity for the light with s- and p-polarization. The preparation of the dielectric mirrors for near infrared region from chalcogenide films seems to be possible exploiting good optical quality of chalcogenide films and their simple deposition.     

Effect of Substrate Temperature on the Properties of Nano-ZnO: In Transparent Conductive Films

High quality transparent conductive In-doped nano-ZnO thin films with In content of 2 at% were prepared by RF magnetron sputtering. The effect of substrate temperature on the structure, electrical and optical properties of nano-ZnO thin films was investigated by XRD, Hall measurement and optical transmittance spectroscopy. It shows that all the films are polycrystalline with hexagonal wurtzite structure and c-axis is perpendicular to the substrate. The grain size of the films changed from 22.4 to 28.7 nm with different substrate temperatures. The lowest resistivity of the films obtained is 3.18×10⁻³ Ω⋅cm as the growth temperature is 100 °C. The transmittance of all the films is about 85% in the visible region, and the optical band gap is in the range of 3.40∼3.45 eV.     

Investigation of annealing-treatment on structural and optical properties of sol-gel-derived zinc oxide thin films

The transparent ZnO thin films were prepared on Si(100) substrates by the sol-gel method. The structural and optical properties of ZnO thin films, submitted to an annealing treatment in the 400–700°C ranges are studied by X-ray diffraction (XRD) and UV-visible spectroscopic ellipsometry (SE). XRD measurements show that all the films are crystallized in the hexagonal wurtzite phase and present a random orientation. Three prominent peaks, corresponding to the (100) phase (2θ ≈ 31.8°), (002) phase (2θ ≈ 34.5°), and (110) phase (2θ ≈ 36.3°) appear on the diffractograms. The crystallite size increases with increasing annealing temperature. These modifications influence the optical properties. The optical constants and thickness of the films have been determined by analysing the SE spectra. The optical bandgap has been determined from the extinction coefficient. We found that the refractive index and the extinction coefficient increase with increasing annealing temperature. The optical bandgap energy decreases with increasing annealing temperature. These mean that the optical quality of ZnO films is improved by annealing.     

Properties of surface treated transparent conducting single walled carbon nanotube films

Transparent conducting single-walled carbon nanotube (SWCNT) films were fabricated using the spin coating technique. UV-ozone treated and poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) coated glass substrates together with SWCNT dispersed in 1,2-dichlorobenzene were used to promote the adhesion of SWCNT at room temperature. The produced film had a sheet resistance of 430 Ω/□ for 80% optical transparency at 550 nm. The spin coated SWCNT film after a post fabricated treatment in a mixer of isopropyl alcohol and nitric acid solution had a sheet resistance as low as 120 Ω/□ for 80% optical transparency at 500 nm. Besides reduction in sheet resistance, we obtained stable and strongly adherent SWCNT films on substrate that could serve as an alternative to transparent conducting oxides in display and optoelectronic applications.     

Size dependent optical characteristics of chemically deposited nanostructured ZnS thin films

ZnS thin films of different thicknesses were prepared by chemical bath deposition using thiourea and zinc acetate as S²⁻ and Zn²⁺ source. The effect of film thickness on the optical and structural properties was studied. The optical absorption studies in the wavelength range 250–750 nm show that band gap energy of ZnS increases from 3·68–4·10 eV as thickness varied from 332–76 nm. The structural estimation shows variation in grain size from 6·9–17·8 nm with thickness. The thermoemf measurement indicates that films prepared by this method are of n-type.     

Influence of gamma radiation on the optical properties of ZnSe nanocrystalline thin films

The effect of gamma (γ) irradiation on the absorption spectra and the optical energy bandwidth of ZnSe nanocrystalline thin films have been studied. Thin films of different thicknesses from 20 to 120 nm were deposited by Inert gas condensation technique at constant temperature of 300 K and under pressure 2 × 10⁻³ Torr of Argon gas flow. The optical transmission (T) and optical reflection (R) in the wavelength range 190–2,500 nm of ZnSe nanocrystalline thin films were measure for unirradiated and irradiated films. The dependence of the absorption coefficient α on photon energy hν was determined for different γ-doses irradiated films. The ZnSe thin films show direct allowed interband transition by γ-doses. Both the absorption coefficient (α) and optical energy bandwidth were found to be γ-dose dependent. The optical energy band width has been decreased by irradiated of γ-doses. The E_gn values of irradiated thin films by 34.5 Gy of γ-doses were recovered to nearly their initial values after 100 days at 300 K.     

Nonresonant optical nonlinearity of ZnO composite nanoparticles with different interfacial chemical environments

 ZnO nanoparticles coated by surfactant molecules were synthesized by microemulsion method. Under proper thermal treatment, a new ZnO/surfactant composite nanoparticles were formed and exhibited an unique optical property. The third-order optical nonlinearity χ⁽³⁾ of ZnO composite nanoparticles with different interfacial chemical environment were measured by single-beam Z-scan technique, the sign and magnitude of both the real and imaginary parts of χ⁽³⁾ at 790 nm were measured to be: –5.2×10^–16 m²/W and 11.6 cm/Gw for ZnO/DBS composite nanoparticles (DBS, dodecyl benzene sulfonate, anionic surfactant); and –2.2x10^–17 m²/W, 0.45 cm/Gw for ZnO/CTAB composite nanoparticles (CTAB, cetyltrimethyl ammonium bromide, cationic surfactant). The ultrafast nonlinear response time (∼250 fs) measured by time-resolved pump-probe technique at excitation wavelength of 647 nm suggests that the optical nonlinearity below band-gap originate mainly from a rapid electronic polarization process or virtual process such as the optical Stark effect. Received: 19 December 1997 / Accepted: 22 December 1997     

Nonlinear optical characterization and measurement of optical limiting threshold of CdSe quantum dots prepared by a microemulsion technique

CdSe quantum dots prepared by micro emulsion technique shows quantum confinement effect and broad emission at 532 nm. These quantum dots have about 4.35 nm size, and they exhibit good nonlinear effects which are measured using z-scan technique. The samples have a reverse saturation in the nonlinear absorption as nonlinear optical absorption coefficient β is 2.545 × 10⁻¹⁰ W m⁻¹ and nonlinear optical refraction coefficient n ₂ is −1.77 × 10⁻¹⁰ esu. The third-order nonlinear optical susceptibility is found to be 4.646 × 10⁻¹¹ esu and also the figure of merit is 2.01 × 10⁻¹² esu m. The optical limiting threshold which is found to be 0.346 GW/cm² makes it a good candidate for device fabrication.     

Growth and optical applications of centimeter-long ZnO nanocombs

Ultralong ZnO nanocombs have been synthesized on silicon substrates with a high growth rate of ∼7 μm/s using a simple “thermal evaporation and condensation” method promoted by Cu catalysts. The lengths of the ZnO nanocombs range from several millimeters to more than one centimeter and the diameters of the branches are about 300 nm. The growth mechanism of the ultralong nanocombs and the catalytic behavior of the copper are discussed. The nanocombs were readily separated and their applications as optical polarizer and grating were investigated. The results show that the ultralong ZnO nanocombs can act as effective optical components in miniaturized integrated optics systems. This article is published with open access at Springerlink.com     

Optical and mechanical properties of diamond like carbon films deposited by microwave ECR plasma CVD

Diamond like carbon (DLC) films were deposited on Si (111) substrates by microwave electron cyclotron resonance (ECR) plasma chemical vapour deposition (CVD) process using plasma of argon and methane gases. During deposition, a d.c. self-bias was applied to the substrates by application of 13·56 MHz rf power. DLC films deposited at three different bias voltages (−60 V, −100 V and −150 V) were characterized by FTIR, Raman spectroscopy and spectroscopic ellipsometry to study the variation in the bonding and optical properties of the deposited coatings with process parameters. The mechanical properties such as hardness and elastic modulus were measured by load depth sensing indentation technique. The DLC film deposited at −100 V bias exhibit high hardness (∼ 19 GPa), high elastic modulus (∼ 160 GPa) and high refractive index (∼ 2·16–2·26) as compared to films deposited at −60 V and −150 V substrate bias. This study clearly shows the significance of substrate bias in controlling the optical and mechanical properties of DLC films.     

Organic–inorganic hybrid mesoporous monoliths for selective discrimination and sensitive removal of toxic mercury ions

Abstract  

The selective optical sensing is attracting strong interest due to the use of “low-tech” spectroscopic instrumentation to detect relevant chemical species in biological and environmental processes. Our development has focused on tailoring specific solid mesoporous monoliths to be used as highly sensitive solid sensors for simple and simultaneous naked-eye detection and removal processes of extremely toxic heavy metal ions such as mercury ions in aquatic samples. The methods are emerging to design optical disc-like sensors by the immobilisation two different organic groups; however, the first organic moiety can enhance the polarity of the inorganic mesoporous disc-like monoliths “additional agents” and the second one can act as a recognition center “probe”. The latter one such as tetraphenylporphine tetrasulfonic acid (TPPS) probe led to facile handling of signal read-out with visual detection of ultra-trace concentrations of mercury ions at the same frequency as the human eye. The facile signaling was quantitatively evident using simple spectrophotometric techniques to indicate the TPPS–Hg(II) ion binding events. Control sensing assays of Hg(II) ions such as contact-time “signal response time”, thickness of support-based sensor, reaction temperature, and pH were established for achieving enhanced signal response and color intensities. Based on our results, these new classes of optical cage sensors exhibited long-term stability of recognition and signaling functionalities of Hg(II) ions that in general provided extraordinary sensitivity, selectivity, reusability, and fast kinetic detection and quantification of Hg(II) ions in our environment.     

Development of picosecond time resolution optical and X-ray streak cameras

We describe the development of an optical and an X-ray streak camera with picosecond time resolution. The entire peripheral electronics and testing systems have been developed indigenously. Both the streak cameras provide ∼ 15 mm/1 ns streak rate with a sweep voltage of ∼ 1 kV amplitude and rise time of 1 ns. The time and spatial resolution of the optical streak camera have been found to be ∼ 17 ps and 100 μm respectively. The sweep pulse generator developed for this purpose provides a step pulse of rise time ≦1 ns and amplitude ∼ 2 kV. The laser diode used for testing the optical streak camera provides multiple pulsation when the pump current is increased beyond a critical threshold.     

Evaluating WRF-Chem multi-scale model in simulating aerosol radiative properties over the tropics — A case study over India

We evaluated the performance of WRF-Chem multi-scale model over the tropics, to simulate the regional distribution and optical properties of aerosols, and its effect on radiation over India for a winter month. The model is evaluated using measurements obtained from upper-air soundings, AERONET sun photometers, various satellite instruments, and pyranometers. The simulated downward shortwave flux was overestimated when the effect of aerosols and clouds, on radiation, was⁻² neglected. The simulated downward shortwave radiation was 1 to 20 Wm closer to the observations when we included aerosol-cloud-radiation interaction in the simulation. The model usually underestimated particulate concentration for the few observations available. This is likely due to turbulent mixing, transport errors and the lack of dust emission/scheme and the secondary organic aerosol treatment in the model. The model efficiently captured the broad regional hotspots such as, higher aerosol optical depth over the northern parts of India, especially over the Indo-Gangetic basin and lower aerosol optical depth over southern parts of India. The regional distribution of aerosol optical depth agreed well with the AVHRR aerosol optical depth and the TOMS aerosol index pattern. The magnitude and wavelength-dependence of simulated aerosol optical depth was also similar to the AERONET observations across India. The difference in surface shortwave radiation between two simulations that included and neglected aerosol-radiation (aerosol-radiation-cloud) interactions⁻² were as high as −25 (−30) Wm⁻². The spatial variations of these differences were also compared with the AVHRR observation. This study suggests that the model is able to qualitatively simulate the distribution of particulates and its impact on radiation over India; however, additional measurements of particulate mass and composition are needed to fully evaluate the model performance.     

Optical characterization of a new donor–acceptor type conjugated polymer derived from 3,4-diphenylthiophene

A new donor–acceptor type poly{2-(3,4-didecyloxythiophen-2-yl)-5-[3,4-diphenyl-5-(1,3,4-oxadiazol-2-yl)thiophen-2-yl]-1,3,4-oxadiazole} (P1) has been designed and synthesized starting from thiodiglycolic acid, 1,2-diphenylethane-1,2-dione, and diethyl oxalate through multi-step reactions using precursor polyhydrazide route. The charge-transporting and linear optical property of the polymer has been investigated by cyclic voltammetric, UV–visible, and fluorescence emission spectroscopic studies. The UV–visible absorption spectrum of polymer in thin film form showed maxima at 420 nm. The polymer displayed bluish-green fluorescence both in solution and thin film form. The optical band gap is determined to be 2.27 eV. Third-order nonlinear optical property of the new polymer has been investigated at 532 nm using single beam Z-scan and degenerate four wave mixing (DFWM) techniques with nanosecond laser pulses. The absorptive nonlinearity observed for the polymer P1 is of optical limiting type, which arises due to an “effective” three-photon absorption (3PA) process. The third-order nonlinear optical susceptibility (χ⁽³⁾) of the polymer is found to be 0.831 × 10^–12 esu. Both linear and nonlinear optical studies revealed that the new polymer (P1) is a promising material for applications in photonic devices.     

Laser interferometer with a high-resolution optical microscope for measuring small line scales

The optical train of an interferometer to certify small-scale measures in the range 1–200 μm with a high-resolution modulation optical microscope for sighting the lines on the measure is described. The circuit of a interference-fringe recording unit is presented. Some results of line scale measurements are reported. Translated from Izmeritel'naya Tekhnika, No 9, pp 27–29, September, 1997     

Temperature-dependence on the optical properties and the phase separation of polymer–fullerene thin films

A detailed study on the thermal transition of poly(3-hexylthiophene) (P3HT) and blends was investigated by differential scanning calorimetry, while the morphological, phase separation and the transformation in the optical properties were probed by thermal-atomic force microscopy (AFM), polarized optical microscopy (POM) and spectroscopic ellipsometry (SE). The inclusion of fullerenes on the polymer structure confirms the formation and evolution of a new endothermic transition at high temperatures. SE revealed that the refractive index and extinction coefficient of the films increased with annealing temperature up to 140 °C due to the suppressed diffusion of PCBM molecules into the blend. Annealing above 140 °C resulted in a decrease in the optical constants due to the formation of large “needle-like” crystals. This is due to the depletion of PCBM clusters near the “needle-like” structures; resulting from the diffusion of the PCBM molecules into the growing PCBM crystals or “needle-like” crystals as is evidenced by in situ thermal-AFM and POM. These findings indicate that annealing temperature of 140 °C is suitable for a P3HT:PCBM film to obtain the desired phase separation for solar cell application.     

Optical properties of lead-bismuth cuprous glasses

The optical transmission and absorption spectra in UV- VIS were recorded in the wavelength range 350–800 nm for different glass compositions in the system (CuO) _x (PbO) ₅₀−x(Bi₂O₃)₅₀ (x = 2.5, 5.0, 7.5, 10.0, 12.5, 15.0, 20.0). Absorption coefficient (α), optical energy gap (E_opt), refractive index (n_D), optical dielectric constant (ε′_∞), measure of extent of band tailing (ΔE), constant (β) and ratio of carrier concentration to the effective mass (N/m*) have been reported. The effects of compositions of glasses on these parameters have been discussed. It has been indicated that a small compositional modification of the glasses lead to an important change in all the optical properties including non-linear behaviour. The optical parameters were found to be almost the same for different glasses in the same family.     

“Thermal Prism” Method for Measuring Thermophysical Properties of Thin Films

Theoretical principles underlying the photothermal method for measuring the thickness and thermal properties of a thin film located between two optical elements (“sandwich”) are analyzed. The method is based on the irradiation of the assembly by repetitive pulse laser radiation. Radiation is absorbed in the film and causes heating of the optical elements by heat conduction. The element is monitored by a narrow beam of a second low-power laser propagating through the heated region. The beam is deflected due to the spatial variation of the refractive index, and the magnitude of the deflection angle as a function of time contains the relaxation and “wave” components. It is shown that the phase of the “wave” component depends on the thickness and thermophysical properties of the film. The thermophysical properties of the film can be determined, provided that the analogous properties of the optical element are measured previously or otherwise known, by comparing experimentally measured values of the phase shift with theoretical values obtained from the analytical solution of the non-stationary two-dimensional heat conduction equation.     

Gamma-ray irradiation induced structural and optical constants changes of thermally evaporated neutral red thin films

Neutral red (NR) is polycrystalline in powder form, it transforms to nanocrystallite phase upon thermal deposition. Gamma-ray irradiation with doses 1.25–6 KGy induced partial transformation of nanocrystallite phase to amorphous structure. The changes of optical constants with γ-ray doses were calculated using spectrophotometer measurements of transmittance and reflectance at normal incidence of light over spectral range 200–2500 nm. The complex refractive index of NR film is highly influenced by exposure to γ-ray irradiation, the onset and optical energy gaps decrease with increasing γ-ray doses, and Urbach tail increases linearly with increasing irradiation dose. The type of electronic transition, oscillator, and electric dipole strengths and dispersion parameters were determined before and after irradiation. The spectral behavior of dielectric constant with γ-ray doses was also estimated.     

Synthesis and characterizations of CdS nanorods by SILAR method: effect of film thickness

In this investigation, we have successfully synthesized CdS nanorods by simple and inexpensive successive ionic layer adsorption and reaction (SILAR) method. The effect of film thickness on the physico-chemical properties such as structural, morphological, wettability, optical, and electrical properties of CdS nanorods has been investigated. The XRD pattern revealed that CdS films are polycrystalline with hexagonal crystal structure. SEM and TEM images showed that CdS film surface are composed of spherical grains along with some spongy cluster and an increase in film thickness up to 1.23 μm causes the formation of matured nanorods having diameter 150–200 nm. The increases in water contact angle form 105° to 130° have been observed as film thickness increases from 0.13 to 1.23 μm indicating hydrophobic nature. The optical band gap was found to be increased from 2.02 to 2.2 eV with increase in film thickness. The films showed the semiconducting behavior with room temperature electrical resistivity in the range of 10⁴–10⁶ Ω cm and have n-type electrical conductivity.