Photovoltaic properties of black silicon microstructured by femtosecond pulsed laser

Prototype devices based on black silicon have been fabricated by microstructuring 250 μm thick multicrystalline n doped silicon wafers using femtosecond pulsed laser in ambient gas of SF₆ to measure its photovoltaic properties. The enhanced optical absorption of black silicon extends across the visible region and all the black silicons prepared in this work exhibit enhanced optical absorption close to 90% from 300 nm to 800 nm. The highest open-circuit voltage (V_oc) and short-circuit current (I_sc) under the illumination of He–Ne continuous laser at 632.8 nm were measured to be 53.3 mV and 0.11 mA, respectively at a maximum power conversion efficiency of 1.44%. Upon excitation with He–Ne continuous laser at 632.8 nm, external quantum efficiency (EQE) of black silicon as high as 112.9% has also been observed. Development of black silicon for photovoltaic purposes could open up a new perspective in achieving high efficient silicon-based solar cell by means of the enhanced optical absorption in the visible region. The current–voltage characteristic and photo responsivity of these prototype devices fabricated with microstructured silicon were also investigated.     

Optical properties of amorphous Si/SiO2superlattice

Amorphous Si/SiO₂(a-Si/SiO₂) superlattices have been fabricated by the magnetron sputtering technique. The superlattice with an Si layer thickness of 1.8 nm has been characterized by transmission electron microscopy (TEM). The result indicates that most of the regions in the Si layer consist of amorphous phase, while regular structure appears in some local regions. This is in agreement with the Raman scattering spectroscopy. The optical absorption spectrum and photoluminescence (PL) spectrum have been measured. Moreover, the third-order optical nonlinearity χ⁽³⁾of this superlattice has been measured. To our knowledge, this is the first investigation of the nonlinear absorption and refractive index of an a-Si/SiO₂superlattice using the Z -scan technique. The real and imaginary parts of χ⁽³⁾have been found to be 1.316  ×  10 ^− 7eus and  − 5.596  ×  10 ^− 7eus, respectively, which are about two orders of magnitude greater than those of porous silicon. The results may be attractive for potential application in electro-optics devices.     

Diffraction patterns and nonlinear optical properties of gold nanoparticles

Stable gold nanoparticles have been prepared by using soluble starch as both the reducing and stabilizing agents; this reaction was carried out at 40 °C for 5 h. The obtained gold nanoparticles were characterized by UV–Vis absorption spectroscopy, transmission electron microscopy (TEM) and z-scan technique. The size of these nanoparticles was found to be in the range of 12–22 nm as analyzed using transmission electron micrographs. The optical properties of gold nanoparticles have been measured showing the surface plasmon resonance. The second-order nonlinear optical (NLO) properties were investigated by using a continuous-wave (CW) He–Ne laser beam with a wavelength of 632.8 nm at three different incident intensities by means of single beam techniques. The nonlinear refractive indices of gold nanoparticles were obtained from close aperture z-scan in order of 10^?7 cm²/W. Then, they were compared with diffraction patterns observed in far-field. The nonlinear absorption of these nanoparticles was obtained from open aperture z-scan technique. The values of nonlinear absorption coefficient are obtained in order of 10^?1 cm/W.     

Nonlinear optical properties of silver colloidal solution by in situ synthesis technique

The silver colloidal solutions were prepared by in situ synthesis technique in the presence of the Polymethyl Methacrylate, which was polymerized by reversible addition-fragmentation transfer. The UV–VIS spectra and transmission electron microscopy had shown the formation of sphere silver nanoparticles with average size of 10 nm. Nonlinear optical properties as a function of silver concentration were studied using Z-scan technique with 13 ns pulse duration at 532 nm. The optical nonlinearity enhancement was observed by increasing the concentration. The third-order nonlinear susceptibility χ⁽³⁾ was measured to 1.045 × 10⁻¹¹ esu when the concentration was 2.13 mg/ml. Besides, the sample was founded to exhibit a shift from saturable absorption to reverse saturable absorption at higher incident laser energy. The reverse saturable absorption was observed to be responsible for the optical limiting characteristics in our experiments.     

Influence of γ-irradiation on the optical parameters of 4-tricyanovinyl-N,N-diethylaniline thin films

Thin films of 4-tricyanovinyl-N,N-diethylaniline (TCVA) were prepared by thermal evaporation technique. The spectral and the optical parameters have been investigated by using the spectrophotometric measurements of both transmittance and reflectance at normal incidence of light in the wavelength range 200–2500 nm. The effect of γ-irradiation on the optical parameters was investigated. It was observed that the increase in γ-irradiation dose caused an increase in the value of absorption index and a shift in the spectrum towards higher wavelengths. Therefore, the value of the optical band gap has decreased from 1.45 eV for as-deposited film to 1.39 eV for film exposed to γ-ray dose of 150 kGy and Urbach tail increased. On the other hand, the dispersion parameters of TCVA films were increased with the increase of the irradiation dose.     

High-spatial-resolution monitoring of strong magnetic field using Rb vapor nanometric-thin cell

We have implemented the so-called λ-Zeeman technique (LZT) to investigate individual hyperfine transitions between Zeeman sublevels of the Rb atoms in a strong external magnetic field B in the range of 2500 ? 5000 G (recently it was established that LZT is very convenient for the range of 10 ? 2500 G). Atoms are confined in a nanometric thin cell (NTC) with the thickness L = λ, where λ is the resonant wavelength 794 nm for Rb D₁ line. Narrow velocity selective optical pumping (VSOP) resonances in the transmission spectrum of the NTC are split into several components in a magnetic field with the frequency positions and transition probabilities depending on the B-field. Possible applications are described, such as magnetometers with nanometric local spatial resolution and tunable atomic frequency references.     

Hyperpolarizability and two photon observation of imidazolium picrate

The computational calculation based on the density functional theory at the basis sets level has been employed to compute the first-order hyperpolarizability of the title material. The nonlinear optical properties of imidazolium picrate was assessed by a open aperture z-scan technique in which Nd:YAG laser pulse at 532 nm wavelength was used as excitation source. Laser damage threshold of the crystal was investigated with Nd:YAG assembly for multiple shots and reported. The changes were observed and analyzed on (0 1 1) and (0 0 1) planes.     

Structural and optical properties of chromotrope 2R thin films of different thicknesses

Chromotrope 2R (CHR) films of different thicknesses have been prepared using spin coater. The material has been characterized using FT-IR, DTA and X-ray diffraction. The XRD of the material in powder and thin film forms showed polycrystalline structure with triclinic phase. Preferred orientation at the (1 1 4) plane is observed for the deposited films. Initial indexing of the XRD pattern was performed using “Crystalfire” computer program. Miller indices, h k l, values for each diffraction line in X-ray diffraction (XRD) spectrum were calculated and indexed for the first time. The DTA thermograms of CHR powder have been recorded in the temperature range 25–350 °C with different heating rates. The spectra of the infra-red absorption allow characterization of vibration modes for the powder and thin film. The effect of film thickness on the optical properties has been studied in the UV-visible-NIR regions. The films show high transmittance exceeding 0.90 in the NIR region λ > 800 nm. The intensity of the absorption peaks for λ < 800 nm are enhanced as the film thickness increase. The absorption bands are attributed to the (π–π*) and (n–π*) molecular transitions. The optical properties have been analyzed according to the single-oscillator model and the dispersion energy parameters as well as the free charge carrier concentration have been determined. The optical energy gap as well as the oscillator strength and electric dipole strength have been calculated.     

Structural and photoluminescence investigation on the hot-wire assisted plasma enhanced chemical vapor deposition growth silicon nanowires

High density of silicon nanowires (SiNWs) were synthesized by a hot-wire assisted plasma enhanced chemical vapor deposition technique. The structural and optical properties of the as-grown SiNWs prepared at different rf power of 40 and 80 W were analyzed in this study. The SiNWs prepared at rf power of 40 W exhibited highly crystalline structure with a high crystal volume fraction, X_C of ～82% and are surrounded by a thin layer of SiO_x. The NWs show high absorption in the high energy region (E>1.8 eV) and strong photoluminescence at 1.73 to 2.05 eV (red–orange region) with a weak shoulder at 1.65 to 1.73 eV (near IR region). An increase in rf power to 80 W reduced the X_C to ～65% and led to the formation of nanocrystalline Si structures with a crystallite size of <4 nm within the SiNWs. These NWs are covered by a mixture of uncatalyzed amorphous Si layer. The SiNWs prepared at 80 W exhibited a high optical absorption ability above 99% in the broadband range between 220 and ～1500 nm and red emission between 1.65 and 1.95 eV. The interesting light absorption and photoluminescence properties from both SiNWs are discussed in the text.     

Self-induced polarization rotation of laser beam in fullerene (C70) solutions

Nonlinear self-rotation of elliptically polarized laser pulses (λ = 532 nm, τ_FWHM ~ 12 ns) in toluene, benzene and binary mixture (toluene + ethanol) solutions of fullerene C₇₀ has been investigated experimentally. Absolute values and signs of the nonlinear refractive indices (n₂) and nonlinear optical susceptibilities χ⁽³⁾(ω, ? ω, ω) of C₇₀ solutions in toluene and benzene at different values of polarization ellipse (θ = 0.2 ÷ 0.8) have been determined. High-resolution transmission electron microscope studies of C₇₀ solutions showed that in toluene + ethanol mixtures ball-shaped C₇₀ clusters are formed with particle sizes in the range ~ 100 ÷ 500 nm. It has been demonstrated, that the clusters sizes depend on the C₇₀ concentration and volume fraction of ethanol in toluene. Correlation between the processes of C₇₀ clusters formation in solutions and the values of polarization self-rotation angle of transmitted laser beam has been demonstrated. Physical mechanisms of laser induced optical activity in fullerene solutions have been discussed.     

Optical temperature sensor through infrared excited blue upconversion emission in Tm3 +/Yb3 + codoped Y2O3

An analysis of the intense blue upconversion emission at 476 and 488 nm in Tm^3 +/Yb^3 + codoped Y₂O₃ under excitation power density of 86.7 W/cm² available from a diode laser emitting at 976 nm, has been undertaken. Fluorescence intensity ratio (FIR) variation of temperature-sensitive blue upconversion emission at 476 and 488 nm in this material was recorded in the temperature range from 303 to 753 K. The maximum sensitivity derived from the FIR technique of the blue upconversion emission is approximately 0.0035 K^? 1. The results imply that Tm^3 +/Yb^3 + codoped Y₂O₃ is a potential candidate for the optical temperature sensor.     

Effect of annealing on structure and optical properties of Ga5Se95 films

To investigate the effect of annealing on the structural and optical properties of a binary compound Ga₅Se₉₅, thin films of Ga₅Se₉₅ have been deposited on quartz substrates at room temperature by the thermal evaporation technique. X-ray diffraction patterns showed that the films before and after annealing at 573 K have polycrystalline texture and exhibit tetragonal structure. The dependences of the optical constants, the refractive index n and extinction coefficient k were studied in the spectral range of 200 nm to 2500 nm. The normal dispersion of the refractive index of the films could be described using the Wemple–DiDomenco single-oscillator model. Analysis of absorption index data reveals that as-deposited Ga₅Se₉₅ films has indirect transitions with optical energy gap of 1.685 eV.     

Advanced multifunctional optical switch for multimode optical fiber networks

In this work, an advanced multifunctional optical switch based on multimode fibers is proposed. It can work as a 3 × 1 optical multiplexer/combiner, a 2 × 2 optical switch, a variable optical attenuator and a variable optical power splitter. All these functionalities can be developed in the same device without any hardware modification, only by using the proper ports and control electronics.The proposed switch has been developed for being used in the visible and near infrared wavelength range: 450–650 nm for optical fiber automobile applications, 650–850 nm for home broadband applications; and 850–1300 nm for multimode fiber access networks. Up to three different types of twisted nematic liquid crystal cells have been designed and fabricated for fulfilling these different wavelength ranges as part of the proposed device.The multifunctional switch has been implemented and experimentally tested. Crosstalk usually better than ? 15 dB at 532 nm, 660 nm and 850 nm, in any state has been measured. Switching is achieved at voltage levels of 4 Vrms. Fiber to fiber insertion losses when operating as a 2 × 2 optical switch, range from 10 to 15 dB within 200 nm wavelength range; with a non-optimized optics for collimation and coupling.     

Titanium oxide thin layers deposed by dip-coating method: Their optical and structural properties

TiO₂ thin films were prepared by sol-gel method. The structural investigations performed by means of X-ray diffraction (XRD) technique and scanning electron microscopy (SEM) showed the shape structure at T = 600 °C. The optical constants of the deposited film were obtained from the analysis of the experimentally recorded transmittance spectral data in the wavelength of 200–3000 nm range. The values of some important parameters of the studied films are determined, such as refractive index n and thickness d. In this work, using the transmission spectra, we have calculated the dielectric constant (ε_∞) for four layered TiO₂ films; a simple relation is suggested to estimate the third-order optical nonlinear susceptibility χ⁽³⁾. It has been found that the dispersion data obeyed the single oscillator of the Wemple–DiDomenico model, from which the dispersion parameters and high-frequency dielectric constant were determined. The estimations of the corresponding band gap E_g, χ⁽³⁾ and ε_∞ are 2.57 eV, 0.021 · 10⁻¹⁰ esu and 5.20, respectively.     

Transparent CdO/n-GaN(0001) heterojunction for optoelectronic applications

Undoped CdO films were prepared by sol–gel method. Transparent heterojunction diodes were fabricated by depositing n-type CdO films on the n-type GaN (0001) substrate. Current–voltage (I–V) measurements of the device were evaluated, and the results indicated a non-ideal rectifying characteristic with I_F/I_R value as high as 1.17×10³ at 2 V, low leakage current of 4.88×10⁻⁶ A and a turn-on voltage of about 0.7 V. From the optical data, the optical band gaps for the CdO film and GaN were calculated to be 2.30 eV and 3.309 eV, respectively. It is evaluated that interband transition in the film is provided by the direct allowed transition. The n-GaN (0001)/CdO heterojunction device has an optical transmission of 50–70% from 500 nm to 800 nm wavelength range.     

Design of multi-layer mirrors for measuring optical phase modulation at 1300 nm

A theoretical design and experimental realization of multi-layer mirrors for Fabry–Perot interferometry and optical telecommunications is described in this work. The mirrors were designed and fabricated by 13 successive thin layers to achieve very high reflectance at optical wavelengths around 1300 nm. Thin layers are ZnS and MgF₂ presenting high and low refractive index, respectively. Layer thickness λ_o/2 at λ_o=656 nm. Experimental results include characterization of transmittance of mirrors around 1300 nm. Additionally, the mirrors were integrated in a Fabry–Perot interferometer to characterize optical sources emitting at 1300 nm. Finally to show a practical application, optical phase modulation was analyzed, using the fabricated mirrors through a scanning Fabry–Perot interferometer acting as high-resolution optical spectrum analyzer (OSA).     

Simulation of 16 × 10 Gb/s WDM system based on optical amplifiers at different transmission distance and dispersion

In this paper, the 16 channel WDM systems at 10 Gb/s have been investigated for the various optical amplifiers and hybrid optical amplifiers and the performance has been compared on the basis of transmission distance and dispersion. The amplifiers EDFA and SOA have been investigated independently and further compared with hybrid optical amplifiers like RAMAN-EDFA and RAMAN-SOA. It is observed that hybrid optical amplifier RAMAN-EDFA provides the highest output power (12.017 and 12.088 dBm) and least bit error rate (10^?40 and 9.08 × 10^?18) at 100 km for dispersion 2 ps/nm/km and 4 ps/nm/km respectively.     

Study on visible luminescence of the Tm3 +: 1D2 → 3 F4 emission state in lead borate titanate aluminumfluoride glasses

This paper reports the visible luminescence properties of ¹D₂ state of Tm^3 + -doped lead borate titanate aluminumfluoride (LBTAFTm) glasses. The absorption and luminescence was analyzed within the frame work of Judd-Ofelt model. The reliability of J-O intensity parameters obtained from the experimental oscillator strengths have satisfactorily been correlated with the calculated oscillator strengths with small r.m.s deviation of ± 0.12 × 10^-6 by the least square fit analysis. Upon 359 nm excitation, the luminescence spectra show only one emission band at 458 nm (blue) corresponding to the ¹D₂ → ³ F₄ transition in the spectral region 400–500 nm. No luminescence quenching has been observed with the increase of Tm^3 + concentration. The decay profiles of the ¹D₂ level have shown single-exponential nature for all the concentrations and the decay times were found to decrease with the increase of concentration. The stimulated emission cross-section (σ_e) for the observed emission transition has also been computed. The large quantum efficiency (η) of the ¹D₂ level suggests the utility of LBTAFTm glass as a potential host for optical device applications at 458 nm emission wavelength.     

Optical and dielectric studies of solution grown glycinium maleate single crystal

Bulk single crystals of glycinium maleate have been grown from aqueous solution by slow evaporation method by optimizing the growth parameters within a period of 15 days. From X-ray diffraction analysis, the crystal was found to crystallize in monoclinic structure (space group C_2/c) with a = 17.866 Å, b = 5.684 Å, c = 17.408 Å and β = 112.65°. Presence of characteristic functional groups was confirmed in FTIR analysis. UV–Vis spectral analysis has revealed the absence of any high absorbance region between the wavelengths ranging from 300 to 900 nm. The optical band gap was calculated and found to be 3.91 eV. The activation energy for conduction at different frequencies was calculated and found to decrease from 0.54 eV to 0.41 eV as frequency increased from 100 Hz to 2 MHz. The dielectric behavior, conduction mechanism and the optical characterization of the glycinium maleate single crystals are being reported for the first time.     

Photo-induced effects on structural and optical properties of Ga15Se81Ag4 chalcogenide thin films

Thin films with thickness of 400 nm have been obtained from the Ga₁₅Se₈₁Ag₄ ternary chalcogenide glass prepared by the melt quenching technique. The behavior of several optical constants has been studied from absorption and reflection spectra as a function of photon energy in the wavelength region 400–1200 nm. The amorphous nature of the sample was examined by X-ray diffraction and non-isothermal DSC measurements. Thin films were illuminated by shining white light using 1500 W tungsten lamp with different exposure time. The ambient temperature during the illumination process was controlled and kept at 348 K, selected by DSC thermogram. Analysis of the optical absorption data shows that the rule of non-direct transition predominates. It is found that the optical band gap decreases by increasing the illumination time. It has also been observed that the value of absorption and extinction coefficients increases while the refractive index decreases by increasing the illumination time from 0 to 150 min. The decrease in optical band gap is explained on the basis of the change in nature of the films, from amorphous to crystalline state, with increase of the illumination time.