Buried crescent InP/InGaAsP/InP heterostructure on <Emphasis Type="Italic">p</Emphasis>-InP for linear edge-emitting diodes

A process has been developed for the liquid phase epitaxy of mesa stripe buried crescent InP/InGaAsP/InP heterostructures with a p–n–p–n/ZnSe leakage current-blocking structure. The salient feature of the process is a discontinuous mesa stripe which alternates with the structure of blocking layers. This technology allows one to fabricate linear edge-emitting diodes, mounted with the mesa stripe down or up, with an emission wavelength λ = 1.3–1.5 μm, high reproducibility, the possibility of coupling more than 50 μW of optical power into single-mode fiber at a current of 100 mA, an emission bandwidth of about 60 nm, and essentially negligible Fabry–Perot modulation.     

Fabrication and spectral characteristics of a laser diode for remote sensing of methane

This paper presents a technological study aimed at producing buried heterostructures for single-mode semiconductor lasers operating in the wavelength range 1651–1662 nm. We have fabricated laser diodes operating at a wavelength corresponding to an absorption band of methane. The effect of temperature on the optical emission spectrum of the laser diodes has been examined. The results demonstrate the possibility of tuning their emission spectrum in the range from 1651 to 1662 nm and producing smart fiber-optic systems for the remote monitoring of the methane concentration.     

Planar buried crescent InP/InGaAsP/InP heterostructure on <Emphasis Type="Italic">p</Emphasis>-InP

A process is described for liquid phase epitaxy of mesa stripe buried crescent InP/InGaAsP/InP heterostructures with a p-n-p-n/ZnSe leakage current blocking structure. The process enables the fabrication of 1.3-μm light-emitting diodes with a low threshold current, stable fundamental transverse mode operation, and high output power. When mounted with the mesa stripe up, the diodes offer an emission power above 30 mW.     

High-temperature buried InP/GaInAsP heterostructure laser diode emitting at 1310 nm

As a continuation of our studies aimed at creating semiconductor lasers based on buried InP/GaInAsP heterostructures, we consider the design and fabrication aspects of 1310-nm laser diodes for operation at elevated temperatures. We report the key features of the fabrication process and parameters of the laser emitters at temperatures of up to 120°C, and present their power-current and spectral characteristics.     

AlInGaPAs/GaAs/Si Heterostructures for Photoelectric Converters Fabricated by Pulsed Laser Deposition

The pulsed laser deposition method has been used to fabricate AlInGaPAs/GaAs/Si nanoheterostructures for cascaded photoelectric converters operating at wavelengths in the range of 300–1300 nm. The structural and luminescent structures of AlInGaPAs nanofilms on GaAs and GaAs on Si were studied. The spectral characteristics of photocells in an AlInGaPAs/GaAs/Si triple-cascade photoelectric converter were examined.

     

Laser diode for remote sensing of methane

This paper examines an approach to the remote sensing of explosion-safe methane concentrations using optical fiber and demonstrates that considerable potential is offered by a system for the remote sensing of methane using a laser diode emitting at 1655 nm. We describe a novel procedure for the growth of epitaxial buried InGaAsP/InP heterostructures for 1655-nm lasers. Single-mode 1655-nm laser diodes have been produced and their parameters have been studied. It is shown that the diodes can be employed in real systems for remote monitoring of methane.     

Tb<Superscript>3+</Superscript> photoluminescence in mesoporous glasses,terbium nitrate and terbium chloride hexahydrates,and coordination compounds

We have compared the intensity and shape of photoluminescence spectra of Tb³⁺ ions in mesoporous glasses, inorganic phosphors (terbium nitrate and terbium chloride hexahydrates), and coordination compounds (terbium benzoate, terbium o-phenoxybenzoate, and terbium terephthalate). The excitation sources used were a repetitively pulsed nitrogen laser with an emission wavelength of 337 nm and cw semiconductor light-emitting diodes with wavelengths of 369 and 385 nm. It has been shown that the highest photoluminescence intensity normalized by the percentage of terbium is offered by terbium terephthalate and terbium- doped mesoporous glass. We are thus led to conclude that introducing terbium terephthalate into the pores of mesoporous photonic glass and mesoporous photonic crystals is a promising approach for obtaining efficient laser operation in the green spectral region on the ⁵D₄–7F₅ transition of the terbium ion.     

Special features of the electrical characteristics of CdTe-based Schottky diodes with almost intrinsic conduction

The electrical properties of Schottky diodes obtained by vacuum deposition of Al on CdTe with weakly pronounced p-type conductivity (room-temperature ρ > 10⁹ Ω cm) have been studied. The diodes exhibit nonrectifying, almost linear current-voltage (I-V) characteristics at bias voltages |V| < 3–5 V, whereas the photo emf amounts to 0.28–0.30 V, which is explained by specific features of the charge transfer mechanism related to a large resistance of the volume part of the diode structure. A decrease in this resistance as a result of irradiation (at photon energies corresponding to an appropriate absorption level) makes the I-V curve shape typical of semiconductor diodes. The results of calculations performed within the framework of the Sah-Noyce-Shockley theory agree with the experimental data.     

Maximizing capacity,flexibility and efficiency in G-PON networks using VCSEL-based OOK and 2/4-PAM formats

This work experimentally demonstrates the potential of multi-level pulse amplitude modulation with direct detection to maximize carrier spectral efficiency and double the Gigabit passive optical networks (G-PON) network data rate. Three scenarios have experimentally been exploited. First, a 1310?nm vertical surface-emitting laser (VCSEL) has directly been modulated with a 10?Gbps OOK data. A receiver sensitivity of ?19.11?dBm is attained, and a successful error free transmission over 22?km SMF fibre achieved, with a transmission penalty of 0.46?dB. To maximize carrier spectral efficiency, 2/4 PAM modulation formats are adopted respectively. A receiver sensitivity of ?14.64 and ?11.63?dBm is attained for 2-PAM and 4-PAM formats respectively. However, a 3.21?km fibre transmission introduces a penalty of 0.64 and 3.30?dB for 2-PAM and 4-PAM formarts respectively. 2/4-PAM modulation formats significantly increase the aggregated data rate at different ONUs within the G-PON without expensive optics investment, though at the cost of reduced transmission reach due to the high bitrate attained. We further demonstrate the design of a software defined digital signal processing assisted receiver to recover the 2/4 PAM transmitted signal without employing costly receiver hardware.     

Synthesis and IR-excited luminescence of (Y1 − x Tm x )2O2S solid solutions

We have studied the key features of the luminescence spectra and kinetics of (Y_{1 ? x} Tm _x )₂O₂S solid solutions in the range 400–2000 nm under laser excitation at 790 and 810 nm. The results have been used to develop a series of IR phosphors “invisible” under laser excitation in the range 790–810 nm and possessing tunable and reproducible relative intensities of three groups of IR luminescence bands in the ranges 770–840, 1360–1520, and 1650–1980 nm, respectively.     

Synthesis and electroluminescence properties of novel silicon-based copolymers containing oxadiazole and fluorene units for PLED

Novel silicon-based copolymers containing an electron-deficient oxadiazole unit and a fluorine unit have been successfully synthesized through the Heck reaction. They are soluble in common organic solvents such as THF, CHCl₃, etc. Their UV–visible absorption spectra exhibit a strong maximum band at the range of 355–381 nm in thin film. Upon a photoexcitation of 350 nm, their photoluminescence spectra show a strong maximum band around 455–475 nm in thin film. The multi-layered light-emitting diodes (LEDs) of Al(200 nm)/Ca(50 nm)/EL polymer(80 nm)/PEDOT(50 nm)/ITO were fabricated. J–V curves show the turn-on voltage in the range of 4.4–7 V. These LEDs emit the white emissive color, due to the combination of a blue electroluminescent (EL) color and a red EL color arising from the formation of a certain charge complex.     

A facile one-step approach to obtaining uniform matchstick-like Ag(2)S-CdS nanoheterostructures

A simple and facile method for synthesizing Ag(2)S-CdS nanoheterostructures has been introduced. With a one-step hydrothermal reaction, Ag(2)S-CdS nanoheterostructures with high uniformity in morphology and structure can be obtained. The Ag(2)S-CdS nanoheterostructures exhibit matchstick-like morphology, composed of spherical Ag(2)S heads and CdS rods. The influences of the reaction temperature, reaction time and molar ratio between Ag and Cd sources were explored, the results suggesting that the growth can be ascribed to the in?situ phase transfer of Ag and Cd sources from the aqueous phase to the organic phase and the subsequent catalyst-assisted growth.     

LEDs (λ<Subscript>max</Subscript> = 3.6 μm) with cone-shaped light-emitting surfaces

A series of light-emitting diodes (LEDs) operating at λ_max = 3.6 μm are created using cone-shaped mesas with heights of 10–130 μm and concave side surfaces. The dependence of the efficiency of room-temperature (T = 298 K) emission on the mesa height at various injection currents has been studied. The character of the observed dependence agrees with the results of theoretical calculations. The output radiation power of LEDs with the maximum mesa height (130 μm) at a pumping current of 220 mA amounts to 53 μW, which is 1.5 times higher than the power of LEDs with a mesa height of 10 μm.     

Photoluminescence of terbium nitrate hexahydrate incorporated into pores of opal photonic crystals

We have measured photoluminescence (PL) and broadband reflection spectra of the surface of an opal photonic crystal (OPC) filled with terbium nitrate hexahydrate (TNH). The excitation sources used were a halogen lamp, lasers with emission wavelengths of 266 and 337 nm, and semiconductor light-emitting diodes (369 and 385 nm). It has been shown that resonance excitation of the TNH (Tb(NO₃)₃ · 6H₂O) filled OPC by pulsed laser light at a wavelength of 266 nm gives rise to superluminescence: an increase in the relative intensity of the 545-nm PL band, corresponding to the Tb^{3+ 5} D ₄–⁷ F ₅ transition. The efficiency of pumping the upper laser level of the OPC increases due to the increase in the density of photon states (Purcell effect) near the bandgap edge in the OPC. The lack of clear superluminescence under excitation by the other excitation sources is related to specific features of the absorption of incident light by the rare-earth ion and to the low output power of the cw light sources.     

On the Action of γ Radiation on Solid C60 and C70 Fullerenes: A Comparison with Graphite Irradiation

C₆₀ and C₇₀ fullerene have been treated in sealed flasks under Ar with γ radiation using radiation dosages ranging from 10 to 1000 kGy. The treated samples studied by electronic and FT-IR spectroscopy have not shown any evidence about fullerenes decomposition or radiopolymerization. However, through Raman spectroscopy it was possible to observe that γ radiation induces C₆₀ dimerization and trimerization. It has additionally been discovered that γ-treated C₆₀ (oligomerized) can be easily photopolymerized in the solid state by post-irradiation with laser light at 514 nm while this phenomenon has not been observed by using laser light at 782 nm and considerably higher laser power. Previously to this study, C₆₀ photopolymerization was known to occur only by using ultraviolet light.

For comparison also graphite was irradiated with 1000 kGy of γ radiation. Irradiated graphite shows considerably changes in its Raman spectrum, showing the formation of glassy carbon domains, perhaps carbon onions. The l_d bandshift to 1310 cm^-1 could be interpreted in terms of formation of hexagonal diamond.     

High-accuracy wavelength-change measurement system based on a Wollaston interferometer, incorporating a self-referencing scheme

A novel wavelength-difference measurement scheme with a Wollaston prism is presented. By using a suitable reference wavelength, a small variation in the signal wavelength can be converted into a relatively larger change in the modulated wavelength, as a result of the so-called fringe beating effect, resulting in enhanced measurement sensitivity by use of autocorrelation and Gaussian filtering techniques. From the results of a simulation carried out, we observed a wavelength variation of 0.01 nm over 15 nm or 0.1 nm over 60 nm for a typical pair of laser diodes with wavelengths of 785 and 810 nm, and wavelength variations of 0.5 nm over 40 nm or 1 nm over 110 nm for 671-and 785-nm wavelengths. These results were partially verified by the experimental results obtained for which a resolution of 0.01 nm over a range of 2.5 nm for the first pair and 0.5 nm over 4 nm for the second pair of laser diodes was seen. The results have applications to the determination of wavelength variations in a wavelength-division multiplexing system or measurement of the wavelength changes induced in a range of optical sensors.     

Implementation of intensity-modulated laser diodes in time-resolved, pump-probe fluorescence microscopy

We present the implementation of intensity-modulated laser diodes for applications in frequency-domain pump-probe fluorescence microscopy. Our technique, which is based on the stimulated-emission approach, uses two sinusoidally modulated laser diodes. One laser (635 nm) excites the chromophores under study, and the other laser (680 nm) is responsible for inducing stimulated emission from excited-state molecules. Both light sources are modulated in the 80-MHz range but with an offset of 5 kHz between them. The result of the interaction of the pump and the probe beams is that a cross-correlation fluorescence signal at 5 kHz is generated primarily at the focal volume. Microscope imaging at the cross-correlation signal results in images with high contrast, and time-resolved high-frequency information can be acquired without high-speed detection. A detailed experimental arrangement of our methodology is presented along with images acquired from a 4.0-mum-diameter fluorescent sphere and TOTO-3-labeled mouse STO cells. (TOTO-3 is a nucleic acid stain.) Our results demonstrate the feasibility of using sinusoidally modulated laser diodes for pump-probe imaging, creating the exciting possibility of high-contrast time-resolved imaging with low-cost laser-diode systems.     

High-power and high-accuracy integrating sphere radiometer: design, characterization, and calibration

We describe the design, characterization, and calibration of a high-power and high-accuracy transfer standard for optical power measurements in fibers based on an integrating sphere radiometer working from -50 to +30 dBm. The integrating sphere radiometer has been calibrated in the spectral range 1250-1650 nm by use of an electrically calibrated pyroelectric radiometer and four tunable laser diodes. The total uncertainty obtained is less than +/-0.8% for these wavelength and power ranges.     

The spectral resolution of high temperature GaAs photon counting soft X-ray photodiodes

Circular mesa GaAs p⁺-i-n⁺ diodes for photon counting soft X-ray spectroscopy have been fabricated and characterised over a temperature range of +80 to -30 °C. The spectroscopic performance of the diodes, as measured by the FWHM of the Mn K_α X-ray line from an ⁵⁵Fe radioisotope, is reported. In addition, we compare the GaAs diodes with previously fabricated and characterised Al_0.8Ga_0.2As p⁺-i-n⁺ diodes of similar geometry.