Optical spectroscopy of single InAs/InP quantum dots around λ=1.55 μm

We discuss the preparation and spectroscopic characterisation of a single InAs/InP quantum dot suitable for long-distance quantum key distribution applications around λ=1.55 μm. The dot is prepared using a site-selective growth technique which allows a single dot to be deposited in isolation at a controlled spatial location. Micro-photoluminescence measurements as a function of exciton occupation are used to determine the electronic structure of the dot. Biexciton emission, shell filling and many-body re-normalization effects are observed for the first time in single InAs/InP quantum dots.     

InGaAsSb negative luminescent devices with built-in cavities emitting at 3.9 μm

An As₂S₃ fiber coupled to an InGaAsSb photodiode was used to record the radiation distribution over the emitting surface in InGaAsSb episide-down-bonded negative luminescence devices (λ=3.9 μm). Emission spectra were recorded under forward and reverse bias and both were modulated by a Fabry–Perot resonator formed by the anode contact and emitting InAs surface in 45-μm thick diodes. The results show that the current/emission distribution crowds in the vicinity of the contact under forward bias, while a uniform current/emission distribution over the emitting surface is seen under reverse bias.     

IR-induced nonlinear optics in Ge-doped Bi12TiO20 large-sized nanocrystallites

Photoinduced non-linear optical effects in large-sized (up to 25 nm) nanocrystallites (NC) of Ge-doped Bi₁₂TiO₂₀ (BTO:Ge) incorporated within olygoether photopolymer matrix have been studied. Photoinduced second harmonic generation (PISHG) was measured. Nd:YAG pulsed laser (λ=1.06 μm) was used as a source of photoinducing light. As a fundamental light source for the SHG and two-photon absorption, Er:LiYF₄ laser (λ=2.065 μm) was used. We have found that with increasing IR pump power density, the output doubled frequency SHG signal (λ=1.03 μm) increases and achieves its maximum value at the pump power density about 0.45 GW/cm² and NC size about 12 nm.The values of second-order optical susceptibilities were almost 20% larger than for the pure BTO NC single crystals. With decreasing temperature below 60 K, the SHG signal increases achieving maximal value at LHeT.     

A DFG spectrometer at 3 μm for high resolution molecular spectroscopy and trace gas detection

We report the realization and characterization of a spectrometer based on difference frequency generation using a periodically poled lithium niobate crystal. As signal and pump lasers we used a diode-pumped Nd–YAG laser (λ=1.064 μm) and an extended cavity semiconductor diode laser (λ=0.785 μm), respectively. The mid-infrared coherent radiation was produced at 3 μm with a maximum power of about 160 nW obtained with 340 mW of signal and only 3.4 mW of pump. That corresponds to an efficiency of 0.01%/Wcm, which is in good agreement with other data available in literature. The generated radiation around 3 μm has allowed us to study fundamental absorption bands of molecules of great atmospheric and physical interest such as water vapor and the hydroxyl free radical. In this work we report preliminary spectroscopic results concerning the ν₁ 5₄₁→6₅₂ H₂O line at 2.968 μm. In particular, for this line we provide the first experimental estimation of self-, N₂- and O₂-broadening coefficients.     

A new method for increasing the laser damage threshold of metal mirrors

In order to increase the damage threshold of metal mirrors we propose to create a special structure on the surface of the mirrors (“photonic surface”). This structure must have the period about λ/2 and will suppress propagation of surface plasmons with the frequency ω₀=2πc/λ along the surface. This structure will also slightly increase the heat removal from the mirror’s surface by the excitation of the thermostimulated plasmon emission from the surface. The heat removal from the surface is estimated and possible implementation of this approach for use with CO₂-lasers (λ=10.6 μm) and Nd-YAG-lasers (λ=1.06 μm) is analyzed.     

Physical processes of current gain in InAs bipolar junction transistors

InAs bipolar junction transistors (BJTs), grown by molecular beam epitaxy, are reported with common emitter current gains (β's) as large as 400. The factors affecting the common emitter current gain have been studied by estimating the magnitudes of the base transport factor (α_T) and emitter injection efficiency (γ). This has been accomplished by studying a sequence of InAs BJTs with varying emitter doping densities, N_E. Minority carrier diffusion length in the base (L_B), α_T, and γ have been extracted from measured electrical characteristics. The results of the study of these InAs BJTs are as follows: L_B≈0.4 μm, α_T≈98% and γ ranges from 92% to nearly 100% depending on N_E. This knowledge of the magnitudes of the injection efficiencies suggests when it would be useful to move from the simple BJT structure to the more advanced heterojunction bipolar transistor (HBT) structure. Lower γ BJTs would be improved, however high-γ BJTs would benefit little, by the use of the widegap emitters of HBTs. The method developed here to estimate γ, α_T and L_B is not specific to InAs BJTs, but should be useful for study of BJTs and HBTs in any material system.     

A pulsed synthesis of β-FeSi2 layers on silicon implanted with Fe ions

Continuous layers and fine-grained films of β-FeSi₂ were synthesized using the implantation of Fe⁺ ions into Si (1 0 0) with subsequent pulsed nanosecond ion-beam treatment of the implanted layers. The X-ray diffraction studies showed that the pulsed ion-beam treatment brings about the formation of a mixture of two phases: FeSi and β-FeSi₂ with strained crystal lattices. Subsequent rapid thermal annealing led to the complete transformation of the FeSi phase into the β-FeSi₂ phase with the formation of a textured layer. The data obtained using Raman spectroscopy corroborate the formation of the β-FeSi₂ phase with a high degree of silicon crystallinity.The results of measuring the optical absorption point to the formation of β-FeSi₂ layers and precipitates with a direct-gap structure, an optical gap of E_g≈0.83 eV. The photoluminescence band peaked at λ≈1.56 μm and caused by direct band-to-band transitions in β-FeSi₂ was observed at temperatures lower than 210 K.     

Computer modeling of MWIR single heterojunction photodetector based on mercury cadmium telluride

In the present paper, an abrupt heterojunction photodetector based on Hg_1 − xCd_xTe (MCT) has been simulated theoretically for mid-infrared applications. A semi-analytical simulation of the device has been carried out in order to study the performance ratings of the photodetector for operation at room temperature. The energy band diagram, carrier concentration, electric field profile, dark current, resistance–area product, quantum efficiency and detectivity have been calculated and optimized as a function of different parameters such as device thickness, applied reverse voltage and operating wavelength. The effect of energy band offsets in conduction and valance band on the transportation of minority carriers has been studied. The influences of doping concentration, electron affinity gradient and the p–n junction position within heterostructure on potential barrier have been analyzed. The optical characterization has been carried out in respect of quantum efficiency, and detectivity of the heterojunction photodetector. In present model the Johnson–Nyquist and shot noise has been considered in calculation of detectivity. The simulated results has been compared and contrasted with the available experimental results. Results of our analytical-cum-simulation study reveal that under suitable biasing condition, the photodetector offers a dark current, I_D ≈ 6.5 × 10⁻¹² A, a zero-bias resistance–area product, R₀A ≈ 11.3 Ω m², quantum efficiency, η ≈ 78%, NEP = 2 × 10⁻¹² W Hz^1/2 and detectivity D^* ≈ 4.7 × 10¹⁰ mHz^1/2/W.     

LEDs and photodetectors for wavelength-division-multiplexed light-wave systems

The very wide transmission window, from 0.8 to 1.6 μm wavelength, in present low-loss optical fibres offers opportunities for wavelength-division-multiplexing (WDM) in light-wave systems. This paper reviews light emitting diode (LED) light sources and photodetectors capable of meeting the power (distance) and bandwidth (bit rate) requirements of such systems. Particular attention will be paid to the new multiplexing dual wavelength LEDs and demultiplexing photodetectors which point a way toward simpler, perhaps more economical, WDM light-wave systems in wide-band loop and inter-office trunking applications.     

Characteristic analysis of the optical delay in frequency response of resonant cavity enhanced (RCE) photodetectors

With consideration of the modulation frequency of the input lightwave itself, we present a new model to calculate the quantum efficiency of RCE p-i-n photodetectors (PD) by superimposition of multiple reflected lightwaves. For the first time, the optical delay, another important factor limiting the electrical bandwidth of RCE p-i-n PD excluding the transit time of the carriers and RC_d response of the photodetector, is analyzed and discussed in detail. The optical delay dominates the bandwidth of RCE p-i-n PD when its active layer is thinner than several 10~nm. These three limiting factors must be considered exactly for design of ultra-high-speed RCE p-i-n PD.     

硅基1.55 μm可调谐共振腔窄带光电探测器的研究

制作了一种低成本硅基1.55 μm可调谐共振腔增强型探测器.首次获得硅基长波长可调谐共振腔探测器的窄带响应,共振峰量子效率达44％,峰值半高宽为12.5nm,调谐范围14.5nm,并且获得1.8 GHz的高频响应.本制作工艺不复杂,成本低,有望用于工业生产.     

Nanometer-scale probing of optical and thermal near-fields with an apertureless NSOM

We have used a home-made apertureless near-field scanning optical microscope (ANSOM) for mapping nanometric steps between SiC and gold regions under visible (λ=655 nm) and infrared (λ=10.6 μm) illumination. The images, obtained with a signal demodulation at the tip oscillation frequency and at higher harmonics, clearly show optical contrasts with a subwavelength resolution of about 30 nm. Other images recorded in the visible on a YBa₂Cu₃O₇ crystal indicate that the tip used in our experiments is able to reveal polarization effects. We also present a near-field thermal optical microscope (NTOM) which operates without any external illumination. In this new kind of microscope, the laser source which is usually used to excite the evanescent waves, is replaced by a simple heating of the sample. The electromagnetic radiation locally scattered by the tip comes from the thermal radiation. Our results with this new technique prove a 200 nm lateral resolution.     

Synthesis of highly oriented TiNx coatings by free-electron laser processing of titanium in nitrogen gas

Titanium substrates were irradiated in pure nitrogen gas by means of a free-electron laser. The treatment resulted in the formation of δ-TiN_x coatings, with surface stoichiometry x≈1 and thickness of about 15 μm. Under specific experimental conditions the nitride phase had a remarkable crystallographic texture with the δ-TiN_x(200) planes parallel to the irradiated surface, and well aligned dendrites growing normal to the surface. The mechanism of the dendritic alignment and the origin of the texture correlate with the existence of a solidification front starting at the surface, which is very peculiar for laser surface treatments. This phenomenon is explained with the help of numerical simulations. PACS 61.80.Ba; 81.15.Fg; 81.65.Lp     

Novel mid-IR laser based on hybrid AlGaAsSb/InAs/CdMgSe heterostructure

We report on the development of a novel design of a mid-IR laser combining III–V and II–VI compounds in a “hybrid” double heterostructure. It possesses large (1.5 eV) potential barriers both for injected electrons and holes, suppressing their leakage from the active region, and provides strong optical confinement. An AlGaAsSb/InAs/CdMgSe laser diode with a III–V/II–VI heterovalent interface at the 0.6 μm-InAs active region has been grown by molecular beam epitaxy on an InAs substrate. Despite a far from optimal defect density at the CdMgSe/InAs interface and high losses inherent for bulk active region of the laser, the structure demonstrates lasing at 2.8 μm (up to 100 K) in the pulsed regime with a threshold current density of 3–4 kA/cm². Type II InSb monolayer insertions into an InAs layer show bright photoluminescence at 3.8 μm (77 K), confirming the great potential of the InAs-based nanostructure active region for longer wavelength applications.     

Mid-infrared lasing from self-consistent quantum wells at a type II single broken-gap heterointerface

A new physical approach for the design of mid-IR lasers operating at 3–5 μm based on type II heterojunctions with effective electron–hole confinement owing to a large asymmetric band-offset at the interface (ΔE_C>0.6 eV and ΔE_V>0.35 eV) has been proposed. The creation of high barriers for carriers leads to their strong accumulation in the active region and increases the quantum emission efficiency of the spatially separated electrons and holes across the heteroboundary due to a tunnel-injection radiative recombination mechanism within the device. An extremely weak reduction of the electroluminescence (EL) intensity for the interface tunnelling-assisted emission band with increasing temperature from 77 to 300 K was observed. This coherent emission (λ=3.146 μm at 77 K) was totally polarised in the plane perpendicular to the p–n heterojunction plane, which means the laser emission was TM-polarised due to tunnelling-assisted light-hole–electron recombination across the interface.     

Refractive index of standard oils as a function of wavelength and temperature

The refractive indices (n) of eight standard oils from Physikalisch Technische Bundesanstalt, Germany were determined with an accuracy of ±1×10⁻⁴ by using Abbe Refractometer. The measurements were performed at temperature 20°C in the spectral range 0.4–0.7 μm. The experimental data were fitted to the simple Cauchy dispersion formula and the results were found to be consistent within the limits of experimental error. In all cases, the refractive index decreased monotonically with increasing wavelength. The refractive indices (n) of these oils have been measured as a function of the temperature t (20°C up to 50°C) at λ=0.589 μm and were found to have linear temperature dependencies. The refractive indices of the studied oils and the uncertainty in their values are calculated at λ=0.589. The Lorentz–Lorenz (L–L) formula has been tested and it was found to be valid with a maximum deviation of 0.4% and was used to calculate the molecular polarizability θ.     

Deflectometer with synthetically generated reference circle for aspheric surface testing

A deflectometer with a synthetically generated reference circle is proposed for aspheric surface testing. Rotation and translation movements are combined to realize laser scanning and make the measurement of the aspheric surface in polar coordinates. It effectively improves the measurement precision for aspheric surfaces with large relative aperture. The measuring equipment is calibrated using a defocused standard spherical surface, and it achieves a precision of λ/5–λ/10 (λ=0.6328 μm), which is close to the precision of the interferometric method (λ/20). This testing technique based on laser deflectometry is capable of measuring most kinds of aspheric surfaces, especially those with large asphericity and those of concave shapes. The feasibility of the technique is shown and experimental results are presented.     

Coherence characteristics of light-emitting diodes

We report the measurement of coherence characteristics of light-emitting diodes (LEDs). Experiments were performed using red and green color LEDs directly illuminating the Young's double slit kept in the far-zone. Fourier transform fringe analysis technique was used for the measurement of the visibility of interference fringes from which the modulus of degree of spectral coherence was determined. Low degree of spectral coherence, typically 0.4 for red and 0.2 for green LED with double-slit separation of 400 μm was observed. A variable slit was then kept in front of the LEDs and the double slit was illuminated with the light coming out of the slit. Experiments were performed with various slit sizes and the visibility of the interference fringes was observed. It was found that visibility of the interference fringes changes drastically in presence of variable slit kept in front of LEDs and a high degree of spectral coherence, typically 0.85 for red and 0.8 for green LED with double-slit separation of 400 μm and rectangular slit opening of 500 μm was observed. The experimental results are compared with the theoretical counterparts. Coherence lengths of both the LEDs were also determined and it was obtained 5.8±2 and 24±4 μm for green and red LEDs, respectively.     

Manifestation of hexagonal-like structure in the second-order nonlinear effects in the GaN nanocrystallites

Contribution of the hexagonal-like structural components to the photoinduced second harmonic generation (SHG) in GaN large-sized nanocrystallites (with sizes about the 10–30 nm) incorporated into the polyvinylalcohol photopolymer matrices is revealed. The SHG measurements were done using pulsed Nd:YAG laser beam (λ=1.06 μm; pulse duration τ=15–50 ps, laser power about 30 MW) as a fundamental ones and a picosecond nitrogen pulsed laser (P=10 MW; λ=0.377 μm; pulse time duration τ=10–25 ps) as a photoinducing one. We have found that with increasing pumping power density the SHG output signal increases and achieves its maximum value for the power density about 2.6 GW/cm² per pulse. The maximal output photoinduced SHG signal was achieved for parallel directions of the pumping and fundamental beam polarizations. The maximal values of the second-order nonlinear susceptibilities were equal to about 1.09 pm/V. We have observed an increase of the output SHG below 30 K and for pump-probe delaying time about 18 ps. Substantial contribution to the SHG of wurtzite-like (hexagonal) structural fragments is shown.