Characteristics of InAlGaAs/InAlAs Superlattice Avalanche Photodiodes for Ultra-low Optical Power Detection in the Near Infrared

Static characteristics of two different structured InAlGaAs/InAlAs superlattice avalanche photodiodes (SLAPDs) cooled by liquid nitrogen were evaluated at a wavelength of 1.5 μm. The dark current of the SLAPD having a thick superlattice layer of 0.504 μm was 5 × 10⁻¹³ A. This was successively reduced by four orders of magnitude compared to that of the thin layer SLAPD of 0.231 μm at a breakdown voltage of around 20 V. The thickened layer was effective in suppressing tunneling dark current. An output current of 1.7×l0⁻¹² A at a bias voltage of 15 V was measured for an optical input with a wavelength of 1.5 μm and a signal power of 1 × 10⁻¹² W. This showed a sharp distinction from the dark current.     

Development of a fast temperature sensor for combustion gases using a single tunable diode laser

The 12 best NIR water transition line pairs for temperature measurements with a single DFB laser in flames are determined by systematic analysis of the HITRAN simulation of the water spectra in the 1–2 μm spectral region. A specific line pair near 1.4 μm was targeted for non-intrusive measurements of gas temperature in combustion systems using a scanned-wavelength technique with wavelength modulation and 2f detection. This sensor uses a single diode laser (distributed-feedback), operating near 1.4 μm and is wavelength scanned over a pair of H₂O absorption transitions (7154.354 cm^-1 & 7153.748 cm^-1) at a 2 kHz repetition rate. The wavelength is modulated (f=500 kHz) with modulation amplitude a=0.056 cm^-1. Gas temperature is inferred from the ratio of the second harmonic signals of the two selected H₂O transitions. The fiber-coupled-single-laser design makes the system compact, rugged, low cost and simple to assemble. As part of the sensor development effort, design rules were applied to optimize the line selection, and fundamental spectroscopic parameters of the selected transitions were determined via laboratory measurements including the temperature-dependent line strength, self-broadening coefficients, and air-broadening coefficients. The new sensor design includes considerations of hardware and software to enable fast data acquisition and analysis; a temperature readout rate of 2 kHz was demonstrated for measurements in a laboratory flame at atmospheric pressure. The combination of scanned-wavelength and wavelength-modulation minimizes interference from emission and beam steering, resulting in a robust temperature sensor that is promising for combustion control applications.PACS 42.62.Fi; 42.55.Px; 42.60.Fc; 39.30+w     

Tuning of long-wavelength emission in InxGa1−xAs quantum dot structures

This work explores the conditions to obtain the extension of the PL emission beyond 1.3 μm in InGaAs quantum dot (QD) structures growth by MOCVD. We found that, by controlling the In incorporation in the barrier embedding the QDs, the wavelength emission can be continuously tuned from 1.25 μm up to 1.4 μm at room temperature. However, the increase in the overall strain of the structures limits the possibility to increase the maximum gain in the QD active device, where an optical density as high as possible is required. By exploring the kinetics of QD surface reconstruction during the GaAs overgrowth, we are able to obtain, for the first time, emission beyond 1.3 μm from InGaAs QDs grown on GaAs matrix. The wavelength is tuned from 1.26 μm up to 1.33 μm and significant improvements in terms of line shape narrowing and room temperature efficiency are obtained. The temperature-dependent quenching of the emission efficiency is reduced down to a factor of 3, the best value ever reported for QD structures emitting at 1.3 μm.     

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.     

GaInAsSb/AlGaAsSb lasers in the wavelength range between 2.73 μm and 2.93 μm

Compressively strained multiple quantum well lasers in the GaInAsSb/AlGaAsSb material system are reported. Indium concentrations between 40% and 47.5% were chosen for the GaInAsSb quantum wells. Compressive strains varied between 1.16% and 1.43%. The lasers worked continuous wave at room temperature up to a wavelength of 2.81 μm. For a laser with 2.93 μm wavelength continuous wave operation was found up to a temperature of −23°C. This laser worked in pulsed operation at 15°C.     

Compact eye-safe laser sources based on OPOs with KTP or PPKTP crystals

We report on compact eye-safe nanosecond laser sources emitting in the 1.5 μm wavelength range based on non-critically phase-matched parametric interaction in optical parametric oscillators (OPOs) with KTP and periodically poled KTP (PPKTP) crystals, pumped by the fundamental frequency of Nd:YAG lasers. As much as 250 μJ signal pulse energy at 1.5 μm wavelength, 6.5 ns FWHM pulse-width, has been obtained in a PPKTP-OPO, extracavity pumped by a Nd:YAG microlaser oscillator–amplifier at 650 μJ pump pulse energy, 8 ns pulse-width. A single signal pulse of 2.7-mJ output energy at 1.57 μm wavelength, less than 5 ns pulse-width, was generated in a KTP-OPO, intracavity pumped by a passively Q-switched Nd:YAG laser.     

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.     

Optically pumped lead chalcogenide infrared emitters on silicon substrates

Two low cost-infrared sources emitting above 4 μm wavelength are described: (i) Double heterostructure or quantum well EuSe/PbSe/Pb_1−xEu_xSe edge emitting lasers on silicon substrates show peak powers up to 200 mW and differential quantum efficiencies up to 20%. They operate up to 250 K when pumped with 870 nm laser diodes (with peak powers of 5.5 W). (ii) A “wavelength transformer”, a EuSe/PbSe/Pb_1−xEu_xSe active resonant cavity with epitaxial bottom and top mirror on a Si(1 1 1) substrate transforms the incoming 870 nm pump radiation into e.g. 4.2 μm wavelength. The device operates at room temperature, and the width and value of the emission wavelength can be tuned by design.     

Excitation characteristics of the IR luminescence in CU2O

Detailed luminescence excitation characteristics of the long-wave (0.93 μm), medium-wave (0.82 μm), and short-wave (0.72 μm) emission peaks in cuprous oxide were studied under pulsed dye laser excitation. The results show that the emission spectrum at 193 K contains only two peaks at 0.93 and 0.72 μm for excitation wavelenghts less than 5800 Å, and that for excitation wavelengths 5800 Å and longer an emission peak also appears at 0.82 μm. This medium wave band increases relatively faster with increase in wavelength until it becomes the dominant luminescent peak at an excitation wavelength of 6350 Å. All three emission peaks have much narrower excitation curves at 300 K than at 77 K, with the peak excitation wavelength shifting to longer wavelengths at higher temperature.     

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.     

Micromachining of copper using Nd:YAG laser radiation at 1064, 532, and 355 nm wavelengths

The interaction phenomena of nanosecond time period Q-switched diode-pumped Nd:YAG laser pulses using 1064, 532 and 355 nm with 0.25 mm thick pure-copper foil was investigated at an incident laser intensity range of 0.5–57.9 GW/cm². For each sample, etch rate and surface structure were determined. Analysis of the results of the tests included scanning electron microscopy (SEM). A maximum etch rate of 13.3 μm per pulse was obtained for the etch rate tests carried out at 532 nm. The maximum etch rate obtainable for 1064 nm was 2.21 μm per pulse, and for 355 nm, 6.68 μm per pulse. The dramatic decrease in etch rate observed when processing at 1064 nm is thought to occur due the highly reflective nature of copper as the interaction wavelength is increased, plus the nature of the plasma formed above the material during the high-intensity laser–material interaction. This plasma then imparts energy to the surface of the processed area leading to surface melting of the area surrounding the hole as can be seen by the SEM photographs.     

Four-channel coarse WDM for inter- and intra-satellite optical communications

A polymer volume grating-based four-channel coarse wavelength division multiplexer (WDM) for inter- and intra-satellite optical communication application is reported for the first time. This compact four-channel WDM device working at 0.83, 1.06, 1.34 and 1.55 μm is designed to build a complete optical link between two satellites, where wavelengths of 0.83 and 1.55 μm are used for data stream channels, 1.06 and 1.34 μm are used for inter- and intra-satellite connection. It is for the first time reported that a WDM device can cover such a large wavelength range in a single substrate. For transverse electric (TE) wave, the channel efficiencies at 0.83, 1.06, 1.34 and 1.55 μm are 55%, 40%, 35% and 45%, respectively. Channel efficiencies for transverse magnetic (TM) waves are 20% lower than those of TE waves on average. Wavelength shifts due to Doppler effect, temperature variations and radiation effects in space can be adequately accommodated.     

Synthesis and efficient field emission of ZnO nanoinjectors

ZnO nanoinjectors were synthesized on Au-coated Si substrate by direct thermal evaporation of zinc powder at a low temperature of 600 °C and atmospheric pressure. Field-emission scanning electron microscopy and X-ray diffraction were applied to study the structural characteristics of the sample. The result indicated that the nanoinjector sample consisted of single-crystalline wurtzite structures which were preferentially oriented in the 0 0 1 direction. The field emission of the sample started at a turn-on field of 1.5 V/μm at a current density of 1 μA/cm², while the emission current density reached about 1 mA/cm² at an applied field of 5.0 V/μm.     

Femtosecond pump and probe measurement of all-optical modulation based on intersubband transition in n-doped quantum wells

An all-optical modulation of interband-resonant light (near-infrared signal light: 800 nm) by intersubband-resonant light (mid-infrared control light: 4–7 μm) in n-doped AlGaAs/GaAs multiple quantum wells is investigated by two-color femtosecond pump–probe experiments at room temperature. The modulation of the near-infrared signal light with an ultrafast recovery as short as 1 ps is successfully observed when the quantum wells are pumped by the mid-infrared control light pulse (4 fJ/μm²). The dependence of the modulation depth on the wavelength of the control light is also measured, which is shown to be consistent with the intersubband absorption spectrum of the quantum wells. The results indicate that the utilization of the intersubband transition is promising for the ultrafast all-optical modulation and switching.     

Continuous wavelength tuning of InAs quantum dots on InP (1 0 0) and (3 1 1)A substrates by chemical-beam epitaxy

The growth of InAs quantum dots (QDs) on InP (1 0 0) and (3 1 1)A substrates by chemical-beam epitaxy is studied. The InAs QDs are embedded in a GaInAsP layer lattice-matched to InP. We demonstrate an effective way to continuously tune the emission wavelength of InAs QDs grown on InP (1 0 0). With an ultra-thin GaAs layer inserted between the QD layer and the GaInAsP buffer, the peak wavelength from the InAs QDs can be continuously tuned from above 1.6 μm down to 1.5 μm at room temperature. The major role of the thin GaAs layer is to greatly suppress the As/P exchange during the deposition of InAs and subsequent growth interruption under arsenic flux, as well as to consume the segregated In layer floating on the GaInAsP buffer. Moreover, it is found that InP (3 1 1)A substrates are particularly promising for formation of uniform InAs QDs. The growth of InAs on InP (3 1 1)A consists of two stages: nanowire formation due to strain-driven growth instability and subsequent QD formation on top of the wires. The excellent size uniformity of the InAs QDs obtained on InP (3 1 1)A manifests itself in the narrow photoluminescence line width of 26 meV at 4.8 K.     

Parameter optimization and characteristic analysis of a polymer microring resonant wavelength multiplexer

Novel formulas of transmission functions are presented, some parameters are optimized, and transmission characteristics are analyzed for a polymer microring resonant wavelength multiplexer around the central wavelength of 1.55 μm with the wavelength spacing of 5.6 nm and with eight vertical output channels. The computed results show that the designed device possesses some excellent features including the 3 dB bandwidth of 0.25 μm, weaker background light of 3.8×10⁻⁴, smaller inserted loss of less than 0.6 dB, and lower crosstalk below −20 dB for every vertical output channel.     

Photoluminescence and excitation spectroscopy of the 1.5 μm Er-related band in MBE-grown GaN layers

The infrared photoluminescence at 1.5 m due to the ⁴I_13/2→⁴I_15/2 transition of Er³⁺ ions has been investigated for GaN:Er³⁺ layers grown by MBE. Low temperature high resolution measurements performed under continuous illumination at the wavelength  nm, resonant to one of the intra-4f-shell transitions, revealed that the 1.5 μm band consists of up to eight individual spectral components. In excitation spectroscopy, a temperature dependence splitting of resonant bands was observed. On the basis of these experimental results, a possible multiplicity of optically active centers formed by Er doping in GaN layers is discussed.     

Enhanced field emission from PbTiO3 nanodots prepared by phase separation approach

Uniformly distributed PbTiO₃ nanodots were successfully prepared by phase separation approach. A precursor sol film was first spin-coated on Si wafer and then spontaneously separated into two distinct phases owing to the Marangoni instability. PT nanodots with tailorable size and density were obtained after further heat treatment. X-ray diffraction analysis indicated that these nanodots showed a perovskite structure. An excellent room temperature field emission property of PbTiO₃ nanodots was observed: the minimum turn-on voltage was about 5.3 V/μm; while the emission current density reached about 270 μA cm⁻² at an applied field of about 9.25 V/μm.     

Quantum dots formed by activated spinodal decomposition of InGa(Al)As alloy on InAs stressors

We have studied quantum dots (QDs) fabricated by activated spinodal decomposition (ASD) of an InGa(Al)As alloy deposited on top of self-organized InAs nanoscale stressors on GaAs substrate. Such a growth sequence results in a strong red shift of the PL emission down to 1.3 μm at 300 K. This red shift is caused by the formation of In-rich areas in the vicinity of the InAs islands, which increase the effective dot size. Beyond a certain critical InAs composition or nominal thickness of the InGa(Al)As layer the PL line shifts back towards higher energies. Adding Al to the alloy increases the red shift for a given In concentration. Room temperature lasing near 1.3 μm with threshold current densities of about 85 A/cm² was achieved for lasers based on three-fold stacked ASD-formed QDs, with a maximum cw output power of 2.7 W.     

Fabrication and low threshold current density CW operation of GaInAsP/InP multiple-reflector microcavity laser

We report CW operation of a GaInAsP/InP multiple-reflector microcavity (MRMC) laser operated at fairly low threshold current density. The threshold current density with broad contact (stripe widthW=240 m, cavity lengthL=60 m) under pulsed operation was 180 A cm^–2 (l _th=20 mA), and was 230 A cm^–2 under CW operation at room temperature operating at 1.52 m wavelength.