Structure optimisation of modern GaAs-based InGaAs/GaAs quantum-dot VCSELs for optical fibre communication

Room-temperature (RT) continuous-wave (CW) performance of modern 1300-nm oxide-confined In(Ga)As/GaAs quantum-dot (QD) vertical-cavity surface-emitting diode lasers (VCSELs) taking advantage of many QD sheets is investigated using our comprehensive self-consistent simulation model to suggest their optimal design. Obviously, quantum dots should be as uniform as possible and as dense as possible to ensure high enough optical gain. Besides, our simulation reveals that efficient and uniform current injection into VCSEL active regions necessary to enhance excitation of the desired fundamental LP₀₁ mode is accomplished in the VCSEL configuration with the broad-area bottom contact and the ring upper one as well as with the oxide aperture localized within the first period of the upper p-type DBR. The doping of the DBR mirrors is chosen as a compromise between their high enough electrical conductivity and low enough free-carrier absorption. The oxide aperture is additionally introducing the radial optical waveguiding. Moreover, our analysis has been concluded that VCSEL active regions should be composed of at least 9 QD sheets to acquire efficient RT CW operation. Furthermore, rather longer optical cavities are recommended in this case because localization of QD sheets should be adjusted to the anti-node positions of the optical cavity standing wave.     

Optimization of GaInNAs quantum-well vertical-cavity surface-emitting laser emitting at 2.33 μm

In the present paper, a comprehensive computer simulation is used to determine optimal structure of the InP-based GaInNAs quantum-well (QW) active region and to investigate a possibility of reaching room-temperature (RT) continuous-wave (CW) single-fundamental-mode 2.33-μm operation of vertical-cavity surface-emitting laser (VCSEL) with such an active region. From among various considered InP-based active regions, the one with the Ga_0.15In_0.85N_0.015As_0.985/Al_0.138Ga_0.332In_0.530As QW, i.e. with barriers lattice matched to InP, seems to be optimal for the 2.33-μm VCSEL performance. Its QW material is chosen for the required long-wavelength emission whereas its barrier is expected to ensure promising laser performance at room and higher temperatures. The latter is mostly connected with the QW conduction band offset equal in the above active region to as much as 413 meV, which is much larger than those of its possible lattice matched to InP competitors, e.g. 276 meV for the Ga_0.47In_0.53As barrier and 346 meV for the Ga_0.327In_0.673As_0.71P_0.29 one. Our simulation reveals that from among various considered structures, a VCSEL with a 4-μm-diameter tunnel junction and two 6-nm Ga_0.15In_0.85N_0.015As_0.985/Al_0.138Ga_0.332In_0.530As QWs exhibits the lowest calculated threshold current of 0.88 mA. Its promising RT CW performance suggests that it may represent a very interesting alternative to GaSb-based VCSELs.     

A nickel hydride complex in the active site of methyl-coenzyme m reductase: implications for the catalytic cycle

Methanogenic archaea utilize a specific pathway in their metabolism, converting C1 substrates (i.e., CO2) or acetate to methane and thereby providing energy for the cell. Methyl-coenzyme M reductase (MCR) catalyzes the key step in the process, namely methyl-coenzyme M (CH3-S-CoM) plus coenzyme B (HS-CoB) to methane and CoM-S-S-CoB. The active site of MCR contains the nickel porphinoid F430. We report here on the coordinated ligands of the two paramagnetic MCR red2 states, induced when HS-CoM (a reversible competitive inhibitor) and the second substrate HS-CoB or its analogue CH3-S-CoB are added to the enzyme in the active MCR red1 state (Ni(I)F430). Continuous wave and pulse EPR spectroscopy are used to show that the MCR red2a state exhibits a very large proton hyperfine interaction with principal values A((1)H) = [-43,-42,-5] MHz and thus represents formally a Ni(III)F430 hydride complex formed by oxidative addition to Ni(I). In view of the known ability of nickel hydrides to activate methane, and the growing body of evidence for the involvement of MCR in "reverse" methanogenesis (anaerobic oxidation of methane), we believe that the nickel hydride complex reported here could play a key role in helping to understand both the mechanism of "reverse" and "forward" methanogenesis.     

Comparative analysis of lasing performance of oxide-confined and proton-implanted vertical-cavity surface-emitting diode lasers

The comprehensive optical-electrical-thermal-recombination self-consistent VCSEL model is used to compare the performance of oxide-confined (OC) and proton-implanted (PI) VCSELs and to optimise their structures. Generally index-guided (IG) OC VCSELs demonstrate lower lasing thresholds whereas both gain-guided (GG) OC and PI ones manifest much better mode selectivity. Therefore, their either low-threshold IG or mode-selective GG versions may be intentionally used for different VCSEL applications. Lasing thresholds of OC IG VCSELs have been found to be very sensitive to the exact localisation of their thin oxide apertures, which should be shifted as close as possible towards the anti-node position. PI VCSELs, on the other hand, are simpler and cheaper in their manufacturing than OC ones. Although lower threshold currents are manifested by PI VCSELs with very thick implanted regions, lower threshold powers are achieved in these devices with much thicker upper unaffected layer used for the radial current flow from the ring contact towards the laser axis. Paradoxically poor thermal properties of PI VCSELs enable lower lasing thresholds of slightly detuned devices. To conclude, cheaper and mode-selective PI VCSELs may be used instead of OC ones in many of their applications provided ambient temperatures and laser outputs are not too high.     

Band diagram determination of MOS structures with different gate materials on 3C-SiC substrate

MOS capacitors were fabricated on 3C-SiC n-type substrate (001) with a 10-μm N-type epitaxial layer. An SiO₂ layer of the thickness t_OX ≈55 nm was deposited by PECVD. Circular Al, Ni, and Au gate contacts 0.7 mm in diameter were formed by ion beam sputtering and lift-off. Energy band diagrams of the MOS capacitors were determined using the photoelectric, electric, and optical measurement methods. Optical method (ellipsometry) was used to determine the gate and dielectric layer thicknesses and their optical indices: the refraction n and the extinction k coefficients. Electrical method of C = f(V_G) characteristic measurements allowed to determine the doping density ND and the flat band voltage VFB in the semiconductor. Most of the parameters which were necessary for the construction of the band diagrams and for determination of the basic physical properties of the structures (e.g. the effective contact potential difference ϕ_MS) were measured by several photoelectric methods and calculated using the measurement data. As a result, complete energy band diagrams have been determined for MOS capacitors with three different gate materials and they are demonstrated for two different gate voltages VG: for the flat-band in the semiconductor (V_G = V_FB) and for the flat-band in the dielectric (V_G = V_G0).     

Analysis of anticipated performance of 650-nm GaInP/AlGaInP quantum-well GaAs-based VCSELs at elevated temperatures

The possibility of application of the 650-nm oxide-confined GaInP/AlGaInP quantum-well vertical-cavity surface-emitting diode lasers (VCSELs) at elevated temperatures as sources of the carrier 650-nm wave in the fibre optical communication using POFs has been investigated with the aid of the comprehensive self-consistent model. An increase in the VCSEL threshold current at higher temperatures has been found to be mostly associated with both the carrier leakage from the valley of the Ga_0.43In_0.57P quantum-well material to the X-valley of the (Al_0.67Ga_0.33)_0.52In_0.48P barriers and the band-to-band absorption within the Ga_0.52In_0.48P layer of the band-gap comparable with the energy of emitted radiation. Nevertheless, the AlGaInP VCSELs exhibit encouraging thermal behaviour with the characteristic temperature T₀ equal to as much as 134 K for the active-region temperatures up to 357 K. For the 5-μm devices, the maximal achievable output has been determined to decrease from a quite high value of 1.0 mW for 293 K to 0.6 mW for 320 K and to still high 0.33 mW for 340 K. However, an efficient operation of the above VCSEL at elevated temperatures requires still some structure modifications leading to a reduction of both the above effects, the electron leakage from the valley and the band-to-band absorption within GaInP layers.     

An attempt to design long-wavelength (>2 μm) InP-based GaInNAs diode lasers

In the present paper, anticipated performance characteristics of various InP-based GaInNAs quantum-well (QW) active regions are determined with the aid of our comprehensive computer model for various sets of parameters (temperature, carrier concentration, QW thickness). It is evident from this analysis that the compressively strained InP-based Ga_0.12In_0.88N_0.02As_0.98/Ga_0.275In_0.725As_0.6P_0.4 QW structure may offer expected lasing emission. Its maximal optical gain of over 2150 cm^?1 has been determined at room temperature for the wavelength of about 2815 nm for the QW thickness of 10 nm and the carrier concentration of 5×10¹⁸ cm^?3. Therefore, the above InP-based QW structure may be successfully applied in compact semiconductor laser sources of the desired radiation of wavelengths longer at room temperature than even 2800 nm. Similar GaAs-based devices emit radiation of distinctly shorter wavelengths, whereas GaSb-based ones avail themselves of more expensive substrates as well as exhibit lower thermal conductivities and worse carrier confinements.     

Energy efficiency of near infrared cobalt luminscence in ZnSe:Co determined by a photoacoustic method

The paper presents results of computations of the energy efficiency of the cobalt luminescence in ZnSe:Co determined by the photoacoustic method. The transmission spectra, photoacoustic experimental and theoretical spectra, and the frequency dependence on the photoacoustic amplitude characteristics are presented. From them, the energy efficiency of Co²⁺ the near infrared luminescence (3200 nm) was computed in the frame of new proposed photoacoustic model of computations of the luminescence energy efficiency.     

Cooling,Spectroscopy and Non‐Sticking of trans‐Stilbene and Nile Red

We create and study trans‐Stilbene and Nile Red in a cryogenic (7 K) cell with a low density helium buffer gas. No molecule–helium cluster formation is observed, indicating limited atom–molecule sticking in this system. We place an upper limit of 5 % on the population of clustered He–trans‐Stilbene, consistent with a measured He–molecule collisional residence time of less than 1 μs. With its very low energy torsional modes, trans‐Stilbene is less rigid than any molecule previously buffer‐gas‐cooled into the Kelvin regime. We also report cooling and gas phase visible spectroscopy of Nile Red, a much larger molecule. Our data suggest that buffer gas cooling will be feasible for a variety of small biological molecules.