Characteristic Temperature T0 Research Articles

Low-threshold 2 µm InAs/InP quantum dash lasers enabled by punctuated growth.

2 µm photonics and optoelectronics is promising for potential applications such as optical communications, LiDAR, and chemical sensing. While the research on 2 µm detectors is on the rise, the development of InP-based 2 µm gain materials with 0D nanostructures is rather stalled. Here, we demonstrate low-threshold, continuous wave lasing at 2 µm wavelength from InAs quantum dash/InP lasers enabled by punctuated growth of the quantum structure. We demonstrate low threshold current densities from the 7.1 µm width ridge-waveguide lasers, with values of 657, 1183, and 1944 A/cm2 under short pulse wave (SPW), quasi-continuous wave (QCW), and continuous wave operation. The lasers also exhibited good thermal stability, with a characteristic temperature T0 of 43 K under SPW mode. The lasing spectra is centered at 1.97 µm, coinciding with the ground-state emission observed from photoluminescence studies. We believe that the InAs quantum dash/InP lasers emitting near 2 µm will be a key enabling technology for 2 µm communication and sensing.

Horizontal flux growth as an efficient preparation method of CeRh2As2 single crystals.

We report an efficient method to obtain CeRh2As2 single crystals with the use of a bismuth flux growth method in a horizontal configuration. Based on our numerous attempts, we found this technique to be scalable and repeatable. The crystals thus obtained are characterized by much sharper phase transitions and distinctly higher characteristic temperatures Tc and T0, compared to previous reports. Moreover, based on our specific heat studies of the obtained single crystals, we also indicate a clear connection between both transition temperatures.

Optimized performance of 905 nm semiconductor lasers by using the high strain quantum well.

We propose and experimentally demonstrate that the lasing power and characteristic temperature (T0) of 905 nm semiconductor lasers can be optimized by use of the high strain quantum well (HSQW). To fix the lasing wavelength around 905 nm, HSQW with a higher ndium (In) content of the InGaAs gain material than that of the commonly used low strain quantum well (LSQW) requires a thickness-reduced quantum well. Thus, the HSQW has the following two advantages: stronger quantum size effects caused by the deep and thin quantum well, and higher compressive strain caused by a high In content of the InGaAs gain material. With the similar epitaxial structure, laser diodes with HSQW have a characteristic temperature T0 of 207 K and can deliver a higher lasing power with less power saturations. The high strain quantum well optimization method can be extended to other laser diodes with a wavelength near 900 nm with low In content InGaAs quantum wells and other similar low-strain gain material systems.

Ultra-high thermal stability InAs/GaAs quantum dot lasers grown on on-axis Si (001) with a record-high continuous-wave operating temperature of 150 °C.

Direct epitaxial growth of group III-V light sources with excellently thermal performance on silicon photonics chips promises low-cost, low-power-consumption, high-performance photonic integrated circuits. Here, we report on the achievement of ultra-high thermal stability 1.3 µm InAs/GaAs quantum dot (QD) lasers directly grown on an on-axis Si (001) with a record-high continuous-wave (CW) operating temperature of 150 °C. A GaAs buffer layer with a low threading dislocation density (TDD) of 4.3 × 106 cm-2 was first deposited using an optimized three-step growth method by molecular beam epitaxy. Then, an eight-layer QD laser structure with p-type modulation doping to enhance the temperature stability of the device was subsequently grown on the low TDD Si-based GaAs buffer layer. It is shown that the QD laser exhibits the ultra-high temperature stability with a characteristic temperature T0=∞ and T1=∞ in the wide temperature range of 10-75 °C and 10-140 °C, respectively. Moreover, a maximum CW operating temperature of up to 150 °C and a pulsed operating temperature of up to 160 °C are achieved for the QD laser. In addition, the QD laser shows a high CW saturation power of 50 mW at 85 °C and 19 mW at 125 °C, respectively.

Ultra-low threshold continuous-wave quantum dot mini-BIC lasers

Highly compact lasers with ultra-low threshold and single-mode continuous wave (CW) operation have been a long sought-after component for photonic integrated circuits (PICs). Photonic bound states in the continuum (BICs), due to their excellent ability of trapping light and enhancing light-matter interaction, have been investigated in lasing configurations combining various BIC cavities and optical gain materials. However, the realization of BIC laser with a highly compact size and an ultra-low CW threshold has remained elusive. We demonstrate room temperature CW BIC lasers in the 1310 nm O-band wavelength range, by fabricating a miniaturized BIC cavity in an InAs/GaAs epitaxial quantum dot (QD) gain membrane. By enabling effective trapping of both light and carriers in all three dimensions, ultra-low threshold of 12 μW (0.052 kW cm−2) is achieved at room temperature. Single-mode lasing is also realized in cavities as small as only 5 × 5 unit cells (~2.5 × 2.5 μm2 cavity size) with a mode volume of 1.16(λ/n)3. The maximum operation temperature reaches 70 °C with a characteristic temperature of T0 ~93.9 K. With its advantages in terms of a small footprint, ultra-low power consumption, and adaptability for integration, the mini-BIC lasers offer a perspective light source for future PICs aimed at high-capacity optical communications, sensing and quantum information.

Enhancement of electrical conductivity of hydrazine-reduced graphene oxide under thermal annealing in hydrogen atmosphere

The effect of further thermal annealing in a hydrogen atmosphere on the electrical properties of hydrazine-reduced graphene oxide (RGO) was investigated. It was found that heat treatment significantly improves the conductivity of RGO and reduces the room temperature resistivity by ten times. The changes become even more significant at the temperature decrease to 4.2 K and can exceed five orders. The variable range hopping conduction mechanism is the major charge transfer in the RGO at low temperatures for all annealing temperatures in the hydrogen of 100–800 °C. It is shown that the characteristic temperature T0 of the hopping transport exhibits an exponential decrease with an increase in the annealing temperature. Low-temperature studies of electric charge transfer reveal that the unified activation energy accompanying the improvement in conductivity is 0.3 eV.

Broad tuning range, high power quantum cascade laser at λ ∼ 7.4 µm.

In this article, we report a high power quantum cascade laser (QCL) at λ∼7.4 µm with a broad tuning range. By carefully designing and optimizing the active region and waveguide structure, a continuous-wave (CW) output power up to 1.36 W and 0.5 W is achieved at 293 K and 373 K which shows the excellent temperature stability. A high wall-plug efficiency (WPE) of 8% and 13.6% in CW and pulsed mode at 293 K are demonstrated. The laser shows a characteristic temperature T0 of 224 K and T1 of 381 K over a temperature range from 283 K to 373 K. In addition, a far field of pure zero order transverse mode and a fairly wide external cavity (EC) tuning range (280 cm-1) from 6.54 µm to 8 µm are achieved in pulsed operation. In addition, an EC single mode output power of 226 mW is obtained under CW operation at 293K.

Anisotropy of the T vs. H phase diagram and the HO/LMAFM phase boundary in URu2−xFexSi2

The correlated f-electron compound URu2Si2 exhibits superconductivity (SC) with a critical temperature Tc = 1.5 K that coexists with the “hidden order” (HO) phase that forms below a characteristic temperature T0 = 17.5 K. The SC of URu2Si2 appears to be spin singlet chiral SC with d-wave order parameter symmetry, and the pairing of SCing electrons may involve spin excitations of the HO phase. Electrical resistance R measurements were performed on single crystal specimens of URu2−xFexSi2 with increasing x throughout the transition from the HO phase to the large moment antiferromagnetic LMAFM phase in high magnetic fields H oriented at various angles θ with respect to the tetragonal c-axis. Measurements of R(θ) at H = 20, 33, 40, and 45 T were conducted in the temperature range 0.33 ≤ T ≤ 20 K and showed θ-dependent behavior in the various phase transitions of URu2−xFexSi2 (HO, LMAFM, spin density wave, Fermi surface reconstruction, etc.). These phase transitions, as functions of T, H, and θ are plotted in a phase diagram of T vs. H//c = Hcosθ for multiple values of x and show that H//c, not θ is a tuning parameter of the URu2−xFexSi2 system throughout all Fe concentrations, as previously found by Scheereret al. for the URu2Si2 parent compound.

Interaction of Heparin with Proteins: Hydration Effects.

We present a thermodynamic investigation of the interaction of heparin with lysozyme in the presence of potassium glutamate (KGlu). The binding constant Kb is measured by isothermal titration calorimetry (ITC) in a temperature range from 288 to 310 K for concentrations of KGlu between 25 and 175 mM. The free energy of binding ΔGb derived from Kb is strongly decreasing with increasing concentration of KGlu, whereas the dependence of ΔGb on temperature T is found to be small. The decrease of ΔGb can be explained in terms of counterion release: Binding of lysozyme to the strong polyelectrolyte heparin liberates approximately three of the condensed counterions of heparin, thus increasing the entropy of the system. The dependence of ΔGb on T, on the other hand, is traced back to a change of hydration of the protein and the polyelectrolyte upon complex formation. This dependence is quantitatively described by the parameter Δw that depends on T and vanishes at a characteristic temperature T0. A comparison of the complex formation in the presence of KGlu with the one in the presence of NaCl demonstrates that the parameters related to hydration are changed considerably. The characteristic temperature T0 in the presence of KGlu solutions is considerably smaller than that in the presence of NaCl solutions. The change of specific heat Δcp is found to become more negative with increasing salt concentration: This finding agrees with the model-free analysis by the generalized van't Hoff equation. The entire analysis reveals a small but important change of the free energy of binding by hydration. It shows that these ion-specific Hofmeister effects can be modeled quantitatively in terms of a characteristic temperature T0 and a parameter describing the dependence of Δcp on salt concentration.

Significantly improved performances of 1.3 μm InAs/GaAs QD laser by spatially separated dual-doping

We report on significantly enhanced performances of 1.3 μm InAs/GaAs quantum dot (QD) lasers by spatially separated dual-doping, including p-type modulation doping in barrier layers and n-type direct doping in QDs simultaneously. The QD lasers are a ridge waveguide of 6 × 1000 μm2 with uncoated facets, whose active region consists of eight stacked InAs QD layers. Compared with the conventional single p-type modulation doped (p-doped) QD laser, the dually doped QD laser achieves a reduced threshold current from 51.07 to 43 mA, an increased single-sided slope efficiency from 0.18 to 0.25 W/A at 25 °C, and an increased characteristic temperature T0 from 654 to 7917 K between 15 and 85 °C. Furthermore, the continuous wave output power of the dually doped QD laser exceeds 20 mW without any attenuation at 85 °C, whereas that of the p-doped one appears to be saturated at 14.08 mW. The results presented here have important implications for realizing high-performance QD lasers emitting at 1.3 μm to various applications.

Mitigating Valence Intersubband Absorption in Interband Cascade Lasers

AbstractInterband cascade lasers (ICLs), especially valued for their low power consumption, are particularly appealing for portable, compact, and battery‐driven trace gas sensors. However, their performance notably degrades outside the 3–4 µm region. Here, a solution to overcome current performance limitations is presented. Simulation results based on the eight‐band k· p method employing a generalized momentum matrix element model identify resonant intersubband absorption in the valence band as the causative underlying mechanism. Experimentally, a direct dependence of this resonant absorption on the thickness of the GaInxSb hole‐quantum well (h‐QW) is confirmed. This is reflected in the improvement of the laser's characteristic temperature T0, threshold current density Jth, slope efficiency η, and output power. Extracted waveguide losses from length‐dependent measurements substantiate the key role of the valence intersubband absorption. While the performance improvement is experimentally verified at 4.35 µm, the simulation results additionally show how to mitigate undesired absorption at longer wavelengths, paving the way towards high‐performance continuous‐wave (cw) operation above 6 µm.

Study of fluctuation conductivity in YBa2Cu3O7−δ films in strong magnetic fields

We report the effect of the ab-plane magnetic field B up to 8 T on the resistivity ρ(T) and fluctuation conductivity σ′'(T) in YBa2Cu3O7-δ thin films. As expected, up to ∼ 2.5 T the magnetic field monotonously increases ρ, the width of the resistive transition, ΔTc, and coherence length along the c axis, ξC(0), but decreases both Tc and the range of superconducting (SC) fluctuations ΔTfl. The fluctuation conductivity exhibits a crossover at characteristic temperature T0 from the 3D Aslamasov-Larkin (AL) theory near Tc to the 2D fluctuation theory of Maki-Thompson (MT). However, at B = 3 T, the MT term is completely suppressed, and above T0 σ′'(T) is unexpectedly described by the fluctuation contribution of 2D AL, suggesting the formation of a 2D vortex lattice in the film under the action of a magnetic field. At the same time, ΔTf sharply increases by a factor of about 7, and ξC(0) demonstrates a very unusual dependence on Tc when B increases above 3 T. Our results demonstrate the possibility of the formation of a vortex state in YBCO and its evolution with increasing B.

Effect of as flux rate during growth interruption on the performances of InAs/InGaAsP/InP quantum dots and their lasers grown by metal-organic chemical vapor deposition

We report the effect of As flux rate (FR) during growth interruption (GI) after quantum dot (QD) deposition on the performances of InAs/InGaAsP/InP QDs and their lasers, grown by metal–organic chemical vapor deposition. The active region consists of seven layers of self-assembled InAs QDs separated by InGaAsP lattice-matched to InP. Room-temperature photoluminescence (PL) measurements show that both the PL peak intensity and emission wavelength of the active region increase with As FR. Temperature-dependent PLs reveal that the activation energies of the QDs from the ground state to higher energy level or barrier become greater with the increase of As FR, indicating less carrier thermal escape from the QDs to their barriers. These above observations can be attributed to the promoted migration and suppressed desorption of surface In atoms, leading to enlarged QDs with deepened energy levels and thus enhanced carrier quantum confinement effect. Moreover, a 6-μm-wide and 3-mm-long ridge-waveguide QD laser with the greatest As FR shows a lowest threshold current of 477 mA under pulse mode at room temperature, corresponding to a threshold current density of 379 A/cm2 per QD layer. Meanwhile, the laser exhibits the highest characteristic temperatures T0 and T1 of 331 K and 775 K, respectively, in the temperature range between 20 °C and 60 °C.

Thermocapillary effect on the dynamics of falling self-rewetting fluid films down a heated vertical cylinder

Nonlinear dynamic analysis of a self-rewetting fluid (SRWF) film flowing down the surface of a vertical cylinder is performed in this paper. The effect of the Biot number, Marangoni number and the substrate curvature on the interfacial evolution patterns and flow stabilities are discussed by linear stability analysis and numerical simulations of the evolution equation of the film thickness. Starting from the characteristic temperature T0 relating to the minimum surface tension and the interfacial temperature Ti, the nonlinear dynamics of the liquid film is investigated numerically in the cases of either Ti>T0 or Ti<T0. Good agreement of linear stability analysis with numerical simulations proves that the LSA could predict the development of thin liquid film flows in the early-time evolution. Through the analysis, we demonstrate that the Marangoni number Ma and the Biot number Bi play contrary roles for the two cases. For Ti<T0, the fingering instability is enhanced by the Marangoni effect while the inverse Marangoni effect stabilizes the interface for Ti>T0. The growth rate changes linearly with the increase of Marangoni number while it changes in forms of arched shapes versus lg(Bi). The growth rate reaches a maximum/minimum value at Bi=1, corresponding to the most/least unstable state. The radius of the cylinder R plays a significant role in the long wavelength modes, showing a stabilizing effect. For perturbations of short waves, increment of R expands the instability region. Nonlinear oscillatory waves (Rayleigh–Plateau instability) appears when the radius is smaller than 1 and fingering pattern tends to occur if the cylinder has a large radius (R≥1).

Long wavelength (λ &amp;gt; 13 μm) quantum cascade laser based on diagonal transition and three-phonon-resonance design

An InP-based quantum cascade laser structure emitting at a wavelength of 13.6 μm is proposed and demonstrated. The active region is based on a diagonal transition and three-phonon-resonance design. A 5 mm long, 30 μm wide high-reflection coated device with a double channel ridge waveguide structure has shown a threshold current density of 3.0 kA/cm2, a dynamic range of 4.4 kA/cm2, a peak output power close to 1 W, and an average optical power up to 11.7 mW at 3% duty cycle, at 293 K. The laser shows a characteristic temperature T0 of 314 K and T1 of 336 K over a temperature range from 283 to 313 K.

Quantum cascade laser with bound-to-quasi-continuum optical transitions at a temperature of up to 371 K

Based on a matched Ga0.47In0.53As/Al0.48In0.52As heteropair, we have developed a quantum cascade laser emitting at a wavelength of 7.4 μm. The chosen heterostructure with a relatively large number of quantum wells and barriers represents two mini-bands separated by a mini-gap with a localised doublet level near the upper mini-band, which provides a wide emission band (∼100 cm−1). In a pulse regime, the maximal laser operation temperature is 371 K. Such a high temperature is explained by two factors: a large energy of the transfer from the doublet to the upper mini-band and a large volt defect. The characteristic temperatures T0 are found, which are equal to 170 K for low (less than 300 K) temperatures and 270 K for the range of 300 − 370 K. In addition, optical cavity losses are determined to be 2.5 and 7.7 cm−1 at temperatures of 80 and 254 K, respectively. The pulse power is 0.3 W at 80 K and 0.05 W at 293 K.

Temperature dependence of the interphase interaction energy of core/shell nanoparticles

Within the framework of the Ising model, in the direct exchange interaction approximation by the “average spin” method, the constant of the interphase exchange interaction A(T), which determines the exchange bias field, is estimated. It was shown that A(T) ~ e(−T/T0), where the characteristic temperature T0 is determined by the constant of the exchange interaction between the atoms of the core and the shell.

14 μm quantum cascade lasers based on diagonal transition and nonresonant extraction

We report InP-based room-temperature high-average-power quantum cascade lasers emitting at 14 μm. Using a novel active region design, a diagonal bound-to-bound lasing transition is guaranteed by efficient electron injection into the upper laser level and fast nonresonant electron extraction through a miniband from the lower laser level. For a 4 mm long and 40 μm wide double channel ridge waveguide laser with 55 stages of the active region, the threshold current density is only 3.13 kA/cm2 at room temperature. At 293 K, the maximum single-facet peak power and average power are up to 830 mW and 75 mW, respectively. The laser exhibits a characteristic temperature T0 of 395 K over a temperature range from 293 to 353 K.

Lasing of a Quantum-Cascade Laser with a Thin Upper Cladding

We present the results of experiments on the fabrication and study of the properties of quantum-cascade lasers of the spectral range of 7–8 μm in the geometry of a waveguide with a thin top cladding based on indium phosphide. The heterostructure is synthesized by molecular beam epitaxy on an InP substrate with an active region based on a heteropair of In0.53Ga0.47As/Al0.48In0.52As solid alloys. Laser emission at a wavelength of 7.8 μm at a temperature of 300 K with a threshold current density of ~6 kA/cm2 was achieved. The values of characteristic temperatures T0 and T1 for the studied quantum-cascade lasers are of the order of 150 and 450 K, respectively. The results obtained confirm that the design of the waveguide with a thin top cladding for devices for detecting liquids, the fabrication of microfluidic devices, and photonic circuits on silicon holds promise.

High temperature characteristics of a 2 μm InGaSb/AlGaAsSb passively mode-locked quantum well laser

A monolithic two-section InGaSb/AlGaAsSb single quantum well mode-locked laser (MLL) emitting at 2 μm is demonstrated. The laser is able to lase in the continuous wave mode up to 80 °C, and passive mode locking operation with a fundamental repetition frequency of ∼18.4 GHz is observed up to 60 °C. The laser has a characteristic temperature T0 of ∼88 K near room temperature, which is only slightly affected by the absorber bias voltage (Va). One consequence of this finding is verified by the temperature-independent power ratios before lasing. The variations of the repetition frequency with gain current (Ig) and temperature (T) have also been systematically investigated. In the bias range in this work, the repetition frequency increases as a whole by more than 30 MHz when the temperature is raised from 20 to 40 °C. Frequency tuning of ∼130 and ∼60 MHz was observed at 20 and 40 °C, respectively. The results and their mechanism analysis provide guidelines for GaSb-based MLLs to better meet the application-required repetition frequencies even with the presence of an unwanted increase in temperature.