Facet Temperature Research Articles

Crystal facet orientation and temperature dependence of charge and spin Hall effects in noncollinear antiferromagnets: A first-principles investigation

Crystal facet orientation and temperature dependence of charge and spin Hall effects in noncollinear antiferromagnets: A first-principles investigation

High-Reliability Uncooled InGaAlAs Lasers with Unpumped Current Blocking Regions Near Cavity Facets

The facet heating of high-power laser diodes significantly influences device reliability. To mitigate facet temperature, current blocking (CB) regions are employed at both cavity facets of the laser. In this paper, we investigate the device characteristics of uncooled InGaAlAs lasers with CB facets. We observe two types of light versus current (LI) curves and propose corresponding physical models. Our findings demonstrate the high reliability performance of uncooled lasers with proper engineering design and CB process.

Catastrophic failure of the back facet in watt-level power long wavelength infrared quantum cascade laser

Quantum cascade lasers (QCLs) suffer from catastrophic failure caused by serious self-heating, thus limiting their output power and working stability. In this study, we observed a distinctive failure morphology on the back facet of watt-level power QCL emitting at λ ∼ 7.7 µm. The failure was caused by a massive localized current and the channel of the massive current can be observed in the cavity. Because the massive current significantly increased temperature nearby, two burned holes were formed around the channel. A 3D thermal model shows that the back facet is a vulnerable location for failure because light absorption by the high-reflectance metal coating increases the facet temperature significantly. However, the starting point of the massive current has a certain distance from the facet which is the hottest location in the cavity. Therefore, we conduct a hypothesis that the cause of the massive current is thermal strain relaxations induced by temperature gradient. We calculated the positions of the relaxation points and one of them correspond with the failure starting point found experimentally. The strain relaxation damaged the active region, thus leading to the formation of the massive current.

Research on transient thermal behavior of semiconductor lasers under pulse current excitation by thermoreflection technique

We measured the response of the facet temperature of the semiconductor laser to a single pulse current with a magnitude of microseconds by using transient thermoreflection method, and then calculated the thermal time constants of the laser. The thermoreflection technique measures the reflectivity of facet which is affected by temperature in real time and thus only measures the thermal parameter. In addition, we select the falling edge of the pulse current where current and light dissipate immediately during the heat decline as the starting point for the study of the thermal response. Therefore, the electro-optical-thermal characteristics of the laser are decoupled in the measurement process. The transient thermal response is formulated as a second-order exponential function of time. The two time constants, i.e τ1 and τ2, represent thermal conductivity performance of laser chip and device package, respectively. We studied the transient thermal behavior of the laser under unsteady state based on the evolution trends of the thermal time constants with amplitude and width of the pulse current. When the pulse is narrow, the heat of the device is concentrated in the active region, even if the current amplitude is relatively large; With the increase of pulse width, the heat generated in the active region diffuses outward to other layers of the chip and heat sink. We also observed the mechanical deformation of the facet in the active region which results from heat accumulation, and the heat generation is related to light intensity.

Research on temperatures located within facet coating layers along z-axis of semiconductor lasers

The critical temperatures of semiconductor laser facet coating layers were studied via measurements from three prongs on the surface (coating exterior boundary), on the facet (coating interior boundary) and along the inner cavity, with temperatures measured via infrared imaging, thermoreflectance and forward-voltage methods, respectively. We subdivided the facet microstructure temperatures and demonstrated the feasibility of facet temperature measurement via thermoreflectance for the reflected light carrying the temperature information mainly comes from the coating interior boundary. Facet reflectivity transients of an AlGaInAs/AlGaAs laser diode are traced using an optical setup featuring fiber transmission and fiber detection. The highest temperature along the z-axis was measured at the facet, with a temperature rise exceeding two times that at the surface, which confirmed the significance of the subdivision, and approaching four times that in the inner cavity. Thermal transient analysis from optical and electrical methods presents similar results; steady temperature measurements as a function of injection current indicate light absorption’s contribution to facet heating.

Sub-field of view surface thermal modeling of Cassini CIRS observations of Rhea during south polar winter

Rhea's exosphere is thought to originate from sources of carbon, water ice and other volatiles that arrived at Rhea by bombardment. Its seasonal variability is directly driven by polar surface temperatures allowing surface adsorption, and the persistence of source volatiles require that seasonal temperatures remain sufficiently cold to retain them. Cassini CIRS detected temperatures in Rhea's winter polar region of 23 K, amongst the coldest measured in the Solar System, but with a relatively large footprint we seek to add value to these observations by modeling sub field-of-view (FOV) temperature distribution and examining how the coldest scenes evolve on a seasonal basis.A simple, rough surface 1-dimensional thermal model is developed using a digital elevation map as input to a thermal model. We compared averaged rough and flat modeled FOVs to CIRS temperatures for a set of case study observations in the south polar region in winter darkness and found they both agree within expected CIRS error in all cases. We develop an asymmetric estimate of CIRS FOV temperature uncertainty, which is particularly important for very cold spectra to accurately represent the sensitivity of the instrument. This approach provides a conservative upper temperature limit to spectral fits whilst highlighting there is often little information to constrain the lower bound of temperature uncertainty in these cases.The distribution of modeled sub-FOV facet temperatures is explored for the full range of azimuth angles and slope gradients. More extensive and cooler temperatures were found in the rough model, complemented by fewer but warmer areas than the flat model. We find temperature contrasts of individual model facets of up to 15 K warmer and 11 K colder within some CIRS FOV when the rough model was compared flat, in a case study of scenes in winter darkness. This is due to the persistence of the seasonal thermal wave beneath the surface. We tested model sensitivity to thermal input parameters (thermal inertia and bolometric Bond albedo). These values are challenging to constrain due to limited observations and measurement noise and are expected to vary with subtle changes in surface characteristics. We found that within 20° of the pole, temperatures do not rise significantly above 80 K all year, implying that a variety of simple organic molecules, linear amides and other carbon-containing compounds would remain stable on a quasi-permanent basis, potentially until photolytic or radiolytic mechanisms liberate by-products which provide a source for exospheric constituents. The simple rough model indicates that some facets experience very different summer/winter season lengths than a flat model can represent, which is important in terms of the exospheric sequestration process.We use threshold temperatures of 55 K for CO2 and 30 K for O2 ice as proxies for their relative stability at the surface. The surface coverage of these temperatures with time was compared to modeled exospheric abundance by Teolis and Waite (2016). They are anti-correlated which is expected, but with slight differences in timings of minima and the equinoctial maxima. We find that the rough model suggests larger areas able to adsorb these species than the flat model, and that the cumulative influence of topography would have a direct relationship on the timing and abundance of the exosphere.We place Rhea's polar environment in context with other icy moons in the Saturnian system using the flat model. The satellites Tethys, Dione and Enceladus experience similar polar thermal regimes to Rhea (with exceptionally cold winters) and would potentially benefit from the consideration of topography in relation to exospheric modeling.

Nondestructive Measurement for Front Facet Temperature of Semiconductor Lasers

A convenient, simple method is proposed to measure the front facet temperature, which is the highest temperature in the semiconductor laser diode (LD), of a GaAs-based laser by employing thermoreflectance technique. Using an optical system featured in fiber connection, we measured the facet reflectivity of the 808-nm AlGaInAs/AlGaAs LD, which gives information about the temperature of the output facet. The fiber system operates at the wavelength of 1550 nm which avoids the absorption of the probe beam by the tested LD and consists of a fiber-coupled 1550 nm laser illuminant and photodiode. All optical elements in the system are connected by the fibers. The current signal collected from the photodiode is related to the facet reflectivity and represents facet temperature. We compared the facet temperatures determined by thermoreflectance technique with the cavity temperature obtained by forward-voltage method and found that the former is as much as three times as the latter.

Modeling and analysis of infrared radiation dynamic characteristics for space micromotion target recognition

The infrared radiation characteristics under micromotion provide an important parameter for space target recognition. To solve the problem of space infrared target recognition over long distances, an infrared radiation model of space target is established by comprehensively considering the target flight scenario, temperature change, target shape and size, and micromotion factor. First, the micromotion model is established, and the multi-target surface is meshed by uniformly sampling the three-dimensional space. Next, the temperature field model of the target surface is established by considering the rapid change in the radiation angle coefficient of the target facets. Then, according to the dynamic angle relationship among the target facet position, radiation vector, and line of sight, the infrared radiation model of the target is established. The effects of micromotion on the surface facet temperature, projected area, and infrared radiation of the target are simulated and analyzed. The experimental results indicate that the temperature range of all targets is 260–323 K under the set parameters and decide the trend of radiation intensity, and the micromotion causes periodic fluctuation of the facets temperature, projected area, and radiation intensity. The shape of the micromotion target and micromotion angle jointly affect the infrared radiation amplitude in the same line of sight observation. The infrared radiation data of multiple space targets in any flight scene can be obtained using the model, which provides support for target detection, tracking, recognition and the design of parameters of infrared detectors.

Large Facet-Specific Built-in Potential Differences Affecting Trap State Densities and Carrier Lifetimes of GaAs Wafers

Previously, intrinsic GaAs {111} wafers showed appreciably better electrical conductivity than the {100} wafer and the least conductive {110} wafer. Such facet dependence can be understood from subtle variation in the bond length, bond geometry, and frontier orbital electron energy distribution for different surface planes. Attempting to gain more insights about the emergence of electrical facet effects, temperature- and voltage-varying impedance measurements were performed on these GaAs wafers using silver as the contacting electrode. Parameters from the impedance and Hall effect measurements allow the presentation of the facet-specific density and energy distribution of trap states located within the GaAs band gap. Charge carrier lifetime information can also be extracted from the impedance data. Unexpectedly, the {100} surface has the largest trap state density, while the {110} surface shows the smallest density of trap states. The {111} surface also gives the longest carrier lifetime. By contrast, the most electrically conductive {111} surface in the undoped Si and Ge wafers exhibits the lowest density of trap states and shortest carrier lifetime, which are considered as characteristics of a surface with a high-conductivity property. Facet-specific GaAs–Ag band diagrams reveal both upward and downward interfacial conduction band bending cases with positive and negative built-in potentials, which resemble n- and p-type semiconductor–metal junction situations. These irregular band diagrams may partly explain the unexpected state trap density and carrier lifetime behaviors seen in the GaAs wafers, making these parameters unsuitable to relate to their electrical conductivity properties.

Output facet heating mechanism for uncoated high power long wave infrared quantum cascade lasers

Output facet temperatures of an uncoated high power continuous-wave quantum cascade laser (QCL) emitting at 8.5 μm were measured by using micro-Raman thermometry. The rate of the measured temperature changes with the injected electrical power increased from 6.5 K/W below the laser threshold to 12.3 K/W above the threshold. In addition, the measured temperature rise exceeded 220 K at an optical power of 0.9 W, well above the model projections based only on Joule heating. Facet oxidation was characterized via x-ray photoelectron spectroscopy measurements at incremental etch depths. While the oxidation reactions of InP and Ga were observed only at the surface level, the measured penetration of native Al2O3 was ∼24 nm. COMSOL thermal modeling demonstrated that light reabsorption by the native Al2O3 layer could well explain the additional temperature rise above the threshold. These results suggest that facet oxidation must be addressed to ensure the reliability of high-power long wave infrared QCLs.

Thermal imaging of buried heterostructure quantum cascade lasers (QCLs) and QCL arrays using CCD-based thermoreflectance microscopy

Thermal profiles of a single-element quantum cascade laser (QCL) and a five-element QCL array at different bias currents, under quasi-continuous-wave (QCW) conditions, are obtained using the charge-coupled device (CCD)-based thermoreflectance imaging technique. Peak temperature changes of 55 K and 105 K are measured on the single-element QCL (operating at 1.2 A) and the central element of the QCL array (operating at 4.2 A), respectively. The average facet temperature of the single QCL device shows a linear relationship with the dissipated power, indicating an effective thermal resistance of Rth = 3.0 ± 0.2 K/W (7%) for the device. The thermal transient behavior of the single QCL device, in response to a 35 μs-wide heating pulse, is also measured. From the transient curve, an effective thermal time constant of τth = 9.5 ± 0.4 μs (4%) is obtained. Experimental results are compared to the results obtained from heat-transfer models for both the single-element and array devices. Thermal profiles show a thermal lensing effect at the facet of the single-element QCL. In the array device, a more pronounced heating is observed at the center of the device while the temperature gradually decreases away from the central element.

Facet Cooling in High-Power InGaAs/AlGaAs Lasers

Several factors limit the reliable output power of a semiconductor laser under CW operation, such as carrier leakage, thermal effects, and catastrophic optical mirror damage (COMD). Ever higher operating powers may be possible if the COMD can be avoided. Despite exotic facet engineering and progress in non-absorbing mirrors, the temperature rise at the facets puts a strain on the long-term reliability of these diodes. Although thermoelectrically isolating the heat source away from the facets with non-injected windows helps lower the facet temperature, data suggests the farther the heat source is from the facets, the lower the temperature. In this letter, we show that longer non-injected sections lead to cooler windows and biasing this section to transparency eliminates the optical loss. We report on the facet temperature reduction that reaches below the bulk temperature in high power InGaAs/AlGaAs lasers under QCW operation with electrically isolated and biased windows. Acting as transparent optical interconnects, biased sections connect the active cavity to the facets. This approach can be applied to a wide range of semiconductor lasers to improve device reliability as well as enabling the monolithic integration of lasers in photonic integrated circuits.

Why is Ice Slippery? Simulations of Shear Viscosity of the Quasi-Liquid Layer on Ice.

The temperature and depth dependence of the shear viscosity (η) of the quasi-liquid layer (QLL) of water on ice-Ih crystals was determined using simulations of the TIP4P/Ice model. The crystals display either the basal {0001} or prismatic {101̅0} facets, and we find that the QLL viscosity depends on the presented facet, the distance from the solid/liquid interface, and the undercooling temperature. Structural order parameters provide two distinct estimates of the QLL widths, which are found to range from 6.0 to 7.8 Å, and depend on the facet and undercooling temperature. Above 260 K, the viscosity of the vapor-adjacent water layer is significantly less viscous than the solid-adjacent layer and is also lower than the viscosity of liquid water.

Remote estimation of complete urban surface temperature using only directional radiometric temperatures

Complete urban surface temperature (Tc) is a key variable for quantifying the urban surface - atmosphere interactions. The estimation of Tc usually requires detailed knowledge of surface geometry and facet temperatures, which are difficult to obtain by remote sensing. Therefore, simple but efficient strategies for the remote estimation of Tc are currently lacking. Facing this challenge, we proposed a comprehensive solution to estimate Tc remotely using only the directional radiometric temperatures (DRTs) without the assistance of ground-based information. By comparing Tc with the nadir temperature (T0), DRTs within the entire hemisphere, and solid-angle integral temperature (SIT), we obtain the following results: (1) The optimal viewing azimuth and zenith angles at which the DRTs are the closest to Tc are φs±90° (φs is the solar azimuth angle) and 45–60°, respectively. (2) The estimation of Tc could be further improved once the DRTs from all-direction are available, and the MAE for the SIT that most closely approximated Tc is only 0.87 K. (3) The MAE of the SIT that best approximates Tc by combining only four DRTs and a kernel-driven model is only slightly higher than that of the specific SIT based on the original DRTs, indicating that the remote estimation of Tc using limited DRTs is plausible. These proposed strategies are generally simple and effective, and can help facilitate the estimation of Tc and therefore help advance the thermal remote sensing of urban climate and environment.

Thermal Behavior of High Power GaAs-Based Laser Diodes in Vacuum Environment

Since 980-nm GaAs-based laser diodes are now implemented in space missions, there is a huge need to study their thermal behavior under vacuum in order to evaluate their reliability. Then spatially resolved temperature variations have been evaluated by thermoreflectance on the front facet under atmospheric and vacuum conditions, and for two different unpumped windows lengths. We show that, in all cases, vacuum induces a front facet temperature variation increase of the laser diodes, making them more sensitive to a catastrophic optical damage.

Parametric study of 1310 nm ridge waveguide AlGaInAs‐InP semi‐conductor laser dynamics

Dynamic performance of directly modulated Fabry-Perot lasers is presented in this work subject to variation of parameters such as cavity length and facet reflectivity. This study is focused on the effect that physical parameters have on the slope of the laser resonance frequency against the square root of the injection current. Temperature variation is also studied to determine modulation bandwidth available for high speed device operation at 10 Gbps. The effect of facet reflectivity and temperature over the laser spectrum and wavelength is also discussed.

Precise Facet Temperature Distribution of High- Power Laser Diodes: Unpumped Window Effect

A thermoreflectance technique is used to evaluate the temperature variations at the output facet of high-power GaAs-based laser diodes emitting at 980 nm. Two kinds of diodes with different unpumped windows (UPWs) are studied to determine the influence of UPW length on the temperature variation. We show that in the vicinity of the active region, where a catastrophic optical damage is most susceptible to occur, the short UPW diode heats much more (up to 40%) than the long UPW one.

High optical strength GaAs-based laser structures

The optical-strength properties of 3S PHOTONICS GaAs-based semiconductor laser structures are presented. After some general considerations about this robustness issue, typical of high-power diode lasers, the performances in both CW and pulsed current injection of our best vertical structures are presented and it is shown that our current generation of 980nm laser diodes can withstand very high facet temperatures (180–200°C) in stable CW operation. Thus, we demonstrate a large margin between the facet temperature in operation conditions and the observed maximum critical value. Maximum CW saturation powers exceeding 3W from a single-lateral mode device have been obtained with the longest laser cavities (7.5mm) at around 5A. Also, very high peak powers are demonstrated in pulsed current experiments opening the way to new specific applications, using available production devices. Specific degradation patterns after high peak current single-pulse induced damage have been observed by cathodo-luminescence imaging and are also discussed. Finally, comparisons between devices emitting at 980nm and 1060nm are considered.

830-nm AlGaAs-InGaAs Graded Index Double Barrier Separate Confinement Heterostructures Laser Diodes With Improved Temperature and Divergence Characteristics

The 830-nm AlGaAs/InGaAs laser diodes (LDs) adopting multistep-graded index double-barrier separate confinement heterostructures (GRIN-DBSCHs) with small divergence beams and improved temperature characteristics under a high-output-power operation are reported. The double-barrier separate confinement heterostructure (DBSCH) design provides good carrier confinement and prevents current leakage by adding a multistep grading layer between cladding and waveguide layers. Simultaneously, the DBSCH design can facilitate reducing the divergence angle at high-power operation and widening the transverse mode distribution to decrease the power density around emission facets. In addition, the p-side doping depth is optimized to effectively raise the barrier height for reducing the electron overflow. Gaussian-like narrow far-field patterns are measured with the full-width at half-maximum vertical divergence angle to be between 11° and 13°. A threshold current of 16.5 mA and a slop efficiency of 0.98 W/A are obtained in the continuous-wave operation condition at room temperature. The maxima optical power densities of 21.5 MW/cm2 per laser facet and good characteristic temperature values of threshold current (T0) and slope efficiency (T1) are achieved.

Stable high optical power in quantum well lasers with profiled reflection and tapered structures

A comprehensive model is presented to study quantum well tapered lasers and quantum well stripe lasers with profiled reflectivity output facets and to obtain lateral stability in high power semiconductor laser. Simulation of semiconductor lasers is performed by numerically solving space-dependent coupled partial differential equations for the complex optical forward and backward waves, carrier density distribution and temperature distribution. The coupled equations are solved by finite difference beam propagation method. The effect of nonlinear parameters like Kerr and linewidth enhancement factors, and precise dependence of linewidth enhancement factor and gain factor on the carrier density and temperature are considered in this paper. We use modal reflector in stripe lasers to confine the lateral mode to the stripe centre and provide the stable operation. We also use unpumped window to reduce the facet temperature and improve the catastrophic optical mirror damage level of tapered lasers.