Thermal and mechanical issues of high-power laser diode degradation

A computational model for the evaluation of the thermomechanical effects that give rise to the catastrophic optical damage of laser diodes has been devised. The model traces the progressive deterioration of the device running in continuous wave conditions. The local heating of the active layer locally leads to the onset of the plastic regime. As a result, dislocations and threads of dislocations grow across the active layers and lead to rapidly growing temperatures in the quantum well. The poor power dissipation under these conditions has been identified as the key factor driving the final degradation of the laserSpanish Government (ENE2014-56069-C4-4-R) and Junta de Castilla y León (VA29U13 and VA081U16). J.L. Pura wishes to acknowledge a grant by the FPU program of the Spanish Government (FPU14/00916

Nanoscale effects on the thermal and mechanical properties of AlGaAs/GaAs quantum well laser diodes: influence on the catastrophic optical damage

Producción CientíficaIn this work we study the catastrophic optical damage (COD) of graded-index separate confinement heterostructure quantum well (QW) laser diodes based on AlGaAs/GaAs. The emphasis is placed on the impact that the nanoscale physical properties have on the operation and degradation of the active layers of these devices. When these laser diodes run in continuous-wave mode with high internal optical power densities, the QW and guide layers can experiment very intense local heating phenomena that lead to device failure. A thermomechanical model has been set up to study the mechanism of degradation. This model has been solved by applying finite element methods. A variety of physical factors related to the materials properties, which play a paramount role in the laser degradation process, have been considered. Among these, the reduced thicknesses of the QW and the guides lead to thermal conductivities smaller than the bulk figures, which are further reduced as extended defects develop in these layers. This results in a progressively deteriorating thermal management in the device. To the best of our knowledge, this model for laser diodes is the first one to have taken into account low scale mechanical effects that result in enhanced strengths in the structural layers. Moreover, the consequences of these conflicting size-dependent properties on the thermo-mechanical behaviour on the route to COD are examined. Subsequently, this approach opens the possibility of taking advantage of these properties in order to design robust diode lasers (or other types of power devices) in a controlled manner.Junta de Castilla y León (programa de apoyo a proyectos de investigación – Ref. Project VA293U13 and VA081U16 (003)Ministerio de Economía, Industria y Competitividad (Proyect ENE2014-56069-C4-4-R

About the physical meaning of the critical temperature for catastrophic optical damage in high power quantum well laser diodes

Producción CientíficaIt is usually assumed that the catastrophic optical damage of high power laser diodes is launched when a critical local temperature (Tc) is reached; temperatures ranging from 120ºC to 200ºC were experimentally reported. However, the physical meaning of Tc in the degradation process is still unclear. In this work we show that, in the presence of a local heat source in the active region, the temperature of the laser structure, calculated using finite element methods, is very inhomogeneously distributed among the different layers forming the device. This is due to the impact that the low dimensionality and the thermal boundary resistances have on the thermal transport across the laser structure. When these key factors are explicitly considered, the quantum well (QW) temperature can be several hundred degrees higher than the temperature of the guides and cladding layers. Due to the size of the experimental probes, the measured critical temperature is a weighted average over the QW, guides and claddings. We show the existence of a great difference between the calculated average temperature, equivalent to the experimentally measured temperature, and the peak temperature localized in the QW. A parallel study on double heterostructure lasers is also included for comparison.Junta de Castilla y León (programa de apoyo a proyectos de investigación – Ref. VA302U13

Thermomechanical issues of high power laser diode catastrophic optical damage

Catastrophic optical damage (COD) of high power laser diodes is a crucial factor limiting ultra high power lasers. The understanding of the COD process is essential to improve the endurance of the high power laser diodes. COD is observed as a process in which the active part of the laser diode is destroyed, forming characteristic defects, the so called dark line defects (DLDs). The DLDs are formed by arrays of dislocations generated during the laser operation. Local heating associated with non-radiative recombination is assumed to be at the origin of the COD process. A summary of the methods used to assess the COD, both in real time and post-mortem is presented. The main approaches developed in recent years to model the heat transport in the laser structures under non homogeneous temperature distribution are overviewed. Special emphasis is paid to the impact of the low dimensionality of QWs in two physical properties playing a major role in the COD process, namely, thermal conductivity and mechanical strength. A discussion about the impact of the nanoscale in both physical properties is presented. Finally, we summarize the main issues of the thermomechanical modelling of COD. Within this model the COD is launched when the local thermal stresses generated around the heat source overcome the yield stress of the active zone of the laser. The thermal runaway is related to the sharp decrease of the thermal conductivity once the onset of plasticity has been reached in the active zone of the laser.Junta de Castilla y León (Projects VA081U16 and VA283P18)Spanish Government (ENE 2014-56069-C4-4-R, ENE 2017-89561-C4-3-R, FPU programme 14/00916)

About the impact of the materials properties in the catastrophic degradation of high power GaAs based laser

Producción CientíficaThe catastrophic degradation of high power lasers depends on both external factors, associated with the technological processes followed to fabricate the laser, and also on intrinsic aspects related to the materials forming the laser structure, more specifically the active zone composed by the QW, guide layers and claddings. The materials properties: optical, thermal and mechanical, play a paramount role in the degradation of the laser. We analyse here how these properties have an impact on the mechanisms responsible for the catastrophic degradation.Junta de Castilla y León (programa de apoyo a proyectos de investigación – Ref. Project VA293U13 and VA081U16 (003))Ministerio de Economía, Industria y Competitividad (Proyect ENE2014-56069-C4-4-R

Thermomechanical degradation of single and multiple quantum well AlGaAs/GaAs laser diodes

The catastrophic degradation of laser diodeswith active zones comprising either single (SQW) or multiple quantum wells (MQW) has been analysed via finite elementmethods. This analysis is based on a physical model that explicitly considers the thermal and mechanical properties of the diode laser structure and the relevant size effects associated with the small thickness of the active layers of the device. The reduced thermal conductivities and the thermal barriers at the interfaces result in a significant local heating process which is accentuated as more quantum wells form the active part of the device. Therefore, in the design of high power devices, the SQW configuration would be more appropriate than the MQW alternative.Junta de Castilla y León (programa de apoyo a proyectos de investigación – Ref VA293U13 and VA081U16 (003)Ministerio de Economía, Industria y Competitividad (ENE2014-56069-C4-4-R

Electromagnetic Field Enhancement on Axially Heterostructured NWs: The Role of the Heterojunctions

Semiconductor nanowires are the building blocks of future nanoelectronic devices. The study of the interaction between nanowires and visible light reveals resonances that promise light absorption/scattering engineering for photonic applications. We carried out experimental measurements through the micro-Raman spectroscopy of different group IV nanowires, both homogeneous Si nanowires and axially heterostructured SiGe/Si nanowires. These experimental measurements show an enhancement of the Raman signal in the vicinity of the heterojunction of SiGe/Si nanowires. The results are analysed in terms of the electromagnetic modelling of the light/nanowire interaction using finite element methods. The presence of axial heterostructures is shown to produce electromagnetic resonances, and the results are understood as a consequence of a finite change in the relative permittivity of the material at the SiGe/Si heterojunction. This effect opens a path to controlling interactions between light and matter at the nanoscale with direct applications in photonic nanodevices.Junta de Castilla y Leo´n (Projects VA293U13, and VA081U16), and Spanish Government (CICYT MAT2010-20441-C02 (01 and 02)). J. L. Pura was granted by the FPU programme (Spanish Government FPU14/00916)

Catastrophic optical damage of high power InGaAs/AlGaAs laser diodes

Producción CientíficaThe defects generated by the catastrophic optical degradation (COD) of high power laser diodes have been examined using cathodoluminescence (CL). Discontinuous dark lines that correspond to different levels of damage have been observed along the ridge. Finite element methods have been applied to solve a physical model for the degradation of the diodes that explicitly considers the thermal and mechanical properties of the laser structure. According to this model, the COD is triggered by a local temperature enhancement that gives rise to thermal stresses leading to the generation of dislocations. Damage is initially localized in the QW, and when it propagates to the waveguide layers the laser ends its life.Junta de Castilla y León (programa de apoyo a proyectos de investigación – Ref. VA302U13

Mechanisms driving the catastrophic optical damage in high power laser diodes

Producción CientíficaThe catastrophic optical damage (COD) of laser diodes consists of the sudden drop off of the optical power. COD is generally associated with a thermal runaway mechanism in which the active zone of the laser is molten in a positive feedback process. The full sequence of the degradation follows different phases: in the first phase, a weak zone of the laser is incubated and the temperature is locally increased there; when a critical temperature is reached the thermal runaway process takes place. Usually, the positive feedback leading to COD is circumscribed to the sequential enhancement of the optical absorption in a process driven by the increase of the temperature. However, the meaning of the critical temperature has not been unambiguously established. Herein, we will discuss about the critical temperature, and the physical mechanisms involved in this process. The influence of the progressive deterioration of the thermal conductivity of the laser structure as a result of the degradation during the laser operation will be addressed.Junta de Castilla y León (programa de apoyo a proyectos de investigación – Ref. VA302U13

Local electric field enhancement at the heterojunction of Si/SiGe axially heterostructured nanowires under laser illumination

Producción CientíficaWe present a phenomenon concerning the electric eld enhancement at the heterojunction region of axially heterostructured Si/SiGe nanowires when the nanowire is illuminated by a focused laser beam. The electric eld is sensed by micro Raman spectroscopy, which permits to reveal the enhancement of the Raman signal arising from the heterojunction region; the Raman signal per unit volume increases at least 10 times with respect to the homogeneous Si, and SiGe nanowire segments. In order to explore the physical meaning of this phenomenon, a 3-dimensional solution of the Maxwell equations of the interaction between the focused laser beam and the nanowire was carried out by nite element methods. A local enhancement of the electric eld at the heterojunction was deduced; however, the magnitude of the electromagnetic eld enhancement only approaches the experimental one when the free carriers are considered, showing enhanced absorption at the carrier depleted heterojunction region. The existence of this e ect promises a way to improve the photon harvesting using axially heterostructured semiconductor NWs.Junta de Castilla y León (programa de apoyo a proyectos de investigación – Ref. VA302U13