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This paper investigates the impact of dislocation density and active layer structure on the degradation mechanisms of 1.3 $\mu \text{m}$ InAs Quantum Dot (QD) lasers for silicon photonics. We analyzed the optical behavior of two sets of samples, having different dislocation densities and different number of quantum dot layers in the active region. The samples were subjected to a short-term step-stress experiment and to long-term constant current operation in order to investigate the dominant degradation processes. The results indicate that: (i) the temperature stability is much higher in the devices grown on native substrate, thanks to the lower defect density; (ii) the roll-off current is considerably higher for the devices with higher number of layers, due to the lower density of carriers in the QDs; (iii) in nominal ground-state operating regime, the degradation rate is limited by the density of dislocations, that may serve as preferential paths for the diffusion of non-radiative recombination centers; (iv) at extreme injection levels and operating temperatures, the devices exhibit a blue shift of the spectral emission; possible explanations for this process are discussed in the paper.

This paper reports on the impact of the quality of the epitaxial structure of InAs Quantum Dot (QD) lasers grown on silicon substrates on lifetime. To this aim, a series of current step-stress and constant-current aging experiments were carried out on two sets of Fabry-Perot QD lasers, emitting at 1.3 μm and featuring two different Threading Dislocations Densities (TDDs) within the epilayers, nominally 6E7 cm-2 (high TDD) grown on Si substrates and 500 cm-2 (low TDD) grown on lattice matched GaAs substrates. The results of the step-stress procedure indicate that i) the high-current optical performance of the lasers is limited by TDD, which reduces the bias range for useful Ground State (GS)-only operation and lowers the roll-off point of the optical output characteristic; ii) TDD contributes to the acceleration of the dominant optical degradation mechanism at high stress current levels, represented for this family of devices by the recombinationenhanced generation of defects in device regions close to the active layers; iii) dislocation density also accelerates optical degradation in GS regime. This process is primarily driven by the diffusion of Non-Radiative Recombination Centers (NRRCs) toward the active region of the devices, as demonstrated by the dependence of threshold current variation on the square root of time. These experimental results show that the presence of TDDs is the main limiting factor for the reliability of QD lasers for epitaxially-integrated silicon photonics applications, further confirming the outcome of previous statistical lifetime analyses carried out on devices featuring similar epitaxial structures.

This paper reports on the degradation processes of heterogeneous III-V/Silicon loop-mirrors laser-diodes designed as the optical sources for next-generation Photonic Integrated Circuits (PICs) operating at 1.55 μm. By submitting the devices to a series of constant-current accelerated aging experiments we were able to identify a common set of degradation mechanisms, including: (i) an increase in threshold current, (ii) a decrease of the sub-threshold emission, (iii) a decrease in the operating voltage of the device and (iv) a small decrease in the slope-efficiency of the laser. The strong correlation between these degradation processes suggests that the loss in optical performance experienced by the devices can be attributed to a decrease in the non-radiative carrier lifetime, as a consequence of the generation/propagation of defects towards the active region of the laser diodes.

This paper reports an extensive analysis of the degradation processes of heterogeneous III-V/Silicon infrared laser diodes designed for integrated telecommunications and interconnects. By submitting the devices to a series of constant current stress tests, a gradual degradation of the main device parameters was observed. In particular, in every stress scenario the devices under test showed: 1) an increase in the threshold current; 2) a decrease of the turn-ON voltage; and 3) an increase in the apparent carrier concentration within the space charge region. The variation of the electrical parameters (turn-ON voltage and apparent charge concentration) was found to be significantly correlated to the optical degradation for long stress times; the results support the hypothesis that degradation originates from an increase in the non-radiative recombination rate, possibly due to the diffusion of defects toward the active region of the devices. In order to investigate the physical origin of those defects, a capacitance deep level transient spectroscopy analysis was performed. The results indicate the presence of five different deep levels, with a main trap located around 0.43 eV above the valence band energy. This trap is compatible with an interface defect located between the In 0.53 Al x Ga 0.47-x As separate confinement heterostructure region and the InP layer.

We demonstrate a novel method that makes an efficient use of laser nonlinear dynamics when subject to optical self-injection for subwavelength displacement sensing purposes. The proposed methodology combines two different phenomena taking place inside the laser cavity: optical self-injection, which results in optical feedback interference, and laser continuous wave frequency modulation, giving rise to a wavelength sweeping effect in the laser’s emission. We present a combination of these phenomena to measure vibration amplitudes below $\lambda /2$ with the resolutions of a few nanometers, bandwidth dependent upon the distance of external target, amplitude, and frequency of current modulation. The basic theoretical details and a mathematical model are presented for the developed measurement principle. Experimental results with the system working as a vibrometer to measure a target vibration of amplitude ${\lambda }/{5}$ (137.5 nm) with a mean peak-to-peak error of 2.4 nm just by pointing the laser diode onto the target and applying some signal processing are also demonstrated.

The research in optoelectronics has recently shown impressive advancements, thanks to the development of disruptive technologies, such as Gallium Nitride LEDs and lasers - that are enabling novel lighting devices - and Silicon photonics - that will change the paradigm in broadband data transmission. Industrial laboratories and academic researchers are working towards the improvement of device performance and reliability. This goal is complicated by the complex nature of optoelectronic devices and systems: a deep knowledge on material properties, device characteristics and system optimization is needed to identify the physical mechanisms that limit the lifetime of the devices, and to address possible solutions. The aim of this thesis is to report on a detailed study of the physical factors that limit the performance and reliability of innovative optoelectronic technologies based on nitrides and arsenides. The investigation was conducted by means of purposely planned Accelerated Life Test (ALT) experiments and short-term stress tests on selected state-of-the-art devices, which let us identify the main physical processes responsible for the long-term degradation of the devices, and pinpointed the weaknesses of modern LEDs when temporary operated outside their Safe Operating Area (SOA) as a consequence of Electrical Over-Stress (EOS) events. With regard to Gallium Nitride (GaN)-based white and visible monochromatic LEDs, we found that under constant near-breakdown reverse bias the devices exhibit a time-dependent degradation phenomenon, which promotes the increase of the leakage current of the device, eventually leading to its catastrophic failure. The peculiar failure statistics, as well as the electro-luminescence and leakage-current data acquired during stress, could be interpreted by considering that highly-depleted GaN under reverse bias may behave like a partially-leaking dielectric that degrades over time due to a defect percolation process, similar to the dielectric degradation-driven breakdown process affecting Silicon MOS devices. On the other hand, high forward-current short-term step-stress on high-power LEDs revealed that sustained operation at driving currents above the maximum rating of the devices can rapidly induce failure in correspondence of a major current injection point, dependent on the specific chip structure, as a consequence of the localized power dissipation and temperature reached due to extreme current crowding effects and to the degradation of the conductivity of extended device regions. A clear dependence of the failure mode on the chip structure and device layout could also be found during the investigation on the effects of single short EOS events of increasing amplitude on high-power white LEDs. The experimental data helped identifying the best LED design to be employed in an electrically critical environment, also showing that EOS-related reliability issues tend to arise more from extrinsic elements of the LED system rather than from the semiconductor chip. Unlike High-Brightness (HB) state-of-the-art LEDs, whose main reliability concern is represented by EOS events, cost-effective mid-power LEDs for lighting applications were found to suffer from gradual degradation processes impacting in the long term on both the electrical an optical characteristics of the device. Moreover, the results of the ALTs highlighted the role of the plastic package in the degradation of the optical properties of the emitted light. The long-term reliability of mid-power LEDs was further investigated at system level by performing a lifetime analysis of commercial LED bulbs employing these devices as primary light sources. The increased complexity of the system under stress negatively impacted on the stability over time of the luminous performances of the luminaries, which was severely affected by the degradation of extrinsic elements like the diffusing dome or the current driver. In Part II of this work, our system-level analysis continued with an extensive investigation on the reliability of blue-emitting phosphors for near-UV laser excitation, as part of a research project performed in collaboration with the New Energy and Industrial Technology Development Organization (NEDO), Japan. By means of a series of pure thermal stress experiments and stress under high levels of optical excitation, we have been able to identify the physical process responsible for the degradation of the phosphors under extreme and more conservative operating conditions. In particular, while the phosphors demonstrated good stability during pure thermal treatment in air up to 300 °C, for temperatures equal to or greater than 450 °C the material exhibited a time-dependent drop in the photo-luminescence, which was attributed to the thermally-induced autoionization of the Eu2+ optically active centers. By means of different material characterization techniques, evidence of this degradation process was also found on samples stressed under moderate 3 W/mm2 – 405 nm optical excitation. This indicated that the optically (and thermally) induced ionization of the optically-active species is the most critical degradation process for this family of phosphorescent material. The operating limits of an improved second generation luminescent material were also investigated by means of short term stress under 405 nm optical excitation. The experimental data showed that, for a given deposition condition, a threshold excitation intensity for continuous pumping exists. Above this threshold, decay of the steady-state photo-luminescence performances and degradation of the material were found to take place, which suggested that the material was being operated in an unuseful excitation regime, mainly limited by the thermal management capabilities of the carrier substrate employed for our experimental purposes rather than from intrinsic properties of the phosphor. Part III of this thesis is devoted to the analysis of the degradation processes of heterogeneous III-V/Silicon infrared (IR) laser diodes designed for integrated telecommunications and interconnects. By submitting the devices to a series of constant current stress tests, a gradual degradation of the main device parameters was observed. In particular, in every stress scenario the devices under test showed (i) an increase in the threshold current, (ii) a decrease in the turn-on voltage, and (iii) an increase in the apparent carrier concentration within the space charge region. The variation of the electrical parameters was found to be significantly correlated to the optical degradation for long stress times; the results support the hypothesis that degradation originates from an increase in the non-radiative recombination rate, possibly due to the diffusion of defects towards the active region of the devices. In order to further investigate the physical origin of the diffusing defects, capacitance Deep-Level Transient Spectroscopy (C-DLTS) analysis was performed. The results indicate the presence of several deep levels, with a main trap located around 0.43 eV above the valence band energy. This trap was found to be compatible with an interface defect characteristic of the quaternary material employed to grow the active region of the device.

International audience; The authors demonstrate that the bits of a Compact Disc – Read Only Memory (CD-ROM) can be read without a photodiode (PD). A commercial CD-ROM drive was used and left unmodified except a beam splitter was removed which would route the light to a PD. This allowed for the reflected light to be incident on the laser within the laser package itself. By monitoring fluctuations in the voltage across the laser diode under constant-current injection, it was possible to read and interpret the bits from a spinning CD-ROM.

With the recent exponential growth of applications using artificial intelligence (AI), the development of efficient and ultrafast brain-like (neuromorphic) systems is crucial for future information and communication technologies. While the implementation of AI systems using computer algorithms of neural networks is emerging rapidly, scientists are just taking the very first steps in the development of the hardware elements of an artificial brain, specifically neuromorphic microchips. In this review article, we present the current state of neuromorphic photonic circuits based on solid-state optoelectronic oscillators formed by nanoscale double barrier quantum well resonant tunneling diodes. We address, both experimentally and theoretically, the key dynamic properties of recently developed artificial solid-state neuron microchips with delayed perturbations and describe their role in the study of neural activity and regenerative memory. This review covers our recent research work on excitable and delay dynamic characteristics of both single and autaptic (delayed) artificial neurons including all-or-none response, spike-based data encoding, storage, signal regeneration and signal healing. Furthermore, the neural responses of these neuromorphic microchips display all the signatures of extended spatio-temporal localized structures (LSs) of light, which are reviewed here in detail. By taking advantage of the dissipative nature of LSs, we demonstrate potential applications in optical data reconfiguration and clock and timing at high-speeds and with short transients. The results reviewed in this article are a key enabler for the development of high-performance optoelectronic devices in future high-speed brain-inspired optical memories and neuromorphic computing., 26 pages, 18 figures, Accepted in Chaos special issue on Time-delay Dynamics

An adaptive autofocus technique for the control of speckle effect in optical feedback interferometry (OFI) is discussed. The beam spot size effect over the OFI signal is presented, justifying the proposed approach. An automated setup for the control of speckle effect using a liquid lens with electro-optical focusing is demonstrated. The spot size is modified according to the signal to noise ratio (SNR) bounds selected for the captured OFI signal, therefore avoiding signal fading caused by speckle effect and possible chaotic behavior because of strong feedback. The quality and shape of the acquired OFI signal when the spot size change takes place shows a transient oscillation without any additional effects over the OFI SNR and the OFI shape. Experimental examples are provided proving the effectiveness of the approach for speckle control in displacement and velocity measurements within a movement range of 20 mm. (C) 2014 Elsevier B.V. All rights reserved.

We present a highly precise method to determine the thermal resistance and the antiguiding factor of (Al,In)GaN laser diodes. Knowing the thermal resistance, we are able to exclude thermal effects and study the charge carrier density dependence of the optical gain and the refractive index. From these properties we determine the antiguiding factor. The method is applied to 489 nm (Al,In)GaN laser diodes with a high charge carrier density in the active region. For these laser diodes we find a high antiguiding factor of 10 +/- 1 at the laser wavelength.

We measure the antiguiding factor of (Al,In)GaN laser diodes emitting in the violet, blue and green spectral range by combining optical gain-spectroscopy with measurements of the charge-carrier induced refractive index change. A precise determination of the thermal resistance of the laser diodes allows us to keep the temperature of the active region constant during the whole measurement and thus to exclude any thermal effect on the refractive index. For laser diodes emitting in the range from 409 to 511 nm, the antiguiding factor at the laser wavelength is 4.1±0.5.