Jens W. Tomm

The physics of catastrophic optical damage in high-power AlGaInP laser diodes

An innovative combination of concepts, namely microphotoluminescence (μPL) mapping, focused ion beam (FIB) microscopy, micro-Raman spectroscopy, and high-speed thermal imaging, was employed to reveal the physics behind catastrophic optical damage (COD), its related temperature dynamics, as well as associated defect and near-field patterns. μPL mapping showed that COD-related defects are composed of highly nonradiative complex dislocations, which start from the output facet and propagate deep inside the cavity. Moreover, FIB analysis confirmed that those dark line defects are confined to the active region, including the quantum wells and partially the waveguide. In addition, the COD dependence on temperature and power was analyzed in detail by micro-Raman spectroscopy and real-time thermal imaging. For AlGaInP lasers in the whole spectral range of 635 to 650 nm, it was revealed that absorption of stimulated photons at the laser output facet is the major source of facet heating, and that a critical facet temperature must be reached in order for COD to occur. A linear relationship between facet temperature and near-field intensity has also been established. This understanding of the semiconductor physics behind COD is a key element for further improvement in output power of AlGaInP diode lasers.

Optically pumped mid infrared emitters built using surface structured PbSe epitaxial layers

Light emitting devices for the infrared spectral region are used in a lot of application fields. In the mid infrared (MIR) region, where a lot of gases show strong absorptions, the optical output power of inexpensive emitters in the relevant wavelength range is too low. An optically pumped emitter for the MIR region around 4 μm based on narrow gap semiconductors is demonstrated. The pumping takes place using inexpensive near-infrared (around 1 μm) high power continuous wave (cw) semiconductors laser. The radiation is converted by the narrow gap semiconductor into the MIR region as spontaneous emission. Molecular beam epitaxy (MBE) grown IV-VI lead chalcogenide-based compounds, especially PbSe, are applied for frequency conversion. The structural and optical quality of these thin film materials is characterized mainly by X-ray defraction measurements (XRD) and photo luminescence (PL) spectroscopy. For high radiation efficiency the outcoupling of the light is enhanced by surface structuring. Useful structures generating high photoluminescence intensity are characterized by IR imaging with an IR camera system being sensitive in the spectral region of interest. Due to the high pumping powers the device design-especially the thermal management of the active PbSe film-plays an important role. We will present a preparation technique for optically pumped, surface structured PbSe emitters in transmission geometry exploiting the transparency of the substrates and glues in the relevant wavelength region. The measured total emission power of the emitters exceeds 0.5 mW. Using an optimised design total emission powers up to 2 mW were achieved.

Diode laser testing by taking advantage of its photoelectric properties

We report on the potential of the photocurrent technique as analytical tool for diode laser testing. The physics involved into the generation of photocurrents as well as experimental requirements for detecting them are highlighted. Based on a number of practical examples, we demonstrate how knowledge about the photoelectrical properties of diode lasers can help to learn about stress and defects within packaged devices or how non-perfect device fabrication may be discovered. These results are discussed in conjunction with device reliability issues.

New approaches towards the understanding of the catastrophic optical damage process in in-plane diode lasers

The microscopic processes accompanying the catastrophic optical damage process in semiconductor lasers are discussed. For 808 and 650 nm edge-emitting broad-area devices relevant parameters such as surface recombination velocities, bulk and front facet temperatures are determined and discussed. Facet temperatures vs. laser output and temperature profiles across laser stripes reveal a strong correlation to near-field intensity and degradation signatures. Furthermore, the dynamics of the fast catastrophic optical damage process is analyzed by simultaneous high-speed infrared thermal and optical imaging of the emitter stripe. The process is revealed as fast and spatially confined. It is connected with a pronounced impulsive temperature flash detected by a thermocamera.

Optical probes as tools for the investigation of aging properties of high-power laser diode arrays

Optical probes are used for the investigation of aging properties of high-power laser diode arrays. Two methods - micro-Raman and laser beam induced current (LBIC) scans are discussed: Micro-Raman spectroscopy - a laboratory based method - was applied in order to investigate facet temperature distributions for fresh and aged high-power laser diode arrays (LDA) under regular operation conditions. Furthermore, facet temperatures were measured for operation close to the catastrophic optical damage (COD) level.For these experimental situation facet temperatures of more than 600 degrees C were obtained. In order to conclude from pure temperature data to the thermal management of the LDA we performed modeling work based on the finite element method. Close to the COD up to 14 percent of the total thermal load of the LDA is concentrated to a very thin region close to the facet. Photocurrent spectra and LBIC were found to indicate aging induced effects such as defect creation as well as strain effects. LBIC - method which also could be used as test method for fabrication processes - allows to map aging induced changes of defect distributions in LDAs. Furthermore, the potential of LBIC as quick strain inspection method is discussed.

Aging behavior of high-power laser arrays monitored by photocurrent spectroscopy

The well-known method of photocurrent spectroscopy, i.e. the measurement of the spectral sensitivity of a laser diode like for a detector, was found to monitor aging properties of (In)AlGaAs/GaAs high power laser diode arrays (LDA) in a convenient way. Photocurrent spectra of LDAs emitting at 808 nm (1.53 eV) were measured in the 0.8 - 3.0 eV photon energy range. Aging induced changes in different spectral regions reveal the influence of different mechanisms affecting the structure. Conclusions on the microscopic nature of the changes are drawn and applications are discussed.

Studies of the degradation mechanisms in high-power diode lasers using multi-channel micro-thermography

We demonstrate the applicability of imaging thermography for investigations of mechanisms associated with gradual degradation in diode lasers. The introduction of two spectral channels provides the means for separate observation of deep level luminescence and thermal radiation emitted according to Plancks law. In the near IR region we found the signal detected by the camera to be mainly affected by mid-gap deep-level luminescence. An intensity increase of the luminescence signal for an aged diode laser compared to an unaged device is noticed. It can be explained by an increase of deep level defect concentration during the aging. In the mid IR, we mainly encounter thermal radiation, which can be used for the analysis of the thermal properties of devices. In present work the thermal behavior of the device subjected to an aging of 3000 hours is analyzed. A significant increase of device temperature is noticed.

Near-field optical-beam-induced current spectroscopy as a tool for analyzing aging processes in diode lasers

We present a near-field optical beam induced current study of aged high power laser diode arrays (LDA). A near-field scanning optical microscope was used as a light source for creating a photocurrent or a photovoltage in the LDA. Mechanisms generating the signal as well as methodical aspects such as resolution limits and application fields of the technique are discussed. The method is demonstrated to provide insight into the microscopic aging properties of LDAs. Both monitoring of the aging status as well as the localization of defects becomes possible.

Defect evolution during catastrophic optical damage in 450-nm emitting InGaN/GaN diode lasers

The catastrophic optical damage (COD) of 450-nm emitting InGaN/GaN diode lasers is investigated with special attention to the kinetics of the process. For this purpose, the COD is triggered artificially by applying individual current pulses. This makes it possible to achieve a sub-µs time resolution for processes monitored by cameras. COD appears as a hot process that involves decomposition of quantum well and waveguide materials. We observe the ejection of hot material from the front facets of the laser. This can be seen in two different wavelength ranges, visible/near infrared and mid infrared. The main contributions identified are both thermal radiation and 450-nm laser light scattered by the emitted material. Defect growth during COD is energized by the optical mode. Therefore, the defect pattern resembles its shape. Ultimately, the loss of material leads to the formation of an empty channel along the laser axis. COD in GaAs and GaN-based devices follows similar general scenarios. After ignition of the process, the defect propagation during the process is fed by laser energy. We observe defect propagation velocities of up to ~30 m/s for GaAs-based devices and 110 m/s for GaN-based devices. The damage patterns of GaN and GaAs-based devices are completely different. For GaN-based devices, the front facets show holes. Behind them in the interior, we find an empty channel at the position of the optical mode surrounded by intact material. In contrast, earlier studies on GaAs-based devices that were degraded under almost identical conditions resulted in molten, phase separated and both recrystallized and amorphous materials with well-defined melting fronts.

Ultrafast carrier dynamics in AlGaN/GaN superlattices by time-dependent reflectivity measurements (Conference Presentation)

Gallium-nitride-based structures have become more and more important in recent years. Especially InGaN/GaN multi-quantum well (MQW) structures are used for optoelectronic devices such as light emitting diodes and diode lasers in the blue and green spectral region and for detectors and power amplifiers. AlGaN/GaN-based structures have the potential to extend optoelectronics towards the ultraviolet spectral region. Thus, carrier dynamics in MQW structures and superlattices containing aluminum are of strong interest.nnThe ultrafast processes of nonequilibrium carriers in such semiconductor superlattices are not yet fully understood. Therefore, we have investigated the carrier dynamics in Al0.18Ga0.82N/GaN superlattice samples by pump-probe measurements. The samples consist of 60 periods with 2 nm barriers and 3 nm quantum wells. A SiN coating prevents degradation effects during excitation [1]. In addition, GaN bulk material was measured.nnFor the measurements, we used an Yb-based oscillator amplifier system (repetition rate 1 MHz) pumping an optical parametric amplifier, allowing second-harmonic wavelengths between 325 nm and 460 nm with a pulse length of 40 fs. Time-dependent pump-probe measurements at room temperature were performed in reflection because absorption in the GaN template between the superlattice and the substrate prevents transmission measurements. After interaction with the sample, the probe beam was spectrally resolved to determine transient spectra. nnIn the measurement, carriers are excited by the pump laser pulse above the superlattice band gap energy. Two processes are involved in the ensuing intra-band relaxation: the first leads to the thermalization of carriers by carrier-carrier scattering, the second is the cooling of carriers by phonon scattering [3]. Due to the polar properties of GaN-based superlattices, one expects a much stronger coupling between electrons and optical phonons compared to GaAs-based systems. This should result in a much faster cooling process. The photo-excited carriers lead to band gap renormalization and therefore to an increase of the refractive index at energies below the band gap and to a decrease of the refractive index at energies above the band gap. These changes manifest themselves in the transient reflectivity measured in our pump-probe experiments.nnExciting 240 meV above the superlattice band gap, we see a decrease in reflectivity of up to 4 percent at an excitation density of 580 µJ/cm2 per pulse, decaying with a time constant of 1.7 ps. Furthermore, carrier cooling rates in superlattices and in bulk materials are compared [3].nnReferencesn[1] C. Netzel, J. Jenschke, F. Brunner, A. Knauer, and M. Weyers; J. Appl. Phys. 120, 095307 (2016)n[2] Jagdeep Shah; Ultrafast Spectroscopy of Semiconductors and Semiconductor Nanostructures, Springer, Berlin (1996)n[3] Y. Rosenwaks, M. C. Hanna, D. H. Levi, D. M. Szmyd, R. K. Ahrenkiel, and A. J. Nozik; Phys. Rev. B 48, 14675 (1993)