Subthreshold Emission Research Articles

Investigation of Current-Driven Degradation of 1.3 μm Quantum-Dot Lasers Epitaxially Grown on Silicon

This work investigates the degradation processes affecting the long-term reliability of 1.3 μm InAs quantum-dot lasers epitaxially grown on silicon. By submitting laser samples to constant-current stress, we were able to identify the physical mechanisms responsible for the optical degradation. More specifically, the samples (i) exhibited a gradual increase in threshold current, well correlated with (ii) a decrease in sub-threshold emission, and (iii) a decrease in slope efficiency. These variations were found to be compatible with a diffusion process involving the propagation of defects toward the active region of the device and the subsequent decrease in injection efficiency. This hypothesis was also supported by the increase in the defect-related current conduction components exhibited by the electrical characteristics, and highlights the role of defects in the gradual degradation of InAs quantum dot laser diodes. Electroluminescence measurements were used to provide further insight in the degradation process.

Measuring the nonlinear phase shift in a mode-locked laser.

The spectrum of a mode-locked laser (MLL) is down-mixed to electrical frequencies using a tunable continuous-wave laser. By characterizing the sub-threshold low-intensity emission relative to the laser mode positions, one can measure the nonlinear phase shift of the MLL while in operation.

Degradation mechanisms of heterogeneous III-V/Silicon loop-mirror laser diodes for photonic integrated circuits

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.

Investigation of the Chromatic Dispersion in Two-Section InAs/GaAs Quantum-Dot Lasers

We present the measurements of the dispersion of InAs/GaAs quantum-dot lasers emitting at 1230 nm (ground state) and 1160 nm (excited state) from the analysis of their subthreshold emission spectra. Measurements from devices with various lengths allow us to deduce that the group velocity dispersion is as high as 2270 fs2mm−1 and is mainly due to the dispersion of bulk GaAs. The gain-induced dispersion varies with the injected current at a rate of $\simeq -2~\text {fs}^{2}$ mA−1mm−1, whereas the effect of a saturable absorber on the dispersion is found to be negligible. These results suggest that the implementation of integrated dispersion compensation could significantly reduce the pulse duration of these lasers in mode-locked regime and lead to an enhancement of the formation of optical frequency combs in these devices.

FERMI LAT STACKING ANALYSIS OF SWIFT LOCALIZED GRBs

ABSTRACT We perform a comprehensive stacking analysis of data collected by the Fermi Large Area Telescope (LAT) of γ-ray bursts (GRBs) localized by the Swift spacecraft, which were not detected by the LAT but which fell within the instrument’s field of view at the time of trigger. We examine a total of 79 GRBs by comparing the observed counts over a range of time intervals to that expected from designated background orbits, as well as by using a joint likelihood technique to model the expected distribution of stacked counts. We find strong evidence for subthreshold emission at MeV to GeV energies using both techniques. This observed excess is detected during intervals that include and exceed the durations typically characterizing the prompt emission observed at keV energies and lasts at least 2700 s after the co-aligned burst trigger. By utilizing a novel cumulative likelihood analysis, we find that although a burst’s prompt γ-ray and afterglow X-ray flux both correlate with the strength of the subthreshold emission, the X-ray afterglow flux measured by Swift’s X-ray Telescope at 11 hr post trigger correlates far more significantly. Overall, the extended nature of the subthreshold emission and its connection to the burst’s afterglow brightness lend further support to the external forward shock origin of the late-time emission detected by the LAT. These results suggest that the extended high-energy emission observed by the LAT may be a relatively common feature but remains undetected in a majority of bursts owing to instrumental threshold effects.

Exploiting broad-area surface emitting lasers to manifest the path-length distributions of finite-potential quantum billiards.

Broad-area vertical-cavity surface-emitting lasers (VCSELs) with different cavity sizes are experimentally exploited to manifest the influence of the finite confinement strength on the path-length distribution of quantum billiards. The subthreshold emission spectra of VCSELs are measured to obtain the path-length distributions by using the Fourier transform. It is verified that the number of the resonant peaks in the path-length distribution decreases with decreasing the confinement strength. Theoretical analyses for finite-potential quantum billiards are numerically performed to confirm that the mesoscopic phenomena of quantum billiards with finite confinement strength can be analogously revealed by using broad-area VCSELs.

Energy and impact angle dependence of sub-threshold external electron emission

The kinetic electron emission induced by Ar+ ions impinging onto a polycrystalline Ag surface is studied as a function of the projectile impact angle in the energy range 5–10keV well below the classical threshold. The resulting emission yield is compared to published model descriptions of sub-threshold electron emission. We find reasonable agreement with the prediction of the hot spot model proposed by Sroubek, which is shown to become equivalent with a Richardson–Dushman-type description of thermionic emission when converted to three dimensions. The model is further extended to describe the impact angle dependence and shown to fit the corresponding experimental data reasonably well.

Direct observation of sub-threshold field emission from silicon nanomembranes

Excited by an energetic electron beam, field enhanced electron emission from a silicon nanomembrane is examined by using a local current probe. The experiments reveal clear transitions from conventional secondary electron emission (SEE) to sub-threshold field electron emission (SFE) and then to conventional field electron emission (FEE). An electron yield of more than 104 is obtained and a model of sub-threshold emission is verified. Ultra sensitive charge/particle detectors could be realized by implementing sub-threshold field emission in nano scale structures, where electron excitations occur within the screen depth of surface electric field.

Manifestation of quantum-billiard eigenvalue statistics from subthreshold emission of vertical-cavity surface-emitting lasers

We report that the subthreshold emission spectra of vertical-cavity surface-emitting lasers (VCSELs) can be analogously used to manifest the quantum-billiard energy spectra. The Fourier-transformed distributions of the subthreshold emission spectra are demonstrated to display various peak structures that are in good agreement with the results of the quantum-billiard model. We also verify that the statistical analyses of the nearest-neighbor eigenvalue spacing distributions obey a Poisson distribution for an equilateral-triangular device and a Wigner distribution for a stadium-shaped device.

Kinetic electron emission from metal surfaces by slow Na+ ions

Kinetic electron emission caused by the impact of singly charged Na ions upon the metal surfaces is analyzed in terms of the recently developed thermal hot-spot model, that heuristically describes the many-electron nature of the emission process. The model accounts for the exponential decrease of electron emission yields at projectile velocities below the thresholds of Auger processes. The agreement with experiments indicates that, in spite of its simplicity, the model well describes the basic physics of sub-threshold electron emission at lowest impact energies for Al and Au surfaces.

Subthreshold field emission from thin silicon membranes

We report on strongly enhanced electron multiplication in thin silicon membranes. The device is configured as a transmission-type membrane for electron multiplication. A subthreshold electric field applied on the emission side of the membrane enhances the number of electrons emitted by two orders of magnitude. This enhancement stems from field emitted electrons stimulated by the incident particles, which suggests that stacks of silicon membranes can form ultrasensitive electron multipliers.

Determination of Compton profiles at solid surfaces from first-principles calculations

Projected momentum distributions of electrons, i.e. Compton profiles above the topmost atomic layer have recently become experimentally accessible by kinetic electron emission in grazing-incidence scattering of atoms at atomically flat single crystal metal surfaces. Sub-threshold emission by slow projectiles was shown to be sensitive to high-momentum components of the local Compton profile near the surface. We present a method to extract momentum distribution, Compton profiles, and Wigner and Husimi phase space distributions from ab-initio density-functional calculations of electronic structure. An application for such distributions to scattering experiments is discussed.

1 /spl times/ 12 VCSEL array with optical monitoring via flip-chip bonding

A vertical-cavity surface-emitting laser (VCSEL) 1 times 12 array flip-chip bonded to a 1 times 12 GaAs monitor photodiode array is demonstrated, and faithful monitoring of the VCSEL threshold current, optical power, and subthreshold emission is recorded. Extremely small monitor crosstalk is measured. This is the first published report of an optically monitored VCSEL array

Self-consistent calculation of current self-distribution effect in GaAs-AlGaAs oxide-confined VCSELs

A three-dimensional electrical-thermal-optical numerical solver is applied to model GaAs-AlGaAs-based top-emitting oxide-confined vertical-cavity surface-emitting lasers (VCSELs) with GaAs multiple-quantum-well active region. Continuous-wave mode of operation is simulated over a range of voltages, covering the subthreshold spontaneous emission and lasing emission. The effect of self-distribution of electrical current is demonstrated for the first time in self-consistent electrical-thermal-optical simulation of VCSELs.

Band alignment at the interfaces of Al2O3 and ZrO2-based insulators with metals and Si

Internal photoemission of electrons was used to determine the band alignment in metal (Mg, Al, Ni, Cu, Au)-oxide-silicon structures with Al2O3- and ZrO2-based insulators. For Al2O3- and ZrO2 layers grown on Si by atomic layer deposition the barrier height between the Si valence band and the oxide conduction band was found to be 3.25 and 3.1 eV, respectively. Thermal oxidation of the Si/oxide stacks results in a barrier height increase to ≈4 eV for both cases due to formation of a silicate interlayer. However, there is a significant sub-threshold electron emission both from silicon and metals, indicating a high density of states in the band gap of the insulators. These states largely determine the electron transport across metal oxides and may also account for a significant dipole component of the potential barrier at the metal/ZrO2 and metal/Al2O3 interfaces.

Slow-ion induced electron emission from clean metal surfaces: “Subthreshold kinetic emission” and “potential excitation of plasmons”

We discuss some recent advances in the understanding of potential (PE) and kinetic electron emission (KE) for impact of slow ions on clean metal surfaces. Below the “classical threshold” for KE by direct momentum transfer to quasi-free metal electrons, electron emission can already occur by three other mechanisms, i.e., electron promotion into the continuum in binary ion-target collisions, quasi-localization of electrons at the surface and, being only relevant at very low impact velocity, a partially localized electronic excitation in the impact zone. Furthermore, we regard potential emission of electrons (PE) from the one-electron decay of plasmons which are excited by impact of slow ions on quasi-free electron metal surfaces (e.g., poly- and mono-crystalline aluminum). Electron spectra for impact of 3–10 keV singly and doubly charged ions show that such plasmons result either from the ion potential energy (“potential excitation of plasmons – PEP”) or, in a secondary process, from sufficiently fast electrons from KE. Contributions from these processes to the respective total electron yield are typically in the percent range. A considerably more pronounced structure in the electron spectra resulting from impact of H + on Al(1 1 1), which has formerly been ascribed also to plasmon decay, is probably caused by diffraction of slow KE electrons undergoing multiple scattering in the uppermost target layers.

Design of InGaN-GaN-AlGaN vertical-cavity surface-emitting lasers using electrical-thermal-optical simulation

A three-dimensional electrical-thermal-optical numerical simulator is developed and applied to model group-III-nitride-based intracavity-contacted vertical-cavity surface-emitting lasers with InGaN multiquantum-well active region. The optical model based on the effective frequency method is combined with an electrical-thermal simulator using the control volume method. Isothermal (pulsed regime imitation) and continuous-wave modes of operation are calculated over a range of voltages, covering subthreshold spontaneous emission and lasing emission. Effects of current crowding at the active region periphery are examined, and in particular, an impact on mode profiles of spatial hole burning superimposed on nonuniform gain distribution is studied. In order to reduce the current crowding and provide more uniform gain distribution within the active region, a semitransparent p-side contact design is proposed.

Near-field spectroscopy of selectively oxidized vertical cavity surface emitting lasers

Selectively oxidized vertical cavity surface emitting lasers (VCSELs) have been studied by spectrally resolved near-field scanning optical microscopy. We have obtained spatially and spectrally resolved images of both subthreshold emission and lasing emission from a selectively oxidized VCSEL operating at a wavelength of 850 nm. Below threshold, highly local high gain regions, emitting local intensity maxima within the active area, were observed; these were found to serve as lasing centers just above threshold. Above threshold, the near-field spatial modal distributions of low order transverse modes were identified by spectrally analyzing the emission; these were found to be complex and somewhat different from those measured in the far field.

Measurement of semiconductor laser gain and dispersion curves utilizing Fourier transforms of the emission spectra

A new technique for the measurement of semiconductor laser gain and dispersion spectra is presented. The technique is based on an analysis of the subthreshold emission spectrum by Fourier transforms. Applications of this method to AlGaInP-based interband laser diodes and mid-infrared intersubband quantum cascade lasers are discussed. A good agreement between the measured dispersion of the refractive index and tabulated values in the literature was found.

Experimental study of resonant photoemission in the metals V, Cr, Mn and Co

We have studied the resonant photoemission of the valence band in V, Cr, Mn and Co transition metals using synchrotron radiation excitation near the 3p ionization thresholds. The effect of the 3p - 3d interaction reaches maximum strength in Cr and Mn. On the other hand, the contribution related to the atomic Fano resonance goes to a minimum in Mn and increases rapidly on moving to Co. This behaviour is also reflected in the values of the effective Coulomb hole - hole interaction which we have calculated from experimental spectra. Furthermore, we have investigated when the Auger transition appears in the vicinity of the 3p ionization thresholds. Our results reveal that V, Cr and Co do not show the so-called subthreshold Auger emission whereas it can possibly be observed in Mn. However, we attribute the unusual appearance of the Auger peak to the wide valence band of Mn rather than the 3p - 3d resonance.