We demonstrate a new generation of composition-tuned, ternary GaAsSb nanowire lasers on silicon with emission wavelengths tuned to below the Si bandgap. By solving previous limitations in the growth of III-As-Sb based nanowires, resonator cavities with extended lengths > 7 µm and high Sb-content (~30%) are realized as a base for bulk-type or quantum-well based nanowire lasers. Bulk GaAsSb nanowire lasers with high radiative efficiency and low threshold are enabled by use of lattice-matched InAlGaAs surface passivation layers. Coaxial InGaAs multi-quantum well (MQW) active regions grown on GaAsSb nanowire templates open further scope of tailoring material gain and lasing wavelength.
Continuous-wave (cw) operation of integrated nanoscale lasers is a key ingredient for on-chip optical processing schemes in Si photonic circuits. Here, we demonstrate cw-lasing from individual InAs nanowires at mid-infrared wavelengths (2.4-2.7 µm) without any external cavity for mode confinement. Using finite difference time domain (FDTD) modelling of the threshold gain, optimal single Fabry-Perot nanowire laser geometries with diameter > 800 nm and lengths of 10-30 µm are realized by site-selective growth methods. Corresponding nanowires exhibit cw-lasing with thresholds around 10-30 kW/cm2 at lasing emission up to 70K.
KEYWORDS: Hot carriers, Nanowires, Solar cells, Semiconductors, Photovoltaics, Diseases and disorders, Crystals, Beam diameter, Time resolved spectroscopy, Spectroscopes
III-V nanowire structures have shown promising results in mitigating hot carrier thermalization rates suitable for hot carrier solar cell applications. This effect is attributed to the spatial confinement of charged particles and the adjustment of material properties in these nanostructures. Furthermore, by designing vertically standing nanowires, it is possible to improve photo-absorption by increasing internal surface reflection. Investigating the properties of hot carriers in core-shell InGaAs nanowires has shown evidence for a strong diameter dependence of these nanostructures. Determining the origin of this effect provides valuable information for the development of efficient hot carrier absorbers for 3rd generation solar cells.
We show by numerical modelling how geometrical parameters of a NW and Si-WG design influence the spontaneous emission enhancement of the QD emitter and the in-coupling efficiencies at the NW-WG interface. First experiments towards the development of an integrated III-V NW-QD system are then presented. Here, we demonstrate a droplet-free site-selective epitaxy of vertical-cavity NW waveguides, where good control of GaAsSb/InGaAs axial heterostructures and their distinct luminescence properties are demonstrated. We also discuss control of Indium incorporation into the InGaAs axial segment, to tune the emission wavelength before optimizing the axial size, progressing towards an axial QD.
Semiconductor nanowires (NWs) have shown robust hot carrier effects due to their small dimensions. Here, we study the cooling mechanisms of hot electrons in the time domain via transient absorption spectroscopy. Probe energies below the bandgap are used to determine the evolution of the carrier effective mass while probe energies above the bandgap track the conduction band occupation. From excitation intensity dependent measurements, we confirm that electron-hole interactions are a major cooling channel at large carrier density, given the high ratio of mh/me of InAs. Our experiments indicate that this cooling channel is amplified in passivated core-shell NWs. We associate this effect with spatial carrier separation caused by Fermi-level pinning in unpassivated NWs. In core-shell NWs, bands are considerably more flat which increases radiative recombination and electron-hole scattering with the latter cooling the hot electron population. Our results highlight the advantages of carrier separation if high carrier densities are to be used for hot phonon bottlenecks.
Nanowire lasers can be monolithically and site-selectively integrated onto silicon photonic circuits. To assess their full potential for ultrafast opto-electronic devices, a detailed understanding of their lasing dynamics is crucial. However, the roles played by their resonator geometry and the microscopic processes that mediate energy exchange between the photonic, electronic, and phononic systems are largely unexplored. Here, we apply femtosecond pump-probe spectroscopy to show that GaAs-AlGaAs core-shell nanowire lasers exhibit unexpected non-equilibrium dynamics occurring over few picosecond timescales. As we will show, these dynamics are intricately linked to the strong interaction between the lasing mode and the gain material arising from their wavelength-scale dimensions. We anticipate that our results will lead to new approaches for ultrafast intensity and phase modulation of chip-integrated nanoscale semiconductor lasers.
We design a Quantum Dot (QD) embedded in a vertical-cavity photonic nanowire (NW), deterministically integrated on a silicon-on-insulator (SOI) waveguide (WG), as a novel quantum light source in a quantum photonic integrated circuit (QPIC). Using a broadband QD emitter, we perform finite-difference time domain simulations to systematically tune key geometrical parameters and explore the coupling mechanisms of the emission, to the NW and WG modes. We find distinct Fabry-Perot resonances in the Purcell enhanced emission that govern the outcoupled power into the fundamental TE mode of the SOI-WG. With an optimized geometry that places the QD emitter in a finite NW in close proximity to the WG, we obtain peak outcoupling efficiencies for polarized emission as high as eighty percent.
We present our recent progress in the monolithic integration and epitaxial gain control of GaAs-based nanowire (NW) lasers on Si photonic platform. First, we describe the challenges in realizing vertical-cavity NW lasers on Si and SOI-based waveguides based on the stringent requirements for high gain, low-loss wave-guiding and coupling efficiency as illustrated by detailed numerical simulations. Consequently, we discuss bottom-up, epitaxial schemes for site-selective integration of individual GaAs NW lasers on planar Si and non-planar Si ridge waveguides under different geometries, demonstrating vertical-cavity NW lasers with remarkably low lasing threshold (< 20 µJ/cm2) and high spontaneous emission coupling factor (B > 0.2) under pulsed optical excitation. First experiments of individual NW-lasers with direct coupling of lasing emission into the underlying Si-ridge waveguides are also shown.
To further improve the threshold gain and lasing characteristics and achieve wavelength tunability, we further tailor the active gain media using low-dimensional systems embedded in the NW resonator cavity. Here, we particularly emphasize the challenges in the growth of GaAs-based cavities that incorporate coaxial GaAs-AlGaAs and InGaAs-AlGaAs multiple quantum well (MQW) heterostructures and illustrate even routes towards quantum-wire or quantum-dot based gain media. Finally, we provide a direct comparison of the lasing characteristics of 3D-bulk like GaAs NW lasers with coaxial NW-MQW heterostructure lasers, and show how the control of composition and structure of the MQW shifts the lasing emission to longer wavelengths.
Semiconductor nanowire (NW) lasers are nanoscale coherent light sources that exhibit a small footprint, low-threshold lasing characteristics, and properties suitable for monolithic integration onto Si photonic circuits. An important milestone on the way towards novel on-chip photonic functionalities is the integration of individual, deterministically addressable NW lasers on Si waveguides with efficient coupling and mode propagation in the underlying photonic circuit. Here, we demonstrate the monolithic integration of single GaAs-based NW lasers directly onto lithographically defined Si ridge waveguides (WG). Subject to optical excitation, the observed lasing behavior shows clear “s-shape”-characteristics, linewidth narrowing and threshold values down to 19.8±1 µJ/cm², which is the lowest value reported to date for this kind of integrated lasing structure. The lasing mode of individual NW lasers is shown to couple efficiently into propagating modes of the underlying orthogonal Si WG, preserving the spectral characteristics during mode pro-pagation in the WG, and in good agreement with Finite-Difference Time-Domain (FDTD) simulations. Using a WG structure with a series of mask openings along the central mode propagation axis, we further illustrate the out-coupling properties of both spontaneous and stimulated emission and demonstrate propagation of the lasing mode over distances > 60 µm, despite absorption in the silicon dominating the propagation losses. By replacing the bulk GaAs as the active gain medium by InxGa1-xAs quantum wells, the emission wavelength can be shifted towards telecommunication bands in order to reduce the absorption losses in silicon. These results pave the way for future on-chip monolithic integration of III-V NW lasers onto silicon based optoelectronic circuits.
III-V semiconductor nanowires (NW) are being considered as future coherent light sources for optoelectronic chips due to their small footprint and high refractive index. The 1D confinement also results in a natural Fabry-Perot resonance cavity. However, the most important feature is the feasibility of direct growth on Si platform. The research carried out in this work consists of time-resolved photoluminescence (TRPL) spectra at different optical excitation powers and temperatures for single GaAs-AlGaAs core-shell nanowire nanolasers on Silicon.
The carrier dynamics response for a single nanolaser below and above the threshold is obtained for different sets of temperatures. The lifetime corresponding to the excitation power below the threshold is of the order of hundreds of picoseconds at all low temperature intervals (4K to 60K). With increasing pump power, the decay time gets shorter until the threshold is achieved. At this point, two lifetimes are obtained for the lasing modes, one of the order of tens of picoseconds (stimulated emission) and another of the order of hundreds of picoseconds (spontaneous emission). A redshift in time-resolved spectra (2-3nm in an interval of 700ps) is measured which disappears at higher temperatures (after 60K). This redshift is a result of the change in refractive index caused by a decrease in carrier density with time. This effect disappears at higher temperatures due to the increase of non-radiative recombination.
Wavelength scale coherent optical sources are vital for a wide range of applications in nanophotonics ranging from metrology and sensing to nonlinear frequency generation and optical switching. In these respects, semiconductor nanowires (NWs) are of particular interest since they represent the ultimate limit of downscaling for photonic lasers with dielectric resonators. By virtue of their unique one-dimensional geometry NW-lasers combine ultra-high modal gain, support low-loss guided modes and facilitate low threshold lasing tuneable across the UV, visible and near infra-red spectral regions. Recently, optically pumped NW lasers have been demonstrated at room temperature and they can now be site-selectively integrated onto silicon substrates. While the fundamental carrier relaxation and gain dynamics of NW-lasers have been explored, the coherent dynamics have hitherto received comparatively little attention. In this contribution, we demonstrate that GaAs-AlGaAs core-shell nanowire lasers grown on silicon are capable of emitting pairs of phase-locked picosecond laser pulses when subject to incoherent pulsed optical excitation. By probing the two-pulse interference that emerges within the homogeneously broadened laser emission, we show that pulse pairs remain mutually coherent over timescales extending beyond ~30ps, much longer than the emitted laser pulse duration (~3 ps). Simulations performed by solving the optical Bloch equations produce good quantitative agreement with experiments, revealing how the phase information is stored in the gain medium close to transparency.
Transverse thermoelectrics promise entirely new strategies for integrated cooling elements for optoelectronics. The recently introduced p × n-type transverse thermoelectric paradigm indicates that the most important step to engineering artificial transverse thermoelectrics is to create alternate p- and n-doped layers with orthogonally oriented anisotropic conductivity. This paper studies an approach to creating extreme anisotropic conductivity in bulk-doped semiconductor thin films via ion implantation. This approach defines an array of parallel conduction channels with photolithographic patterning of an SiO2 mask layer, followed by proton implantation. With a 10 μm channel width and 20 μm pitch, both n-type and p-type Al0.42 Ga0.58As thin films demonstrate a conductivity anisotropy ratio σ /σ⊥ > 104 at room temperature, while the longitudinal resistivity along the channel direction
after implantation only increased by a factor of 3.3 ∼ 3.6. This approach can be readily adapted to other semiconductor materials for artificial p × n-type transverse thermoelectrics as other applications.
We used the two-wire 3ω method to measure the in-plane and out-of-plane thermal conductivity of thin
films and analyzed the error for all fitting parameters. We find the heater half-width, the insulating layer
thickness and the out-of-plane thermal conductivity of the insulating layer the most sensitive parameters in
an accurate fitting. The data of a 2.5 μm GaAs thin film suggests that the phonon mean free path in the
film is limited to the film thickness, far shorter than that in the bulk material at low temperatures.
We present investigations of the propagation length of guided surface plasmon polaritons along Au waveguides on GaAs
and their coupling to near surface InGaAs self-assembled quantum dots. Our results reveal surface plasmon propagation
lengths ranging from 13.4 ± 1.7 μm to 27.5 ± 1.5 μm as the width of the waveguide increases from 2-5 μm. Experiments performed on active structures containing near surface quantum dots clearly show that the propagating plasmon mode
excites the dot, providing a new method to spatially image the surface plasmon mode. We use low temperature confocal
microscopy with polarization control in the excitation and detection channel. After excitation, plasmons propagate along
the waveguide and are scattered into the far field at the end. By comparing length and width evolution of the waveguide
losses we determine the plasmon propagation length to be 27.5 ± 1.5 μm at 830 nm (for a width of 5 μm), reducing to
13.4 ± 1.7 μm for a width of 2 μm. For active structures containing low density InGaAs quantum dots at a precisely controlled
distance 7-120 nm from the Au-GaAs interface, we probed the mutual coupling between the quantum dot and plasmon mode. These investigations reveal a unidirectional energy transfer from the propagating surface plasmon to the quantum dot. The exquisite control of the position and shape afforded by lithography combined with near surface QDs promises efficient on-chip generation and guiding of single plasmons for future applications in nanoscale quantum optics
operating below the diffraction limit.
We report on submicron-thick microcavity light emitting diodes (MCLEDs) emitting at the wavelengths of 415 nm ~
460 nm. These devices were fabricated by flip-chip-bonding, laser lift-off, and thinning processes. Growth of a highquality
AlxGa1-xN interlayer and etch selectivity between N-face GaN and AlxGa1-xN allowed high-precision control of
microcavity thickness, resulting in controlled microcavity effects. Single Fabry-Pérot modes confined in 2λ ~ 2.5λ-thick
MCLEDs gave rise to characteristic angular emission, in contrast to a Lambertian emission. High current operation
(~100 mA) showed robustness of these thin devices with promising the possibility of high-brightness application. We
will discuss design and processing issues regarding photonic-crystal integration towards higher improvements in light
extraction efficiency.
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