In this work, we experimentally study the carrier and refractive index dynamics of InGaAs nanopillar grown on a Si on insulator (SOI) substrate. The recombination process of the InGaAs NP is characterized with different optical techniques. Temperature dependent photoluminescence (PL) at 0.5mW excitation power is carried out to determine the influence of temperature on carrier dynamics. The radiative recombination lifetime has been studied at 7K from time-resolved photoluminescence (TRPL) experiments at a certain excitation power. The optimal combination of pitch (separation between NPs) and diameter in the growth process of this nanostructure has also been measured. These results will contribute to further optimization of the InGaAs nanolaser for integration of III-V optoelectronics on SOI substrates.
This study is focused on the properties of the diamond lattice before and after implantation. The diamond lattices with nitrogen-vacancy centres have very exciting properties and they can be used in a plethora of applications from quantum sensing to biomarkers. Characteristic transmission, scattering and photoluminescence of diamond lattice with nitrogen-vacancy centres (NV-) were studied through different techniques at different temperatures. The luminescence of the synthesised diamonds was studied at a 532nm excitation wavelength and recorded in the range of 500-1100nm. Since its intensity decreases with decreasing the number of nitrogen-vacancy centres. Also, we analysed the luminescence depend on the functional groups attached to the diamond surface.
Raman spectroscopy studies provide interesting results about the phonon confinement effect, structure composition and homogeneity of the material and information about the functional groups attached above the diamond surface. Raman spectra depend on the structure, purity, sp3/sp2 ratio, crystal size and surface chemistry. With increasing sp3 carbon content the intensity of the diamond peak increases, while the D-band in the Raman spectra weakens. Also, we analysed the shifts in the energy and linewidth of the diamond peak in the Raman spectra.
The recent development of novel super-resolution imaging techniques coincides with the efforts to synthesize optically bright and stable biomarkers. In the future, we can use the differently doped nanodiamond fluorophores as biomarkers for sensing and bioimaging.
The integration of III-V optoelectronics on Si substrates is essential for next-generation high-speed communications. The major issue in the integration of III-V semiconductor on Si is the lattice mismatch between Si and the III-V semiconductor material at the interface. The strain induced by the lattice mismatch can be relaxed when a nanostructure, such as a nanopillar (NP), is grown on Si. In this work, we experimentally determine the lasing mode by optically pumping a single InGaAs nanopillar grown on Si on insulator (SOI).
The lasing features of the InGaAs NP are characterized with different optical techniques. Power dependent photoluminescence (PL) at 7K is carried out to determine the lasing threshold by increasing the excitation power. The carrier dynamics below and above the threshold have been studied at 7K from time-resolved photoluminescence (TRPL) experiments at different excitation powers. We have measured a decrease in the carrier lifetimes with a rise in excitation power until the nanostructure starts lasing. The lifetime corresponding to the laser mode is on the order of the sensitivity of the streak camera (±1ps) indicating the extremely short laser lifetime. The InGaAs nanolaser shows a single longitudinal mode because of the small length dimensions (<1μm). The wavelength of the laser mode emission changes with each NP excited due to the slight differences in dimensions between NPs. In addition, the quality of the crystal grown has been studied with temperature-dependent PL. These results will contribute to further optimization of the InGaAs nanolaser for integration of III-V optoelectronics on Si substrates.
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.
Here we describe a uniform diameter, direct bandgap Ge1-xSnx alloy nanowires, with a Sn incorporation up to 9%, the fabricated through a conventional catalytic bottom-up growth paradigm employing innovative catalysts and precursors. Optical characterization by means of temperature dependent photoluminescence is used to identify transition point from indirect to direct badgap of GeSn nanowires.
In this work, we study the optical properties and emission dynamics of the novel nanostructure p-GaAs nanopillars (NPs) on Si. The integration of III-V optoelectronics on Si substrates is essential for next-generation high-speed communications. NPs on Si are good candidates as gain media in monolithically integrated small-scale lasers on silicon. In order to develop this technology, an in-depth knowledge of the NP structure is necessary to resolve its optimal optical properties.
The optical characterization which has been carried out consists of the emission analysis for different NP geometries. We measured NPs with different combinations of pitch (of the order of a few μm) and diameter (of the order of tens of nm). A comparison of intensities for the various NPs provides us with the most efficient geometry. The quality of the crystal grown has been studied from temperature-dependent photoluminescence (PL). A red shift and a significant reduction of the intensity of the NP emission are observed with an increase in temperature. The results also show the presence of two non-radiative recombination channels when the intensity peaks at different temperatures are analyzed with the activation energy function.
In this work we study Ge structures grown on silicon substrates. We use photoluminescence and photoreflectance to determine both direct and indirect gap of Ge under tensile strain. The strain is induced by growing the Ge on an InGaAs buffer layer with variable In content. The band energy levels are modeled by a 30 band k·p model based on first principles calculations. Characterization techniques show very good agreement with the calculated energy values.
A new material pairing is presented for the realisation of sub-wavelength graings in this work and has been used to realise high contrast gratings which operate at wavelengths of 10 μm and greater. The chosen material pairing overcomes the absorption issue which prevents the popular Si/SiO2 pairing from being useful at wavelengths above 6 μm. The obstacles that exist with the currently used grating materials for this wavelength range are described and it is outlined how the chosen materials overcome these issues. It is numerically demonstrated that gratings utilising these materials are capable of wideband high reflectivity. The gratings were fabricated using standard optical photolithography only and it is shown experimentally that the spectral response of gratings which were fabricated show good agreement with theoretically predicted performance
In this work we study Ge transistor structures grown on silicon substrate. We use photoluminescence to determine the band gap of Ge under tensile strain. The strain is induced by growing Ge on an InGaAs buffer layer with variable In content. The band energy levels are modeled using a 30 band k·p model based on first principles calculations. Photoluminescence measurements show a reasonable correspondence with calculated values of the band energies.
Tunable tensile-strained germanium (epsilon-Ge) thin films on GaAs and heterogeneously integrated on silicon (Si) have been demonstrated using graded III-V buffer architectures grown by molecular beam epitaxy (MBE). epsilon-Ge epilayers with tunable strain from 0% to 1.95% on GaAs and 0% to 1.11% on Si were realized utilizing MBE. The detailed structural, morphological, band alignment and optical properties of these highly tensile-strained Ge materials were characterized to establish a pathway for wavelength-tunable laser emission from 1.55 μm to 2.1 μm. High-resolution X-ray analysis confirmed pseudomorphic epsilon-Ge epitaxy in which the amount of strain varied linearly as a function of indium alloy composition in the InxGa1-xAs buffer. Cross-sectional transmission electron microscopic analysis demonstrated a sharp heterointerface between the epsilon-Ge and the InxGa1-xAs layer and confirmed the strain state of the epsilon-Ge epilayer. Lowtemperature micro-photoluminescence measurements confirmed both direct and indirect bandgap radiative recombination between the Γ and L valleys of Ge to the light-hole valence band, with L-lh bandgaps of 0.68 eV and 0.65 eV demonstrated for the 0.82% and 1.11% epsilon-Ge on Si, respectively. The highly epsilon-Ge exhibited a direct bandgap, and wavelength-tunable emission was observed for all samples on both GaAs and Si. Successful heterogeneous integration of tunable epsilon-Ge quantum wells on Si paves the way for the implementation of monolithic heterogeneous devices on Si.
In this work, the optical properties and emission dynamics of core-shell InGaAs/GaAs nanopillars (NPs) have been in-
vestigated using low-temperature photoluminescence (PL) and time-resolved photoluminescence (TRPL). These novel
structures have recently attracted much interest within the silicon photonics scientific community due to their potential
employment as gain medium for monolithically integrated lasers on silicon substrates. The optimization of the emission
properties of these heterostructures is essential to obtain full compatibility with silicon photonics and requires an accurate
tailoring of the pillar geometry (i.e. size, pitch) and composition. Therefore it is critical to gain deeper insight into the
optical and dynamical properties of different NP designs if optimal device performance is to be achieved. The experimental
characterization, carried out on a number of different NP structures with different geometries and compositions, shows that
the time evolution of the emission peak exhibits a strong excitation-dependent blue-shift which can be attributed to the
band-filling effect. Measured emission decay times were strongly geometry-dependent and varied from nanoseconds to
tens of picoseconds. In addition, a dramatic reduction of the decay time was observed for the highest indium concentration
due to the dominant contribution of the strain-induced non-radiative recombination processes.
We report on the development of monolithic two-section dilute nitride passively mode-locked ridge-waveguide lasers.
The dilute nitride material system can cover a wide wavelength range from 1.2 μm to 1.6 μm, while enabling fabrication
on low-cost GaAs substrates. The laser structure comprised 3 GaInNAs quantum wells embedded within GaAs
waveguide and AlGaAs claddings. To achieve mode-locking at 40 GHz repetition rate the laser chips consisted of a 950
μm long gain section and a 90 μm long reverse biased absorber section with a ridge width of 3.5 μm. The mode-locked
laser output exceeded 3 mW per as-cleaved facet with 80 mA current in the gain region and a reverse voltage of 3.8 V
applied to the saturable absorber. The corresponding pulse width was 3.4 ps.
To study the effect of increasing the number of N-related recombination traps present in the proximity of the quantum
wells, we have compared the performance of lasers employing GaAsN or GaAs as quantum well barriers. Time-resolved
photoluminescence measurements revealed that the material comprising GaAsN barriers exhibited a photoluminescence
lifetime of 12 ps with a reverse bias of 5 V. For similar reverse bias, the photoluminescence lifetime for material
comprising GaAs barriers was 108 ps.
High brightness, high power, semiconductor lasers have many potential applications such as: free space communications,
printing, material processing, pumping etc [1]. Such applications require lasers, which are characterized by reliability
and long lifetime.
Catastrophic optical mirror damage (COMD) process is one of the major mechanisms, which drastically limits laser
lifetime and emitted optical power [2]. Mirror degradation and eventually destruction of lasers is caused by facet heating
due to nonradiative surface recombination of carriers. Facet heating reduces the band gap energy, consequently
increasing the absorption coefficient at the facet. The absorbed light and photo-induced electron-hole pair are increased
by the increase in the absorption coefficient. Both effects lead to further nonradiative recombination of carriers which
induces heating and so on, up to degradation of mirror or even destruction of laser. We see that this effect is very
undesirable and knowledge of the temperature dissipation on the surface is very important for improving
semiconductor lasers design. In this work we present the analysis of temperature distribution at the front facet of the
broad area GaAsP/AlGaAs lasers by means of micro-Thermoreflectance (μTR) Spectroscopy.
Several methods proved to be useful in determining the temperature of the laser surface. These are micro-probe band-to-band
photoluminescence, thermoreflectance spectroscopy and Raman spectroscopy [2, 3, 4, 5]. We have used μTR
because it is contactless, non-destructive technique which enables us to obtain temperature distribution in real time.
We have developed resonant-cavity light emitting diodes (RC LED) with very good emission characteristics. RC LEDs proved to be more tolerant to the epitaxial growth parameters and device fabrication procedures. As relatively robust devices they are less sensitive to typical for VCSEL manufacturing challenges and seem to have great potential for applications. Comparing to classical LED the spectrum of RC LED is concentrated into a narrow line with less than 2 nm halfwidth. The RC LED spectrum is determined mainly by the cavity resonance; its width decreases with the increase of the cavity finesse and the intensity increase reflect the on axis cavity enhancement. Additional, favorable RC LED property is its emission characteristic directionality which depends on the tuning between QW emission and cavity resonance. We have optimized the series resistance of diodes. The best results have been obtained for digital alloy graded distributed Bragg reflector (DRB) interfaces. The MBE grown structures were tested extensively prior to the device fabrication by reflectivity and photoluminescence. The assembled diodes were subjected to electrical and optical tests. Generally we have found very good correlation between the results of optical test (PL maps) on as grown wafers and probe tests on final devices.
Elastic Recoil Detection and Rutherford Backscattering methods were used to obtain the depth profiles of hydrogen, oxygen and silicon atoms in porous Si layers on Si substrates. Large values of the oxygen concentration suggest a high oxidation degree of the specific surface of porous Si. It supports our earlier hypothesis, based on experimental evidence, that a formation of silicon oxidation oxides in porous layers is essential to the blue emission of light from this material.
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