High Optical Excitation Research Articles

Numerical study and design of high-efficiency p-In0.1Ga0.9N/i-GaN/n-GaN heterojunction photodiode

This paper reports the performance of a proposed GaN heterojunction p-i-n photodiode with a p-type In0.1Ga0.9N layer. The simulation results show that, compared with the GaN homojunction p-i-n structure, the GaN heterojunction p-i-n structure performs better on the basis of such simulations as photocurrent density and responsivity. The effects of reverse bias voltage, light intensity and i-GaN absorbing layer thickness are explored. The peak responsivity and the cutoff frequency are found to increase remarkably with increasing reverse bias voltage. The degradation of device responsivity and cutoff frequency at high optical excitation power density are also investigated. By increasing the i-GaN absorbing layer thickness, responsivity increases but cutoff frequency decreases. The maximum responsivity is 0.3 A/W at 0.363 μm under an illumination intensity of 105 W/cm2 and an applied reverse bias voltage of −2 V, while the highest cutoff frequency 12 GHz is achieved at an applied reverse bias voltage of −14 V.

Circular array transducer based-photoacoustic/ultrasonic endoscopic imaging with tunable ring-beam excitation

Photoacoustic/ultrasound endoscopic imaging is regarded as an effective method to achieve accurate detection of intestinal disease by offering both the functional and structural information, simultaneously. Compared to the conventional endoscopy with single transducer and laser spot for signal detection and optical excitation, photoacoustic/ultrasound endoscopic probe using circular array transducer and ring-shaped laser beam avoids the instability brought by the mechanical scanning point-to-point, offering the dual-modality imaging with high accuracy and efficiency. Meanwhile, considering the complex morphological environments of intestinal tracts in clinics, developing the probe having sufficient wide imaging distance range is especially important. In this work, we develop a compact circular photoacoustic/ultrasonic endoscopic probe, using the group of fiber, lens and home-made axicon, to generate relatively concentrated ring-shaped laser beam for 360° excitation with high efficiency. Furthermore, the laser ring size can be tuned conveniently by changing the fiber-lens distance to ensure the potential applicability of the probe in various and complex morphological environments of intestines. Phantom experimental results demonstrate imaging distance range wide enough to cover from 12 mm to 30 mm. In addition, the accessibility of the photoacoustic signals of molecular probes in ex vivo experiments at the tissue depth of 7 mm using excitation energy of 5 mJ has also been demonstrated, showing a high optical excitation efficiency of the probe.

Ultrafast scanning electron microscopy with sub-micrometer optical pump resolution

Ultrafast scanning electron microscopy images carrier dynamics and carrier induced surface voltages using a laser pump electron probe scheme, potentially surpassing all-optical techniques in probe resolution and surface sensitivity. Current implementations have left a four order of magnitude gap between optical pump and electron probe resolution, which particularly hampers spatial resolution in the investigation of carrier induced local surface photovoltages. Here, we present a system capable of focusing the laser using an inverted optical microscope built into an ultrafast scanning electron microscopy setup to enable high numerical aperture pulsed optical excitation in conjunction with ultrafast electron beam probing. We demonstrate an order of magnitude improvement in optical pump resolution, bringing this to sub-micrometer length scales. We further show that temporal laser pump resolution can be maintained inside the scanning electron microscope by pre-compensating dispersion induced by the components required to bring the beam into the vacuum chamber and to a tight focus. We illustrate our approach using molybdenum disulfide, a two-dimensional transition metal dichalcogenide, where we measure ultrafast carrier relaxation rates and induced negative surface potentials between different flakes selected with the scanning electron microscope as well as on defined positions within a single flake.

Two-dimensional WSe2 flakes under high power optical excitation

Quasi-2D layers of transition metal dichalcogenides are promising candidates for creating saturable absorbers for pulsed lasers. However, the peculiarities of intense electromagnetic radiation’s influence on such structures have not been thoroughly studied. This paper explores the dynamics of photoexcited carriers in WSe2 flakes through experimental studies. These studies found that WSe2 flakes significantly change their optical properties under the influence of a high-power optical pump, allowed estimating the thermalization time of these structures (about 2 ps), and found that full relaxation takes more than 10 ps. The concentration of carriers in the semiconductor surface layer was estimated to be about 1028 m–3. It was found that standard description models of the optical response based on exciton resonances and absorption by free carriers could not adequately describe the experiments’ results. Thus, for an accurate description of the optical response, it was necessary to consider the effects associated with Coulomb screening that are caused by the high concentration of photo-excited carriers of the optical pumping densities used in this experiment.

Neodymium-doped germanotellurite glasses for laser materials and temperature sensing

GTSN: Nd (GeO2-TeO2-SrF2-Nb2O5–Nd2O3) glasses of varying (0.2–3%) Nd concentration were obtained using the melt-quenching method. Their thermal properties were evaluated using differential scanning calorimetry, showing highly favorable glass thermal stability. Glass structure was analyzed using XRD, IR and Raman spectroscopy. Combining the advantages of germanate, tellurite and fluoride optical systems, a promising glass laser material was achieved, as proven with optical spectroscopy methods, such as absorption, emission and fluorescence lifetime spectroscopies. Utilizing the fluorescence intensity ratios between emission bands from Nd excited levels, several models for luminescent thermometry were established for GTSN: Nd. One of the models was tested in a laser power-dependence experiment, allowing to estimate temperature increase during high power optical excitation. Great thermal stability and spectroscopic performance of the material, along with a built-in method for contactless temperature measurement, make GTSN: Nd a promising material for glass-based optically active elements.

Nonlinear Optical Absorption in GaSe upon Laser Excitation

The nonlinear absorption in GaSe crystals at high optical excitation levels is experimentally studied. A Nd:YAG laser (first and second harmonics, 1064 and 532 nm) and a liquid laser (tuning range 594–643 nm) were used as excitation sources. It is shown that the features observed in the exciton resonance region of the absorption, luminescence, and photoconductivity spectra of GaSe crystals at high optical excitation levels can be explained by exciton–exciton interaction and Coulomb interaction screening by free carriers.

Synthesis and optical properties study of GaAs epitaxial nanoparticles on silicon

The integration of direct bandgap III-V materials on Si is one of the main tasks on the way to the development of cheap and highly effective optoelectronic devices. The goal of this work is to study the morphology and optical properties of GaAs nanoparticles grown on Si(111) by molecular beam epitaxy (MBE). Nanostructure morphology is studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Optical properties are studied by photoluminescence (PL) and Raman spectroscopy. Interestingly, despite large lattice mismatch between the silicon substrate and GaAs no sufficient change of Raman spectra was observed for both continuous layer and nanoparticles indicating that they are relaxed. The room temperature PL signal in the red spectral range was obtained from the epitaxial structure. It is demonstrated that high pump optical excitation of the nanostructures can lead to sufficient change of the PL signal typical for photo-oxidized GaAs.

The role of intervalley phonons in hot carrier transfer and extraction in type-II InAs/AlAsSb quantum-well solar cells

Simultaneous continuous wave (CW) photoluminescence and monochromatic current density-voltage (J-V) measurements of InAs/AlAsSb QW p-i-n diodes reveal stable hot carriers observed at relatively low power (nearly independent of power). This behavior is attributed to preferential scattering of high energy carriers to the upper satellite L- and X-valleys, which inhibits carrier thermalization via LO phonon emission. Although both high electric field and optical excitation are shown to enable stable hot carrier generation in the quantum wells (QWs), the extraction of these carriers is inhibited by the mismatch in valley degeneracy (L to Γ) across the InAs QW/n-AlInAs layers resulting in carrier localization in the QWs in the operating regime of the solar cell.

Terahertz pulse generation from (111)-cut InSb and InAs crystals when illuminated by 1.55-μm femtosecond laser pulses.

Terahertz (THz) pulse generation from p-InAs, p-InSb, and n-InSb epitaxial layers are investigated using 1.55-μm wavelength femtosecond laser pulses for photoexcitation. The samples are of (111) crystallographic orientation resulting in anisotropic photoconductivity. Experiments have shown that THz generation in InAs is mainly due to anisotropic photocurrent in the surface electric field while a dominant mechanism in InSb is optical rectification. At high optical excitation fluencies, InSb is more efficient than p-InAs. In the presence of an external magnetic field, (111) InSb has exhibited promising viability as an alternative to the photoconductive antenna emitter in a THz time-domain-spectroscopy (THz-TDS) system.

Enhancement of Auger recombination induced by carrier localization in InGaN/GaN quantum wells

The origin of efficiency droop in state-of-the-art quality In GaN/GaN and GaN/AlGaN quantum wells (QWs) grown on various crystal planes is studied by means of time-resolved photoluminescence spectroscopy associated with a precise determination of the QW carrier density. In a first set of experiments, it is shown that a polar InGaN/GaN QW under nonresonant high optical excitation shows clear signatures of Auger loss mechanism and thus behaves quite differently compared to its binary based GaN/AlGaN QW counterpart, where no Auger signature is observed. In order to get rid of the impact of the built-in polarization field and illustrate the dominant role of carrier localization, similar experiments have been conducted on two m-plane In GaN/GaN QWs with similar In composition but a different degree of disorder. We demonstrate that carrier localization strongly enhances the Auger recombination process in nonpolar In GaN/GaN QWs. We also show that this effect may be further amplified by the presence of polarization fields on polar QWs. The relaxation of the k-selection rule during the Auger recombination process, resulting from QW potential disorder, can account for the enhancement of the efficiency droop in In GaN/GaN QWs.

Self-formation of ultrahigh-density (1012 cm−2) InAs quantum dots on InAsSb/GaAs(001) and their photoluminescence properties

InAs quantum dots (QDs) with an ultrahigh density of 1 × 1012 cm−2 were fabricated on a 1.25-monolayer-thick InAsSb wetting layer on a GaAs(001) substrate by molecular beam epitaxy. QD formation was initiated by small two-dimensional InAsSb islands. Coalescence and ripening effects involving neighboring QDs were suppressed. Photoluminescence spectra of the QDs shifted continuously to higher energies with increased optical excitation power. This was attributed to the filling of inhomogeneous ground states via tunneling between QDs. Indirect transitions in a type-II band structure were observed for small QDs. In large QDs, direct transitions were also observed at high optical excitation levels.

Luminescence and Lasing in ZnSe Micropowders at High Optical Excitation Levels

Photoluminescence (PL) of ZnSe wide-bandgap semiconductor micropowder was studied at a high optical excitation level by pulsed nanosecond N2-laser emission. A new emission band that appeared on the long-wavelength edge of the PL spectrum at 40–75 meV from the electron–hole plasma (EHP) band depending on the optical excitation level showed that plasmons could participate in recombination processes in the EHP. Random lasing at 475 nm from submicron-sized crystallites in ZnSe powder was produced by the third harmonic of a YAG:Nd3+ laser with an exciting-radiation threshold intensity of 750 kW/cm2. The lasing manifested as a sharp increase of integrated emission intensity, a narrowing of the spectrum, and the appearance in it of localized and extended mode structure. Random lasing was due to feedback of amplified radiation in closely packed active scattering microcrystallites.

Optical nonlinearities in GaSe and InSe crystals upon laser excitation

The nonlinear absorption of light and its temporal evolution in the vicinity of exciton resonance in layered GaSe and InSe crystals under high optical excitation have been experimentally investigated. The decisive factor for the observed temporal dependence of the absorption coefficient and its dependence on the excitation intensity is screening excitons by nonequilibrium-carrier plasma. It is shown that the increase in the transmittance in the absorption-band edge in GaSe with a simultaneous blue shift of the band edge is caused by filling the energy bands under high optical excitation.

Enhanced Terahertz Bandwidth and Power from GaAsBi‐based Sources

Terahertz photoconductive (PC) switches are popular for THz wave generation. Unlike nonlinear crystals, they don't require high optical excitation power and precise phase matching, but their output THz power is limited. Here, the conventional substrate is replaced with enhanced GaAsBi (growth recipe included) and its performance is compared to that of a LT-GaAs PC switch and a nanoplasmonic PC switch. The results show a 0.5 THz increase in bandwidth and 4-fold improvement in emission power for GaAsBi and GaAsBi nanoplasmonic PC switches, respectively As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

GaN-Based Vertical Cavities with All Dielectric Reflectors by Epitaxial Lateral Overgrowth

GaN-based vertical cavities with all dielectric top and bottom distributed Bragg reflectors (DBRs) on c-plane sapphire were investigated under optical injection and compared with those having AlN/GaN bottom and dielectric top DBRs on freestanding GaN. A novel fabrication method employing two epitaxial lateral overgrowth steps is introduced to produce a cavity on bottom dielectric DBRs without the need to remove the sapphire substrate. Under high optical excitation, the cavity with all dielectric DBRs exhibited quality factors up to 1400 and an order of magnitude lower stimulated emission threshold density (5 µJ/cm2) than those employing top dielectric DBRs but semiconductor AlN/GaN bottom DBRs on freestanding GaN. This novel approach is expected to lead to injection vertical cavity lasers with naturally formed nearly defect-free active regions and current confinement without any oxidation steps.

Optically defined plasmonic waveguides in crystalline semiconductors at optical frequencies

High intensity optical excitation to transform a crystalline semiconductor into a plasmonic metal at near-infrared wavelengths is theoretically investigated. A calculated intensity of 51.46  GW/cm2 is sufficient to transform GaAs into metal at 1.55 μm to support plasmonic modes. A practical nanoscale plasmonic gap waveguide is designed based on the GaAs/GaN materials system, demonstrating the capability of obtaining plasmonic waveguiding by high intensity optical excitation. The propagation characteristics of the plasmonic gap mode in the designed waveguide can be dynamically tuned over a broad range of values by varying the intensity of the pump excitation using modest average powers between 15 and 75 mW.

Lasing Action on Whispering Gallery Mode of Self-Organized GaN Hexagonal Microdisk Crystal Fabricated by RF-Plasma-Assisted Molecular Beam Epitaxy

GaN hexagonal microdisks were fabricated on Ti-mask (4 nm in thickness) nanohole-patterned m-plane (10-10) GaN substrates by radio frequency-plasma-assisted molecular beam epitaxy. The GaN hexagonal microdisks, which were supported by ~300 nm-diameter GaN nanocolumns, consisted of thin hexagonal c-plane plates with a hexagon side length of 1-2 m and a typical thickness of 200 nm. The GaN hexagonal microdisks were optically pumped under a high optical excitation density with a 355-nm-wavelength Nd:YAG laser, and ultraviolet lasing actions on the quasi-whispering gallery mode (WGM) were observed at room temperature. The lasing wavelength was 372 nm and the threshold excitation density was approximately 250 kW/cm2. The quasi-WGM resonance was numerically analyzed using a simple plane wave model and a 2-D-flnite difference time domain method, the behaviors of the WGM and quasi WGM were analyzed, revealing that the quasi-WGM has a higher Q-factor, thus, it was clarified that the laser actions of the GaN hexagonal microdisks occurred on the quasi-WGM resonance.

Optical monitoring of nonequilibrium carrier diffusion in single crystalline CVD and HPHT diamonds under high optical excitation

AbstractWe report on a novel approach for the contactless, all‐optical study of ambipolar carrier diffusion in single‐crystalline diamond layers. Using interband two‐photon and single photon absorption (at 351 nm and 213 nm), we created a spatially‐modulated free‐carrier light‐induced transient grating and monitored the in‐plane carrier diffusion in a wide range of injected carrier densities (1015cm–3 to 1018cm–3) and temperatures (80 K to 800 K). A drastic decrease of the ambipolar diffusion coefficient from ∼50 cm2/s at low injections to 6–10 cm2/s at high ones was observed at room temperature and even stronger at lower temperatures. The modelling based on bandgap renormalization and electron–hole scattering provided a fit to the injection‐dependent diffusivity. The determined low‐injection ambipolar mobility data for CVD layer were found in a good agreement with electrical time‐of‐flight measurements. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of n-p-n heterostructures on interface recombination and semiconductor laser cooling

The design of doped n-p-n semiconductor heterostructures has a significant influence on the structures’ nonradiative decay and can also affect their photoluminescence characteristics. Such structures have recently been explored in the context of semiconductor laser cooling. We present a theoretical analysis of optically excited n-p-n structures, focusing mainly on the influence of the layer thicknesses and doping concentrations on nonradiative interface recombination. We find that high levels of n-doping (1019 cm−3) can reduce the minority-carrier density at the interface and increase the nonradiative lifetime. We calculate time-dependent luminescence decay and find them to be in good agreement with experiment for temperatures >120 K, which is the temperature range in which our model assumptions are expected to be valid. A theoretical analysis of the cooling characteristics of n-p-n structures elucidates the interplay of nonradiative, radiative, and Auger recombination processes. We show that at high optical excitation densities, which are necessary for cooling, the undesired nonradiative interface recombination rates for moderate (1017 cm−3) n-doping concentrations are drastically increased, which may be a major hindrance in the observation of laser cooling of semiconductors. On the other hand, high n-doping concentrations are found to alleviate the problem of increased nonradiative rates at high excitation densities, and for the model parameters used in the calculation we find positive cooling efficiencies over a wide range of excitation densities.

Optically tunable nuclear magnetic resonance in a single quantum dot

We report optically detected nuclear magnetic resonance (ODNMR) measurements on small ensembles of nuclear spins in single GaAs quantum dots. Using ODNMR we make direct measurements of the inhomogeneous Knight field from a photoexcited electron which acts on the nuclei in the dot. The resulting shifts of the NMR peak can be optically controlled by varying the electron occupancy and its spin orientation, and lead to strongly asymmetric line shapes at high optical excitation. The all-optical control of the NMR line shape will enable position-selective control of small groups of nuclear spins inside a dot.