Hexagonal Microdisks Research Articles

Stability of Intensity Imbalance between Left‐ and Right‐Circulation Modes in GaN Hexagonal Microdisk

The mechanisms underlying the stability of the intensity imbalance between the left‐ and right‐circulating lasing modes in a GaN hexagonal microdisk resonator are investigated. Finite‐difference time‐domain simulations reveal that the imbalance is determined by the position of the point light source (seed light for lasing) inside the resonator. The coupling mechanism between the resonance modes and light from the source is clarified using numerical calculations. In addition, it is confirmed that light is highly coupled with one of the circulation modes when the source is placed off the bilateral symmetry axis of the mode near the side of the resonator, without any influence of the polarization of the source. The results indicate that the imbalance of the lasing modes observed in the experiments is determined by the position of the seed light near the side of the resonator, which must be spontaneous emission from impurities or defects, such as donor–acceptor pairs, donor‐bounded excitons, basal plane stacking faults, or the transition of free electrons in the conduction band to acceptor levels et al.

Unique Enhancement of the Whispering Gallery Mode in Hexagonal Microdisk Resonator Array with Embedded Ge Quantum Dots on Si.

The coupling between the quantum dots (QDs) and silicon-based microdisk resonator facilitates enhancing the light-matter interaction for the novel silicon-based light source. However, the typical circular microdisks embedded with Ge QDs still have several issues, such as wide spectral bandwidth, difficult mode selection, and low waveguide coupling efficiency. Here, by a promising structural modification based on the mature nanosphere lithography (NSL), we fabricate a large area hexagonal microdisk array embedded with Ge QDs in order to enhance the near-infrared light emissions by a desired whispering gallery modes (WGMs). By comparing circular microdisks with comparable sizes, we found the unique photoluminescence enhancement effect of hexagonal microdisks for certain modes. We have confirmed the WGMs which are supported by the microdisks and the well-correlated polarized modes for each resonant peak observed in experiments through the Finite Difference Time Domain (FDTD) simulation. Furthermore, the unique enhancement of the TE5,1 mode in the hexagonal microdisk is comparatively analyzed through the simulation of optical field distribution in the cavity. The larger enhanced region of the optical field contains more effectively coupled QDs, which significantly enhances the PL intensity of Ge QDs. Our findings offer a promising strategy toward a distinctive optical cavity that enables promising mode manipulation and enhancement effects for large-scale, cost-effective photonic devices.

Effect of temperature on the formation of highly crystalline lead-free perovskite Cs3Sb2Br9 hexagonal microdisks

Effect of temperature on the formation of highly crystalline lead-free perovskite Cs3Sb2Br9 hexagonal microdisks

Excitonic properties of layer-by-layer CVD grown ZnO hexagonal microdisks

We have investigated the excitonic properties of highly crystalline ZnO hexagonal microdisks grown by the chemical vapour deposition technique. It was observed that a suitable negative catalyst like chlorine suppresses the crystal growth along the (0001) direction. We propose a qualitative model for the experimentally observed layer-by-layer growth mechanism of the microdisks. Room temperature photoluminescence of the microdisks manifests a very high near-band-edge (NBE) emission peak in the UV region and a minor defect peak in the visible region. The excitonic emission of the microdisks was studied using the low-temperature photoluminescence down to 83 K, which reveals a surface exciton peak in the NBE region and well fitted higher-order phonon replicas.

InP-on-Si Optically Pumped Microdisk Lasers via Monolithic Growth and Wafer Bonding

On-chip optical light sources are key components in photonic integrated circuits and optical communication. In this paper, we use a novel integration technique called template-assisted selective epitaxy (TASE) to monolithically integrate InP microdisk lasers on silicon. TASE offers several advantages for new device concepts such as lateral doping, dense co-integration of different III–V materials, and in-plane integration with silicon electronics and passive components. Here, we demonstrate room-temperature lasing from InP hexagonal microdisks integrated via TASE. In order to assess and evaluate the viability of TASE, a second InP hexagonal microdisk sample is prepared for comparison using the highly developed and mature direct wafer bonding technique. The lasing performance of the TASE monolithic devices and the bonded microdisk devices is investigated under pulsed optical pumping as a function of temperature and compared. The lasing threshold as well as the light-in light-out curves of our TASE structures compare favorably with the bonded InP hexagonal microdisks. This demonstrates that our TASE approach is a promising technique for the monolithic integration of optical devices on Si.

Electroluminescence from n-ZnO microdisks/p-GaN heterostructure

The light emitting diode structure based on p-GaN film/n-ZnO microdisks quasiarray heterojunction was fabricated. It is shown that the epitaxial quality of p-GaN films upon growth from the vapor phase can lead to growth of the hexagonal microdisk ZnO rather than the vertical nanorods. The density of the microdisks changed across the substrate surface. ZnO hexagonal microdisks are characterized by the average height of about 5 μm with diameters ranging from 25 μm up to 60 μm. The turn-on voltage of the heterojunction of ZnO/GaN (disks/film) is around 5 V. The diode-ideality factor was estimated to be of around 30. The large values of the ideality factors indicate a high density of trap states and also may be connected with the quality of the contacts to the p–n junction. The electroluminescence (EL) spectrum acquired from the p-GaN film/n-ZnO microdisks junction exhibited the bands with maxima at 366, 394 and 495 nm. On the basis of the data of X-ray diffraction, electrical and optical studies these peaks were associated with GaN near-band-edge (NBE) emission, ZnO NBE emission, and emission from the defect levels in ZnO, respectively.

Hydrothermal synthesis and luminescent properties of Y2O3:Eu3+ from waste phosphors

Abstract Europium-doped yttrium oxide (Y2O3:Eu3+) phosphors were successful synthesized by hydrothermal method from waste phosphors in the presence of sodium citrate. The synthesized samples were characterized by X-ray diffraction, scanning electron microscopy and photoluminescence spectroscopy. The experimental results showed that the morphology and photoluminescent properties of the obtained samples had a strong dependence on the concentrate of sodium citrate, the addition amount of NaOH and Eu in the hydrothermal process. Well-dispersed Y2O3:Eu3+ hexagonal microdisk with average diameter of 2 μm were synthesized in the presence of 6 mol% Eu, 37.5 mmol NaOH, molar ratios of Cit3−:Y3+ = 1:1. The photoluminescence properties measurement reveled that Y2O3:Eu3+ phosphors showed a strong red emission corresponding to 5D0–7F2 (613 nm), suggesting promising application in optoelectronics.

Facile synthesis and emission enhancement in NaLuF4 upconversion nano/micro-crystals via Y3+ doping

A series of Y3+-absent/doped NaLuF4:Yb3+, Tm3+ nano/micro-crystals were prepared via a hydrothermal process with the assistance of citric acid. Cubic nanospheres, hexagonal microdisks, and hexagonal microprisms can be achieved by simply adjusting the reaction temperature. The effect of Y3+ doping on the morphology and upconversion (UC) emission of the as-prepared samples were systematically investigated. Compared to their Y3+-free counterpart, the integrated spectral intensities in the range of 445–495 nm from α-, β-, and α/β-mixed NaLuF4:Yb3+, Tm3+ crystals with 40 mol% Y3+ doping are increased by 9.7, 4.4, and 24.3 times, respectively; red UC luminescence intensities in the range of 630–725 nm are enhanced by 4.6, 2.4, and 24.9 times, respectively. It is proposed that the increased UC emission intensity is mainly ascribed to the deformation of crystal lattice, due to the electron cloud distortion in host lattice after Y3+ doping. This paper provides a facile route to achieve nano/micro-structures with intense UC luminescence, which may have potential applications in optoelectronic devices.

Synthesis of hexagonal ZnO microdisk on InN substrate by aqueous method and its optical properties

Hexagonal ZnO microdisks were obtained by using InN/sapphire as substrate via the aqueous method. The growth mechanism of the ZnO microdisks was proposed and demonstrated. Moreover, the optical properties of the ZnO microdisks were investigated by micro-photoluminescence, whispering gallery mode (WGM) lasing from the microdisk was observed at moderate excitation density. It will provide a new way to design microlasers by using low temperature method.

Self-Assembled Microdisk Lasers of Perylenediimides.

Organic solid-state lasers (OSSLs) have been a topic of intensive investigations. Perylenediimide (PDI) derivatives are widely used in organic thin-film transistors and solar cells. However, OSSLs based on neat PDIs have not been achieved yet, owing to the formation of H-aggregates and excimer trap-states. Here, we demonstrated the first PDI-based OSSL from whispering-gallery mode (WGM) hexagonal microdisk (hMD) microcavity of N,N'-bis(1-ethylpropyl)-2,5,8,11-tetrakis(p-methyl-phenyl)-perylenediimide (mp-PDI) self-assembled from solution. Single-crystal data reveal that mp-PDI molecules stack into a loosely packed twisted brickstone arrangement, resulting in J-type aggregates that exhibit a solid-state photoluminescence (PL) efficiency φ > 15%. Moreover, we found that exciton-vibration coupling in J-aggregates leads to an exceptional ultrafast radiative decay, which reduces the exciton diffusion length, in turn, suppresses bimolecular exciton annihilation (bmEA) process. These spectral features, plus the optical feedback provided by WGM-hMD microcavity, enable the observation of multimode lasing as evidenced by nonlinear output, spectral narrowing, and temporal coherence of laser emission. With consideration of high carrier-mobility associated with PDIs, hMDs of mp-PDI are attractive candidates on the way to achieve electrically driven OSSL.

Controlling the morphology and properties of solvothermal synthesized Cu2ZnSnS4 nanoparticles by solvent type

The copper–zinc–tin sulfide Cu2ZnSnS4 (CZTS) semiconductors are recently considered as one of the favorable materials for application as absorber layers in solar cells due to their appropriate direct band gap energy and high optical absorption coefficient. In this study, the effect of solvent type on properties of solvothermal synthesized CZTS nanoparticles has been investigated. Ethylenediamine (EDA), triethylenetetramine (TETA), ethylene glycol (EG), and oleic acid (OA) have been used as the solvent. X-ray diffraction technique and Raman spectroscopy confirmed the formation of crystalline CZTS nanoparticles with kesterite crystal structure in these solvents with the exception of EDA, which forms wurtzite crystal structure. Morphological characterizations show that several distinct morphologies including spherical (70–160nm), nanoplates (∼45nm thickness and more than 1μm length), peculiar flower-like particles (with diameter of ∼0.4–1.5μm), truncated hexagonal disks, irregular particles, and hexagonal microdisks are obtained by varying the solvent type. Optical studies revealed broad absorption of the CZTS particles in the visible region. Compared with other solvents, OA synthesized CZTS particles show higher absorption in the visible region. However, CZTS nanoparticles synthesized by TETA solvent show the most appropriate properties for application as an absorber materials in solar cells due to high crystallinity, low impurity phases, suitable size, and proper band gap energy.

Magnetite Fe3O4 nanoparticles and hematite α-Fe2O3 uniform oblique hexagonal microdisks, drum-like particles and spindles and their magnetic properties

Magnetite Fe3O4 nanoparticles and uniform hematite (α-Fe2O3) oblique hexagonal microdisks, drum-like particles, and spindles have been synthesized via a facile hydrothermal reaction route, in which the mixture solvents of ethylene glycol (EG) and water are used as reaction medium. The phase, size, shape and growth orientation of the synthesized iron oxide crystals were characterized by powder X-ray diffraction and electron microscopy. When the reaction medium is almost composed of EG, a lot of Fe3+ ions reduce to Fe2+ ions due to the effect of EG, resulting in the magnetite Fe3O4 nanoparticles. As the volume ratio of EG/water in the reaction medium is lower than 30:10, the reductive ability of EG is too low to reduce the Fe3+ ions to Fe2+ ions, leading to the hematite α-Fe2O3 crystals. Moreover, since the adsorption of EG on the crystals, the shape of the obtained hematite α-Fe2O3 crystals evolves from oblique hexagonal microdisks to drum-like particles, and spindles due to the decrease of EG in the mixture reaction medium solvent. The magnetic properties of the magnetite Fe3O4 nanoparticles and hematite α-Fe2O3 uniform microcrystals were also investigated by measuring the magnetic hysteresis loops.

Whispering‐Gallery‐Mode Microlaser Based on Self‐Assembled Organic Single‐Crystalline Hexagonal Microdisks

AbstractWhispering‐gallery‐mode (WGM) resonators of semiconductor microdisks have been applied for achieving low‐threshold and narrow‐linewidth microlasers, but require sophisticated top‐down processing technology. Organic single‐crystalline hexagonal microdisks (HMDs) of p‐distyrylbenzene (DSB) self‐assembled from solution can function as WGM microresonators with a cavity quality factor (Q) of 210. Both multiple‐ and single‐mode lasing had been achieved using DSB HMDs with an edge length of 4.3 and 1.2 μm, respectively. These organic microdisks fabricated by bottom‐up self‐assembly approach may offer potential applications as low‐threshold microlaser sources for photonic circuit integration.

Whispering-gallery-mode microlaser based on self-assembled organic single-crystalline hexagonal microdisks.

Whispering-gallery-mode (WGM) resonators of semiconductor microdisks have been applied for achieving low-threshold and narrow-linewidth microlasers, but require sophisticated top-down processing technology. Organic single-crystalline hexagonal microdisks (HMDs) of p-distyrylbenzene (DSB) self-assembled from solution can function as WGM microresonators with a cavity quality factor (Q) of 210. Both multiple- and single-mode lasing had been achieved using DSB HMDs with an edge length of 4.3 and 1.2 μm, respectively. These organic microdisks fabricated by bottom-up self-assembly approach may offer potential applications as low-threshold microlaser sources for photonic circuit integration.

Observation of whispering gallery modes from hexagonal ZnO microdisks using cathodoluminescence spectroscopy

Zinc oxide hexagonal microdisks with diameters ranging from 3 μm up to 15 μm were fabricated by thermal chemical vapour deposition. Optical characterisation of ZnO microdisks was performed using low temperature (80 K) cathodoluminescence (CL) imaging and spectroscopy. The microdisks exhibited green luminescence locally distributed near the hexagonal boundary of the ZnO microdisks. High resolution CL spectra of the ZnO microdisks revealed whispering gallery modes (WGMs) emission. The experimentally observed WGMs were in excellent agreement with the predicted theoretical positions calculated using a plane wave model. This work could provide the means for ZnO microdisk devices operating in the green spectral range.

Controllable fabrication and optical properties of Sn-doped ZnO hexagonal microdisk for whispering gallery mode microlaser

We report a controllable method for fabricating hexagonal Sn doped ZnO microdisks. The photoluminescence mechanism of the Sn doped ZnO microdisks is investigated, the defect emission is attributed to the singly charged oxygen vacancy. Under the excitation of a femtosecond pulsed laser with a wavelength of 325 nm, exciton-exciton collision process is clearly demonstrated, and amplified spontaneous emission is further realized under strong excitation. Using the perfect hexagonal symmetric structure of the Sn doped ZnO microdisks, the whispering-gallery mode lasing with high quality factor and fine mode structure is obtained from a single microdisk.

Bottom‐up engineering of diamond micro‐ and nano‐structures

AbstractEngineering nanostructures from the bottom up enables the creation of carefully sculpted complex structures that are not accessible via top down fabrication techniques, in particular, complex periodic structures for applications in photonics and sensing. In this work a proof of principle that bottom up approach can be adopted and utilized for sculpting devices from diamond is proposed and demonstrated. A realization of periodic structures is achieved by growing nanoscale single crystal diamond through a defined pattern. Optical wave‐guiding of a narrow band emission attributed to the SiV defects in diamond is demonstrated by overgrowth on a thin diamond membrane. In addition, an array of hexagonal microdisks with diameter sizes ranging from 1 to 4 μm is demonstrated. The bottom up approach for diamond opens up new avenues for devices fabrication and sculpting three dimensional structures.

Quasi-Whispering Gallery Mode Lasing Action in an Asymmetric Hexagonal GaN Microdisk

The optical properties of an asymmetric hexagonal GaN microdisk, fabricated via crystal growth by radio-frequency plasma-assisted molecular beam epitaxy (rf-MBE), were investigated by experimental and theoretical methods to clarify the contribution of the light confinement modes of quasi-whispering gallery modes (QWGMs). Under the optically pumped condition, a lasing action was observed from the asymmetric GaN microdisk, indicating that there is a resonant scheme of QWGMs in such a microcavity of a GaN microdisk. In addition, the analysis of the light response of hexagonal GaN microdisk models by a three-dimensional finite difference time-domain method (3D-FDTD) indicates that the QWGMs preferentially contribute to a lasing action even in a microcavity with a regular hexagonal configuration with side lengths of 1 to 2 µm. The results of the experiment and the numerical analysis strongly indicate that it is important to take not only the WGM but also the QWGMs into account to clarify the optical responses of resonant modes in a hexagonal microdisk configuration.

Optically Pumped Lasing Action with Unusual Wavelength of Approximately 390 nm in Hexagonal GaN Microdisks Fabricated by Radio-Frequency Plasma-Assisted Molecular Beam Epitaxy

Hexagonal GaN microdisks exhibiting lasing action with unusual wavelengths of approximately 390 nm under an optically pumped condition have been investigated. The lasing action was caused by the resonant modes of the whispering gallery mode and/or quasi-whispering gallery mode in the hexagonal microdisks. A cross-sectional transmission electron microscopy observation indicated that multiple crystalline boundaries, which are formed by stacking faults, were included in the specific GaN microdisks exhibiting such an unusual lasing action. The origin of the optical gain was discussed, based on the modification of the crystal structure associated with the stacking faults.

Systematical shape evolution of hexagonal NiSe crystals caused by mixed solvents and ammonium chloride

Systematic shape evolution of hexagonal NiSe crystals is realized via a simple solvothermal route in a mixture of NiCl2·6H2O, elemental selenium, hydrazine hydrate and ethylenediamine. By introducing ammonium chloride as electrolyte and varying the volume ratios of hydrazine hydrate to ethylenediamine, shape evolution of hexagonal NiSe crystals from small hexagonal microdisks to hexagonal microdisks in larger width, microspheres, hexagonal prisms and hexagonal bitowers is successfully achieved. X-ray powder diffraction, field emission scanning electron microscopy, energy dispersion spectrometer, transmission electron microscopy and selected area electron diffraction are performed for the analyses of the products. The ionization and hydrolysis of ammonium chloride decrease the nucleation rate of hexagonal NiSe and the diffusion rate of growth resources, while the adsorption of ethylenediamine at {001} facets of hexagonal NiSe crystals inhibits the crystal growth in <001> directions, thus leading to various novel architectures.