Microdisk Structures Research Articles

Wetting of Cell Aggregates on Microdisk Topography Structures Achieved by Maskless Optical Projection Lithography.

Cell aggregates as a 3D culture model can effectively mimic the physiological processes such as embryonic development, immune response, and tissue renewal in vivo. Researches show that the topography of biomaterials plays an important role in regulating cell proliferation, adhesion, and differentiation. It is of great significance to understand how cell aggregates respond to surface topography. Herein, microdisk array structures with the optimized size are used to investigate the wetting of cell aggregates. Cell aggregates exhibit complete wetting with distinct wetting velocities on the microdisk array structures of different diameters. The wetting velocity of cell aggregates reaches a maximum of 293µmh-1 on microdisk structures with a diameter of 2µm and is a minimum of 247µmh-1 on microdisk structures of 20µm diameter, which suggests that the cell-substrates adhesion energy on the latter is smaller. Actin stress fibers, focal adhesions (FAs), and cell morphology are analyzed to reveal the mechanisms of variation of wetting velocity. Furthermore, it is demonstrated that cell aggregates adopt climb and detour wetting modes on small and large-sized microdisk structures, respectively. This work reveals the response of cell aggregates to micro-scale topography, providing guidance for better understanding of tissue infiltration.

An investigation on the effects of geometrical parameters on the sensitivity of micro-disk and micro-ring plasmonic sensors

Geometrical parameters of coupled plasmonic resonators that consist of micro-disk and micro-ring structures have been investigated numerically, and changes in the Q factor, mode volume, and Purcell coefficient of the investigated structure have been calculated according to changes in geometrical variables. Optimal geometrical values for achieving plasmonic modes have been found using simulation results. Additionally, the performance of a single micro-disk, the single micro-ring, and two coupled micro-ring structures as a refractive index sensor has been compared and reported. It has also been shown that although the general shape of the figure of merit (FoM) graph is the same for all tested structures, the FoM of the disk resonator achieves lower values compared to ring and coupled resonators.

Optical characterisation of InGaN-based microdisk arrays with nanoporous GaN/GaN DBRs

Optically pumped whispering gallery mode (WGM) lasing has been observed in many freestanding microdisk structures. Dry etching is normally used to fabricate the microdisks, which causes severe sidewall damage, resulting in degradation of lasing performance, especially for ultra-small electrically-injected devices. In this paper, we demonstrate high quality microdisk cavities with 3.5 µm diameter, by combining a selective overgrowth approach and an epitaxial lattice-matched distributed Bragg reflector (DBR), topped with a highly reflective (>99%) dielectric DBR. InGaN polaritons are found to occur in the high-quality microcavities. WGM modes are measured, with the positions in good agreement with finite difference time domain simulations. Furthermore, lasing behaviour is observed with a threshold at 410 µW and a dominant mode at 488 nm.

Comparison of lasing characteristics of GaN microdisks with different structures

The lasing characteristics of optically-pumped GaN microdisks of different configurations, including microdisks with undercuts, microdisks with cladding layers and thin-film microdisks are investigated in this paper. The microdisks, fabricated from a range of epitaxial structures containing blue-light emitting InGaN/GaN multi-quantum wells grown on Si, sapphire or GaN substrates, undergo different processes to form 8 µm diameter whispering-gallery mode microdisks with different degrees of optical confinement. The microdisks have lasing thresholds ranging from 2.1–8.3 mJ cm−2 and quality factors of 1400–4200. The lasing characteristics are correlated to the material qualities, optical confinement as well as the overlap of the mode with the multi-quantum wells in the microdisk structures. The undercut microdisks benefit from high optical confinement factors but poor overlap factor, while the thin film structures have high overlap factors but low confinement due to absorption by the metallic bonding layers. The findings provide useful insight on ways to optimize GaN microdisk for improving lasing performances.

Label-free detection of virus-like particles employing rotationally symmetric nanowire array based whispering gallery and quasi-whispering gallery resonant modes onto a silicon platform.

In spite of tremendous advancements in modern diagnostics, there is a dire need for reliable, label-free detection of highly contagious pathogens like viruses. In view of the limitations of existing diagnostic techniques, the present theoretical study proposes a novel scheme of detecting virus-like particles employing whispering gallery and quasi-whispering gallery resonant modes of a composite optical system. Whereas whispering gallery mode (WGM) resonators are conventionally realized using micro-disk, -ring, -toroid or spherical structures, the present study utilizes a rotationally symmetric array of silicon nanowires which offers higher sensitivity compared to the conventional WGM resonator while detecting virus-like particles. Notwithstanding the relatively low quality factor of the system, the underlying multiple-scattering mediated photon entrapment, coupled with peripheral total-internal reflection, results in high fidelity of the system against low signal-to-noise ratio. Finite difference time domain based numerical analysis has been performed to correlate resonant modes of the array with spatial location of the virus. The correlation has been subsequently utilized for statistical analysis of simulated test cases. Assuming detection to be limited by resolution of the measurement system, results of the analysis suggest that for only about 5% of the simulate test cases the resonant wavelength shift lies within the minimum detection range of 0.001-0.01 nm. For a single virus of 160 nm diameter, more than 8 nm shift of the resonant mode and nearly 100% change of quality factor are attained with the proposed nanowire array based photonic structure.

Study of Si-Based GeSn Optically Pumped Lasers With Micro-Disk and Ridge Waveguide Structures

Silicon-based monolithic laser has long been desired. Recent demonstration of lasing from direct bandgap group-IV alloy GeSn has opened a completely new venue from the traditional approach of III-V integration on Si. In this paper, high-quality GeSn samples were grown using a multiple-step Sn-enhanced growth recipe with a Sn composition as high as ~20.0%. The GeSn lasers based on waveguide Fabry-Perot and micro-disk cavities have been fabricated and characterized. The ridge waveguide features better local heat dissipation while the micro-disk offers stronger optical confinement as well as strain relaxation. The maximum operating temperature of 260 K from a waveguide laser and a threshold of 108 kW/cm2 at 15 K from a micro-disk laser were achieved. The peak lasing wavelength was obtained up to 3.5 µm with a 100-µm-wide ridge waveguide laser.

Broadband infrared plasmonic metamaterial absorber with multipronged absorption mechanisms.

A broadband plasmonic metamaterial absorber with near perfect multiband absorption in the infrared region is designed using a metal-insulator-metal configuration and fabricated using photolithography. The metal-insulator-metal configuration consists of a Ti microdisk array, a SiO2 insulator spacer, and an Al bottom layer. The multiband absorption occurs with near perfect absorption at 4.8-7.5 µm and 9.7-10.5 µm. Ultra-broadband absorption in the mid-IR wavelength range between 3-14 µm is realized by adding a rough photoresist layer on top of the periodic microdisk structures. The multiband absorption is achieved through the combined mechanisms including plasmonic surface lattice resonance, gap plasmon resonance, Fabry-Perot cavity resonance, and the intrinsic phonon-polariton absorption of SiO2.

GaN-based ultraviolet microdisk laser diode grown on Si

This work reports a demonstration of electrically injected GaN-based near-ultraviolet microdisk laser diodes with a lasing wavelength of 386.3 nm at room temperature. The crack-free laser structure was epitaxially grown on Si substrates using an Al-composed down-graded AlN/AlGaN multilayer buffer to mitigate the mismatches in the lattice constant and coefficient of thermal expansion, and processed into “sandwich-like” microdisk structures with a radius of 12 μm. Air-bridge electrodes were successfully fabricated to enable the device electrical characterization. The electrically pumped lasing of the as-fabricated microdisk laser diodes was evidenced by the rapid narrowing down of electroluminescence spectra and dramatic increase in the light output power, as the current exceeded the threshold of 248 mA.

Realizing Q> 300 000 in diamond microdisks for optomechanics via etch optimization

Nanophotonic structures in single–crystal diamond (SCD) that simultaneously confine and co-localize photons and phonons are highly desirable for applications in quantum information science and optomechanics. Here we describe an optimized process for etching SCD microdisk structures designed for optomechanics applications. This process allows the optical quality factor, Q, of these devices to be enhanced by a factor of 4 over previous demonstrations to Q ∼ 335 000, which is sufficient to enable sideband resolved coherent cavity optomechanical experiments. Through analysis of optical loss and backscattering rates, we find that Q remains limited by surface imperfections. We also describe a technique for altering microdisk pedestal geometry which could enable reductions in mechanical dissipation.

Mid-infrared light emission > 3 µm wavelength from tensile strained GeSn microdisks.

GeSn alloys with Sn contents of 8.4 % and 10.7 % are grown pseudomorphically on Ge buffers on Si (001) substrates. The alloys as-grown are compressively strained, and therefore indirect bandgap. Undercut GeSn on Ge microdisk structures are fabricated and strained by silicon nitride stressor layers, which leads to tensile strain in the alloys, and direct bandgap photoluminescence in the 3-5 µm gas sensing window of the electromagnetic spectrum. The use of pseudomorphic layers and external stress mitigates the need for plastic deformation to obtain direct bandgap alloys. It is demonstrated, that the optically pumped light emission overlaps with the methane absorption lines, suggesting that GeSn alloys are well suited for mid-infrared integrated gas sensors on Si chips.

Excitonic lasing of strain-free InP(As) quantum dots in AlInAs microdisk

Formation, emission, and lasing properties of strain-free InP(As)/AlInAs quantum dots (QDs) embedded in AlInAs microdisk (MD) cavity were investigated using transmission electron microscopy and photoluminescence (PL) techniques. In MD structures, the QDs have the nano-pan-cake shape with the height of ∼2 nm, the lateral size of 20–50 nm, and the density of ∼5 × 109 cm−2. Their emission observed at ∼940 nm revealed strong temperature quenching, which points to exciton decomposition. It also showed unexpected type-I character, indicating In-As intermixing as confirmed by band structure calculations. We observed lasing of InP(As) QD excitons into whispering gallery modes in MD having the diameter of ∼3.2 μm and providing a free spectral range of ∼27 nm and quality factors up to Q∼13 000. Threshold of ∼50 W/cm2 and spontaneous emission coupling coefficient of ∼0.2 were measured for this MD-QD system.

Doping Effects in Direct Bandgap GeSn Light Emitter

N-type doping is necessary for many applications in Ge and GeSn optoelectronics. In recent years the need for energy efficient electronic integrated circuits has led to the exploration of different approaches in order to bring light emitters on chips. Group-IV lasers are being considered, along with heterogeneous integration and flip-chip bonding. To overcome the lack of direct bandgap in Ge, options like injecting high amounts of tensile strain and/or n-type doping were investigated, aiming to achieve a sufficiently large electron population in the Γ-valley to support laser action. Investigations of highly n-doped Ge resulted in strong luminescence enhancement and in an optically pumped laser action. Alternatively, it has been shown that alloying Sn and Ge changes the electronic band structure so that the conduction band at Γ decreases below the L-valley, resulting in a transition from an indirect to a fundamental direct bandgap semiconductor. Optically pumped GeSn lasers were demonstrated, but their operation is still limited to temperatures of about 130 K. The naturally appearing question is whether n-doping can improve the laser performance of these direct bandgap materials, which still have a low directness (the separation between Γ and L valleys is typically 0-50 meV). Calculations indicate an increase of net gain up to a certain doping concentration, beyond which it saturates and then decreases. Here, we concentrate on the experimental study of n-type doping in direct bandgap Ge0.875Sn0.125 alloys. A series of samples with different doping concentrations, in the order of 1018 cm-3, were grown via reactive gas source epitaxy on Si(100) wafers buffered by a Ge virtual substrate. Digermane (Ge2H6), Tin tetrachloride (SnCl4) and phosphine (PH3) were used as precursors. The material is analyzed, regarding crystalline quality, strain and composition, by transmission electron microscopy (TEM), X-ray diffraction reciprocal space mapping (XRD-RSM) and Rutherford Backscattering Spectrometry (RBS). Photoluminescence under a chopped 532 nm laser excitation shows the influence of increased doping concentration on the material quality, regarding carrier recombination due to defects. The optical analysis of the bulk material also shows a very slight change in the bandgap, which can be attributed to bandgap narrowing caused by increasing doping concentration. The impact of the doping concentration is further studied in microdisk resonator lasers. The microdisks are defined by electron beam lithography, and mesas are etched using reactive ion etching (RIE) by Cl2/Ar plasma. To relax the strain of the GeSn layer, and to increase the optical mode confinement, the disks are under-etched by an isotropic CF4 plasma. Finally the structures are passivated with a 10 nm thick Al2O3layer deposited via atomic layer deposition (ALD). The advantage of using microdisk structures is that the deep under-etching of the GeSn disks results in the full strain relaxation of the active layer, eliminating the adverse effect of compressive strain on the band structure. Photoluminescence/laser studies were performed using an excitation with a pulsed Nd:YAG laser at 1064 nm wavelength, and a pulse width of 5 ns. Power and temperature dependence of the laser emission were studied. Figure 1 shows normalized photoluminescence from bulk GeSn and from an under-etched microdisk with a diameter of 8 µm made of n-doped GeSn. The lasing spectrum shows the contribution of multiple modes inside the disk. Undercutting the disks relaxes the strain, resulting in a decreased bandgap leading to a redshift of the laser peak. The photoluminescence peak energies depend on various effects: the strain in the material, the band filling due to high excitation densities and doping, and finally the doping-induced bandgap narrowing, while the laser emission peak position is also influenced by the free-carrier contribution to the refractive index. All these effects will be discussed, based on the systematic analysis of the above described material and laser structures. Additionally, calculations are used to investigate the influence of doping on optical gain. With L-valleys sitting only slightly above, and having a much larger density of states than the Γ valley, the majority of the photo-generated electrons will populate the L valleys if no n-type doping is employed. Only a fraction of electrons will therefore contribute to gain. This can be improved by n-doping, (e.g. by filling the L valleys), as already used in Ge lasers (mandatory due to the indirect bandgap, but also useful in cases of moderate directness). However, additional electrons also increase the free-carrier absorption, and the optimal doping will be a tradeoff between the two effects. Figure 1

Engineering Large In-Plane Tensile Strains in Ge Microdisks, Microrings and Racetrack Optical Cavities

Highly strained Ge micro-cavities are demonstrated, with biaxial equivalent tensile strains > 2 % at the top surface, demonstrating photoluminescence up to the detector cut-off of 2.5 µm wavelength. Ge on Si micro-disk, rings, and racetrack structures are investigated, which have been undercut by wet etching techniques. The anisotropy of the wet etch in the Si leads to the realization of novel, partially suspended structures. The strain distributions in the different cavity structures are discussed and compared with regard to the in-plane uniformity, peak strain level, and effect of the strain distribution on the Ge band-structure.

Temperature characteristics of epitaxially grown InAs quantum dot micro-disk lasers on silicon for on-chip light sources

Temperature characteristics of optically pumped micro-disk lasers (MDLs) incorporating InAs quantum dot active regions are investigated for on-chip light sources. The InAs quantum dot MDLs were grown on V-groove patterned (001) silicon, fully compatible with the prevailing complementary metal oxide-semiconductor technology. By combining the high-quality whispering gallery modes and 3D confinement of injected carriers in quantum dot micro-disk structures, we achieved lasing operation from 10 K up to room temperature under continuous optical pumping. Temperature dependences of the threshold, lasing wavelength, slope efficiency, and mode linewidth are examined. An excellent characteristic temperature To of 105 K has been extracted.

Analysis of Ge micro-cavities with in-plane tensile strains above 2.

Ge on Si micro-disk, ring and racetrack cavities are fabricated and strained using silicon nitride stressor layers. Photoluminescence measurements demonstrate emission at wavelengths ≥ 2.3 μm, and the highest strained samples demonstrate in-plane, tensile strains of > 2 %, as measured by Raman spectroscopy. Strain analysis of the micro-disk structures demonstrate that shear strains are present in circular cavities, which can detrimentally effect the carrier concentration for direct band transitions. The advantages and disadvantages of each type of proposed cavity structure are discussed.

Magnetic domains in H-mediated Zn0.9Co0.1O microdisk arrays

We have fabricated and studied magnetic domains in the periodic ZnCoO microdisk structures at room temperature with MFM technique. The z-component of the remanent magnetic moment is uniform even though the value is much smaller than the saturation magnetic moment.

6H-SiC microdisk torsional resonators in a “smart-cut” technology

We report on experimental demonstration of high frequency torsional resonators based on microdisk structures enabled by a “smart-cut” 6H-silicon carbide (6H-SiC) technology. Circular microdisks axially supported by pairs of thin tethers, with diameters of ∼5–15 μm, exhibit torsional-mode micromechanical resonances with frequency of ∼1–60 MHz, and quality (Q) factors up to 1280 at room temperature in moderate vacuum (∼10 mTorr). Measured intrinsic thermomechanical vibrations of a microdisk with diameter d ≈ 15.9 μm (and triangular cross-section tethers with width wT ≈ 1.5 μm, length LT ≈ 2 μm, and thickness tT ≈ 0.4 μm) demonstrate a torque resolution of ST1/2 ≈ 3.7 × 10−20 (N m)/√Hz, a force sensitivity of SF1/2 ≈ 5.7fN/√Hz, and an angular displacement sensitivity of Sθ1/2 ≈ 4.0 × 10−8 rad/√Hz. By examining devices with varying disk size, different tether shape, width, and length, and by combining experimental data and theoretical calculations, we depict the scaling pathways for ultrasensitive torsional resonant sensors based on this smart-cut 6H-SiC platform.

Growth of GaN Columns and Microstructures on (111) Si by Using Ge Layer via Metal Organic Chemical Vapor Deposition

We demonstrate the successful growth of GaN columnar and microdisk structures on a (111) Ge/Si substrate via metal organic chemical vapor deposition. X-ray diffraction and energy disperse spectroscopy results reveal that the well aligned GaN nanocolumnar structures are grown on a Si substrate by using the Ge layer. The relatively higher growth temperature induces a change in surface morphology of GaN from columnar to microdisk structures. The possible growth mechanism of columnar structured GaN was considered from the Ga-metal/Ge eutectic materials system.

Lasing properties of InP/(Ga0.51In0.49)P quantum dots in microdisk cavities

We investigated the photoluminescence from InP quantum dots incorporated in (Ga${}_{0.51}$In${}_{0.49}$)P microdisk structures. With increasing pump power we observe a transition to stimulated emission indicated by the S shape of the input-output curve. This transition is accompanied with a concentration of the emission to one or a few modes exhibiting quality factors on the order of ${10}^{4}$ at transparency. Time-resolved measurements show that at the same time the photoluminescence decay time considerably decreases. Furthermore, in the transition regime the linewidth of the lasing mode is reduced and the second-order photon correlation function exhibits a reduction of fluctuations as previously reported for lasers with InAs quantum dots in photonic crystal, microdisk and micropillar cavities as the gain medium. The experimental findings are compared with the predictions of a microscopic theory.

Near room temperature continuous-wave laser operation from type-I interband transitions at wavelengths beyond 4 μm

Nonradiative Auger recombination has limited room temperature continuous-wave (cw) operation of type-I interband lasers to wavelengths shorter than 3.36 μm. Using IV–VI semiconductor quantum well microdisk structures, near room temperature laser operation at longer wavelengths is achieved. Their active region consists of type-I single quantum wells of PbSe embedded in PbSrSe barriers. Under optical excitation, single mode cw emission at 4.3 μm is demonstrated up to 2 °C. This proves the feasibility of cw-operation of long wavelength interband lasers up to room temperature.