Microdisk Lasers Research Articles

Spectral Modulation of Optofluidic Coupled-Microdisk Lasers in Aqueous Media.

We present the spectral modulation of an optofluidic microdisk device and investigate the mechanism and characteristics of the microdisk laser in aqueous media. The optofluidic microdisk device combines a solid-state dye-doped polymer microdisk with a microfluidic channel device, whose optical field can interact with the aqueous media. Interesting phenomena, such as mode splitting and single-mode lasing in the laser spectrum, can be observed in two coupled microdisks under the pump laser. We modulated the spectra by changing the gap of the two coupled microdisks, the refractive indices of the aqueous media, and the position of a pump light, namely, selective pumping schemes. This optofluidic microlaser provides a method to modulate the laser spectra precisely and flexibly, which will help to further understand spectral properties of coupled microcavity laser systems and develop potential applications in photobiology and photomedicine.

Perspective: optically-pumped III–V quantum dot microcavity lasers via CMOS compatible patterned Si (001) substrates

Direct epitaxial growth III–V quantum dot (QD) structures on CMOS-compatible silicon substrates is considered as one of the most promising approaches to achieve low-cost and high-yield Si-based lasers for silicon photonic integration. However, epitaxial growth of III–V materials on Si encounters the following three major challenges: high density of threading dislocations, antiphase boundaries and thermal cracks, which significantly degrade the crystal quality and potential device performance. In this review, we will focus on some recent results related to InAs/GaAs quantum dot lasers on Si (001) substrates by III–V/IV hybrid epitaxial growth via (111)-faceted Si hollow structures. Moreover, by using the step-graded epitaxial growth process the emission wavelength of InAs QDs can be extended from O-band to C/L-band. High-performance InAs/GaAs QD micro-disk lasers with sub-milliwatts threshold on Si (001) substrates are fabricated and characterized. The above results pave a promising path towards the on-chip lasers for optical interconnect applications.

Specific Features of the Current–Voltage Characteristic of Microdisk Lasers Based on InGaAs/GaAs Quantum Well-Dots

Injection microlasers with an active region based on arrays of InGaAs/GaAs quantum well-dots, formed by deep etching, have been studied. The manner in which the current–voltage characteristic changes when the diameter microlaser is reduced shows that a nonelectrically conducting layer with thickness of about 1.5 μm is formed near the side surface, which leads to a decrease in the effective current flow area.

Numerical analysis of new output structures changed base on bus waveguide for 1.3 μm InAs/InGaAs quantum dot microdisk lasers on silicon

Abstract We design two types of output waveguide structures based on bus waveguide: waveguide overlapping with microdisk cavity and single-ended output bus waveguide. This is the first time that these two output structures have been designed for microdisk lasers on silicon. Three-dimensional finite-difference time-domain method is used to numerically analyze the optical mode characteristics of the microdisk lasers. The quality factor (Q-factor) and optical mode distribution of microdisk lasers are calculated in detail. The photoelectric properties of microdisk lasers with different output structures are simulated by rate equations, which provide evidence for lasing. For the first time, by comparing the differences between the two output structures, we identify the respective characteristics of these two structures: microdisk lasers with the waveguide overlaps with the microdisk cavity are suitable for vertical coupling devices when the notch width is 3.5 μm and the waveguide width is 1.5 μm. And the single-ended bus waveguide output structure changes the bus waveguide structure from two-ended to single-ended, which meets the requirement of laser directional output.

Evaluation of energy-to-data ratio of quantum-dot microdisk lasers under direct modulation

The energy-to-data ratio (EDR) was evaluated for quantum-dot based microdisk laser directly modulated without external cooling. The experimental values of EDR decrease with decreasing diameter of the microdisk and reach 1.5 pJ/bit for the smallest diameter under study (10.5 μm). In larger microdisks (with a diameter greater than 20 μm), the EDR varies in proportion to the square of the diameter. If this relationship were true for smaller microdisks as well, an EDR value of about 100 fJ would be achieved with a diameter of 4 μm. The observed deviation of EDR from the quadratic dependence on the diameter is associated with an increase in the threshold current density in smaller devices, which in turn may be caused by the contribution of nonradiative recombination on the microresonator sidewalls formed by deep etching.

Control of Whispering Gallery Modes and PT-Symmetry Breaking in Colloidal Quantum Dot Microdisk Lasers with Engineered Notches.

Whispering gallery mode resonators have been demonstrated to be a great way to achieve superior optical cavities with high quality factor and small mode volume. However, due to the high sensitivity of these modes to the properties of the resonator boundary, they are susceptible to parasitic splitting of clockwise and counterclockwise modes. In this work, we investigate the effect of implantation of an engineered notch into the boundary of a circular microdisk resonator fabricated from colloidal quantum dots, which are particularly sensitive to boundary defects. We observed a strong reduction of parasitic mode splitting with introduction of a large engineered notch, as well as enhanced directionality of laser emission. We further investigate the performance of these resonators in evanescently coupled pairs, where the modal interaction allows modulation of laser behavior through variation of the gain and loss induced by the optical pump. We show that two distinct cases of modal interaction can be achieved by adjusting the size of the engineered notch, providing a bridge between intra- and interdisk modal interactions for laser spectral control.

Evaluation of the Impact of Surface Recombination in Microdisk Lasers by Means of High-Frequency Modulation

Microdisk lasers 10–30 μm in diameter operating at room temperature without thermal stabilization and with an active region based on nanostructures of hybrid dimensionality—quantum wells–dots—are investigated. High-frequency measurements of the microlaser response are performed in the direct small-signal modulation mode, which makes it possible to establish the parameters of the operating speed and analyze their dependence on the microlaser diameter. It is found that the K factor is (0.8 ± 0.2) ns, which corresponds to optical losses of ~6 cm–1, and no regular dependence on the diameter is observed. It is found that the low-frequency component of the damping coefficient of relaxation oscillations is inversely proportional to the diameter. This character of the dependence evidences a decrease in the carrier lifetime in small-diameter microcavities, which can be associated with the prevalence of nonradiative recombination on their side walls.

Energy Consumption for High-Frequency Switching of a Quantum-Dot Microdisk Laser

We have studied characteristics of a 23-μm-diameter microdisk laser operating in a regime of direct high-frequency modulation at a stabilized heat-sink temperature of 18°C. It is found that the minimum energy consumption in this regime amounts to ~1.6 pJ/bit and is achieved at a modulation frequency of 4.2 GHz. The maximum modulation frequency is 6.7 GHz, at which the minimum energy consumption is 3.3 pJ/bit.

Optimization design for high-quality factor 1.3 μm InAs/InGaAs quantum dot square microcavity lasers on silicon with output waveguide structures

We, first, demonstrate an optimized structure design and analyze the optical mode properties of 1.3 μm wavelength square microcavity lasers on silicon with InAs/InGaAs quantum-dot active region and mid-point output waveguides. Si-based square microcavities with whispering-gallery-like modes have been proposed for optimizing their quality factors (Q factors). Three-dimensional finite-difference time-domain method is used to numerically analyze the optical mode characteristics. The influences of the side-length of the microcavity, the output waveguide width, and the etching depth on the optical modes of the microcavity are investigated in detail. It indicates that the Q factor increases with increasing etching depth, and decreases rapidly as the waveguide width increases. The results show that with the side length of 18 μm, the waveguide width of 1.0 μm, and the etching depth of 3.5 μm, the Q factor is the highest, and the mode distribution is optimal. The mode wavelength and Q factor are 1305.9 nm and 4694.8, respectively. Advantages of more stable and evenly distributed mode profiles for Si-based square microcavity lasers have been demonstrated, and compared with the Si-based disk microcavity (microdisk) lasers. It promises a potential alternative laser structure for Si-based optoelectronic integration.

Lasing in III–V microdisk core–TiO2 shell lasers

Microdisk lasers having a III–V core coated with a TiO2 shell are experimentally studied under optical pumping. Initial core microdisk lasers with a 5–10 μm diameter comprising five layers of InAs/In0.15Ga0.85As quantum dots demonstrate room temperature lasing with excellent characteristics (threshold, quality factor) at the ground state optical transition in the 1.28–1.30 μm spectral range. Deposition of the TiO2 dielectric shell results in a decimation of the whispering gallery modes of the microdisk resonator and, for thicker (>100 nm) shells, in a blueshift of the lasing wavelength. Using numerical analysis, we demonstrate that coating a III–V core microdisk with a high-index shell causes strong penetration of the second and third radial modes into the shell, whereas the first radial mode remains nearly undisturbed. Though optical loss added by the TiO2 dielectric shell is low (it does not exceed 2 cm−1 even for a 250-nm-thick TiO2 layer), mode leakage to the TiO2 results in a decrease in the optical confinement factor and an increase in the threshold pump power. This effect is more pronounced in microlasers of the smallest diameter studied (5 μm). Thus, in addition to other applications, a TiO2 shell can be used for mode selection in microdisk lasers or for selective outcoupling of the emission to the waveguide structure, which requires proper adjustment of the TiO2 shell thickness and microdisk diameter.

Lasing characteristics of a pyrromethene597-doped microdisk laser fabricated by the ink-jet printing method

A microdisk laser based on the fluorinated hyper branched polymer FC-V-50 was fabricated on an FEP (fluorinated ethylene propylene) substrate with the use of the ink-jet printing method. Pyrromethene597 doped 10 wt.% FC-V-50 solution in ethanol was used for the fabrication of a microdisk laser. The FC-V-50 polymer is characterized by a carboxyl functional group, which can be activated and attached to an amine group found in biomolecules. Based on the lasing characteristics, it is found that the microdisk laser has a relatively low lasing threshold of 34.51 μJ mm−2 and a reasonable Q factor of 7.52 × 103 at 586.59 nm. Therefore, the microdisk can be used for biosensing. The root mean square (RMS) of the surface roughness of the microdisk was estimated to be 6.2 nm. The surface roughness results scattering losses and limits the Q factor. This work reports the first demonstration of a microdisk laser based on FC-V-50 for biosensing applications.

1310 nm InAs quantum-dot microdisk lasers on SOI by hybrid epitaxy

Direct epitaxial growth of O-band InAs/GaAs quantum-dot laser on Si substrates has been rapidly developing over the past few years. But most of current methodologies are not fully compatible with silicon-on-insulator (SOI) technology, which is the essential platform for silicon photonic devices. By implementing an in situ III-V/Si hybrid growth technique with (111)-faceted Si hollow structures, we demonstrate the first optically pumped InAs/GaAs quantum-dot microdisk laser on SOI substrates grown by molecular beam epitaxy (MBE). The microdisk laser on SOI is characterized with threshold pump power as low as 0.39 mW and a Q factor of 3900 at room temperature. Additionally, the compared device performance of InAs quantum-dot microdisk lasers on GaAs, Si (001) and SOI are simultaneously studied with identical epi-structures.

Direct modulation characteristics of microdisk lasers with InGaAs/GaAs quantum well-dots

GaAs-based microdisk lasers with an active region representing a dense array of indium-rich islands (InGaAs quantum well-dots) were studied using direct small-signal modulation. We demonstrate that using dense arrays of InGaAs quantum well-dots enables uncooled high-frequency applications with a GHz-range bandwidth for microdisk lasers. A maximum 3 dB modulation frequency of 5.9 GHz was found in the microdisk with a radius of 13.5 μm operating without a heatsink for cooling. A modulation current efficiency factor of 1.5 GHz/mA1/2 was estimated.

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.

Continuous wave operation of GaAsBi microdisk lasers at room temperature with large wavelengths ranging from 127 to 141 μm

Submicron-meter size GaAsBi disk resonators were fabricated with the GaAsBi/GaAs single-quantum-well (QW)-structure grown by molecular beam epitaxy. The GaAsBi/GaAs QW revealed very broad photoluminescence signals in the wavelength range of 1100–1400 nm at 300 K. The 750 nm diameter and 220 nm thick disk resonators were optically pumped and exhibited lasing characteristics with continuous wave operation at room temperature. To our knowledge, it is the first demonstration of a lasing wavelength longer than 1.3 μm with a maximum value of 1.4 μm in a GaAsBi/GaAs material system. The lasing wavelength spans about 130 nm by adjusting the disk diameter, covering almost the entire O band. The ultrasmall GaAsBi disk lasers may have great potential for highly dense on-chip integration with large tunability in the O band.

Ultra-low threshold InAs/GaAs quantum dot microdisk lasers on planar on-axis Si (001) substrates

Monolithic integration of efficient III–V light-emitting sources on planar on-axis Si (001) has been recognized as an enabling technology for realizing Si-based photonic integrated circuits (PICs). The field of microdisk lasers employing quantum dot (QD) materials is gaining significant momentum because it allows massive-scalable, streamlined fabrication of Si-based PICs to be made cost effectively. Here, we present InAs/GaAs QD microdisk lasers monolithically grown on on-axis Si (001) substrate with an ultra-low lasing threshold at room temperature under continuous-wave optical pumping. The lasing characteristics of microdisk lasers with small diameter (D) around 2 μm and sub-wavelength scale (D∼1.1 μm) are demonstrated, with a lasing threshold as low as ∼3 μW. The promising lasing characteristics of the microdisk lasers with ultra-low power consumption and small footprint represent a major advance towards large-scale, low-cost integration of laser sources on the Si platform.

Unidirectional emission from a PT -symmetric annular microcavity

The introduction of a scattering center to microdisk lasers has been considered as an effective way to tailor their far-field emissions. However, this approach is practically limited by its nearby modes with higher quality ($Q$) factors and worse directionalities. In this research, by applying the simple concept of parity-time ($\mathcal{P}\mathcal{T}$) symmetry, we demonstrate that the resonances with well-designed directionalities can be selectively excited in optical microcavities. Taking the annular microcavities as examples, the unidirectional laser emissions and single-mode operations have been realized simultaneously by breaking the $\mathcal{P}\mathcal{T}$ symmetry. More interestingly, we find that the $\mathcal{P}\mathcal{T}$-symmetric modulations on the imaginary parts of the refractive index in a circular microdisk can affect the internal ray dynamics and thus generate highly directional laser emission. This research can boost both the fundamental studies and the practical applications of $\mathcal{P}\mathcal{T}$-symmetric systems.

On-Chip Real-Time Chemical Sensors Based on Water-Immersion-Objective Pumped Whispering-Gallery-Mode Microdisk Laser.

Optical whispering-gallery-mode (WGM) microresonator-based sensors with high sensitivity and low detection limit down to single unlabeled biomolecules show high potential for disease diagnosis and clinical application. However, most WGM microresonator-based sensors, which are packed in a microfluidic cell, are a “closed” sensing configuration that prevents changing and sensing the surrounding liquid refractive index (RI) of the microresonator immediately. Here, we present an “open” sensing configuration in which the WGM microdisk laser is directly covered by a water droplet and pumped by a water-immersion-objective (WIO). This allows monitoring the chemical reaction progress in the water droplet by tracking the laser wavelength. A proof-of-concept demonstration of chemical sensor is performed by observing the process of salt dissolution in water and diffusion of two droplets with different RI. This WIO pumped sensing configuration provides a path towards an on-chip chemical sensor for studying chemical reaction kinetics in real time.

Epitaxy of Si-Ge-Sn-based heterostructures for CMOS-integratable light emitters

The recent rise of GeSn-based optically pumped lasers have multiplied the efforts to fabricate a fully CMOS compatible and group IV-based light emitter. Their integration with Si-based electronics may yield heavily reduced power consumption in integrated circuits and pave the way towards new sensing or medical applications. Here, we discuss the epitaxy of group IV GeSn and SiGeSn semiconductors and show their suitability for light emitting applications. Double and multi quantum well heterostructures are evaluated, whereby the latter enables an inherently easier control over the formation of deleterious misfit defects. Consequently, microdisk lasers fabricated from those show greatly enhanced light emission and reduced lasing thresholds. The use of in-situ p-i-n doping schemes allow the formation of light emitting diodes, resulting in electrically-enabled light emission already at room temperature.

Robust lasing modes in coupled colloidal quantum dot microdisk pairs using a non-Hermitian exceptional point

Evanescently coupled pairs of microdisk lasers have emerged as a useful platform for studying the non-Hermitian physics of exceptional points. It remains an open question how scalable and versatile such phenomena can be when carried over to other designs. Here we have studied the effect of gain/loss modulation in an evanescently coupled pair of microdisk optical resonators fabricated from solution-processed colloidal quantum dots. The emission spectra of these structures are sensitive to small imperfections, which cause frequency-splitting of the whispering gallery modes. Despite this inherent disorder, we found that when spatially modulating the optical pump to vary the gain differential between the coupled microdisks, the coupling drives the split parasitic intra-cavity modes into coalescence at an exceptional point of the resulting three-mode system. This unusual behavior is rationalized via a Hamiltonian that incorporates the intra-cavity coupling as well as the anisotropic inter-cavity coupling between modes in the microdisk pair.