Channeling chaotic rays into waveguides for efficient collection of microcavity emission

Universal Single-Mode Lasing in Fully Chaotic Billiard Lasers

By numerical simulations and experiments of fully chaotic billiard lasers, we show that single-mode lasing states are stable, whereas multi-mode lasing states are unstable when the size of the billiard is much larger than the wavelength and the external pumping power is sufficiently large. On the other hand, for integrable billiard lasers, it is shown that multi-mode lasing states are stable, whereas single-mode lasing states are unstable. These phenomena arise from the combination of two different nonlinear effects of mode-interaction due to the active lasing medium and deformation of the billiard shape. Investigations of billiard lasers with various shapes revealed that single-mode lasing is a universal phenomenon for fully chaotic billiard lasers.

Manipulating cavity photon dynamics by topologically curved space

Asymmetric microcavities supporting Whispering-gallery modes (WGMs) are of great significance for on-chip optical information processing. We establish asymmetric microcavities on topologically curved surfaces, where the geodesic light trajectories completely reconstruct the cavity mode features. The curvature-mediated photon-lifetime engineering enables the enhancement of the quality factors of periodic island modes by up to 200 times. Strong and weak coupling between modes of very different origins occurs when the space curvature brings them into resonance, leading to fine tailoring of the cavity photon energy and lifetime and the observation of non-Hermitian exceptional point (EP). At large space curvatures, the role of the WGMs is replaced by high-Q periodic modes protected by the high stability of island-like light trajectory. Our work demonstrates interesting physical mechanisms at the crosspoint of optical chaotic dynamics, non-Hermitian physics, and geodesic optical devices, and would initiate the novel area of geodesic microcavity photonics.

Nonconfinement growth of edge-curved molecular crystals for self-focused microlasers

Synthesis of single-crystalline micro/nanostructures with curved shapes is essential for developing extraordinary types of optoelectronic devices. Here, we use the strategy of liquid-phase nonconfinement growth to controllably synthesize edge-curved molecular microcrystals on a large scale. By varying the molecular substituents on linear organic conjugated molecules, it is found that the steric hindrance effect could minimize the intrinsic anisotropy of molecular stacking, allowing for the exposure of high-index crystal planes. The growth rate of high-index crystal planes can be further regulated by increasing the molecular supersaturation, which is conducive to the cogrowth of these crystal planes to form continuously curved-shape microcrystals. Assisted by nonrotationally symmetric geometry and optically smooth curvature, edge-curved microcrystals can support low-threshold lasing, and self-focusing directional emission. These results contribute to gaining an insightful understanding of the design and growth of functional molecular crystals and promoting the applications of organic active materials in integrated photonic devices and circuits.

Direct observation of chaotic resonances in optical microcavities

Optical microcavities play a significant role in the study of classical and quantum chaos. To date, most experimental explorations of their internal wave dynamics have focused on the properties of their inputs and outputs, without directly interrogating the dynamics and the associated mode patterns inside. As a result, this key information is rarely retrieved with certainty, which significantly restricts the verification and understanding of the actual chaotic motion. Here we demonstrate a simple and robust approach to directly and rapidly map the internal mode patterns in chaotic microcavities. By introducing a local index perturbation through a pump laser, we report a spectral response of optical microcavities that is proportional to the internal field distribution. With this technique, chaotic modes with staggered mode spacings can be distinguished. Consequently, a complete chaos assisted tunneling (CAT) and its time-reversed process are experimentally verified in the optical domain with unprecedented certainty.

Superkinetic Growth of Oval Organic Semiconductor Microcrystals for Chaotic Lasing

Synthesis of novel mesoscopic semiconductor architectures continually generates new photonic knowledge and applications. However, it remains a great challenge to synthesize semiconductor microcrystals with smoothly curved surfaces owing to the crystal growth anisotropy. Here, a superkinetic crystal growth method is developed to synthesize 2D oval organic semiconductor microcrystals. The solid source dispersion induces an exceptionally large molecular supersaturation for vapor deposition, which breaks the crystal growth anisotropy. The synthesized stadium-shaped organic semiconductor microcrystals naturally constitute fully chaotic optical microresonators. They support low-threshold lasing on high-quality-factor scar modes localized near the stadium boundary and directional laser emission assisted by the chaotic modes. These results will reshape the understanding of the crystal growth theory and provide valuable guidance for crystalline photonic materials design.

Perpendicular coupler for standing wave excitation and wavelength selection in high-Q silicon microresonators

High quality factor (Q) whispering gallery mode (WGM) resonators have been widely applied in photonics, while the excitation and collection of WGMs are mostly restricted to traveling wave coupler. Here, we experimentally demonstrate a novel on-chip perpendicular coupler (PC) for high-Q (∼1.1 × 10⁵) silicon whispering gallery microresonators. The PC is compact and allows efficiently tunneling coupling between the waveguide and the microresonator, hence it holds great potential for fan-out photonic devices. Drastically different from the traveling wave couplers, standing wave mode can be excited through the PC. In addition, a PC working as an output coupler can also selectively collect the resonance of different wavelengths by locating on different azimuth angles. Our results show the feasibility of such novel coupler for WGM resonators and its potential use in future applications of integrated high Q microresonators.

Regular-Orbit-Engineered Chaotic Photon Transport in Mixed Phase Space

The dynamical evolution of light in asymmetric microcavities is of primary interest for broadband optical coupling and enhanced light-matter interaction. Here, we propose and demonstrate that the chaos-assisted photon transport can be engineered by regular periodic orbits in the momentum-position phase space of an asymmetric microcavity. Remarkably, light at different initial states experiences different evolution pathways, following either regular-chaotic channels or pure chaotic channels. Experimentally, we develop a nanofiber technique to accurately control the excitation position of light in the phase space. We find that the coupling to high-Q whispering gallery modes depends strongly on excitation in islands or chaotic sea, showing a good agreement with the theoretical prediction. The engineered chaotic photon transport has potential in light manipulation, broadband photonic devices, and phase-space reconstruction.

Light Concentration by Metal-Dielectric Micro-Resonators for SERS Sensing

Metal-dielectric micro/nano-composites have surface plasmon resonances in visible and near-infrared domains. Excitation of coupled metal-dielectric resonances is also important. These different resonances can allow enhancement of the electromagnetic field at a subwavelength scale. Hybrid plasmonic structures act as optical antennae by concentrating large electromagnetic energy in micro- and nano-scales. Plasmonic structures are proposed for various applications such as optical filters, investigation of quantum electrodynamics effects, solar energy concentration, magnetic recording, nanolasing, medical imaging and biodetection, surface-enhanced Raman scattering (SERS), and optical super-resolution microscopy. We present the review of recent achievements in experimental and theoretical studies of metal-dielectric micro and nano antennae that are important for fundamental and applied research. The main impact is application of metal-dielectric optical antennae for the efficient SERS sensing.

Highly efficient injection microdisk lasers based on quantum well-dots

We study injection GaAs-based microdisk lasers capable of operating at room and elevated temperatures. A novel type of active region is used, namely InGaAs quantum well-dots representing a dense array of indium-rich islands formed inside an indium-depleted residual quantum well by metalorganic vapor phase epitaxy. We demonstrate a high output power of 18 mW, a differential efficiency of about 31%, and a peak electrical-to-optical power conversion efficiency of 15% in a 31 μm diameter microdisk laser. The continuous-wave lasing is observed up to 110°C.

Spectral engineering for circular-side square microlasers

Spectral engineering has been demonstrated for the circular-side square microlasers with an output waveguide butt-coupled to one vertex. By carefully optimizing deformation parameter and waveguide connection angle, undesired high-order transverse modes are suppressed while the mode Q factors and the transverse-mode intervals are enhanced simultaneously for the low-order transverse modes. Dual-mode lasing with pure lasing spectra is realized experimentally for the circular-side square microlasers with side lengths of 16 μm, and the transverse mode intervals can be adjusted from 0.54 to 5.4 nm by changing the deformation parameter. Due to the enhanced mode confinement, single-mode lasing with a side-mode suppression-ratio of 36 dB is achieved for a 10μm-side-length circular-side square microlaser with a 1.5μm-wide waveguide.

Pair of Exceptional Points in a Microdisk Cavity under an Extremely Weak Deformation

One of the interesting features of open quantum and wave systems is the non-Hermitian degeneracy called an exceptional point, where not only energy levels but also the corresponding eigenstates coalesce. We demonstrate that such a degeneracy can appear in optical microdisk cavities by deforming the boundary extremely weakly. This surprising finding is explained by a semiclassical theory of dynamical tunneling. It is shown that the exceptional points come in nearly degenerated pairs, originating from the different symmetry classes of modes. A spatially local chirality of modes at the exceptional point is related to vortex structures of the Poynting vector.

Counting statistics of chaotic resonances at optical frequencies: Theory and experiments

A deformed dielectric microcavity is used as an experimental platform for the analysis of the statistics of chaotic resonances, in the perspective of testing fractal Weyl laws at optical frequencies. In order to surmount the difficulties that arise from reading strongly overlapping spectra, we exploit the mixed nature of the phase space at hand, and only count the high-Q whispering-gallery modes (WGMs) directly. That enables us to draw statistical information on the more lossy chaotic resonances, coupled to the high-Q regular modes via dynamical tunneling. Three different models [classical, Random-Matrix-Theory (RMT) based, semiclassical] to interpret the experimental data are discussed. On the basis of least-squares analysis, theoretical estimates of Ehrenfest time, and independent measurements, we find that a semiclassically modified RMT-based expression best describes the experiment in all its realizations, particularly when the resonator is coupled to visible light, while RMT alone still works quite well in the infrared. In this work we reexamine and substantially extend the results of a short paper published earlier [L. Wang et al., Phys. Rev. E 93, 040201(R) (2016)2470-004510.1103/PhysRevE.93.040201].

Chaos-assisted broadband momentum transformation in optical microresonators

The law of momentum conservation rules out many desired processes in optical microresonators. We report broadband momentum transformations of light in asymmetric whispering gallery microresonators. Assisted by chaotic motions, broadband light can travel between optical modes with different angular momenta within a few picoseconds. Efficient coupling from visible to near-infrared bands is demonstrated between a nanowaveguide and whispering gallery modes with quality factors exceeding 10 million. The broadband momentum transformation enhances the device conversion efficiency of the third-harmonic generation by greater than three orders of magnitude over the conventional evanescent-wave coupling. The observed broadband and fast momentum transformation could promote applications such as multicolor lasers, broadband memories, and multiwavelength optical networks.

Highly Reproducible Organometallic Halide Perovskite Microdevices based on Top-Down Lithography

Highly reproducible organometallic-halide-perovskite-based devices are fabricated by a manufacturing process, which is demonstrated. Various shapes that are hard to synthesize directly are fabricated, and many unique properties are achieved.The fabrication procedure is utilized to create a photodetector and the detection sensitivity is significantly improved. The results will bring revolutionary advancement to the future of lead-halide-perovskite-based optoelectronic devices.

Postsynthetic and Selective Control of Lead Halide Perovskite Microlasers

The control of photoluminescence and absorption of lead halide perovskites plays a key role in their applications in micro- and nano-sized light emission devices and photodetectors. To date, the wavelength controls of lead halide perovskite microlasers are mostly realized by changing the halide mixture in solution. Herein, we report the postsynthetic and selective control of the optical properties of lead halide perovskites with conventional semiconductor technology. By selectively exposing a CH₃NH₃PbBr₃ microstructure with chlorine in inductively coupled plasma, we find that the wavelengths of absorption, photoluminescence, and laser emissions of exposed structures are blue-shifted around 50 nm. Most importantly, the device characteristics such as the photoluminescence intensities and laser thresholds are well maintained during the reaction process. We believe our finding will significantly boost the practical applications of lead halide perovskite based optoelectronics.

Tunable perovskite microdisk lasers

Perovskite microdisk lasers have been intensively studied recently. But their lasing properties are usually fixed once the devices are synthesized. Here, for the first time, we demonstrated the switchable and tunable perovskite microdisk lasers by surrounding them with 5CB liquid crystals. With the increase of the environmental temperature from 24 °C to 34 °C, the lasing wavelength slightly changed from 552.91 nm to 552.11 nm at the beginning and suddenly shifted to around 552.54 nm at T = 32 °C, where the phase transition of liquid crystals occurs. Our numerical calculation shows that the wavelength shift is caused by the changes of the refractive index of liquid crystals. More than tuning of the wavelength, a more dramatic wavelength transition from ∼554 nm to 550 nm has also been observed. This sudden transition is mainly induced by the reduction of scattering rather than the change in the refractive index when the liquid crystals are changed from the nematic phase to the isotropic phase. We believe that our research can shed light on the applications of perovskite optoelectronics.

The combination of directional outputs and single-mode operation in circular microdisk with broken PT symmetry

Monochromaticity and directionality are two key characteristics of lasers. However, the combination of directional emission and single-mode operation is quite challenging, especially for the on-chip devices. Here we propose a microdisk laser with single-mode operation and directional emissions by exploiting the recent developments associated with parity-time (PT) symmetry. This is accomplished by introducing one-dimensional periodic gain and loss into a circular microdisk, which induces a coupling between whispering gallery modes with different radial numbers. The lowest threshold mode is selected at the positions with least initial wavelength difference. And the directional emissions are formed by the introduction of additional grating vectors by the periodic distribution of gain and loss regions. We believe this research will impact the practical applications of on-chip microdisk lasers.

Experimental Observation of Resonance-Assisted Tunneling

We present the first experimental observation of resonance-assisted tunneling, a wave phenomenon, where regular-to-chaotic tunneling is strongly enhanced by the presence of a classical nonlinear resonance chain. For this we use a microwave cavity made of oxygen free copper with the shape of a desymmetrized cosine billiard designed with a large nonlinear resonance chain in the regular region. It is opened in a region, where only chaotic dynamics takes place, such that the tunneling rate of a regular mode to the chaotic region increases the line width of the mode. Resonance-assisted tunneling is demonstrated by (i) a parametric variation and (ii) the characteristic plateau and peak structure towards the semiclassical limit.

Nonlinear resonance-assisted tunneling induced by microcavity deformation

Noncircular two-dimensional microcavities support directional output and strong confinement of light, making them suitable for various photonics applications. It is now of primary interest to control the interactions among the cavity modes since novel functionality and enhanced light-matter coupling can be realized through intermode interactions. However, the interaction Hamiltonian induced by cavity deformation is basically unknown, limiting practical utilization of intermode interactions. Here we present the first experimental observation of resonance-assisted tunneling in a deformed two-dimensional microcavity. It is this tunneling mechanism that induces strong inter-mode interactions in mixed phase space as their strength can be directly obtained from a separatrix area in the phase space of intracavity ray dynamics. A selection rule for strong interactions is also found in terms of angular quantum numbers. Our findings, applicable to other physical systems in mixed phase space, make the interaction control more accessible.

Thermal and high speed modulation characteristics for AlGaInAs/InP microdisk lasers

Thermal and high speed modulation characteristics are investigated for a unidirectional-emission microdisk laser with a radius of 7 μm surrounded by BCB-cladding layer, with a threshold current of 1.5 mA at the temperature of 287 K. The lasing spectra under different widths of pulsed current are measured to characterize the temperature rise during the pulse period, and the thermal distribution in the microdisk laser is simulated by the finite-element modeling technique. A temperature rise of 25 K is estimated for the microdisk laser biased at 20 mA. Furthermore, small signal modulation response with 3dB bandwidth up to 20 GHz is obtained for the microdisk laser at the biasing current of 18 mA, and eye-diagrams at the modulation bit rates of 20, 25, and 30 GHz are also measured at the temperature of 287 K.

Coherent destruction of tunneling in chaotic microcavities via three-state anti-crossings

Coherent destruction of tunneling (CDT) has been one seminal result of quantum dynamics control. Traditionally, CDT is understood as destructive interference between two intermediate transition paths near the level crossing. CDT near the level anti-crossings, especially the “locking”, has not been thoroughly explored so far. Taking chaotic microcavity as an example, here we study the inhibition of the tunneling via the strong couplings of three resonances. While the tunneling rate is only slightly affected by each strong coupling between two modes, the destructive interference between two strong couplings can dramatically improve the inhibition of the tunneling. A “locking” point, where dynamical tunneling is completely suppressed, has even been observed. We believe our finding will shed light on researches on micro- & nano-photonics.

The impact of emission mechanisms on the long-lived states around avoided resonance crossings in chaotic microcavity

Here we demonstrate the impacts of emission mechanisms on the light confinements in open systems. Taking the oval-shaped cavities as examples, we show that the enhancements in quality (Q) factors are usually associated with the universal emissions. When the coupled resonances have similar far field patterns, the Q factor of the long-lived resonance has the possibility to be enhanced by the coherent destruction at the decay channels. Otherwise, the Q factors of long-lived resonances are usually reduced around the level crossings.

Deformed microdisk coupled to a bus waveguide for applications in resonant filter

Here we explore the applications of a deformed microdisk as a passive photonic element by coupling it to a bus waveguide. We show that deformed microdisk-based resonant filters are able to have transmittance of more than 99%, and the dropped signals can be routed to two different ports for particular applications. Interestingly, our results show that the splitting ratio can be dynamically tuned by locally changing the refractive index of microdisk. Our research opens new opportunities for the applications of deformed microdisks.

Direct-modulated waveguide-coupled microspiral disk lasers with spatially selective injection for on-chip optical interconnects

We investigate direct-modulated waveguide-coupled microspiral disk lasers for on-chip optical interconnects. Microspiral resonators, with a rotationally asymmetric shape and a waveguide directly gapless coupled to the notch, offer a compact unidirectional-emission on-chip laser source. We employ spatially selective injection by means of a ring-shaped p-contact on top of the microdisk rim region to selectively inject current to the whispering-gallery-like modes and thus enhance the laser performance. Here we report room-temperature continuous-wave electrically injected AlGaInAs/InP waveguide-coupled microspiral disk lasers with a disk radius of 30 and 40 μm. For a 30 μm microspiral disk laser gaplessly coupled with a 100 μm-long passive waveguide that is directly connected to an on-chip AlGaInAs/InP photodiode, we estimate a laser output power of at least 200 μW upon a 70 mA injection. We realize small-signal modulation with a 3dB bandwidth exceeding 10 GHz for the 30 μm microspiral disk. We demonstrate an open eye diagram at 15 Gbit/s with a bias current of 90 mA at a stage temperature of 15 °C.

Directional emission of dielectric disks with a finite scatterer in the THz regime

In the Terahertz (THz) domain, we investigate both numerically and experimentally the directional emission of whispering gallery mode resonators that are perturbed by a small scatterer in the vicinity of the resonators rim. We determine quality factor degradation, the modal structure and the emission direction for various geometries. We find that scatterers do allow for directional emission without destroying the resonator's quality factor. This finding allows for new geometries and outcoupling scenarios for active whispering gallery mode structures such as quantum cascade lasers and passive resonators such as evanescent sensors. The experimental results agree well with finite difference time domain simulations.

Analysis of vertical radiation loss and far-field pattern for microcylinder lasers with an output waveguide

Vertical radiation loss and far-field pattern are investigated for microcylinder lasers by 3D FDTD simulation and experimentally. The numerical results show that an output waveguide connected to the microcylinder resonator can result in additional vertical radiation loss for high Q coupled modes and affect the far field pattern. The vertical radiation loss can be controlled by adjusting the up cladding layer thickness. Furthermore, two lobes of vertical far-field patterns are observed for a 15-μm-radius microcylinder laser connected with an output waveguide, which confirms the vertical radiation loss.