On-chip Microcavity Research Articles

All-wet etched high-Q optical microcavity on a silicon chip

Optical microcavities with high quality factors can significantly enhance the intracavity power density and have become a superior platform for studying quantum electrodynamics, nonlinearities, and sensing. Minimized and compact on-chip devices require the integration of microcavities with versatile functions that can be implemented by techniques compatible with semiconductor industries. However, the silicon dry etch technique commonly used to shape silicon wafers limits the massive production of microcavities. Here, an all-wet etching technique is proposed to fabricate on-chip microcavities with the aid of photolithography. On-chip silica microdisks with quality factors of ∼105 are demonstrated, and optical modes with quality factors up to 106 are also observed. The proposed technique opens an avenue for economical, easy handling, and scalable production of on-chip microcavities using foundry-based technologies.

Circularly polarized electroluminescence from a single-crystal organic microcavity light-emitting diode based on photonic spin-orbit interactions

Circularly polarized (CP) electroluminescence from organic light-emitting diodes (OLEDs) has aroused considerable attention for their potential in future display and photonic technologies. The development of CP-OLEDs relies largely on chiral-emitters, which not only remain rare owing to difficulties in design and synthesis but also limit the performance of electroluminescence. When the polarization (pseudospin) degrees of freedom of a photon interact with its orbital angular momentum, photonic spin-orbit interaction (SOI) emerges such as Rashba-Dresselhaus (RD) effect. Here, we demonstrate a chiral-emitter-free microcavity CP-OLED with a high dissymmetry factor (gEL) and high luminance by embedding a thin two-dimensional organic single crystal (2D-OSC) between two silver layers which serve as two metallic mirrors forming a microcavity and meanwhile also as two electrodes in an OLED architecture. In the presence of the RD effect, the SOIs in the birefringent 2D-OSC microcavity result in a controllable spin-splitting with CP dispersions. Thanks to the high emission efficiency and high carrier mobility of the OSC, chiral-emitter-free CP-OLEDs have been demonstrated exhibiting a high gEL of 1.1 and a maximum luminance of about 60000 cd/m2, which places our device among the best performing CP-OLEDs. This strategy opens an avenue for practical applications towards on-chip microcavity CP-OLEDs.

Defect-assisted, spray-printed colloidal quantum dot microlasers for biosensing.

This study successfully implements spectrally distinguishable CdSe-ZnS core-shell colloidal quantum dot (CQD) microlasers by a simple, efficient spray printing technique and demonstrates its potential in biosensing. We have systematically characterized the optical properties of printed microring lasers with diameters less than 60 µm. The smallest structure that can be excited has a diameter as small as 30 µm, which is much smaller than the counterparts prepared by piezoelectric ink-jet printing. The detection sensitivity of 4.54 nm/min/refractive index unit is verified in glucose sensing using a printed CQD microlaser. Biosensing of diverse glucose and bovine serum albumin solutions using printed microlasers with the assistance of defects demonstrates a new, to the best of our knowledge, prototype for the development of high-performance, low-cost on-chip microcavity sensors.

Research advances of ultrahigh-<italic>Q</italic> on-chip microcavity photonics

Optical microcavities can trap light within both spatial and temporal dimensions, giving rise to an enhancement of light-matter interaction, which are employed extensively for fundamental studies of optical physics and photonic applications. In particular, because of its ultrahigh- Q factors and small mode volumes, optical whispering-gallery-mode (WGM) microcavities, confining light by total internal reflection, act as the frontier of the state-of-art microcavity studies. As the recent significant development of the micro-/nano-fabrication technology for semiconductor chips and optical materials, on-chip integration becomes one of the most important trends for WGM microcavities. For example, on-chip optical microcavities have been playing a pivotal role in diverse fields, including photonic chip, broadband integrated optical computing, on-chip optical interconnects, microcomb-based precise measurement, etc. In this review, we introduce the research advances of microlasers, nonlinear optics, photonic integrated circuits, and ultrasensitive optical sensing in ultrahigh- Q on-chip microcavities. The specific progresses are organized as follows: (1) Ultralow-threshold lasing in on-chip WGM microcavity can be achieved by either introducing active materials to the resonator or using intrinsic nonlinearities of the resonator material. Moreover, diverse functional microlasers are also realized, such as unidirectional emission, chaotic laser, symmetry-broken laser, orbit angular momentum laser, parity-time reversal symmetry laser, etc. (2) Numerous optical nonlinear effects with high efficiency have been experimentally generated in WGM microcavities, including second harmonic generation, four-wave mixing, stimulated Brillouin scattering, stimulated Raman scattering. Furthermore, high-performance and multifunctional photonic applications have been developed based on microcavity-engineered nonlinear optics. In particular, optical frequency combs based on Kerr nonlinear effects in WGM microcavities, are applied extensively for integrated precision measurement applications like ultrafast LiDAR, optical clocks, and astronomical spectrum calibration. (3) As one of the most important elements in photonic integrated circuit, the optical microcavity has been widely used in the realization of photonic functional devices, such as signal sources, storage, filter, isolator, and modulator. Moreover, it also has been applied to broadband coupling between individual photonic devices. (4) WGM microcavity sensing attracts much attention due to ultra-high sensitivity and label-free nature. The last decade has witnessed the tremendous progress of on-chip microcavity sensing, and the sensing ability has been widely exploited for detecting single nanoparticles, biological molecules, magnetic fields, angular velocity, temperature, etc. Finally, we provide an outlook on the future developments of on-chip ultrahigh- Q microcavity photonics. While numerous progresses have been achieved in ultrahigh- Q on-chip microcavities, a few challenges and obstacles still exist in practical applications. For example, the optical soliton microcombs suffer from the low conversion efficiency and limited wavelength bandwidth; the bandwidth of the optical isolator by the optical microcavities is narrow; the electro-optic modulator based on microcavities cannot combine the broad modulation bandwidth and low power consumption; it is highly challenging for the specific identification in microcavity biosensing. In order to further push forward the development of on-chip microcavity photonics, new principles, new mechanisms, and new materials are required to be explored. The considerable solution may focus on the non-trivial interaction in complex systems, such as hybrid microcavity structures, regulation of optical field by multiple nonlinear optical effects, optical wave chaos in asymmetric microcavities, non-Hermitian physics, and topological photonics.

Frequency-tuning-induced state transfer in optical microcavities

Quantum state transfer in optical microcavities plays an important role in quantum information processing and is essential in many optical devices such as optical frequency converters and diodes. Existing schemes are effective and realized by tuning the coupling strengths between modes. However, such approaches are severely restricted due to the small amount of strength that can be tuned and the difficulty performing the tuning in some situations, such as in an on-chip microcavity system. Here we propose a novel approach that realizes the state transfer between different modes in optical microcavities by tuning the frequency of an intermediate mode. We show that for typical functions of frequency tuning, such as linear and periodic functions, the state transfer can be realized successfully with different features. To optimize the process, we use the gradient descent technique to find an optimal tuning function for a fast and perfect state transfer. We also showed that our approach has significant nonreciprocity with appropriate tuning variables, where one can unidirectionally transfer a state from one mode to another, but the inverse direction transfer is forbidden. This work provides an effective method for controlling the multimode interactions in on-chip optical microcavities via simple operations, and it has practical applications in all-optical devices.

Free spectral range electrical tuning of a high quality on-chip microcavity.

Reconfigurable photonic circuits have applications ranging from next-generation computer architectures to quantum networks, coherent radar and optical metamaterials. Here, we demonstrate an on-chip high quality microcavity with resonances that can be electrically tuned across a full free spectral range (FSR). FSR tuning allows resonance with any source or emitter, or between any number of networked microcavities. We achieve it by integrating nanoelectronic actuation with strong optomechanical interactions that create a highly geometry-dependent effective refractive index. This allows low voltages and sub-nanowatt power consumption. We demonstrate a basic reconfigurable photonic network, bringing the microcavity into resonance with an arbitrary mode of a microtoroidal optical cavity across a telecommunications fibre link. Our results have applications beyond photonic circuits, including widely tuneable integrated lasers, reconfigurable optical filters for telecommunications and astronomy, and on-chip sensor networks.

Terabit optical OFDM superchannel transmission via coherent carriers of a hybrid chip-scale soliton frequency comb.

We demonstrate seamless channel multiplexing and high bitrate superchannel transmission of coherent optical orthogonal frequency division multiplexing (CO-OFDM) data signals utilizing a dissipative Kerr soliton (DKS) frequency comb generated in an on-chip microcavity. Aided by comb line multiplication through Nyquist pulse modulation, the high stability and mutual coherence among mode-locked Kerr comb lines are exploited for the first time, to the best of our knowledge, to eliminate the guard intervals between communication channels and achieve full spectral density bandwidth utilization. Spectral efficiency as high as 2.625bit/Hz/s is obtained for 180 CO-OFDM bands encoded with 12.75Gbaud 8-QAM data, adding to the total bitrate of 6.885Tb/s within a 2.295THz frequency comb bandwidth. This Letter confirms that high coherence is the key superiority of Kerr soliton frequency combs over independent laser diodes, as a multi-spectral coherent laser source for high-bandwidth high-spectral-density transmission networks.

Intracavity characterization of micro-comb generation in the single-soliton regime.

Soliton formation in on-chip micro-comb generation balances cavity dispersion and nonlinearity and allows coherent, low-noise comb operation. We study the intracavity waveform of an on-chip microcavity soliton in a silicon nitride microresonator configured with a drop port. Whereas combs measured at the through port are accompanied by a very strong pump line which accounts for >99% of the output power, our experiments reveal that inside the microcavity, most of the power is in the soliton. Time-domain measurements performed at the drop port provide information that directly reflects the intracavity field. Data confirm a train of bright, close to bandwidth-limited pulses, accompanied by a weak continuous wave (CW) background with a small phase shift relative to the comb.

Tunable open-access microcavities for on-chip cavity quantum electrodynamics

We report on the development of on-chip microcavities and show their potential as a platform for cavity quantum electrodynamics experiments. Microcavity arrays were formed by the controlled buckling of SiO2/Ta2O5 Bragg mirrors and exhibit a reflectance-limited finesse of 3500 and mode volumes as small as 35λ3. We show that the cavity resonance can be thermally tuned into alignment with the D2 transition of 87Rb and outline two methods for providing atom access to the cavity. Owing to their small mode volume and high finesse, these cavities exhibit single-atom cooperativities as high as C1=65. A unique feature of the buckled-dome architecture is that the strong-coupling parameter g0/κ is nearly independent of the cavity size. Furthermore, strong coupling should be achievable with only modest improvements in mirror reflectance, suggesting that these monolithic devices could provide a robust and scalable solution to the engineering of light-matter interfaces.

Polarization resolved spatial near-field mapping of optical modes in an on-chip rolled-up bottle microcavity

A detailed optical near-field mapping of the spatial distribution of two sets of linearly polarized modes in a vertically rolled-up bottle microcavity is presented. We find different effective axial potential widths depending on the polarization state by coupling a tapered fiber to the on-chip microcavity and show that we can selectively excite modes out of a rich axial mode spectrum. Our results are supported by a theoretical model and help to promote future applications of rolled-up bottle microcavities in on-chip integrated optical networks, where these tube cavities can, in principle, be used to transfer signals horizontally along the tube axis or vertically between stacked photonic layers.

Optomechanical sensing with on-chip microcavities

The coupling between optical and mechanical degrees of freedom has been of broad interest for a long time. However, it is only until recently, with the rapid development of optical microcavity research, that we are able to manipulate and utilize this coupling process. When a high Q microcavity couples to a mechanical resonator, they can consolidate into an optomechanical system. Benefitting from the unique characteristics offered by optomechanical coupling, this hybrid system has become a promising platform for ultrasensitive sensors to detect displacement, mass, force and acceleration. In this review, we introduce the basic physical concepts of cavity optomechanics, and describe some of the most typical experimental cavity optomechanical systems for sensing applications. Finally, we discuss the noise arising from various sources and show the potentiality of optomechanical sensing towards quantum-noise-limited detection.

Mechanism of directional emission from a peanut-shaped microcavity

Collimated directional emission is essentially required an asymmetric resonant cavity. In this paper, we theoretically investigate a type of peanut-shaped microcavity which can support highly directional emission with the emission divergence as small as 2.5o. The mechanism of the collimated emission is explained with the short-term ray trajectory and the intuitive lens model in detail. Wave simulation also confirms these results. This extremely narrow divergence of the emission holds a great potential in highly collimated lasing from on-chip microcavities.

Self-pulsation in fiber-coupled, on-chip microcavity lasers

We report self-pulsation in an erbium-doped silica toroidal microcavity laser coupled to a tapered fiber and investigate the effects of pump power and taper-cavity coupling condition on the dynamic behaviors of the pulse train. The microcavity is pumped at 1444.8 nm, and lasing occurs at 1560.2 nm with a threshold of 12 microW. Experimental results are interpreted within the framework of ion-pair induced self-quenching model.

Chip-based microcavities coupled to nitrogen-vacancy centers in single crystal diamond

Optical coupling of nitrogen-vacancy centers in single-crystal diamond to an on-chip microcavity is demonstrated. The microcavity is fabricated from a hybrid gallium phosphide and diamond material system and supports whispering gallery mode resonances with spectrometer resolution limited Q>25 000.