Hyperband electro-optic modulator based on a two-pulley coupled lithium niobate racetrack resonator.

Integrated optical modulators (IOMs) are crucial components of on-chip photonic circuits. However, most conventional IOMs are restricted to specific spectral bands. Here, we leveraged the wide transparency window of lithium niobate in conjunction with the two-pulley coupled resonator method. This approach led to the development of a hyperband electro-optic (EO) modulator that operates over an expansive spectral range from 775 to 1550 nm on a single device. The demonstrated EO modulator exhibits half-wave voltage-length products of 0.25, 0.93, and 0.68 V·cm at wavelengths of 1539.50, 969.70, and 775.17 nm, respectively.

Quantitative gas pressure measurement by molecular spectroscopy using chip-based supercontinuum in the mid-infrared.

We demonstrate the quantitative pressure measurement of gas molecules in the mid-infrared using chip-based supercontinuum and cepstrum analysis without additional measurements for baseline normalization. A supercontinuum generated in an on-chip waveguide made of chalcogenide glass having high nonlinearity passes through CO gas and provides a transmission spectrum. The gas absorption information is deconvoluted from the original supercontinuum spectral information containing temporal fluctuation by cepstrum analysis and extracted simply by applying a bandpass filter in the temporal domain. The gas pressure estimated from the extracted absorption information is consistent with the value measured by a pressure gauge within a difference of 1.25%, despite spectral fluctuations in the supercontinuum baseline comparable to the spectral depth of the gas absorption lines.

Spontaneous soliton mode-locking of a microcomb assisted by Raman scattering.

We successfully control the interaction dynamics between optical parametric oscillation (OPO) and stimulated Raman scattering, leading to the generation of distinct frequency comb states in a microresonator. Through Raman-scattered photons, a Raman comb with a sech2 envelope is demonstrated having a broad RF beat note linewidth of several hundred kHz. Moreover, under a specific coupling regime, we successfully generate self-locked Raman single-solitons which is confirmed by a narrow RF beat note of 25 Hz. Remarkably, this spontaneous Raman soliton is deterministically generated through adiabatic pump frequency detuning without the requirement of external locking mechanisms. Additionally, we identify a frequency comb with an unconventional envelope that can be fitted with a Lorentzian × sech2 function, generated via an anti-Stokes process with respect to the Raman comb.

Moving-frame imaging of transiting cold atoms for precise long-range transport.

Transporting cold atoms between interconnected vacuum chambers is an important technique for increasing the versatility of cold atom setups, particularly for those that couple atoms to photonic devices. In this report, we introduce a method where we are able to image the atoms at all points during transport via moving optical dipole trap. Cooled 87Rb atoms are transported ∼50 cm into an auxiliary vacuum chamber while being monitored with a moving-frame imaging system for which in-situ characterization of the atom transport is demonstrated. Precise positioning of the atoms near photonic devices is also tested across several tapered fibers showing an axial positioning resolution of ∼450 µm.

Ultrastable microwave and soliton-pulse generation from fibre-photonic-stabilized microcombs.

The ability to generate lower-noise microwaves has greatly advanced high-speed, high-precision scientific and engineering fields. Microcombs have high potential for generating such low-noise microwaves from chip-scale devices. To realize an ultralow-noise performance over a wider Fourier frequency range and longer time scale, which is required for many high-precision applications, free-running microcombs must be locked to more stable reference sources. However, ultrastable reference sources, particularly optical cavity-based methods, are generally bulky, alignment-sensitive and expensive, and therefore forfeit the benefits of using chip-scale microcombs. Here, we realize compact and low-phase-noise microwave and soliton pulse generation by combining a silica-microcomb (with few-mm diameter) with a fibre-photonic-based timing reference (with few-cm diameter). An ultrastable 22-GHz microwave is generated with -110 dBc/Hz (-88 dBc/Hz) phase noise at 1-kHz (100-Hz) Fourier frequency and 10-13-level frequency instability within 1-s. This work shows the potential of fully packaged, palm-sized or smaller systems for generating both ultrastable soliton pulse trains and microwaves, thereby facilitating a wide range of field applications involving ultrahigh-stability microcombs.

Electro-optic control of the external coupling strength of a high-quality-factor lithium niobate micro-resonator.

Controlling the optical coupling between a micro-resonator and waveguide plays a key role in on-chip photonic circuits. Here, we demonstrate a two-point coupled lithium niobate (LN) racetrack micro-resonator that enables us to electro-optically traverse a full set of the zero-, under-, critical-, and over-coupling regimes with minimized disturbance of the intrinsic properties of the resonant mode. The modulation between the zero- and critical-coupling conditions cost a resonant frequency shift of only ∼344.2 MHz and rarely changed the intrinsic quality (Q) factor of 4.6 × 105. Our device is a promising element in on-chip coherent photon storage/retrieval and its applications.

Two-point coupling method to independently control coupling efficiency at different wavelengths.

To efficiently access light waves confined in a high-quality-factor (Q) microcavity over a wide spectral range, it is necessary to independently control coupling efficiency at different wavelengths. Here we suggest an approach to add a degree of freedom to control the coupling efficiency based on a two-point coupling geometry. By changing the phase difference between two paths connecting two coupling points, various combinations of coupling efficiencies at multiple wavelengths can be achieved. An analytic model describing the coupling property is derived and confirmed by experimental results. It is also shown that the coupling property can be modified by adjusting the effective refractive index difference between a waveguide and a resonator.

A squeezed quantum microcomb on a chip.

The optical microresonator-based frequency comb (microcomb) provides a versatile platform for nonlinear physics studies and has wide applications ranging from metrology to spectroscopy. The deterministic quantum regime is an unexplored aspect of microcombs, in which unconditional entanglements among hundreds of equidistant frequency modes can serve as critical ingredients to scalable universal quantum computing and quantum networking. Here, we demonstrate a deterministic quantum microcomb in a silica microresonator on a silicon chip. 40 continuous-variable quantum modes, in the form of 20 simultaneously two-mode squeezed comb pairs, are observed within 1 THz optical span at telecommunication wavelengths. A maximum raw squeezing of 1.6 dB is attained. A high-resolution spectroscopy measurement is developed to characterize the frequency equidistance of quantum microcombs. Our demonstration offers the possibility to leverage deterministically generated, frequency multiplexed quantum states and integrated photonics to open up new avenues in fields of spectroscopy, quantum metrology, and scalable, continuous-variable-based quantum information processing.

Supercontinuum generation in As2S3 waveguides fabricated without direct etching.

We report a supercontinuum generation (SCG) in a waveguide that spontaneously forms without an etching process during the deposition of a core material on a preformed ${\rm{Si}}{{\rm{O}}_2}$ substructure. The mechanism of dispersion control for this new, to the best of our knowledge, type of waveguide is analyzed by numerical simulation, which results in a design rule to achieve a target dispersion profile by adjusting the substructure geometry. SCG is experimentally demonstrated with a waveguide made of ${\rm{A}}{{\rm{s}}_2}{{\rm{S}}_3}$, chalcogenide glass, which has low material absorption over the mid-IR range. A dispersion-controlled waveguide with a length of 10 mm pumped with 77 pJ pulses at a telecommunication wavelength of 1560 nm resulted in a supercontinuum that extends by more than 1.5 octaves.

Self-stabilized soliton generation in a microresonator through mode-pulled Brillouin lasing.

Reducing the complexity required for starting and maintaining a soliton state has been a major task to fully miniaturize soliton microcombs including the accompanying external operating systems. Here we experimentally examine the generative process of a self-stabilized soliton in which a continuous-wave pump detuned on the thermally stable blue side of a resonance generates a Brillouin lasing signal that relays the pump power to the soliton pulses via intracavity mode-coupling without breaking thermal self-stability. Based on a simple setup consisting of a free-running laser and a microcavity without any external feedback systems by virtue of internal thermal locking, single-soliton pulses of 11 GHz repetition rate were deterministically generated. We demonstrate that the single-soliton pulses can be passively maintained over several days in a laboratory environment with a phase noise performance of -137dBc/Hz at 100 kHz.

Hydrophobic passivation of ultra-high-Q silica wedge resonators using hexamethyldisilazane.

Dissipative Kerr solitons in ultra-high-Q resonators are extremely sensitive to the thermal behavior of the resonators. Especially for resonators with hydrophilic surfaces, moisture continuously adsorbs on their surfaces and causes additional absorption loss that results in an excessive thermal shift of resonance frequency. This change makes soliton mode locking more challenging or even impossible. Here, we report hydrophobic monolayer passivation using hexamethyldisilazane on ultra-high-Q silica wedge resonators. It was experimentally confirmed that the Q-factor and dispersion were maintained after passivation, and excess thermal shift by moisture was inhibited for more than three days in the atmosphere. Soliton mode locking was successfully performed with the resonator one month after passivation.

Universal light-guiding geometry for on-chip resonators having extremely high Q-factor.

By providing an effective way to leverage nonlinear phenomena in integrated devices, high-Q optical resonators have led to recent advances in on-chip photonics. However, developing fabrication processes to shape any new material into a resonator with extremely smooth surfaces on a chip has been an exceptionally challenging task. Here, we describe a universal method to implement ultra-high-Q resonators with any new material having desirable properties that can be deposited by physical vapor deposition. Using this method light-guiding cores with surface roughness on the molecular-scale are created automatically on pre-patterned substrates. Its efficacy has been verified using As2S3, a chalcogenide glass that has high-nonlinearity. The Q-factor of the As2S3 resonator so-developed approached the propagation loss record achieved in chalcogenide fibers which were limited by material losses. Owing to the boosted Q-factor, lasing by stimulated Brillouin scattering has been demonstrated with 100 times lower threshold power than the previous record.

Transition of pulsed operation from Q-switching to continuous-wave mode-locking in a Yb:KLuW waveguide laser.

We report on the diverse pulsed operation regimes of a femtosecond-laser-written Yb:KLuW channel waveguide laser emitting near 1040 nm. By the precise position tuning of a carbon-nanotube-coated saturable absorber (SA) mirror, the transition of the pulsed operation from Q-switching, Q-switched mode-locking and finally sub-GHz continuous-wave mode-locking are obtained based on the interplay of dispersion and mode area control. The Q-switched pulses exhibit typical fast SA Q-switched pulse characteristics depending on absorbed pump powers. In the Q-switched mode-locking, amplitude modulations of the mode-locked pulses on the Q-switched envelope are observed. The radio-frequency spectrum represents the coexistence of Q-switching and mode-locking signals. In the purely mode-locked operation, the waveguide laser generates 2.05-ps pulses at 0.5 GHz.

A novel nucleic acid amplification system based on nano-gap embedded active disk resonators.

Recent advances in nucleic acid based testing using bio-optical sensor approaches have been introduced but most are based on hybridization between the optical sensor and the bio-molecule and not on an amplification mechanism. Direct nucleic acid amplification on an optical sensor has several technical limitations, such as the sensitivity of the temperature sensor, instrument complexity, and high background signal. We here describe a novel nucleic acid amplification method based on a whispering gallery mode active resonator and discuss its potential molecular diagnostic application. By implanting nanoclusters as active compounds, this active resonator operates without tapered fiber coupling and emits a strong photoluminescence signal with low background in the wavelength of low absorption in an aqueous environment that is typical of biosensors. Our method also offers an extremely low detection threshold down to a single copy within 10 min due to the strong light-matter interaction in a nano-gap structure. We envision that this active resonator provides a high refractive index contrast for tight mode confinement with simple alignment as well as the possibility of reducing the device size so that a point-of-care system with low-cost, high-sensitivity and simplicity.

Balanced Detection Method Using Optical Affinity Sensors for Quick Measurement of Biomolecule Concentrations.

To determine the concentration of biomolecules using a label-free optical biosensor, it is necessary to measure the serial signal from the reaction starting point, which is inconvenient for practical applications. Here, we propose an alternative detection method for determining the concentration of a biomolecule. The method, which is derived from the fraction bound equation of the Langmuir adsorption model, determines the concentration relative to a reference sample with required accuracy, with a single measurement at any point in time. We also experimentally demonstrated the method and its accuracy by detecting streptavidin-biotin complexes using on-chip optical sensors based on active disk resonators integrated with microfluidic circuits. By performing the proposed method in a simultaneous parallel measurement scheme, signal fluctuations evenly induced in the detectors by external perturbations could be automatically suppressed, similar to the balanced detection method. We expect our approach to be applicable to practical applications where fast and accurate detection responses are needed.

On-chip label-free biosensing based on active whispering gallery mode resonators pumped by a light-emitting diode.

Biosensing based on whispering-gallery mode (WGM) resonators has been continuously studied with great attention due to its excellent sensitivity guaranteeing the label-free detection. However, its practical impact is insignificant to date despite notable achievements in academic research. Here, we demonstrate a novel practical platform of on-chip WGM sensors integrated with microfluidic channels. By placing silicon nanoclusters as a stable active compound in micro-resonators, the sensor chip can be operated with a remote pump and readout, which simplifies the chip integration and connection to the external setup. In addition, silicon nanoclusters having large absorption cross-section over broad wavelength range allow active sensing for the first time with an LED pump in a top-illumination scheme which significantly reduces the complexity and cost of the measurement setup. The nano-slot structure of 25 nm gap width is embedded in the resonator where the target bio-molecules are selectively detected with the sensitivity enhanced by strongly confined mode-field. The sensitivity confirmed by real-time measurements for the streptavidin-biotin complex is 0.012 nm/nM, improved over 20 times larger than the previously reported WGM sensors with remote readout.

High quality chalcogenide-silica hybrid wedge resonator.

Chalcogenide glasses, with high nonlinearity and low loss, have captured research interest as an integrated device platform for near- and mid-infrared nonlinear optical devices. Compared to silicon-based microfabrication technologies, chalcogenide fabrication processes are less mature and a major challenge is obtaining high quality devices. In this paper, we report a hybrid resonator design leveraging a high quality silica resonator to achieve high Q factors with chalcogenide. The device is composed of a thin chalcogenide layer deposited on a silica wedge resonator. The hybrid resonators exhibit loaded Q factors up to 1.5 x 105 in the near-infrared region. We also measured the effective thermo-optic coefficient of the device to be 5.5x10-5/K, which agreed well with the bulk value. Thermal drift of the device can be significantly reduced by introducing a titanium dioxide cladding layer with a negative thermo-optic coefficient.

Electro-optical frequency division and stable microwave synthesis.

Optical frequency division by using frequency combs has revolutionized time keeping and the generation of stable microwave signals. We demonstrate optical frequency division and microwave generation by using a tunable electrical oscillator to create dual combs through phase modulation of two optical signals that have a stable difference frequency. Phase-locked control of the electrical oscillator by means of optical frequency division produces stable microwaves. Our approach transposes the oscillator and frequency reference of a conventional microwave frequency synthesizer. In this way, the oscillator experiences large phase noise reduction relative to the frequency reference. The electro-optical approach additionally relaxes the need for highly linear photodetection of the comb mode spacing. As well as simplicity, the technique is also tunable and scalable to higher division ratios.

Design and characterization of whispering-gallery spiral waveguides.

Whispering gallery delay lines have demonstrated record propagation length on a silicon chip and can provide a way to transfer certain applications of optical fiber to wafer-based systems. Their design and fabrication requires careful control of waveguide curvature and etching conditions to minimize connection losses between elements of the delay line. Moreover, loss characterization based on optical backscatter requires normalization to account for the impact of curvature on backscatter rate. In this paper we provide details on design of Archimedean whispering-gallery spiral waveguides, their coupling into cascaded structures, as well as optical loss characterization by optical backscatter reflectometry.

Low-noise Brillouin laser on a chip at 1064 nm.

We demonstrate narrow-linewidth-stimulated Brillouin lasers at 1064 nm from ultra-high-Q silica wedge disk resonators on silicon. Fundamental Schawlow-Townes frequency noise of the laser is on the order of 0.1 Hz2/Hz. The technical noise spectrum of the on-chip Brillouin laser is close to the thermodynamic noise limit of the resonator (thermorefractive noise) and is comparable to that of ultra-narrow-linewidth Nd:YAG lasers. The relative intensity noise of the Brillouin laser also is reduced by using an intensity-stabilized pump laser. Finally, low-noise microwave synthesis up to 32 GHz is demonstrated by heterodyne of first and third Brillouin Stokes lines from a single resonator.