Nonlinear Brillouin Research Articles

Storing light near an exceptional point

Photons with zero rest mass are impossible to be stopped. However, a pulse of light can be slowed down and even halted through strong light-matter interaction in a dispersive medium in atomic systems. Exceptional point (EP), a non-Hermitian singularity point, can introduce an abrupt transition in dispersion. Here we experimentally observe room-temperature storing light near an exceptional point induced by nonlinear Brillouin scattering in a chip-scale 90-μm-radius optical microcavity, the smallest platform up to date to store light. Through nonlinear coupling, a Parity-Time (PT) symmetry can be constructed in optical-acoustical hybrid modes, where Brillouin scattering-induced absorption (BSIA) can lead to both slow light and fast light of incoming pulses. A subtle transition of slow-to-fast light reveals a critical point for storing a light pulse up to half a millisecond. This compact and room-temperature scheme of storing light paves the way for practical applications in all-optical communications and quantum information processing.

Enhancement of Brillouin nonlinearities with a coupled resonator optical waveguide.

Stimulated Brillouin scattering (SBS) is a nonlinear optical phenomenon mediated from the coupling of photons and phonons. It has found applications in various realms, yet the acousto-optic interaction strength remains relatively weak. Enhancing the SBS with resonant structures could be a promising solution, but this method faces strict constraints in operational bandwidth. Here, we present the first demonstration to our knowledge of the broadband enhancement of Brillouin nonlinearities by a suspended coupled resonator optical waveguide (CROW) on an SOI platform. By comprehensively balancing the Brillouin gain and operational bandwidth, a 3-fold enhancement for the Brillouin gain coefficient (GB) and a broad operational bandwidth of over 80 GHz have been achieved. Furthermore, this 1.1 mm device shows a forward Brillouin gain coefficient of 2422 m-1W-1 and a high mechanical quality factor (Qm) of 1060. This approach marks a pivotal advancement toward wide bandwidth, low energy consumption, and compact integrated nonlinear photonic devices, with potential applications in tunable microwave photonic filters and phonon-based non-reciprocal devices.

Soliton Microcombs Multiplexing Using Intracavity-Stimulated Brillouin Lasers.

Solitons in microresonators have spurred intriguing nonlinear optical physics and photonic applications. Here, by combining Kerr and Brillouin nonlinearities in an over-modal microcavity, we demonstrate spatial multiplexing of soliton microcombs under a single external laser pumping operation. This demonstration offers an ideal scheme to realize highly coherent dual-comb sources in a compact, low-cost and energy-efficient manner, with uniquely low beating noise. Moreover, by selecting the dual-comb modes, the repetition rate difference of a dual-comb pair could be flexibly switched, ranging from 8.5 to 212MHz. Beyond dual-comb, the high-density mode geometry allows the cascaded Brillouin lasers, driving the co-generation of up to 5 space-multiplexing frequency combs in distinct mode families. This Letter offers a novel physics paradigm for comb interferometry and provides a widely appropriate tool for versatile applications such as comb metrology, spectroscopy, and ranging.

Modulation of Brillouin Optomechanical Interactions via Acoustoelectric Phonon-Electron Coupling

Optomechanical Brillouin nonlinearities---arising from the coupling between traveling photons and phonons---have become the basis for a range of powerful optical signal processing and sensing technologies. The dynamics of such interactions are largely set and limited by the host material's elastic, optical, and photoelastic properties, which are generally considered intrinsic and static. Here we propose and theoretically show that it is feasible to dynamically reconfigure the Brillouin nonlinear susceptibility in transparent semiconductors through acoustoelectric phonon-electron coupling. Acoustoelectric interactions permit a wide range of tunability of the phonon dissipation rate and velocity, perhaps the most influential parameters in the effective optomechanical cooperativity and Brillouin nonlinear susceptibility. We develop a Hamiltonian-based analysis that yields self-consistent dynamical equations and noise coupling, allowing us to explore the physics of such acoustoelectrically enhanced Brillouin (AEB) interactions and show that they give rise to a significant enhancement of the performance of Brillouin-based photonic technologies. Moreover, we show that these AEB effects can drive systems into regimes of fully coherent scattering that resemble the dynamics of optical parametric processes, significantly different than the incoherent traditional Brillouin limit. We propose and computationally explore a particular semiconductor heterostructure in which the acoustoelectric interaction arises from a piezoelectric phonon-electron coupling. We find that this system provides the necessary piezoelectric and carrier response (${k}^{2}\ensuremath{\approx}6\mathrm{%}$), favorable semiconductor materials properties, and large optomechanical confinement and coupling [$|{g}_{0}|\ensuremath{\approx}8000$ (rad/s)$\sqrt{\mathrm{m}}$] sufficient to demonstrate acoustoelectrically enhanced optomechanical interactions.

Photonic circuits for laser stabilization with integrated ultra-high Q and Brillouin laser resonators

The integration of stabilized lasers, sources that generate spectrally pure light, will provide compact, low-cost solutions for applications including quantum information sciences, precision navigation and timing, metrology, and high-capacity fiber communications. We report a significant advancement in this field, demonstrating stabilization of an integrated waveguide Brillouin laser to an integrated waveguide reference cavity, where both resonators are fabricated using the same CMOS-compatible integration platform. We demonstrate reduction of the free running Brillouin laser linewidth to a 292 Hz integral linewidth and carrier stabilization to a 4.9 × 10−13 fractional frequency at 8 ms reaching the cavity-intrinsic thermorefractive noise limit for frequencies down to 80 Hz. We achieve this level of performance using a pair of 56.4 × 106 quality factor Si3N4 waveguide ring-resonators that reduce the high-frequency noise by the nonlinear Brillouin process and the low-frequency noise by Pound–Drever–Hall locking to the ultra-low loss resonator. These results represent an important step toward integrated stabilized lasers with reduced sensitivity to environmental disturbances for atomic, molecular, and optical physics (AMO), quantum information processing and sensing, and other precision scientific, sensing, and communications applications.

Cavity optomechanics in a fiber cavity: the role of stimulated Brillouin scattering

We study the role of stimulated Brillouin scattering in a fiber cavity by numerical simulations and a simple theoretical model and find good agreement between experiment, simulation and theory. We also investigate an optomechanical system based on a fiber cavity in the presence of the nonlinear Brillouin scattering. Using simulation and theory, we show that this hybrid optomechanical system increases optomechanical damping for low mechanical resonance frequencies in the unresolved sideband regime. Furthermore, optimal damping occurs for blue detuning in stark contrast to standard optomechanics. We investigate whether this hybrid optomechanical system is capable of cooling a mechanical oscillator to the quantum ground state.

Ridge-type suspended waveguide Brillouin laser

As is well known, the on-chip waveguide with high Brillouin gain has many applications in the field of photonics. Brillouin lasers on silicon substrates are widely used in frequency tunable laser emission, mode-locked pulsed lasers, low-noise oscillators and optical gyroscopes. However, in a silicon-based Brillouin laser, a long waveguide length is still used to achieve Brillouin laser output, which is not conducive to on-chip integration. In this work is proposed a new type of waveguide structure consisting of chalcogenide As<sub>2</sub>S<sub>3</sub> rectangles and an air slit. Owing to the existence of the air gap, the radiation pressure makes the enhancement of Brillouin nonlinearity much higher than the enhancement caused only by the material nonlinearity. This makes the Brillouin gain reach 1.78 × 10<sup>5</sup> W<sup>–1</sup>·m<sup>–1</sup>, which is nearly 10 times larger than the previously reported backward SBS gain of 2.88 × 10<sup>4</sup> W<sup>–1</sup>·m<sup>–1</sup>, resulting in phonon frequency tuning in a 4.2–7.0 GHz range. This method provides a new idea for designing nano-scaled optical waveguides for forward stimulated Brillouin scattering, and at the same time, this enhanced broadband coherent phonon emission paves the way for improving the hybrid on-chip CMOS signal processing technology.

Phonon-induced anomalous gauge potential for photonic isolation in frequency space

Photonic gauge potentials are crucial for manipulating charge-neutral photons like their counterpart electrons in the electromagnetic field, allowing the analogous Aharonov–Bohm effect in photonics and paving the way for critical applications such as photonic isolation. Normally, a gauge potential exhibits phase inversion along two opposite propagation paths. Here we experimentally demonstrate phonon-induced anomalous gauge potentials with noninverted gauge phases in a spatial-frequency space, where near-phase-matched nonlinear Brillouin scatterings enable such unique direction-dependent gauge phases. Based on this scheme, we construct photonic isolators in the frequency domain permitting nonreciprocal propagation of light along the frequency axis, where coherent phase control in the photonic isolator allows switching completely the directionality through an Aharonov–Bohm interferometer. Moreover, similar coherent controlled unidirectional frequency conversions are also illustrated. These results may offer a unique platform for a compact, integrated solution to implement synthetic-dimension devices for on-chip optical signal processing.

Demonstration of Forward Brillouin Gain in a Hybrid Photonic–Phononic Silicon Waveguide

Silicon-based stimulated Brillouin scattering (SBS) promotes the on-chip all-optical signal processing network by interfacing silicon photonic and phononic technologies. Controllable and strong Brillouin coupling in silicon is a key requirement for this purpose. Here, we demonstrate traveling-wave forward SBS and Brillouin gain through a class of hybrid photonic-phononic silicon waveguides on the silicon-on-insulator (SOI) platform. This design combines the advantages of a silicon ridge waveguide and phononic crystal slab, allowing the independent control on the confined optical and acoustic modes. The strong and tailorable Brillouin nonlinearity is demonstrated via the heterodyne four-wave mixing spectroscopy. Three-tone gain experiment reveals a small-signal Stokes gain of 0.9 dB in a 1.085 cm length straight waveguide device at moderate pump power. The limiting factors and further improvements of net Brillouin amplification in our system are also discussed. This design can also be applied to the intermodal SBS as well as other silicon-based material platforms, and thus it offers the pathway toward on-chip microwave photonic filters, Brillouin amplifiers, and nonreciprocal devices.

Synthetic Anti-PT Symmetry in a Single Microcavity.

Non-Hermitian systems based on parity-time (PT) and anti-PT symmetry reveal rich physics beyond the Hermitian regime. So far, realizations of such symmetric systems have been limited to the spatial domain. Here we theoretically and experimentally demonstrate synthetic anti-PT symmetry in a spectral dimension induced by nonlinear Brillouin scattering in a single optical microcavity, where Brillouin scattering induced transparency or absorption in two spectral resonances provides the optical gain and loss to observe a phase transition between two symmetry regimes. This scheme provides a new paradigm towards the investigation of non-Hermitian physics in a synthetic photonic dimension for all-optical signal processing and quantum information science.

Nonlinear Brillouin spectroscopy: what makes it a better tool for biological viscoelastic measurements.

Brillouin spectroscopy is an emerging tool in biomedical imaging and sensing. It is capable of assessing the high-frequency viscoelastic longitudinal modulus with microscopic spatial resolution. Nonlinear Brillouin spectroscopy based on impulsive stimulated Brillouin scattering offers a number of significant advantages over conventional spontaneous and stimulated Brillouin scattering. In this report, we evaluate the accuracy of Brillouin shift measurements in spontaneous and nonlinear Brillouin microscopy by calculating the Allan variance for both CW excited spontaneous Brillouin measurements and nonlinear Brillouin scattering measurements made with both nanosecond and picosecond pulse excitation. We find that impulsive stimulated Brillouin spectroscopy is superior to spontaneous Brillouin spectroscopy in terms of the accuracy of such measurements and demonstrate its application for assessing tiny changes in Brillouin frequency shifts associated with low concentrations of biologically relevant solutions.

Brillouin Dynamic Gratings-A Practical Form of Brillouin Enhanced Four Wave Mixing in Waveguides: The First Decade and Beyond.

Brillouin-Enhanced Four-Wave-Mixing techniques, which couple four optical beams through Brillouin nonlinearity, have gained popularity in the 1980’s largely owing to their phase conjugation properties. Experiments were mainly conducted in liquid cells. The interest in Brillouin-Enhanced Four-Wave-Mixing has reawakened in the 2000’s, following the quest for dynamically reconfigurable gratings in optical fibers. Termed Brillouin Dynamic Grating this time around, it is, in fact, an acoustic wave, optically generated by stimulated Brillouin scattering process between two pump waves. The acoustic wave either carries the coherent information encoded by the pump beams, or in the case of sensing applications, its properties are determined by the environmental parameters. This information, in turn, is imparted to the third phase-matched optical probe wave through the elasto-optic effect. Over the last decade, this mechanism allowed for the realization of many all-optical signal processing functions and has proven instrumental in distributed sensing applications. This paper describes the basics, as well as the state of the art, of BDG-based applications in optical fibers. It also surveys the efforts being done to carry over these concepts to the photonic chip level.

Demonstration of forward cascaded Brillouin lasing using a silicon-based rectangular spiral microring resonator assisted by EDFA

We experimentally demonstrate the forward cascaded Brillouin lasing exploiting a silicon-based rectangular spiral microring resonator assisted by an erbium-doped fiber amplifier. To realize the enhanced Brillouin nonlinearity, the optical and acoustic fields are effectively confined by partly suspending the spiral resonator. The free spectral range is precisely designed to match the half of the Brillouin frequency shift to guarantee Brillouin laser oscillation. The fabricated spiral resonator is incorporated in a fiber loop to serve as not only a resonance-enhanced element to generate the internal pump lasing for Brillouin scattering but also a Brillouin gain medium to excite Brillouin lasing. The spiral structure allows a 0.6368 cm long resonator waveguide to be enclosed in a small footprint of 250 × 330 μm2. Four anti-Stokes and three Stokes lasing lines are obtained with the Brillouin frequency shift of about 12.0463 GHz. The proposed approach provides a potential way to implement Brillouin lasing on a silicon-based chip.

On-chip inter-modal Brillouin scattering

Brillouin nonlinearities—which result from coupling between photons and acoustic phonons—are exceedingly weak in conventional nanophotonic silicon waveguides. Only recently have Brillouin interactions been transformed into the strongest and most tailorable nonlinear interactions in silicon using a new class of optomechanical waveguides that control both light and sound. In this paper, we use a multi-mode optomechanical waveguide to create stimulated Brillouin scattering between light-fields guided in distinct spatial modes of an integrated waveguide for the first time. This interaction, termed stimulated inter-modal Brillouin scattering, decouples Stokes and anti-Stokes processes to enable single-sideband amplification and dynamics that permit near-unity power conversion. Using integrated mode multiplexers to address separate optical modes, we show that circulators and narrowband filters are not necessary to separate pump and signal waves. We also demonstrate net optical amplification and Brillouin energy transfer as the basis for flexible on-chip light sources, amplifiers, nonreciprocal devices and signal-processing technologies.

Brillouin backward scattering in the nonlinear interaction of a short-pulse laser with an underdense transversely magnetized plasma with nonextensive distribution

Nonlinear Brillouin backward scattering of a linearly polarized short laser pulse propagating through a homogenous nonextensive distributed plasma in the presence of a uniform magnetic field perpendicular to both the direction of propagation and electric vector of the radiation field is investigated theoretically when ponderomotive relativistic and nonlinearity effects up to third order are taken into account. The governing equations for nonlinear wave in the context of nonextensive statistics are given, and the nonextensive coupled equations describing the nonlinear Brillouin backward scattering instability are solved by the Fourier transformation method, and the growth rate of the nonlinear Brillouin backward scattering instability is obtained. The results in the case q→1 are consistent with those in the framework of the Maxwellian distribution. It is found that the instability growth rate increases on increasing plasma density, radiation field amplitude, and nonextensive parameter, while the instability growth rate shows a decrease due to the presence of external magnetic field.

Impulsive Brillouin microscopy

Brillouin scattering has been emerging as a viable tool for microscopy. However, most of the work done has been with the use of spontaneous Brillouin scattering, which has several hindrances to its use. In this work, we propose and demonstrate nonlinear Brillouin scattering as a solution to many of these hindrances. Here we demonstrate fast two-dimensional microscopic optical imaging of materials’ mechanical properties for the very first time (to our knowledge) using nonlinear Brillouin scattering. Impulsive stimulated Brillouin scattering (ISBS) was used in an optical configuration that is capable of providing accurate local assessment of viscoelastic properties faster than conventional Brillouin spectroscopy. This proof-of-principle imaging experiment has been demonstrated for materials of known properties and microfluidic devices. Applications to noninvasive biomedical imaging are discussed. The fast acquisition times and strong signal of ISBS coupled with the ability of Brillouin scattering to easily measure materials’ viscoelastic properties make this an attractive technique for biological use.

Stimulated Brillouin Scattering Microscopic Imaging.

Two-dimensional stimulated Brillouin scattering microscopy is demonstrated for the first time using low power continuous-wave lasers tunable around 780 nm. Spontaneous Brillouin spectroscopy has much potential for probing viscoelastic properties remotely and non-invasively on a microscopic scale. Nonlinear Brillouin scattering spectroscopy and microscopy may provide a way to tremendously accelerate the data aquisition and improve spatial resolution. This general imaging setup can be easily adapted for specific applications in biology and material science. The low power and optical wavelengths in the water transparency window used in this setup provide a powerful bioimaging technique for probing the mechanical properties of hard and soft tissue.

Sub-optical wavelength acoustic wave modulation of integrated photonic resonators at microwave frequencies.

Light-sound interactions have long been exploited in various acousto-optic devices based on bulk crystalline materials. Conventionally, these devices operate in megahertz frequency range where the acoustic wavelength is much longer than the optical wavelength and a long interaction length is required to attain significant coupling. With nanoscale transducers, acoustic waves with sub-optical wavelengths can now be excited to induce strong acousto-optic coupling in nanophotonic devices. Here we demonstrate microwave frequency surface acoustic wave transducers co-integrated with nanophotonic resonators on piezoelectric aluminum nitride substrates. Acousto-optic modulation of the resonance modes at above 10 GHz with the acoustic wavelength significantly below the optical wavelength is achieved. The phase and modal matching conditions in this scheme are investigated for efficient modulation. The new acousto-optic platform can lead to novel optical devices based on nonlinear Brillouin processes and provides a direct, wideband link between optical and microwave photons for microwave photonics and quantum optomechanics.

Multiwavelength fiber laser employing a nonlinear Brillouin optical loop mirror: experimental and numerical studies.

We numerically and experimentally study a multiwavelength fiber laser (MWFL) employing a nonlinear Brillouin optical loop mirror (NBOLM). Taking into account the impact of stimulated Brillouin scattering (SBS) effect on nonlinear polarization evolution, we present the power transmission equation of Stokes lines from the NBOLM. Thereafter, we combine the power transmission equation, coupled wave equations of SBS process in NBOLM, rate and power propagation equations in the erbium-doped fiber (EDF) to build up a model for the MWFL. Using this model, we can explain the impacts of EDF pump power, input polarization state and quarter-wave-plate angle on the number and amplitude flatness of output Stokes lines. Furthermore, the results from numerical calculations are verified by the experimental measurements.

Nonlinear Brillouin amplification of finite-duration seeds in the strong coupling regime

Parametric plasma processes received renewed interest in the context of generating ultra-intense and ultra-short laser pulses up to the exawatt-zetawatt regime. Both Raman as well as Brillouin amplifications of seed pulses were proposed. Here, we investigate Brillouin processes in the one-dimensional (1D) backscattering geometry with the help of numerical simulations. For optimal seed amplification, Brillouin scattering is considered in the so called strong coupling (sc) regime. Special emphasis lies on the dependence of the amplification process on the finite duration of the initial seed pulses. First, the standard plane-wave instability predictions are generalized to pulse models, and the changes of initial seed pulse forms due to parametric instabilities are investigated. Three-wave-interaction results are compared to predictions by a new (kinetic) Vlasov code. The calculations are then extended to the nonlinear region with pump depletion. Generation of different seed layers is interpreted by self-similar solutions of the three-wave interaction model. Similar to Raman amplification, shadowing of the rear layers by the leading layers of the seed occurs. The shadowing is more pronounced for initially broad seed pulses. The effect is quantified for Brillouin amplification. Kinetic Vlasov simulations agree with the three-wave interaction predictions and thereby affirm the universal validity of self-similar layer formation during Brillouin seed amplification in the strong coupling regime.