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Surface acoustic wave (SAW) devices are ubiquitously used for signal processing and filtering, as well as mechanical, chemical and biological sensing, and show promise as quantum transducers. However, nowadays most SAWs are excited and driven via electromechanical coupling and interdigital transducers (IDTs), limiting operation bandwidth and flexibility. Novel ways to coherently excite and detect SAWs all-optically interfaced with photonic integrated circuits are yet elusive. Backward Stimulated Brillouin scattering (SBS) provides strong coherent interactions between optical and acoustic waves in chip-scale waveguides, however, demonstrations have been limited to single longitudinal waves in the waveguide core. Here, we numerically model and experimentally demonstrate surface acoustic wave stimulated Brillouin scattering (SAW-SBS) on a photonic chip. We designed and fabricated tailored waveguides made out of GeAsSe glass that show good overlap between SAWs at 3.81 GHz and guided optical modes, without requiring a top cladding. We measure a 225 W$^{-1}$m$^{-1}$ Brillouin gain coefficient of the surface acoustic resonance and linewidth narrowing to 40 MHz. Experimentally accessing this new regime of stimulated Brillouin scattering opens the door for novel on-chip sensing and signal processing applications, strong Brillouin interactions in materials that do not provide sufficient acoustic guidance in the waveguide core as well as excitation of surface acoustic waves in non-piezoelectric materials., Comment: 7 pages, 4 figures

Signal processing based on stimulated Brillouin scattering (SBS) is limited by the narrow linewidth of the optoacoustic response, which confines many Brillouin applications to continuous wave signals or optical pulses longer than several nanoseconds. In this work, we experimentally demonstrate Brillouin interactions at the 150 ps time scale and a delay for a record 15 ns which corresponds to a delay of 100 pulse widths. This breakthrough experimental result was enabled by the high local gain of the chalcogenide waveguides as the optoacoustic interaction length reduces with pulse width. We successfully transfer 150ps-long pulses to traveling acoustic waves within a Brillouin-based memory setup. The information encoded in the optical pulses is stored for 15 ns in the acoustic field. We show the retrieval of eight amplitude levels, multiple consecutive pulses and low distortion in pulse shape. The extension of Brillouin-based storage to the ultra-short pulse regime is an important step for the realisation of practical Brillouin-based delay lines and other optical processing applications., Comment: 6 pages, 6 figures

Non-reciprocal optical components such as isolators and circulators are crucial for preventing catastrophic back-reflection and controlling optical crosstalk in photonic systems. While non-reciprocal devices based on Brillouin intermodal transitions have been experimentally demonstrated in chip-scale platforms, harnessing such interactions has required a suspended waveguide structure, which is challenging to fabricate and is potentially less robust than a non-suspended structure, thereby limiting the design flexibility. In this paper, we numerically investigate the performance of a Brillouin-based isolation scheme in which a dual-pump-driven optoacoustic interaction is used to excite confined acoustic waves in a traditional ridge waveguide. We find that acoustic confinement, and therefore the amount of Brillouin-driven mode conversion, can be enhanced by selecting an appropriate optical mode pair and waveguide geometry of two arsenic based chalcogenide platforms. Further, we optimize the isolator design in its entirety, including the input couplers, mode filters, the Brillouin-active waveguide as well as the device fabrication tolerances. We predict such a device can achieve 30 dB isolation over a 38 nm bandwidth when 500 mW pump power is used; in the presence of a +/- 10 nm fabrication-induced width error, such isolation can be maintained over a 5-10 nm bandwidth.

Vital sign detection is used across ubiquitous scenarios in medical and health settings. Contact and wearable sensors have been widely deployed. However, they are unsuitable for patients with burn wounds or infants with insufficient attaching areas. Contactless detection can be achieved using camera imaging, but it is susceptible to ambient light conditions and creates privacy concerns. Here, we report the first demonstration of a photonic radar for non-contact vital signal detection to overcome these challenges. This photonic radar can achieve millimeter range resolution based on synthesized radar signals with a bandwidth of up to 30 GHz. The high resolution of the radar system enables accurate respiratory detection from breathing simulators and a cane toad as a human proxy. Moreover, we demonstrated that the optical signals generated from the proposed system can enable vital sign detection based on light detection and ranging (LiDAR). This demonstration reveals the potential of a sensor-fusion architecture that can combine the complementary features of radar and LiDAR for improved sensing accuracy and system resilience. The work provides a novel technical basis for contactless, high-resolution, and high-privacy vital sign detection to meet the increasing demands in future medical and healthcare applications.

Photonic generation of radio-frequency signals has shown significant advantages over the electronic counterparts, allowing the high precision generation of radio-frequency carriers up to the terahertz-wave region with flexible bandwidth for radar applications. Great progress has been made in photonics-based radio-frequency waveform generation. However, the approaches that rely on sophisticated benchtop digital microwave components, such as synthesizers and digital-to-analog converters have limited achievable bandwidth and thus resolution for radar detections. Methods based on voltage-controlled analog oscillators exhibit high time-frequency non-linearity, causing degraded sensing precision. Here, we demonstrate, for the first time, a photonic stepped-frequency (SF) waveform generation scheme enabled by MHz electronics with a tunable bandwidth exceeding 30 GHz and intrinsic time-frequency linearity. The ultra-wideband radio-frequency signal generation is enabled by using a polarization-stabilized optical cavity to suppress intra-cavity polarization-dependent instability; meanwhile, the signal's high-linearity is achieved via consecutive MHz acousto-optic frequency-shifting modulation without the necessity of using electro-optic modulators that have bias-drifting issues. We systematically evaluate the system's signal quality and imaging performance in comparison with conventional photonic radar schemes that use high-speed digital electronics, confirming its feasibility and excellent performance for high-resolution radar applications.

Optical modulation plays arguably the utmost important role in microwave photonic (MWP) systems. Precise synthesis of modulated optical spectra dictates virtually all aspects of MWP system quality including loss, noise figure, linearity, and the types of functionality that can be executed. But for such a critical function, the versatility to generate and transform analog optical modulation is severely lacking, blocking the pathways to truly unique MWP functions including ultra-linear links and low-loss high rejection filters. Here we demonstrate versatile RF photonic spectrum synthesis in an all-integrated silicon photonic circuit, enabling electrically-tailorable universal analog modulation transformation. We show a series of unprecedented RF filtering experiments through monolithic integration of the spectrum-synthesis circuit with a network of reconfigurable ring resonators.

Electrodes in close proximity to an active area of a device are required for sufficient electrical control. The integration of such electrodes into optical devices can be challenging since low optical losses must be retained to realise high quality operation. Here, we demonstrate that it is possible to place a metallic shallow phosphorus doped layer in a silicon micro-ring cavity that can function at cryogenic temperatures. We verify that the shallow doping layer affects the local refractive index while inducing minimal losses with quality factors up to 10$^5$. This demonstration opens up a pathway to the integration of an electronic device, such as a single-electron transistor, into an optical circuit on the same material platform., Comment: 6 figures

We formulate theoretically and demonstrate experimentally an all-optical method for reconstruction of the amplitude, phase and coherence of frequency combs from a single-shot measurement of the spectral intensity. Our approach exploits synthetic frequency lattices with pump-induced spectral short- and long-range couplings between different signal components across a broad bandwidth of of hundreds GHz in a single nonlinear fiber. When combined with ultra-fast signal conversion techniques, this approach has the potential to provide real-time measurement of pulse-to-pulse variations in the spectral phase and coherence properties of exotic light sources., Comment: 15 pages, 4 figures

Geometrical dimensionality plays a fundamentally important role in the topological effects arising in discrete lattices. While direct experiments are limited by three spatial dimensions, the research topic of synthetic dimensions implemented by the frequency degree of freedom in photonics is rapidly advancing. The manipulation of light in such artificial lattices is typically realized through electro-optic modulation, yet their operating bandwidth imposes practical constraints on the range of interactions between different frequency components. Here we propose and experimentally realize all-optical synthetic dimensions involving specially tailored simultaneous short- and long-range interactions between discrete spectral lines mediated by frequency conversion in a nonlinear waveguide. We realize triangular chiral-tube lattices in three-dimensional space and explore their four-dimensional generalization. We implement a synthetic gauge field with nonzero magnetic flux and observe the associated multidimensional dynamics of frequency combs, all within one physical spatial port. We anticipate that our method will provide a new means for the fundamental study of high-dimensional physics and act as an important step towards using topological effects in optical devices operating in the time and frequency domains., Comment: 20 pages, 6 figures

Acoustic waves can serve as memory for optical information, however, acoustic phonons in the GHz regime decay on the nanosecond timescale. Usually this is dominated by intrinsic acoustic loss due to inelastic scattering of the acoustic waves and thermal phonons. Here we show a way to counteract the intrinsic acoustic decay of the phonons in a waveguide by resonantly reinforcing the acoustic wave via synchronized optical pulses. This scheme overcomes the previous constraints of phonon-based optical signal processing for light storage and memory. We experimentally demonstrate on-chip storage up to 40 ns, four times the intrinsic acoustic lifetime in the waveguide. We confirm the coherence of the scheme by detecting the phase of the delayed optical signal after 40 ns using homodyne detection. Through theoretical considerations we anticipate that this concept allows for storage times up to microseconds within realistic experimental limitations while maintaining a GHz bandwidth of the optical signal. The refreshed phonon-based light storage removes the usual bandwidth-delay product limitations of e.g. slow-light schemes., Comment: 6 pages, 5 figures, BS and MM contributed equally

Higher-order soliton dynamics, specifically soliton compression and fission, underpin crucial applications in ultrafast optics, sensing, communications, and signal processing. Bragg solitons exploit the strong dispersive properties of periodic media near the photonic band edge, enabling soliton dynamics to occur on chip-scale propagation distances and opening avenues to harness soliton compression and fission in integrated photonic platforms. However, implementation in CMOS-compatible platforms has been hindered by the strong nonlinear loss that dominates the propagation of high-intensity pulses in silicon and the low-optical nonlinearity of traditional silicon nitride. Here, we present CMOS-compatible, on-chip Bragg solitons, with the largest soliton-effect pulse compression to date with a factor of x5.7, along with the first time-resolved measurements of soliton fission on a CMOS-compatible platform. These observations were enabled by the combination of unique cladding-modulated Bragg grating design, the high nonlinearity and negligible nonlinear loss of compositionally engineered ultra-silicon-rich nitride (USRN: Si7N3)., Comment: 20 pages including the 2 pages of Supplementary Material. Manuscript includes 6 figures and 1 supplementary figure

Integrated silicon microwave photonics offers great potential in microwave phase shifter elements, and promises compact and scalable multi-element chips that are free from electromagnetic interference. Stimulated Brillouin scattering, which was recently demonstrated in silicon, is a particularly powerful approach to induce a phase shift due to its inherent flexibility, offering an optically controllable and selective phase shift. However, to date, only moderate amounts of Brillouin gain has been achieved and theoretically this would restrict the phase shift to a few tens of degrees, significantly less than the required 360 degrees. Here, we overcome this limitation with a phase enhancement method using RF interference, showing a 360 degrees broadband phase shifter based on Brillouin scattering in a suspended silicon waveguide. We achieve a full 360 degrees phase-shift over a bandwidth of 15 GHz using a phase enhancement factor of 25, thereby enabling practical broadband Brillouin phase shifter for beam forming and other applications., Comment: \copyright 2019 Optical Society of America. Users may use, reuse, and build upon the article, or use the article for text or data mining, so long as such uses are for non-commercial purposes and appropriate attribution is maintained. All other rights are reserved

This article offers an extensive survey of results obtained using hybrid photonic crystal fibers (PCFs) which constitute one of the most active research fields in contemporary fiber optics. The ability to integrate novel and functional materials in solid- and hollow-core PCFs through various post-processing methods has enabled new directions towards understanding fundamental linear and nonlinear phenomena as well as novel application aspects, within the fields of optoelectronics, material and laser science, remote sensing and spectroscopy. Here the recent progress in the field of hybrid PCFs is reviewed from scientific and technological perspectives, focusing on how different fluids, solids and gases can significantly extend the functionality of PCFs. In the first part of this review we discuss the most important efforts by research groups around the globe to develop tunable linear and nonlinear fiber-optic devices using PCFs infiltrated with various liquids, glasses, semiconductors and metals. The second part is concentrated on the most recent and state-of-the-art advances in the field of gas-filled hollow-core PCFs. Extreme ultrafast gas-based nonlinear optics towards light generation in the extreme wavelength regions of vacuum ultraviolet (VUV), pulse propagation and compression dynamics in both atomic and molecular gases, and novel soliton - plasma interactions are reviewed. A discussion of future prospects and directions is also included.

Entangled multiphoton states lie at the heart of quantum information, computing, and communications. In recent years, topology has risen as a new avenue to robustly transport quantum states in the presence of fabrication defects, disorder and other noise sources. Whereas topological protection of single photons and correlated photons has been recently demonstrated experimentally, the observation of topologically protected entangled states has thus far remained elusive. Here, we experimentally demonstrate the topological protection of spatially-entangled biphoton states. We observe robustness in crucial features of the topological biphoton correlation map in the presence of deliberately introduced disorder in the silicon nanophotonic structure, in contrast with the lack of robustness in nontopological structures. The topological protection is shown to ensure the coherent propagation of the entangled topological modes, which may lead to robust propagation of quantum information in disordered systems., Comment: 17 pages, 5 figures

We theoretically investigate a new class of silicon waveguides for achieving Stimulated Brillouin Scattering (SBS) in the mid-infrared (MIR). The waveguide consists of a rectangular core supporting a low-loss optical mode, suspended in air by a series of transverse ribs. The ribs are patterned to form a finite quasi-one-dimensional phononic crystal, with the complete stopband suppressing the transverse leakage of acoustic waves, and confining them to the core of the waveguide. We derive a theoretical formalism that can be used to compute the opto-acoustic interaction in such periodic structures, and find forward intramodal-SBS gains up to $1750~\text{m}^{-1}\text{W}^{-1}$, which compares favorably with the proposed MIR SBS designs based on buried germanium waveguides. This large gain is achieved thanks to the nearly complete suppression of acoustic radiative losses.

Wavelength-scale SBS waveguides are enabling novel on-chip functionalities. The micro- and nano-scale SBS structures and the complexity of the SBS waveguides require a characterization technique to monitor the local geometry-dependent SBS responses along the waveguide. In this work, we experimentally demonstrate detection of longitudinal features down to 200$\mu$m on a silicon-chalcogenide waveguide using the Brillouin optical correlation domain analysis (BOCDA) technique. We provide simulation and analysis on how multiple acoustic and optical modes and geometrical variations influence the Brillouin spectrum., Comment: 8 pages, 6 figures

Breaking the symmetry between forward and backward propagating optical modes is of fundamental scientific interest and enables crucial functionalities, such as isolators, circulators, and duplex communication systems. Whereas there has been progress in achieving optical isolation on-chip, integrated broadband non-reciprocal signal processing functionalities that enable transmitting and receiving via the same low-loss planar waveguide, without altering the frequency or mode of the signal, remain elusive. Here, we demonstrate a non-reciprocal delay scheme based on the uni-directional transfer of optical data pulses to acoustic waves in a chip-based integration platform. We experimentally demonstrate that this scheme is not impacted by simultaneously counter-propagating optical signals. Furthermore, we achieve a bandwidth more than an order of magnitude broader than the intrinsic opto-acoustic linewidth, linear operation for a wide range of signal powers, and importantly, show that this scheme is wavelength preserving and avoids complicated multi-mode structures.., Comment: 8 pages, 6 figures, Moritz Merklein and Birgit Stiller contributed equally to this work

Recent advances in design and fabrication of photonic-phononic waveguides have enabled stimulated Brillouin scattering (SBS) in silicon-based platforms, such as under-etched silicon waveguides and hybrid waveguides. Due to the sophisticated design and more importantly high sensitivity of the Brillouin resonances to geometrical variations in micro- and nano-scale structures, it is necessary to have access to the localized opto-acoustic response along those waveguides to monitor their uniformity and maximize their interaction strength. In this work, we design and fabricate photonic-phononic waveguides with a deliberate width variation on a hybrid silicon-chalcogenide photonic chip and confirm the effect of the geometrical variation on the localized Brillouin response using a distributed Brillouin measurement., Comment: 5 pages, 4 figures

Storing and delaying optical signals plays a crucial role in data centers, phased array antennas, communication and future computing architectures. Here, we show a delay scheme based on cascaded Brillouin light storage, that achieves multi-stage delay at arbitrary positions within a photonic integrated circuit. Importantly these multiple resonant transfers between the optical and acoustic domain are controlled solely via external optical control pulses, allowing cascading of the delay without the need of aligning multiple structural resonances along the optical circuit., Comment: 5 pages, 5 figures, Birgit Stiller and Moritz Merklein contributed equally to this work

Stimulated Brillouin scattering drives a coherent interaction between optical signals and acoustic phonons and this effect can be used for storing optical information in acoustic waves. An important consideration arises when multiple optical frequencies are simultaneously employed in the Brillouin process: in this case the acoustic phonons that are addressed by each optical wavelength can be separated by frequencies far smaller than the acoustic phonon linewidth, potentially leading to crosstalk between the optical modes. Here we extend the concept of Brillouin-based light storage to multiple wavelength channels. We experimentally and theoretically show that the accumulated phase mismatch over the length of the spatially extended phonons allows each optical wavelength channel to address a distinct phonon mode, ensuring negligible crosstalk, even if the phonons overlap in frequency. Moreover, we demonstrate that the strict phase matching condition enables the preservation of the coherence of the opto-acoustic transfer at closely spaced multiple acoustic frequencies. This particular phase-mismatch for broad-bandwidth pulses has far-reaching implications allowing dense wavelength multiplexing in Brillouin-based light storage, multi-frequency Brillouin sensing, multi-wavelength Brillouin lasers, parallel microwave processing and quantum photon-phonon interactions., Comment: 11 pages, 12 figures, Birgit Stiller and Moritz Merklein contributed equally to this work

Modern fiber-optic coherent communications employ advanced spectrally-efficient modulation formats that require sophisticated narrow linewidth local oscillators (LOs) and complex digital signal processing (DSP). Here, we establish a novel approach to carrier recovery harnessing large-gain stimulated Brillouin scattering (SBS) on a photonic chip for up to 116.82 Gbit/sec self-coherent optical signals, eliminating the need for a separate LO. In contrast to SBS processing on-fiber, our solution provides phase and polarization stability while the narrow SBS linewidth allows for a record-breaking small guardband of ~265 MHz, resulting in higher spectral-efficiency than benchmark self-coherent schemes. This approach reveals comparable performance to state-of-the-art coherent optical receivers without requiring advanced DSP. Our demonstration develops a low-noise and frequency-preserving filter that synchronously regenerates a low-power narrowband optical tone that could relax the requirements on very-high-order modulation signaling and be useful in long-baseline interferometry for precision optical timing or reconstructing a reference tone for quantum-state measurements., Comment: Part of this work has been presented as a postdealine paper at CLEO Pacific-Rim'2017 and OSA Optica

We suggest and experimentally realize a spectral photonic lattice - a signal can hop between discrete frequency channels, driven by nonlinear interaction with stronger pump lasers. By controlling the complex envelope and frequency separations of multiple pumps, it is possible to introduce non- local hopping and to break time-reversal symmetry, which opens up new possibilities for photonic quantum simulation. As two examples, we observe a spectral quantum walk and demonstrate the discrete Talbot effect in the spectral domain, where we find novel instances containing asymmetry and periodicities not possible in spatial lattices.

The interaction of optical and acoustic waves via stimulated Brillouin scattering (SBS) has recently reached on-chip platforms, which has opened new fields of applications ranging from integrated microwave photonics and on-chip narrow-linewidth lasers, to phonon-based optical delay and signal processing schemes. Since SBS is an effect that scales exponentially with interaction length, on-chip implementation on a short length scale is challenging, requiring carefully designed waveguides with optimized opto-acoustic overlap. In this work, we use the principle of Brillouin optical correlation domain analysis (BOCDA) to locally measure the SBS spectrum with high spatial resolution of 800 {\mu}m and perform a distributed measurement of the Brillouin spectrum along a spiral waveguide in a photonic integrated circuit (PIC). This approach gives access to local opto-acoustic properties of the waveguides, including the Brillouin frequency shift (BFS) and linewidth, essential information for the further development of high quality photonic-phononic waveguides for SBS applications., Comment: 7 pages, 5 figures

The ability to generate complex optical photon states involving entanglement between multiple optical modes is not only critical to advancing our understanding of quantum mechanics but will play a key role in generating many applications in quantum technologies. These include quantum communications, computation, imaging, microscopy and many other novel technologies that are constantly being proposed. However, approaches to generating parallel multiple, customisable bi- and multi-entangled quantum bits (qubits) on a chip are still in the early stages of development. Here, we review recent developments in the realisation of integrated sources of photonic quantum states, focusing on approaches based on nonlinear optics that are compatible with contemporary optical fibre telecommunications and quantum memory infrastructures as well as with chip-scale semiconductor technology. These new and exciting platforms hold the promise of compact, low-cost, scalable and practical implementations of sources for the generation and manipulation of complex quantum optical states on a chip, which will play a major role in bringing quantum technologies out of the laboratory and into the real world., Comment: 27 pages, 6 figures, 209 references, review paper published on accepted article previews

We report record low loss silicon-on-sapphire nanowires for applications to mid infrared optics. We achieve propagation losses as low as 0.8dB/cm at 1550nm, 1.1 to 1.4dB/cm at 2080nm and < 2dB/cm at = 5.18 microns., Comment: 9 pages, 6 figures, 18 references

Controlling and manipulating quanta of coherent acoustic vibrations - phonons - in integrated circuits has recently drawn a lot of attention, since phonons can function as unique links between radiofrequency and optical signals, allow access to quantum regimes and offer advanced signal processing capabilities. Recent approaches based on optomechanical resonators have achieved impressive quality factors allowing for storage of optical signals. However, so far these techniques have been limited in bandwidth and are incompatible with multi-wavelength operation. In this work, we experimentally demonstrate a coherent buffer in an integrated planar optical waveguide by transferring the optical information coherently to an acoustic hypersound wave. Optical information is extracted using the reverse process. These hypersound phonons have similar wavelengths as the optical photons but travel at 5-orders of magnitude lower velocity. We demonstrate the storage of phase and amplitude of optical information with GHz-bandwidth and show operation at separate wavelengths with negligible cross-talk., Comment: 9 pages, 6 figures, Moritz Merklein and Birgit Stiller contributed equally to this work

We demonstrate the operation of an all-optical buffer based on temporal cavity solitons stored in a nonlinear passive fiber ring resonator. Unwanted acoustic interactions between neighboring solitons are suppressed by modulating the phase of the external laser driving the cavity. A new locking scheme is presented that allows the buffer to operate with an arbitrarily large number of cavity solitons in the loop. Experimentally, we are able to demonstrate the storage of 4536 bits of data, written all-optically into the fiber ring at 10 Gb/s, for 1 minute., Comment: 5 pages, 4 figures

Inelastic scattering processes such as Brillouin scattering can often function in cascaded regimes and this is likely to occur in certain integrated opto-acoustic devices. We develop a Hamiltonian formalism for cascaded Brillouin scattering valid for both quantum and classical regimes. By regarding Brillouin scattering as the interaction of a single acoustic envelope and a single optical envelope that covers all Stokes and anti-Stokes orders, we obtain a compact model that is well suited for numerical implementation, extension to include other optical nonlinearities or short pulses, and application in the quantum-optics domain. We then theoretically analyze intra-mode forward Brillouin scattering (FBS) for arbitrary waveguides with and without optical dispersion. In the absence of optical dispersion, we find an exact analytical solution. With a perturbative approach, we furthermore solve the case of weak optical dispersion. Our work leads to several key results on intra-mode FBS. For negligible dispersion, we show that cascaded intra-mode FBS results in a pure phase modulation and discuss how this necessitates specific experimental methods for the observation of fibre-based and integrated FBS. Further, we discuss how the descriptions that have been established in these two classes of waveguides connect to each other and to the broader context of cavity opto-mechanics and Raman scattering. Finally, we draw an unexpected striking similarity between FBS and discrete diffraction phenomena in waveguide arrays, which makes FBS an interesting candidate for future research in quantum-optics., Comment: 18 pages, 8 figures

We demonstrate correlated photon pair generation via spontaneous four-wave mixing in a low-loss double-stripe silicon nitride waveguide with a coincidence-to-accidental ratio over 10. The coincidence-to-accidental ratio is limited by spontaneous Raman scattering, which can be mitigated by cooling in the future. This demonstration suggests that this waveguide structure is a potential platform to develop integrated quantum photonic chips for quantum information processing., Comment: 7 pages, 4 figures

Cascaded stimulated Brillouin scattering (SBS) is a complex nonlinear optical process that results in the generation of several optical waves that are frequency shifted by an acoustic resonance frequency. Four-wave mixing (FWM) between these Brillouin shifted optical waves can create an equally spaced optical frequency comb with a stable spectral phase, i.e. a Brillouin frequency comb (BFC). Here, we investigate phase-locking of the spectral components of BFCs, considering FWM interactions arising from the Kerr-nonlinearity as well as from coupling by the acoustic field. Deriving for the first time the coupled-mode equations that include all relevant nonlinear interactions, we examine the contribution of the various nonlinear processes to phase-locking, and show that different regimes can be obtained that depend on the length scale on which the field amplitudes vary.

We study the impact of structural variations (that is slowly varying geometry aberrations and internal strain fields) on the width and shape of the stimulated Brillouin scattering (SBS) resonance in nanoscale waveguides. We find that they lead to an homogeneous resonance broadening through two distinct mechanisms: firstly, the acoustic frequency is directly influenced via mechanical nonlinearities; secondly, the optical wave numbers are influenced via the opto-mechanical nonlinearity leading to an additional acoustic frequency shift via the phase-matching condition. We find that this second mechanism is proportional to the opto-mechanical coupling and, hence, related to the SBS-gain itself. It is absent in intra-mode forward SBS, while it plays a significant role in backward scattering. In backward SBS increasing the opto-acoustic overlap beyond a threshold defined by the fabrication tolerances will therefore no longer yield the expected quadratic increase in overall Stokes amplification. Finally, we illustrate in a numerical example that in backward SBS and inter-mode forward SBS the existence of two broadening mechanisms with opposite sign also opens the possibility to compensate the effect of geometry-induced broadening. Our results can be transferred to other micro- and nano-structured waveguide geometries such as photonic crystal fibres., Comment: 18 pages, 6 figures

Temporal optical solitons have been the subject of intense research due to their intriguing physics and applications in ultrafast optics and supercontinuum generation. Conventional bright optical solitons result from the interaction of anomalous group-velocity dispersion and self-phase modulation. Here we report the discovery of an entirely new class of bright solitons arising purely from the interaction of negative fourth-order dispersion and self-phase modulation, which can occur even for normal group-velocity dispersion. We provide experimental and numerical evidence of shape-preserving propagation and flat temporal phase for the fundamental pure-quartic soliton and periodically modulated propagation for the higher-order pure-quartic solitons. Using analytic theory, we derive the approximate shape of the fundamental pure-quartic soliton exhibiting excellent agreement with our experimental observations. Our discovery, enabled by the unique dispersion of photonic crystal waveguides, could find applications in communications and ultrafast lasers.

We derive a set of design guidelines and a figure of merit to aid the engineering process of on-chip waveguides for strong Stimulated Brillouin Scattering (SBS). To this end, we examine the impact of several types of loss on the total amplification of the Stokes wave that can be achieved via SBS. We account for linear loss and nonlinear loss of third order (two-photon absorption, 2PA) and fifth order, most notably 2PA-induced free carrier absorption (FCA). From this, we derive an upper bound for the output power of continuous-wave Brillouin-lasers and show that the optimal operating conditions and maximal realisable Stokes amplification of any given waveguide structure are determined by a dimensionless parameter $\mathcal{F}$ involving the SBS-gain and all loss parameters. We provide simple expressions for optimal pump power, waveguide length and realisable amplification and demonstrate their utility in two example systems. Notably, we find that 2PA-induced FCA is a serious limitation to SBS in silicon and germanium for wavelengths shorter than 2200nm and 3600nm, respectively. In contrast, three-photon absorption is of no practical significance.

We demonstrate the first functional signal processing device based on stimulated Brillouin scattering in a silicon nanowire. We use only 1 dB of on-chip SBS gain to create an RF photonic notch filter with 48 dB of suppression, 98 MHz linewidth, and 6 GHz frequency tuning. This device has potential applications in on-chip microwave signal processing and establishes the foundation for the first CMOS-compatible high performance RF photonic filter.

Integrated sources of indistinguishable photons have attracted a lot of attention because of their applications in quantum communication and optical quantum computing. Here, we demonstrate an ultra-compact quantum splitter for degenerate single photons based on a monolithic chip incorporating Sagnac loop and a micro-ring resonator with a footprint of 0.011 mm2, generating and deterministically splitting indistinguishable photon pairs using time-reversed Hong-Ou-Mandel interference. The ring resonator provides enhanced photon generation rate, and the Sagnac loop ensures the photons travel through equal path lengths and interfere with the correct phase to enable the reversed HOM effect to take place. In the experiment, we observed a HOM dip visibility of 94.5 +- 3.3 %, indicating the photons generated by the degenerate single photon source are in a suitable state for further integration with other components for quantum applications, such as controlled-NOT gates.

Instantaneous frequency measurement (IFM) of microwave signals is a fundamental functionality for applications ranging from electronic warfare to biomedical technology. Photonic techniques, and nonlinear optical interactions in particular, have the potential to broaden the frequency measurement range beyond the limits of electronic IFM systems. The key lies in efficiently harnessing optical mixing in an integrated nonlinear platform, with low losses. In this work, we exploit the low loss of a 35 cm long, thick silicon waveguide, to efficiently harness Kerr nonlinearity, and demonstrate the first on-chip four-wave mixing (FWM) based IFM system. We achieve a large 40 GHz measurement bandwidth and record-low measurement error. Finally, we discuss the future prospect of integrating the whole IFM system on a silicon chip to enable the first reconfigurable, broadband IFM receiver with low-latency., Comment: 13 pages, 7 figures

We study the impact of two-photon absorption (2PA) and fifth-order nonlinear loss such as 2PA-induced free-carrier absorption in semiconductors on the performance of Stimulated Brillouin Scattering devices. We formulate the equations of motion including effective loss coefficients, whose explicit expressions are provided for numerical evaluation in any waveguide geometry. We find that 2PA results in a monotonic, algebraic relationship between amplification, waveguide length and pump power, whereas fifth-order losses lead to a non-monotonic relationship. We define a figure of merit for materials and waveguide designs in the presence of fifth-order losses. From this, we determine the optimal waveguide length for the case of 2PA alone and upper bounds for the total Stokes amplification for the case of 2PA as well as fifth-order losses. The analysis is performed analytically using a small-signal approximation and is compared to numerical solutions of the full nonlinear modal equations.

Highly selective and reconfigurable microwave filters are of great importance in radio-frequency signal processing. Microwave photonic (MWP) filters are of particular interest, as they offer flexible reconfiguration and an order of magnitude higher frequency tuning range than electronic filters. However, all MWP filters to date have been limited by trade-offs between key parameters such as tuning range, resolution, and suppression. This problem is exacerbated in the case of integrated MWP filters, blocking the path to compact, high performance filters. Here we show the first chip-based MWP band-stop filter with ultra-high suppression, high resolution in the MHz range, and 0-30 GHz frequency tuning. This record performance was achieved using an ultra-low Brillouin gain from a compact photonic chip and a novel approach of optical resonance-assisted RF signal cancellation. The results point to new ways of creating energy-efficient and reconfigurable integrated MWP signal processors for wireless communications and defence applications., Comment: 16 pages, 5 figures

We develop a systematic theory for optimising single-photon frequency conversion using optical Bragg scattering. The efficiency and phase-matching conditions for the desired Bragg scattering conversion as well as spurious scattering and modulation instability are identified. We find that third-order dispersion can suppress unwanted processes, while dispersion above the fourth order limits the maximum conversion efficiency. We apply the optimisation conditions to frequency conversion in highly nonlinear fiber, silicon nitride waveguides and silicon nanowires. Efficient conversion is confirmed using full numerical simulations. These design rules will assist the development of efficient quantum frequency conversion between multicolour single photon sources for integration in complex quantum networks., Comment: 9 pages, 14 figures

The growing requirement for photon pairs with specific spectral correlations in quantum optics experiments has created a demand for fast, high resolution and accurate source characterization. A promising tool for such characterization uses the classical stimulated process, in which an additional seed laser stimulates photon generation yielding much higher count rates, as recently demonstrated for a $\chi^{(2)}$ integrated source in A.~Eckstein \emph{et al.}, Laser Photon. Rev. \textbf{8}, L76 (2014). In this work we extend these results to $\chi^{(3)}$ sources, demonstrating spectral correlation measurements via stimulated four-wave mixing for the first time in a integrated optical waveguide, namely a silicon nanowire. We directly confirm the speed-up due to higher count rates and demonstrate that additional resolution can be gained when compared to traditional coincidence measurements. As pump pulse duration can influence the degree of spectral entanglement, all of our measurements are taken for two different pump pulse widths. This allows us to confirm that the classical stimulated process correctly captures the degree of spectral entanglement regardless of pump pulse duration, and cements its place as an essential characterization method for the development of future quantum integrated devices., Comment: 10 pages, 4 figures

We apply the moment method to nonlinear pulse propagation in silicon waveguides in the presence of two-photon absorption, free-carrier dispersion and free-carrier absorption. The evolution equations for pulse energy, temporal position, duration, frequency shift and chirp are obtained. We derive analytic expressions for the free-carrier induced blueshift and acceleration and show that they depend only on the pulse peak power. Importantly, these effects are independent of the temporal duration. The moment equations are then numerically solved to provide fast estimates of pulse evolution trends in silicon photonics waveguides. We find that group-velocity and free-carrier dispersion dominate the pulse dynamics in photonic crystal waveguides. In contrast, two-photon and free-carrier absorption dominate the temporal dynamics in silicon nanowires. To our knowledge, this is the first time the moment method is used to provide a concise picture of multiphoton and free-carrier effects in silicon photonics. The treatment and conclusions apply to any semiconductor waveguide dominated by two-photon absorption., Comment: 10 pages, 8 figures

Recent theoretical studies of Stimulated Brillouin Scattering (SBS) in nanoscale devices have led to an intense research effort dedicated to the demonstration and application of this nonlinearity in on-chip systems. The key feature of SBS in integrated photonic waveguides is that small, high-contrast waveguides are predicted to experience powerful optical forces on the waveguide boundaries, which are predicted to further boost the SBS gain that is already expected to grow dramatically in such structures because of the higher mode confinement alone. In all recent treatments, the effect of radiation pressure is included separately from the scattering action that the acoustic field exerts on the optical field. In contrast to this, we show here that the effects of radiation pressure and motion of the waveguide boundaries are inextricably linked. Central to this insight is a new formulation of the SBS interaction that unifies the treatment of light and sound, incorporating all relevant interaction mechanisms --- radiation pressure, waveguide boundary motion, electrostriction and photoelasticity --- from a rigorous thermodynamic perspective. Our approach also clarifies important points of ambiguity in the literature, such as the nature of edge-effects with regard to electrostriction, and of body-forces with respect to radiation pressure. This new perspective on Brillouin processes leads to physical insight with implications for the design and fabrication of SBS-based nanoscale devices.

The ability to use coherent light for material science and applications is directly linked to our ability to measure short optical pulses. While free-space optical methods are well-established, achieving this on a chip would offer the greatest benefit in footprint, performance, flexibility and cost, and allow the integration with complementary signal processing devices. A key goal is to achieve operation at sub-Watt peak power levels and on sub-picosecond timescales. Previous integrated demonstrations require either a temporally synchronized reference pulse, an off-chip spectrometer, or long tunable delay lines. We report the first device capable of achieving single-shot time-domain measurements of near-infrared picosecond pulses based on an ultra-compact integrated CMOS compatible device, with the potential to be fully integrated without any external instrumentation. It relies on optical third-harmonic generation in a slow-light silicon waveguide. Our method can also serve as a powerful in-situ diagnostic tool to directly map, at visible wavelengths, the propagation dynamics of near-infrared pulses in photonic crystals., Comment: 20 pages, 6 figures, 38 references

A key resource for quantum optics experiments is an on-demand source of single and multiple photon states at telecommunication wavelengths. This letter presents a heralded single photon source based on a hybrid technology approach, combining high efficiency periodically poled lithium niobate waveguides, low-loss laser inscribed circuits, and fast (>1 MHz) fibre coupled electro-optic switches. Hybrid interfacing different platforms is a promising route to exploiting the advantages of existing technology and has permitted the demonstration of the multiplexing of four identical sources of single photons to one output. Since this is an integrated technology, it provides scalability and can immediately leverage any improvements in transmission, detection and photon production efficiencies., Comment: 5 pages, double column, 3 figures

Stimulated Brillouin scattering (SBS) and Kerr-nonlinear four wave-mixing (FWM) are among the most important and widely studied nonlinear effects in optical fibres. At high powers SBS can be cascaded producing multiple Stokes waves spaced by the Brillouin frequency shift. Here, we investigate the complex nonlinear interaction of the cascade of Stokes waves, generated in a Fabry-Perot chalcogenide fibre resonator through the combined action of SBS and FWM. We demonstrate the existence of parameter regimes, in which pump and Stokes waves attain a phase-locked steady state. Real-time measurements of 40ps pulses with 8GHz repetition rate are presented, confirming short-and long-term stability. Numerical simulations qualitatively agree with experiments and show the significance of FWM in phase-locking of pump and Stokes waves. Our findings can be applied for the design of novel picosecond pulse sources with GHz repetition rate for optical communication systems.

We experimentally demonstrate phase sensitive amplification (PSA) in a silicon photonic crystal waveguide based on pump-degenerate four-wave mixing. An 11 dB phase extinction ratio is obtained in a record compact 196 {\mu}m nanophotonic device due to broadband slow-light, in spite of the presence of two-photon absorption and free-carriers. Numerical calculations show good agreement with the experimental results.

Harnessing nonlinear optical effects in a photonic chip scale has been proven useful for a number of key applications in optical communications. Microwave photonics can also benefit from the adoption of such a technology, creating a new concept of nonlinear integrated microwave photonics. Here, we discuss the potential of on-chip nonlinear processing towards the creation of robust and multifunctional microwave photonic (MWP) processors. We also highlight key recent results in the field, including frequency agile MWP filters and ultra-wideband signal generators., Comment: 5 pages, 3 figures, invited paper at 2013 IEEE Topical Meeting on Microwave Photonics

We report a simple technique in microwave photonic (MWP) signal processing that allows the use of an optical filter with a shallow notch to exhibit a microwave notch filter with anomalously high rejection level. We implement this technique using a low-loss, tunable Si3N4 optical ring resonator as the optical filter, and achieved an MWP notch filter with an ultra-high peak rejection > 60 dB, a tunable high resolution bandwidth of 247-840 MHz, and notch frequency tuning of 2-8 GHz. To our knowledge, this is a record combined peak rejection and resolution for an integrated MWP filter., Comment: 8 pages, 5 figures

We report a novel class microwave photonic (MWP) notch filter with a very narrow isolation bandwidth (10 MHz), an ultrahigh stopband rejection (> 60 dB), a wide frequency tuning (1-30 GHz), and flexible bandwidth reconfigurability (10-65 MHz). This record performance is enabled by a new concept of sidebands amplitude and phase controls using an electro-optic modulator and an optical filter. This new concept enables energy efficient operation in active MWP notch filters, and opens up the pathway to enable low-power nanophotonic devices as high performance RF filters., Comment: 4 pages, 5 figures

We examine the effect of acoustic mode confinement on Stimulated Brillouin Scattering in optical waveguides that consist of a guiding core embedded in a solid substrate. We find that SBS can arise due to coupling to acoustic modes in three different regimes. First, the acoustic modes may be guided by total internal reflection; in this case the SBS gain depends directly on the degree of confinement of the acoustic mode in the core, which is in turn determined by the acoustic V-parameter. Second, the acoustic modes may be leaky, but may nevertheless have a sufficiently long lifetime to have a large effect on the SBS gain; the lifetime of acoustic modes in this regime depends not only on the contrast in acoustic properties between the core and the cladding, but is also highly dependent on the waveguide dimensions. Finally SBS may occur due to coupling to free modes, which exist even in the absence of acoustic confinement; we find that the cumulative effect of coupling to these non-confined modes results in significant SBS gain. We show how the different acoustic properties of core and cladding lead to these different regimes, and discuss the feasibility of SBS experiments using different material systems.