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IntroductionBrillouin scattering, named after Leon Brillouin [1], is the process of inelastic scattering, in which light interacts with moving density fluctuations, acoustic phonons. As a result of such interaction, light changes its path and acquires fre-quency shifts due to the Doppler effect. Brillouin scatter-ing forms a basis for a number of important applications in fields ranging from material sciences to photonics and quantum mechanics. For example, material stiffness can be determined by the measurement of the frequency shift upon scattering [2]. Common photonics applications include distributed sensing [3], narrow-linewidth single- and multi-frequency lasers [4-6], and gyroscopes [7,8]. Depending on the power of the driving optical field, Brillouin scattering can be categorized in two regimes: spontaneous and stimulated. For spontaneous scattering (valid for low to moderate light intensities) acoustic pho-nons are generated from thermal noise [1,9]. In the pro-cess of stimulated Brillouin scattering (SBS), however, a high-intensity optical field can change the medium’s den-sity via electrostriction [9,10]. The first experimental demonstration of SBS was by by Chiao et. al. in 1964 [10]. At this point the fist laser, ruby laser [11], had already been constructed, producing high power coherent light necessary for the SBS experiments. In the late 1960s SBS was also demonstrated in liquids [12] and gases [13]. With the advent of low-loss optical fibres in the early 1970s, SBS was observed in a germa-nium-doped silica fibre at sub-watt optical powers [14]. The relatively low power threshold was associated with the preservation of the light confinement in the fiber core over a long distance.

Recent advances in nanofabrication have opened new

opportunities for photonic integration, leading to the re-vival of SBS research. Depending on the confinement of optical and acoustic modes, Brillouin gain can be orders of magnitude larger in nanostructures [15-19] compared to standard optical fibers [10], leading to compact, power-efficient devices. Figure 1 summarises the history of SBS research from the first prediction to the present time.

Fig. 1. Timeline of major advances in SBS theory and experiments.

The University of Sydney hosts a new research activ-ity in nonlinear optical phononics and SBS under the leadership of ARC Laureate Fellow, Professor Benjamin J. Eggleton. The group (Fig. 2) engages in theoretical and experimental research to explore novel effects associated with SBS in highly-nonlinear nanoscale circuits and opto-mechanical structures [20]. In collaboration with the La-ser Physics Centre at the Australian National University, researchers have developed integrated chalcogenide glass platforms for on-chip SBS. In 2011 the Sydney Univer-sity group was the first to demonstrate SBS using on-chip

Driving acoustic waves optically on a chipIrina V. Kabakova, David Marpaung, Christopher G. Poulton and Benjamin J. Eggleton ARC Centre for Ultrahigh bandwidth Devices for Optical Systems (CUDOS) and Institute of Photonics and Optical Sciences (IPOS), School of Physics, University of Sydney, New South Wales 2006, Australia kabakova@physics.usyd.edu.au

Stimulated Brillouin scattering (SBS) is an important nonlinear effect in optical fibers and waveguides that has been traditionally exploited for high-sensitivity distributed sensing, coherent lasers and gyroscopes. Recent advances in nanofabrication led to the extensive growth of SBS research. The ability to generate SBS in nanoscale devices has opened numerous opportunities for photonic integration, resulting in on-chip Brillouin lasers, microwave generation, Brillouin cooling and quantum optomechanics. In this paper we briefly describe principles of inelastic Brillouin scattering and review our recent developments in optical signal manipulation and lasing using on-chip SBS.

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waveguides. Now we are expanding the range of on-chip applications to include SBS lasers and frequency combs, microwave photonics signal processing, non-reciprocity, SBS isolators, Brillouin dynamic gratings, and the design and fabrication of novel optomechanical nanostructures. In this paper, we briefly revisit the fundamentals of SBS, review our recent progress in on-chip SBS applications, and discuss our future directions.

Fig. 2. Nonlinear phononics group at the University of Sydney. Back row from left to right: Dr Feng Gao, Dr Ravi Pant, Prof. Ben Eggleton, Iman Aryanfar, Dr Alvaro Casas-Bedoya, Dr David Marpaung, Dr Irina Kabakova, Dr Darren Hudson, A/Prof. Chris Poulton. Front row: Matt Collins, Thomas Buettner, Neetesh Singh.

Principles Of SBSThe physical origin of SBS can be understood in terms of the thermodynamically linked processes of electrostriction, the tendency of a material to be compressed in the pres-ence of an optical intensity gradient, and the photoelastic effect, which is the change in electric permittivity in re-sponse to a change in material density [9]. The basic process, illustrated for backward stimulated Brillouin scattering (BSBS), is shown in Fig. 3: a pump signal at frequency ωp is counter-propagated with a probe signal at frequency ωs. The interference between the two waves will have frequency

 0>−=Δ SP ωωω , and the

beat pattern will move at speed   kv ΔΔ= /ω , where

 sp kkk −=Δ is the difference in propagation constants

of the pump and the signal. Due to electrostriction the maxima of the beat pattern will cause density changes in the material. If  v matches the acoustic velocity,

 va ≈ v , induced density fluctuations

will drive the acoustic mode resonantly. For this condition the frequency difference   ωΔ is the Brillouin frequency shift

 BΩ and the wave at the probe frequency is known

as the Stokes wave. Through the photoelastic effect, the density fluctuations are translated into the refractive index

grating, moving in the same direction as the pump wave. By scattering on such a grating, the pump will produce a frequency down-shifted wave, at the same frequency as the probe, thus amplifying the probe.

Fig. 3. In SBS a counter-propagating optical pump (ωp) and a probe (ωs) interfere in a medium and create a moving beat pattern (

 SP ωωω −=Δ ). This beat pattern excites a

density fluctuation (acoustic wave). If   ωΔ coincides with one of the acoustic modes (

 BΩ≈Δω ), the acoustic wave

is resonantly amplified, leading to the pump’s efficient scattering.

SBS Gain And NanostructuresThe efficiency of the SBS process is characterized by a gain parameter

 effeffpB ALPgG /= , where gB is the Brillouin

gain factor that is determined by the material properties and the device design, PP is the peak power of the pump wave, Leff is the effective propagation length, and Aeff is the optical mode area [9]. For fixed pump power, high Brillouin gain can be achieved by: (1) using long-length waveguides, or (2) reducing the cross-section of the opti-cal waveguide, while keeping the device length short. As modern technology moves towards energy-efficient, on-chip integrated photonics and optoelectronics, the sec-ond scenario becomes preferable.

“...SBS can be understood in terms of the thermodynamically linked processes of electrostriction...and the photoelastic effect...”

Figure 4 shows different SBS platforms that exploit strong light confinement in devices with a small mode area. These include: (1) dual-nanoweb [21] and suspend-ed core [22] fibres, (2) embedded [15] and suspended [19] waveguides/nanowires, and (3) high-finesse whisper-ing gallery mode (WGM) resonators [16,18]. Advances in nanofabrication, in particular in the development of novel material platforms, and in optical loss reduction, have played a critical role in achieving strong optoacoustic coupling in photonic micro- and nanostructures.

Device miniaturization allowed demonstration of novel effects, including quantum optomechanical cooling that exploits radiation pressure forces on the nanoscale [23]. Because light carries momentum, it induces pres-

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sure on boundaries that it travels through. It has been recently shown for silicon nanowires on a silicon-nitride membrane that the radiation pressure forces can enhance SBS. Radiation pressure induced interaction between the optical and acoustic vibrations has been explored for real-izing the quantum ground state [24], on-chip microwave sources[16] and optomechanical oscillators [23].

SBS Applications Using Chalcogenide ChipThe choice of a material platform plays an important role in realizing SBS on a chip. Since SBS is the process of op-toacoustic interaction, both modes (optical and acoustic) should be guided in a structure and, more importantly, should spatially overlap. It is also helpful if the material of choice has high refractive index, since the gain factor gB scales with the refractive index as

 gB ∝n

8.

“The chalcogenide photonic chip...is an excellent platform for exploring photon-phonon interactions...”

The chalcogenide photonic chip, developed by the ARC Centre for Ultrahigh bandwidth Devices for Op-tical Systems (CUDOS), is an excellent platform for ex-ploring photon-phonon interactions on a length scale of only a few centimetres [15]. Chalcogenide As2S3 glass has a high refractive index of n=2.38, that leads to about 70 times larger Brillouin gain factor compared to silica glass. Additionally, As2S3 rib waveguides guide well both optical and acoustic waves and have a mode overlap integral close to one (Fig. 5).

By harnessing SBS on a chalcogenide chip, the Univer-sity of Sydney researchers have demonstrated the broad range of optical and microwave functions including dy-namic gratings, slow and fast light, microwave photonic filters, cascaded SBS, narrow-linewidth Brillouin lasing and Brillouin frequency comb generation. Some of these applications are briefly discussed below.

Slow and fast lightSlow and fast light refers to a phenomenon in which group velocity of the light pulse (velocity of the envelope of a wave packet) becomes much slower or faster than the phase velocity of light in the medium c/n. This can be achieved by exploiting strong dispersion in the vicinity of an optical or electro-magnetic resonance of any type. Some of the techniques include the use of refractive index periodic structures, electro-magnetically induced trans-parency or Brillouin gain dispersion. SBS is known to be the most flexible approach because it allows wavelength flexibility and the delay time can be tuned optically. On-chip SBS slow and fast light for optical delays have been pioneered by Pant et. al. and a record slow-down factor of 130 has been demonstrated [25].

Microwave signal processing and generationIn the electromagnetic spectrum, microwaves occupy a frequency range from 300 MHz to 300 GHz. These frequencies are important for satellite and wireless com-munications; they are also used in radars and medicine (as an alternative to surgery). Typically, low frequency microwaves are generated and processed using electronics. Multi-GHz frequencies are, however, more challenging due to impairments such as electromagnetic interference and large loss at high frequencies. Using photonic systems, these challenges can be mitigated.

SBS is the natural choice of a physical process to con-sider for microwave generation because most materials exhibit Brillouin frequency shifts in the range of 5 to 20 GHz. Thus, beating of the pump and the probe results in a gigahertz microwave signal. Cascading SBS and pro-ducing high-order Stokes waves, allows the generation of several microwave tones at once. The Brillouin frequency shift can be tuned in a small range by changing the am-bient temperature or applying stress to the SBS material. These techniques can be useful for fine-tuning the fre-quency of a microwave generator. Cascaded SBS with up to 3rd order Stokes waves have been demonstrated on a chalcogenide photonic chip by Pant et. al. [26]

Fig. 4. Examples of micro- and nanostructures that exploit strong light confinement of the optical mode to enhance opto-acoustic interactions. These include: dual-nanoweb [21] and suspended core [22] fibers, a Si nanowire on a Si3N4 slot membrane [19], As2S3 rib waveguides [15], a CaF2 microtoroid [18] and a wedge resonator [16].

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One of the key components in microwave transmis-sion systems is a narrowband filter. This is used to increase the signal-to-noise ratio of a desired signal by filtering an unwanted background. The requirements for a microwave filter are quite strict: it has to be tunable in a broad range of frequencies (tens of GHz), but must have narrow pass-band (MHz). Techniques based on SBS can satisfy both requirements since SBS is wavelength-independent pro-cess with the gain bandwidth of only a few tens of MHz.

Photonic-chip based microwave notch filters with unprecedented characteristics have been recently demon-strated by Marpaung et. al. [27]. This filter has a wide tun-ability range (1-20 GHz), large extinction ratio of more than 55dB, and a 3dB bandwidth of 31-77 MHz.

Dynamic grating and optical information storageThe moving density grating, formed via electrostriction and photoelastic effects from two-wave interference (pump and Stokes waves), is known as a Brillouin dynamic grating (BDG). Unlike stationary refractive index grating such as Bragg gratings, a BDG moves at the speed of sound and exists only while optical waves are in the medium. The dynamic nature of BDGs is attractive for optical switching since the function can be achieved without modification of the transmission channel. BDGs have been realized in chalcogenide waveguides on a chip by Pant et. al. [28]

BDG can also be used to store information about opti-

cal signals for as long as phonon lifetimes (several nano-seconds). If a read-wave is launched in to the channel dur-ing this time, the phase and amplitude information of the write-signal can be retrieved.

Summary and Future Directions In summary, we have shown that the chalcogenide photon-ic chip is the ideal platform for harnessing SBS on a length-scale of a few centimetres. A rich variety of optical func-tions can be achieved through photon-phonon interaction on a chip. Due to space limitations, we discussed only a few of them - optical delay lines [25], microwave generation [26] and filtering [27] and dynamic gratings [28]. For fur-ther details refer to our recent review paper [20].

Further miniaturisation of photonic structures and ex-ploration of the novel physics related to radiation pressure forces on the nanoscale are an ongoing interest. Develop-ment of different material platforms and hybrid integra-tion of nano-circuits is yet another focus of our research which will allow realization of SBS in materials that natu-rally have poor acoustic properties.

In our future work we will explore spontaneous Brillouin scattering for measurements of the mechanical properties of different types of materials including bio-logical tissues. Brillouin spectroscopy [2] is based on the detection of the Brillouin frequency shift

 BΩ , which pro-

vides information about the sound velocity and the elastic modulus of the medium.

The non-contact and non-invasive character of Brillouin spectroscopy measurement is well-suited for differentiation between healthy and pathological tissues. The benefits of this technique have been demonstrated for early diagnostics of ocular diseases such as cataract, pres-byopia and corneal ectasia [30,31].

AcknowlegementFunding from the Australian Research Council (ARC) through its Laureate Project FL120100029 is grate-fully acknowledged. This research was also supported by the ARC Center of Excellence for Ultrahigh band-width Devices for Optical Systems (project number CE110001018). We acknowledge the contributions of our collaborators Dr Duk-Yong Choi, Dr Steve Madden, and Prof. Barry Luther-Davies at the Australia National University.

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Fig. 5. Schematic of a chalcogenide photonic chip with computed optical and acoustic modes. Two applications of on-chip SBS are optical delay lines [26] and microwave generation [27], respectively.

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AUTHOR BIOGRAPHIES

Dr Irina Kabakova is a Research Associate at CUDOS, School of Physics, Sydney University. She received her MSc from Moscow State University (2007) and her PhD degree from the University of Sydney (2012) for the work in nonlinear all-optical switching using speciality fiber Bragg gratings. Currently Irina is working in the area of nonlinear opto-acoustic interactions with the focus on Brillouin single-line and comb sources.

Additionally, she is starting a new research activity in Brillouin microscopy.

Dr David Marpaung joined CUDOS in August 2012 as a research fellow working on photonic chip based nonlinear signal processing for high speed coherent optical communication systems. He received his Ph.D. degree in electrical engineer-ing from the University of Twente, the Netherlands in 2009. From August 2009 until July 2012 he was a postdoctoral researcher in the University of Twente within the framework of the European

Commission FP7 funded project SANDRA. David’s main research interest is the use of integrated photonic circuits for microwave photonic signal processing and high capacity optical communications.

Chris Poulton is the Associate Professor at the School of Mathematical Sciences at UTS. He re-ceived his PhD from the University of Sydney in 2000 for his work on electromagnetic and elasto-dynamic wave propagation in periodic materials. After that Chris works as a Lecturer on Applied Mathematics at the University of Liverpool (UK), and later in the Institute for High-frequency and Quantum Electronics at the University of

Karlsruhe, Germany. At the beginning of 2006 he joined the Max Planck Research Group (photonics and new materials) in Erlangen, Germany, where he worked on plasmonic interactions, nonlinear effects and guidance in photonic crystal fibres. In 2007 he joined the faculty at the School of Mathematical Sciences at UTS. His cur-rent research interests are in metamaterials, nonlinear effects in structured media, and in phonon-photon interactions within nanophotonic structures.

Professor Benjamin Eggleton is an ARC Laure-ate Fellow and Professor of Physics at the Univer-sity of Sydney and Director of the ARC Centre for Ultrahigh-Bandwidth Devices for Optical Systems (CUDOS). He obtained his Ph.D. degtree in Physics from the University of Sydney, in 1996 and then joined Bell Laboratories, Lucent Technologies as a Postdoctoral Member of Staff. In 2000, he was promoted to Director within the Specialty Fiber

Business Division of Bell Laboratories, where he was engaged in forward-looking research supporting Lucent Technologies business in optical fiber devices. He re-turned to the University of Sydney in 2003 as the founding Director of CUDOS and Professor in the School of Physics. Eggleton is a Fellow of the Australian Institute of Physics, the Optical Society of America, IEEE and ATSE. He was the recipient of the 2011 Eureka Prize for Leadership in Science and the Walter Boas Medal of the Aus-tralian Institute of Physics and has received numerous other awards for his research. Eggleton has published about 360 journal papers which have been cited >12,000 times with an h-number of > 53. He was President of the Australian Optical Society and is currently Editor-in-Chief for Optics Communications.