High Brillouin Research Articles

High pressure and high temperature Brillouin scattering measurements of pyrope single crystals using flexible CO2 laser heating systems

Single-crystal Brillouin scattering measurements are important for interpreting seismic velocities within the Earth and other planetary interiors. These measurements are rare, however, at temperatures above 1000 K, due to the fact that the transparent samples cannot be heated by common laser heating systems operating at a wavelength on the order of 1 μm. Here we present Brillouin scattering data on pyrope collected at pressures up to 23.8 GPa and temperatures between 850 and 1900 K using a novel CO2 laser heating system confined in either a flexible hollow silica waveguide or an articulated arm with mirrors mounted in each junction to direct the laser to the exit point. Pyrope has been chosen because it has been extensively studied at high pressures and moderate temperatures and therefore it is an excellent sample for bench-marking the CO2 laser heating system. The new high-temperature velocity data collected in this study allow the room pressure thermal parameters of pyrope to be constrained more tightly, resulting in values that reproduce the temperature dependence of the unit-cell volume of pyrope measured in recent studies at ambient pressure. Aggregate wave velocities of pyrope calculated along an adiabat using the thermoelastic parameters determined in this study are larger than those obtained using published values, implying that velocities for many mantle components may be underestimated at mantle temperatures because high temperature experimental data are lacking.

Low-loss compact chalcogenide microresonators for efficient stimulated Brillouin lasers.

Chalcogenide glasses (ChGs) possess a high elasto-optic coefficient, making them ideal for applications in microwave photonics and narrow-linewidth lasers based on stimulated Brillouin scattering (SBS). However, current As2S3-based integrated devices suffer from poor stability and low laser-induced damage threshold, and planar ChG devices feature limited quality factors. In this Letter, we propose and demonstrate a high-quality integrated GeSbS ChG Brillouin photonic device. By introducing Euler bending structures, we suppress high-order optical modes and reduce propagation losses in a finger-shaped GeSbS microresonator, resulting in a compact footprint of 3.8 mm2 and a high intrinsic quality factor of 5.19 × 106. The combination of GeSbS material's high Brillouin gain and the resonator's high-quality factor enables the generation of stimulated Brillouin lasers with a low threshold of 0.96 mW and a fundamental linewidth of 58 Hz. Moreover, cascaded stimulated Brillouin lasers can be realized up to the seventh order, yielding microwave beat frequencies up to 40 GHz.

Chaos-Assisted Dynamical Tunneling in Flat Band Superwires.

Recent theoretical investigations have revealed unconventional transport mechanisms within high Brillouin zones of two-dimensional superlattices. Electrons can navigate along channels we call superwires, gently guided without brute force confinement. Such dynamical confinement is caused by weak superlattice deflections, markedly different from the static or energetic confinement observed in traditional wave guides or one-dimensional electron wires. The quantum properties of superwires give rise to elastic dynamical tunneling, linking disjoint regions of the corresponding classical phase space, and enabling the emergence of several parallel channels. This paper provides the underlying theory and mechanisms that facilitate dynamical tunneling assisted by chaos in periodic lattices. Moreover, we show that the mechanism of dynamical tunneling can be effectively conceptualized through the lens of a paraxial approximation. Our results further reveal that superwires predominantly exist within flat bands, emerging from eigenstates that represent linear combinations of conventional degenerate Bloch states. Finally, we quantify tunneling rates across various lattice configurations and demonstrate that tunneling can be suppressed in a controlled fashion, illustrating potential implications in future nanodevices.

Slow-light-enhanced Brillouin scattering with integrated Bragg grating.

Advancements in photonic integration technology have enabled the effective excitation of simulated Brillouin scattering (SBS) on a single chip, boosting Brillouin-based applications such as microwave photonic signal processing, narrow-linewidth lasers, and optical sensing. However, on-chip circuits still require large pump power and centimeter-scale waveguide length to achieve a considerable Brillouin gain, making them both power-inefficient and challenging for integration. Here, we exploit the slow-light effect to significantly enhance SBS, presenting the first, to the best of our knowledge, demonstration of a slow-light Brillouin-active waveguide on the silicon-on-insulator (SOI) platform. By integrating a Bragg grating with a suspended ridge waveguide, a 2.1-fold enhancement of the forward Brillouin gain coefficient is observed in a 1.25 mm device. Furthermore, this device shows a Brillouin gain coefficient of 1,693 m-1W-1 and a mechanical quality factor of 1,080. The short waveguide length reduces susceptibility to inhomogeneous broadening, enabling the simultaneous achievement of a high Brillouin gain coefficient and a high mechanical quality factor. This approach introduces an additional dimension to enhance acousto-optic interaction efficiency in the SOI platform and holds significant potential for microwave photonic filters and high spatial resolution sensing.

Extreme thermodynamics in nanolitre volumes through stimulated Brillouin–Mandelstam scattering

Examining the physical properties of materials—particularly of toxic liquids—under a wide range of thermodynamic states is a challenging problem due to the extreme conditions the material has to experience. Such temperature and pressure regimes, which result in a change in the refractive index and sound velocity, can be accessed by optoacoustic interactions such as Brillouin–Mandelstam scattering. Here we demonstrate the Brillouin–Mandelstam measurements of nanolitre volumes of liquids in extreme thermodynamic regimes. This is enabled by a fully sealed liquid-core optical fibre containing carbon disulfide. Within this waveguide, which exhibits tight optoacoustic confinement and a high Brillouin gain, we are able to conduct spatially resolved measurements of the local Brillouin response, giving us access to a resolved image of the temperature and pressure values along the liquid channel. We measure the material properties of the liquid core at very large positive pressures (above 1,000 bar) and substantial negative pressures (below –300 bar), as well as explore the isobaric and isochoric regimes. The extensive thermodynamic control allows the tunability of the Brillouin frequency shift of more than 40% using only minute volumes of liquid.

Comparative first-principles structural and vibrational properties of rutile and anatase TiO2

The structural and vibrational properties of two polymorphs of TiO2, rutile and anatase, have been investigated by first-principles methods at different levels of exchange-correlational (XC) energy functionals in density functional theory (DFT). Reports in the literature to date are contradictory regarding the stability of the rutile phase using DFT XC-functionals more sophisticated than simple local-density approximation. Here the PBEsol generalized gradient approximation (GGA), TPSS meta-GGA, and HSE06 hybrid functionals have been employed to demonstrate the XC-functional effects on the calculated structural, phonon and thermodynamic properties of rutile and anatase TiO2. Lattice and elastic parameters correctly calculated with these XC-functionals show good agreement with the experimental values. Calculated phonon frequencies generated stable phonon dispersion relations for both rutile and anatase TiO2 when correctly converged, in agreement with the experimental observations. The phonon frequencies along high symmetry Brillouin zone paths and their corresponding phonon density of states showed sensitivity to different levels of XC-functional employed in phonon dispersion prediction. Nevertheless, the thermodynamic properties of rutile and anatase TiO2 estimated by harmonic approximations are in excellent experimental agreement and are effectively invariant to the level of theory employed in the DFT XC-functional.

Polarization-maintaining photonic crystal fiber with high Brillouin gain coefficient for Brillouin lasers.

Stimulated Brillouin scattering (SBS) is effective for realizing a laser with an ultra-narrow linewidth. Although photonic crystal fiber (PCF) is considered an excellent medium to achieve SBS, it does not meet the requirements of low loss, large birefringence, and ease of fabrication. We propose a polarization-maintaining PCF (PM-PCF) structure and theoretically analyze the effects of the geometric structural parameters of the PM-PCF on various optical properties. Our theoretical analysis and experimental results contribute to the advancement of the field of ultra-narrow linewidth fiber lasers, distributed fiber sensing, and fiber-optic gyroscopes related to SBS.

Annular pupil confocal Brillouin–Raman microscopy for high spectral resolution multi-information mapping

Abstract Brillouin–Raman combined confocal spectroscopy is a novel and powerful technique for providing non-contact and direct readout of the micro-regional chemical and mechanical properties of a material, and thus used in a broad range of applications, including material characterization in manufacturing and biological imaging. However, the inadequate spectral and spatial resolution restricts the further development of combined spectral technology. In this paper, an annular pupil confocal Brillouin–Raman microscopy (APCBRM) scheme is proposed to achieve high-spectral-resolution Brillouin spectral detection and high-lateral-resolution Brillouin, Raman, and 3D topography mapping. The use of an annular pupil significantly suppresses the spectral broadening caused by a high-numerical-aperture objective lens and compresses the full width at half maximum of the Brillouin spectrum by 22.1 %, effectively improving the Brillouin spectral resolution. In addition, the size of the excitation spot is compressed, and the lateral resolutions in Brillouin and Raman spectroscopy increased to about 353.2 nm and 347.1 nm, respectively. Thus, high lateral resolution and Brillouin spectral resolution are achieved simultaneously. Furthermore, the high-precision confocal focusing system based on reflected light realizes real-time focusing during scanning and three-dimensional topography mapping. These results demonstrate that APCBRM has excellent potential for applications in the fields of novel materials, precision machining, and biomedicine.

Polarization-maintaining photonic crystal fiber with high Brillouin gain coefficient for Brillouin lasers

Polarization-maintaining photonic crystal fiber with high Brillouin gain coefficient for Brillouin lasers

Novel Ge-As-Se chalcogenide glass for potential high Brillouin gain coefficient of fiber

Two series of glasses compositions, xGeSe2-(100-x) As2Se3 (x = 15,42,63,79,94 mol%) and Ge30AsySe70-y (y = 4,10,15,20 mol%), were selected for optimization with a high Brillouin gain coefficient(gB) by considering both the acousto-optic parameters and the stability of glass against crystallization. The variation in its thermal, mechanical, and material acousto-optic properties were systematically investigated. In the xGeSe2-(100-x) As2Se3 series of glasses, the gradual increase of GeSe2, resulted in a significant increase in elastic modulus and longitudinal sound velocity (VL), whereas refractive index(n) and acoustic attenuation(α) decreased rapidly. The decrease in n, and the increase in VL were not conducive to the calculation of the gB. Furthermore, in the Ge30AsySe70-y series of glasses, n increased to a maximum of 2.62, and α decreased to a minimum of 215 dB/cm/GHz2, with increasing As content. The highest gB = 33.77 × 10−11m/W was obtained in Ge30As20Se50 at 1550 nm, and this value was ∼2.3 times greater than that obtained in As2S3 (gB = 14.43 × 10−11m/W) and ∼1.6 times greater than that obtained in As2Se3 (gB = 20.89 × 10−11m/W) chalcogenide (ChG) glass. On the basis of Ge30As20Se50 host glass, a chalcogenide single-refractive-index fiber was drawn with a lowest optical loss of 0.7 dB/m at 6.5 μm. The above results provided an important reference for the further development of new chalcogenide glass fiber with high gB, and broadened the application in Brillouin lasers.

Enabling Variable High Spatial Resolution Retrieval From a Long Pulse BOTDA Sensor

Spatial resolution (SR) is one of the most important parameters of Brillouin optical time-domain analysis (BOTDA) sensors, which determines the minimum length that a perturbation event can be distinguished. In the field of Internet of Things (IoT), there is an urgent need for sensors with large-scale high-precision sensing capability for scenarios, such as intelligent monitoring of production lines and urban infrastructure. Conventionally, the SR is normally restricted to be longer than 1 m due to the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 10$ </tex-math></inline-formula> -ns acoustic lifetime limitation in silica optical fibers. For long-distance smart monitoring systems, the SR is generally on the order of several meters or even worse. However, it does not meet the needs of many applications. Therefore, there is an urgent need to achieve SR in the submeter magnitude. In this work, for the first time to the best of our knowledge, we propose a convolutional neural network (CNN) to process the data of conventional BOTDA sensors, which achieves unprecedented performance improvement that allows to directly retrieve submeter SR from the sensing system that use long pump pulses. By using the simulated Brillouin gain spectrums (BGSs) as the CNN input and the corresponding high SR Brillouin frequency shift (BFS) as the output target, the trained CNN is able to obtain an SR higher than the theoretical value determined by the pump pulse width. In the experiment, the CNN accurately retrieves 0.5-m hotspots from the measured BGS with pump pulses from 20 to 50 ns, and the acquired BFS is in great agreement with 45/40 ns differential pulse-width pair (DPP) measurement results. Compared with the DPP technique, the proposed CNN demonstrates a twofold improvement in BFS uncertainty with only half the measurement time. In addition, by changing the training data sets, the proposed CNN can obtain tunable high SR retrieval based on conventional BOTDA sensors that use long pulses without any requirement of hardware modifications. It is worth mentioning that the proposed method is also applicable to larger pulse widths to retrieve a submeter SR. The proposed data post-processing approach paves the way to enable novel high SR BOTDA sensors, which brings substantial improvement over the state-of-the-art techniques in terms of system complexity, measurement time, reliability, etc.

Transition metal carbonate precursors as cathode materials for lithium ion batteries: first principles study

Lithium-ion batteries (LIBs) are frequently regarded as the best batteries ever made due to their significant energy density, low lithium reduction potential, and small size. LIBs are structurally comprised of functional parts of cathodes, anodes, separator, and electrolyte. Cathode materials are among the best possibilities of achieving the remarkable energy densities for LIBs due to their capacitance value exceeding 250 mAh/g. This study investigates the structural, electronic, mechanical and thermodynamic properties of two candidate materials: Ni0.2Mn0.5Co0.2CO3 and Ni0.2Mn0.5Co0.2O2 by means of DFT-based computational simulations. In particular, structural parameters, density of states, elastic constants and phonon dispersion curves were calculated to mimic their stability. From our results we found that the density of states reveal no energy band gap at the Fermi line for both materials, which indicate metallic characteristic. We also note that from the phonon dispersions, Ni0.2Mn0.5Co0.2CO3 shows no negative vibrations along the high Brillouin zone as compared to Ni0.2Mn0.5Co0.2O2 which displayed negative vibration. This implies that Ni0.2Mn0.5Co0.2CO3 is vibrationally stable while Ni0.2Mn0.5Co0.2O2 is unstable.

A Novel Nonlinear Optical Limiter Based on Stimulated Brillouin Scattering in Highly-Nonlinear Fiber

A novel nonlinear optical limiter (NOL) based on stimulated Brillouin scattering (SBS) in highly nonlinear fiber was proposed and experimentally demonstrated at 1550 nm wavelength. The nonlinear optical limiting effects of HNLF were characterized and demonstrated theoretically and experimentally. In a proof-of-concept experiment, we verified that the NOL based on a 50 m HNLF has excellent limiting performance due to its small effective area and high Brillouin gain coefficient. The linear transmittance and lowest nonlinear transmittance of the NOL were 87.5% and 11.9%, respectively.

Compact Brillouin comb laser in Chalcogenide fiber assisted by four-wave mixing

We experimentally demonstrate a multiwavelength Brillouin comb generation employing a segment of As2S3 chalcogenide fiber as the Brillouin gain medium. The extremely high nonlinear and Brillouin gain coefficients allow the As2S3 fiber length to be as short as 4.2 m. A compact Brillouin comb laser composed of multiple Brillouin Stokes and anti-Stokes channels is generated from the As2S3 fiber either with or without ring cavity feedback through the interaction of stimulated Brillouin scattering and four-wave mixing effects. Different output couplings are used to extract the Brillouin comb laser when the cavity feedback is connected. A high laser power of 927 mW and multiple laser channels of 20 are achieved with an output coupling of 90 % and 10 %, respectively. A trade-off between the power threshold, laser channel number, output power and slope efficiency should be considered when optimizing the optical performance of the multiwavelength Brillouin laser.

High spatial resolution fast Brillouin optical time-domain analysis enabled by frequency-agility digital optical frequency comb.

A significant spatial resolution enhancement scheme for digital optical frequency comb (DOFC)-based fast Brillouin optical time-domain analysis (BOTDA) is proposed and experimentally demonstrated by using frequency-agility probes, without sacrificing the frequency resolution. The proposed system ensures high spatial resolution by using short frame duration, meanwhile enabling high frequency resolution retrieval of the Brillouin gain spectrum using frequency interleaving of multiple frequency-agility DOFC probes. Additionally, quadratic phase coding is introduced to release the influence of the high peak to average power ratio of the probes. Eventually, the proposed BOTDA sensor achieves a record 5-m spatial resolution over 10-km fiber with less than 2-MHz frequency uncertainty, and a 1-GHz dynamic measurement range. For proof of concept, 10-Hz vibration sensing is also successfully demonstrated at a 40-Hz sampling rate, showing great potential for fast measurement. It is worth mentioning that a higher spatial resolution can be achieved by using more frequency-agility DOFC probes, albeit at the expense of increasing the measurement time.

Cascade Brillouin Lasing in a Tellurite-Glass Microsphere Resonator with Whispering Gallery Modes.

Brillouin microlasers based on microresonators with whispering gallery modes (WGMs) are in high demand for different applications including sensing and biosensing. We fabricated a microsphere resonator with WGMs from a synthesized high-quality tellurite glass with record high Q-factors for tellurite microresonators (Q ≥ 2.5 × 107), a high Brillouin gain coefficient (compared to standard materials, e.g., silica glasses), and a Brillouin frequency shift of 9 ± 0.5 GHz. The high density of excited resonance modes and high loaded Q-factors allowed us to achieve experimentally cascade Stokes-Brillouin lasing up to the 4th order inclusive. The experimental results are supported by the results of the theoretical analysis. We also theoretically obtained the dependences of the output Brillouin powers on the pump power and found the pump-power thresholds for the first five Brillouin orders at different values of pump frequency detuning and Q-factors, and showed a significant effect of these parameters on the processes under consideration.

Design of photonic crystal fiber amplifier based on stimulated Brillouin amplification for orbital angular momentum

A probe made of amino acids is arranged in a linear chain and joined together by peptide bonds between the carboxyl and amino groups of adjacent amino acid residues. The sequence of amino acids in a protein is determined by a gene and encoded in the genetic code. This can happen either before the protein is used in the cell, or as part of control mechanisms. In order to transmit and amplify high-purity orbital angular momentum mode, a photonic crystal fiber amplifier based on stimulated Brillouin amplification is proposed and designed in this paper. The transmission properties of the photonic crystal fiber amplifier are systematically analyzed by using the finite element method in the C-band. The results show that this photonic crystal fiber amplifier can support the transmission and amplification of 66 orbital angular momentum modes, and all values of the purity of the orbital angular momentum modes supported by this amplifier are higher than 99.4%. By systematically analyzing the Brillouin gain spectra of orbital angular momentum modes with different topological charges, it is found that they have all high Brillouin gain coefficients (&gt; 7 × 10&lt;sup&gt;–9&lt;/sup&gt; m/W) which are 4–5 orders of magnitude higher than the existing OAM amplifiers with the best performance, thus higher signal gain can be obtained. The comprehensive performance of the proposed photonic crystal fiber amplifier is superior to that of the existing optical fiber amplifiers based on stimulated Brillouin amplification and the optical fiber amplifiers doped with rare-earth ions. This makes the amplification and long-distance transmission of OAM mode stable and accurate and provides a possibility for designing the orbital angular momentum mode laser system.

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&lt;sub&gt;2&lt;/sub&gt;S&lt;sub&gt;3&lt;/sub&gt; 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&lt;sup&gt;5&lt;/sup&gt; W&lt;sup&gt;–1&lt;/sup&gt;·m&lt;sup&gt;–1&lt;/sup&gt;, which is nearly 10 times larger than the previously reported backward SBS gain of 2.88 × 10&lt;sup&gt;4&lt;/sup&gt; W&lt;sup&gt;–1&lt;/sup&gt;·m&lt;sup&gt;–1&lt;/sup&gt;, 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.

Propagation of waves in high Brillouin zones: Chaotic branched flow and stable superwires

We report unexpected classical and quantum dynamics of a wave propagating in a periodic potential in high Brillouin zones. Branched flow appears at wavelengths shorter than the typical length scale of the ordered periodic structure and for energies above the potential barrier. The strongest branches remain stable indefinitely and may create linear dynamical channels, wherein waves are not confined directly by potential walls as electrons in ordinary wires but rather, indirectly and more subtly by dynamical stability. We term these superwires since they are associated with a superlattice.

Brillouin Spectroscopy as an Innovative Tool to Investigate Biomechanical Properties of Native Human Aortic Valve and Bioprostheses Tissue

Objective: Proper aortic valve (AV) function is enabled by a complex multi-layered extracellular matrix structure and embedded valvular interstitial and endothelial cells (VICs/VECs). In a highly dynamic environment, VICs and VECs respond to biomechanical stimuli thereby influencing valvular matrix remodelling and pathological changes. Thus, it is of cardinal importance to understand the mechanobiology of AV tissue. It is hypothesized that biomechanical properties of native human AV as well as biological AV prostheses tissue can be determined using Brillouin spectroscopy. Methods: Native AV tissue (n=4) and explanted AV prosthesis (n=1) were obtained from operating theatre after AV replacement, 1mm-thick vibratome cuts were prepared (cross section in radial direction) and directly analysed by means of a confocal Brillouin microscope (10x/0.25NA). Here, the inelastically back-scattered laser light (780nm) is analysed by a custom-build Brillouin spectrometer based on a two-stage virtually imaged phased array (VIPA) set-up. Brillouin maps have been acquired pixel-by-pixel (acquisition time 1s/pixel). Tissue slides adjacent to the vibratome sections were fixed and examined histologically. Results: In stenotic AV tissue mineralized regions in the fibrosa exhibit higher Brillouin shift values compared to non-mineralized spongiosa (Figure 1A). The matrix of the examined explanted AV bioprosthesis (glutaraldehyde-fixed porcine AV) showed higher Brillouin shift values than the massive endocarditic vegetation covering both surfaces of the cusp. Calcified regions at the coaptation area were correlated with higher Brillouin shift values (Figure 1B). Conclusions: Brillouin spectroscopy enables detection of differentiated degrees of tissue stiffness within native stenotic and fibrotic human AV. It represents a promising tool to analyze biomechanical properties of human AV. Figure 1. Procedure of Brillouin spectroscopy on (a) stenotic human aortic valve and (b) aortic valve prothesis (from porcine AV cusps) with florid endocarditis and subsequent histological analysis, high Brillouin shift values indicate enhanced stiffness