High-resolution Brillouin Research Articles

Monitoring cis-to-trans isomerization of azobenzene using Brillouin microscopy

Brillouin spectroscopy is commonly used to study the acoustic properties of materials. Here we explored its feasibility in studying the photoinduced isomerization of azobenzene. The isomerization of azobenzene changes the solution elastic modulus, and Brillouin scattering is sensitive to these changes. In this study, we experimentally demonstrated the photoswitching of azobenzene in DMSO using our home-made virtually imaged phased array-based high-resolution optical Brillouin spectrometer, and confirmed the results by ultraviolet–visible spectrophotometry. Remarkable Brillouin frequency shift variations were quantitatively recorded upon irradiation, and it was found that this method can indeed be used to monitor the isomerization process in situ. Importantly, our strategy also allows us to provide the relationship between the fraction of trans- and cis- azobenzene and the Brillouin frequency shift. This shows that Brillouin spectroscopy has broad prospects for the characterization of azobenzene isomerization and other photoresponsive materials.

Test and Analysis on Extended Temperature Rise of 110 kV Transformer Based on Distributed Temperature Sensing

Transformer plays a vital role in grids and its internal hot-spot temperature is a crucial monitoring parameter. In this paper, three groups of temperature rise tests with the load rate of 70%, 100%, and 125% were carried out on a 110 kV transformer with built-in distributed sensing fiber. Based on the distributed temperature sensing technology, the temperature distribution of winding was acquired. Accuracy of IEC, Susa, and Swift thermal models was compared. To eliminate the non-negligible error in cooling process, a modified-Susa model based on driving factor and steady-state temperature rise was proposed. Using this, the top oil temperature prediction error during operation and cooling is reduced. In addition, the real-time tracking of hot spots and analysis of overall temperature gradient of windings was implemented. The accuracy of different hot spot calculation models was also compared. Results show that the prediction accuracy of each model is satisfactory during the operation, but the error during the cooling process cannot be ignored. The refined temperature distribution of the winding was also obtained by the high-resolution Brillouin optical time-domain analysis sensing method. Periodic temperature fluctuations and local hot spots inside windings are perceived, which is confirmed to be closely related to winding geometry.

High-resolution line-scan Brillouin microscopy for live imaging of mechanical properties during embryo development

Brillouin microscopy can assess mechanical properties of biological samples in a three-dimensional (3D), all-optical and hence non-contact fashion, but its weak signals often lead to long imaging times and require an illumination dosage harmful for living organisms. Here, we present a high-resolution line-scanning Brillouin microscope for multiplexed and hence fast 3D imaging of dynamic biological processes with low phototoxicity. The improved background suppression and resolution, in combination with fluorescence light-sheet imaging, enables the visualization of the mechanical properties of cells and tissues over space and time in living organism models such as fruit flies, ascidians and mouse embryos.

High-resolution line-scan Brillouin microscopy for live-imaging of mechanical properties during embryo development

High-resolution line-scan Brillouin microscopy for live-imaging of mechanical properties during embryo development

Quasi-Distributed Refractive Index Sensing by Stimulated Brillouin Scattering in Tapered Optical Fibers

In this paper, we demonstrate that multiple tapers in optical fibers allow for quasi-distributed refractive index sensing via a high spatial resolution Brillouin Optical Frequency-Domain Analysis (BOFDA) configuration. We first characterize, theoretically and experimentally, the variation of the Brillouin frequency shift (BFS) with the diameter of the tapered fiber. Then, we characterize the dependence of the BFS from the outer refractive index in an optical fiber taper with a waist diameter of 10 µm. Finally, we show that more tapers can be realized along the same fiber, in order to provide multi-point refractive index sensing.

Distributed Liquid Level Sensor Based on Brillouin Optical Frequency-Domain Analysis

We propose and demonstrate a distributed liquid level sensor based on high-resolution Brillouin Optical Frequency-Domain Analysis and an actively heated cobalt-doped optical fiber. The liquid level position is determined by localizing the abrupt temperature change occurring along the active fiber at the liquid/air interface, through distributed temperature sensing. Differently from previous works based on the use of optically heated fibers, in the proposed configuration no separate laser source was employed to heat the fiber. In practice, a single laser source was adopted for both optical heating and distributed temperature sensing. The experiments report the measurement of the liquid level position with 5 mm level resolution along a 9-cm long heated fiber. The paper also discusses the limitations of the proposed method. In particular, the negative impact of spontaneous Brillouin scattering, in terms of measurement noise, is revealed by the measurements. We show that the spontaneous Brillouin scattering induces spurious components in the frequency-domain, affecting the accuracy of temperature measurements.

Dispersion of the surface phonons in semiconductor/topological insulator Si/Bi2Te3 heterostructure studied by high resolution Brillouin spectroscopy

The dynamics and dispersion of surface phonons in heterostructure semiconductor/ topological insulator Si/Bi2Te3 was investigated using high resolution Brillouin light scattering method in the GHz frequency range. Both Rayleigh and Sezawa surface acoustic waves have been observed for wave vectors ranging from 0.006 to 0.023 nm−1. Anomaly in dispersion relations ω(q) for both surface waves were detected for the wave vector q = 0.016 nm−1. The finite element method (FEM) was used to simulate the observed shapes of ω(q) and to find the deformation profiles of surface acoustic waves. We attribute the observed changes to the coupling between low energy electrons and surface phonons. The coupling between helical Dirac states and surface phonons is discussed in the frame of accessible theoretical models.

Label-free histological imaging of tissues using Brillouin light scattering contrast.

Brillouin light scattering offers a unique label-free approach to measure biomechanical properties non-invasively. While this technique is used in biomechanical analysis of cells and tissues, its potential for visualizing structural features of tissues based on the biomechanical contrast has not been much exploited. Here, we present high-resolution Brillouin microscopy images of four basic tissue types: muscular, connective, epithelial, and nervous tissues. The Brillouin contrast distinguishes between muscle fiber cells and endomysium in skeletal muscle and reveals chondrocytes along with spatially varying stiffness of the extracellular matrix in articular cartilage. The hydration-sensitive contrast can visualize the stratum corneum, epidermis, and dermis in the skin epithelium. In brain tissues, the Brillouin images show the mechanical heterogeneity across the cortex and deeper regions. This work demonstrates the versatility of using the Brillouin shift as histological contrast for examining intact tissue substructures via longitudinal modulus without the need for laborious tissue processing steps.

Fiber-based angular filtering for high-resolution Brillouin spectroscopy in the 20-300 GHz frequency range.

Brillouin spectroscopy emerges as a promising non-invasive tool for nanoscale imaging and sensing. One-dimensional semiconductor superlattice structures are eminently used for selectively enhancing the generation or detection of phonons at few GHz. While commercially available Brillouin spectrometers provide high-resolution spectra, they consist of complex experimental techniques and are not suitable for semiconductor cavities operating at a wide range of optical wavelengths. We develop a pragmatic experimental approach for conventional Brillouin spectroscopy that can integrate a widely tunable excitation-source. Our setup combines a fibered-based angular filtering and a spectral filtering based on a rotating single etalon and a double grating spectrometer for sequential reconstruction of Brillouin spectra. This configuration allows probing confined acoustic phonon modes in the 20-300 GHz frequency range with excellent laser rejection and high spectral resolution. Remarkably, our scheme based on the excitation and collection of the enhanced Brillouin scattering signals through the optical cavity allows for better angular filtering with decreasing phonon frequency. It can be implemented for the study of cavity optomechanics and stimulated Brillouin scattering over broadband optical and acoustic frequency ranges.

Surface phonons in topological insulator Bi2Te3 investigated by Brillouin light scattering

High resolution Brillouin spectroscopy was used for the first time to study the dispersion and anisotropy of surface phonons in the single crystal of topological insulator Bi2Te3. Two surface acoustic waves have been observed, which distinguishes this material from other metals or nontransparent materials. The modes were assigned as Rayleigh waves. The obtained results were then simulated by Finite Element Method. The layered structure of the unit cell proposed in simulation reproduced quite well experimental results of the modes dispersion and anisotropy.

Investigation of phonons and magnons in [Ni80Fe20/Au/Co/Au]10 multilayers

The dispersion relations of surface acoustic waves and spin waves propagating in magnetic [Ni80Fe20/Au/Co/Au]10 multilayers deposited on a silicon substrate have been investigated by high-resolution Brillouin spectroscopy. We measured the spectra of spin waves for two canonical geometries where the spin waves propagate with the wave vector oriented parallel and perpendicular to the direction of static magnetization in-plane magnetized Py layer. We investigated experimentally the crossing of phononic and magnonic dispersion relations and discussed the mechanism of magnetoelastic interaction in this system.

Assessment of corneal substrate biomechanics and its effect on epithelial stem cell maintenance and differentiation

Whilst demonstrated extensively in vitro, the control of cell behaviour via modulation of substrate compliance in live tissues has not been accomplished to date. Here we propose that stem cells can be regulated solely through in situ modulation of tissue biomechanics. By first establishing, via high-resolution Brillouin spectro-microscopy, that the outer edge (limbus) of live human corneas has a substantially lower bulk modulus compared to their centre, we then demonstrate that this difference is associated with limbal epithelial stem cell (LESC) residence and YAP-dependent mechanotransduction. This phenotype-through-biomechanics correlation is further explored in vivo using a rabbit alkali burn model. Specifically, we show that treating the burnt surface of the cornea with collagenase effectively restores the tissue’s mechanical properties and its capacity to support LESCs through mechanisms involving YAP suppression. Overall, these findings have extended implications for understanding stem cell niche biomechanics and its impact on tissue regeneration.

In- and out-of-plane longitudinal acoustic-wave velocities and elastic moduli in h-BN from Brillouin scattering measurements

The elastic constants of high-quality, single-crystal hexagonal boron nitride (h-BN) have been measured by means of high resolution Brillouin spectroscopy using a modified reflected light microscope. The sound propagation velocity in the c-axis direction and perpendicular to the c-axis have been obtained from the Brillouin frequency shift, with a proper account taken of the vast difference between the ordinary and extraordinary refractive indices recently reported in the highly anisotropic layered h-BN crystal. The elastic constants c11 and c33 obtained from the Brillouin experiments are somewhat lower than previous determinations based on inelastic x-ray measurements and confirm the overestimation of the h-BN elastic constants by ab initio calculations based on density functional perturbation theory in the local density approximation.

High-Pass Filtering for Accurate Reconstruction of the Brillouin Frequency Shift Profile From Brillouin Optical Frequency Domain Analysis Data

In this paper, we propose and demonstrate a new method to reconstruct the Brillouin frequency shift profile in high spatial resolution Brillouin optical frequency domain analysis (BOFDA) sensors. The method aims to compensate the distorting terms affecting BOFDA measurements, which originate from the modulation impressed on the acoustic wave involved in the scattering phenomenon. We show that these terms can be easily removed by applying a high-pass filter to the data acquired in the frequency-domain, at the cost of a degradation of the signal-to-noise ratio. A numerical analysis, as well as experimental tests carried out at 8-mm spatial resolution, validate the proposed technique.

The electron-phonon interaction at deep Bi2Te3-semiconductor interfaces from Brillouin light scattering

It is shown that the electron-phonon interaction at a conducting interface between a topological insulator thin film and a semiconductor substrate can be directly probed by means of high-resolution Brillouin light scattering (BLS). The observation of Kohn anomalies in the surface phonon dispersion curves of a 50 nm thick Bi2Te3 film on GaAs, besides demonstrating important electron-phonon coupling effects in the GHz frequency domain, shows that information on deep interface electrons can be obtained by tuning the penetration depth of optically-generated surface phonons so as to selectively probe the interface region, as in a sort of quantum sonar.

High-resolution Brillouin optical correlation domain analysis with no spectral scanning.

Distributed Brillouin fiber sensors typically rely on the reconstruction of the steady-state Brillouin gain spectrum (BGS), through spectral scanning of the frequency offset between the pump and signal waves. In this work, we propose and demonstrate an alternative approach, in which the local Brillouin frequency shift (BFS) is extracted from temporal transient analysis of the step response of the amplified signal wave. Measurements are taken at only two arbitrary frequency offsets between pump and signal. No spectral scanning and no prior knowledge of a reference BGS are necessary. The principle is supported by analytic and numeric solutions of the differential equations of stimulated Brillouin scattering. The BFS of a 2 meters-long fiber under test was measured with 1 MHz accuracy and a dynamic range of 200 MHz. Transient measurements were also performed in a Brillouin optical correlation domain analysis (B-OCDA) experiment with 4 cm resolution, standard deviation of 2.4 MHz and 100 MHz dynamic range. A 4 cm-wide hot-spot was properly identified in the measurements. Multiple correlation peaks could be addressed in a single flight of a pump pulse. The results represent the first B-OCDA that is free of spectral scanning. This new measurement concept may be applicable to random-access distributed and dynamic monitoring of sound and vibration.

Full scale strain monitoring of a suspension bridge using high performance distributed fiber optic sensors

This paper investigated field monitoring of a 1108 m suspension bridge during an assessment load test, using integrated distributed fibre-optic sensors (DFOSs). In addition to the conventional Brillouin time domain analysis system, a high spatial resolution Brillouin system using the differential pulse-width pair (DPP) technique was adopted. Temperature compensation was achieved using a Raman distributed temperature sensing system. This is the first full scale field application of DFOSs using the Brillouin time domain analysis technique in a thousand-meter-scale suspension bridge. Measured strain distributions along the whole length of the bridge were presented. The interaction between the main cables and the steel-box-girder was highlighted. The Brillouin fibre-optic monitoring systems exhibited great facility for the purposes of long distance distributed strain monitoring, with up to 0.05 m spatial resolution, and 0.01 m/point sampling interval. The performance of the Brillouin system using DPP technique was discussed. The measured data was also employed for assessing bridge design and for the assessment of structural condition. The results show that the symmetrical design assumptions were consistent with the actual bridge, and that the strain values along the whole bridge were within the safety range. This trial field study serves as an example, demonstrating the feasibility of highly dense strain and temperature measurement for large scale civil infrastructures using integrated DFOSs.

Brillouin Optical Correlation Domain Analysis Addressing 440 000 Resolution Points

The distributed Brillouin analysis of an 8.8-km-long fiber with a spatial resolution of 2 cm is presented. All 440 000 potential resolution points are addressed in the measurement. A 7-cm-long hot-spot, located toward the output end of the pump wave, is properly identified in the experiment. The experimental error in the estimate of the local values of the Brillouin frequency shift is ±3.5 MHz. The analysis is based on the simultaneous generation and analysis of Brillouin interaction in more than 2000 correlation peaks, induced by periodic phase modulation of the pump and signal waves. The Brillouin amplifications at individual peaks are resolved using radar-like coding of pump wave magnitude by a 10000 bit-long aperiodic sequence, and postdetection compression at the receiver end. Extensive numerical simulations of the Brillouin interactions over kilometers of fiber with centimeter resolution are reported as well. The results are at the state of the art for high-resolution distributed Brillouin sensors.

Surface acoustic wave velocity and elastic constants of cubic GaN

We present high-resolution surface Brillouin scattering measurements on cubic GaN layers grown on GaAs substrate. By using a suitable scattering geometry, scattering by surface acoustic waves is recorded for different azimuthal angles, and the surface acoustic wave velocities are determined. A comparison of experimental results with numerical simulations of the azimuthal dependence of the surface wave velocity shows good agreement and allows a consistent set of elastic constants for c-GaN to be determined.

Acoustic and relaxation behaviors of polydimethylsiloxane studied by using brillouin and dielectric spectroscopies

The temperature dependences of the acoustic properties and the dielectric relaxation times of polydimethylsiloxane were investigated by using high-resolution Brillouin and broadband dielectric spectroscopies. The longitudinal sound velocity showed a large increase upon approaching the glass transition temperature while the acoustic absorption coefficient exhibited a maximum at ~263 K. Comparison of these results with previous ultrasonic data revealed a substantial frequency dispersion of the acoustic properties of this silicone-based elastomer. The relaxation times derived from the acoustic absorption peaks were consistent with the temperature dependence of the dielectric relaxation time of the structural a process, indicating a strong coupling between the acoustic waves and the segmental motions of the main chains.