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Optical Fiber-Based Probe Enables Phonon Imaging in 3D

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NOTTINGHAM, England, May 17, 2021 — A University of Nottingham research team has developed a phonon probe device that can simultaneously access 3D spatial information and mechanical properties from microscopic objects. The probe operates at the GHz range of the acoustic spectrum. At such frequencies, the wavelength of sound becomes comparable to ultraviolet optical wavelengths and therefore provides an opportunity for high-resolution imaging.

The phonon probe, which supports applications in biological metrology, health care, and precision manufacturing, uses time-resolved Brillouin scattering to pump GHz frequency ultrasound from the tip of a 125-μm-diameter optical fiber into a specimen. It uses a pulsed laser to detect a high-frequency ultrasound wave as the wave travels through the specimen. This time-of-flight acoustic signature simultaneously encodes the local mechanical properties and the spatial profile of the specimen. When the probe is scanned, these two properties can be resolved in 3D with optical lateral resolution and with axial resolution dictated by the sub-µm acoustic wavelengths.

The researchers applied their phonon probe to the parallel elastography-profilometry of objects as small as 10 × 2 μm (radius and height). The probe demonstrated 2.5-μm lateral resolution and was able to measure object height with 45-nm precision, which is over an order of magnitude smaller than the probe’s optical wavelength. To show the endoscopic potential for the device, the researchers extrapolated the single fiber to tens of thousands of fibers in an imaging bundle.

(a) Layout of the optical fiber system used to generate and detect GHz frequency phonons from the tip of a single-mode optical fiber (inset). Scanning the distal end of the ultrasonic probe in space with respect to a microscopic object (inset) allows mapping of elastic (b, overlaid onto brightfield image) and topographic (c) information with high resolution. Scale bars: 10 µm. Courtesy of La Cavera, S., Pérez-Cota, F., Smith, R.J. et al.
(a) Layout of the optical fiber system used to generate and detect GHz-frequency phonons from the tip of a single-mode optical fiber (inset). Scanning the distal end of the ultrasonic probe in space with respect to a microscopic object (inset) allows mapping of elastic (b, overlaid onto brightfield image) and topographic (c) information with high resolution. Scale bars: 10 μm. Courtesy of S. La Cavera et al.
To their knowledge, the researchers said, the device is the first optical fiber-based ultrasonic imaging tool capable of resolving biological cell-size objects. They believe that the phonon probe could be the highest-resolution optical fiber-based, ultrasonic 3D-imaging device currently available. The phonon probe offers noncontact operation, label-free contrast, and high resolution — a combination that could allow it to supplement bench-top profilometry equipment such as atomic force microscopy, stylus profilometry, and optical profilometry.


The photon probe’s ability to measure subsurface mechanical properties, its biocompatibility, and its endoscopic potential could make it useful for the type of in vivo measurements required for minimally invasive point-of-care diagnostics; the building blocks of disease can be traced to the subcellular level and are intertwined with mechanical properties. An endoscopic device that could access this regime could accelerate the development of elasticity-based diagnostics, members of the research team reported.

Beyond clinical health care, the fields of tissue engineering and precision manufacturing could also use the high-resolution tool for superficial diagnostics.

The research was published in Light: Science & Applications (www.doi.org/10.1038/s41377-021-00532-7).

Published: May 2021
Glossary
phonon
A phonon is a quantum of vibrational energy associated with the periodic motion of atoms or molecules in a crystalline lattice. In simpler terms, phonons are quanta of lattice vibrations in a solid material. They represent the collective vibrational modes of atoms or groups of atoms in a crystal lattice and play a crucial role in understanding the thermal and mechanical properties of materials. Key points about phonons include: Quantization of vibrational energy: Similar to photons for...
probe
Acronym for profile resolution obtained by excitation. In its simplest form, probe involves the overlap of two counter-propagating laser pulses of appropriate wavelength, such that one pulse selectively populates a given excited state of the species of interest while the other measures the increase in absorption due to the increase in the degree of excitation.
optical fiber
Optical fiber is a thin, flexible, transparent strand or filament made of glass or plastic used for transmitting light signals over long distances with minimal loss of signal quality. It serves as a medium for conveying information in the form of light pulses, typically in the realm of telecommunications, networking, and data transmission. The core of an optical fiber is the central region through which light travels. It is surrounded by a cladding layer that has a lower refractive index than...
metrology
Metrology is the science and practice of measurement. It encompasses the theoretical and practical aspects of measurement, including the development of measurement standards, techniques, and instruments, as well as the application of measurement principles in various fields. The primary objectives of metrology are to ensure accuracy, reliability, and consistency in measurements and to establish traceability to recognized standards. Metrology plays a crucial role in science, industry,...
endoscope
An endoscope is a medical device used for visualizing and examining the interior of hollow organs, body cavities, or confined spaces within the body. It consists of a flexible or rigid tube equipped with an optical system and illumination sources, allowing healthcare professionals to observe internal structures, perform diagnostic procedures, and guide surgical interventions without the need for invasive surgery. endoscope suppliers → Key components and features of an...
Research & TechnologyeducationEuropephononprobeoptical fiberbiological metrologymetrologymetrology and researchendoscopeOptoacoustic imagingphotoacousticBiophotonicsmedicalBrillouin light scatteringBrillouin optomechanicsoptical material propertiesmanufacturingtissue engineering

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