Optical Fiber-Based Probe Enables Phonon Imaging in 3D

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 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).