(Nanowerk Spotlight) The ability to precisely measure and manipulate matter at the nanoscale has emerged as a frontier of paramount importance. From unraveling the fundamental physics of quantum systems to engineering the next generation of ultrahigh density microchips, the capacity to interact with the world at its most minute scales underpins some of the most transformative breakthroughs of our era. However, achieving high precision typically requires complex and costly interferometric systems that impose stringent demands on measurement setups.
In the quest for more practical alternatives, researchers have explored various approaches. Some have leveraged the interaction between nanostructures like silicon particles and the local polarization of structured light fields to create novel displacement sensors. Others have harnessed the power of optical metasurfaces, such as in the development of an “optical ruler” for lateral nanoscale displacement measurements. Yet these methods often rely on specific nanostructures or relatively expensive, intricate components, limiting their broad applicability.
Structured light – optical fields with customized spatiotemporal properties – has emerged as a promising avenue for simplifying and enhancing optical sensing systems. By tailoring the amplitude, phase, and polarization of light, researchers can unlock novel capabilities in areas like optical manipulation, biomedical imaging, communications, and sensing.
In particular, 2D structured light has found extensive use in sensing applications due to its ability to enable simple, fast, and accurate measurements. However, the lack of depth information in these 2D patterns constrains their utility for 3D position sensing at the nanoscale.
Now, a research team has unveiled a new type of structured light that could help overcome these limitations. As reported in a paper in Advanced Functional Materials (“Designed 3D Dumpling-Shaped Femtosecond Laser Structured Light Field for Nanoscale Sensing”), the scientists have discovered and characterized a “dumpling-shaped structured light field” (DSLF) that forms when focusing special cylindrical lens beams under high numerical aperture (NA) conditions. This unique 3D light field consists of two perpendicular line-shaped focal regions – a straight line and a curved line – whose relative orientation can be precisely tuned by modulating the phase of the input beam.
The optical system for generating the 3D dumpling-shaped beam. a) Experiment setup: Fs laser, femtosecond laser; HWP, half-wave plate; PBS, polarizing beam splitter; M, mirror; BE, beam expander; SLM, spatial light modulator; DM, dichroic mirror; OL, objective lens; L, lens; CCD, charge coupled device. b) The only-phase hologram of a cylindrical lens loaded on the SLM. c) The designed DSLF is generated after focusing by an objective lens. (Image: Reproduced with permission from Wiley-VCH Verlag )
To investigate the propagation and focusing properties of the DSLF, the researchers conducted comprehensive simulations involving both scalar and vector diffraction theory. They identified a key relationship between the phase profile of the beam illuminating the objective lens and the morphology of the resulting light field, experimentally validating their predictions through direct imaging.
Exploiting this understanding, the team demonstrated the ability to flexibly control the DSLF’s shape – accessing configurations ranging from an “upright dumpling” to a flattened line focus to an “inverted dumpling” – by adjusting the focal length of the phase mask encoding the cylindrical lens. The researchers then applied the DSLF to laser direct writing via two-photon polymerization, showcasing its potential for rapidly generating complex 3D microstructures with submicron features in a photosensitive polymer.
But perhaps the most exciting application lies in optical sensing. Through experiments and simulations, the scientists discovered that the relative lengths of the two focal lines vary in a predictable manner as the DSLF is defocused, with the most dramatic changes occurring within about 100 nm of the focal plane. By simply imaging the reflected DSLF and analyzing the shape of the resulting light pattern, they realized it was possible to detect incredibly subtle displacements and vibrations of a target surface.
Putting this concept into practice, the researchers demonstrated a displacement sensor with an axial resolution of just 10 nm – about 1/80th of the wavelength of the probing light. Notably, this is 10-20 times finer than the optical resolution of the imaging system itself, enabled by the sensitive response of the DSLF’s structure to defocus. The team further showcased the system’s capabilities by using it to directly visualize minute vibrations induced by gently tapping on the optical table.
Compared to conventional interferometric techniques, this approach offers a much simpler and more stable architecture, requiring only a standard microscope and camera to achieve nanometric precision. The researchers propose that this simplicity, combined with the system’s ability to directly distinguish positive and negative displacements, could make it an attractive alternative to costlier and more complex methods in many application areas.
Looking ahead, the scientists envision that this work could open up new possibilities not just in metrology and sensing, but also in areas like optical manipulation, microscopy, and laser materials processing. With further development, the unique 3D properties of the DSLF could potentially be harnessed to enable more advanced multiphoton fabrication schemes, exotic optical trapping configurations, or novel forms of super-resolution imaging.
At the same time, the researchers emphasize that their study represents just one realization of the broader concept of employing structured light for enhanced optical measurements. They suggest that exploring other classes of 3D structured fields, enabled by alternative beam shaping techniques or even machine learning-based inverse design, could uncover a rich new landscape of opportunities at the intersection of metrology, sensing, imaging, and fabrication.
Through innovations like the “dumpling-shaped” light field, scientists continue to push the boundaries of what is possible with optical tools at the micro and nanoscale. By cleverly shaping light to probe and manipulate the world in new ways, they are not only enhancing the precision and functionality of existing technologies, but also opening doors to entirely new capabilities. As research in this field advances, we can expect to see structured light play an increasingly central role in empowering scientific discovery and technological progress across a multitude of domains.
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