Optical lens

Many Optical lens I won’t say more. Let me just mention an intriguing optical lens I recently came across: the microsphere superlens.

1. Isn’t an optical lens just a glass sphere??? Of course, I’m not talking about a glass sphere… Question: How small are microsphere-based super‑optical lenses? They range from 2 to 9 µm, whereas the diameter of an average human hair is typically between 60 and 90 µm—just pluck a strand of hair to get a sense of this scale. Don’t underestimate such tiny microspheres; they can be used for super‑resolution imaging! Researchers employed silica microspheres with diameters of 2–9 µm (you can think of them as miniature spheres) to achieve white‑light microscopy, achieving a minimum lateral resolution of 50 nm. As we know, the imaging resolution of white light—the standard illumination source in conventional optical microscopes—is limited by the diffraction limit, generally considered to be about half the wavelength, or roughly 250 nm. This diffraction‑limit formula should be familiar to everyone:

2. Optical lens The experimental setup for the diffraction‑limit formula is remarkably simple… how simple, exactly? You simply place a microsphere on the stage of an ordinary optical microscope—more precisely, adhere it directly to the sample holder. Is that really how you obtained it? Yes, it is. At this point, Zhihu pops up and hits me right in the knee: “Isn’t NC just a water‑based journal?” So far, this paper’s Google Scholar citation count stands at 65 per year. But dear reader, this probably isn’t the cheapest optical super‑resolution system in history… Once the system architecture is established, the choice of sample also matters greatly. What kind of sample should one select for super‑resolution experiments? It’s tempting to assume that gratings or nanopores fabricated via FIB or similar techniques could yield structures below the diffraction limit. Yet the authors actually hit upon using a Blu‑ray disc! That’s arguably the most inexpensive sample imaginable. Subsequently, they tested the performance of this super‑resolution system: the microsphere–optical lens combination exhibited excellent super‑resolution capabilities under both reflection‑illumination and transmission‑illumination modes of the microscope.

The aforementioned paper represents pioneering work in super-resolution imaging using microspheres; subsequent developments have explored super-resolution imaging with microsphere‑based optical lenses in various material and liquid immersion configurations, which we will not discuss here. This technology has now been commercialized. The optical lens is quite compact and electronically controlled, making it suitable for microscopic imaging of biological tissues. It can also be employed to image physical microstructures such as nanogaps, nanopores, and nano‑patterns—though these are presented here solely for illustrative purposes, with attention to the schematics and scale.

Some friends are interested in this type of microsphere. Optical lens I’m interested in the principles underlying super-resolution systems. Let me briefly introduce them: as early as 2004, a paper reported the phenomenon of photon nanofocusing jets formed when light scatters off microspheres. This effect exhibits sub-diffraction‑limited characteristics at the full width at half maximum of the nanoscale emission intensity distribution, indicating that such microspheres can be used to achieve optical super‑resolution. Beyond wavelength, the behavior also depends on the microsphere’s refractive index— which is itself influenced by the surrounding medium—and its size. Although we typically observe the focal spot of a high‑refractive‑index microsphere located inside the sphere, increasing the refractive index of the surrounding medium can shift the nanofocusing jet outward, beyond the sphere’s surface. From a physical optics perspective, the super‑resolution process can be analyzed as follows: fine surface features give rise to exponentially decaying evanescent waves, which belong to the near field. Conventional objectives generally cannot capture these evanescent waves, since they encode high‑frequency information about the object’s structure, while the objective’s numerical aperture is limited and only resolves far‑field signals. When a dielectric medium—namely, the microsphere—lies between the near‑field and far‑field collectors, it acts as a transducer, converting the near‑field radiation from the microstructure into the far field, where it can then be collected by the objective. In this way, we gain access to the object’s high‑frequency components—its fine details—enabling super‑resolution imaging. (Of course, your objective cannot become the limiting factor in this process; in experiments, you typically use a 100× objective with an NA of 0.9–1.) You might think this experiment is straightforward. Indeed, scientific research often requires careful observation and thorough exploration; you’ll find that simple things are not always easy.