Subwavelength imaging and detection using adjustable and movable droplet microlenses

We developed adjustable and movable droplet microlenses consisting of a liquid with a high refractive index. The microlenses were prepared via ultrasonic shaking in deionized water and the diameter of the microlenses ranged from 1 to 50 μm. By stretching the microlenses, the focal length can be adjusted from 13 to 25 μm. With the assistance of an optical tweezer, controllable assembly and movement of microlens arrays were also realized. The results showed that an imaging system combined with droplet microlenses provided a resolution of ~80 nm and an effective numerical aperture of 4.19 under white-light illumination. Using the droplet microlenses, fluorescence emission at 550 nm from CdSe@ZnS quantum dots were efficiently excited and collected. Moreover, Raman scattering signals from a silicon wafer were enhanced by ~19 times. The presented droplet microlenses may offer new opportunities for flexible liquid devices in subwavelength imaging and detection.