Reconfigurable Size‐Sorting of Micronanoparticles in Chalcogenide Waveguide Array

Abstract

Since the experimental demonstration of optical tweezers in 1970s, [1] it has been rapidly developed into noninvasive and versatile tool to manipulate atoms [2] and biomolecules. [3,4] Particularly, the optical trapping is based upon the gradient force whose direction and magnitude are determined by the local field gradient. [5][6][7] In order to manipulate multiple entities with the different sizes (i.e., viruses and DNAs), the optical tweezers need to be assisted with the other forces. [8] For example, by combining the quasi-Bessel beam with the forces induced by both fluid and photoresist, sub-100 nm particles can be separated. [9] Other schemes, like acoustics [10,11] and microfluidics, [12][13][14] are also promising for separating biomolecules with diameter of %μm. [15] Ultimately, whereas, optical tweezers face two critical challenges; first, the diffraction limits how tightly the beam can be focused hence limiting the trapping strength; second, the short focal depth of trapping area forbids the continuously optical transportation of nanoparticles using free space light. [16,17] As such, the diffraction limit of optical tweezers restrains the manipulated particles' size to be micrometer. Thus, optical trapping and long-distance transporting of particles with the size ranging from nanometer to micrometer by free-space beam is extremely formidable. To manipulate subwavelength particles, optical waveguides (WGs) confining light beam within solid structures have been intensively investigated. Such a light confinement can lead to a self-consistent beam that indefinitely transmits through the WG without loss or varying its form. [18,19] To this end, optical forces produced by the different kinds of dielectric nanostructures like channel WG, [20][21][22] WG loops, [23] rib WG, [24] and slot WG [16,17] have attracted intense attentions. These structures produce E-field decaying exponentially within a region above the diffraction limits. They can trap nanometersized objects to WG's surface and transport them by transmission fields. Most WG structures are often designed to manipulate the nanoparticles and employed rarely for microscale objects. In the meantime, the size of fluidic channels cannot diminish from nanoscale biomolecules (i.e., DNA and viruses) to microscale liquid droplets. [25] This issue has been somewhat resolved by manipulating and sorting microsphere that acts as carriers of biomolecules with a quantity of DNAs or proteins attaching on them. [26,27] The new-generation "lab-on-chip" sorting system [28,29] will merge optical manipulation, microfluidic, and some other techniques to obtain size-sorting of micronanoobjects over a single chip. Such a system entails tunable manipulation techniques that can convey and trap different sized particles freely between regimes.The chalcogenide alloys, pioneered by Ovshinsky, [30] are well known for their successful applications in phase-change memory and rewritable optical discs due to the advantages of rapid switching speed, excellent scalability, high cyclabil...