Enhance The Backaction Force Mediated By Photonic Nanojet With A Broadband Supercontinuum Source

Abstract

Optical radiation force originates from the photon momentum transfer to the absorptive microparticle, and has inspired important applications, including atom cooling and the Bose-Einstein condensates. However, the momentum flux points forward, and microparticle thus experiences a forward radiation force (‘positive force’). In practice, the negative optical force (‘negative force’), which points backwards, has also attracted extensive research efforts, including the tailoring of the beam wavefront, the polarization, and the background medium. The dielectric microparticle concentrates the light into a photonic nanojet with transverse size smaller than the wavelength. The molecules heat up and lead to temperature rise inside the photonic nanojet. We have observed the backaction force of the dielectric particles under a mode-locked laser oscillating at 1.57m. Since the polymer microsphere presents chromatic dispersion, the photonic nanojet dimension extends along the longitudinal direction when the incident laser adopts the supercontinuum source. We built an all-fiber mode-locked laser (MLL) as seed light with central wavelength of 1.55 µm and repetition rate of 44 MHz. The pulses were first chirped by dispersion compensating fiber (DCF) before amplification using an erbium doped fiber amplifier (EDFA, IPG photonics). The amplified laser was compressed by passing through a single mode optical fiber (SMF). The supercontinuum light was created through self-phase modulation in the high-nonlinear optical fiber (HNLF-SPINE, 50 m). Due to self-phase modulation in the fiber, the spectrum broadens from ~ 50 nm to ~ 400 nm. The collimated supercontinuum beam directly entered the cuvette containing the dielectric microparticle suspension. In contrast, we bypassed the HNLF and applied the MLL for the experiment. The detection arm is orthogonal to the supercontinuum beam, similar to the light-sheet fluorescence microscopy, but with separate visible light for illumination. In the presence of either supercontinuum or MLL laser, the particles are all attracted to the laser source. In contrast to single beam optical trap, the photonic nanojet mediated optical backaction provides parallel manipulation of all particles simultaneously. The speed increases with laser power for both the MLL and the supercontinuum cases, moreover, the magnitude of force under supercontinuum illumination is greater than that under MLL. Such force enhancement attributes to the spreading of the photonic nanojet with broadband spectrum, which suggests the possibility to augment the backaction by shaping the spectrum of the laser. Since the spectrum of our supercontinuum source only covers ~ 400 nm, the magnitude of the backaction force can be further enhanced by using a laser with broader spectrum. In conclusion, we built a supercontinuum source with spectrum spanning ~ 400 nm in high nonlinear fiber, and applied the supercontinuum beam to enhance the backaction force owing to the increased volume of photonic nanojet. Such backaction force mediated by the supercontinuum source may find various applications, e.g., large-scale particle manipulation, and particle classification.