Total longitudinal momentum in a dispersive optical waveguide

A broadband and low-power light-control-light effect in a fiber-optic nano-optomechanical system

The coupling of the optical and mechanical degrees of freedom using optical force in nano-devices offers a novel mechanism to implement all-optical signal processing. However, the ultra-weak optical force requires a high pump optical power to realize all-optical processing. For such devices, it is still challenging to lower the pump power and simultaneously broaden the bandwidth of the signal light under processing. In this work, a simple and cost-effective optomechanical scheme was demonstrated that was capable of achieving a broadband (208 nm) and micro-Watt (∼624.13 μW) light-control-light effect driven by a relatively weak optical force (∼3 pN). In the scheme, a tapered nanofiber (TNF) was evanescently coupled with a substrate, allowing the pump light guided in the TNF to generate a strong transverse optical force for the light-control-light effect. Additionally, thanks to the low stiffness (5.44 fN nm^-1) of the TNF, the light-control-light scheme also provided a simple method to measure the static weak optical force with a minimum detectable optical force down to 380.8 fN. The results establish TNF as a cost-effective scheme to break the limitation of the modulation wavelength bandwidth (MWB) at a low pump power and show that the TNF-optic optomechanical system can be well described as a harmonic oscillator.

Polarization-Dependent Lateral Optical Force of Subwavelength-Diameter Optical Fibers

It is highly desirable to design optical devices with diverse optomechanical functions. Here, we investigate lateral optical force exerted on subwavelength-diameter (SD) optical fibers harnessed by input light modes with different polarizations. It is interesting to find that input light modes of circular or elliptical polarizations would bring about lateral optical force in new directions, which has not been observed in previous studies. By means of finite-difference time-domain (FDTD) simulations, detailed spatial distributions of the asymmetric transverse force density are revealed, meanwhile dependence of optical force on input light polarizations, fiber diameters, and inclination angles of fiber endfaces are all carefully discussed. It is believed that polarization-sensitive reflection, refraction, and diffraction of optical fields occur at the interface, i.e., fiber oblique endfaces, resulting in asymmetrically distributed optical fields and thereafter non-zero transverse optical force. We believe our new findings could be helpful for constructing future steerable optomechanical devices with more flexibility.

Transverse optical forces and sideways deflections in subwavelength-diameter optical fibers

We investigate transverse optical forces exerted on the endface of subwavelength-diameter (SD) optical fiber by using a finite-difference time-domain (FDTD) method. Detailed spatial distributions of transverse optical force along the fiber axis can now be accessible, based on which the dependence of transverse optical force on transverse cross sections, oblique-cut endfaces and high-order mode are carefully studied. Our numerical results demonstrate that either asymmetric cross section or oblique-cut endface would dominantly contribute to the transverse optical force and the corresponding sideways deflection of SD fiber, which is in good agreement with previous experimental observations. The novel behavior of transverse optical force by the high-order mode would give rise to new guidelines for constructing high-performance optomechanical devices.