The emerging dual-focus lenses are drawing increasing attention recently due to their wide applications in both academia and industries, including laser cutting systems, microscopy systems, and interferometer-based surface profilers. In this paper, a miniature electrically tunable rotary dual-focus lens is developed. Such a lens consists of two optical elements, each having an optical flat surface and one freeform surface. The two freeform surfaces are initialized with the governing equation Ar2θ (A is the constant to be determined, r and θ denote the radii and angles in the polar coordinate system) and then optimized by ray tracing technique with additional Zernike polynomial terms for aberration correction. The freeform surfaces are achieved by a single-point diamond turning technique and then a PDMS-based replication process is utilized to materialize the final lens elements. To drive the two coaxial elements to rotate independently, two MEMS thermal rotary actuators are developed and fabricated by a standard MUMPs process. The experimental results show that the MEMS thermal actuator provides a maximum rotation angle of about 8.2 degrees with an input DC voltage of 6.5 V, leading to a wide tuning range for both the two focal lengths of the lens. Specifically, one focal length can be tuned from about 30 mm to 20 mm while the other one can be adjusted from about 30 mm to 60 mm.
Photonic crystal split-beam nanocavities allow for ultra-sensitive optomechanical transductions but are degraded due to their relatively low optical quality factors. We report our recent work in designing a new type of one-dimensional photonic crystal split-beam nanocavity optimized for an ultra-high optical quality factor. The design is based on the combination of the deterministic method and hill-climbing algorithm. The latter is the simplest and most straightforward method of the local search algorithm, which provides the local maximum of the chosen quality factors. This split-beam nanocavity is made up of two mechanical uncoupled cantilever beams with Bragg mirrors patterned onto it and separated by a 75 nm air gap. Experimental results emphasize that the quality factor of the second order TE mode can be as high as 19,900. Additionally, one beam of the device is actuated in the lateral direction with the aid of a NEMS actuator and the quality factor maintains quite well even there’s a lateral offset up to 64 nm. We also apply Fano resonance to further increase the Q-factor by constructing two interfering channels. Before tuning, the original Q-factor is 60,000; it’s noteworthy that the topmost Q-factor reaches 67,000 throughout out-of-plane electrostatic force tuning. The dynamic mechanical modes of two devices is analyzed as well. Potentially promising applications, such as ultra-sensitive optomechanical torque sensor, local tuning of fano resonance, all-optical-reconfigurable filters etc, are foreseen.
Nowadays, nano-electro-mechanical systems (NEMS) actuators using electrostatic forces are facing the bottleneck of the
electromagnetic interference which greatly degrades their performances. On the contrary, the hybrid circuits driven by
optical gradient forces which are immune to the electromagnetic interference show prominent advantages in
communication, quantum computation, and other application systems. In this paper we propose an optical actuator
utilizing the optical gradient force generated by a hetero-structure photonic crystal cavity. This type of cavity has a
longitudinal air-slot and characteristics of ultrahigh quality factor (Q) and ultra-small mode volume (V) which is capable
of producing a much larger force compared with the waveguide-based structures. Due to the symmetry property,
attractive optical gradient force is generated. Additionally, the optomechanical coefficient (gom) of this cavity is two
orders of magnitude larger than that of the coupled nanobeam photonic crystal cavities. The 2D hetero-structure cavity,
comb drives, folded beam suspensions and the displacement sensor compose the whole device. The cavity serves as the
optical actuator whilst the butt-coupled waveguide acts as the displacement sensor which is theoretically proved to be
insensitive to the temperature variations. As known, the thermo-optic effect prevails especially in the cavity-based
structures. The butt-coupled waveguide can be used to decouple the thermal effect and the optoemchanical effect (OM)
with the aid of comb drives. The results demonstrate that the proposed optical gradient force actuator show great
potential in the future of all-optical reconfigurable circuits.