Chan Choy Chum

Hybrid bilayer plasmonic metasurface efficiently manipulates visible light

Two highly coupled plasmonic metasurfaces exhibit much higher conversion efficiency and extinction ratio than individual ones. Metasurfaces operating in the cross-polarization scheme have shown an interesting degree of control over the wavefront of transmitted light. Nevertheless, their inherently low efficiency in visible light raises certain concerns for practical applications. Without sacrificing the ultrathin flat design, we propose a bilayer plasmonic metasurface operating at visible frequencies, obtained by coupling a nanoantenna-based metasurface with its complementary Babinet-inverted copy. By breaking the radiation symmetry because of the finite, yet small, thickness of the proposed structure and benefitting from properly tailored intra- and interlayer couplings, such coupled bilayer metasurface experimentally yields a conversion efficiency of 17%, significantly larger than that of earlier single-layer designs, as well as an extinction ratio larger than 0 dB, meaning that anomalous refraction dominates the transmission response. Our finding shows that metallic metasurface can counterintuitively manipulate the visible light as efficiently as dielectric metasurface (~20% in conversion efficiency in Lin et al.’s study), although the metal’s ohmic loss is much higher than dielectrics. Our hybrid bilayer design, still being ultrathin (~λ/6), is found to obey generalized Snell’s law even in the presence of strong couplings. It is capable of efficiently manipulating visible light over a broad bandwidth and can be realized with a facile one-step nanofabrication process.

High Aspect Subdiffraction-Limit Photolithography via a Silver Superlens

Photolithography is the technology of choice for mass patterning in semiconductor and data storage industries. Superlenses have demonstrated the capability of subdiffraction-limit imaging and been envisioned as a promising technology for potential nanophotolithography. Unfortunately, subdiffraction-limit patterns generated by current superlenses exhibited poor profile depth far below the requirement for photolithography. Here, we report an experimental demonstration of sub-50 nm resolution nanophotolithography via a smooth silver superlens with a high aspect profile of ~45 nm, as well as grayscale subdiffraction-limit three-dimensional nanopatterning. Theoretical analysis and simulation show that smooth interfaces play a critical role. Superlens-based lithography can be integrated with conventional UV photolithography systems to endow them with the capability of nanophotolithography, which could provide a cost-effective approach for large scale and rapid nanopatterning.

Nano-antenna in a photoconductive photomixer for highly efficient continuous wave terahertz emission

We report highly efficient continuous-wave terahertz (THz) photoconductive antenna based photomixer employing nano-gap electrodes in the active region. The tip-to-tip nano-gap electrode structure provides strong THz field enhancement and acts as a nano-antenna to radiate the THz wave generated in the active region of the photomixer. In addition, it provides good impedance matching to the THz planar antenna and exhibits a lower RC time constant, allowing more efficient radiation especially at the higher part of the THz spectrum. As a result, the output intensity of the photomixer with the new nano-gap electrode structure in the active region is two orders of magnitude higher than that of a photomixer with typical interdigitated electrodes. Significant improvement in the THz emission bandwidth was also observed. An efficient continuous wave THz source will greatly benefit compact THz system development for high resolution THz spectroscopy and imaging applications.

Plasmon-induced transparency in coupled triangle-rod arrays

We demonstrate polarization-dependent plasmon-induced transparency in coupled triangle-rod arrays. The observed phenomenon is the result of the destructive interference between the bright and dark resonators in this coupled system, which is verified through the numerical simulations using the finite-difference time-domain (FDTD) method. By precisely controlling the structural parameters of the coupled triangle-rod system, the plasmon-induced transparency can be effectively manipulated. This plasmonically coupled nanostructure could be potentially useful for designing and developing artificial plasmonic molecules and metamaterials with desired functions, which may further find promising applications in biosensing, nanoparticle trapping and optical filters.

Optical Magnetic Resonances in Subwavelength Ag–MgF2–Ag Grating Structures

Optical magnetic responses were demonstrated in subwavelength Ag–MgF2–Ag grating structures for transverse magnetic-polarized light. The subwavelength Ag–MgF2–Ag grating structures were fabricated using e-beam lithography followed by a lift-off process. By fixing the Ag–MgF2–Ag strip dimension, the effect of the stripe width on the magnetic resonances was compared for two different grating pitches. With further reduced grating pitch, we pushed the optical magnetic resonances to near UV (deep blue). Numerical simulations confirmed our experimental observations and were in good agreement with the experimental results.

Sub-wavelength nano-electrode structures to improve the performance of terahertz photomixers

By utilizing the sub-wavelength metallic structures in the active region of the photomixer, the confinement and concentration of electric field from optical pump lasers on a photoconductive substrate can be efficiently achieved as these sub-wavelength metallic structures are exhibiting the nano-antenna effect over a high index photoconductive substrate. Designing the sub-wavelength metallic structures, branch-like nano-electrodes structures, a new strategy to improve carrier capture was developed in which more carrier collection points occupy across the area of the pumping laser source. These branch-like nano-electrode structures were found to improve THz emission intensity of a photomixer by approximately one order of magnitude and optical-to-THz conversion efficiency by 10 times higher than that of photomixer with one row of nano-electrodes separated by the same 100nm gap. The enhancement is attributed to a more efficient collection of generated carriers due to a more intense electric field under the branch-like nano-electrodes structures. This is coupled with increased effective areas where strong tip-to-tip THz field enhancements were observed. The more efficient THz photomixer will greatly benefit the development of continuous wave THz imaging and spectroscopy system.

Retraction Note to: Retraction: Greatly enhanced continuous-wave terahertz emission by nano-electrodes in a photoconductive photomixer

enhancement compared with a photomixer with conventional interdigitated electrodes, which we used as a reference device. We estimated the powers of the devices based on the blackbody power of a Hg lamp, as described in the Methods section and shown in Fig. 3b of this Article. After receiving a communication from J. Mangeney, R. Colombelli, E. Peytavit, J. F. Lampin, M. Jarrahi, S. Barbieri, M. Wanke and M. A. Belkin, we measured the absolute output of our device using a loaned pyrodetector (Gentec-EO, THZ1.5B-BL-USB). We measured the absolute output power to be about 1 mW, instead of 100 mW as initially estimated and shown in Fig. 3b. Due to this error, we wish to retract the Article, even though the reported relative enhancement and the idea of using nanogap electrodes are still valid. We apologize to the readers for any adverse consequences that may have resulted from the paper’s publication. Retraction: Greatly enhanced continuous-wave terahertz emission by nano-electrodes in a photoconductive photomixer