Chee Leong Tan

Localized surface plasmon resonance with broadband ultralow reflectivity from metal nanoparticles on glass and silicon subwavelength structures

Metal nanoparticles (NPs) are well known to increase the efficiency of photovoltaic devices by reducing reflection and increasing light trapping within device. However, metal NPs on top flat surface suffer from high reflectivity losses due to the backscattering of the NPs itself. In this paper, we experimentally demonstrate a novel structure that exhibits localized surface plasmon resonance (LSPR) along with broadband ultralow reflectivity over a wide range of wavelength. Experimental results show that by depositing Ag NPs and Au NPs onto glass subwavelength structures (SWS) the backscattering effect of NPs can be suppressed, and the reflections can be considerably reduced by up to 87.5% and 66.7% respectively, compared to NPs fabricated on a flat glass substrate. Broadband ultralow reflection (< 2%) is also observed in the case of Ag NPs and Au NPs fabricated on cone shaped SWS silicon substrate over a wavelength range from 200 nm to 800 nm. This broadband ultralow reflectivity of Ag NPs and Au NPs on silicon SWS structure leads to a substantial enhancement of average absorption by 66.53% and 66.94%, respectively, over a broad wavelength range (200-2000 nm). This allows light absorption by NPs on SWS silicon structure close to 100% over a wavelength range from 300 nm to 1000 nm. The mechanism responsible for the increased light absorption is also explained.

High-responsivity plasmonics-based GaAs metal-semiconductor-metal photodetectors

We report the experimental characterization of high-responsivity plasmonics-based GaAsmetal-semiconductor-metalphotodetector (MSM-PD) employing metal nano-gratings. Both the geometry and light absorption near the designed wavelength are theoretically and experimentally investigated. The measured photocurrent enhancement is 4-times in comparison with a conventional single-slit MSM-PD. We observe reduction in the responsivity as the bias voltage increases and the input light polarization varies. Our experimental results demonstrate the feasibility of developing a high-responsivity, low bias-voltage high-speed MSM-PD.

Optical absorption enhancement of hybrid-plasmonic-based metal-semiconductor-metal photodetector incorporating metal nanogratings and embedded metal nanoparticles

We propose and numerically demonstrate a high absorption hybrid-plasmonic-based metal semiconductor metal photodetector (MSM-PD) comprising metal nanogratings, a subwavelength slit and amorphous silicon or germanium embedded metal nanoparticles (NPs). Simulation results show that by optimizing the metal nanograting parameters, the subwavelength slit and the embedded metal NPs, a 1.3 order of magnitude increase in electric field is attained, leading to 28-fold absorption enhancement, in comparison with conventional MSM-PD structures. This is 3.5 times better than the absorption of surface plasmon polariton (SPP) based MSM-PD structures employing metal nanogratings and a subwavelength slit. This absorption enhancement is due to the ability of the embedded metal NPs to enhance their optical absorption and scattering properties through light-stimulated resonance aided by the conduction electrons of the NPs.

Bi-SERS sensing and enhancement by Au-Ag bimetallic non-alloyed nanoparticles on amorphous and crystalline silicon substrate

We have demonstrated Au-Ag bimetallic non-alloy nanoparticles (BNNPs) on thin a-Si film and c-Si substrate for high SERS enhancement, low cost, high sensitivity and reproducible SERS substrate with bi-SERS sensing properties where two different SERS peak for Au NPs and Ag NPs are observed on single SERS substrate. The isolated Au-Ag bimetallic NPs, with uniform size and spacing distribution, are suitable for uniform high density hotspot SERS enhancement. The SERS enhancement factor of Au-Ag BNNPs is 2.9 times higher compared to Ag NPs on similar substrates due to the increase of the localized surface plasmon resonance effect. However there is a decrement of SERS peak intensity at specific wavenumbers when the surrounding refractive index increases due to out-phase hybridization of Au NPs. The distinct changes of the two different SERS peaks on single Au-Ag BNNPs SERS substrate due to Au and Ag NPs independently show possible application for bi-molecular sensing.

Metal Nano-Grating Optimization for Higher Responsivity Plasmonic-Based GaAs Metal-Semiconductor-Metal Photodetector

To improve the responsivity of the metal semiconductor metal photodetector (MSM-PD), we propose and demonstrate the use of sub-wavelength slits in conjunction with nano-structured the metal fingers that enhance the light transmission through plasmonic effects. A 4-finger plasmonics-based GaAs MSM-PD structure is optimized geometrically using a 2-D Finite Difference Domain (FDTD) method and developed, leading to more than 7-times enhancement in photocurrent in comparison with the conventional MSM-PD of similar dimensions at a bias voltage as low as 0.3 V. This enhancement is attributed to the coupling of the surface plasmon polaritons (SPPs) with the incident light through the nano-structured metal fingers. This work paves the way for the development of high-responsivity, high-sensitivity, low bias-voltage high-speed MSM-PDs and CMOS-compatible GaAs-based optoelectronic devices.

Design of high-sensitivity plasmonics-assisted GaAs metal-semiconductor-metal photodetectors

In this paper, we use the finite difference timedomain (FDTD) method to optimize the light absorption of an ultrafast plasmonic GaAs metal-semiconductor-metal photodetector (MSM-PD) employing metal nano-gratings. The MSM-PD is optimized geometrically, leading to improved light absorption near the designed wavelength of GaAs through plasmon-assisted electric and magnetic field concentration through a subwavelength aperture. Simulation results show up to 10-times light absorption enhancement at 867 nm due to surface plasmon polaritons (SPPs) propagation through the metal nano-grating, in comparison to conventional MSM-PD.

Light absorption enhancement in metal-semiconductor-metal photodetectors using plasmonic nanostructure gratings

In this paper, we adopt the finite difference time-domain (FDTD) method to optimize the absorption of a novel metal-semiconductor-metal photodetector (MSM-PD) structure based on the use of a double-layer nanostructured metal grating. The metal fingers of the MSM-PDs are etched with appropriate depths to maximize light absorption through plasmonic effects. Simulation results show 40 times enhancement in 980nm light trapping due to extraordinary optical signal propagation through the nanostructured double-metal grating, in comparison to conventional MSM-PDs.

Impact of Nanograting Phase-Shift on Light Absorption Enhancement in Plasmonics-Based Metal-Semiconductor-Metal Photodetectors

The finite difference time-domain (FDTD) method is used to simulate the light absorption enhancement in a plasmonic metal-semiconductor-metal photodetector (MSM-PD) structure employing a metal nanograting with phase shifts. The metal fingers of the MSM-PDs are etched at appropriate depths to maximize light absorption through plasmonic effects into a subwavelength aperture. We also analyse the nano-grating phase shift and groove profiles obtained typically in our experiments using focused ion beam milling and atomic force microscopy and discuss the dependency of light absorption enhancement on the nano-gratings phase shift and groove profiles inscribed into MSM-PDs. Our simulation results show that the nano-grating phase shift blue-shifts the wavelength at which the light absorption enhancement is maximum, and that the combined effects of the nano-grating groove shape and phase shift degrade the light absorption enhancement by up to 50%.

Absorption enhancement of MSM photodetector structure with a plasmonic double grating structure

We present finite difference time domain simulation to analyze the optical absorption enhancement of metal-semiconductor-metal photo detectors employing double plasmonic grating structures. Simulation results show that the combination of a subwavelength aperture and double nano-structured metal grating results in up to 25 times enhancement in optical absorption, in comparison to MSM photodetector structures employing only a subwavelength aperture. This improvement of the absorption enhancement is due to the coupling out function of the bottom grating structure which distributes the light to both side of the subwavelength aperture.

Groove shape-dependent absorption enhancement of 850 nm MSM photodetectors with nano-gratings

Finite difference time-domain (FDTD) analysis is used to investigate the light absorption enhancement factor dependence on the groove shape of the nano-gratings etched into the surfaces of metal-semiconductor-metal photodetector (MSM-PD) structures. By patterning the MSM-PDs with optimized nano-gratings a significant improvement in light absorption near the design wavelength is achieved through plasmon-assisted electric field concentration effects. Simulation results show about 50 times light absorption enhancement for 850 nm light due to improved optical signal propagation through the nano-gratings.