E. P. Li

Nanyang Technological University, Singapore, Singapore

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Publications (36)53.69 Total impact

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    ABSTRACT: In this paper, we rotate an array of asymmetrical double layer of 4-strips windmill structure to investigate its effect on the chirality and sensitivity detection of biomolecular structures. The structure is made up of silver material with 300nm pitch and 10nm separation between layers. The spectrum shows two resonance modes on 600THz and 900THz with linear polarized light normally incident on the structure. We investigate the CD by rotating one of the layers with respect to the horizontal axis of the other layer by the angle θ. It is observed that the CD spectra at different angles are different. The rotation resulted in larger wavelength shift of the CD spectra. In addition, the CD also increases with the rotating angle given a larger absorption difference between the left and right handed circular polarized light.
    SPIE NanoScience + Engineering; 09/2013
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    ABSTRACT: In this paper, the effect of tapered asymmetrical gammadion on the optical rotational properties and sensitivity detection of biomolecular structures is presented. The asymmetrical structure is made up of gold material on a glass structure and immersed in water. The chirality of the un-rotated array is first determined by measuring the circular dichroism (CD) spectrum. Three modes, arising from Bloch periodic theory and surface resonance mode are observed. Then each asymmetrical gammadion structure is tapered at the arms. Tapering fraction, which define the ratio of tapered end to the untapered end is used for defining the new design. The designs are then fabricated using e-beam lithography and tested using the polarimeter for CD spectra. The spectra show that the three CD modes changes in degree amplitude and wavelength, especially for smaller tapering fractions.
    Proc SPIE 09/2013;
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    ABSTRACT: In this paper, the near field distribution patterns formed from nanostripe corral and half spiral are investigated. Various near field distribution patterns are generated owning to the interference of propagating surface plasmon waves that emerged from the nanoslits or nanostripe. The half spiral nanoslits are illuminated with Stokes polarizations. Each polarization state shows a different field pattern at different locations on the surface of metal film. This is due to the excitation of surface plasmon waves at different parts of the nanostructures when illuminated with different types of polarization states. The same Stokes polarization states are also illuminated on a nanostripe corral structure. In this case, dipolar field distributions are observed when illuminated with different linear polarization states, while optical vortices are observed for circular polarization. It is believed that these interesting field patterns due to different arrangements of nanostructures could be used for near field imaging and polarization sensing.
    Applied Physics A 09/2013; 112(3). · 1.69 Impact Factor
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    ABSTRACT: We theoretically and experimentally investigate the influence of the incident angle of illumination on the fluorescence enhancement for two kinds of gold nanohole arrays - a gold nanohole array on glass substrate (Au-hole array), and a gold coated photoresist nanohole array (Au/PR-hole array). Our simulations show that the absorption spectra for the Au/PR-hole array vary substantially with angle of incidence, whereas the corresponding spectra for the Au-hole array are marginally affected. The experimental results reveal fluorescence enhancement up to 14 and 12 times for the Au-hole array at an incident angle of 70 degrees and the Au/PR-hole array at an incident angle of 20 degrees, respectively. Moreover, a model sandwich immunoassay for the detection of prostate specific antigen (PSA) shows about 3-fold higher sensitivity on Au/PR-hole array as compared to planar gold at 70 degrees. To the best of our knowledge this is the first investigation reporting on the relationship between the incident angle and the fluorescence enhancement on gold nanohole arrays. Our findings provide a solid foundation for designing a highly efficient fluorescence detection system with oblique angular illumination. (C) 2013 Elsevier B.V. All rights reserved.
    Sensors and Actuators B Chemical 09/2013; 186:205-211. · 3.84 Impact Factor
  • E. H. Khoo, I. Ahmed, E. P. Li
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    ABSTRACT: Tapered nanobumps are placed on the circumference of optical vortex to manipulate the field amplitude using circular polarized light. Tapered nanobump produces stronger field enhancement due to higher charge density at the tapered end. The geometrical parameters of the tapered nanobumps are optimized to achieve highest field enhancement. The electric field is enhanced or diminished by illuminating with left or right circular polarized lightwave. Additional nanobumps are added to provide field enhancement at different parts of the vortex. This setup can be used to control the position of nanoparticles for analysis, and is useful for sensing and catalysis applications.
    Applied Physics Letters 04/2013; 102(13). · 3.52 Impact Factor
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    ABSTRACT: The dynamic-thermal electron-quantum medium finite-difference time-domain (DTEQM-FDTD) method is used for efficient analysis of mode profile in elliptical microcavity. The resonance peak of the elliptical microcavity is studied by varying the length ratio. It is observed that at some length ratios, cavity mode is excited instead of whispering gallery mode. This depicts that mode profiles are length ratio dependent. Through the implementation of the DTEQM-FDTD on graphic processing unit (GPU), the simulation time is reduced by 300 times as compared to the CPU. This leads to an efficient optimization approach to design microcavity lasers for wide range of applications in photonic integrated circuits.
    Optics Express 03/2013; 21(5):5910-5923. · 3.55 Impact Factor
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    ABSTRACT: The polarization of electromagnetic (EM) wave plays an essential role in the application of optoelectronics, life science microscopy and photographic display[1]. The conversion of EM wave in one polarized direction to its orthogonal direction is usually quite weak in natural materials. This paper presents a tunable metamaterial which rotates the polarized direction up to 20° in THz regime. The metamaterial can be transformed to its non-superimposable mirror image and tunes the polarization angle, which can be applied as tunable wave plate and optical switch in THz regime.
    Micro Electro Mechanical Systems (MEMS), 2013 IEEE 26th International Conference on; 01/2013
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    ABSTRACT: This paper briefly overviews the research and development (R&D) of antennas and computational electromagnetics in public research institutions in Singapore. The R&D activities of local universities and research institutes in particular, from Agency of Science, Technology and Research (A*STAR) in these areas are briefed with the selected projects from 10 MHz to 10 THz.
    Antennas and Propagation (EuCAP), 2013 7th European Conference on; 01/2013
  • Ping Bai, Lin Wu, Er Ping Li
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    ABSTRACT: We pattern a metal film into different nanostructures to investigate the influence of surface plasmonic modes and the effects on the sensitivity to the change of environment's refractive indexes. We find out that 1-D nanoslit array structures and grating structures have much higher sensitivity than 2-D nanoisland array structures and nanowindow array structures when detect the molecules with moderate dimensions such as PLK1 mRNA.
    Nanoelectronics Conference (INEC), 2013 IEEE 5th International; 01/2013
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    ABSTRACT: We experimentally demonstrate a micromachined switchable metamaterial with dual mode resonance which is induced at THz regime under oblique incidence. Here, we explore, both theoretically and experimentally, the dynamic dual mode switching by reshaping metamaterial elements using micromachined actuators. The mode switching allows robust control over the transmission and the reflection of the metamaterial at 0.76 THz and 1.16 THz. Such switchable dual mode metamaterial promises wide applications in optical switches, tunable filters, and THz detectors.
    Applied Physics Letters 10/2012; 101(15). · 3.52 Impact Factor
  • E. H. Khoo, Z. Guo, I. Ahmed, E. P. Li
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    ABSTRACT: In this paper, the near field distribution patterns excited from half spiral nanoslits and gratings are investigated. The various near field distribution patterns observed are due to the interference of propagating surface plasmon emerging from the nanoslits or gratings. The half spiral nanoslits are incident with left and right-handed circular polarization. The resulting focal spots are found at different positions for left (LHC) and right-handed circular (RHC) polarizations. This is due to the change in phase difference of propagating surface plasmon waves emerging from the nanoslit when excited by different circular polarizations. The distance between the focal spots for left and right-handed polarizations is λspp/2. In addition, the half spiral nanoslit is also illuminated with linear polarization in different rotational angles. This paper also includes the near field distributions that result from the interference of surface plasmon polariton fields with partial spiral shape. It is believed that these interesting field patterns due to different arrangements of nanoslits could be used for trapping molecules, near field imaging and sensing.
    Proc SPIE 10/2012;
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    ABSTRACT: In this paper, small plasmonic nanobumps, which consist of metal/dielectric layers are placed on the ring of optical vortex to enhance electric field ampltiude. In this paper, a plasmonic nanobump is placed on the ring of smaller optical vortex. The smaller optical vortex form from the resultant topological phase between the handedness of the incident circular polarized light and the nanoslits spiral. Different designs of plasmonic nanobump are investigated, and tapered nanobump produced higher field enhancement due to higher surface charge density at the tapering end. Higher field intensity at the tip of the plasmonic nanobump produces lower potential, which attract nanoparticle to the region. The optical force increases by the square of the electric field amplitude. This high electric field intensity at the plasmonic nanobump functions as attractive node, which trap molecules inside the optical vortex. Additional plasmonic nanobumps are added onto the other locations of the optical vortex to manipulate the particle trapping positions. This allows the precise control of molecule's position and movement for imaging, characterization and analysis, which is useful for mobile lab-on-chip devices.
    Proc SPIE 10/2012;
  • Lin Wu, Ping Bai, Er Ping Li
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    ABSTRACT: The surface plasmon (SP) resonance excited at subwavelength cylindrical hole arrays milled in metal films is systematically studied by solving the three-dimensional Maxwell’s equations using the finite element method. The absorption spectrum of the hole arrays, combined with the electric-field distribution, is employed to investigate the plasmon resonance of the patterned metal film. It is found that (i) an SP resonance correlates to a resonant peak in the absorption spectrum, but not all the peaks in the spectrum correlates to the plasmon resonances; (ii) the size variation of the hole array will shift the resonant wavelength, i.e., an increment of 100 nm in the pitch p, the hole diameter d, and the hole depth t leads to a redshift of 60–70, 30–40, or 10–20 nm in the resonant wavelength, respectively; (iii) the maximum enhancement of the electric field on the surface of the metal film corresponds to the highest absorption peak, which can be achieved by designing the p, d, and t of the hole array; and (iv) for small holes (e.g., d=125 nm) or shallow holes (e.g., t=100 nm), the absorption characteristics of the hole arrays are particularly important as some resonant peaks are missing in their transmission spectra. Our finding is of particular importance in applications such as SP resonance based sensing.
    Journal of the Optical Society of America B 04/2012; 29(4). · 2.21 Impact Factor
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    ABSTRACT: A novel CMOS-compatible infrared Si-waveguide photodetector, which utilizes photoemission from electrically floating metal silicide nanoparticles embedded in the space charge region of a Si p-n junction, is proposed. Numerical simulations show that absorption of infrared radiation in the particles can be dramatically enhanced due to resonant excitation of the localized surface plasmons on the particles, thus enabling to shrink the detector's footprint to a submicron scale. A proof-of-concept device is fabricated using standard CMOS technology, exhibiting a peak responsivity of ∼30 mA/W and a 3-dB bandwidth of ∼6 GHz.
    03/2012;
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    L. Wu, P. Bai, X. Zhou, E. P. Li
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    ABSTRACT: Reflection and transmission modes are studied in a surface plasmon resonance (SPR) sensor, where a nanohole-array-patterned gold film is deposited on a glass substrate, and the analyte in an aqueous medium is bound onto the patterned gold film. The reflection (transmission) mode means that light illuminates the structure from the analyte (substrate) side, and the detection is fixed at the analyte side. The two modes are studied by calculating absorption (and transmission) spectra and optical field distributions of the sensor. This study shows that both modes can excite the SPR on the analyte/gold interface, which is useful for sensing. However, the optical field distribution at the resonant wavelength is different for the two modes, and the effect is dependent on the gold film thickness. Comparing the sensing performance, the reflection mode gives larger resonant optical field enhancements (32 as compared with 22), but the transmission mode uses much thinner gold films (60 nm as compared with 150 nm). The optical power flow is analyzed to understand the fundamental difference between the two modes. These findings are particularly useful to guide the design of metallic nanostructure-based plasmonic sensing systems.
    IEEE Photonics Journal 01/2012; 4(1):26-33. · 2.36 Impact Factor
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    ABSTRACT: In this paper, the microfluidic switchable absorptive metamaterials (MMs) filter in the gigahertz region is designed. It consists of a microfluidic network between the electrical resonator and the metal plate. The absorption frequency of the tunable MMs absorber can be tuned by pumping different liquids into the microchannel using the microfluidic technology. Based on the designed absorptive MMs, the absorptive peak can be shifted by introducing a microfluidic layer with frequency tunability up to 20% and absorption change up to 90%. By introducing the tunability in absorption frequency of MMs, it has great potential in the applications of filter and sensor / detector for biomedical applications.
    Solid-State Sensors, Actuators and Microsystems Conference (TRANSDUCERS), 2011 16th International; 07/2011
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    ABSTRACT: We propose a quarter-wave plate based on nanoslits and analyze it using a semianalytical theory and simulations. The device comprises two nanoslits arranged perpendicular to one another where the phases of the fields transmitted by the nanoslits differ by λ/4. In this way, the polarization state of the incident light can be changed from linear to circular or vice versa. The plasmonic nanoslit wave plate is thin and has a subwavelength lateral extent. We show that the predictions for the phase shift obtained from a semianalytical model are in very good agreement with simulations by the finite difference time domain method.
    Optics Letters 07/2011; 36(13):2498-500. · 3.39 Impact Factor
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    ABSTRACT: We study the generation of surface plasmon polariton and its extraction efficiency from a microdisk-based plasmonic source to a metal-insulator-metal (MIM) waveguide. The field distribution is no longer a whispering gallery mode commonly found in microdisk lasers but hybridized with surface plasmon polariton. We observe the steady-state intensity of the surface plasmon polariton mode inside the plasmonic microdisk structure at 1.47- μm wavelength. This field intensity is extracted efficiently to a MIM plasmonic waveguide through a nanometer gap. The extraction efficiency reaches 60%.
    IEEE Photonics Journal 07/2011; · 2.36 Impact Factor
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    ABSTRACT: We present a three-dimensional finite difference time domain (FDTD) method on graphics processing unit (GPU) for plasmonics applications. For the simulation of plasmonics devices, the Lorentz-Drude (LD) dispersive model is incorporated into Maxwell equations, while the auxiliary differential equation (ADE) technique is applied to the LD model. Our numerical experiments based on typical domain sizes as well as plasmonics environment demonstrate that our implementation of the FDTD method on GPU offers significant speed up as compared to the traditional CPU implementations.
    Progress In Electromagnetics Research 01/2011; 116:441-456. · 5.30 Impact Factor
  • E. H. Khoo, I. Ahmed, E. P. Li
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    ABSTRACT: In this paper we demonstrate how an elliptically shaped semiconductor microcavity can be used to generate surface plasmons (SP) mode by pumping current and injecting optical pulse. After achieving stable lasing mode, external magnetic field is applied to a small elliptical confined area on the elliptical microcavity. The applied magnetic field produces Lorentz torque and "pushes" the electrons to the edge of the microcavity. Strong electron plasma is built up on the boundary of the microcavity and air interface as more electrons accumulate. The laser light source interacts with the electron plasma at the boundary of microcavity and excites surface plasmon mode. The direct excitation of SPP modes could be used to extract the laser light from elliptical microcavity source and results in a lower coupling loss and higher efficient small coupling system.
    Proc SPIE 08/2010;