Microwave photonic systems have huge potential for both existing and future applications, including radar, radiofrequency sensing and modern wireless communications due to their distinct advantages in terms of ultra-wide bandwidth, flexible tunability, and immunity to electromagnetic interference. There is a strong research trend in microwave photonic systems towards integration and miniaturization, resulting in multiple radio frequency functions on a single chip which is both compact and light weight. Thus integrated microwave photonics has attracted a lot of attentions and achieves significant improvements in last ten years. In this paper, we will review some research progresses on silicon-based integrated microwave photonics in our group, including highly efficient micro heater on silicon photonic chip, chip-scale microwave waveform generation, on-chip true time delay, and microwave photonic processing and measurement. Our schemes are all fabricated on silicon-on-insulator chips and have advantages of compactness and capability to integrate with electronic units. These chips may motivate the great application potentials in silicon-based integrated microwave photonics.
Interference of surface plasmon (SP) waves plays a key role in light transmission through a subwavelength aperture surrounded by groove structures. In order to characterize interference of the hole and groove-generated SP waves, their phase information was carefully investigated using finite difference time domain simulations. In a structure with only one groove, constructive interference of the generated SP waves will enhance transmitted light by a factor of 5.4 compared with that of a single hole. Increasing the groove number to 3 in the design, which supports constructive interference of SP waves, will enhance the transmission coefficient to 10.5 times that for the single-hole transmission coefficient.
We theoretically investigate the propagation of graphene plasmon polaritons in graphene nanoribbon waveguides
and experimentally observe the excitation of the graphene plasmon polaritons in a continuous graphene
monolayer. We show that graphene nanoribbon bends do not induce any additional loss and nanofocusing occurs
in a tapered graphene nanoriboon, and we experimentally demonstrate the excitation of graphene plasmon
polaritonss in a continuous graphene monolayer assisted by a two-dimensional subwavelength silicon grating.
Nowadays, bringing photovoltaics to the market is mainly limited by high cost of electricity produced by the
photovoltaic solar cell. Thin-film photovoltaics offers the potential for a significant cost reduction compared to
traditional photovoltaics. However, the performance of thin-film solar cells is generally limited by poor light
absorption. We propose an ultrathin-film silicon solar cell configuration based on SOI structure, where the light
absorption is enhanced by use of plasmonic nanostructures. By placing a one-dimensional plasmonic nanograting
on the bottom of the solar cell, the generated photocurrent for a 200 nm-thickness crystalline silicon solar cell
can be enhanced by 90% in the considered wavelength range. These results are paving a promising way for the
realization of high-efficiency thin-film solar cells.
Active resonance tuning is highly desired for the applications of plasmonic structures, such as optical switches
and surface enhanced Raman substrates. In this paper, we demonstrate the active tunable plasmonic structures,
which composed of monolayer arrays of metallic semishells with dielectric cores on stretchable elastic
substrates. These composite structures support Bragg-type surface plasmon resonances whose frequencies are
sensitive to the arrangement of the metallic semishells. Under uniaxial stretching, the lattice symmetry of these
plasmonic structures can be reconfigured from hexagonal to monoclinic lattice, leading to not only large but also
polarization-dependent shifts of the resonance frequency. The experimental results are supported by the numerical
simulations. Our structures fabricated using simple and inexpensive self-assembly and lift-transfer techniques
can open up applications of the stretch-tunable plasmonic structures in sensing, switching, and filtering.
Thin-film photovoltaics offers the potential for a significant cost reduction compared to traditional photovoltaics. However, the performance of thin-film solar cells is limited by poor light absorption. We have devised an ultra-thin-film silicon solar cell configuration assisted by plasmonic nanostructures. By placing a one-dimensional plasmonic nanograting on the bottom of the solar cell, the generated photocurrent for a 200 nm-thickness crystalline silicon solar cell can be enhanced by 90% in the considered wavelength range, while keeping insensitive to the incident angle. These results are paving a promising way for the realization of high-efficiency thin-film solar cells.
We discuss optical properties of ultrathin metal films, with particular attention to the phenomenon of quenched
transmission. Transmission of light through an optically ultrathin metal film with a thickness comparable to its
skin depth is significant. We demonstrate the quenched transmission through the ultrathin metal films when they
are periodically modulated. We also discuss the physics behind it and explain how this abnormal phenomenon
is ascribed to surface plasmon resonance effects.
We point out an apparently overlooked consequence of the boundary conditions obeyed by the electric displacement vector at air-metal interfaces: the continuity of the normal component combined with the quantum mechanical penetration of the electron gas in the air implies the existence of a surface on which the dielectric function vanishes. This, in turn, leads to an enhancement of the normal component of the total electric field. We study this effect for a planar metal surface, with the inhomogeneous electron density accounted for by a Jellium model. We also illustrate the effect for equilateral triangular nanoislands via numerical solutions of the appropriate Maxwell equations, and show that the field enhancement is several orders of magnitude larger than what the conventional theory predicts.
We study the importance of taking the nonlocal optical response of metals into account for accurate determination
of optical properties of nanoplasmonic structures. Here we focus on the computational physics aspects of this
problem, and in particular we report on the nonlocal-response package that we wrote for state-of the art numerical
software, enabling us to take into account the nonlocal material response of metals for any arbitrarily shaped
nanoplasmonic structures, without much numerical overhead as compared to the standard local response. Our
method is a frequency-domain method, and hence it is sensitive to possible narrow resonances that may arise
due to strong electronic quantum confinement in the metal. This feature allows us to accurately determine which
geometries are strongly affected by nonlocal response, for example regarding applications based on electric field
enhancement properties for which metal nanostructures are widely used.
By structural engineering of sub-wavelength apertures, we numerically demonstrate that transmission through
apertures can be significantly enhanced. Based on equivalent circuit theory analysis, structured apertures are
obtained with a 1900-fold transmission enhancement factor. We show that the enhancement is due to the
excitation of the strong localized resonant modes of the structured apertures.
Optically pumped polymer photonic crystal band-edge dye lasers are presented. The photonic crystal is a rectangular
lattice providing laser feedback as well as an optical resonance for the pump light. The lasers are defined in a thin film of
photodefinable Ormocore hybrid polymer, doped with the laser dye Pyrromethene 597. A compact frequency doubled
Nd:YAG laser (352 nm, 5 ns pulses) is used to pump the lasers from above the chip. The laser devices are 450 nm thick
slab waveguides with a rectangular lattice of 100 nm deep air holes imprinted into the surface. The 2-dimensional
rectangular lattice is described by two orthogonal unit vectors of length a and b, defining the ΓP and ΓX directions. The
frequency of the laser can be tuned via the lattice constant a (187 nm - 215 nm) while pump light is resonantly coupled
into the laser from an angle (θ) depending on the lattice constant b (355 nm). The lasers are fabricated in parallel on a 10
cm diameter wafer by combined nanoimprint and photolithography (CNP). CNP relies on a UV transparent quartz
nanoimprint stamp with an integrated metal shadow mask. In the CNP process the photonic crystal is formed by
mechanical deformation (imprinting) while the larger features are defined by UV exposure through the combined
In this paper, we investigate the capacitance tuning of nanoscale split-ring resonators. Based on a simple LC
circuit model (LC-model), we derive an expression where the inductance is proportional to the area while the
capacitance reflects the aspect ratio of the slit. The resonance frequency may be tuned by the slit aspect
ratio leaving the area, the lattice constant Λ, and nearest-neighbor couplings in periodic split-ring resonator
structures invariant. Experimental data as well as numerical simulation data, verify the predictions of the simple
Optofluidic devices exploit the characteristics of liquids to achieve a dynamic adaptation of their optical properties. The
use of liquids allows for functionalities of optical elements to be created, reconfigured or tuned. We present an overview
of our work on fluid-control of optical elements and highlight the benefits of an optofluidic approach, focusing on
optofluidic cavities created in silicon photonic crystal (PhC) waveguide platforms. These cavities can be spatially and
spectrally reconfigured, thus allowing a dynamic control of their optical characteristics. PhC cavities are major building
blocks in many applications, from microlasers and biomedical sensor systems to optical switches and integrated circuits.
In this paper, we show that PhC microcavities can be formed by infusing a liquid into a selected section of a uniform
PhC waveguide and that the optical properties of these cavities can be tuned and adapted. By taking advantage of the
negative thermo-optic coefficient of liquids, we describe a method which renders PhC cavities insensitive to temperature
changes in the environment. This is only one example where the fluid-control of optical elements results in a
functionality that would be very hard to realize with other methods and techniques.
We report on experimental realization of the Fang Ag superlens structure  suitable for further processing and
integration in bio-chips by replacing PMMA with a highly chemical resistant cyclo-olefin copolymer, mr-I T85 (Micro
Resist Technology, Berlin, Germany). The superlens was able to resolve 80 nm half-pitch gratings when operating at a
free space wavelength of 365 nm.
Fang et al. used PMMA since it enables the presence of surface plasmons at the PMMA/Ag interface at 365 nm and
because it planarizes the quartz/chrome mask. If the superlens is to be integrated into a device where further processing
is needed involving various organic polar solvents, PMMA cannot be used. We propose to use mr-I T85, which is highly
chemically resistant to acids and polar solvents.
Our superlens stack consists of a quartz/chrome grating mask, a 40 nm layer of mr-I T85, 35 nm Ag, and finally 70 nm
of the negative photoresist mr-UVL 6000 (Micro Resist). A 50 nm layer of aluminium on top of the quartz/chrome mask
reflected all light that did not penetrate through the mask openings thereby reducing waveguiding in the top resist layer.
The exposures took place in a UV-aligner at 365 nm corresponding to the excitation wavelength of the surface plasmons
at the mr-I T85/Ag interface. Supporting COMSOL simulations illustrate the field intensity distribution inside the resist
as well as the presence of surface plasmons at the mr-I T85/Ag boundary. AFM scans of the exposed structure revealed
80 nm gratings.
The properties of crossing for two perpendicular subwavelength plasmonic slot waveguides are theoretically
investigated. Results show when encountering a nano intersection the crosstalk for the direct crossing is around
25%, almost the same as throughput. In terms of symmetry considerations and resonant-tunnelling effect, we
design two types of compact cavity-based structures. Our results show that the crosstalk is eliminated and the
throughput reaches the unity on resonance. Simulation results are in agreement with those from coupled-mode
theory. Taking material loss into account, the symmetry properties of the modes are preserved and the crosstalk
remains suppressed. Our results may open a way to construct nanoscale crossings for high-density nanoplasmonic
The properties of guided plasmon polaritons supported by a triangular metallic waveguide are presented. The waveguide examined is a metal core with equilateral triangular cross section embedded in an infinite lossless dielectric media. Based on the rotation symmetry of the waveguide, the sketch of the supported fundament modes is given. The fundamental modes can be constructed by a proper combination of the corner modes and surface modes, which can be supported by isolated metal corners and metallic-dielectric interface respectively. The mode properties of the metallic waveguide, e.g., the dispersion and propagation length with the size of the metal core, mode field orientation and field distribution profiles are addressed by using a finite element method. The numerical singularities of the optical field are removed by smoothing the corners with an appropriate arc at the nano meter scale. The guided modes supported by the structure are determined and characterized for both subwavelength and suprawavelength. We find that the corner modes exist in both regimes, while the surface modes only appear in the suprawavelenth. Our results also show that the mode properties preserve a certain kind of symmetry of the waveguides. The degenerate modes exist both for the corner guided modes and for surface guided modes. The first fundamental corner modes is a polarization-independent mode without the cut-off size of the waveguides. Calculations also show how sensitively the mode changes with the corner sharpness. The propagation constant of the corner modes is sensitive to the corner sharpness, while the side modes are unaffected.
In this paper we theoretically discuss how a strongly dispersive photonic crystals environment may be used to
enhance the light-matter interactions, thus potentially compensating for the reduced optical path in typical lab-on-
a-chip systems. Combining electromagnetic perturbation theory with full-wave electromagnetic simulations
we address the prospects for slow-light enhancement of Beer-Lambert absorption and photonic band-gap based
Channel drop filters with ring/disk resonators in a plasmon-polaritons metal are studied. It shows that light can be efficiently dropped. Results obtained by the finite difference time domain method are consistent with those from the coupled mode theory. It also shows, without considering the loss of the metal, that the quality factor for the channel drop system can be very high. The quality factor decreases significantly if we take into account the loss, which also leads to a weak drop efficiency.
Optical microcavities based on zero-group-velocity surface modes
in photonic crystals are studied. It is shown that high quality
factors can be easily obtained for such microcavities in photonic
crystals. With increasing of the cavity length, the quality factor
is gradually enhanced and the resonant frequency converges to that
of the zero-group-velocity surface mode in the photonic crystals.
Different with other microcavities mentioned in the literature,
microcavities proposed in this paper can be considered as open
cavities in the sense that one of the in-plane boundaries is
exposed to air.
The coupling efficiency between external plane waves and the Bloch waves in photonic crystals are investigated. It is found that the coupling coefficient is highly angular dependent even for an interface between air n=1 and a photonic crystal with effective index -1. It is also shown that, for point imaging by a photonic crystal slab owing to the negative refraction, the influence of the surface termination to the transmission and the imaging quality is significant. Finally, we present results demonstrating experimentally negative refraction in a two-dimensional photonic crystal.