The magneto-optical effect has been used to control the propagation of surface plasmon polaritons in plasmonic waveguides. Here we investigate single-interface metal-dielectric and metal-dielectric-metal plasmonic waveguides in which either the dielectric or the metal is a magneto-optical material. We derive the dispersion relation of these waveguides, and investigate the effect of an externally applied static magnetic field. We find that in metal-dielectric-metal waveguide structures in which the dielectric is a magneto-optical material, the symmetry of the structure prohibits any non-reciprocal propagation in the system. Moreover, the induced change in the propagation constant of the supported modes in the presence of an externally applied static magnetic field is relatively small. In addition, we find that using a magneto-optical metal in a single-interface metal-dielectric plasmonic waveguide results in non-reciprocal propagation of the plasmonic modes along the interface. We also find that in metal-dielectric-metal plasmonic waveguides in which the metal is a magneto-optical material, the propagation constant of the supported modes is dependent on the relative direction of the applied magnetic fields to the upper and lower metal regions. If the applied magnetic fields to the two metal regions are equal and in the same direction, the induced changes in the propagation constants of the modes propagating in the positive and negative directions are the same. On the other hand, if the directions of the applied external magnetic fields are opposite, the propagation constants of the modes propagating in the positive and negative directions are different. We finally investigate Fabry-Perot cavity magneto-optical switches.
Reflection occurs at an air-material interface. The development of antireflection schemes, which aims to cancel such reflection, is important for a wide variety of applications including solar cells and photodetectors. Recently, it has been demonstrated that a periodic array of resonant subwavelength objects placed at an air-material interface can significantly reduce reflection that otherwise would have occurred at such an interface. Here, we introduce the theoretical condition for complete reflection cancellation in this resonant antireflection scheme. Using both general theoretical arguments and analytical temporal coupled-mode theory formalisms, we show that in order to achieve perfect resonant antireflection, the periodicity of the array needs to be smaller than the free-space wavelength of the incident light for normal incidence, and also the resonances in the subwavelength objects need to radiate into air and the dielectric material in a balanced fashion. Our theory is validated using first-principles full-field electromagnetic simulations of structures operating in the infrared wavelength ranges. For solar cell or photodetector applications, resonant antireflection has the potential of providing a low-cost technique for antireflection that does not require nanofabrication into the absorber materials, which may introduce detrimental effects such as additional surface recombination. Our work here provides theoretical guidance for the practical design of such resonant antireflection schemes.
We consider light trapping in photonic crystals. Using temporal coupled-mode theory and assuming that the active material is weakly absorbing, we show that the upper bound of the angle-integrated light trapping absorption enhancement is proportional to the photonic density of states. The tight bound can be reached if all the modes supported by the structure are coupled to external radiation. We discuss the roles of van Hove singularity, effective medium theory, and periodicity. By appropriate design, the angle-integrated absorption enhancement could surpass the conventional limit substantially in two dimension and marginally in three dimension.
State of art III-V multi-junction solar cells have demonstrated a record high efficiency of 43.5%. However, these cells
are only applicable to high concentration systems due to their high cost of substrates and epitaxial growth. We
demonstrate thin film flexible nanostructure arrays for III-V solar cell applications. Such nanostructure arrays allow
substrate recycling and much thinner epitaxial layer thus could significantly reduce the cost of traditional III-V solar
cells. We fabricate the GaAs thin film nanostructure arrays by conformally growing GaAs thin film on nanostructured
template followed by epitaxial lift-off. We demonstrate broadband optical absorption enhancement of a film of GaAs
nanostructure arrays over a planar thin film with equal thickness. The absorption enhancement is about 300% at long
wavelengths due to significant light trapping effect and about 30% at short wavelengths due to antireflection effect from
tapered geometry. Optical simulation shows the physical mechanisms of the absorption enhancement. Using thin film
nanostructure arrays, the III-V solar system cost could be greatly reduced, leading to low $/W and high kW/kg flexible
We observe from simulations that a doubly resonant structure can exhibit spectral behavior analogous to
electromagnetically induced transparency, as well as superscattering, depending on the excitation. We develop a
coupled-mode theory that explains this behavior in terms of the orthogonality of the radiation patterns of the
eigenmodes. These results provide insight in the general electromagnetic properties of photonic nanostructures and
We use a rigorous electromagnetic approach to develop a light-trapping theory, which reveals that the
conventional limit can be substantially surpassed in nanophotonic regimes, opening new avenues for highly efficient
Based on the effects of photonic transitions, we show that a linear, broadband and nonreciprocal on-chip optical isolation
can be accomplished by dynamic refractive index modulations. Such scheme allows for on-chip optical isolation using
standard CMOS fabrication process. We also show how to use photonic transition to create on-chip tunable resonance
with quality factor and resonant separately controllable.
We consider an aperiodic array of coupled metallic waveguides with varying subwavelength widths. For an incident
plane wave, we numerically demonstrate that a focus of as small as one hundredth of a wavelength can be achieved for a
focal distance that is much longer than the wavelength. Moreover, the focusing behavior can be controlled by changing
either the incident wavelength, or the angle of incidence, thus providing the capability of nanoscale beam steering. We
show that the behavior of such subwavelength focusing can be understood using Hamiltonian optics.
We report the first experimental demonstration of far-field lensing using a plasmonic slit array. We implement a planar
nano-slit lens using a combination of thin film deposition and focused ion beam milling. Our lens structures consist of
optically thick gold films with micron-size arrays of closely-spaced, nanoscale slits of varying widths milled using a
focused ion beam. We demonstrate experimentally that it acts as a far-field cylindrical lens for light at optical
frequencies. We show excellent agreement between the full electromagnetic field simulations of the design, which
include both evanescent and propagating modes, and the far-field, diffraction-limited confocal measurements on
manufactured structures. The flexibility offered by these slit-based planar lenses allows for the design of microlenses
that compensate for oblique illumination in integrated opto-electronic systems, such as complementary metal-oxide
semiconductor (CMOS) image sensors.
Achieving on-chip optical signal isolation is a fundamental difficulty in integrated photonics . The need to overcome
this difficulty, moreover, is becoming increasingly urgent, especially with the emergence of silicon nano-photonics [2-
4], which promise to create on-chip optical systems at an unprecedented scale of integration. In spite of many efforts,
there have been no techniques that provide complete on-chip signal isolation using materials or processes that are
fundamentally compatible with silicon CMOS process. Here we introduce an isolation mechanism based on indirect
interband photonic transition. Photonic transition, as induced by refractive index modulation , has been recently
observed experimentally in silicon nanophotonic structures . Here we show that a linear, broad-band, and nonreciprocal
isolation can be accomplished by spatial-temporal modulations that simultaneously impart frequency and
wavevector shifts during the photonic transition process. We further show that non-reciprocal effect can be
accomplished in dynamically-modulated micron-scale ring-resonator structures.
The authors show that the incorporation of gain media in only a selected device area can annul the effect of material loss,
and enhance the performance of loss-limited plasmonic devices. In addition, they demonstrate that optical gain provides a
mechanism for on/off switching in metal-dielectric-metal (MDM) plasmonic waveguides. The proposed gain-assisted plasmonic switch consists of a subwavelength MDM plasmonic waveguide side-coupled to a cavity filled with semiconductor material. In the absence of optical gain in the semiconductor material filling the cavity, an incident optical wave in the plasmonic waveguide remains essentially undisturbed by the presence of the cavity. Thus, there is almost complete transmission of the incident optical wave through the plasmonic waveguide. In contrast, in the presence of optical gain in the semiconductor material filling the cavity, the incident optical wave is completely reflected. They show that the principle of operation of such gain-assisted plasmonic devices can be explained using a temporal coupled-mode theory. They also show that the required gain coefficients are within the limits of currently available semiconductor-based optical gain media.
We propose to exploit the unique properties of surface plasmons to enhance the signal-to-noise ratio of mid-infrared
photodetectors. The proposed photodetector consists of a slit in a metallic slab filled with absorptive semiconductor
material. Light absorption in the slit is enhanced due to Fabry-Perot resonances. Further absorption enhancement is
achieved by surrounding the slit with a series of periodic grooves that enable the excitation of surface plasmons that
carry electromagnetic energy towards the slit. Using this scheme, we design and optimize a photodetector operating at
lambdao = 9.8 microns
with a roughly 250 times enhancement in the absorption per unit of volume of semiconductor material
compared to conventional photodetectors operating at the same wavelength.