We review our recent studies about perfect absorbers by plasmonic and all-dielectric metasurface and its applications. We demonstrate refractory plasmonic thermal radiation emitters at mid-IR wavelength. In addition, we review photothermal control of light in silicon Huygens’ metasurface perfect absorbers based on degenerate critical coupling and its applications to all-optical switching of light as well as intensity modulation by quadrupole resonance.
Through the combination of nanoscale Mie-resonance and photothermal/thermo-optical effect, the nonlinear index n2 of both plasmonic and metal nanostructure can be enhanced by more than three orders of magnitude. We discovered various types of nonlinearity that include saturation, suppression, and reverse saturation in nanostructures. Through a similar mechanism, we also achieved optical bistability in a nanoscale resonator with a record-low Q-factor (<10) and observed large nonlinearity with hysteretic behavior. These tunable optical nonlinearities with low requirements on sample size and shape open new possibilities for the design of photonic devices and metal/semiconductor super-resolution.
Through the combination of nanoscale Mie-resonance and photothermal/thermo-optical effect, plus a nanosecond excitation source that matches the thermal relaxation time of a silicon nanostructure, we demonstrated an ultra-large nonlinear index n2 = 1 um^2/mW, six-orders larger than the value in bulk. Under a confocal laser scanning scheme, unexpected sharp transition of scattering intensity is unveiled, suggesting a rapid temperature transient. The super-continuum wavelength tunability offers high-efficiency excitation among nano-silicon with various sizes. This robust and ultra-large nonlinearity shall be useful in optical switching and super-resolution mapping of semiconductor nanophotonic structures.
In this study, we propose the concept of generating transient nonlinearity via nonlinear carrier lifetime variation based on Auger recombination in silicon nanostructures. The nonlinear Auger lifetime variation creates a common crossing point for all pump-probe transient traces at different pump fluences, presenting a fluence-independent property. Furthermore, we observe that sub-linear and super-linear responses exist before and after the crossing point, revealing an unconventional temporal tunability of Auger-induced transient nonlinearity. Leveraging the combination of a laser scanning microscope and pump-probe technique, these temporally transient nonlinear behaviors are applicable to spatial resolution enhancement beyond the diffraction limit.
We study the scattering behavior of silicon nanoblocks in various displacements with respect to the optical axis of a tightly focused linearly polarized Gaussian beam. Experimentally, the laser scanning image of a single nanoblock deviates significantly from coherent image convolution. Theoretically, with exact Cartesian multipole decomposition, the results are explained through generation of high-order multipoles at large focus displacement and multipole interference. Surprisingly, due to the high-order multipoles, the efficiency of photothermal nonlinearity and Raman scattering are better with displaced focus. Our result extends Mie theory with displaced tight focus, opening up new opportunities in nanoscale light-matter interactions.
In this study, we found giant photothermal nonlinearity with 𝑛2 = 10-1𝜇𝑚2/𝑚𝑊 in ~100𝑛𝑚 silicon nanoblocks, based on Mie-resonance enhanced absorption and efficient temperature increase via thermal insulation. Through a continuouswave pump-probe setup, we demonstrated an ultrasmall high-contrast all-optical switch with 90% modulation depth. Due to the 0.001𝜇𝑚3 small geometrical size, thermal dissipation is as fast as nanosecond, leading to modulation speed at GHz, which is much faster than other thermal optic switches. The large and fast all-optical switching could open the possibility toward high-density integrated photonic nanocircuits based entirely on silicon.
KEYWORDS: Silicon, Nanostructures, Scattering, Nonlinear optics, Temperature metrology, Raman scattering, Integrated photonics, High temperature raman spectroscopy, Control systems
We recently achieved n_2 ~10-1 μm^2/mW on a single silicon Mie resonator, i.e. five orders-of-magnitude improvement of silicon nonlinearity. Here we present the direct evidence quantitatively linking the nonlinearity to temperature rise with 10K precision, and unravel that the huge optical nonlinearity is due to nonlinear temperature rise, coupled with nonlinear absorption, resulting in 1000K increase with ~10 mW/μm^2 excitation. We developed corresponding numerical simulation tools that confirm our observations and can be adapted to explain general nanostructure heating.
Aiming to develop realistic high-resolution photo images for visual anti-counterfeit media, in this study, we fabricated the world’s smallest level of 8K photo with color pixel composed of metal-insulator-metal (MIM) disk-based structure. In addition, we devised a method of preprocessing photo data in order to express realistic images with structural color pixels.
A metal-insulator-metal (MIM) structure is a fundamental plasmonic structure that has been studied widely since the
early stage of plasmonics. It enables us to confine surface plasmon polariton (SPP) and concentrate light into nano-space
beyond the diffraction limit. A finite-length MIM structure is considered to be a Fabry-Perot resonator of SPP as a
nanocavity. Here, we review our recent studies about active metasurface based on a reconfigurable metal-air-metal
(MAM) nanocavity which modify reflection or absorption spectra in scattering by changing a gap distance. Such
reconfigurable MAM nanocavity becomes promising candidate for various applications such as plasmonic color or sky
radiator from visible to infrared range.
Long-range surface plasmon polariton (LRSP) with lower attenuation and longer propagation length than conventional
surface plasmon polariton (SPP) on a single interface has been studied as new elements for integrated optical circuits.
LRSP exhibits on a metal film bounded by dielectric and can be expected to edge closer to practical use due to the long
propagation length. It is theoretically predicted that the propagation length of LRSP greatly increases as the lateral width
of a metal film decreases. Such mode propagating in a metal slab waveguide with extremely long propagation length is
named “super LRSP”. Here, we fabricate silver slab waveguide and observe the tendency of propagation properties of
super LRSP.
The spectral control of thermal radiation is demonstrated using an array of sprit-ring resonators as plasmonic metasurface. Thermally excited plasmonic resonant modes in a meta-atom are found to enhance thermal radiation at specific wavelengths. From numerical simulations, the emissivity at the resonance can be tuned to blackbody level by tuning the coupling between a sprit-ring and a metal substrate. Since the thickness of the metasurface is significantly small compared to the wavelength, these devices are promising candidates for extremely thin film thermal emitters or efficient frequency-converters for wide range of the frequency from infrared to visible.
Metal gap optical waveguides support propagation and strong confinement of coupled surface plasmon polariton
in nano-region. We study efficient transmission through a plasmonic T-branch with a mesa structure in metal
gap optical waveguides. Transmissivity through the branch with various mesa geometries is investigated by
numerical simulations. It is found that transmissivity through the branch can be improved by introducing thin
metal barrier into dielectric gap. We can achieve more than four times enhancement in the transmissivity.
KEYWORDS: 3D displays, 3D image processing, Mirrors, Image display, Semiconductor lasers, Luminescence, Ultraviolet radiation, Commercial off the shelf technology, Goggles
We have developed a new type of volumetric three-dimensional (3D) image display which has a 360-degree,
3D volume group-viewing capability without the use of any special viewing aids or goggles. In our system, a
clear thin photopolymer sheet dissolving Lanthanide(III) complexes is used. The Lanthanide(III) complexes
used in our system are Eu(TTA)3 Phen, designed for 615nm luminescence (red) against an exciting light of
395nm. Arbitrary luminous point is identified by three dimensional control of the excitation position of the
Lanthanide(III) complexes dissolved photopolymer sheet. The specific point (x,y,z) is excited by laser diode
beam. The direction of the exciting laser beam is controlled by two galvano mirrors so as to excite the specific
two-dimensional point (x,y). The depth direction (z) of the 3D object is specified by controlling two galvano
mirrors in synchronization with a photopolymer sheet rotation. The 3D images are expressed as an aggregate
of these luminous point. In the experiment, we observed static 3D objects of a triangle. We also observed
3D animation of a 3D Lissajous curve. Displayed 3D objects integrated with real background because rotating
photopolymer sheet is transparent (transparent 3D display).
Wedged negative dielectric gap waveguide has been studied theoretically.
FDTD simulations show adiabatic wavenumber conversion in coupled mode of surface plasmon polariton along the wedged structure.
In addition, field enhancement has been observed at the output of the wedged structure even in large propagation loss of metals.
We propose wedged negative dielectric gap waveguide as an efficient nano-optical coupler between optical fibers to nano-optical circuits.
Propagation modes in metal gap waveguide formed on a prism have been studied experimentally by using attenuated total reflection method.
Coupled modes of surface plasmon polariton in two metal/dielectric interfaces are studied experimentally by using attenuated total reflection (ATR) method. Metal/dielectric interfaces are fabricated as metal gap waveguides on a high refractive index prism. ATR spectra are analyzed by mode analysis for the metal gap waveguide. A tapered metal gap waveguide is proposed as an effcient coupler from optical fibers to nano-optical circuits. FDTD simulations show adiabatic conversion of wavenumber of coupled surface plasmon polariton.
Negative dielectric planar waveguides is reviewed toward silicon nanophotonics from the point of view of two-dimensional optical waves. Excitation, index-guiding, squeeze of optical beam width, adiabatic mode conversion of two-dimensional optical waves are discussed theoretically. The efficient excitation method of surface plasmon polariton is proposed as an adiabatic coupler from optical fibers. This is a gateway from conventional dielectric waveguides to nanophotonics. Low-dimensional optical waves at exotic interfaces are studied for the interface including non-metallic negative dielectrics, negative permeability materials and left-handed materials. Future perspectives toward polaritonics is discussed.
Index-guide of surface plasmon polariton in negative dielectric gap waveguides is studied theoretically. Concepts of low-dimensional optical waves and wavenumber surface are introduced for simplicity. Propagation properties of two-dimensional optical waves in a metal gap waveguide are calculated by means of an effective index method and FDTD calculations. A wave front of two-dimensional optical waves expands as cylindrical waves and optical beams confined in the gap have the two-dimensional diffraction limit. It is shown that the beam width of two-dimensional optical waves can be squeezed to nanometer order by decreasing the gap distance. A dielectric core embedded in the gap can be used as an efficient nano-optical waveguide.
Metal optical waveguides and the applications to nano-optical circuits are studied. The concept of "low-dimensional optical waves" is introduced for simplicity. Propagation modes of slab and cylindrical waveguides are analyzed theoretically.
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