A cantilever-based microring laser structure was proposed for easily integrating III-V active layer into mechanically stretchable substrates. Local strain gauges were demonstrated by embedding cantilever-based microring lasers in a deformable polymer substrate. The characterizations of microscale local strain gauges had been studied from both simulated and experimental results. The lasing wavelength of strain gauges was blue-shift and linear tuned by stretching the flexible substrate. Gauge factor being ~11.5 nm per stretching unit was obtained for a cantilever-based microring laser with structural parameters R=1.25 μm, W<sub>1</sub>=450 nm and W<sub>2</sub>=240 nm. Such microring lasers embedded in a flexible substrate are supposed to function not only as strain gauges for monitoring the micro- or nano-structured deformation, but also tunable light sources for photonic integrated circuits
Fivefold photonic crystal (PC) structure is originally presented with theoretical investigation about its photonic band gap through transmission spectrum calculated by finite-difference time-domain methods. The existence of PC band gap makes it possible for defect structures with high-quality factor. Two types of fivefold PC microcavity are proposed of which the quality factor, normalized frequency, and electromagnetic field profile have been simulated. With some preliminary structural parameters optimization, monopole mode in one-defect fivefold PC microcavity with a high-Q factor (∼2212) and whispering gallery mode in six-defect fivefold PC microcavity with a high-Q factor (∼13,300) are obtained.
We designed two kinds of hybrid plasmonic annular resonators with different cross-sectional shapes, i.e., a square and circle called “square ring” and “circle ring” resonators, respectively. Both resonators feature an ultracompact mode volume of ∼10−4 μm3 and a relatively high-quality factor of ∼102 at a submicron footprint within our studied wavelength range from 400 to 900 nm. Their performance as defined by the Q/V ratio (quality factor over mode volume) is enhanced considerably with a reduction in their physical dimensions. There exists critical annular radii, which increase from 400 to 600 nm with an increase in the azimuthal numbers from m=7 to m=10, if the two types of rings are compared with the same mode numbers and same ring thickness of 120 nm. Below the critical radii, the circle ring resonator outperforms the square ring resonator in terms of the Q/V ratio, and the difference in Q/V of the two types of rings increases rapidly with the decrease of the radii. On the other hand, they have critical annular radii of ∼250 nm, below which the square ring resonator outperforms the circle ring resonator at the wavelengths of 490 and 595 nm; however, the difference in Q/V of the two types of rings remains small within the radii range we consider. It is suggested that, in practice, with the consideration of the wavelength of green emission for these two ring structures with radii from 100 to 500 nm and ring thickness ∼120 nm, they have a negligible difference in Q/V performance.
A polarization-independent metamaterial near-perfect absorber is numerically studied in the infrared range in a two-perpendicular-nanorod design. It is shown that the absorptance and the peak wavelength associated with magnetic resonance are sensitive to the nanorod length, the thickness, and the refractive index of the spacer, while only being slightly affected by the period and the distance between neighboring nanorods. This design shows two absorption peaks with absorptance values of 89% and 83% at the wavelengths of 1.24 and 1.46 μm, respectively. Furthermore, the absorptance and the peak wavelength associated with magnetic resonance show negligible dependence on the polarization angle. These properties are advantageous for applications, including thermal sensing and selective emitters in thermophotovoltaics.
Ambient light is destructive to the reflective type projection system’s contrast ratio which has great influence on the
image quality. In contrast to the conventional front projection, short-throw projection has its advantage to reject the
ambient light. Fresnel lens-shaped reflection layer is adapted to direct light from a large angle due to the low lens throw
ratio to the viewing area. The structure separates the path of the ambient light and projection light, creating the chance to
solve the problem that ambient light is mixed with projection light. However, with solely the lens-shaped reflection layer
is not good enough to improve the contrast ratio due to the scattering layer, which contributes a necessarily wide viewing
angle, could interfere with both light paths before hitting the layer. So we propose a new design that sets the draft angle
surface with absorption layer and adds an angle-selective absorber to separate these two kinds of light. The absorber is
designed to fit the direction of the projection light, leading to a small absorption cross section for the projection light and
respectfully big absorption cross section for the ambient light. We have calculated the design with Tracepro, a ray tracing
program and find a nearly 8 times contrast ratio improvement against the current design in theory. This design can
hopefully provide efficient display in bright lit situation with better viewer satisfaction.
Optical absorption improvement and cost reduction of thin-film solar cells have been long-time issues. These two aims are achieved simultaneously by combining metallic nanoribbons and dielectric gratings at the front side of ultrathin-film amorphous silicon solar cells. Surface-plasmon-polariton waves excited by the nanoribbons at the long wavelength co-operates with Uller-Zenneck waves and cavity resonances excited by the gratings at the short wavelength with little cross-effect, leading to a complementary absorption enhancement of 31% when compared to planar structure. In addition, this design exhibits wide-angle absorption as well as a high fabrication tolerance. Compared to the previous work combining different mechanisms, this design provides fewer fabrication steps and an easier approach. Moreover, the nanoribbons can be used as a transparent conducting electrode for a low-cost alternative to expensive indium tin oxide thin-film.
Surface plasmon resonances of optical bowtie nanoantennas with symmetry breaking are studied numerically using the finite-element method. Beginning with both x-axial and y-axial symmetry, bowtie structures are reshaped by varying two parameters (edge lengths and bow angles) to create various symmetries to achieve controllable resonant modes and gap enhancement in the visible and infrared wavelength range. The four edges’ coupling is the main factor contributing to the final fundamental resonances. Double fundamental resonances can be achieved in bowtie structures with x-axial or y-axial symmetry. These properties can guide both the engineering and the fabrication of plasmonic nanoantennas.
Thin film absorber structure is becoming one of the hot topics recently for its various sub-wavelength applications such as photo detector, thermo photovoltaic cells, thin-film thermal emitters, and multi-color filters. In this work, we extended the design principles of single band absorbers to tri-band absorbers based on single nanorod unit which exhibits localized surface plasmon resonances (LSPs). By varying the geometric parameters of nanorod arrays, the absorption spectrum can be tailored. Detailed study was carried out by using finite difference time domain (FDTD) method to reveal the mechanism of high absorption. Even broader absorption could be realized based on this work.
Nanolasers have shown their potential in optical communication and information storage, due to their tiny footprint,
potential high modulation rate and light spot under diffraction limit. In recent years, many structures use metal as whole
or part of the cavity to achieve light confinement at subwavelength scale. In this paper, we propose and compare two
novel types of hybrid plasmonic lasers, both with an ultrathin insulator layer sandwiched by a ring shape semiconductor
and planar silver layer. The lasers differ in their cross-section curvature on the interface of metal and insulator. Finite
difference time domain (FDTD) method is used to calculate and optimize these two ring laser structures. The resonant
wavelength is set around 490 nm. The ultrathin thickness of the insulator layer makes photonic modes hybridize with
surface plasmon plaritons (SPPs) at Ag-insulator interface, which confines the light field strongly in the ultrathin layer.
The SPPs carry high momentum and high effective refractive index to TM mode. Whispering gallery mode is achieved
according to strong feedback at the ring boundary by total internal reflection. The ring lasers have relatively high Q
factors, approaching 100, at 250 nm radius and mode confinement around λ<sup>2</sup>/360. The mode volume can be shrunk to
0.1(λ/n)3 and 0.01(λ/n)3 respectively, which leads to Purcell factors around 70 for square cross-section and 380 for circle cross-section. We discuss the curvature effects on the mode volume and on the quality factor which accounts for the high Purcell factor for the circle cross-section.
We numerically study the absorption enhancement of amorphous Si (α-Si) solar cells, in which a dual grating structure combining front dielectric grating and back metal grating is proposed to improve light absorption in the 300-900 nm wavelength range. The front dielectric grating scatters the incident light into active layer which can reduce reflection without much energy loss, especially at the short wavelengths. The back metal grating causes the absorption enhancement at long wavelengths due to the excitation of surface plasmon polaritons (SPPs) at the interface of metal/semiconductor and/or photonic modes in the active layer. When these two gratings are combined, a large, broadband absorption enhancement over the entire spectrum can be realized. For better comparison, the flat structure without any gratings is chosen as a reference. In our work, the absorption enhancement of the solar cells with dual gratings is superior to the structures with a front dielectric or back metal grating alone in almost over the entire wavelength range 300-900 nm. For wavelengths in the range 300-900 nm, 72.4% absorptivity is observed for 100-nmthickness flat α-Si solar cell, 76.9% and 75.1% for front and back grating cases, and up to 82.6% for dual grating case at the grating period of 360 nm.
A cavity enhanced one-dimensional grating structure is proposed to improve the light absorption within the α-Si thin film solar cell. Typically, dielectric or metal structure including gratings is added for the light absorption enhancement. Not only does the structure form the guided modes, and increase the surface area/surface angle, but also the thin film itself forms a cavity allowing light trapping for better absorption. However, the structure is optimized in these two mechanisms separately. In this paper, finite element method (FEM) was used to optimize thicknesses of two cavities and then combine them into a one –dimensional grating structure. Comparing to the flat thin film solar cell, we have get absorption enhancement factors of 1.12 and 1.51 normalized for the AM 1.5 spectrum for 300 nm to 950 nm by the two proposed structures.
The authors propose a type of plasmonic ring laser which has the footprint smaller than previous published devices, showing the potential to be a single-mode ultra-compact light source. In this structure, CdS gain medium and Ag substrate are separated by an ultrathin MgF<sub>2</sub> layer. The short distance between high-index CdS material and silver makes photonic modes of CdS ring hybridize with surface plasmon plaritons (SPPs) of the Ag-MgF<sub>2</sub> interface, which leads to strong light confinement in this thin MgF<sub>2</sub> gap region. The surface plasmons of this structure carry high momentum, which leads to strong feedback at the ring boundary by total internal reflection forming whispering gallery like mode. Finite difference time domain (FDTD) method is used to calculate and optimize the plasmonic ring geometry. With a 15 nm thick MgF<sub>2</sub> layer, the ring’s outer and inner radius can be shrunk to 290 nm and 170 nm with quality factors of 70 at the resonant wavelength of 514 nm. We fix ring width and reduce MgF<sub>2</sub> thickness and ring radius to get better confinement. When MgF<sub>2</sub> thickness is 5 nm, the outer and inner radius are set as 310 nm and 190 nm respectively, Q factors can reach 93. Free spectral range (FSR) of the ring is around 45 nm, which shows a good ability to generate single mode signal during a large wavelength range. The circled and confined optical fields can significantly enhance light-matter interactions and getting high Purcell factors.
As the desire growing of the thin film absorption structure for various sub-wavelength applications such as photo
detector, thin-film thermal emitters, thermo photovoltaic cells, and multi-color filters, we proposed a type of subwavelength
multi-branch dimers which exhibit several tunable dipole-dipole-like plasmonic resonances and integrated it
into metal-insulator-metal structure as the top layer. The structures are studied through numerical calculation by finite
element method. When normal incident is considered, the novel structure shows three absorption peaks in the considered
wavelength range. One peak has near-perfect absorption and the other two also show excellent absorption.. When
different angle oblique incident is considered, the absorption only has slight change, which is useful to an ultrathin
absorber structure. In addition, we find that the thickness of the dielectric layer can tune the absorption rates for each
absorption peak. In general, the multi-branch dimers can easily tune its absorption rates and spectrum via the change of
their geometric parameters such as branch lengths, branch angles, and dielectric layer thickness.
We describe the performance of submicron microdisk and photonic crystal lasers fabricated within InGaP/InGaAlP
quantum well material. The smallest lasers, with diameters of approximately 600 nm, feature ultra-small mode volumes
and exhibit single mode operation at low threshold powers. Their small cavity volumes of approximately 0.03 μm<sup>3</sup> for
microdisk lasers and 0.01 μm<sup>3</sup> for photonic crystal lasers enable them to be used as spectroscopic sources. Here we
demonstrate the fabrication and characterization of visible, monolithically fabricated, submicron mode volume lasers.
We demonstrated a continuously tunable optofluidic distributed feedback (DFB) dye laser on a monolithic
poly(dimethylsiloxane) (PDMS) elastomer chip. The optical feedback was provided by a phase-shifted higher order
Bragg grating embedded in the liquid core of a single mode buried channel waveguide. We achieved nearly 60nm
continuously tunable output by mechanically varying the grating period with two dye molecules Rhodamine 6G (Rh6G)
and Rhodamine 101 (Rh101). Single-mode operation was obtained with <0.1nm linewidth. Because of the higher order
grating, a single laser, when operated with different dye solutions, can provide tunable output covering from near UV to
near IR spectral region. The low pump threshold (< 1uJ) makes it possible to use a single high energy pulsed laser to
pump hundreds of such lasers on a chip. An integrated array of five DFB dye lasers with different lasing wavelengths
was also demonstrated. Such laser arrays make it possible to build highly parallel optical sensors on a chip. The laser
chip is fully compatible with PDMS based soft microfluidics.
"Optofluidics" is the marriage of optics, optoelectronics and nanophotonics with fluidics. Such integration represents a new approach for dynamic manipulation of optical properties at length scales both greater than and smaller than the wavelength of light with applications ranging from reconfigurable photonic circuits to fluidically adaptable optics to high sensitivity bio-detection currently under development. The capabilities in terms of fluidic control, mixing, miniaturization and optical property tuning afforded by micro-, nano- and electro-fluidics combined with soft lithography based fabrication provides an ideal platform upon which to build such devices. In this paper we provide a general overview of some of the important issues related to the fabrication, integration and operation of optofluidic devices and present three comprehensive application examples: nanofluidically tunable photonic crystals, optofluidic microscopy and DFB dye lasers.