We discuss asymmetric reflectance in surface plasmon Bragg gratings incorporating optical gain, referred to as
active asymmetric surface plasmon Bragg gratings. It is shown that balanced modulation of index and gain/loss with
quarter pitch spatial shift causes unidirectional coupling between contra-propagating modes in long-range surface
plasmon polariton Bragg gratings. Such gratings operate at the breaking threshold of parity-time symmetry
(exceptional point). Two active asymmetric surface plasmon Bragg gratings designs are proposed and their
performance is examined through modal and transfer matrix method computations.
In this paper, we review our recent work on active plasmonic structures composed of optically pumped dye molecules infiltrated in a polymer host as the cladding of long-range surface plasmon polariton (LRSPP) structures. In particular, concepts for distributed Bragg and distributed feedback (DBR/DFB) lasers, and a spatially non-reciprocal Bragg grating (NRBG) are reviewed. The LRSPP Bragg grating is a fundamental element in these devices which is created by stepping the width of a metal stripe to produce modulation of refractive index. The gain medium in all of these active devices is assumed to be a thin film (~1μm) of polymer (poly (methyl methacrylate)) doped with organic laser dye molecules IR- 140. The gain medium is assumed pumped optically through the top of the devices via 10 ns laser pulses at 810 nm with 500 kW/cm2 power intensity to enable stimulated emission at 880 nm. The maximum material gain coefficient of this medium was measured independently as 68 cm-1.
Applications that involve the use of hydrogen gas (H2) have an inherent risk in that hydrogen is combustible in air and
hence accurate detection of its concentration is critical for safe operation.
Long-Range Surface Plasmon Polaritons (LRSPPs) are optical surface waves that are guided along thin metal films or
stripes which are symmetrically cladded by a dielectric and have been demonstrated to be highly sensitive for biological
and chemical sensing.
The sensor presented here consists of a gold (Au) stripe suspended on an ultrathin Cytop membrane. This architecture is
referred to as the membrane waveguide and has previously been demonstrated to support LRSPP propagation. Hydrogen
sensing is achieved by overlaying a palladium (Pd) patch on a straight waveguide section, which induces a measureable
insertion loss change under the presence of hydrogen.
The design and optimization of the sensor through finite element method (FEM) simulation will be discussed. This will
include the design of the optimal waveguide geometry along with the design of an integrated grating coupler for
broadside light coupling. In addition, details on the fabrication process are presented.
Incorporation of a solid-state gain medium in the cladding of a Long Range Surface Plasmon Polariton (LRSPP)
waveguide in order to create a single-mode near-infrared laser source is proposed. LRSPP Bragg gratings based on
stepping the width of the metal strip are used to form the laser’s cavity. Three laser configurations are presented: The
first 2 lasers employ DBRs (Distributed Bragg Reflectors) in ECL (External Cavity Laser) architecture while the third is
based on the DFB (Distributed Feedback) configuration. All 3 configurations are thermally tunable by heating the
gratings directly by injecting current. The lasers are convenient to fabricate leading to inexpensive sources that could be
used in optical integrated circuits or waveguide biosensors.
There is considerable interest in Long-Range Surface Plasmon-Polariton (LRSPP) waves, which inherently
have a low attenuation, for sensing applications. LRSPPs propagating along thin metal stripes arranged as integrated
optical structures, such as Mach-Zehnder interferometers (MZIs), are or particular interest for biosensing. In order for
sensors to be functional, high-quality metal stripe waveguides integrated with microfluidic channels are required. Wafer-based
fabrication processes have been developed and implemented to fabricate thin (35 nm) Au stripes and features
embedded into CYTOP claddings, with etched microfluidic channels exposing sections of the stripes for sensing.
CYTOP is a fluoropolymer having a refractive index close to that of water, motivating its use as a cladding material. The
fabrication processes developed are discussed and measurements (physical and optical) on operating sensors are
presented.
The design of gratings for broadside coupling of a Gaussian beam to a long-range surface plasmon polariton (LRSPP) waveguide is explored. The waveguide is a gold (Au) slab supported by a thin Cytop membrane bounded by air and forms a waveguide structure for potential use as a gas sensor. Grating coupler designs are proposed and modeled in two dimensions using the finite element method (FEM). A large design space of varying grating dimensions and input positions is examined and the resulting simulations predict an input coupling efficiency of approximately 29%. Fabrication of these gratings is also examined through standard optical lithography.
The use of hydrogen (H2) as a clean energy source is gaining significant global interest. Hydrogen gas can be
combustible in air at concentrations starting at 4%, so a low cost, compact and reliable leak detector for hydrogen gas
integratable into systems is desired. A Long Range Surface Plasmon Polariton (LRSPP) membrane waveguide structure
is discussed as a hydrogen sensor. Palladium on a silicon dioxide free-standing membrane is proposed as the waveguide
structure. Palladium absorbs hydrogen thereby inducing a detectable change in its permittivity. The design of straight
waveguide and Mach-Zehnder Interferometer (MZI) architectures are discussed. Finite element method (FEM)
simulations are conducted to choose appropriate designs to maximize sensor sensitivity.
A Long Range Surface Plasmon Polariton (LRSPP) gold waveguide supported by a thin suspended CYTOP membrane
is discussed with respect to biosensing applications. This structure allows for refractive index symmetry through
immersion in liquid or gaseous environments. The amorphous fluoropolymer CYTOP is used for the membrane material
due to its desirable optical properties. The fabrication steps for the membranes and waveguides are described along with
the membranes' mechanical properties determined experimentally through bulge testing. These structures are promising
for applications in biological and chemical sensing.
Metal-Semiconductor-Metal photodetectors (MSM-PDs) have been demonstrated with ease of fabrication, low capacitance, and faster responses compared to PIN photodetectors. Si and Ge are two of the CMOS compatible materials for sensing area of the photodetector. Ge, because of its higher mobility and absorption at 1.55μm wavelength is an attractive material of choice. In the outlined work, an interdigitated electrode MSM photodetector with a-Ge:H (amorphous-Ge:hydrogenated) as the sensing material has been recognized as a promising candidate for near infrared photodetection. Hydrogenating Ge generally helps improve material characteristics because it increases life time of photocarriers. Ge was sputter deposited with different H2 concentrations of 0%, 5%, 10%, 15%, and 25% in the plasma gas. The highest hydrogen concentration showed the highest responsivity among other detectors showing that hydrogen helps to reduce the number of defects within the a-Ge film and therefore increase the life time of carriers. Results show that highest photocurrent belongs to a sample with 25% H2 concentration in the plasma with a responsivity of 2mA/W and dark current of 11.6μA at 5v for a device area of 95×110 (μm)2.
There is currently considerable research underway to utilize Long-range Surface Plasmon-Polariton (LRSPP) waves,
which inherently have a low attenuation, to conduct biosensing using integrated optical structures such as the Mach-
Zehnder interferometer (MZI). In order for the sensor to be functional and biosensing to occur, many technological
elements are required including high-quality optical waveguide structures integrated with microfluidic channels, stable
low-noise interrogation optoelectronics, and external fluidic components. The processes involved in the fabrication of a
variety of devices along with the devices themselves and the optical set-ups used for their interrogation are described.
Measured insertion losses for uncladded, cladded and channelled devices are compared to theoretical results.
A silicon-on-insulator (SOI) rib waveguide integrated with a Bragg grating and nickel chromium heating elements has
been fabricated. The heaters enable tuning of the Bragg wavelength through a thermo-optic effect. As the temperature
is increased, the Bragg wavelength also increases due to the increasing silicon refractive index and effective mode index
of the guide. The device is designed so that an additional processing step can remove the buried oxide beneath the
grating to form a suspended waveguide. This structure would further decrease the power input required to tune the filter,
and would allow the waveguide to buckle at a critical temperature. In this work the un-released structure is fabricated
and mounted in a standard dual in-line pin (DIP) package to allow optical and electrical characterization. Test results
demonstrating thermo-optic tuning show a 1 nm shift in Bragg wavelength with a power input of 58 mW.
The effect of thermal tuning on the optical properties of an SOI based suspended waveguide is analysed. This analysis is based on the model that a fixed-fixed suspended beam, which forms the optical waveguide, will buckle when thermal expansion causes an axial stress that exceeds the critical buckling pressure of the beam. The analysis of the waveguide response will be broken up into the pre- and post-buckle stages of thermal actuation. Each stage of actuation will have a separate relationship for the shift in optical response as a function of temperature, which will include a combination of the thermo-optic, photo-elastic, and thermo-elastic effects. Given a corrugated, or "Bragg grating" version of the waveguide, it will be shown that thermally tuning the Bragg wavelength involves a change in index via a change in temperature and stress, and a change in grating pitch via a change in temperature. Particular attention will be paid to the evolving stress field over the length of the waveguide and its relationship to the stress-optic effect. It will also be shown that the pre- and post-buckle temperatures are path-dependant. Finally, examples of device implementation will be explored.
We present characterization of a surface micro-machined microbolometer featuring a number of unique features. The active resistor layer is amorphous GexSi1-xOy grown by reactively co-sputtering Ge and Si in an oxygen background. Complete control over Ge, Si, and O content using this technique allows control of both temperature coefficient of resistance and resistivity of the material, enabling optimization of material characteristics for bolometer applications. The resistor layer is combined with top and bottom NiCr metalization to form a tuned absorber for 10 μm radiation, eliminating requirements for additional absorber layers or for carefully controlled air gap thickness. Characterization of device noise and performance is presented.
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