Norman R. Fong

Fabrication of surface plasmon waveguides on thin CYTOP membranes

The fabrication of a membrane supported long-range surface plasmon polariton waveguide for use as a biosensor is described. This type of device can be completely immersed in the sensing environments to create symmetrical waveguide surroundings. The membrane is created from the fluoropolymer CYTOP which has strong physical properties and a low index of refraction. The fabrication steps of this device are described along with examples of completed structures.

Fabrication of long-range surface plasmon hydrogen sensors on Cytop membranes integrating grating couplers

The fabrication process for a long-range surface plasmon polariton hydrogen sensor is presented. The device, referred to as the cladded membrane waveguide, features a 5 μm wide and 20 nm thick gold stripe embedded in a 160 nm free standing Cytop membrane. Broadside excitation and output are achieved with integrated grating couplers. Hydrogen sensitivity is provided by an overlaid 5 nm thick palladium patch, which acts as a transduction medium. The device is fabricated by integrating several process techniques including blind through-wafer alignment, optical photolithography, overlaid electron beam lithography, metal lift-off, and through-substrate silicon wet etching. Fabricated results are presented along with a detailed discussion. The devices are characterized optically via a cutback measurement with the measured waveguide attenuation being consistent with simulated values.

Mechanical Properties of Thin Free-Standing CYTOP Membranes

Mechanical bulge testing of thin free-standing CYTOP membranes for use with surface plasmon polariton (SPP) biosensors was conducted. Curve fitting of experimental data to a known deflection model allowed for the extraction of a Youngs modulus of 1.3 ± 0.1 GPa and residual stress of 16.9 ± 1.3 MPa. The fracture stress for the membranes was observed to be 96.9 ± 8.4 MPa, which occurs at the membrane edges.

Characterization of grating-coupled long range surface plasmon polariton membrane waveguides

The first demonstration of grating-coupled long range surface plasmon polaritons in cladded free-standing membrane waveguides is presented. Two different waveguide structures are explored: the first is a gold (Au) stripe embedded in a thin Cytop free-standing membrane, the other being the same structure but with a thin palladium (Pd) over-layer. The waveguides are excited with integrated grating couplers designed for a working wavelength of 1550 nm. The waveguides are characterized by applying a cutback technique with the Au waveguide loss measured as 3.4 dB/mm and the Pd/Au waveguide loss as 57 dB/mm. The wavelength dependency of the weakly reflecting optical cavity is also observed with a free spectral range of ~3.6 nm and a finesse of 2.1.

Modeling of long range surface plasmon polariton cladded membrane waveguides with integrated grating couplers as hydrogen sensors

The design of a long range surface plasmon polariton cladded membrane waveguide with grating couplers is proposed. The device consists of a gold stripe embedded in a thin Cytop membrane with a palladium over-layer and can be used as a hydrogen sensor. Input and output light coupling is achieved through integrated gold grating couplers directly on the waveguides. The design is approached through finite element method modeling. Waveguide and sensor designs are compared and discussed via a 2D modal analysis. The design and optimization of input and output grating couplers are also presented.

Long range surface plasmon polariton waveguides for hydrogen sensing

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.

Grating Coupler Excitation of Membrane Supported Long Range Surface Plasmons

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.

Design of hydrogen gas sensors based on surface plasmon waveguides

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.