Alireza Hassani

Porous polymer fibers for low-loss Terahertz guiding.

We propose two designs of effectively single mode porous polymer fibers for low-loss guiding of terahertz radiation. First, we present a fiber of several wavelengths in diameter containing an array of sub-wavelength holes separated by sub-wavelength material veins. Second, we detail a large diameter hollow core photonic bandgap Bragg fiber made of solid film layers suspended in air by a network of circular bridges. Numerical simulations of radiation, absorption and bending losses are presented; strategies for the experimental realization of both fibers are suggested. Emphasis is put on the optimization of the fiber geometries to increase the fraction of power guided in the air inside of the fiber, thereby alleviating the effects of material absorption and interaction with the environment. Total fiber loss of less than 10 dB/m, bending radii as tight as 3 cm, and fiber bandwidth of approximately 1 THz is predicted for the porous fibers with sub-wavelength holes. Performance of this fiber type is also compared to that of the equivalent sub-wavelength rod-in-the-air fiber with a conclusion that suggested porous fibers outperform considerably the rod-in-the-air fiber designs. For the porous Bragg fibers total loss of less than 5 dB/m, bending radii as tight as 12 cm, and fiber bandwidth of approximately 0.1 THz are predicted. oupling to the surface states of a multilayer reflector facilitated by the material bridges is determined as primary mechanism responsible for the reduction of the bandwidth of a porous Bragg fiber. In all the simulations, polymer fiber material is assumed to be Teflon with bulk absorption loss of 130 dB/m.

Design of the microstructured optical fiber-based surface plasmon resonance sensors with enhanced microfluidics.

The concept of a Microstructured Optical Fiber-based Surface Plasmon Resonance sensor with optimized microfluidics is proposed. In such a sensor plasmons on the inner surface of large metallized channels containing analyte can be excited by a fundamental mode of a single mode microstructured fiber. Phase matching between plasmon and a core mode can be enforced by introducing air filled microstructure into the fiber core, thus allowing tuning of the modal refractive index and its matching with that of a plasmon. Integration of large size microfluidic channels for efficient analyte flow together with a single mode waveguide of designable effective refractive index is attractive for the development of integrated highly sensitive MOF-SPR sensors operating at any designable wavelength.

Photonic bandgap fiber-based Surface Plasmon Resonance sensors

The concept of photonic bandgap fiber-based surface plasmon resonance sensor operating with low refractive index analytes is developed. Plasmon wave on the surface of a thin metal film embedded into a fiber microstructure is excited by a leaky Gaussian-like core mode of a fiber. We demonstrate that by judicious design of the photonic crystal reflector, the effective refractive index of the core mode can be made considerably smaller than that of the core material, thus enabling efficient phase matching with a plasmon, high sensitivity, and high coupling efficiency from an external Gaussian source, at any wavelength of choice from the visible to near-IR. To our knowledge, this is not achievable by any other traditional sensor design. Moreover, unlike the case of total internal reflection waveguide-based sensors, there is no limitation on the upper value of the waveguide core refractive index, therefore, any optical materials can be used in fabrication of photonic bandgap fiber-based sensors. Based on numerical simulations, we finally present designs using various types of photonic bandgap fibers, including solid and hollow core Bragg fibers, as well as honeycomb photonic crystal fibers. Amplitude and spectrum based methodologies for the detection of changes in the analyte refractive index are discussed. Furthermore, sensitivity enhancement of a degenerate double plasmon peak excitation is demonstrated for the case of a honeycomb fiber. Sensor resolutions in the range 7 * 10(-6) -5 * 10(-5) RIU were demonstrated for an aqueous analyte.

Design criteria for microstructured-optical-fiber-based surface-plasmon-resonance sensors

Design strategies for microstructured-optical-fiber (MOF-) based surface-plasmon-resonance (SPR) sensors are presented. In such sensors, plasmons on the inner surface of the large metallized channels containing analyte can be excited by a fundamental mode of a single-mode microstructured fiber. Phase matching between a plasmon and a core mode can be enforced by introducing air-filled microstructures into the fiber core. Particularly, in its simplest implementation, the effective refractive index of a fundamental mode can be lowered to match that of a plasmon by introducing a small central hole into the fiber core. Resolution of the MOF-based sensors is demonstrated to be as low as 3×10−5 RIU, where RIU means refractive index unit. The ability to integrate large-size microfluidic channels for efficient analyte flow together with a single-mode waveguide of designable modal refractive index is attractive for the development of integrated highly sensitive MOF-SPR sensors operating at any designable wavelength.

Low loss porous terahertz fibers containing multiple subwavelength holes

We propose a porous polymer terahertz fiber with a core composed of a hexagonal array of subwavelength air holes. Numerical simulations show that the larger part of guided power propagates inside the air holes within the fiber core, resulting in suppression of the bulk absorption losses of the core material by a factor of ∼10–20. Confinement of terahertz power in the subwavelength holes greatly reduces effective refractive index of the guided mode but not as much as to considerably increase modal radiation losses due to bending. As a result, tight bends of several centimeter bending radii can be tolerated.

Photonic crystal fiber-based plasmonic sensors for the detection of biolayer thickness

The application of metallized photonic crystal fibers in surface plasmon resonance sensors of biolayer thickness is demonstrated. By the judicious design of photonic crystal fibers, the effective refractive index of the fundamental core mode can be tuned to enable efficient phase matching with a plasmon anywhere from the visible to near IR. Among other advantages of the presented sensors we find high sensitivity in the visible and near-IR spectral regions, as well as high coupling efficiency from an external Gaussian beam. Based on the numerical simulations, we present designs using various types of photonic crystal fibers, including holey fibers with and without defect, as well as honeycomb photonic crystal fibers. We find that in addition to the fundamental plasmonic excitation, higher order plasmonic modes can also be excited. In principle, using several plasmonic excitations at the same time can enhance sensor detection limit. Both amplitude and spectral-based methodologies for the detection of changes in the biolayer thickness are discussed. Sensor resolutions of the biolayer thickness as high as 0.039-0.044 nm are demonstrated in the whole 600-920 nm region. Finally, we perform analysis of the effect of imperfections in the metal layer geometry on the sensor sensitivity.

Photonic Crystal Fiber and Waveguide-Based Surface Plasmon Resonance Sensors for Application in the Visible and Near-IR

Abstract In the proposed photonic crystal waveguide-based surface plasmon resonance (SPR) sensor, a plasmon wave on the surface of a thin metal film is excited by a Gaussian-like leaky mode of an effectively single-mode photonic crystal waveguide. By judicious design of a photonic crystal waveguide, the effective refractive index of a core mode can be made considerably smaller than that of the core material, thus enabling efficient phase-matching with a plasmon, high sensitivity, and high coupling efficiency from an external Gaussian source, at any wavelength of choice from the visible to near infrared (IR), which is, to our knowledge, not achievable by any other design. Moreover, unlike the case of total internal reflection (TIR) waveguide-based sensors, a wide variety of material combinations can be used to design photonic crystal waveguide-based sensors as there is no limitation on the value of the waveguide core refractive index. This sensor design concept was implemented using planar multilayer photonic crystal waveguides, solid and hollow core Bragg fibers, as well as microstructured photonic crystal fibers. Amplitude and spectral-based methodologies for the detection of changes in the analyte refractive index were devised. Sensor resolution as low as 8.3·10−6 refractive-index unit (RIU) was found for aqueous analyte.

Surface Plasmon Resonance-like integrated sensor at terahertz frequencies for gaseous analytes.

Plasmon-like excitation at the interface between fully polymeric fiber sensor and gaseous analyte is demonstrated theoretically in terahertz regime. Such plasmonic excitation occurs on top of a ~30µm ferroelectric PVDF layer wrapped around a subwavelength porous polymer fiber. In a view of designing a fiber-based sensor of analyte refractive index, phase matching of a plasmon-like mode with the fundamental core guided mode of a low loss porous fiber is then demonstrated for the challenging case of a gaseous analyte. We then demonstrate the possibility of designing high sensitivity sensors with amplitude resolution of 3.4·10-4 RIU, and spectral resolution of 1.3·10-4 RIU in THz regime. Finally, novel sensing methodology based on detection of changes in the core mode dispersion is proposed.

Guiding in the visible with "colorful" solid-core Bragg fibers.

We report on the fabrication and characterization of solid-core all-polymer Bragg fibers consisting of a large-diameter polymethyl methylacrylate (PMMA) core surrounded by 50 alternating PMMA/Polystyrene (PS) polymer layers. By modifying the reflector layer thickness we illustrate that bandgap position can be adjusted at will in the visible. Moreover, such fibers are intensely colored in both the transmission and the outside reflection modes. Potential applications of such fibers are discussed.

Boundary integral method for the challenging problems in bandgap guiding, plasmonics and sensing

A boundary integral method [1] for calculating leaky and guided modes of microstructured optical fibers is presented. The method is rapidly converging and can handle a large number of inclusions (hundreds) of arbitrary geometries. Both, solid and hollow core photonic crystal fibers can be treated efficiently. We demonstrate that for large systems featuring closely spaced inclusions the computational intensity of the boundary integral method is significantly smaller than that of the multipole method. This is of particular importance in the case of hollow core band gap guiding fibers. We demonstrate versatility of the method by applying it to several challenging problems.