Hamid Pahlevaninezhad

Nanoplasmonic Terahertz Photoconductive Switch on GaAs

Low-temperature (LT) grown GaAs has a subpicosecond carrier response time that makes it favorable for terahertz photoconductive (PC) switching. However, this is obtained at the price of lower mobility and lower thermal conductivity than GaAs. Here we demonstrate subpicosecond carrier sweep-out and over an order of magnitude higher sensitivity in detection from a GaAs-based PC switch by using a nanoplasmonic structure. As compared to a conventional GaAs PC switch, we observe 40 times the peak-to-peak response from the nanoplasmonic structure on GaAs. The response is double that of a commercial, antireflection coated LT-GaAs PC switch.

Two-wire waveguide for terahertz

We present a rigorous theoretical analysis of the two-wire waveguide. Obtaining the attenuation constant in terms of the dimensions of the waveguide analytically, we show that the absorption coefficient can be less than 0.01 cm(-1), with the appropriate values of the dimensions.

A high-efficiency fiber-based imaging system for co-registered autofluorescence and optical coherence tomography

We present a power-efficient fiber-based imaging system capable of co-registered autofluorescence imaging and optical coherence tomography (AF/OCT). The system employs a custom fiber optic rotary joint (FORJ) with an embedded dichroic mirror to efficiently combine the OCT and AF pathways. This three-port wavelength multiplexing FORJ setup has a throughput of more than 83% for collected AF emission, significantly more efficient compared to previously reported fiber-based methods. A custom 900 µm diameter catheter ‒ consisting of a rotating lens assembly, double-clad fiber (DCF), and torque cable in a stationary plastic tube ‒ was fabricated to allow AF/OCT imaging of small airways in vivo. We demonstrate the performance of this system ex vivo in resected porcine airway specimens and in vivo in human on fingers, in the oral cavity, and in peripheral airways.

Coupling of terahertz waves to a two-wire waveguide.

We calculate theoretically the coupling of a terahertz wave from a dipole into a two-wire waveguide. The field transmission and reflection are obtained using a Single Mode Matching (SMM) technique at the input port of the two-wire waveguide. The results show more than 70 percent coupling efficiency for the waveguide using 500 μm radii wires with 2mm center-to-center separation and the exciting field cross section of 1mm × 1mm. The results also show good agreement with the full-wave numerical simulations using the Finite Element Method (FEM).

Endoscopic Doppler optical coherence tomography and autofluorescence imaging of peripheral pulmonary nodules and vasculature

We present the first endoscopic Doppler optical coherence tomography and co-registered autofluorescence imaging (DOCT-AFI) of peripheral pulmonary nodules and vascular networks in vivo using a small 0.9 mm diameter catheter. Using exemplary images from volumetric data sets collected from 31 patients during flexible bronchoscopy, we demonstrate how DOCT and AFI offer complementary information that may increase the ability to locate and characterize pulmonary nodules. AFI offers a sensitive visual presentation for the rapid identification of suspicious airway sites, while co-registered OCT provides detailed structural information to assess the airway morphology. We demonstrate the ability of AFI to visualize vascular networks in vivo and validate this finding using Doppler and structural OCT. Given the advantages of higher resolution, smaller probe size, and ability to visualize vasculature, DOCT-AFI has the potential to increase diagnostic accuracy and minimize bleeding to guide biopsy of pulmonary nodules compared to radial endobronchial ultrasound, the current standard of care.

Single-walled carbon nanotubes as base material for THz photoconductive switching: a theoretical study from input power to output THz emission.

This paper studies the relation between photoexcitation of a single-walled carbon nanotube (SWNT) based device, and its THz output power in the context of THz photoconductive (PC) switching and THz photomixing. A detailed approach of calculating output THz power for such a device describes the effect of each parameter on the performance of the THz PC switch and highlights the design dependent achievable limits. A numerical assessment, with typical values for each parameter, shows that-subject to thermal stability of the device-SWNT based PC switch can improve the output power by almost two orders of magnitudes compared to conventional materials such as LT-GaAs.

Fiber-optic polarization diversity detection for rotary probe optical coherence tomography

We report a polarization diversity detection scheme for optical coherence tomography with a new, custom, miniaturized fiber coupler with single mode (SM) fiber inputs and polarization maintaining (PM) fiber outputs. The SM fiber inputs obviate matching the optical lengths of the X and Y OCT polarization channels prior to interference and the PM fiber outputs ensure defined X and Y axes after interference. Advantages for this scheme include easier alignment, lower cost, and easier miniaturization compared to designs with free-space bulk optical components. We demonstrate the utility of the detection system to mitigate the effects of rapidly changing polarization states when imaging with rotating fiber optic probes in Intralipid suspension and during in vivo imaging of human airways.

Coregistered autofluorescence-optical coherence tomography imaging of human lung sections.

Abstract. Autofluorescence (AF) imaging can provide valuable information about the structural and metabolic state of tissue that can be useful for elucidating physiological and pathological processes. Optical coherence tomography (OCT) provides high resolution detailed information about tissue morphology. We present coregistered AF-OCT imaging of human lung sections. Adjacent hematoxylin and eosin stained histological sections are used to identify tissue structures observed in the OCT images. Segmentation of these structures in the OCT images allowed determination of relative AF intensities of human lung components. Since the AF imaging was performed on tissue sections perpendicular to the airway axis, the results show the AF signal originating from the airway wall components free from the effects of scattering and absorption by overlying layers as is the case during endoscopic imaging. Cartilage and dense connective tissue (DCT) are found to be the dominant fluorescing components with the average cartilage AF intensity about four times greater than that of DCT. The epithelium, lamina propria, and loose connective tissue near basement membrane generate an order of magnitude smaller AF signal than the cartilage fluorescence.

Efficient terahertz slot-line waveguides.

We present two solutions to the challenge of radiation loss of slot-lines at terahertz frequencies: using a slot-line in a homogeneous medium, and using a slot-line on a layered substrate. A theoretical analysis of the slot-line in a homogeneous medium as a terahertz transmission line is presented. The absorption coefficient is obtained in terms of the waveguide dimensions using the field distribution of the slot-line. Results show that the slot-line in a homogeneous medium and the slot-line on a layered substrate can be effective transmission lines for terahertz waves with 2 cm(-1) and 3 cm(-1) absorption due to conductor loss. Full-wave numerical simulations using the Finite Element Method (FEM) are applied to validate the theory.

Advances in Terahertz Waveguides and Sources

In this paper, we review recent progress toward efficient and versatile devices for terahertz applications. Low-loss transmission lines and waveguides are discussed, as well as potentially high-power photomixing devices based on carbon nanotubes.