Andrey Markov

Label-free bacteria detection using evanescent mode of a suspended core terahertz fiber

We propose for the first time an E. coli bacteria sensor based on the evanescent field of the fundamental mode of a suspended-core terahertz fiber. The sensor is capable of E. coli detection at concentrations in the range of 104-109 cfu/ml. The polyethylene fiber features a 150 μm core suspended by three deeply sub-wavelength bridges in the center of a 5.1 mm-diameter cladding tube. The fiber core is biofunctionalized with T4 bacteriophages which bind and eventually destroy (lyse) their bacterial target. Using environmental SEM we demonstrate that E. coli is first captured by the phages on the fiber surface. After 25 minutes, most of the bacteria is infected by phages and then destroyed with ~1μm-size fragments remaining bound to the fiber surface. The bacteria-binding and subsequent lysis unambiguously correlate with a strong increase of the fiber absorption. This signal allows the detection and quantification of bacteria concentration. Presented bacteria detection method is label-free and it does not rely on the presence of any bacterial “fingerprint” features in the THz spectrum.

Two-wire terahertz fibers with porous dielectric support

A practical plasmonic THz fiber is described that features two metallic wires held together by the porous dielectric cladding. High porosity is required in order to guarantee low loss and low dispersion of guided modes.

Hybrid metal wire–dielectric terahertz waveguides: challenges and opportunities [Invited]

In this review we evaluate recent experimental and theoretical progress in the development of wire-based waveguides used for practical low-loss and low-dispersion delivery of terahertz radiation. Waveguides considered in this review utilize plasmonic modes guided in the air gap between two parallel wires. The two parallel wires are, in turn, encapsulated inside of a low-loss, low-refractive-index micro- or nano-structured cladding that provides mechanical stability and isolation from the environment. We describe two alternative techniques that may be used to encapsulate the two-wire waveguides while minimizing the negative impact of dielectric cladding on the optical properties of the waveguide. The first technique uses low-density foam as a cladding material, while the other uses air-filled microstructured plastic claddings to support metallic wires. Additionally, we offer a detailed analysis of the modal properties of wire-based waveguides, compare them with the properties of a classic two-wire waveguide, and present several strategies for the improvement of hybrid waveguide performance. Using the resonant dependence of the confinement properties of some hybrid plasmonic modes also allows us to propose their use in terahertz refractometry. Finally, we demonstrate that wire-based porous waveguides can have a very large operational bandwidth while supporting tightly confined, air-bound modes at both high and low frequencies. This is possible as, at higher frequencies, hybrid fibers can support ARROW-like low-loss air-bound modes while changing their guidance mechanism to plasmonic confinement in the inter-wire air gap at lower frequencies.

Graded index porous optical fibers – dispersion management in terahertz range

Graded index porous fiber incorporating an air-hole array featuring variable air-hole diameters and inter-hole separations is proposed. We experimentally demonstrate smaller pulse distortion, larger bandwidth and higher excitation efficiency compared to fibers with uniform porosity.

Resonant THz sensor for paper quality monitoring using THz fiber Bragg gratings.

THz Bragg gratings were fabricated by using CO2 laser inscription. The simulated and experimental results demonstrate potential of such gratings in paper thickness monitoring, with experimental spectral sensitivities of ~ -0.67 GHz / 10 μm.

Hybrid plasmonic terahertz fibers for sensing applications

A hybrid plasmonic THz fiber featuring two metallic wires in a porous dielectric cladding is studied for resonant sensing applications. In our design, introduction of even lossless analytes into the fiber core leads to significant changes in the modal losses, which is used as a transduction mechanism.

Planar Porous THz Waveguides for Low-Loss Guidance and Sensing Applications

Planar porous dielectric waveguides featuring periodic sequence of deeply subwavelength air/dielectric bi-layers are proposed, fabricated and characterized in view of their potential applications as low-loss waveguides and sensors in the THz spectral range. The waveguide design maximizes the fraction of power guided in the air to reduce waveguide loss due to material absorption, as well as to provide a conveniently accessible microfluidic channels for sensor measurements.

Time Resolved Dynamic Measurements at THz Frequencies Using a Rotary Optical Delay Line

Fabrication, characterization, and applications of a fast rotary linear optical delay line (FRLODL) for THz time-domain spectroscopy are presented. The FRLODL features two reflective surfaces with spatially separated incoming and outgoing beams. It has been manufactured using CNC machining. A linear dependence of the optical delay on the rotation angle allows a straightforward extraction of the conversion factor between the acquisition time (in ms) and the terahertz pulse time (in ps). We also discuss the accuracy of the rotary delay line detailing the possible sources of imprecision. The FRLODL has been tested using rotation speeds of up to 48 Hz, corresponding to an acquisition rate of up to 192 Hz with four blades incorporated on the same disk. At high speeds we observe a decrease of the bandwidth due to the limitations of the electronics, in particular, the transimpedance amplifier. An error analysis is performed by experimentally evaluating the signal-to-noise ratio and the dynamic range. With regard to the applications of the FRLODL, we first present observation of the evaporation of liquids, namely water, acetone and methanol. We then demonstrate monitoring of the spray painting process. Finally, detection of fast moving objects at 1 m/s and their thickness characterization are presented.

Photonic bandgap plasmonic waveguides

A novel type of plasmonic waveguides is proposed featuring an “open” design with convenient access to a plasmonic mode. Its optical properties are investigated experimentally and numerically confirming PBG guidance in a broad spectral range.

Low-loss terahertz waveguide Bragg grating using a two-wire waveguide and a paper grating

We propose a low-loss terahertz waveguide Bragg grating (TWBG) fabricated using a plasmonic two-wire waveguide and a micromachined paper grating for potential applications in terahertz (THz) communications. Two TWBGs were fabricated with different periods and lengths. Transmission spectra of these TWBGs show 16 dB loss and 14 dB loss in the middle of their respective stop bands at 0.637 and 0.369 THz, with Q factors of 142 and 105, respectively. Insertion loss of 1-4 dB in the whole 0.1-0.7 THz region was also measured. Finally, TWBG modal dispersion relations, modal loss, and field distributions were studied numerically, and low-loss, high-coupling-efficiency operation of TWBGs was confirmed.