Characterization of Silver Nanowire Layers in the Terahertz Frequency Range.

Thin layers of silver nanowires are commonly studied for transparent electronics. However, reports of their terahertz (THz) properties are scarce. Here, we present the electrical and optical properties of thin silver nanowire layers with increasing densities at THz frequencies. We demonstrate that the absorbance, transmittance and reflectance of the metal nanowire layers in the frequency range of 0.2 THz to 1.3 THz is non-monotonic and depends on the nanowire dimensions and filling factor. We also present and validate a theoretical approach describing well the experimental results and allowing the fitting of the THz response of the nanowire layers by a Drude-Smith model of conductivity. Our results pave the way toward the application of silver nanowires as a prospective material for transparent and conductive coatings, and printable antennas operating in the terahertz range-significant for future wireless communication devices.

Single-walled carbon nanotube layers for millimeter-wave beam steering.

The ability to efficiently transmit and manipulate high-frequency signals poses major challenges resulting in a lack of active and reconfigurable millimeter-wave and terahertz devices that are needed to enable beyond-5G broadband communication systems. Here, thin single-walled carbon nanotube (SWCNT) layers are introduced as a tunable impedance surface for millimeter-waves. Carbon nanotube layers are integrated with dielectric rod waveguides. Their surface impedance, tuned by light, is shown to modify the wave propagation inside the waveguide. A direct application of the effect is a phase shifter, demonstrated experimentally and by numerical simulations. Additionally, an antenna array of two dielectric waveguides, one covered in SWCNTs, is designed and fabricated. The proof-of-concept illustrates optically-controlled beam steering enabled by carbon nanotubes, and directions for further device optimizations are provided. These findings demonstrate thin SWCNT layers as an optically-reconfigurable element, suitable for broadband millimeter-wave communications.

Wavelength-dependent photoconductivity of single-walled carbon nanotube layers.

A number of electronic devices such as phase shifters, polarizers, modulators, and power splitters are based on tunable materials. These materials often do not meet all the requirements namely low losses, fast response time, and technological compatibility. Novel nanomaterials, such as single-walled carbon nanotubes, are therefore widely studied to fill this technological gap. Here we show how the dielectric constant of single-walled carbon nanotube layers can be substantially modified by illuminating them due to unique light-matter interactions. We relate the optical excitation of the nanotube layers to the illumination wavelength and intensity, by resistance and capacitance measurements. The dielectric constant is modified under laser illumination due to the change of material polarization and free carrier generation, and is shown to not be temperature-related. The findings indicate that SWCNT layers are a prospective tunable optoelectronic material for both high and low frequency applications.

Optically controlled dielectric properties of single-walled carbon nanotubes for terahertz wave applications.

Materials with tunable dielectric properties are valuable for a wide range of electronic devices, but are often lossy at terahertz frequencies. Here we experimentally report the tuning of the dielectric properties of single-walled carbon nanotubes under light illumination. The effect is demonstrated by measurements of impedance variations at low frequency as well as complex dielectric constant variations in the wide frequency range of 0.1-1 THz by time domain spectroscopy. We show that the dielectric constant is significantly modified for varying light intensities. The effect is also practically applied to phase shifters based on dielectric rod waveguides, loaded with carbon nanotube layers. The carbon nanotubes are used as tunable impedance surface controlled by light illumination, in the frequency range of 75-500 GHz. These results suggest that the effect of dielectric constant tuning with light, accompanied by low transmission losses of the carbon nanotube layer in such an ultra-wide band, may open up new directions for the design and fabrication of novel Terahertz and optoelectronic devices.

Coherent properties of super-continuum containing clearly defined solitons.

With the use of numerical simulations based on generalized nonlinear Schrödinger equation, we study for the first time the coherence of super-continuum (SC), generated in tapered and cobweb fibers in the regime with clearly defined solitions in spectrum. We suggest a simple model, which explains the influence of pump pulses power and duration on SC coherence. A possibility of concerned SC generation regime application in optical frequencies metrology is discussed.

Modelling of high-power supercontinuum generation in highly nonlinear, dispersion shifted fibers at CW pump.

For the first time a remarkably exact match was achieved of the results from modelling of CW-pumped SC in a highly non-linear fibre with experiment (A.K. Abeeluck et al. Opt. Lett. 29, 2163-2165 (2004)) where a wide-band SC in the 1200-1780-nm range was reported. Our simulation results show that decay of CW pump radiation into a train of sub-picosecond pulses induced by the modulation instability leads to formation of optical solitons. Energy and carrier frequency of the solitons are random parameters because of quantum noise in the pump radiation. We found that a relatively smooth SC spectrum obtained by us from modelling and observed experimentally comes from averaging of a large number of soliton spectra and the spectrum of short-wavelength non-soliton radiation that is generated because of resonant pumping of energy from solitons.