Functionalised liquid crystals for manipulating terahertz radiation

New materials and techniques are needed to advance terahertz science and technology. Liquid crystals have been widely used for devices such as phase shifters and light modulators in the optical and terahertz regime. Liquid crystals doped with ferroelectric nanoparticles have improved of the birefringence in optical regime. In this work, the method of doping liquid crystals with ferroelectric nanoparticles in order to improve their birefringence in the terahertz regime is being investigated. The nanoparticles are synthesised using the sol gel technique and then characterised to check their ferroelectric properties. The new liquid crystals/nanoparticles suspensions were characterised with terahertz time domain spectroscopy and an algorithm was developed to determine the refractive index of the materials. In order to thoroughly understand the interactions between the liquid crystal molecules and the nanoparticles, several liquid crystals were doped with nanoparticles. After the doping of liquid crystals with nanoparticles the absorption spectrum of the new suspensions and the birefringence were measured. An increase in the birefringence of the liquid crystals was observed, most significantly in liquid crystals 2020 and E7. The other materials explored in this work are azobenzene complex dyes. The azobenzene complex dyes, PAADs, have been used for aligning of liquid crystals in the optical regime and also for manipulation of light polarisation and create complex beam shapes. Here I present a study of ultra-thin films of photo-aligning materials, in the optical regime with a future target to use them for applications in the terahertz spectral range. Finally, graphene is investigated as a zero-gap semiconductor for terahertz emission via the use of ultrafast diffusion carrier currents

Orbital angular momentum in the near-field of a fork grating

Light beams with Orbital Angular Momentum (OAM) are explored in applications from microscopy to quantum communication, while the Talbot effect revives in applications from atomic systems to x-ray phase contrast interferometry. We evidence the topological charge of an OAM carrying THz beam in the near-field of a binary amplitude fork-grating by means of the Talbot effect, which we show to persist over several fundamental Talbot lengths. We measure and analyze the evolution of the diffracted beam behind the fork grating in Fourier domain to recover the typical donut-shaped power distribution, and we compare experimental data to simulations. We isolate the inherent phase vortex using the Fourier phase retrieval method. To complement the analysis, we assess the OAM diffraction orders of a fork grating in the far-field using a cylindrical lens.ISSN:1094-408

Real-Time High Resolution THz Imaging with a Fiber-Coupled Photo Conductive Antenna and an Uncooled Microbolometer Camera

We present a real-time THz imaging method using a commercial fiber-coupled photo conductive antenna as the THz source and an uncooled microbolometer camera for detection. This new combination of state-of-the-art components is very adaptable due to its compact and uncooled radiation source, whose fiber coupling allows for a flexible placement. Using a camera with high sensitivity renders real-time imaging possible. As a proof-of-concept, the beam shape of a THz Time Domain Spectrometer was measured. We demonstrate real time imaging at nine frames per second and show its potential for practical applications in transmission geometry covering both material science and security tasks. The results suggest that hidden items, complex structures and the moisture content of (biological) materials can be resolved. We discuss the limits of the current setup, possible improvements and potential (industrial) applications, and we outline the feasibility of imaging in reflection geometry or extending it to multi-spectral imaging using band pass filters

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Travel through a stack of Fourier transforms behind a double bifurcation fork grating highlighting Talbot recurrences (from experimentaldata)

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Travel through a stack of Fourier transforms behind a single bifurcation fork grating highlighting Talbot recurrences (from experimental data)

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Travel through a stack of Fourier transforms behind a single bifurcation fork grating highlighting Talbot recurrences (from simulated data)

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Travel through a stack of Fourier transforms behind a double bifurcation fork grating highlighting Talbot recurrences (from simulated data)

Fabrication of freestanding photonic devices combining polymer films with microfabrication techniques and 3D printing

We report a technological concept for freestanding photonic elements based on metamaterials fabricated on polymer films by clean-room processes and framed using 3D printing. A spin-coated cyclic olefin copolymer (TOPAS) of variable thickness down to one micrometer was used as the substrate onto which metamaterials were fabricated using optical lithography. We demonstrate the possibility of applying a second TOPAS layer to protect the device or to allow for stacking another metamaterial layer. To obtain freestanding elements, frames were 3D printed directly on top of the metamaterial before lift-off from the carrier wafer. This ensured maintaining the flatness of the elements. Both the cleanroom process and the 3D printing enabled the design and manufacturing of elements in different sizes and shapes, e.g., to adapt to specific experimental set-ups and holder geometries or to be compatible with standard optical mounts. While TOPAS is transparent for wavelengths from UV to the far infrared, except for a few infrared absorption lines, we illustrate the concept with the simulation and manufacturing of THz band-pass filters. The performance of the fabricated filters was assessed using THz time-domain spectroscopy. The process is scalable to other wavelength ranges and has the potential for upscaling in manufacturing.ISSN:1094-408

Terahertz refractive index matching solution

ISSN:1094-408