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Andreas Klein

Andreas K. Klein
School of Engineering and Computing Sciences
Durham University
South Road, DH1 3LE Durham
United Kingdom
Contact: andreas.klein@durham.ac.uk
Design and Fabrication of Terahertz Cavities

 This project addresses the need for compact and portable THz Lasers. To achieve this, a broad material range is investigated from low cost solutions to common semiconductor materials such as silicon or III/V semiconductors. The initial focus is on Distributed Bragg Reflectors and their electrical or micromechanical tunability. The tunability is desirable for both matching the cavity to the gain material and also future applications, e.g. spectroscopy. Another field of interest are surface plasmon polariton (SPP) structures for THz applications; this includes filters, slow light structures and beam steering devices. Again the tunability of such devices is investigated.

The main instruments used for measurements are THz-Time Domain Spectroscopy (0.1 THz - 3.5 THz) and a Vector Network Analyser (0.75 THz – 1.1 THz).

Photonic Crystals

While there are many examples for photonic crystal cavities with high quality factors, most of them do not allow the incorporation of other components in the photonic crystal, i.e. they consist of a small defect which provides the high quality mode but only have a small spacial extension. Our research focuses on the development of photonic crystal cavities which are large enough to incorporate devices and still provide a high quality factor. 
The resulting photonic crystal slabs are therefore not only optimised to provide strong feedback, but also to have a large physical extension which makes the integration of active devices as easy as possible. While this makes experiments more feasible, it comes with a trade-off. The modes in a photonic crystal are standing waves forming in the regions of different refractive index. Due to the finite extension of a photonic crystal additional modes can form which can decrease the it's performance. Therefore, material, size and height of a photonic crystal have to be tailored for every frequency and application. 
Modes forming in a photonic crystal slab

Illustration of a photonic crystal with indicated fundamental
(green) and parasitic modes (red)

Further applications are possible, e.g. in spectroscopy. We demonstrated the increased sensitivity in the detection of chemicals in solution with THz radiation when a microfluidic channel is placed in the cavity of a photonic crystal. The increased absorption probability in the cavity leads to an 8 fold increase in sensitivity if the surrounding photonic crystal is opimised to the application. 
Photonic Crystal Enhanced THz Spectroscopy
Illustration of a microfluidic channel (blue) placed in the defect
of a photonic crystal to enhance the sensitivity during spectroscopy

Spoof Surface Plasmon Polariton Stuctures

The concept of spoof SPPs extends the concept of plasmoncis to all frequencies and additionally gives the possibility to manipulate the properties purely over the geometry of a sample. We investigate spoof-SPP slow light-structures at THz frequencies for single frequencies or multiple frequencies, so called rainbow slow-light structures. The Bandwidth and strength of the slow-light effect can be influenced, additionally we investigate switchable and tunable plasmonic structures.
Spoof Surface Plasmon Polariton THz Structure
Illustration of a slow-light spoof surface plasmon polariton 
structure for THz frequencies