Prof. Francisco García-Vidal (Universidad Autónoma de Madrid), Prof. Luis Martín-Moreno (Universidad de Zaragoza)
Development of a photonic infrastructure for the guiding and confinement of THz radiation over sub-wavelength length scales.
In particular, we want to achieve:
- Design, fabrication and experimental characterization of microstructured conducting surfaces for guiding and confinement of surface plasmon polaritons (SPPs) near 1 THz
- Development of efficient broadband coupling of T-rays to SPPs
- Development of efficient near-field probing techniques
- Demonstration of basic on-chip optical components such as sources, lenses and mirrors as well as cavity field enhancement for biological and chemical sensing
- Exploration of electrical/optical control of SPP propagation in doped semiconductors
Regular structuring of metallic surfaces with holes allows the establishment of electromagnetic surface waves ("spoof surface plasmon polaritons") and thus the creation of designed electromagnetic modes, enabling control over THz radiation.
Terahertz or T-rays are electromagnetic waves with frequencies in the region between those used for high-speed electronics on the lower and photonics on the higher end. They fall between the microwave and the mid-infrared regions of the spectrum. The generation, detection and routing of T-rays is of major interest for applications as diverse as materials characterization, biomedical applications, security imaging, and biochemical detection.
We want to extend concepts used at visible and near-infrared frequencies based on surface plasmon polaritons (SPPs) - electromagnetic surface waves occurring at interfaces between a conductor and a dielectric - for the confinement and guiding of electromagnetic energy to the THz regime of the spectrum. However, while conventional metals such as gold or silver can sustain SPPs at their interfaces with energy confinement far below the diffraction limit, at THz frequencies these surface waves are highly delocalized since the excitation takes place at frequencies far below the intrinsic plasma frequency of the metal. Therefore, many of the advantages of plasmonics such as deep sub-wavelength energy confinement for guiding and single-molecule sensing vanish at lower frequencies.
Our approach therefore centres on the creation of designer "meta-surfaces", where the geometry instead of the intrinsic materials properties leads to the establishament of highly confined surface waves. In fact, even perfect conductors can sustain such waves, which due to their similarity with SPPs at visible frequencies are termed "spoof surface plasmon polaritons".
Dispersion relation for spoof SPPs on the surface of a perfect conductor corrugated with a regular array of square holes. The black curves show the dispersion relation for infinitely deep holes (dotted curve) and holes of a depth of 25 microns (solid curve) calculated using an analytical model. Results from finite difference time domain simulations are also shown (dots). The insets show the distribution of the electric field in top and side view at the zone boundary.
A simple example is a flat interface of a perfect conductor perforated with a regular, two-dimensional array of square holes (insets). The figure shows the dispersion relation of these modes computed both using an analytical model (black curves) and finite-difference time-domain simulations (dots). The dispersion curves lie to the right of the air light line (grey), highlighting the fact that the waves are confined to the interface.
After modeling and fabrication using state-of-the-art microfabrication techniques, a fiber-coupled THz time-domain spectroscopy setup is used for experimental characterization of the fabricated structures.
"Terahertz pulse propagation using plasmon-polariton-like surface modes on structured conductive surfaces"
S.A. Maier & S.R. Andrews
Appl. Phys. Lett. 88 (2006) 251120