This review provides a perspective on the recent developments in the transmission of light through subwavelength apertures in metal films. The main focus is on the phenomenon of extraordinary optical transmission in periodic hole arrays, discovered over a decade ago. It is shown that surface electromagnetic modes play a key role in the emergence of the resonant transmission. These modes are also shown to be at the root of both the enhanced transmission and beaming of light found in single apertures surrounded by periodic corrugations. This review describes both the theoretical and experimental aspects of the subject. For clarity, the physical mechanisms operating in the different structures considered are analyzed within a common theoretical framework. Several applications based on the transmission properties of subwavelength apertures are also addressed.

http://digital.csic.es/bitstream/10261/47904/1/Light%20passing.pdf

Light passing through subwavelength apertures

Time-resolved, pulsed terahertz spectroscopy has developed into a powerful tool to study charge carrier dynamics in semiconductors and semiconductor structures over the past decades. Covering the energy range from a few to about 100 meV, terahertz radiation is sensitive to the response of charge quasiparticles, e.g., free carriers, polarons, and excitons. The distinct spectral signatures of these different quasiparticles in the THz range allow their discrimination and characterization using pulsed THz radiation. This frequency region is also well suited for the study of phonon resonances and intraband transitions in low-dimensional systems. Moreover, using a pump-probe scheme, it is possible to monitor the nonequilibrium time evolution of carriers and low-energy excitations with sub-ps time resolution. Being an all-optical technique, terahertz time-domain spectroscopy is contact-free and noninvasive and hence suited to probe the conductivity of, particularly, nanostructured materials that are difficult or impossible to access with other methods. The latest developments in the application of terahertz time-domain spectroscopy to bulk and nanostructured semiconductors are reviewed.

/pdf/carrier-dynamics-in-semiconductors-studied-with-time-1u551csnya.pdf

Carrier dynamics in semiconductors studied with time-resolved terahertz spectroscopy

We report on a new class of polymer photonic crystal fibers for low-loss guidance of THz radiation. The use of the cyclic olefin copolymer Topas, in combination with advanced fabrication technology, results in bendable THz fibers with unprecedented low loss and low material dispersion in the THz regime.We demonstrate experimentally how the dispersion may be engineered by fabricating both high- and low-dispersion fibers with zero-dispersion frequency in the regime 0.5-0.6 THz. Near-field, frequencyresolved characterization with high spatial resolution of the amplitude and phase of the modal structure proves that the fiber is single-moded over a wide frequency range, and we see the onset of higher-order modes at high frequencies as well as indication of microporous guiding at low frequencies and high porosity of the fiber. Transmission spectroscopy demonstrates low-loss propagation (< 0.1 dB/cm loss at 0.6 THz) over a wide frequency range.

/pdf/bendable-low-loss-topas-fibers-for-the-terahertz-frequency-427i253lrq.pdf

Bendable, low-loss Topas fibers for the terahertz frequency range

In the last decades, many research teams working at Terahertz frequencies focused their efforts on surpassing the diffraction limit. Numerous techniques have been investigated, combining methods existing at optic wavelength with THz system such as Time Domain Spectroscopy. The actual development led on one side to a resolution as high as λ/3000 and one the other side to a video-rate recording. The purpose of this paper is to give an overview of the history of the field, to describe the different approaches, to give examples of existing applications and to draw the perspective for this research area.

/pdf/review-of-near-field-terahertz-measurement-methods-and-their-47k96sf8gv.pdf

Review of Near-Field Terahertz Measurement Methods and Their Applications: How to Achieve Sub-Wavelength Resolution at THz Frequencies

T-ray wavelengths are long enough to pass through dry, nonpolar objects opaque at visible wavelengths, but short enough to be manipulated by optical components to form an image. Sensing in this band potentially provides advantages in a number of areas of interest to security and defense such as screening of personnel for hidden objects and the retection of chemical and biological agents. Several private companies are developing smaller, reliable cheaper systems allowing for commercialization and this motivates us to review a number of promising applications within this paper. While there are a number of challenges to be overcome there is little doubt that T-ray technology will play a significant role in the near future for advancement of security, public health, and defense.

/pdf/t-ray-sensing-and-imaging-2wc4eoviaq.pdf

T-Ray Sensing and Imaging

Using terahertz-light excitation, we have measured with sub-wavelength spatial, and sub-cycle temporal resolution the time- and frequency-dependent electric-field and surface-charge density in the vicinity of small metallic holes. In addition to a singularity like concentration of the electric field near the hole edges, we observe, that holes can act as differential operators whose near-field output is the time-derivative of the incident electric field. Our results confirm the well-known predictions made by Bouwkamp, Philips Res. Rep. 5, 321-332 (1950), and reveal, with unprecedented detail, what physically happens when light passes through a small hole.

/pdf/advanced-terahertz-electric-near-field-measurements-at-sub-2ke4x2dmwd.pdf

Advanced terahertz electric near-field measurements at sub-wavelength diameter metallic apertures

We discuss a mode expansion technique to rigorously model the diffraction from three-dimensional pits and holes in a perfectly conducting layer with finite thickness. On the basis of our simulations we predict extraordinary transmission through a single hole, caused by the Fabry-Perot effect inside the hole. Furthermore, we study the fundamental building block for extraordinary transmission through hole arrays: two and three holes. Coupled electromagnetic surface waves, the perfect conductor equivalent of a surface plasmon, are found to play a key role in the mutual interaction between two or three holes.

/pdf/extraordinary-transmission-through-1-2-and-3-holes-in-a-3u8g8lvskl.pdf

Extraordinary transmission through 1, 2 and 3 holes in a perfect conductor, modelled by a mode expansion technique

Erratum: Advanced terahertz electric near-field measurements at sub-wavelength diameter metallic apertures(Opt. Express (2008), 16, (7407-7417)).

We present THz near-field measurements and calculations of the electric field around metal objects with sizes comparable to the wavelength. Several examples are presented, such as the fields of one metal sphere, and of several spheres. We show how the near-field depends on the THz polarization and frequency and on the distance between the spheres. Our results are compared with Mie theory describing the scattering of light by particles. The enhancements of the field due to a sphere are compared with those generated by a metal tip used for near-field imaging.

Terahertz near-field measurements of field enhancement near metal objects

The longitudinal components of orthogonal-circularly polarized fields carry a phase singularity that changes sign depending on the polarization handedness. The addition of orbital angular momentum adds to or cancels this singularity and results in polarization-dependent scattering through round and square apertures, which we demonstrate analytically, numerically, and experimentally. By preparing the incident polarization and arranging the configuration of sub-wavelength apertures, we produce shadow-side scattered fields with arbitrary phase vorticity.

J. M. Brok

Papers

Advanced terahertz electric near-field measurements at sub-wavelength diameter metallic apertures

Extraordinary transmission through 1, 2 and 3 holes in a perfect conductor, modelled by a mode expansion technique

Erratum: Advanced terahertz electric near-field measurements at sub-wavelength diameter metallic apertures(Opt. Express (2008), 16, (7407-7417)).

Terahertz near-field measurements of field enhancement near metal objects

Electromagnetic Spin-Orbit Interactions via Scattering