Tiny Transmission Lines Nanofocus IR Light
USURBIL, Spain, April 6, 2011 — Experiments conducted at nanoGUNE, a new nanoscience and nanotechnology center, show that infrared light can be transported and nanofocused with miniature transmission lines consisting of two closely spaced metal nanowires. This innovation could have implications in spectroscopy and sensing applications.
In conventional optical instruments, light cannot be focused to spot sizes smaller than half its wavelength because of diffraction effects. An important approach to overcoming the diffraction limit is based on optical antennas, which can concentrate light in nanoscale-size spots, which are orders of magnitude smaller than is achievable with conventional lenses. Tiny objects such as molecules or semiconductor nanoparticles that are placed into these so-called "hot spots" of the antenna can efficiently interact with light. Thus, optical antennas boost single-molecule spectroscopy or the sensitivity of optical detectors. However, the hot spot is bound to the antenna structure, which limits flexibility in designing nanooptical circuits.
This image shows the concept and design of the device. (Images: Martin Schnell, CIC nanoGUNE)
The researchers at nanoGUNE adapted the concept of classic transmission lines to the infrared frequency range. Transmission lines are specialized cables for carrying, for example, radio-frequency signals. A simple form consists of two metal wires running closely in parallel, also called ladder line. This structure was widely used in former times for connecting the radio receiver or television set to the rooftop antenna. Applied at MHz frequencies, where typical wavelengths are in the range of centimeters to several meters, it is a prime example for transport of energy in waveguides of strongly subwavelength-scale diameter.
This is a near-field microscopy image of the tapered transmission line structure, taken at 9.3-µm wavelength (30 THz). It shows the infrared field intensity along the transmission line, revealing the tiny infrared hot spot at the taper apex.
In their experiments, the researchers demonstrated that infrared light can be transported in the same way, by scaling down the size of the transmission lines to below 1 µm. To that end, they fabricated two metal nanowires connected to an infrared antenna. The antenna captures infrared light and converts it into a propagating surface wave traveling along the transmission line. By gradually reducing the width of the transmission line, or “tapering,” the researchers demonstrated that the infrared surface wave is compressed to a tiny spot at the taper apex with a diameter of only 60 nm. This tiny spot is 150 times smaller than the free-space wavelength, emphasizing the extreme subwavelength-scale focus achieved in the experiments. The researchers applied their recently introduced near-field microscopy technique to mapping the various electrical field components of the infrared focus with nanoscale resolution.
Nanofocusing of infrared light with transmission lines has important implications in spectroscopy and sensing applications. By connecting a transmission line to the antenna, the infrared light captured by the nanoantenna can be transported over significant distances and nanofocused in a remote place.
"This opens new pathways for the development of infrared nanocircuits," said Rainer Hillenbrand, leader of the Nanooptics Group at nanoGUNE. "It is amazing that the classical radio-frequency concepts still work at infrared frequencies. Near-field optical microscopy techniques urgently seek for new ways to confine light down to the nanometer scale."
"The concept of tapered transmission lines is a promising way to achieve this. Acting as an ultrasmall torch, it conducts infrared light exactly to the spot under analysis," said Martin Schnell, who performed the experiments.
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