- Microscopic Antennas for Light Beat Fiber Optics
- Optical signals use light not only for telecommunications, but for sensors that can detect even scant amounts of toxins. Most optical signals are routed down fibers like a conduit, but now Rice University researchers are proposing to use microscopic optical antennas to receive light in the same way a cell phone's antenna receives radio waves.Electromagnetic radiation runs the gamut from long-wavelength radio waves to short-wavelength light waves, but antennas are usually thought of as useful only for radio waves. Since an antenna's size is related to the wavelength it is intended to receive, radio antennas must be a millimeter or longer. Since light's wavelength is measured in nanometers, antennas scaled down to that size should be able to receive the nanometer wavelengths of light.
Now, Rice University researchers have proven this concept by crafting optical antennas at nanometer wavelengths, permitting the microscopic antennas to receive light waves the same way that cell phone antennas receive radio waves.
An artist's rendering of how plasmons in a pair of gold sub-nanometer electrodes concentrate light from a laser (source: Natelson Lab/Rice University).Cell phone antennas work by providing propagating electromagnetic waves with a physical "sounding board," allowing them to lock onto the signal to receive it. The metallic structure of the antenna, when resonating with the incoming electromagnetic waves, produces an oscillating voltage for the cell phone’s radio frequency (RF) front-end circuitry.
For the microwave frequencies used by cell phones, that wavelength is measured in millimeters and corresponds to the length of the phone's internal antenna.
Rice University researchers mimicked the way a radio-wave antenna works—downsizing it into a tiny nanometer-sized structure that appropriately matches the wavelength of light being received. The antenna, instead of being a solid piece of metal, was fabricated in a manner similar to modern "slot" antennas that cuts an antenna-shaped slot in a solid piece of metal. Rice's optical antenna used a slot-like "nanogap" between gold electrodes, which permitted the laser to induce a voltage characteristic of an antenna.
Scanning electron microscope (SEM) image of gold tips in a nanogap device used in experiments to capture and amplify light (source: Natelson Lab/Rice University).
For its demonstration, Rice University used a laser to illuminate its optical antenna, resulting in a signal at its output that was amplified a million times. The mechanism by which the laser beam was amplified harnessed what are called plasmons—collective oscillations of free electrons. The laser caused the plasmons to oscillate at the same frequency as the light, thereby producing evanescent electromagnetic fields that were thousands of times stronger than those of the original laser.
Next, the researchers want to demonstrate the optical antennas for ultra-sensitive sensors. Optical sensors today use spectroscopy to analyze substances to determine their makeup. But with an amplification factor of a million, the Rice University researchers hope to craft optical sensors so sensitive that they can detect even a single molecule of a toxin.