[ad_1]
The interaction of light with matter is the basis of spectroscopy, a set of techniques at the heart of physics and chemistry. From infrared light to X-rays, a wide range of wavelengths is used to stimulate vibrations, electronic transitions and other processes, thus probing the world of atoms and molecules.
However, the terahertz (THz) region is a less used form of light. Located on the electromagnetic spectrum between infrared and microwave, the THz radiation has the right frequency (about 10 ^ 12 Hz) to excite molecular vibrations. Unfortunately, its long wavelength (several hundred micrometers) is about 100,000 times the typical molecular size, making it impossible to focus the THz beams on a single molecule by conventional optics. Only large sets of molecules can be studied.
Recently, a team led by the Institute of Industrial Science (IIS) of the University of Tokyo has found a solution to this problem. In a Nature Photonics study, they showed that THz radiation can effectively detect the motion of individual molecules, overcoming the conventional diffraction limit for focusing light beams. In fact, the method was sensitive enough to measure the tunnel of a single electron.
The IIS team presented a nanoscale design known as a single molecule transistor. Two adjacent metal electrodes, the source and the drain of the transistor, are placed on a thin silicon wafer shaped like a "bow tie". Then, simple molecules – in this case C60, aka fullerene – are deposited in the spaces below the nanometer between the source and the drain. The electrodes act as antennas to focus the THz beam on isolated fullerenes.
"Fullerenes absorb focused THz radiation, causing them to oscillate around their center of mass," says lead author Shaoqing Du. "The ultrafast molecular oscillation increases the electrical current in the transistor, in addition to its inherent conductivity." Although this current change is tiny – in the order of femto-amps (fA) – it can be measured precisely with the same electrodes as those used to trap molecules. In this way, two vibration peaks at about 0.5 and 1 THz were shown.
In fact, the measurement is sensitive enough to measure a slight division of absorption peaks, caused by the addition or subtraction of a single electron. When C60 oscillates on a metal surface, its vibrational quantum (vibron) can be absorbed by an electron in the metal electrode. Thus stimulated, the electrons move towards the molecule C60. The resulting charged C60-molecule vibrates at a slightly lower frequency than C60 neutral, thereby absorbing a different frequency of THz radiation.
In addition to providing insight into tunneling, the study demonstrates a practical method for obtaining electronic and vibronic information on molecules that only weakly absorb THz photons. This could pave the way for wider use of THz spectroscopy, an underdeveloped method, complementary to both visible light and X-ray spectroscopy, highly relevant to nanoelectronics and quantum computing.
Source link
