In a central processing unit (CPU), data is processed and carried by moving electrons. And in electronic circuits, these electrons move in the desired direction with the application of an electric field. This is controlled by the CPU’s clock, based on an oscillating signal, and produces a fixed sine wave to enable on-off switching of electron motion at an order of gigahertz.
According to the researchers, however, an oscillating field of light with a frequency of petahertz could enable the petahertz operation of on-off switching, leading to data processing speed enhancements to one million times faster than what’s currently possible in conventional computers. That is, if researchers can figure out how to move electrons with this light frequency.
That’s exactly what Japanese researchers claim that they’ve done. The researchers, hailing from Tohoku University, Nagoya University, Institute for Molecular Science, Okayama Science University, and Chuo University, say that they have succeeded in moving electrons in an organic superconductor in a specific direction via the irradiation of ultrashort laser pulses.
Manipulating Current with Light Pulses
Ohm’s law says that an induced current, and consequently the velocity of electrons, is proportional to the electric current that is applied. This can change somewhat if electrons are scattered—something that also determines a material’s resistivity.
An illustration of frequency doubling induced by a petahertz non-linear current in an organic superconductor, κ- BEDT-TTF compounds. Image used courtesy of Nature
However, if an electric current can be applied on a timescale shorter than the scattering time, electrons in a solid don’t have sufficient time to be averaged. Instead, they accelerate and generate what’s known as a polarized net current.
This is what the researchers attempted to achieve in this study: a scattering-free current that uses ultrashort laser pulses that are significantly shorter than electron scattering time in organic superconductors—around 40 femtoseconds.
Second-Harmonic Generation: the Speedy Detection Method
This task is easier said than done.
One major obstacle is that electric detection of such a short-time current is impossible. To overcome this, the researchers employed an optical detection method in their study called second-harmonic generation.
The top image shows a non-linear electric displacement D(t) induced by electric field; the bottom shows non-linear light-induced current. Image used courtesy of Nature
Second-harmonic generation (SHG), also known as “frequency doubling,” is a nonlinear optical process where two photons of the same frequency interact with a nonlinear material. It has long since been known as a method for detecting the breaking of electronic symmetry, such as the microscopic dipole moment seen in ferroelectrics.
SHG can also be induced by the polarized current, which is another type of electronic symmetry breaking.
Computers at the Speed of Petahertz
In this study, the researchers used an ultrashort laser with a 6 fs pulse width on an organic centrosymmetric superconductor, κ-(BEDT-TTF)2Cu[N(CN)2]Br. Contrary to what one would expect (because an SHG is usually only generated in materials where spatial symmetry is broken), an SHG was exhibited in the centrosymmetric material.
This indicates that the light irritation generates a polarized net current.
According to the research team, it may be possible to make computers with an operation speed of petahertz, which is a million times faster than gigahertz, with further understanding of the scattering-free petahertz current.
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