A researcher from India has taken the first definitive step to produce high-speed electronic devices that can operate one million times faster than modern electronics. At Max Planck Institute of Quantum Optics in Garching, Germany, Manish Garg and other researchers used laser light to generate very high frequency electric current inside a solid material. The electrons were found to be moving at a speed close to 1015 (one million billion) hertz; the best achievable speed in modern transistors is only 109 (one billion) hertz. The results were published in Nature.
Conventionally, the motion of electrons (conductivity) is achieved by applying voltage. But Dr. Garg and others controlled the motion of electrons inside the solid material by using laser pulses. “Light waves are electromagnetic in nature and have very high oscillation frequency of electric and magnetic fields. This ultrahigh frequency of light waves can be used to drive and control electron motion in semiconductors. Electronics when driven by such light waves will be inherently faster than current state of electronics,” says Dr. Garg, who is the first author of the paper.
The conductivity increases by more than 19 orders of magnitude in presence of the laser pulse.“When we shine high intensity laser on silicon dioxide nanofilm charge carriers (electrons) are generated. When the electrons move in the presence of electric field of the laser it generates current,” he says. “Initially, the nanofilm behaves like an insulator but when we shine high intensity laser it behaves like a conductor. The conductivity increases by more than 19 orders of magnitude in presence of the laser pulse.”
The performance of high-speed circuits rely on how quickly electric current can be turned on and off inside a material. “We showed that we could turn the conductivity of silicon dioxide nanofilm from zero to very high values in a time interval of 30 attoseconds (an attosecond is 1×10-18 of a second), which is one million times faster than modern electronics,” he says.
The high speed of electrons was achieved only in the presence of the laser pulse. Once the pulse (flash) is gone, the nanofilm is restored to its original configuration (insulating). “The whole idea of doing light-wave-electronics is based solely on this fact. So in terms of binary logic, the nanofilm will be in a ‘zero’ state (insulating) when there is no laser and when it interacts with laser it would become conducting ‘one state’” he says.
The very short time interval needed to turn silicon dioxide from an insulator to a conductor was possible as the team used high intensity and extremely short laser pulses and silicon dioxide was in the form of a nanofilm. In the bulk form, silicon dioxide tends to get damaged by high intensity laser as the material tends to accumulate heat produced by the laser pulse. But as a nanofilm, silicon dioxide becomes nearly transparent to laser and absorbs less heat and therefore gets less damaged.
“In our earlier work, which was also published in Nature, we obtained signatures of very high frequency current, but we were not able to measure it. But now we are able to measure current in real time by measuring the time-structure of emitted extreme ultraviolet radiation using an attosecond streak camera,” he says. Current produced in the nanofilm manifests as extreme UV radiation.
The coupling of dipole moment of electrons inside the nanofilm with the electric field of the laser pulse gets the electrons to make a transition from the valence band to the conduction band whereas the motion of electrons inside the conduction band gives rise to an electric current.
“We envision in future we will be able to use transistors driven by laser pulses instead of electronic transistors in electronic devices. The technical challenges is to make use of high frequency currents to perform logic operations similar to the ones performed inside an electronic transistor and also make it feasible on integrated chips,” Dr. Garg says.
Scientists have long debated whether transition of electrons from the valence to the conduction band or the motion of electrons inside the conduction band gives rise to extreme to extreme ultraviolet radiation. “Our study has resolved this by showing that the motion of electrons in the conduction band is the real mechanism for extreme UV radiation,” he says. “When we resolved this we found the motion of electrons in the conduction band is the same as the motion of electrons in modern electronics.”