Scientists Achieve Breakthrough in Polariton Laser Technology
An international scientific team has modeled and conducted an experiment in the course of which they have managed to produce an electrically pumped spin-polarized polariton laser. This allows for a reduction in the laser’s energy consumption levels and control over the output polarization. This is achieved thanks to the use of magnetic materials in the device’s contacts: the electrons that come into contact with the laser have a preferred spin direction, which allows for effective spin pumping. Polariton lasers are very promising for the very reason that they do not require high amounts of energy. In addition, they work at room temperatures. Due to this, they can be used in portable electronics, optical computers and communication devices. Results of the experiment have been published in Physical Review Letters.
The primary advantage of polariton lasers is their low energy consumption. A regular laser is based on the induced radiation effect: if a high-energy excited electron is hit by a photon, the electron “falls” into a low-energy state, producing two photons that are identical to the original one. A cascade of such processes results in the formation of a large number of identical, coherent photons that form the laser emission.
To generate a laser beam, the population inversion condition has to be met: the electron density on the high-energy level must be higher than on the low-energy level. In this way, it is important to “pump into” the system an amount of additional energy that is required to transfer enough electrons onto the high-energy level. The minimum amount of energy required for the formation of laser radiation is referred to as the lasing-action threshold: this value, among other things, determines a laser’s minimum energy requirement.
There is also another process which concerns the type of emission that is called spontaneous: in that case, an electron on the high-energy level can, at a random time, emit a photon and fall into a low-energy state. The issue with spontaneous emission is that the exiting photons tend to be incoherent, with a random phase. This issue can be avoided if all the electrons are put into the same quantum-mechanical state, which would also cause all exiting photons to be identical. Unfortunately, this cannot be done with electrons, as they cannot assume the same quantum-mechanical state according to the so-called Pauli Exclusion Principle, which states that two or more identical fermions (particles with half-integer spin) cannot occupy the same quantum state within a quantum system simultaneously.
A polariton laser setup. Credit: bibo.kz
In polariton lasers, this limitation is overcome in the following manner: electrons in such systems, by interacting with each other and the light, form composite particles – exciton polaritons. These particles have a full spin, and are thus left unaffected by the Pauli Principle; at low temperatures, they can assume the same quantum-mechanical state. Such a state of matter in which a large fraction of particles occupy the same quantum state is referred to as the Bose-Einstein condensate. If polaritons can disappear from the condensate while spontaneously emitting photons that exit through the cavity face of a laser, the laser output will be coherent just like in a regular laser. Except that the lasing-action threshold does not need be reached any longer! In real life, of course, some amount of energy will still be required, but it will be substantially (by several magnitudes) less than for what is needed for regular, semiconductor-based lasers.
The first polariton lasers have been built in early 00’s; they worked at ultra-low temperatures of several Kelvin and had to be pumped by another laser. In the recent years, both of these issues have been solved: in 2013 an electrically pumped polariton laser that could operate at room temperature was demonstrated. The last remaining issue is that of controlling the polarization of the emission.
“In a polariton laser, two Bose-Einstein condensates tend to form: one with upward-directed spin polaritons, and another with downward-directed ones. Both are emitting independently: as a result, the emitted light’s polarization is linear, and its direction is random. If we could manage to mostly pump one of the condensates, it would allow us to receive a stable, circularly-polarized emission and to also lower the lasing-action threshold, further reducing the laser’s energy consumption. Such spin-selective pumping is quite easy to implement when dealing with optics, but electric spin-polarized pumping has not yet been done,” – comments Ivan Iorsh, one of the article’s authors and an associate professor at ITMO University’s Department of Nanophotonics and Metamaterials.
This is exactly what was accomplished by the international team that also included physicists from the University of Michigan, Nanyang Technological University, University of Southhampton and St. Petersburg State University. The researches have used a ferromagnetic material as a contact in their laser setup, which was used to create a magnetic field. Electrons that entered this system had their spin polarization defined by the ferromagnetic material, which they passed on to the polaritons that formed a condensate. This led to a stable, elliptical polarization of the resulting emission and the lowering of the threshold.
By controlling the spin using a magnetic field, it is also possible to manipulate the light’s polarization. This means that optical signals can be coded through electrical ones. In that case, the direction of polarization would substitute the ones and zeroes – such a setup can be implemented on a microchip with low power consumption that will work at room temperature.
The results of this research were showcased in an experiment at the University of Michigan. The team from ITMO University and St. Petersburg State University modeled the system.
“The experiment has fully confirmed the behavior predicted by our modeling. It’s always amazing to see an experiment confirm a theoretical prediction. The discovered effect is very important for spintronics – the science of coding information not through electrical charges, but through the spin. The main issue there is the inevitable spin relaxation – the loss of spin polarization by electrons due to their interactions with the crystal grating. We have demonstrated the opposite effect – the increase and amplification of spin polarization, which could open up completely new opportunities for application in devices,” – comments Alexey Kavokin, head of the Spin Optics Laboratory at St.Petersburg State University.
He added that another promising area of development in science that is related to polariton lasers is that of quantum simulators based on polariton condensates. Researchers are currently racing to create the first quantum processor. Google’s artificial intelligence laboratory has collected 49 qubits, Mikhail Lukin’s teams at Harvard – 51. The hundred-qubit threshold will likely be crossed in the coming months. Still, the practical application of such systems is highly limited: Google’s superconductor-based processor only works at ultra-low temperatures (less than one degree Kelvin), while Mikhail Lukin’s qubits are based on cold atoms, which can only be kept together in lab conditions.
“In this context, polaritons offer an alternative platform for quantum computations. It is a semiconductor platform, and those are relatively cheap and are easy to integrate into the existing processors. And the main thing that our work with our colleagues from Michigan has shown is that polariton condensates are fine with room temperatures. I’m sure that a semiconductor platform for quantum technology can be created in Russia in a short time. We might even come out ahead of Google!” – adds Kavokin.
The scientist notes that, per his opinion, in the next two or three years polariton lasers will likely see practical application. This is mostly related to their use in the creation of macroscopic multiparticle wave functions at room temperatures. The use of polariton lasers in quantum computing is also a promising venture.
Reference: Room Temperature Spin Polariton Diode Laser, Aniruddha Bhattacharya, Md Zunaid Baten, Ivan Iorsh, Thomas Frost, Alexey Kavokin, Pallab Bhattacharya, 2017, Physical Review Letters.