At the beginning of last year, the coronavirus pneumonia-19 epidemic closed the experiment of SLAC National Accelerator Laboratory of the Ministry of Energy, and Shambhu Ghimire's research team was forced to find another way to study an interesting research goal: a quantum material called topological insulator (TIs) can conduct electricity on its surface, but not through its interior.
Two years ago, Denitsa Baykusheva, a researcher at the Swiss National Science Foundation, joined his team at the Stanford Pulse Institute, with the goal of finding a way to generate higher harmonics or HHG in these materials as a tool to study their behavior. In HHG, the laser will be transformed into higher energy and higher frequency by material irradiation, which is called harmonic, just like pressing the guitar string will make a higher tone. TIs is the cornerstone of spintronics, quantum sensing and quantum computing. If this can be done, it will provide scientists with new tools to study these and other quantum materials.
With the cessation of the experiment, she and her colleagues turned to theory and computer simulation, and proposed a new formula for generating HHG in topological insulators. The results show that circularly polarized light rotating in the direction of laser beam will generate clear and unique signals from the conductive surface and the interior of TI (bismuth selenide), which will actually enhance the signals from the surface.
The picture above shows how the circularly polarized laser (above) detects the topological insulator (black), which is a quantum material that conducts electricity on its surface but does not pass through it. Light passes through a process called higher harmonic generation, which makes the electrons in the material fly away, recombine and emit light (white light) with higher energy and frequency. By analyzing the emitted light, scientists can measure the spin and momentum of electrons in materials. SLAC's experiments confirm that these signals are the only features of topological surfaces. Source: Greg Stewart /SLAC National Accelerator Laboratory.
When the laboratory reopened for experiments and safety precautions were taken against coronavirus pneumonia, Baykusheva began to test this formula for the first time. In a paper published in Nano Letters today, the research team reported that these tests were carried out as expected and the first unique signature was generated from the topological surface.
"This material looks very different from any other material we have tried," said Ghimire, principal researcher of PULSE. "It's really exciting to find a new material whose optical response is different from any other material."
In the past ten years, Ghimire and David Reis, director of PULSE, have done a series of experiments, which prove that HHG can be generated in a way that was previously thought impossible or even impossible: injecting laser into crystals and freezing argon or semiconductor materials with thin atoms. Another study describes how to use HHG to generate attosecond laser pulses, which can be used to observe and control the movement of electrons by irradiating the laser with ordinary glass.
This arrow pattern reflects the combination of spin and momentum of electrons on the surface of topological insulators. Topological insulator is a kind of quantum material, which conducts current on its surface rather than through its interior. SLAC experiments show that the coupling of circularly polarized laser with this spin polarization produces a unique high-order harmonic generation mode, which is the characteristic of topological surface. Source: Denitsa Baykusheva/ Stanford Pulse Institute.
However, quantum materials are firmly opposed to analysis in this way, and the splitting characteristics of topological insulators pose a special problem.
"When we irradiate TI with laser, both the surface and the interior will generate harmonics. The challenge is how to separate them. "
He explained that the key discovery of the team is that circularly polarized light interacts with the surface and the interior in a completely different way, which promotes the generation of higher harmonics from the surface and gives it unique characteristics. In turn, these interactions are formed by two basic differences between the surface and the interior: the degree of electron spin polarization (for example, clockwise or counterclockwise) and the symmetry type of atomic lattice.
Schematic diagram of SLAC high power laser laboratory experimental device. Scientists use circularly polarized laser to detect topological insulators, which are quantum materials that conduct electricity on their surfaces, but do not pass through them. A process called harmonic generation converts laser light into higher energy and frequency, or harmonics. This creates a polarization mode (arrow) in the detector, revealing the spin and momentum of electrons in the conductive surface-the unique characteristics of the topological surface. Source: Shambhu Ghimire/ Stanford Pulse Institute.
Ghimire said that since the group published the formula of realizing high hydrogen and high mercury on TIs earlier this year, two other research groups in Germany and China have reported the creation of high hydrogen and high mercury in topological insulators. But both experiments were conducted with linearly polarized light, so they didn't see the enhanced signal generated by circularly polarized light. He said that this kind of signal is a unique feature of topological surface state.
Because intense laser can turn electrons in the material into soup-plasma-the research team must find ways to change the wavelength of their high-power titanium sapphire laser, so that it can be extended by 10 times, thus reducing the energy of 10 times. They also use very short laser pulses to reduce the damage to samples, which has the added benefit of allowing them to capture the behavior of materials at shutter speeds equivalent to millionths and billionths of a second.
"The advantage of using HHG is that it is an ultra-fast detector," Ghimire said. "Now that we have determined this new method for detecting topological surface states, we can use it to study other interesting materials, including topological states induced by intense laser or chemical methods."
Researchers from the Institute of Materials and Energy Sciences (SIMES) of Stanford University, the University of Michigan at ann arbor and pohang university of science and technology contributed to this work.