Taking advantage of ultrafast time-resolved pump detection technology, a collaborated research led by Prof. SHENG Zhigao's team from High Magnetic Field Center, Hefei Institutes of Physical Science (HFIPS) of Chinese Academy of Sciences (CAS) realized the birefringence modulation of GHz frequency induced by ultrafast coherent phonons in SrTiO3 crystals.
According to paper published in the Advanced Science, its operating frequency proved to be much higher than the cutoff frequency of the commercial photoelastic modulators in the market.
A specific material with birefringence can shapes light. The photoelastic modulator based on birefringence modulation technology is one of the core components of modern optical technology. At present, most photoelastic modulators use the mechanical stress provided by piezoelectric materials to drive photoelastic crystals to achieve birefringence modulation, and their working frequency is limited by the resonant frequency of photoelastic / piezoelectric crystals, which is generally of the order of kHz. Therefore, it is urgent to develop birefringent materials and modulation techniques with GHz operating frequency.
"We found the GHz optical birefringence effect induced by ultrafast coherent phonons in perovskite SrTiO3 crystals and optically manipulated it," said SHENG Zhigao, corresponding author of the paper, "with the help of the ultrafast pump-probe system in our high magnetic field magneto-optical laboratory."
They did lots of material screening and technical exploration.
At first, the team used ultrafast laser pulses to generate coherent acoustic phonons with low damping in the transducer / SrTiO3 heterostructure.
After screening a series of materials, they found that LaRhO3 semiconductor thin films as transducers can obtain relatively high photon-phonon energy conversion efficiency.
Subsequently, in the optimized heterostructure, it is found that ultrafast coherent acoustic phonons can induce optical birefringence with GHz frequency in stress-sensitive SrTiO3 crystals.
Moreover, the research team realized the optical manipulation of coherent phonons and their induced GHz birefringence by double-pump technique.
This discovery reveals a mechanism of ultrafast optical birefringence modulation and provides a technical basis for the application of GHz high frequency acousto-optic devices.
a.Schematic diagram of the principle of coherent acoustic phonon induced GHz frequency optical birefringence.b.Behaviors of optical birefringence with different lattice orientations of STO. (Image by SUN Tao)