News Source: School of Physics

Recently, Chen Yuhui, a young researcher in Professor Zhang Xiangdong's group at School of Physics, Beijing Institute of Technology cooperated with researchers from Lund University in Sweden and University of Otago in New Zealand, made important progress in the study of optical multibody problems. For the first time, it was discovered experimentally that there is an intrinsically unstable phase in optical non-equilibrium system. Related research results were published in the top international journal in Physical Review Letters.

Because of its rich connotation and wide application prospects, the many-body problem has always been a key content of condensed matter physics research. In the fields of thermodynamics, electricity, magnetism including superfluidity, superconductivity, ferromagnetism and so on. However, for the many-body problems of optical drive, since the optical system is naturally a non-equilibrium system and an open system with inputs and outputs at the same time, its complexity is more than that of the traditional multi-body problem. As a result, people's understanding of the optical multi-body problem is far inferior to the many body problems of heat, electricity and magnetism. At present, only a few eigenphases of optical many-body problems such as the general pure absorption phase and intrinsic bistable phase have been discovered.

On the other hand, with the increasing influence of photonic technology on daily life, people's demand for high brightness light sources and strong nonlinear switches have become more prominent, and these devices rely on optical multi-body systems to enhance the interaction between light and matter. The research on the eigen phase of optical multi-body systems has attracted people's attention again due to its huge potential in application.

Figure 1 Experimental observation results of the intrinsically unstable phase of an optical multibody system. (a) and (c) are the experimental devices. (b) and (d) are the measurement results of the transmission spectrum.

When the team members studied the transmission experiment of high-concentration erbium doped YSO crystal, they found that due to the dipole interaction between erbium ions, the response of the system is no longer has time invariance, and its optical response cannot be expressed as a stable function in time domain. This interaction phase is called Intrinsic Optical Instability. For the unstable optical system, the traditional theories and experiments generally believe that additional positive feedback is required to achieve, such as optical chaos in optical resonators; but in these traditional systems, the response function of the system remains time-invariant, and the instability of the overall output stems from the slight deviation of the input signal being continuously amplified during the positive feedback process. However, the research of the team members revealed that the material system itself has an inherently unstable phase, at this time, the response of the system is not time invariant; even if the input signal is absolutely stable, it will not correspond to a stable output, which is fundamentally different from the general optical chaos and other phenomena. The Intrinsic Optical Instability is caused by many body interactions in quantum mechanics.

BIT is the first unit of this work, Researcher Chen Yuhui from the School of Physics and Dr. Sebastian Horvath of Lund University in Sweden are the co first authors of this work, and Researcher Chen Yuhui and Professor Zhang Xiangdong are the corresponding authors. This work is supported by the National Natural Science Foundation of China, the academic initiation program for young teachers of BIT and the youth science and technology innovation program of BIT.

Related article link:

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.126.110601