Terahertz emission in polaritonic systems with nitrides
Oleksandr Kyriienko, Ivan A. Shelykh, and Alexey V. Kavokin
The problem of creation of an efficient THz source is of high importance in modern physics and technology. High-power emitters with frequency tunable in the terahertz range are required for applications in information communication technologies, medicine, and security. However, while for optical spectrum and radio-wave frequencies there are numerous efficient sources of radiation, the THz range of spectrum remains the least explored. None of the existing THz emitters universally satisfies the application requirements. The problem is challenging, since the efficiency of semiconductor THz sources is limited due to the short life-time of involved electronic states (typically, fractions of a nanosecond) compared to a long typical time of spontaneous emission of a THz photon (typically milliseconds).
The existing terahertz sources are based on various operation principles (1). The standard solid-state oscillators such as the Gunn diode or tunnelling diode are restricted only to generation in the lower boundary of the THz frequency window. The group of emitters based on the free electron lasers are bulky, and cannot be integrated easily in the optoelectronic schemes. Finally, the semiconductor sources of THz radiation are represented by several groups of emitters. First, exploiting the intersubband transition of the quantum well, together with electron tunnelling through the structure, one can realize quantum cascade lasers (QCL) (2). In such waveguide structures one can achieve comparably high output power (up to 50 mW), but overall efficiency remains low due to the small spontaneous emission rate. Another drawback of the system is the ultra-low operating temperature of the device, and current research is focused on its raise. Alternatively, the semiconductor THz emission source can be done using downshift optical to THz range conversion. This can be realized if a semiconductor with a large difference of electron and hole mobility is illuminated by a short laser pulse. The resultant density oscillations with THz frequency last for a few periods, and lead to classical dipole emission. However, the power of such an emitter is limited to nanowatt range and can not be easily tuned.
In the following section we review different mechanisms which were proposed for THz emission generation using cavity polaritons, and discuss their advantages over previous schemes. First, we present a broad introduction of the physics of excitons in semiconductor materials. Then, we introduce strong-light matter coupling and discuss properties of emergent polaritonic states. Next, we describe currently existing proposals of THz emitters based on polaritons. Finally, we conclude that realization of presented proposals with nitrides will lead to a significant step towards realization of efficient room-temperature THz sources, including lasers.