Thermal conduction and radiation
Thermal conduction control in phononic metamaterials
We are researching phononic metamaterial using superlattice(SL). Using a phonon superlattice, the phonon group velocity can be manipulated in a specific frequency band. Multiple superlattices that reflect different frequencies can be used to suppress the thermal conductivity(κ) in the simulation. Currently, we are investigating to demonstrate theoretical models experimentally. If we can control thermal current, we can apply a technique that dissipates heat using a metamaterial with phonon transmission in LED and other electronic devices.
Meanwhile, we designed phonon laser, equipped with the superlattice working as a phononic distributed Bragg reflector (DBR) in the acoustic phonon spectrum. The superlattice with alternating values of acoustic impedances will be designed to realize the perfect reflection/transmission at certain frequencies.
A new concept of thermal radiations
Additional emphasis is placed on the extraction of phonons into the air whose efficiency is extremely low in clear contrast to electromagnetic waves; This is because the difference between the acoustic impedance (~10^4) and the air (1), which is a concept corresponding to the phonon refractive index, is very large. As a result, most of the phonons are trapped inside the semiconductor device. On the other hand, the electromagnetic waves is easily extracted into the air due to the relatively small refractive index difference (solid = 3, air = 1). From the viewpoint of the phonon spectrum, the thermal band of the phonon (0.1-100 THz) is mostly prohibited from the air. Efficient approach in this case is the phonon-electromagnetic wave conversion, it is different from 'plank black body radiation theory'. Specifically, phonon-electromagnetic wave conversion occurs before the thermal equilibrium time-scale. In this way, we now establish and develop related concepts of thermal radiation.
The terahertz (THz) electromagnetic wave radiation by electron-phonon interaction has been reported when an acoustic wave packet crosses an interface of polar materials, transient polarization currents are induced in polar material. However, it has not been reported in non-polar materials yet.
Details about THz electromagnetic wave radiation in polar materials:
- M. R. Armstrong et al., Nature Physics 5, 285 (2009)
- H. Jeong, Y. D. Jho et al., Phys. Rev. Lett. 114, 043603 (2015)
- H. Jeong, Y. D. Jho et al., Phys. Rev. B 94, 024307 (2016)
We are researching THz electromagnetic wave radiation induced by the interactions between AC phonons and electrons in non-polar material, such as Si. In non-polar material, AC wave packets are generated via deformation potential coupling with photocarriers. When the AC wave packet encounter the electron reservoir, the phonon-induced electric fields due to deformation potential accelerates the electrons, generating the THz electromagnetic wave pulse, as illustrated in figure 1.
This AC-phonon-induced THz radiation was experimentally observed in the figure 2. As shown in figure 2, the temporal period of the THz wave pulses wave 'T' determined by the period of AC wave packet, and the corresponding spectrum change was also confirmed.
Luminescence Studies under the Electron-Phonon Interaction toward Thermal Engineering
Regardless of the recent advancement in realizing generation and propagation mechanism of phonon in quantum scale structures, highly efficiency heat removal interface capable of operating at room temperature are elusive till now, so our resolution is to fill this gap. In typical LED device pyramidal pattern arrays were proved competent for enhancing the light extraction efficiency and we are considering the same structure from the viewpoint of heat. Now in this LEDs phonons are proved as the source of heat, which are generated as heat oscillation in the active layer region of MQWs, our purpose here is to quantify the focused phonons behavior in the high phonon density region at the tip of the cone and phonon-photon conversion in pyramidal structure.
We experimentally confirmed the enhanced phonon assisted Anti-Stokes photoluminescence at room temperature from the GaN hexagonal pyramids which indicates to a high intensity phonon region in those pyramids. Also through time resolved photoluminescence measurement we detected a time delay in ASPL signal appearance which is possibly due to focusing of propagating acoustic phonons generated in MQWs. Through quantifying the phonons anharmonic behavior in the high phonon density region at the tip of the cone by means of experimental techniques like Pump-probe spectroscopy, TCSPC etc., and identifying the mechanisms, the characteristics of phonon-photon conversion could be optimized.
In addition, this phonon lens can eventually lead us towards the “phonon diode”, with which it could be possible to control the heat flow in one direction and revolutionize the heat dissipation from optoelectronic devices. The proposed phonon focusing structure and reflectors along with the acoustically mismatched heterostructures can be incorporated to the building blocks of future phonon-engineered nanodevices.