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Photoelectric modulation of perovskite coupled to phonon resonator


Recently, researchers from the Max Planck Institute for Polymers in Germany and Pohang University of Science and Technology in South Korea have explored the effect of resonator-phonon coupling on the instantaneous conductivity of organic-inorganic mixed peroxides by measuring the ultrafast photoconductivity reaction of peroxides in a tunable F-P terahertz cavity, which is of great significance for tunable switching and frequency control devices.

Background:

The selective interaction of coherent electromagnetic radiation with the movement of specific microscopic materials can induce superconducting optical phase transitions and affect molecular reaction pathways. The quantum nature of light has recently received more and more attention, and the use of a vacuum state can avoid the limitations caused by energy dissipation. The interaction between the quantum field and the two-level system leads to the mixing of multiple states. In general, material-field interactions are weak and only slightly modulate material behavior (e.g., Lamb shift in hydrogen atoms). To enhance the coupling, the material can be placed in a cavity that is tuned to the transition resonance between the two energy levels of the material. It has been reported that a strong mixture of field states and material states using this method can produce polarized exciton states of multiple material systems (inorganic, organic, and mixed perovskite materials) from the visible band to terahertz frequencies. This approach can significantly affect the rate of chemical reactions, supramolecular self-assembly, and conductivity.

Innovative research

The authors investigate the resonant interaction between terahertz fields and organic-inorganic perovskites in cavities, and how it affects the electrical and optical properties of the system. In addition, methylammonium iodide lead (MAPI, (CH3NH3)PbI3) perovskites, which consist of CH3NH3+ cations embedded in a negatively charged inorganic PbI3- sublattice, have been studied. Due to the action of local charge, the MAPI lattice is highly sensitive to external electric fields, and its phonon mode has high optical intensity. On the one hand, the high strength of the MAPI phonon modes makes it easier for them to be strongly coupled to the resonant terahertz modes of the cavity. On the other hand, due to the strong electron-phonon interaction, the phonons significantly affect the charge mobility in MAPI, resulting in polaron formation and electron-phonon scattering. The latter is the main mechanism that inhibits the movement of free charges in perovskites. In harmonic systems, electron-phonon scattering occurs in a dark longitudinal (LO) phonon pattern that cannot be coupled with a transverse electromagnetic field. However, significant dissonance in MAPI perovskites leads to a mix of longitudinal and bright transverse (TO) phonon modes, and even to the localized character of these vibrations. The latter may achieve various scattering pathways for electrons by breaking the rules of conservation of momentum. Therefore, the coupling of the bright phonons of MAPI perovskites with the electromagnetic field can affect the density of states and properties of the TO and LO phonons, as well as their interaction with charge carriers (Fig. 1a).

The authors tuned the cavities to resonate with the phonon mode of MAPI perovskites to enhance their interaction with the terahertz field. The cavity consists of two molten silica substrates, each with a thin (190 nm) ITO layer deposited (Figure 1b). ITO is transparent in the visible spectrum but is electrically conductive, so it can reflect most of the terahertz radiation. This design enables photo-excitation of charge carriers in perovskites coupled to terahertz cavities and detection of charge carrier mobility using terahertz pulses. It can provide an opportunity to control the conductivity of perovskites and the terahertz response of perovskite cavity systems.

Figure 1. Electron-phonon coupling in a terahertz tuning cavity

The authors characterize the performance of the cavity in the terahertz frequency range by using the transmission of a terahertz time-domain spectroscopy chamber. Figures 2a-f show the time distribution and spectrum of terahertz pulses transmitted through the cavities between the mirrors. Since perovskites are in the frequency range of THz pulses, they have two strong modes at ~1 THz and ~2 THz (Figure 2g), which are assigned to Pb-I-Pb bending vibration and Pb–I bond stretching.

Figure 2. Optical properties of terahertz cavities and perovskites

The authors inserted a ~1 μm thick polycrystalline sample supported on a 1 μm thick SiNx membrane into the cavity and adjusted the first cavity mode (mode order m=1) to close the resonance with the 1 THz phonon (Figure 3a). With this cavity length, the second cavity mode (m=2) resonates with the 2THz perovskite phonon. However, due to the different spatial distribution of the intracavity field, the interaction of the electromagnetic field in the first and second cavity modes with the thin film sample is different. In the center of the cavity, the electric field is maximum for m=1 and zero for m=2 mode. Therefore, the strength of the interaction between the different modes (the product of the electric field and the transition dipole moment) depends mainly on the position of the sample in the cavity. The response of the perovskite cavity system shows great variability when the cavity length and the position of the perovskite within the cavity change, as shown in the simulations in Figure 3b and the experimental results in Figure 3c.

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Figure 3. Frequency- and time-resolved spectra of perovskite cavity systems

Fig.4 Cavity tunable photoconductance enhancement of perovskite cavity system

Since the perovskite does not change significantly inside the terahertz cavity, whether it is in the ground state or excited state, the response of the entire perovskite cavity system will show significant changes. The results in the figure 3D show that the system is capable of tunable control of terahertz field modulation. However, modulation is not limited to the maximum. The data in the time domain is analyzed, i.e., the data before the Fourier transform that generates the spectrum in the graph. For both system configurations, Figure 4a shows the electric field of the THz pulse and its photoinduced change measured at 30 ps after sample excitation. Before the terahertz pulse reaches its maximum, the modulation of the field is weak for both the on- and off-resonant perovskite cavity configurations. In the vicinity of the pulse maximum, the field changes by about 5-10% in a resonant perovskite cavity system (Figure 4b). After about 0.5 ps, the modulation increases to 20% (Figure 4c) and reaches 40% about 1.5 ps after the pulse reaches maximum (Figure 4d). According to the results shown in Figure 4b-d, tuning the cavity to resonate with the 1THz perovskite mode can increase the modulation by a factor of 1.5, 2, and 3 over the duration of the THz pulse. In summary, the on-demand tunability of this ultrafast terahertz field modulation can benefit photonic integrated devices and optical communication modulation.

The article was published in Light: Science & Applications, a top international academic journal, with the title of “Controlling the electro-optic response of a semiconducting perovskite coupled to a phonon-resonant cavity”. Lucia Di Virgilio is the first author and Maksim Grechko and Mischa Bonn are the corresponding authors. (Source: LightScienceApplications WeChat public account)

Related Paper Information:https://www.nature.com/articles/s41377‍-023-01232-0

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