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1. Observation of the polaronic character of excitons in a two-dimensional semiconducting magnet CrI3

   https://doi.org/10.1038/s41467-020-18627-x  

   Jin, Wencan ; Kim, Hyun Ho ; Ye, Zhipeng ; Ye, Gaihua ; Rojas, Laura ; Luo, Xiangpeng ; Yang, Bowen ; Yin, Fangzhou ; Horng, Jason Shih An ; Tian, Shangjie ; et al ( September 2020 , Nature Communications)

   Exciton dynamics can be strongly affected by lattice vibrations through electron-phonon coupling. This is rarely explored in two-dimensional magnetic semiconductors. Focusing on bilayer CrI₃, we first show the presence of strong electron-phonon coupling through temperature-dependent photoluminescence and absorption spectroscopy. We then report the observation of periodic broad modes up to the 8th order in Raman spectra, attributed to the polaronic character of excitons. We establish that this polaronic character is dominated by the coupling between the charge-transfer exciton at 1.96 eV and a longitudinal optical phonon at 120.6 cm⁻¹. We further show that the emergence of long-range magnetic order enhances the electron-phonon coupling strength by ~50% and that the transition from layered antiferromagnetic to ferromagnetic order tunes the spectral intensity of the periodic broad modes, suggesting a strong coupling among the lattice, charge and spin in two-dimensional CrI₃. Our study opens opportunities for tailoring light-matter interactions in two-dimensional magnetic semiconductors.
2. Ultra-low-power nonvolatile integrated photonic switches and modulators based on nanogap-enhanced phase-change waveguides

   https://doi.org/10.1364/OE.411254  

   Zhang, Jieying ; Zheng, Jiajiu ; Xu, Peipeng ; Wang, Yanqun ; Majumdar, Arka ( November 2020 , Optics Express)

   We propose a nanogap-enhanced phase-change waveguide with silicon PIN heaters. Thanks to the enhanced light-matter interaction in the nanogap, the proposed structure exhibits strong attenuation (Δα = ∼35 dB/µm) and optical phase (Δn_eff = ∼1.2) modulation atλ = 1550 nm when achieving complete phase transitions. We further investigate two active optical devices based on the proposed waveguide, including an electro-absorption modulator and a 1 × 2 directional-coupler optical switch. Finite-difference time-domain simulation of the proposed modulator shows a high extinction ratio of ∼17 dB at 1550 nm with an active segment of volume only ∼0.004λ³. By exploiting a directional coupler design, we present a 1 × 2 optical switch with an insertion loss of < 4 dB and a compact coupling length of ∼ 15 µm while maintaining small crosstalk less than −7.2 dB over an optical bandwidth of 50 nm. Thermal analysis shows that a 10 V pulse of 30 ns (1×1 modulator) and 55 ns (1×2 switch) in duration is required to raise the GST temperature of the phase-change waveguide above the melting temperature to induce the amorphization; however, the complete crystallization occurs by applying a 5 V pulse of 180 ns (1×1 modulator) and a 6 V pulse of 200 ns (1×2 switch),more »respectively.

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3. Experimental observation of topological Z2 exciton-polaritons in transition metal dichalcogenide monolayers

   https://doi.org/10.1038/s41467-021-24728-y  

   Li, Mengyao ; Sinev, Ivan ; Benimetskiy, Fedor ; Ivanova, Tatyana ; Khestanova, Ekaterina ; Kiriushechkina, Svetlana ; Vakulenko, Anton ; Guddala, Sriram ; Skolnick, Maurice ; Menon, Vinod M. ; et al ( July 2021 , Nature Communications)

   The rise of quantum science and technologies motivates photonics research to seek new platforms with strong light-matter interactions to facilitate quantum behaviors at moderate light intensities. Topological polaritons (TPs) offer an ideal platform in this context, with unique properties stemming from resilient topological states of light strongly coupled with matter. Here we explore polaritonic metasurfaces based on 2D transition metal dichalcogenides (TMDs) as a promising platform for topological polaritonics. We show that the strong coupling between topological photonic modes of the metasurface and excitons in TMDs yields a topological polaritonic Z₂phase. We experimentally confirm the emergence of one-way spin-polarized edge TPs in metasurfaces integrating MoSe₂and WSe₂. Combined with the valley polarization in TMD monolayers, the proposed system enables an approach to engage the photonic angular momentum and valley and spin of excitons, offering a promising platform for photonic/solid-state interfaces for valleytronics and spintronics.
4. Nonreciprocal directional dichroism of a chiral magnet in the visible range

   https://doi.org/10.1038/s41535-020-0224-6  

   Yokosuk, Michael O. ; Kim, Heung-Sik ; Hughey, Kendall D. ; Kim, Jaewook ; Stier, Andreas V. ; O’Neal, Kenneth R. ; Yang, Junjie ; Crooker, Scott A. ; Haule, Kristjan ; Cheong, Sang-Wook ; et al ( April 2020 , npj Quantum Materials)

   Nonreciprocal directional dichroism is an unusual light–matter interaction that gives rise to diode-like behavior in low-symmetry materials. The chiral varieties are particularly scarce due to the requirements for strong spin–orbit coupling, broken time-reversal symmetry, and a chiral axis. Here we bring together magneto-optical spectroscopy and first-principles calculations to reveal high-energy, broadband nonreciprocal directional dichroism in Ni₃TeO₆with special focus on behavior in the metamagnetic phase above 52 T. In addition to demonstrating this effect in the magnetochiral configuration, we explore the transverse magnetochiral orientation in which applied field and light propagation are orthogonal to the chiral axis and, by so doing, uncover an additional configuration with a unique nonreciprocal response in the visible part of the spectrum. In a significant conceptual advance, we use first-principles methods to analyze how the Ni²⁺d-to-don-site excitations develop magneto-electric character and present a microscopic model that unlocks the door to theory-driven discovery of chiral magnets with nonreciprocal properties.
5. Molecular polariton electroabsorption

   https://doi.org/10.1038/s41467-022-35589-4  

   Cheng, Chiao-Yu ; Krainova, Nina ; Brigeman, Alyssa N. ; Khanna, Ajay ; Shedge, Sapana ; Isborn, Christine ; Yuen-Zhou, Joel ; Giebink, Noel C. ( December 2022 , Nature Communications)

   We investigate electroabsorption (EA) in organic semiconductor microcavities to understand whether strong light-matter coupling non-trivially alters their nonlinear optical [$${\chi }^{(3)}\left(\omega,{{{{\mathrm{0,0}}}}}\right)$$${\chi }^{\left(3\right)}\left(\omega ,\mathrm{0,\; 0}\right)$] response. Focusing on strongly-absorbing squaraine (SQ) molecules dispersed in a wide-gap host matrix, we find that classical transfer matrix modeling accurately captures the EA response of low concentration SQ microcavities with a vacuum Rabi splitting of$$\hslash \Omega \approx 200$$$\hslash \Omega \approx 200$meV, but fails for high concentration cavities with$$\hslash \Omega \approx 420$$$\hslash \Omega \approx 420$meV. Rather than new physics in the ultrastrong coupling regime, however, we attribute the discrepancy at high SQ concentration to a nearly dark H-aggregate state below the SQ exciton transition, which goes undetected in the optical constant dispersion on which the transfer matrix model is based, but nonetheless interacts with and enhances the EA response of the lower polariton mode. These results indicate that strong coupling can be used to manipulate EA (and presumably other optical nonlinearities) from organic microcavities by controlling the energy of polariton modes relative to other states in the system, but it does not alter the intrinsic optical nonlinearity of the organic semiconductor inside the cavity.