Plasmon-phonon coupling between mid-infrared chiral metasurfaces and molecular vibrations

The Strong Coupling Effect between Metallic Split-Ring Resonators and Molecular Vibrations in Polymethyl Methacrylate

We propose and study a nanoscale strong coupling effect between metamaterials and polymer molecular vibrations using metallic split-ring resonators (SRRs). Specifically, we first provided a numerical investigation of the SRR design, which was followed by an experimental demonstration of strong coupling between mid-infrared magnetic dipole resonance supported by the SRRs fabricated on a calcium fluoride substrate and polymethyl methacrylate molecular vibrations at 1730 cm-1. Characterized by the anti-crossing feature and spectral splitting behaviors in the transmission spectra, these results demonstrate efficient nanoscale manipulation of light-matter interactions between phonon vibrations and metamaterials.

Observation of nonvanishing optical helicity in thermal radiation from symmetry-broken metasurfaces

Spinning thermal radiation is a unique phenomenon observed in condensed astronomical objects, including the Wolf-Rayet star EZ-CMa and the red degenerate star G99-47, due to the existence of strong magnetic fields. Here, by designing symmetry-broken metasurfaces, we demonstrate that spinning thermal radiation with a nonvanishing optical helicity can be realized even without applying a magnetic field. We design nonvanishing optical helicity by engineering a dispersionless band that emits omnidirectional spinning thermal radiation, where our design reaches 39% of the fundamental limit. Our results firmly suggest that metasurfaces can impart spin coherence in the incoherent radiation excited by thermal fluctuations. The symmetry-based design strategy also provides a general pathway for controlling thermal radiation in its temporal and spin coherence.

Molecular Plasmonics with Metamaterials

Molecular plasmonics, the area which deals with the interactions between surface plasmons and molecules, has received enormous interest in fundamental research and found numerous technological applications. Plasmonic metamaterials, which offer rich opportunities to control the light intensity, field polarization, and local density of electromagnetic states on subwavelength scales, provide a versatile platform to enhance and tune light-molecule interactions. A variety of applications, including spontaneous emission enhancement, optical modulation, optical sensing, and photoactuated nanochemistry, have been reported by exploiting molecular interactions with plasmonic metamaterials. In this paper, we provide a comprehensive overview of the developments of molecular plasmonics with metamaterials. After a brief introduction to the optical properties of plasmonic metamaterials and relevant fabrication approaches, we discuss light-molecule interactions in plasmonic metamaterials in both weak and strong coupling regimes. We then highlight the exploitation of molecules in metamaterials for applications ranging from emission control and optical modulation to optical sensing. The role of hot carriers generated in metamaterials for nanochemistry is also discussed. Perspectives on the future development of molecular plasmonics with metamaterials conclude the review. The use of molecules in combination with designer metamaterials provides a rich playground both to actively control metamaterials using molecular interactions and, in turn, to use metamaterials to control molecular processes.

Dual-band selective circular dichroism in mid-infrared chiral metasurfaces

Most chiral metamaterials and metasurfaces are designed to operate in a single wavelength band and with a certain circular dichroism (CD) value. Here, mid-infrared chiral metasurface absorbers with selective CD in dual-wavelength bands are designed and demonstrated. The dual-band CD selectivity and tunability in the chiral metasurface absorbers are enabled by the unique design of a unit cell with two coupled rectangular bars. It is shown that the sign of CD in each wavelength band can be independently controlled and flipped by simply adjusting the geometric parameters, the width and the length, of the vertical rectangular bars. The mechanism of the dual-band CD selection in the chiral metasurface absorber is further revealed by studying the electric field and magnetic field distributions of the antibonding and bonding modes supported in the coupled bars under circularly polarized incident light. Furthermore, the chiral resonance wavelength can be continuously increased by scaling up the geometric parameters of the metasurface unit cell. The demonstrated results will contribute to the advance of future mid-infrared applications such as chiral molecular sensing, thermophotovoltaics, and optical communication.

Infrared Open Cavities for Strong Vibrational Coupling

Arrays of subwavelength plasmonic nanoparticles exhibiting narrowband lattice resonances are referred to as open cavities because of their ability to strongly couple with electronic excitations in molecular chromophores. However, realization of these ideas in the mid-infrared spectral region has been limited. We demonstrated a dramatic reduction in the bandwidth of lattice resonances in large-area arrays of half-wavelength mid-infrared antennas, reaching resonance quality factors above 200. By tuning the wavelength of the antenna-lattice resonances (ALR) to match the transition frequency of the molecular vibrational modes, we achieved a strong coupling between the ALR and the carbonyl stretching excitation in a thin film of (poly)methyl methacrylate (PMMA) polymer deposited on the array. Splitting of the polaritonic transitions, reduction of their bandwidth below that of the bare molecular transition, and characteristic dispersion confirmed the strong coupling regime. Our results pave the way for exciting research on the many-body correlated dynamics of vibrational polaritons.

Coupled plasmonic systems: controlling the plasmon dynamics and spectral modulations for molecular detection

This review describes recent studies on coupled plasmonic systems for controlling plasmon dynamics and molecular detection using spectral modulations. The plasmon dephasing time can be controlled by weak and strong coupling regimes between the plasmonic nanostructures or localized surface plasmon resonances (LSPRs) and the other optical modes such as microcavities. The modal coupling induces near-field enhancement by extending the plasmon dephasing time to increase the near-field enhancement at certain wavelengths resulting in the enhancement of molecular detection. On the other hand, the interaction between LSPR and molecular excited or vibrational states also modulates the resonance spectrum, which can also be used for detecting a small number of molecules with a subtle change in the spectrum. The spectral modulation is induced by weak and strong couplings between LSPRs and the electronic or vibrational states of molecules, and this method is sensitive enough to measure a single molecule.

Chiral plasmonic metasurface absorbers in the mid-infrared wavelength range

Chiral metamaterials in the mid-infrared wavelength range have tremendous potential for studying thermal emission manipulation and molecular vibration sensing. Here, we present one type of chiral plasmonic metasurface absorber with high circular dichroism (CD) in absorption of more than 0.56 across the mid-infrared wavelength range of 5-5.5 µm. The demonstrated chiral metasurface absorbers exhibit a maximum chiral absorption of 0.87 and a maximum CD in absorption of around 0.60. By adjusting the geometric parameters of the unit cell structure of the metasurface, the chiral absorption peak can be shifted to different wavelengths. Due to the strong chiroptical response, the thermal analysis of the designed chiral metasurface absorber further shows the large temperature difference between the left-handed and right-handed circularly polarized light. The demonstrated results can be utilized in various applications such as molecular detection, mid-infrared filter, thermal emission, and chiral imaging.