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We demonstrate ultrastrong coupling between intersubband transitions in single semiconductor quantum well and resonant modes of photonic nanocavities implemented on both rigid and flexible substrates. The Rabi splitting reaches 27% of the intersubband transition frequency.

Nanophotonic resonators can confine light to deep-subwavelength volumes with highly enhanced near-field intensity and therefore are widely used for surface-enhanced infrared absorption spectroscopy in various molecular sensing applications. The enhanced signal is mainly contributed by molecules in photonic hot spots, which are regions of a nanophotonic structure with high-field intensity. Therefore, delivery of the majority of, if not all, analyte molecules to hot spots is crucial for fully utilizing the sensing capability of an optical sensor. However, for most optical sensors, simple and straightforward methods of introducing an aqueous analyte to the device, such as applying droplets or spin-coating, cannot achieve targeted delivery of analyte molecules to hot spots. Instead, analyte molecules are usually distributed across the entire device surface, so the majority of the molecules do not experience enhanced field intensity. Here, we present a nanophotonic sensor design with passive molecule trapping functionality. When an analyte solution droplet is introduced to the sensor surface and gradually evaporates, the device structure can effectively trap most precipitated analyte molecules in its hot spots, significantly enhancing the sensor spectral response and sensitivity performance. Specifically, our sensors produce a reflection change of a few percentage points in response to trace amounts of the amino-acid proline or glucose precipitate with a picogram-level mass, which is significantly less than the mass of a molecular monolayer covering the same measurement area. The demonstrated strategy for designing optical sensor structures may also be applied to sensing nano-particles such as exosomes, viruses, and quantum dots.

Existing techniques for optical trapping and manipulation of microscopic objects, such as optical tweezers and plasmonic tweezers, are mostly based on visible and near‐infrared light sources. As it is in general more difficult to confine light to a specific length scale at a longer wavelength, these optical trapping and manipulation techniques have not been extended to the mid‐infrared spectral region or beyond. Here, it is shown that by taking advantage of the fact that many materials have large permittivity dispersions in the mid‐infrared region, optical trapping and manipulation using mid‐infrared excitation can achieve additional functionalities and benefits compared to the existing techniques in the visible and near‐infrared regions. In particular, it is demonstrated that by exploiting the exceedingly high field confinement and large frequency tunability of mid‐infrared graphene plasmonics, high‐performance and versatile mid‐infrared plasmonic tweezers can be realized to selectively trap or repel nanoscale objects of different materials in a dynamically reconfigurable way. This new technique can be utilized for sorting, filtering, and fractionating nanoscale objects in a mixture.