Enhanced chirality induced in a composite structure consisting of α-MoO3 film and a silver metasurface.

Chiral structures have been widely used in many fields, such as biosensing and analytical chemistry. In this paper, the chiral response of a composite structure consisting of α-M o O 3 film and a silver (Ag) metasurface is studied. First, the effect of the thickness of α-M o O 3 film on the circular dichroism (CD) is discussed, and it is found that CD can reach 0.93 at a wavelength of 9.6 µm when the thickness of α-M o O 3 film is 6.075 µm. To better understand the physical mechanism, we analyze the transverse electric and transverse magnetic wave components in the transmitted wave for the whole structure and each layer. One can see that the strong chirality of the structure is attributed to the polarization conversion of α-M o O 3 film and the selective transmissivity of Ag ribbons. In addition, the influence of the filling factor of the Ag ribbons on chirality is also studied. This work combines hyperbolic material α-M o O 3 with Ag ribbons to enhance CD. Also, it provides greater freedom in the tuning of chirality. We believe that this work not only deepens the understanding of the chiral response of anisotropic materials, but also gives promise for its applications in the fields of polarization optics and biosensing.

Detection of Green Asparagus in Complex Environments Based on the Improved YOLOv5 Algorithm.

An improved YOLOv5 algorithm for the efficient recognition and detection of asparagus with a high accuracy in complex environments was proposed in this study to realize the intelligent machine harvesting of green asparagus. The coordinate attention (CA) mechanism was added to the backbone feature extraction network, which focused more attention on the growth characteristics of asparagus. In the neck part of the algorithm, PANet was replaced with BiFPN, which enhanced the feature propagation and reuse. At the same time, a dataset of asparagus in complex environments under different weather conditions was constructed, and the performance variations of the models with distinct attention mechanisms and feature fusion networks were compared through experiments. Experimental results showed that the mAP@0.5 of the improved YOLOv5 model increased by 4.22% and reached 98.69%, compared with the YOLOv5 prototype network. Thus, the improved YOLOv5 algorithm can effectively detect asparagus and provide technical support for intelligent machine harvesting of asparagus in different weather conditions and complex environments.

A pixelated frequency-agile metasurface for broadband terahertz molecular fingerprint sensing.

Terahertz (THz) plasmonic resonance based on an arbitrarily designed resonance metasurface is the key technique of choice for enhancing fingerprint absorption spectroscopy identification of biomolecules. Here, we report a broadband THz micro-photonics sensor based on a pixelated frequency-agile metasurface and illustrate its application ability to enhance and differentiate the detection of broadband absorption fingerprint spectra. The design uses symmetrical metal C-shape resonators with the functional graphene micro-ribbons selectively patterned into the gaps. A strong electric resonance with a high quality factor was formed, consisting of an electric dipole mode associated with the excitation of a dark toroidal dipole (TD) mode through the coupling from the electric dipole moment of the individual frequency-agile meta-unit. The resonance positions are nearly linearly modulated with the varying Fermi level of graphene. The configuration arranges a certain metapixel of the metasurface to multiple response spectra assembling a one-to-many mapping between spatial and spectral information which is instrumental in greatly shrinking the actual size of the sensor. By the synchronous regulation of graphene and C-shape rings, we have obtained highly surface-sensitive resonances over a wide spectral range (∼1.5 THz) with a spectral resolution less than 20 GHz. The target multiple enhanced absorption spectrum of glucose molecules is read out in a broadband region with high sensitivity. More importantly, the design can be extended to cover a larger spectral region by altering the range of geometrical parameters. Our microphotonic technique can resolve absorption fingerprints without the need for spectrometry and frequency scanning, thereby providing an approach for highly sensitive and versatile miniaturized THz spectroscopy devices.

Investigation of a new graphene strain sensor based on surface plasmon resonance.

The high confinement of surface plasmon polaritons in graphene nanostructures at infrared frequencies can enhance the light-matter interactions, which open up intriguing possibilities for the sensing. Strain sensors have attracted much attention due to their unique electromechanical properties. In this paper, a surface plasmon resonance based graphene strain sensor is presented. The considered sensing platform consists of arrays of graphene ribbons placed on a flexible substrate which enables efficient coupling of an electromagnetic field into localized surface plasmons. When the strain stretching is applied to the configuration, the localized surface plasmon resonance frequency sensitively shift. The strain is then detected by measuring the frequency shifts of the localized plasmon resonances. This provides a new optical method for graphene strain sensing. Our results show that the tensile direction is the key parameter for strain sensing. Besides, the sensitivity and the figure of merit were calculated to evaluate the performance of the proposed sensor. The calculated figure of merit can be up to two orders of magnitude, which could be potentially useful from a practical point of view.

Cathodoluminescence nanoscopy of open single-crystal aluminum plasmonic nanocavities.

Exact understanding of the plasmon response of aluminum (Al) nanostructures in deep subwavelengths is critical for the design of Al based plasmonic applications, such as the emission control of quantum dots and surface-enhanced resonance Raman scattering in the ultraviolet (UV) range. Here, the plasmonic properties of open triangle cavities patterned by a focused ion beam in single-crystal bulk Al were explored using cathodoluminescence. The resonant modes were determined by experimental spectra and deep subwavelength real-space mode patterns ranging from the visible to the UV, which agreed well with full-wave electromagnetic simulations. The dispersion relation of the cavity modes was consistent with that at the interface between Al and vacuum, showing strong electromagnetic field confinement in the cavities. Open Al triangle cavities provided room for the interaction between optical emitters and confined electromagnetic fields, paving the way for plasmonic devices for a variety of applications, such as plasmonic light-emitting devices or nanolasers in the UV range.

Dynamic spontaneous emission control of an optical emitter coupled to plasmons in strained graphene.

Spontaneous emission control of an optical emitter is critical for many applications, such as in the fields of sensing, integrated photonics and quantum optics. Integrating optical emitters with a mechanical system can provide an avenue for strain sensors as well. Here, the dynamic spontaneous emission modification of an emitter coupled to graphene by uniaxial strain is demonstrated. Our results show that the emission rate can be controlled by tuning the strain of graphene, which depends on the polarized orientation of the emitter. More specifically, the decay rate can be enhanced for several times if the emitter is polarized perpendicular to graphene under strain. Azimuthal angle dependent oscillation of decay rate exists for the emitter polarized parallel to the graphene. Moreover, the controllable decay of the emitter comes from the anisotropic plasmons excitation in strained graphene, which is verified by the corresponding isofrequency contours of plasmons. The strain engineering provides a new platform for dynamic spontaneous emission modulation of emitters coupled with graphene, which opens up intriguing possibilities for the design of strain sensors and quantum devices.

Unidirectional excitation of graphene plasmons in Au-graphene composite structures by a linearly polarized light beam.

Controllable manipulation of propagating surface plasmon polaritons is critical in plasmonics and important for nanophotonic applications. Here, we demonstrate theoretically that graphene plasmons (GPs) can be unidirectionally excited in an Au-graphene composite structure by a linearly polarized optical wave at the wavelength of 10.2 µm. The unidirectional ratio can reach as large as 900 with the incidence angle at 37.7° off normal, which is obtained by the angular spectrum of GPs. Moreover, the physical mechanism behind the unidirectional excitation is revealed to be the interference between anti-symmetric and symmetric amplitude distributions of GPs, which are induced by the gold rod antenna under the normal and grazing illuminations, respectively.

Substrate carrier concentration dependent plasmon-phonon coupled modes at the interface between graphene and semiconductors.

The coupled modes between graphene plasmons and surface phonons of a semiconductor substrate are investigated, which can be efficiently controlled by carrier injection of the substrate. A new physical mechanism on tuning plasmon-phonon coupled modes (PPCMs) is proposed due to the fact that the energy and lifetime of substrate surface phonons depend a lot on the carrier concentration. Specifically, the change of dispersion and lifetime of PPCMs can be controlled by the carrier concentration of the substrate. The energy of PPCMs for a given momentum increases as the carrier concentration of the substrate increases. On the other hand, the momentum of PPCMs for a given energy decreases when the carrier concentration of the substrate increases. The lifetime of PPCMs is always larger than the intrinsic lifetime of graphene plasmons without plasmon-phonon coupling.

Mid-infrared plasmon induced transparency in heterogeneous graphene ribbon pairs.

The control of coherent phenomena in graphene structures is proposed. Specifically, plasmon induced transparency (PIT) effect is investigated in a kind of simple graphene structures - graphene ribbon pairs. The transparency effect are understood by the mode coupling between dipolar and quadrupole plasmons modes in graphene ribbons. By using bias voltage tuning or geometry parameters changing, the PIT effect can be effectively controlled, which is based on the frequency tuning of dipolar or quadrupole modes in ribbons. These properties make these structures possess applications in two-dimensional plasmonics devices in mid-infrared range. In addition, the tuning of PIT in graphene ribbon pairs opens an avenue for active coherent control in plasmonics.

Surface plasmon modes in graphene wedge and groove waveguides.

Surface plasmon modes at terahertz-infrared waveband in subwavelength graphene wedge and groove waveguides are investigated, which can be categorized into perfect electric conductor and perfect magnetic conductor symmetric modes with different propagation characteristics. The electromagnetic near-fields are localized strongly in different regions for these two kinds of modes. Moreover, these modes can be interpreted by the folded graphene ribbon modes. The brim width of the waveguides and the Fermi energy of the graphene strongly influence the dispersion and propagation distances of the plasmon modes, which can be used for tuning the plasmon modes in graphene wedge and groove waveguides efficiently.