Perfect excitation and attenuation-free propagation of graphene surface polaritons under optical admittance matching.

Graphene supports both transverse magnetic and electric modes of surface polaritons due to the intraband and interband transition properties of electrical conductivity. Here, we reveal that perfect excitation and attenuation-free propagation of surface polaritons on graphene can be achieved under the condition of optical admittance matching. With both vanished forward and backward far-field radiation, incident photons are fully coupled to surface polaritons. This requires an exact match between the admittance difference of sandwiching media and the conductivity of graphene, resulting in no decay of propagating surface polaritons. The dispersion relation has a completely different line shape for structures that support compared to those that do not support admittance matching. This work promotes complete comprehension of the excitation and propagation behaviors of graphene surface polaritons and may further inspire ideas for research on surface waves on two-dimensional materials.

Visible-Band Chiroptical Meta-devices with Phase-Change Adjusted Optical Chirality.

Low-cost large-area chirality meta-devices (CMDs) with adjustable optical chirality are of great interest for polarization-sensitive imaging, stereoscopic display, enantioselectivity analysis, and catalysis. Currently, CMDs with adjusted chiroptical responses in the mid-infrared to terahertz band have been demonstrated by exploiting photocarriers of silicon, pressure, and phase-change of GSTs but are still absent in the visible band, which in turn limits the development of chiral nanophotonic devices. Herein, by employing a phase-change material (Sb2S3), we present a protocol for the fabrication of wafer-scale visible-band enantiomeric CMDs with handedness, spectral, and polarization adjustability. As measured by circular dichroism, the chirality signs of CMDs enantiomers can be adjusted with Sb2S3 from amorphous to crystalline, and the chirality resonance wavelength can also be adjusted. Our results suggest a new type of meta-devices with adjustable chiroptical responses that may potentially enable a wide range of chirality nanophotonic applications including highly sensitive sensing and surface-enhanced nanospectroscopy.

Enhancing electromagnetic field gradient in tip-enhanced Raman spectroscopy with a perfect radially polarized beam.

Tip-enhanced Raman spectroscopy (TERS) is a promising label-free super-resolving imaging technique, and the electric field gradient of nanofocusing plays a role in TERS performance. In this paper, we theoretically investigated the enhancement and manipulation of the electric field gradient in a bottom-illumination TERS configuration through a tightly focused perfect radially polarized beam (PRPB). Improvement and manipulation in electric field enhancement and field gradient of the gap-plasmon mode between a plasmonic tip and a virtual surface plasmons (SPs) probe are achieved by adjusting the ring radius of the incident PRPB. Our results demonstrate that the method of optimizing the ring radius of PRPB is to make the illumination angle of incident light as close to the surface plasmon resonance (SPR) excitation angle as possible. Under the excitation of optimal parameters, more than 10 folds improvement of field enhancement and 3 times of field gradient of the gap-plasmon mode is realized compared with that of the conventional focused RPB. By this feat, our results indicate that such a method can further enhance the gradient Raman mode in TERS. We envision that the proposed method, to achieve the dynamic manipulation and enhancement of the nanofocusing field and field gradient, can be more broadly used to control light-matter interactions and extend the reach of tip-enhanced spectroscopy.

Circular nanocavity substrate-assisted plasmonic tip for its enhancement in nanofocusing and optical trapping.

Plasmonic tip nanofocusing has widely been applied in tip-enhanced Raman spectroscopy, optical trapping, nonlinear optics, and super-resolution imaging due to its capability of high local field enhancement. In this work, a substrate with a circular nanocavity is proposed to enhance the nanofocusing and optical trapping characteristics of the plasmonic tip. Under axial illumination of a tightly focused radial polarized beam, the circular nanohole etched on a metallic substrate can form a nanocavity to induce an interference effect and further enhance the electric field intensity. When a plasmonic tip is placed closely above such a substrate, the electric field intensity of the gap-plasmon mode can further be improved, which is 10 folds stronger than that of the conventional gap-plasmon mode. Further analysis reveals that the enhanced gap-plasmon mode can significantly strengthen the optical force exerted on a nanoparticle and stably trap a 4-nm-diameter dielectric nanoparticle. Our proposed method can improve the performance of tip-enhanced spectroscopy, plasmonic tweezers and extend their applications. We anticipate that our methods allow simultaneously manipulating and characterizing single nanoparticles in-situ.

Extracting epsilon-near-zero wavelength of ultrathin plasmonic film.

Strong optical nonlinearities of plasmonic thin films exist at their epsilon-near-zero (ENZ) wavelengths, which are essential to be acquired first for the design and fabrication of ENZ photonic devices. However, it has been challenging to obtain the ENZ wavelength precisely when the film thickness is reduced to tens of nanometers or less. By enhancing both electric field intensity and light-matter interaction distance in the film, we propose that the ENZ wavelength and the medium model of ultrathin films can be extracted accurately from the transmittance and reflectance spectra under oblique light excitation. A characteristic valley in the transmittance spectrum, which originates from the increased light absorption caused by the ENZ electric field enhancement, can be used to determine the ENZ wavelength with significantly improved fitting accuracy of the Drude parameters. The work in this paper provides an accurate and effective method for the acquisition of ENZ wavelength and will contribute to the research of nonlinear plasmonic devices.

Refractive index sensing and imaging based on polarization-sensitive graphene.

Graphene exhibits extraordinary opto-electronic properties due to its unique dynamic conductivity, bringing great value in optical sensing, surface plasmon modulation and photonic devices. Based on the polarization-sensitive absorption of graphene working at near infrared to ultraviolet wavelengths, we theoretically investigate the refractive index sensing and imaging mechanism under oblique and tight focusing incidences of light respectively. We demonstrate that such graphene-based methods can provide ultrahigh refractive index resolution (∼2.09×10-8 RIU) for label-free sensing, and high transverse spatial resolution (∼200 nm) and large longitudinal detecting length (∼750 nm) for imaging under 532 nm incident wavelength. The proposed methods could potentially guide future researches in graphene optical detection, non-invasive biological sensing and imaging, and other applications.

High-performance imaging of cell-substrate contacts using refractive index quantification microscopy.

Non-invasive imaging of living cells is an advanced technique that is widely used in the life sciences and medical research. We demonstrate a refractive index quantification microscopy (RIQM) that enables label-free studies of glioma cell-substrate contacts involving cell adhesion molecules and the extracellular matrix. This microscopy takes advantage of the smallest available spot created when an azimuthally polarized perfect optical vortex beam (POV) is tightly focused with a first-order spiral phase, which results in a relatively high imaging resolution among biosensors. A high refractive index (RI) resolution enables the RI distribution within neuronal cells to be monitored. The microscopy shows excellent capability for recognizing cellular structures and activities, demonstrating great potential in biological sensing and live-cell kinetic imaging.

Graphene-Based Confocal Refractive Index Microscopy for Label-Free Differentiation of Living Epithelial and Mesenchymal Cells.

Label-free imaging and investigation of living cells are significant for many biomedical studies. It has been challenging to detect the epithelial-mesenchymal transition of cells in situ without affecting cellular activity. Here, we present a common-path differential confocal microscope based on the polarization-sensitive absorption of graphene to realize high-performance refractive index imaging and differentiation of living colorectal cancer cells (HCT116) with large detecting depth (1.29 µm), excellent refractive index resolution (2.86 × 10-5 RIU), and high spatial resolution (727 nm) simultaneously. Compared with epithelial (parental HCT116) cells, mesenchymal (paclitaxel-resistant HCT116) cells manifest generally lower refractive index values through the refractive index statistics, which is due to the stronger migration ability and weaker surface adherence of mesenchymal cells. The graphene-based microscopy provides an effective label-free approach to high-resolution imaging and study of living cell kinetics, and we expect it to be widely used in the research fields of pathology, tumorigenesis, and chemotherapy.

Real-time subcellular imaging based on graphene biosensors.

Non-invasive living cell microscopy in real time is essential for a wide variety of biomedical research. Here, we present a subcellular refractive index imaging technique for living cells based on a graphene biosensor system. Owing to the optical reflectivity differences of graphene to s- and p-polarizations, a 45° generalized-cylindrical-vector-polarized laser beam is employed to demodulate the reflected cylindrical vector beam for differential detecting. Benefitting from the vector beam-enabled common-path graphene biosensor, the imaging spatial resolution and refractive index sensitivity are noticeably improved. Subcellular refractive index mapping of live human colonic cancer cells is perfectly achieved without inducing any cell damage. Furthermore, real-time monitoring of an individual cell is also performed with the disassembly of the cell nucleolus clearly observed. This technique would be a promising tool for the study of living cell morphology, kinetics, and pathology, and for other biomedical research.