Infrared and optical spectroscopy represents one of the most informative methods in advanced materials research. As an important branch of modern optical techniques that has blossomed in the past ...decade, scattering‐type scanning near‐field optical microscopy (s‐SNOM) promises deterministic characterization of optical properties over a broad spectral range at the nanoscale. It allows ultrabroadband optical (0.5–3000 µm) nanoimaging, and nanospectroscopy with fine spatial (<10 nm), spectral (<1 cm−1), and temporal (<10 fs) resolution. The history of s‐SNOM is briefly introduced and recent advances which broaden the horizons of this technique in novel material research are summarized. In particular, this includes the pioneering efforts to study the nanoscale electrodynamic properties of plasmonic metamaterials, strongly correlated quantum materials, and polaritonic systems at room or cryogenic temperatures. Technical details, theoretical modeling, and new experimental methods are also discussed extensively, aiming to identify clear technology trends and unsolved challenges in this exciting field of research.
Scattering‐type scanning near‐field optical microscopy (s‐SNOM) allows ultrabroadband optical (0.5–3000 µm) nanoimaging and nanospectroscopy with fine spatial (<10 nm), spectral (<1 cm−1), and temporal (<10 fs) resolution. Recent progress in the s‐SNOM technique and its application in material research at room and cryogenic temperatures is summarized.
Infrared spectroscopy, especially for molecular vibrations in the fingerprint region between 600 and 1,500 cm(-1), is a powerful characterization method for bulk materials. However, molecular ...fingerprinting at the nanoscale level still remains a significant challenge, due to weak light-matter interaction between micron-wavelengthed infrared light and nano-sized molecules. Here we demonstrate molecular fingerprinting at the nanoscale level using our specially designed graphene plasmonic structure on CaF2 nanofilm. This structure not only avoids the plasmon-phonon hybridization, but also provides in situ electrically-tunable graphene plasmon covering the entire molecular fingerprint region, which was previously unattainable. In addition, undisturbed and highly confined graphene plasmon offers simultaneous detection of in-plane and out-of-plane vibrational modes with ultrahigh detection sensitivity down to the sub-monolayer level, significantly pushing the current detection limit of far-field mid-infrared spectroscopies. Our results provide a platform, fulfilling the long-awaited expectation of high sensitivity and selectivity far-field fingerprint detection of nano-scale molecules for numerous applications.
Due to the two-dimensional character of graphene, the plasmons sustained by this material have been invariably studied in supported samples so far. The substrate provides stability for graphene but ...often causes undesired interactions (such as dielectric losses, phonon hybridization, and impurity scattering) that compromise the quality and limit the intrinsic flexibility of graphene plasmons. Here, we demonstrate the visualization of plasmons in suspended graphene at room temperature, exhibiting high-quality factor Q~33 and long propagation length > 3 μm. We introduce the graphene suspension height as an effective plasmonic tuning knob that enables in situ change of the dielectric environment and substantially modulates the plasmon wavelength, propagation length, and group velocity. Such active control of micrometer plasmon propagation facilitates near-unity-order modulation of nanoscale energy flow that serves as a plasmonic switch with an on-off ratio above 14. The suspended graphene plasmons possess long propagation length, high tunability, and controllable energy transmission simultaneously, opening up broad horizons for application in nano-photonic devices.
A new hybridized plasmon–phonon polariton mode in graphene/h‐BN van der Waals heterostructures is presented, featuring the ultrahigh field confinement characteristic of the graphene plasmon and the ...long lifetime property of the h‐BN transverse optical phonon. This enables an ultralong hybrid plasmon lifetime of up to 1.6 ps (with ultrahigh mode confinement up to >l02/7000 and ultrasmall group velocity down to 0.001c, where c is the speed of light in vacuum), superior to any localized plasmon ever demonstrated.
Most van der Waals crystals present highly anisotropic optical responses due to their strong in-plane covalent bonding and weak out-of-plane interactions. However, the determination of the ...polarization-dependent dielectric constants of van der Waals crystals remains a nontrivial task, since the size and dimension of the samples are often below or close to the diffraction limit of the probe light. In this work, we apply an optical nano-imaging technique to determine the anisotropic dielectric constants in representative van der Waals crystals. Through the study of both ordinary and extraordinary waveguide modes in real space, we are able to quantitatively determine the full dielectric tensors of nanometer-thin molybdenum disulfide and hexagonal boron nitride microcrystals, the most-promising van der Waals semiconductor and dielectric. Unlike traditional reflection-based methods, our measurements are reliable below the length scale of the free-space wavelength and reveal a universal route for characterizing low-dimensional crystals with high anisotropies.
Cherenkov radiation (CR) excited by fast charges can serve as on-chip light sources with a nanoscale footprint and broad frequency range. The reversed CR, which usually occurs in media with the ...negative refractive index or negative group-velocity dispersion, is highly desired because it can effectively separate the radiated light from fast charges thanks to the obtuse radiation angle. However, reversed CR at the mid-infrared remains challenging due to the significant loss of conventional artificial structures. Here we observe mid-infrared analogue polaritonic reversed CR in a natural van der Waals (vdW) material (i.e., α-MoO
), whose hyperbolic phonon polaritons exhibit negative group velocity. Further, the real-space image results of analogue polaritonic reversed CR indicate that the radiation distributions and angles are closely related to the in-plane isofrequency contours of α-MoO
, which can be further tuned in the heterostructures based on α-MoO
. This work demonstrates that natural vdW heterostructures can be used as a promising platform of reversed CR to design on-chip mid-infrared nano-light sources.
Flexible plasmonic devices with electrical tunability are of great interest for diverse applications, such as flexible metamaterials, waveguide transformation optics, and wearable sensors. However, ...the traditional flexible metal–polymer plasmonic structures suffer from a lack of electrical tunability. Here the first flexible, electrically tunable, and strain‐independent plasmons based on graphene–mica heterostructures are experimentally demonstrated. The resonance frequency, strength, quality factor, electrical tunability, and lifetime of graphene plasmons exhibit no visible change at bending radius down to 1 mm and after 1000 bending cycles at a radius of 3 mm. The plasmon‐enhanced infrared spectroscopy detection of chemicals is also demonstrated to be unaffected in the flexible graphene–mica heterostructures. The results provide the basis for the design of flexible active nanophotonic devices such as plasmonic waveguides, resonators, sensors, and modulators.
Flexible and electrically tunable plasmonic devices are fabricated on graphene–mica heterostructures. The devices display high tolerance to bending and have unchanged plasmon frequency, intensity, quality factor, and tunability with bending radii down to 1 mm. Plasmon‐enhanced infrared absorption is demonstrated on the bending device. The flexible graphene plasmon provides the basis for wearable sensors and flexible active nanophotonic devices.
In this paper, a simple and novel micro-displacement sensor based on tilted fiber Bragg grating (TFBG) is proposed and studied. The transversal compression imposed on TFBG causes the changes of ...grating angle, index, and length, resulting in a considerable spectrum change of the cladding modes. The spectrum amplitude, extinction ratio, and central wavelength are analyzed with the change of the transversal compression. These three parameters are found to have repeatable linear relationship with the transversal load. Moreover, the extinction ratio exhibits a high displacement sensitivity of 18 dB/mm.
Due to weak interactions between micrometer‐wavelength infrared (IR) light and nanosized samples, a high signal to noise ratio is a prerequisite in order to precisely characterize nanosized samples ...using IR spectroscopy. Traditional micrometer‐thick window substrates, however, have considerable IR absorption which may introduce unavoidable deformations and interruptions to IR spectra of nanoscale samples. A promising alternative is the use of a suspended graphene substrate which has ultrahigh IR transmittance (>97.5%) as well as unique mechanical properties. Here, an effective method is presented for fabrication of suspended graphene over circular holes up to 150 µm in diameter to be utilized as a transparent substrate for IR spectroscopy. It is demonstrated that the suspended graphene has little impact on the measured IR spectra, an advantage which has led to the discovery of several missing vibrational modes of a 20 nm thick PEO film measured on a traditional CaF2 substrate. This can provide a better understanding of molecules' fine structures and status of hanging bands. The unique optical properties of suspended graphene are determined to be superior to those of conventional IR window materials, giving this new substrate great potential as part of a new generation of IR transparent substrates, especially for use in examining nanoscale samples.
Large area suspended graphene (150 µm in diameter) is fabricated and utilized as a transparent substrate for IR spectroscopy. It is experimentally demonstrated to have little interference on the measured IR spectra, which displays several missing vibrational modes of the 20 nm thick PEO film when measured on a CaF2 substrate. It holds great potential for next‐generation infrared substrates for nanoscale measurement.
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