Infrared spectroscopy is a powerful tool widely used in research and industry for a label-free and unambiguous identification of molecular species. Inconveniently, its application to spectroscopic analysis of minute amounts of materials, for example, in sensing applications, is hampered by the low infrared absorption cross-sections. Surface-enhanced infrared spectroscopy using resonant metal nanoantennas, or short "resonant SEIRA", overcomes this limitation. Resonantly excited, such metal nanostructures feature collective oscillations of electrons (plasmons), providing huge electromagnetic fields on the nanometer scale. Infrared vibrations of molecules located in these fields are enhanced by orders of magnitude enabling a spectroscopic characterization with unprecedented sensitivity. In this Review, we introduce the concept of resonant SEIRA and discuss the underlying physics, particularly, the resonant coupling between molecular and antenna excitations as well as the spatial extent of the enhancement and its scaling with frequency. On the basis of these fundamentals, different routes to maximize the SEIRA enhancement are reviewed including the choice of nanostructures geometries, arrangements, and materials. Furthermore, first applications such as the detection of proteins, the monitoring of dynamic processes, and hyperspectral infrared chemical imaging are discussed, demonstrating the sensitivity and broad applicability of resonant SEIRA.

We demonstrate the use of femtosecond direct laser writing lithography for a fast and homogeneous large-area fabrication of plasmonic nanoantennas on a substrate by creating a patterned polymer as an etch mask on a metal layer. Subsequent argon ion beam etching provides plasmonic nanoantennas with feature sizes below the diffraction limit of the laser light. They exhibit tunable high-quality plasmon resonances in the mid-infrared spectral range, which are ideally suited for surface-enhanced infrared absorption (SEIRA). In the present work, we demonstrate reliable, fast, and low-cost fabrication of a wide variety of antenna arrays and examine particularly the influence of plasmonic coupling between neighboring antennas on the SEIRA enhancement effect. Specifically, we measure the enhanced infrared vibrational bands of a 5 nm thick 4,4′-bis(N-carbazolyl)-1,1′-biphenyl layer evaporated on arrays with different longitudinal and transversal spacings between antennas. An optimum SEIRA enhancement per antenna of 4 orders of magnitude is found close to the collective plasmon excitation in the nanoantenna array, rather than at the highest antenna density. Our method establishes a low-cost replacement technique for electron beam lithography. Simple, fast, and straightforward fabrication of optimized SEIRA antenna arrays with cm 2 areas, which can be used in real-world applications such as chemical and biological vibrational sensing, is now possible.

Highly efficient counter-propagating fiber-based optical traps are presented which utilize converging beams from fibers with 3D printed diffractive Fresnel lenses on their facet. The use of a converging beam instead of diverging beam in dual-fiber traps creates a strong trapping efficiency in both the axial and the transverse directions. Converging beams with a numerical aperture of up to 0.7 are produced by diffractive Fresnel lenses. These lenses also provide a large focal distance of up to 200 μm in a moderately high refractive index medium. Fabrication of such diffractive lenses with microsized features at the tip of a fiber is possible by femtosecond two photon lithography. In comparison to chemically etched fiber tips, the normalized trap stiffness of dual-fiber tweezers is increased by a substantial factor of 35−50 when using a converging beam produced by diffractive Fresnel lenses. The large focal length provided by these diffractive structures allows working at a large fiber-to-fiber distance, which leads to larger space and the freedom to combine other spectroscopy and analytical methods in combination with trapping.

We demonstrate second-harmonic generation with ultranarrow resonances in hybrid plasmon-fiber cavities, formed by depositing single-crystalline gold nanorods onto the surface of tapered microfibers with diameters in the range of 1.7-1.8 μm. The localized surface plasmon mode of the single gold nanorod efficiently couples with a whispering gallery mode of the fiber, resulting in a very narrow hybrid plasmon-fiber resonance with a high quality factor Q of up to 250. When illuminated with a tunable 100 fs laser, a sharp SHG peak narrower than half of the spectral width of the impinging laser emerges, superimposed on a broad multiphoton photoluminescence background. The enhancement of the SHG peak of the hybrid system is typically 1000-fold when compared to that of a single gold nanorod alone. Tuning the laser over the hybrid resonance enables second-harmonic spectroscopy and yields an ultranarrow line width as small as 6.4 nm. We determine the second-harmonic signal to rise with the square of the laser power, while the multiphoton photoluminescence background rises with powers between 4 and 6, indicating a very efficient higher-order process. A coupled anharmonic oscillator model is able to describe the linear as well as second-harmonic resonances very well. Our work will open the door to the simultaneous utilization of narrow whispering gallery resonances together with high plasmonic near-field enhancement and should allow for nonlinear sensing and extremely efficient nonlinear light generation from ultrasmall volumes.

Weak vibrational signals in the infrared and terahertz spectral region can be enhanced by orders of magnitude when employing the electromagnetic near fields of plasmonic nanostructures. This approach is known as antenna-assisted surface-enhanced infrared absorption (SEIRA) and allows for a broad range of possible sensing applications. In the present work, we investigate the scaling of the SEIRA enhancement with wavelength, particularly toward the molecular fingerprint region (500−1500 cm −1 ). We apply the concept of SEIRA to perform resonant antenna-enhanced spectroscopy of molecules in a spectral range from 4.5 to 45 THz (6.7−67 μm wavelength, 150−1500 cm −1 ) using a standard Fourier transform infrared spectrometer. We fabricate arrays of rectangular gold antennas by electronbeam lithography and coat them with 30 nm thick layers of the fullerenes C 60 and C 70 , respectively. For the single digit THz measurements, we utilize spin-coated amino acids, particularly threonine. The resonances of the structures are tailored to spectrally match the molecular absorption features. An increased SEIRA enhancement of 2 orders of magnitude is found for antennas resonant at 6.7 THz when compared to 45 THz, corresponding to a λ 3 scaling over a frequency range of 1 order of magnitude. This scaling behavior is in excellent agreement with both numerical simulations and classical antenna theory. Further increase toward the single-digit THz region will yield the potential for ultrasensitive THz spectroscopy.