Correction: Jaik et al. Photomotion of Hydrogels with Covalently Attached Azo Dye Moieties-Thermoresponsive and Non-Thermoresponsive Gels. Gels 2022, 8, 541.

In the original publication [...].

Photomotion of Hydrogels with Covalently Attached Azo Dye Moieties-Thermoresponsive and Non-Thermoresponsive Gels.

The unique photomotion of azo materials under irradiation has been in the focus of research for decades and has been expanded to different classes of solids such as polymeric glasses, liquid crystalline materials, and elastomers. In this communication, azo dye-containing gels are obtained by photocrosslinking of non-thermoresponsive and lower critical solution temperature type thermoresponsive copolymers. These are analysed with light microscopy regarding their actuation behaviour under laser irradiation. The influences of the cloud-point temperature and of the laser power are investigated in a series of comparative experiments. The thermoresponsive hydrogels show more intense photoactuation when the cloud-point temperature of the non-crosslinked polymer is above, but closer to, room temperature, while higher laser powers lead to stronger motion, indicating a photothermal mechanism. In non-thermoresponsive gels, considerably weaker photoactuation occurs, signifying a secondary mechanism that is a direct consequence of the optical field-azo dye interaction.

Biosensing with a scanning planar Yagi-Uda antenna.

We investigate a model bioassay in a liquid environment using a z-scanning planar Yagi-Uda antenna, focusing on the fluorescence collection enhancement of ATTO-647N dye conjugated to DNA (deoxyribonucleic acid) molecules. The antenna changes the excitation and the decay rates and, more importantly, the emission pattern of ATTO-647N, resulting in a narrow emission angle (41°) and improved collection efficiency. We efficiently detect immobilized fluorescently-labeled DNA molecules, originating from solutions with DNA concentrations down to 1 nM. In practice, this corresponds to an ensemble of fewer than 10 ATTO-647N labeled DNA molecules in the focal area. Even though we use only one type of biomolecule and one immobilization technique to establish the procedure, our method is versatile and applicable to any immobilized, dye-labeled biomolecule in a transparent solid, air, or liquid environment.

Ultrafast single-photon detection at high repetition rates based on optical Kerr gates under focusing: erratum.

Two typos are corrected, and the linear refractive index n is removed from the expressions of the phase shift in Opt. Lett.46, 560 (2021)OPLEDP0146-959210.1364/OL.414895. The removal of n reduces the gate efficiency, but it does not affect the general findings. Here, we present the corrected equations and the corresponding new numerical results, showing that increasing the pulse energy from 1.8 nJ to 4 nJ leads to nearly the same results of Opt. Lett.46, 560 (2021)OPLEDP0146-959210.1364/OL.414895.

Ultrafast single-photon detection at high repetition rates based on optical Kerr gates under focusing.

The ultrafast detection of single photons is currently restricted by the limited time resolution (a few picoseconds) of the available single-photon detectors. Optical gates offer a faster time resolution, but so far they have been applied mostly to ensembles of emitters. Here, we demonstrate through a semi-analytical model that the ultrafast time-resolved detection of single quantum emitters can be possible using an optical Kerr shutter at gigahertz rates under focused illumination. This technique provides sub-picosecond time resolution, while keeping a gate efficiency at around 85%. These findings lay the ground for future experimental investigations on the ultrafast dynamics of single quantum emitters, with implications for quantum nanophotonics and molecular physics.

Focused Gaussian beam in the paraxial approximation.

A focused Gaussian beam represents a case of highly practical importance in many areas of optics and photonics. We derive analytical expressions for a focused Gaussian beam in the paraxial approximation, considering an arbitrary lens filling factor. We discuss the role of higher-order Bessel functions of the first kind in defining the electric field in the focal region.

Planar antenna designs for efficient coupling between a single emitter and an optical fiber.

Fluorescence detection is a well-established readout method for sensing, especially for in-vitro diagnostics (IVD). A practical way to guide the emitted signal to a detector is by means of an optical fiber. However, coupling fluorescence into a fiber is challenging and commonly lacks single-molecule sensitivity. In this work, we investigate specific fiber geometries, materials and coatings that in combination with a planar Yagi-Uda antenna reach efficient excitation and collection. The simulation of a practical setting determines more than 70% coupling efficiency for a horizontally oriented dipole, with respect to the planar antenna, emitting at 700 nm and embedded in polyvinyl alcohol (PVA). Moreover, the coupling efficiency would only scale by a factor of 2/3 for emitters with random orientation, as a result of the antenna geometry. These findings are relevant for single-molecule detection with fiber optics and have implications for other applications involving the coupling of light with nano-scale sources and detectors. Scanning the surface of a sample with such fibers could also be advantageous for imaging techniques to provide a low background noise and a high resolution.

Plasmon-Assisted Suppression of Surface Trap States and Enhanced Band-Edge Emission in a Bare CdTe Quantum Dot.

Colloidal quantum dots have emerged as a versatile photoluminescent and optoelectronic material. Limitations like fluorescence intermittency, nonradiative Auger recombination, and surface traps are commonly addressed by growing a wide-band-gap shell. However, the shell isolates the excitonic wave function and reduces its interaction with the external environment necessary for different applications. Furthermore, their long emission lifetime hinders their use in high-speed optoelectronics. Here, we demonstrate a high degree of control on the photophysics of a bare core CdTe quantum dot solely by plasmon coupling, showing that more than 99% of the surface defect-state emission from a trap-rich quantum dot can be quenched. Moreover, the band-edge state excitonic and biexcitonic emission rates are Purcell enhanced by 1460- and 613-fold, respectively. Our findings show how plasmon coupling on bare quantum dots could make chemical approaches developed for improving their optical properties unnecessary, with implications for nanoscale lasers, light-emitting devices, solar cells, and ultrafast single-photon sources.

Beaming light from a quantum emitter with a planar optical antenna.

The efficient interaction of light with quantum emitters is crucial to most applications in nano and quantum photonics, such as sensing or quantum information processing. Effective excitation and photon extraction are particularly important for the weak signals emitted by a single atom or molecule. Recent works have introduced novel collection strategies, which demonstrate that large efficiencies can be achieved by either planar dielectric antennas combined with high numerical aperture objectives or optical nanostructures that beam emission into a narrow angular distribution. However, the first approach requires the use of elaborate collection optics, while the latter is based on accurate positioning of the quantum emitter near complex nanoscale architectures; hence, sophisticated fabrication and experimental capabilities are needed. Here we present a theoretical and experimental demonstration of a planar optical antenna that beams light emitted by a single molecule, which results in increased collection efficiency at small angles without stringent requirements on the emitter position. The proposed device exhibits broadband performance and is spectrally scalable, and it is simple to fabricate and therefore applies to a wide range of quantum emitters. Our design finds immediate application in spectroscopy, quantum optics and sensing.

Enhancement of the intrinsic fluorescence of adenine using aluminum nanoparticle arrays.

This study demonstrates the metal-enhanced fluorescence of adenine using aluminum nanoparticle arrays in the deep UV range. It achieves the reproducible intensity enhancement of intrinsic fluorescence up to 80 on well-defined aluminum nanoparticle arrays at 257 nm excitation. In addition to a high signal enhancement, a strong modification of the fluorescence emission spectrum of adenine is observed. This study illustrates that the label-free detection of DNA bases and proteins that have low intrinsic fluorescence and absorption bands in the deep UV range can be facilitated using aluminum nanostructures.

Towards deep-UV surface-enhanced resonance Raman spectroscopy of explosives: ultrasensitive, real-time and reproducible detection of TNT.

We report ultrasensitive and label-free detection of 2,4,6-trinitrotoluene (TNT) deposited by drop coating using deep-ultraviolet surface-enhanced resonance Raman scattering (DUV-SERRS). Well-defined aluminum nanoparticle arrays as the SERRS substrate at 257 nm excitation wavelength enabled highly reproducible and real-time detection of TNT down to the detection limit of the attogram level in quantity. This extreme sensitivity can be further improved by optimization of the nanostructured substrates. DUV-SERRS promises to have a large impact on public safety and security, as it can be readily extended to other explosives and hazardous materials.

Large suppression of quantum fluctuations of light from a single emitter by an optical nanostructure.

We investigate the reduction of the electromagnetic field fluctuations in resonance fluorescence from a single emitter coupled to an optical nanostructure. We find that such hybrid systems can lead to the creation of squeezed states of light, with quantum fluctuations significantly below the shot-noise level. Moreover, the physical conditions for achieving squeezing are strongly relaxed with respect to an emitter in free space. A high degree of control over squeezed light is feasible both in the far and near fields, opening the pathway to its manipulation and applications on the nanoscale with state-of-the-art setups.

Coherent interaction of light with a metallic structure coupled to a single quantum emitter: from superabsorption to cloaking.

We provide a general theoretical platform based on quantized radiation in absorptive and inhomogeneous media for investigating the coherent interaction of light with material structures in the immediate vicinity of quantum emitters. In the case of a very small metallic cluster, we demonstrate extreme regimes where a single emitter can either counteract or enhance particle absorption by 3 orders of magnitude. For larger structures, we show that an emitter can eliminate both scattering and absorption and cloak a plasmonic antenna. We provide physical interpretations of our results and discuss their applications in active metamaterials and quantum plasmonics.

Photophysics of single silicon vacancy centers in diamond: implications for single photon emission.

Single silicon vacancy (SiV) color centers in diamond have recently shown the ability for high brightness, narrow bandwidth, room temperature single photon emission. This work develops a model describing the three level population dynamics of single SiV centers in diamond nanocrystals on iridium surfaces including an intensity dependent de-shelving process. Furthermore, we investigate the brightness and photostability of single centers and find maximum single photon rates of 6.2 Mcps under continuous excitation. We investigate the collection efficiency of the fluorescence and estimate quantum efficiencies of the SiV centers.

Metallodielectric hybrid antennas for ultrastrong enhancement of spontaneous emission.

We devise new optical antennas that reduce the excited-state radiative lifetimes of emitters to the order of 100 fs while maintaining quantum efficiencies of about 80% at a broadband operation. Here, we combine metallic nanoparticles with planar dielectric structures and exploit design strategies from plasmonic nanoantennas and concepts from cavity quantum electrodynamics to maximize the local density of states and minimize the nonradiative losses incurred by the metallic constituents. The proposed metallodielectric hybrid antennas promise an important impact on various fundamental and applied research fields, including photophysics, ultrafast plasmonics, bright single-photon sources, and Raman spectroscopy.

Deep-UV surface-enhanced resonance Raman scattering of adenine on aluminum nanoparticle arrays.

We report the ultrasensitive detection of adenine using deep-UV surface-enhanced resonance Raman scattering on aluminum nanostructures. Well-defined Al nanoparticle arrays fabricated over large areas using extreme-UV interference lithography exhibited sharp and tunable plasmon resonances in the UV and deep-UV wavelength ranges. Theoretical modeling based on the finite-difference time-domain method was used to understand the near-field and far-field optical properties of the nanoparticle arrays. Raman measurements were performed on adenine molecules coated uniformly on the Al nanoparticle arrays at a laser excitation wavelength of 257.2 nm. With this technique, less than 10 amol of label-free adenine molecules could be detected reproducibly in real time. Zeptomole (~30,000 molecules) detection sensitivity was readily achieved proving that deep-UV surface-enhanced resonance Raman scattering is an extremely sensitive tool for the detection of biomolecules.

Optical antennas as nanoscale resonators.

Recent progress in nanotechnology has enabled us to fabricate sub-wavelength architectures that function as antennas for improving the exchange of optical energy with nanoscale matter. We describe the main features of optical antennas for enhancing quantum emitters and review the designs that increase the spontaneous emission rate by orders of magnitude from the ultraviolet up to the near-infrared spectral range. To further explore how optical antennas may lead to unprecedented regimes of light-matter interactions, we draw a relationship between metal nanoparticles, radio-wave antennas and optical resonators. Our analysis points out how optical antennas may function as nanoscale resonators and how these may offer unique opportunities with respect to state-of-the-art microcavities.

Nanofocusing radially-polarized beams for high-throughput funneling of optical energy to the near field.

We theoretically show that a weakly-focused radially polarized beam can excite surface-plasmon-polaritons in metal nanowires and nanocones with efficiencies of the order of 90% and large bandwidths. The coupling mechanism relies on the formation of a standing wave on the nanowire facet, which imposes a relationship between the operating wavelength and the nanowire radius. An immediate application of this finding is nanofocusing of optical energy for implementations of ultra-fast and high-throughput linear and nonlinear nanoscopies, optical nanolithographies, quantum nano-optics and photochemistry at the nanoscale.

Magnetic metamaterials in the blue range using aluminum nanostructures.

We report an experimental and theoretical study of the optical properties of two-dimensional arrays of aluminum nanoparticle in-tandem pairs. Plasmon resonances and effective optical constants of these structures are investigated, and strong magnetic response as well as negative permeability is observed down to 400 nm wavelength. Theoretical calculations based on the finite-difference time-domain method are performed for various particle dimensions and lattice parameters, and are found to be in good agreement with the experimental findings. The results show that metamaterials operating across the whole visible wavelength range are feasible.

Highly efficient interfacing of guided plasmons and photons in nanowires.

Successful exploitations of strongly confined surface plasmon-polaritons critically rely on their efficient and rapid conversion to lossless channels. We demonstrate a simple, robust, and broadband butt-coupling technique for connecting a metallic nanowire and a dielectric nanofiber. Conversion efficiencies above 95% in the visible and close to 100% in the near-infrared can be achieved with realistic parameters. Moreover, by combining butt-coupling with nanofocusing, we propose a broadband high-throughput near-field optical microscope.