Band 8, Heft 6

The coupling between light and collective oscillations of free carriers at metallic surfaces and nanostructures is at the origin of one of the main fields of nanophotonics: plasmonics. The potential applications offered by plasmonics range from biosensing to solar cell technologies and from nonlinear optics at the nanoscale to light harvesting and extraction in nanophotonic devices. Heavily doped semiconductors are particularly appealing for the infrared spectral window due to their compatibility with microelectronic technologies, which paves the way toward their integration in low-cost, mass-fabricated devices. In addition, their plasma frequency can be tuned chemically, optically, or electrically over a broad spectral range. This review covers the optical properties of the heavily doped conventional semiconductors such as Ge, Si, or III–V alloys and how they can be successfully employed in plasmonics. The modeling of their specific optical properties and the technological processes to realize nanoantennas, slits, or metasurfaces are presented. We also provide an overview of the applications of this young field of research, mainly focusing on biosensing and active devices, among the most recent developments in semiconductor plasmonics. Finally, an outlook of further research directions and the potential technological transfer is presented.

Coherent Raman scattering (CRS) techniques are recognized for their ability to induce and detect vibrational coherences in molecular samples. The generation of coherent light fields in CRS produces much stronger signals than what is common in incoherent Raman spectroscopy, while also enabling direct views of evolving molecular vibrations. Despite the attractive attributes of CRS spectroscopy, the technique’s sensitivity is insufficient for performing measurements on single molecules, thus precluding the ability to coherently drive, manipulate and observe individual vibrational quantum oscillators with light. The single-molecule sensitivity that has been achieved in surface-enhanced Raman scattering (SERS) with the aid of plasmonic antennas suggests that a similar approach may be used to push CRS techniques to the single-molecule detection limit. Compared with SERS, however, experimental successes in surface-enhanced coherent Raman scattering (SE-CRS) are few, and a theoretical understanding of surface-enhancement in CRS is still incomplete. In this review, we discuss some of the principles and challenges in SE-CRS and summarize the latest advances in the quest of performing routine CRS experiments on single molecules.

Since the time of their introduction, optical tweezers (OTs) have grown to be a powerful tool in the hands of biologists. OTs use highly focused laser light to guide, manipulate, or sort target objects, typically in the nanoscale to microscale range. OTs have been particularly useful in making quantitative measurements of forces acting in cellular systems; they can reach inside living cells and be used to study the mechanical properties of the fluids and structures that they contain. As all the measurements are conducted without physically contacting the system under study, they also avoid complications related to contamination and tissue damage. From the manipulation of fluorescent nanodiamonds to chromosomes, cells, and free-swimming bacteria, OTs have now been extended to challenging biological systems such as the vestibular system in zebrafish. Here, we will give an overview of OTs, the complications that arise in carrying out OTs in vivo , and specific OT methods that have been used to address a range of otherwise inaccessible biological questions.

Optical losses of a host material together with the total internal reflection phenomenon can significantly reduce photoluminescence external quantum efficiency of embedded light-emitters. This is not only the case for light-emitting color centers in thin layers of nanocrystalline diamond, but also for silicon nanocrystals in silica dioxide matrices and for some types of perovskite materials. Here, we show that a significant boost (more than 100-fold enhancement) of the directional light emission efficiency from light-emitters in diamond can be achieved by using two-dimensional photonic crystal slabs (PhCs) to extract the light emission into vertical direction (resonant extraction) and at the same time to couple the excitation beam into the structure (resonant excitation). We have further shown that this so-called resonant extraction and excitation scheme provides the highest enhancement when the overlap between the electric field distribution of extraction leaky mode and the region of the excited light-emitters is maximized. This can be achieved by using the same type of the photonic mode for both extraction and excitation, and by optimizing the thickness of a diamond layer. The usage of the same type of modes appears to be more significant than tuning of the Q-factors of the excitation and extraction leaky modes individually. The results of our measurements are supported by the outputs of computer simulations. Our findings may be helpful in designing future PhCs for extraction of luminescence originating from various optoelectronic and sensor devices making use of the unique properties of the diamond. Moreover, our concept can be easily extended to other light-emitting materials with optical losses.

Diffractive approaches are needed when refractive microlenses reach their focusing limit at the micron-scale in visible light. Previously, we have reported on micron-sized optical lenses based on the diffraction of metallic nanowires. Here, we extend our study to lenses based on pairs of subwavelength dielectric scatterers. Using simulations by two-dimensional finite element method, we demonstrate that focusing holds for pair spacings as small as the wavelength-size. For pairs with distances between inner walls larger than about 1.2λ, the scattered waves generate a quadratic phase modulation on the total propagating field leading to a diffraction-limited focusing i.e. an effective optical lens effect with high numerical aperture. In addition, they have low sidelobe intensities, long depths of focus, and they have a low sensitivity with polarization. For pairs with inner wall distances smaller than about 1.2λ, the focusing phase modulation is accumulated during the propagation through the dielectric pair structure. In this work, we report only on the experimental demonstration for the case of larger wall separation to emphasize on the scattered wave effect on micro-focusing. A pair of parallel polymer lines (cylindrical lens), and a grid of polymer lines (square microlens array) with 2 μm-spacing were fabricated by two-photon induced polymerization. Their focal lengths are comparable to their separating distances, their spot-sizes are 0.37 μm and 0.28 μm at wavelength 530 nm, and their focusing efficiencies are 70% and 60%, respectively.

We propose a homogenization scheme for metamaterials that utilizes causality to determine their effective parameters. By requiring the Kramers-Kronig causality condition in the homogenization of metamaterials, we show that the effective parameters can be chosen uniquely, in contrast to the conventional parameter retrieval method which has unavoidable phase ambiguity arising from the multivalued logarithm function. We demonstrate that the effective thickness of metamaterials can also be determined to a specific value by saturating the minimum-error condition for the causality restriction. Our causal homogenization provides a robust and accurate characterization method for metamaterials.

In this paper, the enhanced terahertz (THz) electromagnetically induced transparency (EIT) resonance is achieved based on the coupled electric field dropping effect within the undulated meta-surface. It is found that the height difference between the inner ring and the outer split ring could lead to a significant coupled mode with electric field dropping distribution. When the height of the inner ring increases, the electric field of the inner ring gradually cascades to the outer ring just as a waterfall so that the coupling between the two rings in a unit cell and two adjacent units is enhanced. Both the simulation and experimental results show that a nearly 95% transparency window which is twice that of traditional EIT can be observed by applying such coupled electric field dropping effect, which may provide a promising way to develop the high resonance intensity meta-surface in the THz region.

Bessel beams have attracted considerable interest because of their unique non-diffractive, self-healing characteristics. Different approaches have been proposed to generate Bessel beams, such as using axicons, diffractive optical elements, composite holograms, or spatial light modulators. However, these approaches have suffered from limited numerical aperture, low efficiency, polarization-dependent properties, etc. Here, by utilizing dielectric Huygens metasurfaces as ultrathin, compact platforms by integrating the functionalities of Dammann gratings and axicons, we successfully demonstrate multiple Bessel beam generation with polarization-independent property. The number of two-dimensional arrays can be controlled flexibly, which can enhance information capacity with a total efficiency that can reach 66.36%. This method can have various applications, such as parallel laser fabrication, efficient optical tweezers, and optical communication.

Metallic helical metamaterials have become the prominent candidates for circular polarizers and other optical-chiral devices as they exhibit strong circular dichroism at a broad operation bandwidth. However, the rapid fabrication of an intertwined double helix with multiple pitch numbers and excellent mechanical strength, electrical conductivity and surface smoothness remains a challenge. We propose and realize the single-exposure femtosecond laser photoreduction of a freestanding, three-dimensional silver double-helix microstructure by the double-helix focal field intensity engineered with a spatial light modulator. At the same time, the photoreduction solution and the laser repetition rate are optimized to further tackle the surface roughness and the thermal flow problems. As a result, the silver double-helix array with the enhanced quality exhibits pronounced optical chirality in a wide wavelength range from 3.5 to 8.5 μm. This technique paves a novel way to easily and rapidly fabricate metallic metamaterials for chiro-optical devices in the mid-infrared regime.

We present a feasible way to strongly enhance Raman signals via introducing an ultra-thin dielectric film in the dual-layer plasmonic hotspots structure, which forms a quasi-three-dimensional structure. The Raman intensity was obtained with an enhancement factor of 735% for the dual-layer metal structure buffered with an ultra-thin silicon film. Moreover, the silicon layer based surface-enhanced Raman scattering (SERS) substrate provided a Raman signal two to five times larger than that of the silica buffered substrate. These distinct responses confirm that the ultra-thin high-index semiconductor film has the capability of additionally enhancing Raman scattering. Otherwise, the upper and lower metal clusters can support multiple kinds of plasmonic resonances, which produce a remarkable physical enhancement of the Raman signals. Besides these impressive optical properties, the substrates have prominent advantages on structural features, since the fabrication process can be fulfilled simply, suggesting a feasible way for a large-area and low-cost SERS platform. The findings may pave an avenue to achieve insights on the dielectric enhanced Raman scattering and hold potential applications in optoelectronics, such as environmental and health sensors.

Relativity theory severely restricts the ability to perform nonlocal measurements in quantum mechanics. Studying such nonlocal schemes may thus reveal insights regarding the relations between these two fundamental theories. Therefore, for the last several decades, nonlocal measurements have stimulated considerable interest. However, the experimental implementation of nonlocal measurements imposes profound restrictions because the interaction Hamiltonian cannot contain, in general, nonlocal observables such as the product of local observables belonging to different particles at spacelike-separated regions. In this work, we experimentally realize a scheme for nonlocal measurements with the aid of probabilistic quantum erasure. We apply this scheme to the tasks of performing high-accuracy nonlocal measurements of the parity, as well as measurements in the Bell basis, which do not necessitate classical communication between the parties. Unlike other techniques, the nonlocal measurement outcomes are available locally (upon successful postselection). The state reconstructed via performing quantum tomography on the system after the nonlocal measurement indicates the success of the scheme in retrieving nonlocal information while erasing any local data previously acquired by the parties. This measurement scheme allows to realize any controlled-controlled-gate with any coupling strength. Hence, our results are expected to have conceptual and practical applications to quantum communication and quantum computation.

Optical pulling forces exerted on small particles can be achieved by tailoring the properties of the electromagnetic field, the particles themselves, or the surrounding environment. However, the nonlinear optical effect of the surrounding environment has been largely neglected. Herein, we report the optical pulling forces on a Rayleigh particle immersed in a nonlinear optical liquid using high-repetition-rate femtosecond laser pulses. The analytic expression of time-averaged optical forces allows us to better understand the underlying mechanism of the particle transportation. It is shown that the two-photon absorption of the surrounding liquid gives rise to a negative radiation force. Transversely confined Rayleigh particles can be continuously dragged towards the light source during a pulling process.