Volume 8, Issue 2

Quantum cascade lasers (QCLs) are becoming a key tool for plenty of applications, from the mid-infrared (mid-IR) to the THz range. Progress in related areas, such as the development of ultra-low-loss crystalline microresonators, optical frequency standards, and optical fiber networks for time and frequency dissemination, is paving the way for unprecedented applications in many fields. For most demanding applications, a thorough control of QCLs emission must be achieved. In the last few years, QCLs’ unique spectral features have been unveiled, while multifrequency QCLs have been demonstrated. Ultra-narrow frequency linewidths are necessary for metrological applications, ranging from cold molecules interaction and ultra-high sensitivity spectroscopy to infrared/THz metrology. A review of the present status of research in this field is presented, with a view of perspectives and future applications.

It is now well-appreciated that a bandlimited wave can possess oscillations much more rapidly than those predicted by the bandlimit itself, in a phenomenon known as superoscillation. Such superoscillations are required to be of dramatically smaller amplitude than the signal they are embedded in, and this has initially led researchers to consider them of limited use in applications. However, this view has changed in recent years and superoscillations have been employed in a number of systems to beat the limits of conventional diffraction theory. In this review, we discuss the current state of research on superoscillations in terms of superresolved imaging and subwavelength focusing, including the use of special non-diffracting and Airy beams to carry transverse superoscillating patterns. In addition, we discuss recent analogous works on using superoscillations to break the temporal resolution limit, and also consider the recently introduced inverse of superoscillations, known as suboscillations.

Multimode silicon photonics is attracting more and more attention because the introduction of higher-order modes makes it possible to increase the channel number for data transmission in mode-division-multiplexed (MDM) systems as well as improve the flexibility of device designs. On the other hand, the design of multimode silicon photonic devices becomes very different compared with the traditional case with the fundamental mode only. Since not only the fundamental mode but also the higher-order modes are involved, one of the most important things for multimode silicon photonics is the realization of effective mode manipulation, which is not difficult, fortunately because the mode dispersion in multimode silicon optical waveguide is very strong. Great progresses have been achieved on multimode silicon photonics in the past years. In this paper, a review of the recent progresses of the representative multimode silicon photonic devices and circuits is given. The first part reviews multimode silicon photonics for MDM systems, including on-chip multichannel mode (de)multiplexers, multimode waveguide bends, multimode waveguide crossings, reconfigurable multimode silicon photonic integrated circuits, multimode chip-fiber couplers, etc. In the second part, we give a discussion about the higher-order mode-assisted silicon photonic devices, including on-chip polarization-handling devices with higher-order modes, add-drop optical filters based on multimode Bragg gratings, and some emerging applications.

Chirality, a property of broken mirror symmetry, prevails in nature. Chiral molecules show different biochemical behaviors to their mirror molecules. For left or right circularly polarized lights, the fundamental chiral states of electromagnetic fields interact differently with chiral matter, and this effect has been used as a powerful tool for the detection of chiral molecules. This optical sensing, also termed chiral sensing, is not only easy to implement but also non-invasive to the analytes. However, the measurements made by the optical sensing of chiral molecules are challenging, as chiroptical signals are extremely weak. Recent years have seen active research efforts into metamaterial and plasmonic platforms for manipulating local fields to enhance chiroptical signals. This metamaterial approach offers new possibilities of chiral sensing with high sensitivity. Here, we review the recent advances in chiral sensing using metamaterial and plasmonic platforms. In addition, we explain the underlying principles behind the enhancement of chiroptical signals and highlight practically efficient chiral sensing platforms. We also provide perspectives that shed light on design considerations for chiral sensing metamaterials and discuss the possibility of other types of chiral sensing based on resonant metamaterials.

We propose a deterministic method to measure the optical permittivity of two-dimensional (2D) materials without a priori knowledge of the electronic transitions over the spectral window of interest. Using the thin-film approximation, we show that the ratio of reflection coefficients for s and p polarization can give a unique solution to the permittivity of 2D materials within the measured spectral window. The uniqueness and completeness of our permittivity measurement method do not require a priori knowledge of the electronic transitions of a given material. We experimentally demonstrate that the permittivity of monolayers of MoS 2 , WS 2 , and WSe 2 in the visible frequency range can be accurately obtained by our method. We believe that our method can provide fast and reliable measurement of the optical permittivity of newly discovered 2D materials.

As two groups of bases in fibers, cylindrical vector (CV) modes and the orbital angular momentum (OAM) modes can be transformed into each other. Several transformation relations have been studied in previous works, such as σ^+OAM+l=HEl+1,meven+iHEl+1,modd.${\hat \sigma ^ + }OA{M_{ + l}} = HE_{l + 1,m}^{even} + iHE_{l + 1,m}^{odd}.$ However, these relations are discussed in the limitation of equal amplitude, limited phase difference (kπ2, k∈Z)$\left( {{{k\pi } \over 2},{\rm{ }}k \in Z} \right)$ and finite (generally two) mode bases. Complete connection between the CV and OAM modes has not been found. In this paper, a four-dimensional complex space model is constructed to describe arbitrary CV and OAM modes. The reliability of the model is verified by previously reported results and our experiment results. The complete transformation relation between the CV modes and OAM modes is well described in the model. Furthermore, two common kinds of relations have been researched, that is, a single arbitrary polarized OAM mode and two arbitrary orthogonal polarized OAM modes and their corresponding CV modes. These two kinds of states include most of previously reported states, and some new states have not been reported.

Magneto-optical spectroscopy based on the transverse magneto-optical Kerr effect (TMOKE) is a sensitive method for investigating magnetically-ordered media. Previous studies were limited to the weak coupling regime where the spectral width of optical transitions considerably exceeded the Zeeman splitting in magnetic field. Here, we investigate experimentally and theoretically the transverse Kerr effect in the vicinity of comparatively narrow optical resonances in confined quantum systems. For experimental demonstration we studied the ground-state exciton resonance in a (Cd,Mn)Te diluted magnetic semiconductor quantum well, for which the strong exchange interaction with magnetic ions leads to giant Zeeman splitting of exciton spin states. For low magnetic fields in the weak coupling regime, the Kerr effect magnitude grows linearly with increasing Zeeman splitting showing a dispersive S-shaped spectrum, which remains almost unchanged in this range. For large magnetic fields in the strong coupling regime, the magnitude saturates, whereas the spectrum becomes strongly modified by the appearance of two separate peaks. TMOKE is sensitive not only to the sample surface but can also be used to probe in detail the confined electronic states in buried nanostructures if their capping layer is sufficiently transparent.

All-solution-processed flexible organic solar cells (FOSCs) with high power conversion efficiency (PCE) are the prerequisite for application in low-cost, large-area, flexible, photovoltaic devices. In this work, high-performance, top-illuminated FOSCs using ultrathin Ag-modified graphite-coated poly(ethylene terephthalate) (PET) substrates are demonstrated. The ultrathin Ag-modified graphite/PET substrates have excellent electric conductivity, mechanical flexibility, and easy processability for FOSCs. A PCE of 5.31% for FOSCs, based on the blend system poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo [1,2-b:4,5-b′]dith-iophene- co -3-fluorothieno[3,4-b]thiophene-2-carboxylate]: [6,6]-phenyl-C7l-but-yric acid methyl ester, having a bilayer of MoO x /Ag upper transparent anode is demonstrated. Top-illuminated FOSCs with a transparent upper electrode of solution-processed Ag nanowires also yielded a PCE of 3.76%. All-solution-processed FOSCs exhibit excellent mechanical flexibility and retain >81% of the initial efficiency after 500 cycles of bending test. Furthermore, graphite-based electrodes demonstrate good heat-insulation properties. The outcomes of this work offer an alternative to fabricate high-performance, all-solution-processable, top-illuminated FOSCs, providing a commercially viable approach for application in large-area solar cells that can be prepared by printing and roll-to-roll fabrication processes.

Plasmonic sensing has a great potential in the portable detection of human tumor markers, among which the carcinoembryonic antigen (CEA) is one of the most widely used in clinical medicine. Traditional plasmonic and non-plasmonic methods for CEA biosensing are still not suitable for the fast developing era of Internet of things. In this study, we build up a cost-effective plasmonic immunochip platform for rapid portable detection of CEA by combining soft nanoimprint lithography, microfluidics, antibody functionalization, and mobile fiber spectrometry. The plasmonic gold nanocave array enables stable surface functionality, high sensitivity, and simple reflective measuring configuration in the visible range. The rapid quantitative CEA sensing is implemented by a label-free scheme, and the detection capability for the concentration of less than 5 ng/ml is achieved in clinical experiments, which is much lower than the CEA cancer diagnosis threshold of 20 ng/ml and absolutely sufficient for medical applications. Clinical tests of the chip on detecting human serums demonstrate good agreement with conventional medical examinations and great advantages on simultaneous multichannel detections for high-throughput and multi-marker biosensing. Our platform provides promising opportunities on low-cost and compact medical devices and systems with rapid and sensitive tumor detection for point-of-care diagnosis and mobile healthcare.

We propose a vortex beam generator based on a nanometer spiral slit and explore the propagation rule of the topological charge. Compared to the common methods of generation of a vortex beam with a fixed topological charge, the optical vortex generated by the proposed vortex beam generator has the topological charge varying with the propagation distance. The value of topological charge can be modulated by the geometric charge of the spiral slit and the propagation distance. Theoretical analysis predicts the variation rule of the topological charge of vortex beam in the near field, and numerical simulations and experimental measurement verify the proposed scheme. Discussion on the shape and structure of the spiral slit is also presented. This work provides the theoretical foundation for the generation of a vortex field with variable topological charge. The simple geometry of the vortex beam generator and the flexible modulation of the topological charge must inspire applications of the vortex beam.

In this article, we present a multiperiodic nanohole array structure for improved sensing. The structure consists a series of rows of nanoholes, each having a different period in an ascending order. A monochromatic source illuminates the structure, and a resonance condition is met for the row having a momentum matching Bloch wave, which leads to extraordinary optical transmission. With this new plasmonic structure, the sensing signal can be retrieved using the spatial position of the transmission maxima. This setup requires a simple optical setup while achieving increased resolution and accuracy. A resolution of 4.6×10 −6 refractive index units is achieved, which is comparable to surface plasmon resonance system based on the Kretchmann configuration.

Metasurfaces exploit optical phase, amplitude, and polarization engineering at subwavelength dimensions to achieve unprecedented control of light. The realization of all dielectric metasurfaces has led to low-loss flat optical elements with functionalities that cannot be achieved with metal elements. However, to reach their ultimate potential, metasurfaces must move beyond static operation and incorporate active tunability and reconfigurable functions. The central challenge is achieving large tunability in subwavelength resonator elements, which requires large optical effects in response to external stimuli. Here we study the thermal tunability of high-index silicon and germanium semiconductor resonators over a large temperature range. We demonstrate thermal tuning of Mie resonances due to the normal positive thermo-optic effect (dn/dT>0) over a wide infrared range. We show that at higher temperatures and longer wavelengths, the sign of the thermo-optic coefficient is reversed, culminating in a negative induced index due to thermal excitation of free carriers. We also demonstrate the tuning of high-order Mie resonances by several linewidths with a temperature swing of ΔT<100 K. Finally, we exploit the large near-infrared thermo-optic coefficient in Si metasurfaces to realize optical switching and tunable metafilters.