Table of contents

The topological transition of an iso-frequency contour will provide a powerful control for the interaction between light and matter. For example the transition of iso-frequency contour from an elliptical dispersion to a hyperbolic dispersion can result in many interesting optical phenomena including super-resolution, optical switching and collimation. In recently published literature, it has been shown that another transition from the metal-type hyperbolic dispersion to dielectric-type hyperbolic dispersion can be realized in the microwave circuit-based metamaterials. Particularly, the transition point corresponds to a new class of metamaterials with two intersecting linear dispersions, which can be called linear-crossing metamaterials (LCMMs). Because of the linear dispersion, LCMMs have been demonstrated to possess many unusual properties such as directional propagation and slab-focusing with a partial cloaking effect. In this perspective, we will demonstrate that the multilayered structure composed of ε-negative material and μ-negative material can mimic the LCMM in the optical regime. Based on this effective LCMM, we study the slab-imaging with a partial cloaking effect. In addition, we reveal that with the aid of LCMM, the Bessel beam with self-healing can be realized by a point source. The results show that LCMMs would be very useful in a variety of applications such as 50/50 beam splitters, focusing and non-diffraction beams.

Demand for wireless services is increasing rapidly, due to deep penetration of mobile terminals, such as smartphones. Radio spectrum congestion is an important issue, even for conventional mobile services. One of the possible solutions would be to use wired and wireless seamless networks consisting of radio and optical links, where huge data traffic can be offloaded from radio-waves in the air to optical signals over fibres. This paper provides seamless network configurations which can be applied to 90-GHz high-speed wireless links and high-resolution radars. We focus on applications for public transportation systems including airport runways, railways, etc.

In this article, the potential of photonic integrated circuits (PICs) for modern gas sensing applications is discussed. Optical detection systems can be found at the high-end of the currently available gas detectors, and PIC-based optical spectroscopic devices promise a significant reduction in size and cost. The performance of such devices is reviewed here. This discussion is not limited to one semiconductor platform, but includes several available platforms operating from the visible wavelength range up to the long wavelength infrared. The different platforms are evaluated regarding their capabilities in creating a fully integrated spectroscopic setup, including light source, interaction cell and detection unit. Advanced spectroscopy methods are assessed regarding their PIC compatibility. Based on the comparison of PICs with state-of-the-art bulk optical devices, it can be concluded that they can fill the application space of compact and low cost optical gas sensors.

Recent advances in light-sheet microscopy have enabled sensitive imaging with high spatiotemporal resolution. However, the creation of thin light-sheets for high axial resolution is challenging, as the thickness of the sheet, field of view and confinement of the excitation need to be carefully balanced. Some of the thinnest light-sheets created so far have found little practical use as they excite too much out-of-focus fluorescence. In contrast, the most commonly used light-sheet for subcellular imaging, the square lattice, has excellent excitation confinement at the cost of lower axial resolving power. Here we leverage the recently discovered Field Synthesis theorem to create light-sheets where thickness and illumination confinement can be continuously tuned. Explicitly, we scan a line beam across a portion of an annulus mask on the back focal plane of the illumination objective, which we call C-light-sheets. We experimentally characterize these light-sheets and demonstrate their application on biological samples.

Fiber Bragg grating (FBG) sensors are typically bonded on the surface of a structure using an adhesive to collect ultrasonic waves for damage detection in structural health monitoring applications. However, the ultrasonic wave transfer from structure to optical fiber suffers signal attenuation due to the adhesive bond layer, which has a significantly different acoustic impedance than the optical fiber. Therefore, this paper develops a systematic procedure to fabricate an aligned carbon nanotube (CNT)-wrapped FBGs for acoustic impedance matching. Specifically, we first develop an automated CNT winding system to fabricate CNT-wrapped FBGs with varying CNT layer thickness, which are bonded to an aluminum plate for ultrasonic sensitivity testing. We demonstrate that CNT wrapped FBGs do not necessarily produce an increased sensitivity as compared to a reference polyimide-coated FBG, however some outliers are observed with a significant improvement. Using a scanning electron microscopy we examine the cross-section of CNT/adhesive layers, identifying a unique CNT/adhesive bonding morphology with a stiff exterior shell and a relatively compliant inner layer. Finite element simulation validates that this two-layered bonding geometry is most likely the source of the increased FBG ultrasonic sensitivity for the outliers.

A gain-switched Dy³⁺-doped ZBLAN fiber laser operating at 2.943 μm is experimentally reported for the first time to the best of our knowledge. The laser was pumped by a 1.1 μm Q-switched ytterbium (III) fiber laser constructed in-house. A stable pulse train is achieved with repetition rates spanning between 25 and 100 kHz. For the repetition rate of 50 kHz, stable 183 ns pulses with an energy of 0.72 μJ and peak power of 4 W are recorded. By using a longer length of Dy³⁺-doped ZBLAN fiber, gain-switched operation was achieved at a wavelength larger than 3 μm.

We use a differential construction procedure to construct three starting solutions for a thermal infrared telescope design problem. We further refine these solutions with a simple optimization procedure. We show that this hybrid method is interesting by its flexibility that makes it suitable to find solutions to problems with tight volume constraints and its ease of use with little designer interaction required. Taking advantage of a fully automated process, we are able to investigate for each solution and its starting geometry, the impact of the Zernike order on the final performance obtained. Finally we show the systems proposed are sufficient for thermal infrared applications and while not as performant as the three mirror system used for comparison, their simplicity makes them an interesting alternative.

Optical fiber sensors of hydrogen gas (H₂) are conventionally based on the reaction of a sensitive material deposited on the surface of a fiber. Long-term applications of H₂ monitoring require more robust configurations, less sensitive to the degradations of the sensitive layer. To overcome this issue, we develop disruptive polarisation-maintaining optical fibers composed of a sensitive material (Palladium, Pd) integrated into the silica cladding. We present the development of two Panda-type optical fibers with or without embedded Pd particles. These fibers have been fabricated for evaluating, through the measurement of the birefringence, the contribution of Pd particles on the detection of H₂ gas. We have specially developed a gas chamber for measuring on-line the detection of H₂ during its diffusion into the fiber. Dynamic comparisons between both fibers demonstrate the contribution of Pd particles resulting in a faster response time (of about 20 h for our experimental conditions). These results pave the way to the realization of robust optical fibers with enhanced sensitivity to H₂ gas for developing sensing systems compatible with long-term hydrogen monitoring applications in extreme and harsh environments, such as radioactive waste repositories.

A new concept of an all optical, dual-fiber-based Pirani thermal vacuum gauge is proposed and demonstrated. The configuration utilizes two fibers: one that produces heat, and one that responds to the resultant thermal exchange. The temperature of the latter fiber is a function of the heat transfer through the gas in which it resides. The active heat-generating fiber is a luminescence-quenched, heavily Yb³⁺-doped optical fiber that efficiently produces thermal energy when optically pumped. The temperature sensor is implemented with a conventional commercial fiber Bragg grating. Both fibers are inserted into a custom vacuum chamber whose internal pressure can be carefully controlled. Performance of the system is characterized with pressures ranging from 20 mTorr to Standard Pressure. The proposed system may also be used as a sensitive flow rate sensor.

The performance of organic–inorganic metal halide perovskites-based (MHPs) photovoltaic devices critically depends on the design and material properties of the interface between the light-harvesting MHP layer and the electron transport layer (ETL). Therefore, the detailed insight into the transfer mechanisms of photogenerated carriers at the ETL/MHP interface is of utmost importance. Owing to its high charge mobilities and well-matched band structure with MHPs, titanium dioxide (TiO₂) has emerged as the most widely used ETL material in MHPs-based photovoltaic devices. Here, we report a contactless method to directly track the photo-carriers at the ETL/MHP interface using the technique of low-temperature electron paramagnetic resonance (EPR) in combination with in situ illuminations (Photo-EPR). Specifically, we focus on a model nanohybrid material consisting of TiO₂-based nanowires (TiO₂NWs) dispersed in the polycrystalline methylammonium lead triiodide (MAPbI₃) matrix. Our approach is based on observation of the light-induced decrease in intensity of the EPR signal of paramagnetic Ti³⁺ (${\boldsymbol{S}}=1/2$) in non-stoichiometric TiO₂NWs. We associate the diminishment of the EPR signal with the photo-excited electrons that cross the ETL/MHP interface and contribute to the conversion of Ti³⁺ states to EPR-silent Ti²⁺ states. Overall, we infer that the technique of low-temperature Photo-EPR is an effective strategy to study the transfer mechanisms of photogenerated carriers at the ETL/MHP interface in MAPbI₃-based photovoltaic and photoelectronic systems.

Optical anisotropy is essential for the polarization-sensitive optoelectronic devices. Recently, in-plane anisotropy is demonstrated in various 2D layered materials. It has been proved that organic-inorganic perovskites possess excellent optical properties; however, the anisotropy of organic-inorganic perovskites is rarely reported because of the in-plane isotropic structure. Here, we report a large optical anisotropy of one-dimensional organic lead iodine perovskites C₄N₂H₁₄PbI₄ crystals, including emission, excitation and reflection anisotropy. An emission linear dichroic ratio of 5.5 and an excitation linear dichroic ration of 7 are achieved respectively, which are much larger than the in-plane anisotropy of 2D layered materials. The large optical anisotropy can be ascribed to the anisotropic dipole moment in the unique 1D chain structure. In addition, the PL of the 1D C₄N₂H₁₄PbI₄ crystal is dominated by the broadband self-trapped exciton emission due to the quantum confinement effect and strong electron-phonon interaction. Our results advocate that 1D perovskites are promising in the application of broadband polarization-sensitive optoelectronic devices.

The potential of discrete and distributed fiber-based sensors exploiting the Rayleigh scattering signature of doped amorphous silica is investigated for the real time monitoring of molecular hydrogen (H₂) detection. We showed that the impact of the refractive index changes induced by the H₂ diffusion into the silica host matrix can be used to detect and quantify this gas presence through two approaches: first via the related fiber length variation and second through the observed spectral shift. Comparing the obtained results with H₂ diffusion calculations, we can estimate the sensor sensitivity thresholds to be ∼10¹⁶n_molecule cm⁻³ for the distributed measurements (spatial resolution better than 1 mm) and below ∼10¹⁹n_molecule cm⁻³ for the discrete-one. The presented architecture of the sensor is well adapted to the monitoring of slowly evolving H₂ concentrations such as the ones expected in nuclear waste repositories as the time response of the sensor remains limited by the diffusion of the gas within the optical fiber. These threshold values and time responses can be easily improved by optimizing the length, the composition and/or the geometry of the sensing fiber.

This paper reports a wavelength-stabilized high-power diode laser emitting up to 14 W CW in the 9xx nm range. Wavelength stabilization is achieved by a distributed Bragg reflector (DBR) monolithically integrated in the diode laser chip. Key features are identical layer epitaxy (ILE) and the use of a multiple-order electron beam lithography (EBL) optical confining grating. ILE avoids any regrowth or complex technology processes, while EBL multiple-order grating allows narrow-band back reflection and effective lateral optical confinement, and makes it possible to stabilize multiple wavelengths on the same wafer using a manufacturable and reliable technology. DBR diode lasers with different pitches, whose wavelengths were 3 nm spaced, were fabricated and high spectral purity (95% optical power within about 0.6 nm) and wavelength stability were measured. Moreover, the high uniformity of performances across the wafer with different emitted wavelengths demonstrates the maturity of the proposed technology for high-yield, high-volume laser diode production for wavelength-stabilized applications. A multi-emitter module, including ten DBR diode lasers, collimating and focusing optics, showed 100 W CW wavelength-stabilized output power at 14 A in a 135 μm core optical fiber within 0.17 NA. Single diode lasers, or multi-emitter modules, can be used to combine high-power optical beams by wavelength division multiplexing (WDM) using dichroic optics, scaling up beam power to the kW range and maintaining optical beam quality.

Surface coupling of single quantum emitters to optical cavities consisting of a photonic crystal slab is a delicate yet crucial task for photonic quantum applications. By coupling through the evanescent surface field only small Purcell factors can be achieved. Here, we propose to introduce a pit in the slab to position the emitter closer to the mode field maximum. Photonic crystal slab L3 cavities are investigated with respect to quality factor and Purcell effect, using finite element calculations in the frequency domain. That way the spatial distribution of the Purcell factor can be calculated. Introducing a small sized pit to the surface of the photonic crystal cavity can evoke subwavelength field effects, confining the field maximum inside the pit. By engineering a pit in the center of the cavity the Purcell factor can be increased from 176 to 1331, albeit reducing the Q factor from 20769 to 16696.

Flexible and stretchable photonics are emerging fields aiming to develop novel applications where the devices need to conform to uneven surfaces or whenever lightness and reduced thickness are major requirements. However, owing to the relatively small refractive index of transparent soft matter including most polymers, these materials are not well adapted for light management at visible and near-infrared frequencies. Here we demonstrate simple, low cost and efficient protocols for fabricating Si_1−xGe_x-based, sub-micrometric dielectric antennas over record scales (50 mm wafers) with ensuing hybrid integration into different plastic supports. The transfer process has a near-unity yield: up to 99.94% for disordered structures and 99.5% for the ordered counterpart. Finally, we benchmark the optical quality of the dielectric antennas with light scattering measurements, demonstrating the control of the islands structural color and the onset of sharp Mie modes after encapsulation in plastic. Thanks to the ease of implementation of our fabrication methods, these results are relevant for the integration of SiGe-based dielectric Mie resonators in flexible substrates over large surfaces.

Based on the characteristics of ferrofluids, a monolithic optofluidic device for magnetic field sensing is proposed and demonstrated. The device consists of a Fabry–Pérot interferometer, composed by an optical waveguide orthogonal to a microfluidic channel, which was fabricated inside a fused silica substrate through femtosecond laser micromachining. The interferometer was first optimized by studying the influence of the waveguide writing parameters on its spectral properties. Waveguides written at higher pulse energies led to a decrease of the signal-to-noise ratio, due to an enhancement of micrometer sized defects associated with Mie scattering. Fringe visibility was also maximized for waveguides written at lower scanning speeds. Making use of the tunable refractive index property exhibited by magnetic fluids, the interferometer was then tested as a magnetic field sensor by injecting a ferrofluid inside the microfluidic channel. A linear sensitivity of −0.25 nm/mT was obtained in the 9.0–30.5 mT range with the external field parallel to the waveguide axis.