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Synthetic aperture radar (SAR) systems employ a Linearly Chirped Microwave Waveform Generator (LCMWG) with large time–bandwidth product (TBWP), to provide a wide range resolution. Photonics has now been recognized as a disruptive approach to achieve high performance at bandwidth of few tens of gigahertz, with light and compact architectures, due to the typical photonics benefits, such as electromagnetic interference immunity, small power consumption, small footprint, and high immunity to vibration/shock and radiation. In this article, we report on the photonic generation of a high-frequency LCMW, with a large TBWP (102–103), using a chip-scaled architecture, based on a frequency-tunable optoelectronic oscillator (OEO) and a recirculating phase modulation loop (RPML). A new configuration of the OEO employing an ultrahigh Q-factor resonator has been conceived to allow the oscillator working in Ka band at 40 GHz or even more, with very low phase noise. Key building block of the RPML is a phase modulator driven by an engineered parabolic split waveform. The ultra-large pulse compression rate (PCR) >> 102, together with large signal purity, was also obtained, making the proposed architecture particularly suitable for SAR systems with large range resolution demand, such as Earth surveillance and monitoring.

A hybrid titanium dioxide/silicon rich nitride ring resonator with the unique feature of being simultaneously athermal and polarization-insensitive is reported for the first time to our knowledge. Although its potential application domain is extremely wide, the designed integrated microphotonic cavity, having a racetrack shape, is intended for notch filtering in a microwave photonic passband filter. A careful selection of the CMOS-compatible material system and an innovative design approach have allowed a very low dependence of the filtering shape on the input beam polarization and, simultaneously, a thermal drift of the resonance wavelength 4, 4 nm, and 30.7 dB, respectively.

The design, fabrication, and optical characterization of the sensing element of a photonic InP-based gyroscope intended for applications in the field of aerospace and defense are reported in this paper. The sensing element is a spiral resonator coupled to a straight bus waveguide through a multimode interference coupler and exhibits a Q factor of approximately 600 000 with a footprint of approximately 10 mm 2. The design of each component of the sensor is based on some well-established numerical methods such as the Finite Element Method, the beam propagation method, and the film mode matching method. The spiral cavity was designed using the standard transfer matrix method. The selected fabrication process, which is an enhanced version of the standard COBRA process, allows the monolithic integration of the sensing element with the other active components of the gyroscope, e.g., lasers, photodiodes, and modulators. Each component of the fabricated sensing element was optically characterized using an appropriate setup, which was also used for the optical characterization of the whole sensor. Based on the results of the characterization, the gyro performance was evaluated, and a way to improve both the resolution and the bias drift, i.e., down to 10°/h and 1°/h, respectively, was also clearly identified. The achieved results demonstrate, for the first time, the actual feasibility of a photonic gyro-on-chip through a well-established InP-based generic integration process.

The design of a new graphene-based continuously tunable optical delay line formed by two vertically stacked microring resonators coupled to a straight waveguide is proposed. High values of delay time (τg = 360 ps) and a wide tuning range (Δτg = 230 ps) have been calculated, due to the graphene sandwiched between the stacked ring resonators, which also provides an electrooptical tuning of the delay with low energy consumption (Eswitch = 3.4 pJ) and fast switching time (tswitch -1 ps/μm2 has been calculated, which corresponds to an enhancement of about a factor 4 compared with the state-of-the-art of the integrated optical delay lines, also providing a switching time several times faster. Such performance, together with a small device footprint (3 μm2), gives a significant contribution to the state-of-the-art of optical delay lines, confirming the suitability of the graphene-based resonant cavity as a high-efficient optical delay line for applications in which fast tuning and wide range of tunability are required, e.g., phased array antennas.

The ability to confine light at the nanoscale continues to excite the research community, with the ratio between quality factor Q and volume V, i.e., the Q/V ratio, being the key figure of merit. In order to achieve strong light-matter interaction, however, it is important to confine a lot of energy in the resonant cavity mode. Here, we demonstrate a novel cavity design that combines a photonic crystal nanobeam cavity with a plasmonic bowtie antenna. The nanobeam cavity is optimised for a good match with the antenna and provides a Q of 1700 and a transmission of 90%. Combined with the bowtie, the hybrid photonic-plasmonic cavity achieves a Q of 800 and a transmission of 20%, both of which remarkable achievements for a hybrid cavity. The ultra-high Q/V of the hybrid cavity is of order of 106 (λ/n)−3, which is comparable to the state-of-the-art of photonic resonant cavities. Based on the high Q/V and the high transmission, we demonstrate the strong efficiency of the hybrid cavity as a nanotweezer for optical trapping. We show that a stable trapping condition can be achieved for a single 200 nm Au bead for a duration of several minutes (ttrap > 5 min) and with very low optical power (Pin = 190 μW).

A novel photonic/plasmonic cavity based on a 1-D photonic crystal cavity vertically coupled to a plasmonic gold structure is reported. The design has been optimized to achieve an ultra-high Q/V ratio, therefore improving the light-matter interaction and making the device suitable for optical trapping applications. Accurate 3-D finite element method (FEM) simulations have been carried out to evaluate the device behavior and performance. The device shows Q = 2:8 × 103 and V = 4 × 10-4(λ=n)3, which correspond to a Q=V = 7 × 106(λ=n)-3 with a resonance transmission around 50% at λR = 1589:62 nm. A strong gradient of the optical energy has been observed in the metal structure at the resonance, inducing a strong optical force and allowing a single particle trapping with a diameter less than 100 nm. The device turns out very useful for novel biomedical applications, such as proteomics and oncology.

This paper reports the numerical and experimental results of a high-Q silica-onsilicon spiral resonator to be used in microoptical gyroscopes having a potential resolution 2 waveguiding spiral cavity as sensing element for gyro applications is given, and results of its optical characterization are provided. Quality factor, finesse, free spectral range, and thermal stability have been measured, clearly showing the potential of the device for gyro applications. The effect of coupling tuning through micrometer scale heaters and the supported eigenstates of polarization have also been experimentally investigated. The thermal stabilization of the silica chip is realized using a thermoelectric cooler co-packaged with the resonant cavity. The Q-factor of the spiral exceeds 106, and the thermal drift of the resonance frequency is very low (

A guided-wave chemical sensor for the detection of environmental pollutants or biochemical substances has been designed. The sensor is based on an asymmetric directional coupler employing slot optical waveguides. The use of a nanometer guiding structure where optical mode is confined in a low-index region permits a very compact sensor (device area about 1200 μm2) to be realized, having the minimum detectable refractive index change as low as 10-5. Silicon-on-Insulator technology has been assumed in sensor design and a very accurate modelling procedure based on Finite Element Method and Coupled Mode Theory has been pointed out. Sensor design and optimization have allowed a very good trade-off between device length and sensitivity. Expected device sensitivity to glucose concentration change in an aqueous solution is of the order of 0.1 g/L.

In general, the papers in this special issue focus on recent advances, including modeling, development, and fabrication strategies, as well as applications in strategic sectors, such as the case of safety and security field trials.