Since ancient times, plasmonic structural coloring has inspired humanity; glassmakers achieved vibrant colors by doping glass with metal nanoparticles to craft beautiful objects such as the Roman Lycurgus cup and stained glass. These lovely color filtering effects are a consequence of the resonant coupling of light and free electrons in metal nanoparticles, known as surface plasmons. Thanks to the continuing improvement of nanofabrication technology, the dimensions of nanoparticles and structures can now be precisely engineered to form “optical nanoantennas,” allowing for control of optical response at an unprecedented level. Recently, the field of plasmonic structural coloring has seen extensive growth. In this review, we provide an up-to-date overview of various plasmonic color filtering approaches and highlight their uses in a broad palette of applications. Various surface plasmon resonance modes employed in the plasmonic color filtering effect are discussed. We first review the development of the pioneering static plasmonic colors achieved with invariant optical nanoantennas and ambient environment, then we address a variety of emerging approaches that enable dynamic color tuning, erasing, and restoring. These dynamic color filters are capable of actively changing the filtered colors and carrying more color information states than the static systems. Thus, they open an avenue to high-density data storage, information encryption, and plasmonic information processing. Finally, we discuss the challenges and future perspectives in this exciting research area.

Colors with plasmonic nanostructures: A full-spectrum review

The environmental concerns in the current century is not only limited to the polluting effect of the fossil fuel consumption but also the recycling challenges of waste turns to be a substantial challenges of the industry. Recycling of colored discarded materials is very difficult because of the problems in relation to the dissociation of diverse chemical compounds present in the colorant agents. Single or double component materials which could create various colors by geometrical changes can be a great solution to the mentioned limitations. Metasurfaces’ and metamaterials’ structural color therefore draws attention as they enable generation of vivid colors only by geometrical arrangement of metals which not only ease the recycling but at the same time enhance the mechanical stability of the colors. In this review, the progress in the field of plasmonic metasurface- and metamaterial-based structural colors is reviewed.

Review of Metasurface Plasmonic Structural Color

Polarization modulation of two phase-correlated, orthogonally polarized wavelengths by a parabolic waveform is a promising way to generate linear-frequency-modulated (LFM) signals, but the time-bandwidth product (TBWP) of the generated LFM signal is intrinsically limited by the achievable modulation index of the polarization modulator (PolM). In this paper, an approach to increase the TBWP of the LFM signal generated by polarization modulation is proposed and comprehensively studied by splitting the electrical parabolic waveforms into N pieces with identical amplitude. Applying the split parabolic signal to the PolM, the total equivalent phase shift would be boosted by N /2 times. As a result, the bandwidth as well as the TBWP of the generated LFM signal is increased by N /2 times. An experiment is carried out. As compared to the scheme using an unsplit parabolic signal, the TBWP is improved by more than 500 times. The relationships between the bandwidth, the time duration, and the TBWP of the generated signal with the parameters of the electrical waveform generator are discussed.

Photonic Generation of Linear-Frequency-Modulated Waveforms With Improved Time-Bandwidth Product Based on Polarization Modulation

Cognitive radio is considered as a possible disruptive force to improve the spectral resource efficiency through sensing and interacting with the environment. Traditional electrical technologies to implement the cognitive radio face challenges in terms of bandwidth, resolution, and speed. In this paper, the concept and architecture of broadband cognitive radio systems enabled by photonics are proposed. Key microwave photonic techniques for the architecture are reviewed, including the photonics-based spectrum sensing, the photonic arbitrary waveform generation, the photonics-based self-interference cancellation processing, and the microwave photonic dechirp processing and radar imaging. A preliminary demonstration of a cognitive radar system enabled by photonics is performed. The future possible research directions on this topic are discussed.

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Broadband Cognitive Radio Enabled by Photonics

Optoelectronic oscillators (OEOs) have been widely investigated to generate ultra-pure single-frequency microwave signals. Here, we propose and experimentally demonstrate a dual-chirp Fourier domain mode-locked (FDML) OEO to generate dual-chip microwave waveforms. In the proposed FDML OEO, a frequency-scanning dual-passband microwave photonics filter based on phase-modulation-to-intensity-modulation conversion using an optical notch filter and two laser diodes is incorporated into the OEO cavity. Fourier domain mode-locking operation is achieved by synchronizing the scanning period of the filter to the cavity round-trip time. Tunable dual-chirp microwave waveforms with a large time-bandwidth product are generated directly from the FDML OEO cavity in the experiment, which can be used in modern radar systems to improve its range-Doppler resolution.

Dual-chirp Fourier domain mode-locked optoelectronic oscillator

We present a photonic method for the generation of dual-chirp signals. In this approach, a single-tone RF signal is first modulated using a modulator in order to generate two optical carriers, which are further modulated by a linearly chirped signal by another modulator. Square-law detection in a photodiode then generates a dual-chirp signal whose center frequency and time-bandwidth product (TBWP) are fourfold increased. The dual-chirping and the improved TBWP can improve the range resolution of the radar system and help avoid the range measurement error caused by the range-Doppler coupling effect. The generation of bandwidth-quadrupled S-band dual-chirp waveforms is demonstrated experimentally. The processing of the dual-chirp signal is also discussed, in order to show its advantages over single-chirp signals. We believe that the proposed approach is a potential solution for the generation of dual-chirp signals with high center frequency and large bandwidth in modern radar systems.

Photonic Generation of Dual-Chirp Waveforms With Improved Time-Bandwidth Product

Photonic generation of chirped microwave signals with high time-bandwidth product

Influence of structural parameters to polarization-independent color-filter behavior in ultrathin Ag films

Duty cycle dependency of the optical transmission spectrum in an ultrathin nanostructured Ag film

In recent years, some approaches to realize photonic analog-to-digital conversion based on spectral encoding have been proposed. In these approaches, the amplitude information of input analog signals is first converted into the frequency information of the optical carrier via a nonlinear process, which is equivalent to optical frequency modulation; then, the spectral information of the optical carrier is quantized and encoded with the help of an optical filter group. In this letter, we propose, for the first time to our knowledge, that the time-frequency uncertainty principle puts a fundamental limit on this type of photonic analog-to-digital converters (ADCs). The condition for achieving acceptable performance in terms of effective number of bits is discussed. The presented theoretical results are verified by numerical simulations and a proof-of-concept experiment in electrical domain. We believe that the presented result is important to the design of spectral-encoding-based photonic ADCs.

Yuxiao Xu

Papers

Photonic Generation of Dual-Chirp Waveforms With Improved Time-Bandwidth Product

Photonic generation of chirped microwave signals with high time-bandwidth product

Influence of structural parameters to polarization-independent color-filter behavior in ultrathin Ag films

Duty cycle dependency of the optical transmission spectrum in an ultrathin nanostructured Ag film

Time-Frequency Uncertainty in the Photonic A/D Converters Based on Spectral Encoding