Most optical systems involve a combination of lenses separated by free-space regions where light acquires the required angle-dependent phase delay for a certain functionality. Very recently, flat-optics structures have been proposed to compress these large free-space volumes and miniaturize the overall optical system. However, these early designs can only replace freespace volumes of limited length, or operate in a very narrow angular range, or require a high-index background. These issues raise questions about the applicability of these devices in practical scenarios. Here, we first derive a fundamental trade-off between the length of compressed free space and the operating angular range, which explains some of the limitations of earlier designs, and we then propose a solution to relax this trade-off using nonlocal metasurface structures composed of suitably coupled resonant layers. This strategy, inspired by coupled-resonator-based band-pass microwave filters, allows replacing free-space volumes of arbitrary length over wide angular ranges, and with very high transmittance. Finally, we theoretically demonstrate, for the first time, the potential of combining local and nonlocal metasurfaces to realize compact, fully solid-state, planar structures for focusing, imaging, and magnification, in which the focal length of the lens (and, hence, its magnifying power) does not dictate the actual distance at which focusing is achieved. Our findings are expected to extend the reach of the field of metasurfaces and open new unexplored opportunities.

Causality -the principle stating that the output of a system cannot precede the input -is a universal property of nature. Here, we extend the concept of causality, and its implications, from the temporal to the spectral domain, leveraging the peculiar properties of time-modulated non-Hermitian wave-physics systems, with particular emphasis on photonic systems. Specifically, we uncover the existence of a broad class of complex time-modulated metamaterials which obey the time-domain equivalent of the well-established frequency-domain Kramers-Kronig relations. We find that, in the scattering response of such time-modulated systems, the output frequencies are inherently prohibited from spectrally preceding the input frequencies, hence we refer to these systems as 'spectrally causal'. We explore the consequences of this newly introduced concept for several relevant applications, including broadband perfect absorption, temporal cloaking of an 'event', and truly unidirectional propagation along a synthetic dimension. By extending the concept of causality into the spectral domain and providing new tools to extend the field of temporally modulated metamaterials ("chrono-metamaterials") into the complex realm, our findings not only deepen our understanding of spectral scattering dynamics, but may also open unexplored opportunities and enable relevant technological advances in various areas of photonics and, more broadly, of wave physics and engineering.Significance Statement -The causality principle -an effect cannot temporally precede its cause -is a fundamental property of nature and underlies several constraints on the properties of physical materials. Here, we extend the notion of causality from the temporal to the spectral domain in temporally modulated wave-physics systems. Specifically, we uncover the existence of a broad class of time-modulated photonic systems that are "spectrally causal" in the sense that the output wave cannot contain oscillations with frequencies (colors) lower than the frequencies of the input wave oscillations. As an important consequence, wave reflections are then automatically minimized. This new class of timemodulated metamaterials has relevant implications for broadband perfect absorption, invisibility, unidirectional propagation, and may open unexplored opportunities in wave physics and engineering.

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Oxidative carbonylation of methane is an appealing approach to the synthesis of acetic acid but is limited by the demand for additional reagents. Here, we report a direct synthesis of CH3COOH solely from CH4 via photochemical conversion without additional reagents. This is made possible through the construction of the PdO/Pd–WO3 heterointerface nanocomposite containing active sites for CH4 activation and C–C coupling. In situ characterizations reveal that CH4 is dissociated into methyl groups on Pd sites while oxygen from PdO is the responsible for carbonyl formation. The cascade reaction between the methyl and carbonyl groups generates an acetyl precursor which is subsequently converted to CH3COOH. Remarkably, a production rate of 1.5 mmol gPd–1 h–1 and selectivity of 91.6% toward CH3COOH is achieved in a photochemical flow reactor. This work provides insights into intermediate control via material design, and opens an avenue to conversion of CH4 to oxygenates.

Abstract-In this paper, a second-order decoupling design using a resonator and an interdigital capacitor is proposed for an MIMO antenna pair in mobile terminals. The proposed antenna pair consists of an interdigital capacitor and an open loop resonator. By properly combining the responses of the resonator and interdigital capacitor, a second-order decoupling performance can be achieved. Meanwhile, isolation between the two antennas is increased by at least 15 dB within the frequency band of interest, from −5 dB to −20 dB. Moreover, the decoupled antenna pair maintains good impedance matching performance from 2.4 GHz to 2.5 GHz. The proposed decoupled antenna pair and its coupled counterpart have been fabricated and measured. The measured results agree with the simulation ones. The proposed MIMO antenna pair is an eligible candidate for Wi-Fi MIMO applications in the 2.4 GHz band.

Cognitive abilities are impaired in patients with pituitary adenoma. However, studies on attention processing impairment in preoperative patients and attention processing recovery after transsphenoidal adenomectomy are lacking. The study aims to identify the electrophysiological change that relates to attention processing in pituitary patients before and after treatment. Twenty five preoperative pituitary patients and 25 follow-up postoperative patients were recruited. 27 healthy controls (HCs) were matched to the patients with age, gender, and education. Event-related potentials were used to investigate the attention processing in the preoperative patients, postoperative patients, and HCs. Across three groups, all emotional stimuli evoked P200 components. Compared with the HCs or postoperative patients, the amplitudes of P200 in the preoperative patients were higher. Moreover, The amplitudes of P200 decreased in the postoperative patients, which were similar to that in the HCs. The attention processing was improved after surgery, but no significant differences were detected between the postoperative patients and HCs. Abnormal hormone levels may be relevant to the factor that impair attention processing. Compared with that of the HCs and postoperative patients, the P200 component elicited by negative stimuli is higher in preoperative patients, which may illustrate compensatory activity after attention impairments. Furthermore, these data indicate that improvements in attention processing may be attributed to the amelioration of endocrine disorders. This study shows that the P200 component may be used to diagnose attention processing in preoperative pituitary patients and prove the improvement of attention processing in postoperative patients.

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