Parity-time (PT) symmetric periodic structures, near the spontaneous PT-symmetry breaking point, can act as unidirectional invisible media. In this regime, the reflection from one end is diminished while it is enhanced from the other. Furthermore, the transmission coefficient and phase are indistinguishable from those expected in the absence of a grating. The phenomenon is robust even in the presence of Kerr nonlinearities, and it can also effectively suppress optical bistabilities.

The global COVID-19 pandemic caused by the SARS-CoV-2 virus has made the development of a vaccine a top biomedical priority. In this study, we developed a series of DNA vaccine candidates expressing different forms of the SARS-CoV-2 Spike (S) protein and evaluated them in 35 rhesus macaques. Vaccinated animals developed humoral and cellular immune responses, including neutralizing antibody titers comparable to those found in convalescent humans and macaques infected with SARS-CoV-2. Following vaccination, all animals were challenged with SARS-CoV-2, and the vaccine encoding the full-length S protein resulted in >3.1 and >3.7 log10 reductions in median viral loads in bronchoalveolar lavage and nasal mucosa, respectively, as compared with sham controls. Vaccine-elicited neutralizing antibody titers correlated with protective efficacy, suggesting an immune correlate of protection. These data demonstrate vaccine protection against SARS-CoV-2 in nonhuman primates.

A safe and effective vaccine for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) may be required to end the coronavirus disease 2019 (COVID-19) pandemic 1 – 8 . For global deployment and pandemic control, a vaccine that requires only a single immunization would be optimal. Here we show the immunogenicity and protective efficacy of a single dose of adenovirus serotype 26 (Ad26) vector-based vaccines expressing the SARS-CoV-2 spike (S) protein in nonhuman primates. 52 rhesus macaques were immunized with Ad26 vectors encoding S variants or sham control and were challenged with SARS-CoV-2 by the intranasal and intratracheal routes 9 , 10 . The optimal Ad26 vaccine induced robust neutralizing antibody responses and provided complete or near-complete protection in bronchoalveolar lavage and nasal swabs following SARS-CoV-2 challenge. Vaccine-elicited neutralizing antibody titers correlated with protective efficacy, suggesting an immune correlate of protection. These data demonstrate robust single-shot vaccine protection against SARS-CoV-2 in nonhuman primates. The optimal Ad26 vector-based vaccine for SARS-CoV-2, termed Ad26.COV2.S, is currently being evaluated in clinical trials.

Recent advancements in computational inverse design have begun to reshape the landscape of structures and techniques available to nanophotonics. Here, we outline a cross section of key developments at the intersection of these two fields: moving from a recap of foundational results to motivation of emerging applications in nonlinear, topological, near-field and on-chip optics.The development of devices in nanophotonics has historically relied on intuition-based approaches, the impetus for which develops from knowledge of some a priori known physical effect. The specific features of such devices are then typically calculated and matched to suitable applications by tuning small sets of characteristic parameters. This approach has had a long track record of success, giving rise to a rich and widely exploited library of templates that includes multilayer thin films 1 , Fabry-Perot 2 and microring resonators 3 , silicon waveguides 4,5 , photonic crystals 6 , plasmonic nanostructures 7 , and nanobeam cavities 8 , top of Fig. 1. Combining the principles of index guiding and bandgap engineering, along with material resonances, this collection of designs enables remarkable manipulation of light over bands of frequencies spanning from the ultra-violet to the mid infrared: group velocity can be slowed by more than two orders of magnitude 9 , light confined to volumes thousands of times smaller than its free-space wavelength 10 , and resonances made to persist in micron sized areas for tens of millions of cycles 11 .Yet, as the scope of nanophotonics broadens to include large bandwidth or multi-frequency applications, nonlinear phenomena, and dense integration, continuing with this prototypical approach poses a challenge of increasing complexity. For instance, consider the design of a wavelength-scale structure for enhancing nonlinear interactions 12 , discussed below. Even in the simplest case, several interdependent characteristics must be simultaneously optimized, among which are large quality factors at each individual wavelength and nonlinear overlaps, which must be controlled in as small a volume as possible. In such a situation, the templates of the aforementioned standard library offer no clear or best way to proceed; there is no definite reason to expect that an optimal design can be found in any of the traditional templates, or that such a design necessarily exists. Moreover, the performance of a given nonlinear device is likely to be highly dependent on the particular characteristics of the problem, and as greater demands are placed on functionality it becomes increasingly doubtful that any one class of structures will have the broad applicability of past devices. This lack of evident strategies for broadband applications also brings to attention the space of structures included in the standard photonic library. Predominately, traditional designs are repetitive mixtures and combinations of highly symmetric shapes described by a small collection of parameters. Since intuition-based optimization is then carried out ...

Coronavirus disease 2019 (COVID-19) in humans is often a clinically mild illness, but some individuals develop severe pneumonia, respiratory failure and death1–4. Studies of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection in hamsters5–7 and nonhuman primates8–10 have generally reported mild clinical disease, and preclinical SARS-CoV-2 vaccine studies have demonstrated reduction of viral replication in the upper and lower respiratory tracts in nonhuman primates11–13. Here we show that high-dose intranasal SARS-CoV-2 infection in hamsters results in severe clinical disease, including high levels of virus replication in tissues, extensive pneumonia, weight loss and mortality in a subset of animals. A single immunization with an adenovirus serotype 26 vector-based vaccine expressing a stabilized SARS-CoV-2 spike protein elicited binding and neutralizing antibody responses and protected against SARS-CoV-2-induced weight loss, pneumonia and mortality. These data demonstrate vaccine protection against SARS-CoV-2 clinical disease. This model should prove useful for preclinical studies of SARS-CoV-2 vaccines, therapeutics and pathogenesis.

We show both theoretically and experimentally that a pair of inductively coupled active LRC circuits (dimer), one with amplification and another with an equivalent amount of attenuation, display all the features which characterize a wide class of non-Hermitian systems which commute with the joint parity-time PT operator: typical normal modes, temporal evolution, and scattering processes. Utilizing a Liouvilian formulation, we can define an underlying PT -symmetric Hamiltonian, which provides important insight for understanding the behavior of the system. When the PT -dimer is coupled to transmission lines, the resulting scattering signal reveals novel features which reflect the PT -symmetry of the scattering target. Specifically we show that the device can show two different behaviors simultaneously, an amplifier or an absorber, depending on the direction and phase relation of the interrogating waves. Having an exact theory, and due to its relative experimental simplicity, PT -symmetric electronics offers new insights into the properties of PT -symmetric systems which are at the forefront of the research in mathematical physics and related fields.

We present a computational framework for efficient optimization-based "inverse design" of large-area "metasurfaces" (subwavelength-patterned surfaces) for applications such as multiwavelength/multi-angle optimizations, and demultiplexers. To optimize surfaces that can be thousands of wavelengths in diameter, with thousands (or millions) of parameters, the key is a fast approximate solver for the scattered field. We employ a "locally periodic" approximation in which the scattering problem is approximated by a composition of periodic scattering problems from each unit cell of the surface, and validate it against brute-force Maxwell solutions. This is an extension of ideas in previous metasurface designs, but with greatly increased flexibility, e.g. to automatically balance tradeoffs between multiple frequencies or to optimize a photonic device given only partial information about the desired field. Our approach even extends beyond the metasurface regime to non-subwavelength structures where additional diffracted orders must be included (but the period is not large enough to apply scalar diffraction theory).

We formulate and exploit a computational inverse-design method based on topology optimization to demonstrate photonic crystal structures supporting complex spectral degeneracies. In particular, we discover photonic crystals exhibiting third-order Dirac points formed by the accidental degeneracy of monopolar, dipolar, and quadrupolar modes. We show that, under suitable conditions, these modes can coalesce and form a third-order exceptional point, leading to strong modifications in the spontaneous emission (SE) of emitters, related to the local density of states. We find that SE can be enhanced by a factor of 8 in passive structures, with larger enhancements ∼ ffiffiffiffiffi n 3 p possible at exceptional points of higher order n. [3,4], and as precursors to nontrivial topological effects [5][6][7]. Recent work also showed that Dirac-point degeneracies can give rise to rings of exceptional points [8]. An exceptional point (EP) is a singularity in a non-Hermitian system where two or more eigenvectors and their corresponding complex eigenvalues coalesce, leading to a nondiagonalizable, defective Hamiltonian [9,10]. EPs have been studied in various physical contexts, most notably lasers and atomic as well as molecular systems [11,12]. In recent decades, interest in EPs has been reignited in connection with non-Hermitian parity-time symmetric systems [13], especially optical media involving carefully designed gain and loss profiles [14][15][16][17][18][19][20], where they can lead to intriguing phenomena such as excess noise [21,22], chiral modes [23], directional transport [24,25], and anomalous lasing behavior [26][27][28]. Also recently, it became possible to directly observe EPs in photonic crystals (PhCs) [8] and optoelectronic microcavities [29]. Thus far, however, the main focus of these works has been the effect of second-order exceptional points (EP2s) realized through photonic radiations, where only two modes coalesce; apart from a few mathematical analyses [30][31][32] or works focused on acoustic systems [33], there has been little or no investigation into the design and consequences of EPs of higher order (where more than two modes collapse).In this Letter, we formulate and exploit a powerful inverse-design method, based on topology optimization (TO), to develop complex photonic crystals supporting Dirac points formed out of the accidental degeneracy [34] of modes belonging to different symmetry representations. We show that such higher-order Dirac points can be exploited to create third-order exceptional points (EP3s) along with complex contours of EP2s. Furthermore, we consider possible enhancements and spectral modifications in the spontaneous emission (SE) rate of emitters, showing that the local density of states (LDOS) at an EP3 (14) can be enhanced eightfold (in passive systems) and can exhibit a cubic Lorentzian spectrum under special conditions. More generally, we find enhancement factors ∼ ffiffiffiffiffi n 3 p with increasing EP order n. Although the area of photonic inverse design is not new ...