The ideas of topology have found tremendous success in Hermitian physical systems, but even richer properties exist in the more general non-Hermitian framework. Here, we theoretically propose and experimentally demonstrate a new topologically-protected bulk Fermi arc which-unlike the well-known surface Fermi arcs arising from Weyl points in Hermitian systems-develops from non-Hermitian radiative losses in photonic crystal slabs. Moreover, we discover half-integer topological charges in the polarization of far-field radiation around the Fermi arc. We show that both phenomena are direct consequences of the non-Hermitian topological properties of exceptional points, where resonances coincide in their frequencies and linewidths. Our work connects the fields of topological photonics, non-Hermitian physics and singular optics, and paves the way for future exploration of non-Hermitian topological systems.In recent years, topological physics has been widely explored in closed and lossless Hermitian systems, revealing novel phenomena such as topologically non-trivial band structures [1][2][3][4][5][6][7][8][9] and promising applications including backscattering-immune transport [10][11][12][13][14][15][16][17][18][19][20][21]. However, most systems, particularly in photonics, are generically non-Hermitian due to radiation into open space or material gain/loss. NonHermiticity enables even richer topological properties, often with no counterpart in Hermitian frameworks [22][23][24][25]. One such example is the emergence of a new class of degeneracies, commonly referred to as exceptional points (EPs), where two or more resonances of a system coalesce in both eigenvalues and eigenfunctions [26][27][28]. So far, isolated EPs in parameter space [29][30][31][32][33][34][35] and continuous rings of EPs in momentum space [36][37][38] have been studied across different wave systems due to their intriguing properties, such as unconventional transmission/reflection [39][40][41], relations to parity-time symmetry [42][43][44][45][46][47][48], as well as their unique applications in sensing [49,50] and single-mode lasing [51][52][53].Here, we theoretically design and experimentally realize a new configuration of isolated EP pairs in momentum space, which allows us to reveal the unique topological signatures of EPs in the band structure and far-field polarization, and to extend topological band theory into the realm of non-Hermitian systems. Specifically, we demonstrate that a Dirac point (DP) with nontrivial Berry phase can split into a pair of EPs [54][55][56] when radiation loss-a form of non-Hermiticity-is added to a 2D-periodic photonic crystal (PhC) structure. The EPpair generates a distinct double-Riemann-sheet topology in the complex band structure, which leads to two novel consequences: bulk Fermi arcs and polarization half charges. First, we discover and experimentally demonstrate that this pair of EPs is connected by an open-ended isofrequency contourwe refer to it as a bulk Fermi arc-in direct contrast to the common intuiti...

N-doped porous carbon produced via chemical activation of polypyrrole functionalized graphene sheets shows selective adsorption of CO(2) (4.3 mmol g(-1)) over N(2) (0.27 mmol g(-1)) at 298 K. The potential for large scale production and facile regeneration makes this material useful for industrial applications.

Exciton-polaritons in a microcavity are composite two-dimensional bosonic quasiparticles, arising from the strong coupling between confined light modes in a resonant planar optical cavity and excitonic transitions, typically using excitons in semiconductor quantum wells (QWs) placed at the antinodes of the same cavity. Quantum phenomena such as Bose-Einstein condensation (BEC) [1, 2], superfluidity [3], quantized vortices [4][5][6][7][8], and macroscopic quantum states [9, 10] have been realized at temperatures from tens of Kelvin up to room temperatures [11][12][13], and polaritonic devices such as spin switches [14] and optical transistors [15] have also been reported. Many of these effects of exciton-polaritons depend crucially on the polariton-polariton interaction strength. Despite the importance of this parameter, it has been difficult to make an accurate experimental measurement, mostly because of the difficulty of determining the absolute densities of polaritons and bare excitons. Here we report the direct measurement of the polariton-polariton interaction strength in a very high-Q microcavity structure. By allowing polaritons to propagate over 40 µm to the center of a lasergenerated annular trap, we are able to separate the polariton-polariton interactions from polariton-exciton interactions. The interaction strength is deduced from the energy renormalization of the polariton dispersion as the polariton density is increased, using the polariton condensation as a benchmark for the density. We find that the interaction strength is about two orders of magnitude larger than previous theoretical estimates, putting polaritons squarely into the strongly-interacting regime. When there is a condensate, we see a sharp transition to a different dependence of the renormalization on the density, which is evidence of many-body effects.Much of the physics of polaritons is dominated by the fact that they have extremely light effective mass. When an exciton is mixed with a cavity photon to become an excitonpolariton, it has an effective mass about four orders of magnitude less than a vacuum electron, and about three orders of magnitude less than a typical semiconductor quantum well exciton. (The Supplementary Information gives a basic introduction to the properties of exciton-polaritons.) Therefore one can view the polaritons as excitons which are given much longer diffusion length, with propagation distance of polaritons up to millimeters [16,17]; this may have implications for solar cells, which depend crucially on the diffusive 2 migration of excitons [18]. Alternatively, exciton-polaritons can be viewed as photons with nonlinear interactions many orders of magnitude higher than in typical optical materials, due to their excitonic components [19]. The light effective mass of the polaritons (typically 10 −8 that of a hydrogen atom) allows for quantum phenomena to be realized at much higher temperatures than in cold atomic gases.Interactions among polaritons. The interaction of exciton-polaritons is presumed to come...

The experimental realization of Bose-Einstein condensation (BEC) with atoms and quasiparticles has triggered wide exploration of macroscopic quantum effects. Microcavity polaritons are of particular interest because quantum phenomena such as BEC and superfluidity can be observed at elevated temperatures. However, polariton lifetimes are typically too short to permit thermal equilibration. This has led to debate about whether polariton condensation is intrinsically a nonequilibrium effect. Here we report the first unambiguous observation of BEC of optically trapped polaritons in thermal equilibrium in a high-Q microcavity, evidenced by equilibrium Bose-Einstein distributions over broad ranges of polariton densities and bath temperatures. With thermal equilibrium established, we verify that polariton condensation is a phase transition with a well-defined density-temperature phase diagram. The measured phase boundary agrees well with the predictions of basic quantum gas theory. DOI: 10.1103/PhysRevLett.118.016602 The realization of exciton-polariton condensation in semiconductor microcavities from liquid-helium temperature [1,2] all the way up to room temperature [3][4][5] presents great opportunities both for fundamental studies of many-body physics and for all-optical devices on the technology side. Polaritons in a semiconductor microcavity are admixtures of the confined light modes of the cavity and excitonic transitions, typically those of excitons in semiconductor quantum wells placed at the antinodes of the cavity. Quantum effects such as condensation [1][2][3][4][5], superfluidity [6], and quantized vortices [7][8][9][10][11] have been reported. The dual light-matter nature permits flexible control of polaritons and their condensates, facilitating applications in quantum simulation. It is also straightforward to measure the spectral functions, Aðk; ωÞ, of polaritons, which can provide insights into the dynamics of many-body interactions in polariton systems. For cold atoms, the equilibrium occupation numbers can be measured [12], but the spectral function is not readily accessible. Observations of non-Hermitian physics [13] and phase frustration [14] have shown that polaritons are an important complement to atomic condensates.However, in most previous experiments, the lifetime of the polaritons in microcavities has been 30 ps or less [15] due to leakage of the microcavity. Thus, although there have been claims to partial thermalization of polaritons [16,17], no previous work has unambiguously shown a condensation in thermal equilibrium, leading to the common description of polariton condensates as "nonequilibrium condensates" [18][19][20]. The theory of nonequilibrium condensation is still an active field [21][22][23][24]. Although polariton experiments and theory have shown that a great number of canonical features of condensation persist in nonequilibrium, e.g., superfluid behavior [22,23], some aspects may not [25,26], and debates persist over whether polariton condensates can be called Bose-Einstein c...

This corrects the article DOI: 10.1103/PhysRevLett.118.016602.

The detection of colon cancer using endoscopy is widely used, but the interpretation of the diagnosis is based on the clinician's naked eye. This is subjective and can lead to false detection. Here we developed a rapid and accurate molecular fluorescence imaging technique using antibody-coated quantum dots (Ab-QDs) sprayed and washed simultaneously on colon tumor tissues inside live animals, subsequently excited and imaged by endoscopy. QDs were conjugated to matrix metalloproteinases (MMP) 9, MMP 14, or carcinoembryonic antigen (CEA) Abs with zwitterionic surface coating to reduce nonspecific bindings. The Ab-QD probes can diagnose tumors on sectioned mouse tissues, fresh mouse colons stained ex vivo and also in vivo as well as fresh human colon adenoma tissues in 30 min and can be imaged with a depth of 100 μm. The probes successfully detected not only cancers that are readily discernible by bare eyes but also hyperplasia and adenoma regions. Sum and cross signal operations provided postprocessed images that can show complementary information or regions of high priority. This multiplexed quantum dot, spray-and-wash, and endoscopy approach provides a significant advantage for detecting small or flat tumors that may be missed by conventional endoscopic examinations and bestows a strategy for the improvement of cancer diagnosis.

We report multistate optical switching among high-order bouncing-ball modes ("ripples") and whispering-gallerying modes ("petals") of exciton-polariton condensates in a laser-generated annular trap. By tailoring the diameter and power of the annular trap, the polariton condensate can be switched among different trapped modes, accompanied by redistribution of spatial densities and superlinear increase in the emission intensities, implying that polariton condensates in this geometry could be exploited for a multistate switch. A model based on non-Hermitian modes of the generalized Gross-Pitaevskii equation reveals that this mode switching arises from competition between pump-induced gain and in-plane polariton loss. The parameters for reproducible switching among trapped modes have been measured experimentally, giving us a phase diagram for mode switching. Taken together, the experimental result and theoretical modeling advances our fundamental understanding of the spontaneous emergence of coherence and move us toward its practical exploitation. Condensates

No abstract