Matter structured on a length scale comparable to or smaller than the wavelength of light can exhibit unusual optical properties. Particularly promising components for such materials are metal nanostructures, where structural alterations provide a straightforward means of tailoring their surface plasmon resonances and hence their interaction with light. But the top-down fabrication of plasmonic materials with controlled optical responses in the visible spectral range remains challenging, because lithographic methods are limited in resolution and in their ability to generate genuinely three-dimensional architectures. Molecular self-assembly provides an alternative bottom-up fabrication route not restricted by these limitations, and DNA- and peptide-directed assembly have proved to be viable methods for the controlled arrangement of metal nanoparticles in complex and also chiral geometries. Here we show that DNA origami enables the high-yield production of plasmonic structures that contain nanoparticles arranged in nanometre-scale helices. We find, in agreement with theoretical predictions, that the structures in solution exhibit defined circular dichroism and optical rotatory dispersion effects at visible wavelengths that originate from the collective plasmon-plasmon interactions of the nanoparticles positioned with an accuracy better than two nanometres. Circular dichroism effects in the visible part of the spectrum have been achieved by exploiting the chiral morphology of organic molecules and the plasmonic properties of nanoparticles, or even without precise control over the spatial configuration of the nanoparticles. In contrast, the optical response of our nanoparticle assemblies is rationally designed and tunable in handedness, colour and intensity-in accordance with our theoretical model.

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Our calculations show that a nonchiral nanocrystal is able to dramatically change the circular dichroism (CD) of a chiral molecule when the nanocrystal and molecule form a complex and couple via dipole and multipole Coulomb interactions. Plasmon resonances of metal nanocrystals in the nanocrystal-molecule complex result in both the resonant enhancement of CD signals of molecules and the appearance of new spectral structures. Two mechanisms, in which a nanocrystal can influence the CD effect, have been identified. The first mechanism is the plasmon-induced change in the electromagnetic field inside the chiral molecule. The second is the optical absorption of the nanocrystal-molecule complex due to the chiral currents inside the metal nanocrystal induced by the dipole of the chiral molecule. The first mechanism creates a change in the angle between the effective electric and magnetic dipoles of the molecule. This mechanism can lead to symmetry breaking and to a plasmon-induced CD signal of the nonchiral molecule. Both mechanisms create interesting Fano-like shapes in the CD spectra. Importantly, the second mechanism gives the main contribution to the CD signal at the plasmon frequency when the absorption band of the chiral molecule is far from the plasmon resonance. This may happen in many cases since many biomolecules are optically active in the UV range, whereas plasmon resonances in commonly used nanometals are found at longer wavelengths. As concrete examples, the paper describes alpha-helix and calixarene ligand molecules coupled with metal nanocrystals. The above results are also applied to complexes incorporating semiconductor nanocrystals. The results obtained here can be used to design a variety of hybrid nanostructures with enhanced and tailored optical chirality in the visible wavelength range.

A reconfigurable plasmonic nanosystem combines an active plasmonic structure with a regulated physical or chemical control input. There have been considerable e orts on integration of plasmonic nanostructures with active platforms using topdown techniques. The active media include phase-transition materials, graphene, liquid crystals and carrier-modulated semiconductors, which can respond to thermal 1 , electrical 2 and optical stimuli 3-5 . However, these plasmonic nanostructures are often restricted to two-dimensional substrates, showing desired optical response only along specific excitation directions. Alternatively, bottom-up techniques o er a new pathway to impart reconfigurability and functionality to passive systems. In particular, DNA has proven to be one of the most versatile and robust building blocks 6-9 for construction of complex three-dimensional architectures with high fidelity 10-14 . Here we show the creation of reconfigurable three-dimensional plasmonic metamolecules, which execute DNA-regulated conformational changes at the nanoscale. DNA serves as both a construction material to organize plasmonic nanoparticles in three dimensions, as well as fuel for driving the metamolecules to distinct conformational states. Simultaneously, the threedimensional plasmonic metamolecules can work as optical reporters, which transduce their conformational changes in situ into circular dichroism changes in the visible wavelength range.Circular dichroism (CD), that is, differential absorption of left-and right-handed circularly polarized light, of natural chiral macromolecules is highly sensitive to their three-dimensional (3D) conformations 15 . Taking a similar strategy, we create 3D reconfigurable plasmonic chiral metamolecules 4,16 , whose conformation changes are highly correlated with their pronounced and distinct CD spectral changes in the visible wavelength range. Figure 1a shows the design schematic. Two gold nanorods (AuNRs) are hosted on a reconfigurable DNA origami template 7,10 , which consists of two 14-helix bundles (80 nm × 16 nm × 8 nm) folded from a long single-stranded DNA (ssDNA) scaffold with the help of hundreds of staple strands 13 . The two origami bundles are linked together by the scaffold strand passing twice between them at one point. To ensure the mobility of the DNA bundles and avoid the formation of a Holliday junction 17 , 8 unpaired bases are introduced to each ssDNA connector (Supplementary Note 1). Twelve binding sites are extended from each origami bundle for robust assembly of one AuNR (38 nm × 10 nm) functionalized with complementary DNA (Supplementary Note 2). The surface to surface distance of the two AuNRs is roughly 25 nm. Owing to close proximity, the excited plasmons in the two AuNRs can be strongly coupled 18 . The two crossed AuNRs constitute a 3D plasmonic chiral object [19][20][21][22] , which generates a theme of handedness when interacting with left-and right-handed circularly polarized light, giving rise to strong CD. Left-handedRight-handed Two gold nanorods (...

Making use of self-assembly techniques, we realize nanoscopic semiconductor quantum rings in which the electronic states are in the true quantum limit. We employ two complementary spectroscopic techniques to investigate both the ground states and the excitations of these rings. Applying a magnetic field perpendicular to the plane of the rings, we find that, when approximately one flux quantum threads the interior of each ring, a change in the ground state from angular momentum ᐉ 0 to ᐉ 21 takes place. This ground state transition is revealed both by a drastic modification of the excitation spectrum and by a change in the magnetic-field dispersion of the single-electron charging energy. PACS numbers: 73.20.Dx, 03.65.Bz, 78.66.Fd The fascination of ringlike atomic and quantum structures dates back to Kekulé's famous proposal of the structure of benzene [1]. Particularly interesting are the magnetic properties of such nonsimply connected quantum systems, which are related to the possibility of trapping magnetic flux in their interior. Trapping of a single flux quantum in a small molecule such as benzene is impossible with the magnetic fields available in today's laboratories. In recent years, however, the availability of submicron solid-state ring structures has triggered a strong interest in the magnetic properties of rings, especially in view of the fact that, even in the presence of scattering, the many-particle ground state becomes chiral in a magnetic field, which leads to so-called "persistent currents" [2]. The large body of theoretical work on the properties of quantum rings [3] is accompanied by a number of ground breaking experimental investigations of the magnetic and transport properties of rings [4]. These studies have been carried out in the mesoscopic range, where scattering still influences the phase coherent transport, and a large number of quantum states are filled. To the best of our knowledge, no spectroscopic data is available on rings in the scatter-free, few-electron quantum limit. Furthermore, despite a strong theoretical interest [5,6], the only data available on the excitations of rings were taken on macroscopic structures [7].Here, we report on the spectroscopy of the ground states and excitations of self-assembled, nanoscopic InGaAs quantum rings, occupied with one or two electrons each, and subjected to magnetic fields 0 # B # 12 T, corresponding to 0-1.5 flux quanta threading the interior of the ring. In both ground state and excitation spectroscopies we observe characteristic changes at about B 8 T which are attributed to the development of a magneticfield-induced chiral ground state.The quantum rings are fabricated by solid-source molecular-beam epitaxy, using the Stranski-Krastanov growth mode, which has now become a well-established technique for the fabrication of high-quality, selfassembled semiconductor nanostructures [8]. Recently, we reported on a remarkable change in morphology when InAs self-assembled dots, grown on GaAs, are covered with a thin layer of GaAs and annealed...