Jinwei Shi

Chirality detection of enantiomers using twisted optical metamaterials

Many naturally occurring biomolecules, such as amino acids, sugars and nucleotides, are inherently chiral. Enantiomers, a pair of chiral isomers with opposite handedness, often exhibit similar physical and chemical properties due to their identical functional groups and composition, yet show different toxicity to cells. Detecting enantiomers in small quantities has an essential role in drug development to eliminate their unwanted side effects. Here we exploit strong chiral interactions with plasmonic metamaterials with specifically designed optical response to sense chiral molecules down to zeptomole levels, several orders of magnitude smaller than what is typically detectable with conventional circular dichroism spectroscopy. In particular, the measured spectra reveal opposite signs in the spectral regime directly associated with different chiral responses, providing a way to univocally assess molecular chirality. Our work introduces an ultrathin, planarized nanophotonic interface to sense chiral molecules with inherently weak circular dichroism at visible and near-infrared frequencies.

Method for measuring the threshold value of stimulated Brillouin scattering in water

A new method for accurate measurement of the threshold value of stimulated Brillouin scattering (SBS) in water in terms of pump laser intensity is investigated. The threshold value of SBS is determined by the point of the deviation of the value of the attenuation coefficients of wide- and narrow-linewidth lasers rather than the intensity of the backscattered SBS signal in the material.

High performance plasmonic random laser based on nanogaps in bimetallic porous nanowires

A plasmonic random laser is fabricated using gold-silver bimetallic porous nanowires with abundant nanogaps that provide strong feedback or gain channels for coherent lasing from dye molecules. The strong confinement of the nanogaps allows the bimetallic porous nanowire-based random laser, which is pumped by ns pulses, to operate with a very low threshold and extremely low concentrations of Rhodamine 6 G (as low as 0.067 mM). This random laser can be used as a pump source for another coherent random laser based on oxazine. These results provide a basis for studies of coherent random lasing pumped by another random laser.

White light emission with red-green-blue lasing action in a disordered system of nanoparticles

White light emission from a disordered system with intensity feedback is investigated. The gain material is an ethanol solution with three laser dyes (Coumarin 440, Coumarin 6, Oxazine), and titanium dioxide (TiO2) nanoparticles to provide feedback. A single pulsed laser beam at 355 nm with 8 ns pulse duration is used to pump the dyes. Coumarin 440 and Coumarin 6 are excited first by the pump beam, and part of the Coumarin 6 peak pumps Oxazine. Bright white light emission is obtained by combining blue, green, and red beams with a threshold effect. The working properties and system emission characteristics are discussed.

Experimental investigation on line width compression of stimulated Brillouin scattering in water

Line width compression of stimulated Brillouin scattering (SBS) in water was investigated experimentally. The results show that, when the water temperature is low, the compressing effect is obvious. However, when the temperature is higher than 25 °C, the line width is almost not compressed in water. Also, the pulse duration compression of SBS was measured simultaneously, and appeared an inherent relation to line width compression. It reveals that the line width can be simply measured by measuring the compressed pulse duration of SBS.

Single quantum dot controls a plasmonic cavity’s scattering and anisotropy

Significance We experimentally demonstrate that a single semiconductor quantum dot placed in close proximity to a plasmonic cavity (i.e., a spherical metallic nanoparticle) can be used to control the scattering spectrum and anisotropy of the latter. The scattering spectrum of the hybrid structure features a Fano resonance mediated by single photon absorption/scattering. This result is highly counterintuitive because the scattering cross sections of these two nanoparticles differ by four orders of magnitude. Our work represents a critical step toward realizing quantum plasmonic nanostructures that are capable of producing scattered light, which, depending on its polarization state, obeys either quantum or classical statistics. Furthermore, our work enables a hybrid orientation sensor unaffected by photobleaching of quantum dots. Plasmonic cavities represent a promising platform for controlling light–matter interaction due to their exceptionally small mode volume and high density of photonic states. Using plasmonic cavities for enhancing light’s coupling to individual two-level systems, such as single semiconductor quantum dots (QD), is particularly desirable for exploring cavity quantum electrodynamic (QED) effects and using them in quantum information applications. The lack of experimental progress in this area is in part due to the difficulty of precisely placing a QD within nanometers of the plasmonic cavity. Here, we study the simplest plasmonic cavity in the form of a spherical metallic nanoparticle (MNP). By controllably positioning a semiconductor QD in the close proximity of the MNP cavity via atomic force microscope (AFM) manipulation, the scattering spectrum of the MNP is dramatically modified due to Fano interference between the classical plasmonic resonance of the MNP and the quantized exciton resonance in the QD. Moreover, our experiment demonstrates that a single two-level system can render a spherical MNP strongly anisotropic. These findings represent an important step toward realizing quantum plasmonic devices.

Interplay Between Optical Bianisotropy and Magnetism in Plasmonic Metamolecules

The smallness of natural molecules and atoms with respect to the wavelength of light imposes severe limits on the nature of their optical response. For example, the well-known argument of Landau and Lifshitz and its recent extensions that include chiral molecules show that the electric dipole response dominates over the magneto-electric (bianisotropic) and an even smaller magnetic dipole optical response for all natural materials. Here, we experimentally demonstrate that both these responses can be greatly enhanced in plasmonic nanoclusters. Using atomic force microscopy nanomanipulation technique, we assemble a plasmonic metamolecule that is designed for strong and simultaneous optical magnetic and magneto-electric excitation. Angle-dependent scattering spectroscopy is used to disentangle the two responses and to demonstrate that their constructive/destructive interplay causes strong directional scattering asymmetry. This asymmetry is used to extract both magneto-electric and magnetic dipole responses and to demonstrate their enhancement in comparison to ordinary atomistic materials.

Alignment-free three-dimensional optical metamaterials

Three-dimensional optical metamaterials based on multilayers typically rely on critical vertical alignment to achieve the desired functionality. Here the conditions under which three-dimensional metamaterials with different functionalities may be realized without constraints on alignment are analyzed and demonstrated experimentally. This study demonstrates that the release of alignment constraints for multilayered metamaterials is allowed, while their anomalous interaction with light is preserved.

Cavity coupling in a random laser formed by ZnO nanoparticles with gain materials

Cavity coupling in a random laser with a weakly scattering disordered structure formed by ZnO nanoparticles is observed experimentally. The lasing characteristics are quite different from those of a traditional random laser. It is found that the threshold of coherent radiation with gain materials in such a structure is considerably low, and the emission spectrum and the threshold of each peak are orientationally uniform; the possible positions of the coherent peaks are fixed. These characteristics will be very useful in its applications. A new physical mechanism, cavity coupling, is suggested to discuss the lasing system. Nano-scale scatterers play an important role in providing randomly distributed feedback.

Cascade-pumped random lasers with coherent emission formed by Ag–Au porous nanowires

A series of sequentially cascade-pumped random lasers is reported. It consists of three random lasers in which the Ag-Au bimetallic porous nanowires play the role of scatterers, and the gain materials are coumarin 440 (C440), coumarin 153 (C153), and rhodamine 6G (R6G), respectively. The random laser with C440 is first pumped by a 355 nm pulsed laser. The emission of C440 pumps the C153, and the emission of C153 pumps the R6G sequentially. Low-threshold coherent emissions from the three random lasers are observed. The cascade-pumped random lasers can be achieved easily with low cost and can be used in applications conveniently.