Mengxin Ren

Giant nonlinear optical activity in a plasmonic metamaterial

In 1950, a quarter of a century after his first-ever nonlinear optical experiment when intensity-dependent absorption was observed in uranium-doped glass, Sergey Vavilov predicted that birefringence, dichroism and polarization rotatory power should be dependent on light intensity. It required the invention of the laser to observe the barely detectable effect of light intensity on the polarization rotatory power of the optically active lithium iodate crystal, the phenomenon now known as the nonlinear optical activity, a high-intensity counterpart of the fundamental optical effect of polarization rotation in chiral media. Here we report that a plasmonic metamaterial exhibits nonlinear optical activity 30 million times stronger than lithium iodate crystals, thus transforming this fundamental phenomenon of polarization nonlinear optics from an esoteric phenomenon into a major effect of nonlinear plasmonics with potential for practical applications.

Nanostructured Plasmonic Medium for Terahertz Bandwidth All-Optical Switching

Periodic nanostructuring can enhance the optical nonlinearity of plasmonic metals by several orders of magnitude. By patterning a gold film, the largest sub-100 femtosecond nonlinearity is achieved, which is suitable for terahertz rate all-optical data processing as well as ultrafast optical limiters and saturable absorbers.

Tunable Band-Stop Filters for Graphene Plasmons Based on Periodically Modulated Graphene.

Tunable band-stop filters based on graphene with periodically modulated chemical potentials are proposed. Periodic graphene can be considered as a plasmonic crystal. Its energy band diagram is analyzed, which clearly shows a blue shift of the forbidden band with increasing chemical potential. Structural design and optimization are performed by an effective-index-based transfer matrix method, which is confirmed by numerical simulations. The center frequency of the filter can be tuned in a range from 37 to 53 THz based on the electrical tunability of graphene, while the modulation depth (−26 dB) and the bandwidth (3.1 THz) of the filter remain unchanged. Specifically, the bandwidth and modulation depth of the filters can be flexibly preset by adjusting the chemical potential ratio and the period number. The length of the filter (~750 nm) is only 1/9 of the operating wavelength in vacuum, which makes the filter a good choice for compact on-chip applications.

Linearly polarized light emission from quantum dots with plasmonic nanoantenna arrays.

Polarizers provide convenience in generating polarized light, meanwhile their adoption raises problems of extra weight, cost, and energy loss. Aiming to realize polarizer-free polarized light sources, herein, we present a plasmonic approach to achieve direct generation of linearly polarized optical waves at the nanometer scale. Periodic slot nanoantenna arrays are fabricated, which are driven by the transition dipole moments of luminescent semiconductor quantum dots. By harnessing interactions between quantum dots and scattered fields from the nanoantennas, spontaneous emission with a high degree of linear polarization is achieved from such hybrid antenna system with polarization perpendicular to antenna slot. We also demonstrate that the polarization is engineerable in aspects of both spectrum and magnitude by tailoring plasmonic resonance of the antenna arrays. Our findings will establish a basis for the development of innovative polarized light-emitting devices, which are useful in optical displays, spectroscopic techniques, optical telecommunications, and so forth.

Flexible modulation of plasmon-induced transparency in a strongly coupled graphene grating-sheet system.

General actively tunable near-field plasmon-induced transparency (PIT) systems based on couplings between localized plasmon resonances of graphene nanostructures not only suffer from interantenna separations of smaller than 20 nm, but also lack switchable effect about the transparency window. Here, the performance of an active PIT system based on graphene grating-sheet with near-field coupling distance of more than 100 nm is investigated in mid-infrared. The transparency window in spectrum is analyzed objectively and proved to be more likely stemmed from Aulter-Townes splitting. The proposed system exhibits flexible tunability in slow-light and electro-optical switches, promising for practical active photonic devices.

Broadband asymmetric transmission of optical waves from spiral plasmonic metamaterials

This study theoretically demonstrates a broadband circular asymmetric transmission effect on the basis of two dimensional planar spiral metamaterials. We found that by increasing the number of turns in a single spiral unit cell, the bandwidth of the asymmetric transmission effect will be broaden dramatically and reach to about 940 nm in the near-infrared spectral range.

Reconfigurable metasurfaces that enable light polarization control by light

Plasmonic metasurfaces have recently attracted much attention because of their novel characteristics with respect to light polarization and wave front control on deep-subwavelength scales. The development of metasurfaces with reconfigurable optical responses is opening new opportunities in high-capacity communications, real-time holograms and adaptive optics. Such tunable devices have been developed in the mid-infrared spectral range and operated in light intensity modulation schemes. Here we present a novel optically reconfigurable hybrid metasurface that enables polarization tuning at optical frequencies. The functionality of tuning is realized by switching the coupling conditions between the plasmonic modes and the binary isomeric states of an ethyl red switching layer upon light stimulation. We achieved more than 20° nonlinear changes in the transmitted polarization azimuth using just 4 mW of switching light power. Such design schemes and principles could be easily applied to dynamically adjust the functionalities of other metasurfaces.

Nanoscale beam splitters based on gradient metasurfaces

Beam splitters are essential components in various optical and photonic applications, for example, interferometers, multiplexers, and so on. Present beam splitters based on cubes or plates are normally bulky. Realizing beam splitters in nanoscales is useful to reduce the total size of photonic devices. We demonstrate here a beam splitter with nanoscale thickness based on a gradient metasurface comprising lithium niobate cylinder arrays. Since one unit cell of metasurface comprising two cylinder rows shows two opposite phase gradients, the incident light is split into different directions according to the generalized Snells law. The split ratio is proven to be effectively tunable.

Displacement sensor based on plasmonic slot metamaterials

In this paper, we demonstrate a plasmonic type displacement sensor based on slot metamaterials. The sensors are formed by arranging metamaterial arrays with different dimension parameters adjacently. Hence, the measured spectra would be modified as a result of moving the sensors across the detecting area of the spectrometer. From the spectral changes, the displacement amount could be retrieved. The sensor is demonstrated to be capable of recognizing a displacement of 200 nm, which is equal to the period of the metamaterial lattice, and the sensitivity is largely dependent on the shape and size of the acquisition area of the spectrometer used for spectra analysis.

Tailorable reflection of surface plasmons in defect engineered graphene

The electrical, optical, mechanical and thermal properties of graphene can be significantly altered by defects, thus engineering the defects in graphene is promising for applications in functionalized materials and nanoscale devices. Here the propagations of surface plasmon waves near graphene defect boundaries created by ion beams are studied. Specifically, plasmon reflections are observed near the induced defect boundaries for the first time, which implies that ion-irradiation induced defects act as efficient scattering centers for the plasmonic waves, just like the native grain boundaries. Moreover, engineering the defects with varied ion doses results in tailorable plasmon reflection properties due to changed defect degrees. The controllable plasmon reflections near ion induced defect boundaries open up a new avenue for plasmon wave engineering.