Hyesog Lee

Far-field optical hyperlens magnifying sub-diffraction-limited objects

The diffraction limit of light, which is causd by the loss of evanescent waves in the far field that carry high spatial frequency information, limits the resolution of optical lenses to the order of the wavelength of light. We report experimental demonstration of the optical hyperlens for sub-diffraction-limited imaging in the far field. The device magnifies subwavelength objects by transforming the scattered evanescent waves into propagating waves in an anisotropic medium and projects the high-resolution image at far field. The optical hyperlens opens up possibilities in applications such as real-time biomolecular imaging and nanolithography.

Development of optical hyperlens for imaging below the diffraction limit

We report here the design, fabrication and characterization of optical hyperlens that can image sub-diffraction-limited objects in the far field. The hyperlens is based on an artificial anisotropic metamaterial with carefully designed hyperbolic dispersion. We successfully designed and fabricated such a metamaterial hyperlens composed of curved silver/alumina multilayers. Experimental results demonstrate far-field imaging with resolution down to 125nm at 365nm working wavelength which is below the diffraction limit.

Tuning the focus of a plasmonic lens by the incident angle

We report the experimental realization of tuning the focus position of a plasmonic lens by adjusting the angle of the incident light similar to conventional lenses. A circular slit in silver film acts as both a surface plasmon polariton coupler and a plasmonic focusing lens. At small incident angles, the plasmonic lens has a very good focus with the position depending only on the angle of the incident beam. Numerical simulations of the focusing properties, including polarization dependence, agree well with experimental observations. This tunable plasmonic lens can be used in nanoscale photonics, biological sensing, and manipulation.

Experimental studies of far-field superlens for sub-diffractional optical imaging.

Contrary to the conventional near-field superlensing, subwavelength superlens imaging is experimentally demonstrated in the far-field. The key element is termed as a Far-field SuperLens (FSL) which consists of a conventional superlens and a nanoscale coupler. The evanescent fields from the object are enhanced and then converted into propagating fields by the FSL. By only measuring the propagating field in the far-field, the object image can be reconstructed with subwavelength resolution. As an example of this concept, we design and fabricate a silver structured one dimensional FSL. Experimental results show that feature resolution of better than 50nm is possible using current FSL design.

Broad Band Two-Dimensional Manipulation of Surface Plasmons

A plasmonic interference pattern can be formed when multiple surface plasmon waves overlap coherently. Utilizing a sharp edge coupling mechanism, we experimentally demonstrate plasmonic interference patterns that can be designed at will by shaping the edges in a metallic film. The patterns can also be dynamically tailored by adjusting the wavelength, the polarization, and the incident angle of the excitation light beam. Possessing the subdiffraction limited feature resolution, this dynamical manipulation method of surface plasmon patterns will have profound potentials in nanolithography, particle manipulation, and other related fields.

Deep subwavelength nanolithography using localized surface plasmon modes on planar silver mask

The development of a near-field optical lithography is presented in this paper. By accessing short modal wavelengths of localized surface plasmon modes on a planar metallic mask, the resolution can be significantly increased while using conventional UV light source. Taking into account the real material properties, numerical studies indicate that the ultimate lithographic resolution at 20nm is achievable through a silver mask by using 365nm wavelength light. The surface quality of the silver mask is improved by adding an adhesion layer of titanium during the mask fabrication. Using a two-dimensional hole array silver mask, we experimentally demonstrated nanolithography with half-pitch resolution down to 60nm, far beyond the resolution limit of conventional lithography using I-line (365nm) wavelength.

Tuning the far-field superlens: from UV to visible.

A far-field optical superlens, which is able to form sub-diffraction- limited images in the far field at UV wavelength, was recently demonstrated. In current work we present two methods to tune the working wavelength from UV to visible by tuning either the permittivity of the surrounding medium or that of the metal. A practical design is provided for each method. The tunable far-field superlens enables possible applications of the far-field superlens in sub-diffraction-limited imaging and sensing over a wide range of wavelength.

Near-field Moiré effect mediated by surface plasmon polariton excitation

We have demonstrated a surface plasmon polariton mediated optical Moiré effect by inserting a silver slab between two subwavelength gratings. Enhancement of the evanescent fields by the surface plasmon excitations on the silver slab leads to a remarkable contrast improvement in the Moiré fringes from two subwavelength gratings. Numerical calculations, which agree very well with the experimental observation of evanescent-wave Moiré fringes, elucidate the crucial role of the surface plasmon polaritons. The near-field Moiré effect has potential applications to extend the existing Moiré techniques to subwavelength characterization of nanostructures.

Dislocation microstructures on flat and stepped Si surfaces: Guidance for growing high‐quality GaAs on (100) Si substrates

Type‐I dislocations at the GaAs/Si interface are beneficial because they effectively relax the mismatched stress, but do not propagate into the GaAs film. Accordingly, the best way to grow a low defect density GaAs film on a Si substrate is to form as many as possible type‐I dislocations or, equivalently, to suppress other kinds of defects. The high‐resolution transmission electron microscopy study shows that most of the type‐I dislocations are formed at the double step on a Si surface. It is further determined that the silicon surface steps are mainly due to the substrate tilting instead of the heating before growth. Based on our study, the (100) Si substrate with double steps along both [110] and [110] axes provides the best condition for growing low defect density GaAs on Si substrates.

Gate-controlled magnetic properties of the magnetic semiconductor (Zn,Co)O

Electric field-controlled ferromagnetism of (Zn,Co)O is demonstrated via anomalous Hall effect measurements. The electron carrier concentration in this material is 1.65x10(20) cm(-3) as measured via ordinary Hall effect at 4 K, and an anomalous Hall effect is observed up to 6 K, but with no hysteresis at any temperature. With positive electric gate field, the carrier concentration is increased by approximately 2%, resulting in a clear magnetic hysteresis at 4 K. The ability to reversibly induceeliminate ferromagnetism by applied gate field alone, measured via the effect on the carriers, is a clear sign of carrier-induced ferromagnetism in this system.