Zhaowei Liu

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.

Superlenses to overcome the diffraction limit

The imaging resolution of conventional lenses is limited by diffraction. Artificially engineered metamaterials now offer the possibility of building a superlens that overcomes this limit. We review the physics of such superlenses and the theoretical and experimental progress in this rapidly developing field. Superlenses have great potential in applications such as biomedical imaging, optical lithography and data storage.

Optical Negative Refraction in Bulk Metamaterials of Nanowires

Negative refraction in metamaterials has generated great excitement in the scientific community. Although negative refraction has been realized in microwave and infrared by using metamaterials and by using two-dimensional waveguide structures, creation of a bulk metamaterial showing negative refraction at visible frequency has not been successful, mainly because of the significant resonance losses and fabrication difficulties. We report bulk metamaterials made of nanowires that show such negative refraction for all incident angles in the visible region. Moreover, the negative refraction occurs far from any resonance, resulting in a low-loss and a broad-band propagation at visible frequencies. These remarkable properties can substantially affect applications such as imaging, three-dimensional light manipulation, and optical communication.

Hyperlenses and metalenses for far-field super-resolution imaging

The resolution of conventional optical lens systems is always hampered by the diffraction limit. Recent developments in artificial metamaterials provide new avenues to build hyperlenses and metalenses that are able to image beyond the diffraction limit. Hyperlenses project super-resolution information to the far field through a magnification mechanism, whereas metalenses not only super-resolve subwavelength details but also enable optical Fourier transforms. Recently, there have been numerous designs for hyperlenses and metalenses, bringing fresh theoretical and experimental advances, though future directions and challenges remain to be overcome.

Spherical hyperlens for two-dimensional sub-diffractional imaging at visible frequencies

Hyperlenses have generated much interest recently, not only because of their intriguing physics but also for their ability to achieve sub-diffraction imaging in the far field in real time. All previous efforts have been limited to sub-wavelength confinement in one dimension only and at ultraviolet frequencies, hindering the use of hyperlenses in practical applications. Here, we report the first experimental demonstration of far-field imaging at a visible wavelength, with resolution beyond the diffraction limit in two lateral dimensions. The spherical hyperlens is designed with flat hyperbolic dispersion that supports wave propagation with very large spatial frequency and yet same phase speed. This allows us to resolve features down to 160 nm, much smaller than the diffraction limit at visible wavelengths, that is, 410 nm. The hyperlens can be integrated into conventional microscopes, expanding their capabilities beyond the diffraction limit and opening a new realm in real-time nanoscopic optical imaging.

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.

Rapid growth of evanescent wave by a silver superlens

Recent theoretical work suggested the possibility of constructing a super-resolution diffraction-free lens by using a negative refractive index medium (NRIM). The key proposition is that evanescent waves can be greatly enhanced by increasing the thickness of the NRIM. We report here experimental evidence that confirms that the transmission of evanescent waves rapidly grows with the film thickness up to about 50 nm, after which it decays as loss becomes significant. These findings represent the first step toward the understanding and realization of a diffraction-free lens.

Theory of the transmission properties of an optical far-field superlens for imaging beyond the diffraction limit

A conventional optical superlens for imaging beyond the diffraction limit produces images only in the near-field zone of the superlens. In contrast, an optical far-field superlens (FSL) device has a remarkable transmission property that leads to a one-to-one relationship between the far-field and the near-field angular spectra. This property makes the device suitable for imaging beyond the diffraction limit from far-field measurement. This specific FSL is composed of a properly designed periodically corrugated metallic slab-based superlens. Through the numerical design and parameter study, we show that the transmission property of this FSL is based on a specific strong-broadband wavenumber excitation of surface-plasmon polaritons supported by the nanostructured metallic grating.

Resonant and non-resonant generation and focusing of surface plasmons with circular gratings

We report the generation and focusing of surface plasmon polariton (SPP) waves from normally incident light on a planar circular grating milled into a silver film. The focusing mechanism is explained by using a simple coherent interference model of SPP generation on the circular grating by the incident field. Experimental results concur well with theoretical predictions and highlight the requirement for the phase matching of SPP sources in the grating to achieve the maximum enhancement of the SPP wave at the focal point. NSOM measurements show that the plasmonic lens achieves more than a 10-fold intensity enhancement over the intensity of a single ring of the in-plane field components at the focus when the grating design is tuned to the SPP wavelength. We discuss the techniques adaptability for surface enhanced nano-scale spectroscopy.

Large positive and negative lateral optical beam displacements due to surface plasmon resonance

We report abnormally large positive and negative lateral optical beam shifts at a metal–air interface when the surface plasmon resonance of the metal is excited. The optimal thickness for minimal resonant reflection is identified as the critical thickness above which a negative beam displacement is observed. Experimental results show good agreement with theoretical predictions and the large observed bidirectional beam displacements also indicate the existence of forward and backward surface propagating waves at the surface plasmon resonance of the metal.