Yu-Chieh Cheng

Flat Focusing Mirror

The control of spatial propagation properties of narrow light beams such as divergence, focusing or imaging are main objectives in optics and photonics. In this letter, we propose and demonstrate experimentally a flat focusing mirror, based on an especially designed dielectric structure without any optical axis. More generally, it also enables imaging any light pattern in reflection. The flat focusing mirror with a transversal invariance can largely increase the applicability of structured photonic materials for light beam propagation control in small-dimension photonic circuits.

Beam focalization in reflection from flat dielectric subwavelength gratings

We experimentally demonstrate the recently predicted effect of near-field focusing for light beams from flat dielectric subwavelength gratings (SWGs). This SWGs were designed for visible light 532 nm and fabricated by direct laser writing in a negative photoresist, with the refractive index n=1.5 and the period d=314  nm. The laterally invariant gratings can focus light beams without any optical axis to achieve the transversal invariance. We show that focal distances can be obtained up to 13 μm at normal reflection for TE polarization.

Controllable light diffraction in woodpile photonic crystals filled with liquid crystal

An approach to switching between different patterns of light beams transmitted through the woodpile photonic crystals filled with liquid crystals is proposed. The phase transition between the nematic and isotropic liquid crystal states leads to an observable variation of the spatial pattern transmitted through the photonic structure. The transmission profiles in the nematic phase also show polarization sensibility due to refractive index dependence on the field polarization. The experimental results are consistent with a numerical calculation by Finite Difference Time Domain method.

Acoustically penetrable sonic crystals based on fluid-like scatterers

We propose a periodic structure that behaves as a fluid–fluid composite for sound waves, where the building blocks are clusters of rigid scatterers. Such building-blocks are penetrable for acoustic waves, and their properties can be tuned by selecting the filling fraction. The equivalence with a fluid–fluid system of such a doubly periodic composite is tested analytical and experimentally. Because of the fluid-like character of the scatterers, sound structure interaction is negligible, and the propagation can be described by scalar models, analogous to those used in electromagnetics. As an example, the case of focusing of evanescent waves and the guided propagation of acoustic waves along an array of penetrable elements is discussed in detail. The proposed structure may be a real alternative to design a low contrast and acoustically penetrable medium where new properties as those shown in this work could be experimentally realized.

Negative Goos-Hänchen shift in reflection from subwavelength gratings

Abstract. A negative Goos–Hänchen (GH) shift of light beam reflecting from planar microphotonic structures consisting of two parallel dielectric subwavelength diffraction gratings is predicted. The phenomenon occurs because the energy flow of the mode guided in the subwavelength structure is predominantly directed in the opposite direction to the energy flow of the incident beam. The numerical calculations show that the negative GH shift can reach approximately six times that of the wavelength in realistic structures, so that the phenomenon can be experimentally observed. In addition to the negative lateral shift, the focusing of the beam in reflection from the grating is also observed as a secondary effect.

Unlocked evanescent waves in periodic structures

We predict the existence of evanescent modes with unlocked phases in two-dimensional (2D) dielectric periodic structures. Contrary to what is known for one-dimensional structures, where evanescent fields lock to the host modulation, we show that in 2D systems a new class of evanescent modes exists with an unlocked real part of the wave vector. Hence, beams constructed from such unlocked evanescent waves can exhibit spatial effects. A significant focalization of a beam propagating within the band gap of a flat photonic crystal slab is also shown. The predicted phenomenon is expected to be generic for spatially modulated materials.

Flat focusing in reflection from a chirped dielectric mirror with a defect layer

Abstract. Recently, the principle of flat focusing based on one-dimensionally chirped dielectric mirrors has been proposed and experimentally demonstrated. The flat chirped mirror causes anomalous diffraction of the beam during reflection, opposite to the normal diffraction in free space propagation. The anomalous diffraction compensates the normal diffraction of the beam resulting in focusing after reflection. For a better focusing performance and for a larger near-field focal distance, a stronger anomalous diffraction is required. We show that the anomalous diffraction can be enhanced by introducing a defect layer in the chirped mirror, as the structure becomes similar to a Gires–Tournois interferometer. The focal distance can be substantially increased due to the defect layer. In our specific structure, the focal distance shows an increase from 19 to 39  μm, numerically.

Flat focusing mirrors

We present the flat focusing mirror - the dielectric mirrors capable to focus and image the narrow beams in reflections. The mirrors are flat, and transversally invariant, i.e. have no optical axis. We consider two different configurations of such mirrors: 1) Bragg-like mirrors, consisting of structure of specially designed dielectric layers; 2) periodically modulated surface on a subwavelength scale. We also discuss the combined case - the longitudinally chirped photonic crystals.

Beam focalization by chirped mirrors

A novel application of chirped dielectric mirrors for narrow beam focalization is proposed and demonstrated numerically and experimentally. Analogy to temporal dispersion compensation by chirped dielectric mirrors is discussed.

Near field focusing of beams reflected by flat mirror

Photonic crystals (PhCs) attracted a great amount of attention because of their capability to control the light propagation, in particular due to photonic band-gaps for plane waves in frequency domain. Spatial propagation of beams has been also considered in PhCs bringing to realization, among others, a negative refraction and consequently, allowing to build a flat PhC lens with normal- as well as with subwavelength resolution.