Phase conjugation of an optical near field

Optical Time Reversal from Time-Dependent Epsilon-Near-Zero Media

Materials with a spatially uniform but temporally varying optical response have applications ranging from magnetic field-free optical isolators to fundamental studies of quantum field theories. However, these effects typically become relevant only for time variations oscillating at optical frequencies, thus presenting a significant hurdle that severely limits the realization of such conditions. Here we present a thin-film material with a permittivity that pulsates (uniformly in space) at optical frequencies and realizes a time-reversing medium of the form originally proposed by Pendry [Science 322, 71 (2008)SCIEAS0036-807510.1126/science.1162087]. We use an optically pumped, 500 nm thick film of epsilon-near-zero (ENZ) material based on Al-doped zinc oxide. An incident probe beam is both negatively refracted and time reversed through a reflected phase-conjugated beam. As a result of the high nonlinearity and the refractive index that is close to zero, the ENZ film leads to time reversed beams (simultaneous negative refraction and phase conjugation) with near-unit efficiency and greater-than-unit internal conversion efficiency. The ENZ platform therefore presents the time-reversal features required, e.g., for efficient subwavelength imaging, all-optical isolators and fundamental quantum field theory studies.

A Phase-Conjugate-Mirror Inspired Approach for Building Cloaking Structures with Left-handed Materials

A phase conjugate mirror (PCM) has a remarkable property of cancellation the back-scattering wave of the lossless scatterers. The similarity of a phase conjugate mirror to the interface of a matched RHM (right-handed material) and a LHM (left-handed material) prompts us to explore the potentials of using the RHM-LHM structure to achieve the anti-scattering property of the PCM. In this paper, we present two such structures. The first one is a RHM-LHM cloaking structure with a lossless arbitrary-shape scatterer imbedded in the RHM and its left-handed duplicate imbedded in the matched LHM. It is shown that such a structure is transparent to the incident electromagnetic (EM) field. As a special case of this structure, we proposed an EM tunnel that allows EM waves to spatially transport to another location in space without significant distortion and reflection. The second one is an RHM-PEC (perfect electric conductor)-LHM cloaking structure, which is composed of a symmetric conducting shell embedded in the interface junction of an RHM and the matched LHM layer. Such a structure presents an anomalously small scattering cross-section to an incident propagating EM field, and the interior of the shell can be used to shield small objects (size comparable to the wavelength) from interrogation. We report the results of 2D finite-element-method (FEM) simulations that were performed to verify our idea, and discuss the unique properties of the proposed structures as well as their limitations.

Recording of optical near fields in remote locations by near-field holography

We propose a method of near-field recording in a space that is quite apart from the original source (generator) of optical near fields. The method is based on the recently developed technique of near-field holography. Experiments based on our method have shown that near fields that originate from sub-diffraction-limit-sized objects can be stored in a photorefractive crystal 2 mm apart from the crystal surface, resulting in the retrieval of sub-diffraction-limit-sized spots. This means that our scheme can provide a method for multilayer (stackwise) near-field storage and, thus, contribute to a significant enhancement of the storage capacity of near-field optical memory.

Imaging tip formation in single-mode optical fibres

The formation of probe tips is a crucial step in all forms of scanning probe microscopy (SPM). In this work single-mode optical fibres are chemically etched in a variable temperature bath of etchant solution (HF acid buffered with ammonium fluoride) to produce tips for optical SPM. Tip evolution is monitored by prematurely truncating the etching process and imaging the tip end-structure using atomic force microscopy (AFM). In the case of a visible regime single-mode fibre the AFM images show a remarkable ring structure in the central cladding region and a tip structure in the core with a central depression; this serves to demonstrate the efficacy of chemical etching for converting compositional variation to three-dimensional topography. In the case of a standard, single-mode optical communications fibre the (projected) tip cone angle is assessed from AFM images in the early stages of tip formation. Values of the cone angle thus determined, for different etch conditions, are compared to those predicted by a model in which the independently determined core and cladding etch rates, and core diameter are the sole determinants of the final tip geometry. The model was devised in the context of etching multi-mode fibres and is shown to be valid here for single-mode fibres within the range of experimental accuracy and etch conditions examined.

Chemical etching of optical fibre tips--experiment and model

The formation of chemically etched fibre tips for use in optical scanning probe microscopy is addressed. For tips formed at a cleaved fibre end in the bulk of a buffered HF acid solution the morphological features (tip height, cone angle) are found to depend strongly on the temperature and etchant composition. The tip formation process is analysed and explained in terms of a simple model in which the only pertinent physical parameters are the fibre core diameter and etch rates of the fibre core and cladding. The etch rates are determined in separate experiments as a function of temperature (in the range 24-50 degrees C) for etchant solutions of de-ionised water: 50% HF acid: 40% NH4F in the volume ratio 1:1:X for X = 2, 4 and 6, and used in the model to yield a correct description of the experimental tip cone angles. The model is successfully extended to the intriguing case of negative tip formation which initiates in a normal, positive tip structure. By contrast, tip formation in the meniscus region of a bare fibre/etchant/organic solvent system is found to be independent of etchant composition and temperature.

Characterization of near-field optical probes

Radiation and collection characteristics of four different near-field optical-fiber probes, namely, three uncoated probes and an aluminum-coated small-aperture probe, are investigated and compared. Their radiation properties are characterized by observation of light-induced topography changes in a photosensitive film illuminated with the probes, and it is confirmed that the radiated optical field is unambiguously confined only for the coated probe. Near-field optical imaging of a standing evanescent-wave pattern is used to compare the detection characteristics of the probes, and it is concluded that, for the imaging of optical-field intensity distributions containing predominantly evanescent-wave components, a sharp uncoated tip is the probe of choice. Complementary results obtained with optical phase-conjugation experiments with the uncoated probes are discussed in relation to the probe characterization.