JEOS:RP - Journal of the European Optical Society Rapid publications
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Resonant two-beam interferometric sensor independent of intracavity losses
Get PDFWe study a resonant interferometric sensor with two cavity eigenstates. The finesse associated to one eigenstate is significantly lower than the empty cavity value, because of the presence of a lossy intracavity sample. We show theoretically that the sensitivity of the interferometer only depends on the empty cavity finesse when the low-finesse eigenfrequency is locked to resonance. This is experimentally demonstrated and a resolution of 60 pm is reported. Our method can be applied to any resonant two-beam interferometer
Rigorous modeling and physical interpretation of terahertz near-field imaging
Get PDFApertureless scanning near-field optical microscopy (SNOM) operating with terahertz (THz) laser pulses is a subject of great research interest. The Mie scattering theory is commonly used to explain the features of the optical waves produced by field interactions with SNOM tips and microstructures. However, since Mie scattering fails with SNOMs at submillimeter wavelengths, a rigorous model and analysis are desirable to assess the feasibility of the THz tip-enhanced scanning near-field techniques. In this paper, we present a numerical simulation of an apertureless SNOM imaging system in the THz band. A 2-dimensional model based on the finite element method (FEM) is investigated and discussed. The modeling results are in good agreement with the experimental data obtained for this system at 2 THz radiation [H.-T. Chen at al., Phys. Rev. Lett. 93, 267401 (2004)]. Additionally, a physical interpretation using the antenna theory is successfully confirmed by the simulation results
Duty cycle tolerant binary gratings for fabricable short period phase masks
Get PDFWavelength scale 1D binary gratings of rectangular corrugation profile are often used as diffractive elements acting on incident free space waves under different incidence angle, wavelength and polarization. Their optical function is best understood by considering the interplay of the grating modes propagating up and down the periodic walls and slits of the segmented structure. The interference conditions between modes depend on the difference between the effective index of the interfering modes and on their relative amplitude. This difference and relative amplitude depend critically on the ratio between the wall and slit widths which is difficult to control technologically. The condition for a wide tolerance of the effective index difference and for a balanced mode excitation on the wall/slit ratio is found analytically and once for all for a wide class of 1D gratings. It is also found that TE interference elements may exhibit a very wide wall/slit ratio tolerance domain
Photopolymers: beyond the standard approach to photosensitisation
Get PDFA photopolymerizable material for use in the visible normally consists of a dye sensitizer, a free radical generator, monomer/monomers and, optionally, a binder, and is prepared usually as a dry or liquid film. Upon illumination with light of appropriate wavelength, a photopolymerisation reaction is triggered, leading to a change in the film’s refractive index. Because the presence of the dye is essential, the film is usually made sensitive to light during its preparation. Here we separate the film preparation and the sensitisation processes. In this way the photopolymerisation process can be used to detect dye labelled analytes, providing an alternative to fluorescence detection methods that offers visual, easy to interpret information. Our approach also allows for a precise control of the spatial location of the photoinduced refractive index changes that can be utilised for holography, lithography and photonic device fabrication
Gain-controlled wave chaos in a chaotic optical fibre
Get PDFIn this paper, we present a non-standard fibre amplifier specially designed to amplify scar modes of a multimode chaotic optical fibre. More precisely, we introduce Ytterbium in the optical fibre as a gain medium localised on the maximum of intensity of the scar modes. After briefly recalling the relevance of a chaotic optical fibre as a device to visualise quantum chaos, we describe the amplification process of scars. We present some numerical results that demonstrate the selective amplification of scar modes, with an amplification rate proportional to the overlap between these modes and the gain area
Anamorphic zoom system based on liquid crystal displays
Get PDFIn this work we present an anamorphic zoom system, based on liquid crystal displays, which allows changing the magnification and distortion of an image very quickly, without mechanical parts and keeping the output plane stationary. The anamorphic lenses that make up the optical processor are obtained by displaying a combination of convergent-divergent cylindrical lenses, with arbitrary orientations, onto the spatial light modulators. In order to illustrate the capabilities of the system, some experimental results are show
Design of passive ring resonators to be used for sensing applications
Get PDFIn this paper we report on the effects of two optical beams counterpropagating in a passive ring resonator that is the building block of a lot of sensing applications. By using the transfer matrix method in combination with the coupled mode theory, the analytical expressions of the power transfer functions for drop and through port configurations are derived in both cases of single beam and double beams inside the ring. The implemented model has shown some improvements in the resonator performance, such as the increase of the transmission power and the reduction of the linewidth, when the interaction between the two beams is considered, with respect to the single beam ring resonator configuration
Increasing the lateral resolution of scanning microscopes by a factor of two using 2-Image microscopy
Get PDFIncreasing the resolution of optical microscopes is a challenging task for studying the cell machinery at the molecular level. 4Pi or TIRF microscopies permit one to reduce the axial dimension of the detection volume. To reduce its lateral dimension, we have proposed a solution in which the scanning head of a 4Pi microscope or of a confocal microscope is coupled to an interferometer. With this technique two beams coming from the source produce two images that are superimposed coherently. For this reason, one can call this technique 2-Image microscopy. It has been shown that with 2-Image microscopy, the complete use of the spatial frequencies collected by the objective allows to reach a 1.22 lambda/4NA lateral resolution. This improvement is independent of the excitation mode and is effective with incoherent light such as fluorescent or chemiluminescent (i.e. without optical excitation) samples. In this paper, we present an interferometric set-up and a modulation technique that make benefit fully from the advantages of 2-Image microscopy
Imaging the coupling of terahertz radiation to a high electron mobility transistor in the near-field
Get PDFWe used AlGaN/GaN high electron mobility transistors as room-temperature direct detectors of radiation at 0.15 THz from a free electron laser, hence 5 times higher than their cutoff frequency of 30 GHz. By near-field active mapping we investigated the antenna-like coupling of the radiation to the transistor channel. We formulate a model for the detection based on self-mixing in the transistor channel. The noise equivalent power is found in the range of 10^{-7} W/Hz^{0.5} without any optimization of the device responsivity. Present day AlGaN/GaN fabrication technology may provide operation at higher frequency, integration of amplifiers for improved responsivity and fast switches for multiplexing, which make the detector here described the basic element of a monolithic terahertz focal plane array
Scattering cancellation by metamaterial cylindrical multilayers
Get PDFIn this paper, we present the theoretical analysis and the design of cylindrical multilayered electromagnetic cloaks based on the scattering cancellation technique. We propose at first the analysis and the design of bi-layered cylindrical shells, made of homogenous and isotropic metamaterials, in order to effectively reduce the scattered field from a dielectric cylindrical object. The single shell and the bi-layered shell cases are compared in terms of scattering reduction and loss effects. The comparison shows that the bi-layered configuration exhibits superior performances. The scattering cancellation approach, is, then, extended to the case of generic multilayered cylindrical shells, considering again homogeneous and isotropic metamaterials. The employment of the proposed technique to the case of cloaking devices working at multiple frequencies is also envisaged and discussed. Finally, some practical layouts of cylindrical electromagnetic cloaks working at optical frequencies are also proposed. In these configurations, the homogenous and isotropic metamaterials are replaced by their actual counterparts, obtained using alternating stacked plasmonic and non-plasmonic layers. The theoretical formulation and the design approaches presented throughout the paper are validated through proper full-wave numerical simulations