217 research outputs found
Combination of scanning probe technology with photonic nanojets
No full textLight focusing through a microbead leads to the formation of a photonic nanojet functional for enhancing the spatial resolution of traditional optical systems. Despite numerous works that prove this phenomenon, a method to appropriately translate the nanojet on top of a region of interest is still missing. Here, by using advanced 3D fabrication techniques we integrated a microbead on an AFM cantilever thus realizing a system to efficiently position nanojets. This fabrication approach is robust and can be exploited in a myriad of applications, ranging from microscopy to Raman spectroscopy. We demonstrate the potential of portable nanojets by imaging different sub-wavelength structures. Thanks to the achieved portability, we were able to perform a detailed optical characterization of the resolution enhancement induced by the microbead, which sheds light into the many contradictory resolution claims present in literature. Our conclusions are strongly supported by rigorous data analysis and by numerical simulations, all in perfect agreement with experimental results
Light coupling structures and switches for plasmonic coaxial waveguides
No full textWe introduce wavelength-scale light coupling structures and switches for plasmonic coaxial waveguides. We first consider single-slit structures optimized for a wavelength of 1550 nm and find that, when the slit is on resonance, the coupling to the plasmonic coaxial waveguide is maximized. We also observe that for optimized double- and triple-slit structures, the coupling efficiency is enhanced compared to the single-slit structure by factors of ˜3.02 and ˜4.21, respectively. We find that, in the case of double- and triple-slit structures, the surface plasmons excited at the metal-air interface enhance light coupling to the plasmonic coaxial waveguide via the slits. In addition, we investigate slit-based outcoupling structures for light extraction from the waveguide into a free space. We observe that while the far-field radiation pattern of single-slit structures is symmetric, double- and triple-slit structures have asymmetric radiation patterns. We also show that by exciting the incoupling slit structures at proper angles, we can excite only the right- or the left-propagating mode of the plasmonic coaxial waveguide. We finally design compact plasmonic switches consisting of a plasmonic coaxial waveguide side-coupled to a periodic array of two open-circuited coaxial stub resonators. Such a structure is based on a plasmonic analog of electromagnetically induced transparency and supports a slow-light mode. The space between the metallic parts is filled with an active material with a tunable refractive index. We show that the modulation depth of this structure is large enough for optical switching applications
Theoretical investigation of fabrication-related disorders on the properties of subwavelength metal-dielectric-metal plasmonic waveguides
No full textWe theoretically investigate the effect of fabrication-related disorders on subwavelength metal-dielectric-metal plasmonic waveguides. We use a Monte Carlo method to calculate the roughness-induced excess attenuation coefficient with respect to a smooth waveguide. For small roughness height, the excess optical power loss due to disorder is small compared to the material loss in a smooth waveguide. However, for large roughness height, the excess attenuation increases rapidly with height and the propagation length of the optical mode is severely affected. We find that the excess attenuation is mainly due to reflection from the rough surfaces. However, for small roughness correlation lengths, enhanced absorption is the dominant loss mechanism due to disorder. We also find that the disorder attenuation due to reflection is approximately maximized when the power spectral density of the disordered surfaces at the Bragg spatial frequency is maximized. Finally, we show that increasing the modal confinement or decreasing the guide wavelength, increase the attenuation due to disorder
Active plasmonic devices enhanced by waveguide dispersion engineering
No full textPlasmonic devices, based on surface plasmons propagating at metal-dielectric interfaces, have shown the potential to manipulate light at deep subwavelength scales. One of the main challenges in plasmonics is achieving active control of optical signals. In this paper, we introduce active plasmonic devices enhanced by waveguide dispersion engineering. We consider plasmonic waveguide systems consisting of a metal-dielectric-metal waveguide (MDM) side-coupled to arrays of MDM stub resonators. The MDM waveguide and stubs are filled with an active material whose absorption coefficient can be modified with an external control beam. Such plasmonic waveguide systems can be engineered to support slowlight modes. We find that, as the slowdown factor increases, the sensitivity of the effective index of the mode to variations of the refractive index of the active material increases. Such slow-light enhancements of the sensitivity to refractive index variations lead to enhanced performance of active plasmonic devices such as switches. To demonstrate this, we consider absorption switches based on Fabry-Perot cavity structures, consisting of slow-light plasmonic waveguide systems sandwiched between two conventional MDM waveguides. We find that increased slowdown factor leads to increased induced change of the propagation length of the slow-light mode for a given refractive index variation, and therefore to increased modulation depth. Compared to conventional MDM absorption switches, slow-light enhanced switches achieve significantly higher modulation depth with moderate insertion loss. We use a scattering matrix theory to account for the behavior of the devices which is in excellent agreement with numerical results obtained with the finitedifference frequency-domain method. © 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE)
Efficient design of nanoscale metal-dielectric-metal plasmonic waveguide devices
No full textWe develop a method for the efficient design of nanoscale metal-dielectric-metal plasmonic waveguide devices, based on the space-mapping algorithm and transmission line theory models. The method requires full-wave simulations of only a few candidate structures. © OSA 2013
Microsphere embedded in cantilever opens the AFM to high resolution optical microscopy
No full textSummary form only given. The combination of the AFM technique and the sphere-mediated microscopy (SMM) [1] opens a new opportunity to the Atomic Force Microscopy (AFM). With the help of a tipless AFM cantilever is possible to place and scan a microspheres (MS) close to the surface. From the optical point of view, when a MS is close to a surface act as high NA nanolenses whose optical characteristics define the maximum attainable resolution.We performed a detailed measurement of the spatial resolution in SMM by imaging a calibration target made of gratings with different periodicity. Images of the test gratings with and without the microsphere allowed a full characterization of the spatial frequency response of our microscope (modulation transfer function or MTF) and the consequent quantitative determination of the enhancement in resolution induced by the microsphere[2]
Efficient optimization of nanoplasmonic devices using space mapping
No full textWe show that the space-mapping algorithm, originally developed for microwave circuit optimization, can enable the efficient optimization of nanoplasmonic devices. Space-mapping utilizes a physics-based coarse model to approximate a fine model accurately describing a device. The main concept in the algorithm is to find a mapping that relates the fine and coarse model parameters. If such a mapping is established, we can then avoid using the direct optimization of the computationally expensive fine model to find the optimal solution. Instead, we perform optimization of the computationally efficient coarse model to find its optimal solution, and then use the mapping to find an estimate of the fine model optimal. In this paper, we demonstrate the use of the space mapping algorithm for the optimization of metal dielectric- metal plasmonic waveguide devices. In our case, the fine model is a full-wave finite-difference frequency domain (FDFD) simulation of the device, while the coarse model is based on the characteristic impedance and transmission line theory. We show that, if we simply use the coarse model to optimize the structure without space mapping, the response of the structure obtained substantially deviates from the target response. On the other hand, using space mapping we obtain structures which match very well the target response. In addition, full-wave FDFD simulations of only a few candidate structures are required before the optimal solution is reached. In comparison, a direct optimization using the fine FDFD model in combination with a genetic algorithm requires thousands of full-wave FDFD simulations to reach the same optimal. © 2013 SPIE
Compact multisection cavity switches in metal-dielectric-metal plasmonic waveguides
No full textWe introduce a compact absorption switch consisting of a plasmonic metal-dielectric-metal waveguide coupled to a multisection cavity. The optimized multisection cavity switch leads to greatly enhanced modulation depth compared to an optimized Fabry-Perot cavity switch. © 2014 OSA
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