The propagation of electromagnetic waves can be manipulated at the nanoscale by surface plasmons supported by ultra thin metal layers. An adhesion layer, with thickness in the order of few nanometerss is used for depositing ultra thin metal gold layers. Cr and Ti are the most popular metallic adhesion layers. Apart from them, a non metallic silane based wetting layer like (3-Aminopropyl)trimethoxysilane (APTMS) can be used. The behaviour of the propagating surface plasmons due to the influence of these adhesion layers has not been thoroughly investigated. To study the influence of the adhesion layers on propagating plasmons for use in plasmonic and metamaterial applications,we experimentally compared the performances of the ultra-thin gold layers using Cr and APTMS adhesion layers and without any adhesion layer. We show that the gold layers using APTMS adhesion exhibit short range surface plasmon polaritons (SR-SPPs) with characteristics close to the theoretical calculations, considering an ideal gold film.
In this work, we report on fabrication of deep-profile one- and two-dimensional lattices made from Al-doped ZnO (AZO). AZO is considered as an alternative plasmonic material having the real part of the permittivity negative in the near infrared range. The exact position of the plasma frequency of AZO is doping concentration dependent, allowing for tuning possibilities. In addition, the thickness of the AZO film also affects its material properties. Physical vapor deposition techniques typically applied for AZO coating do not enable deep profiling of a plasmonic structure. Using the atomic layer deposition technique, a highly conformal deposition method, allows us to fabricate high-aspect ratio structures such as one-dimensional lattices with a period of 400 nm and size of the lamina of 200 nm in width and 3 μm in depth. Thus, our structures have an aspect ratio of 1:15 and are homogeneous on areas of 2×2 cm2 and more. We also produce two-dimensional arrays of circular nanopillars with similar dimensions. Instead of nanopillars hollow tubes with a wall thickness on demand from 20 nm up to a complete fill can be fabricated.
This paper is devoted to experimental and theoretical studies of nonlinear propagation of a long-range surface plasmon
polariton (LRSPP) in gold strip waveguides. The plasmonic waveguides are fabricated in house, and contain a gold layer,
tantalum pentoxide adhesion layers, and silicon dioxide cladding. The optical characterization was performed using a
high power picosecond laser at 1064 nm. The experiments reveal two nonlinear optical effects: nonlinear power
transmission and spectral broadening of the LRSPP mode in the waveguides. Both nonlinear optical effects depend on
the gold layer thickness. The theoretical model of these effects is based on the third-order susceptibility of the constituent
materials. The linear and nonlinear parameters of the LRSPP mode are obtained, and the nonlinear Schrödinger equation
is solved. The dispersion length is much larger than the waveguides length, and the chromatic dispersion does not affect
the propagation of the plasmonic mode. We find that the third-order susceptibility of the gold layer has a dominant
contribution to the effective third-order susceptibility of the LRSPP mode. The real part of the effective third-order
susceptibility leads to the observed spectral broadening through the self-phase modulation effect, and its imaginary part
determines the nonlinear absorption parameter and leads to the observed nonlinear power transmission. The experimental
values of the third-order susceptibility of the gold layers are obtained. They indicate an effective enhancement of the third-order
susceptibility for the gold layers, comparing to the bulk gold values. This enhancement is explained in terms of the
change of the electrons motion.
We report on theoretical analysis and experimental validation of the applicability of the effective medium approximation
to deeply subwavelength (period ⩽λ/30) all-dielectric multilayer structures. Following the theoretical prediction of the
anomalous breakdown of the effective medium approximation [H. H. Sheinfux et al., Phys. Rev. Lett. 113, 243901
(2014)] we thoroughly elaborate on regimes, when an actual multilayer stack exhibits significantly different properties
compared to its homogenized model. Our findings are fully confirmed in the first direct experimental demonstration of
the breakdown effect. Multilayer stacks are composed of alternating alumina and titania layers fabricated using atomic
layer deposition. For light incident on such multilayers at angles near the total internal reflection, we observe pronounced
differences in the reflectance spectra (up to 0.5) for structures with different layers ordering and different but still deeply
subwavelength thicknesses. Such big reflectance difference values resulted from the special geometrical configuration
with an additional resonator layer underneath the multilayers employed for the enhancement of the effect. Our results are
important for the development of new homogenization approaches for metamaterials, high-precision multilayer
ellipsometry methods and in a broad range of sensing applications.
In this work, we show theoretically and confirm experimentally that thin metal membranes patterned with an array of
slot dimers (or their Babinet analogue with metal rods) can function as a versatile spectral and polarization filter. We
present a detailed covariant multipole theory for the electromagnetic response of an arbitrary dimer based on the Green
functions approach. The theory confirms that a great variety of polarization properties, such as birefringence, chirality
and elliptical dichroism, can be achieved in a metal layer with such slot-dimer patterning (i.e. in a metasurface). Optical
properties of the metasurface can be extensively tuned by varying the geometry (shape and dimensions) of the dimer, for
example, by adjusting the sizes and mutual placement of the slots (e.g. inter-slot distance and alignment angle). Three
basic shapes of dimers are analyzed: II-shaped (parallel slots), V-shaped, and T-shaped. These particular shapes of
dimers are found to be sensitive to variations of the slots lengths and orientation of elements. Theoretical results are well
supported by full-wave three-dimensional simulations. Our findings were verified experimentally on the metal
membranes fabricated using UV lithography with subsequent Ni growth. Such metasurfaces were characterized using
time-domain THz spectroscopy. The samples exhibit pronounced optical activity (500 degrees per wavelength) and high
transmission: even though the slots cover only 4.3 % of the total membrane area the amplitude transmission reaches 0.67
at the resonance frequency 0.56 THz.
We focus on plasmonic modulators with a gain core to be implemented as active nanodevices in photonic integrated circuits. In particular, we analyze metal–semiconductor–metal (MSM) waveguides with InGaAsP-based active material layers. A MSM waveguide enables high field localization and therefore high modulation speed. The modulation is achieved by changing the gain of the core that results in different transmittance through the waveguide. Dependences on the waveguide core size and gain values of various active materials are studied. The effective propagation constants in the MSM waveguides are calculated numerically. We optimize the structure by considering thin metal layers. A thin single metal layer supports an asymmetric mode with a high propagation constant. Implementing such layers as the waveguide claddings allows to achieve several times higher effective indices than in the case of a waveguide with thick (>50 nm) metal layers. In turn, the high effective index leads to enhanced modulation speed. We show that a MSM waveguide with the electrical current control of the gain incorporates compactness and deep modulation along with a reasonable level of transmittance.
Metamaterials are artificially designed media that show averaged properties not yet encountered in nature. Among such
properties, the possibility of obtaining optical magnetism and negative refraction are the ones mainly exploited but
epsilon-near-zero and sub-unitary refraction index are also parameters that can be obtained. Such behaviour enables
unprecedented applications. Within this work, we will present various aspects of metamaterials research field that we deal with at our department. From the modelling part, we will present tour approach for determining the field enhancement in slits that have
dimensions in the 104 times smaller than the incident wavelength. This huge difference makes it almost impossible for
commercial software to handle thus analytical approached have to be employed. From the fabrication point of view, various 2D and 3D high resolution patterning techniques are used. The talk will describe the ones available within our group. We will present the electron-beam lithography approach for fabricating nano-antennae to be used in coupling of plasmonics waveguides to/from free space. Also, a 3D technique based on twophoton-polymerisation and isotropic metal deposition to fabricate metal-covered 3D photonic crystals will be discussed. From the measuring side we will present two THz based setups for obtaining material’s characteristics, both in the low as well as in the high THz range, thus having the possibility of describing a material from 0.1 to 10THz.
In order to improve the photoconversion efficiency, we consider the possibility of increasing the photocurrent in solar
cells exploiting the electron photoemission from small metal nanoparticles into a semiconductor. The effect is caused by
the absorption of photons and generation of local surface plasmons in the nanoparticles with optimized geometry. An
electron photoemission from metal into semiconductor occurs if photon energy is larger than Schottky barrier at the
metal-semiconductor interface. The photocurrent resulting from the absorption of photons with energy below the
bandgap of the semiconductor added to the solar cell photocurrent can extend spectral response range of the device.
We study the effect on a model system, which is a Schottky barrier n-GaAs solar cell, with an array of Au nanoparticles
positioned at the interface between the semiconductor and the transparent top electrode. Based on the simulations, we
chose to study disk-shaped Au nanoparticles with sizes ranging from 25nm to 50nm using electron beam lithography.
Optical characterization of the fabricated devices shows the presence of LSP resonance around the wavelength of
1250nm, below the bandgap of GaAs.
High-index-contrast grating mirrors featuring beam steering abilities for the transmitted beam as well as high reflectivity
over a broad bandwidth are suggested. Gratings designed to provide control over the wave front of the transmitted beam
are numerically investigated. The proposed structures are then fabricated for experimental characterization. The
measurements performed show the beam steering ability of the suggested HCG designs and are also in good agreement
with the theoretical predictions. General design rules to engineer these HCG structures for different applications are
derived. These grating mirrors would have a significant impact on low cost laser sources fabrication, since a more
efficient integration of optoelectronic modules can be achieved by avoiding expensive external lens systems.
In this paper we present the whole fabrication and characterization cycle for obtaining 3D metal-dielectric woodpile
structures. The optical properties of these structures have been measured using different setups showing the need of
considering e.g. border effects when planning their use in real-life devices. It was found that the behavior of the
structures close to the edge is very different from the one in the middle. The existence of special features in the former
spectra still needs to be completely understood and explained.
The concept of metamaterials (MTMs) is acknowledged for providing new horizons for controlling electromagnetic
radiations thus their use in frequency ranges otherwise difficult to manage (e.g. THz radiation) broadens our possibility
to better understand our world as well as opens the path for new applications.
THz radiation can be employed for various purposes, among them the study of vibrations in biological molecules,
motion of electrons in semiconductors and propagation of acoustic shock waves in crystals.
We propose here a new THz fractal MTM design that shows very high transmission in the desired frequency range as
well as a clear differentiation between one polarisation and another. Based on theoretical predictions we fabricated and
measured a fractal based THz metamaterial that shows more than 60% field transmission at around 1THz for TE
polarized light while the TM waves have almost 80% field transmission peak at 0.6THz. One of the main characteristics
of this design is its tunability by design: by simply changing the length of the fractal elements one can choose the
operating frequency window. The modelling, fabrication and characterisation results will be presented in this paper.
Due to the long wavelength of THz radiation, the resolution requirements for fabrication of metamaterials are within the
optical lithography range. However, the high aspect ratio of such structures as well as the substrate thickness pose
challenges in the fabrication process.
The measurements were made using terahertz time domain spectroscopy (THz-TDS) that allows us to obtain both the
amplitude and phase of the transmission function. The experimental results are in very good agreement with theoretical
calculations based on finite-difference time-domain simulations.
Within the last years, interest in photonic wires and photonic crystals grew due to their demonstrated ability
of controlling light propagation and characteristics. One of the limitations of such devices is due to the induced
roughness during the fabrication process. Generally, an increase in roughness leads to loss increase thus limiting
the propagation length and postponing the commercialization of such structures. In this paper we present a
new algorithm for measuring the sidewall roughness of our devices based on atomic force microscope (AFM)
approach. Using this algorithm, the roughness can be quantified and thus actions in decreasing it can be taken
improving the device's performance.
Since the diffraction gratings were invented, their use in various security systems has been exploited. Their big advantage is the low production cost and, in the same time, the difficulty of replicating them. Most of the nowadays security systems are using those gratings to prove their originality. They can be seen on all the CDs, DVDs, most of the major credit cards and even on the wine bottles. In this article we present a new way of making such gratings without changing the production steps but generating an even more difficult to be replicated item. This new way consists not only in changing the grating period so that various false colours can be seen, but also their orientation so that for a complete check of the grating it should be seen under a certain solid angle. In the same time, one can also keep the possibility to change the grating period so this way various colours can be seen for each angle variation. By combining these two techniques (changing period and changing the angle ones) one can indeed create different images for each view angle and thus increasing the security of the object. In the same time, as can be seen, from the fabrication point of view no further complications appear. The production steps are identical, the only difference being the pattern. The resolution of the grating is not increased necessarily so neither from this point of view will complications appear.
In this work we present a numerical evaluation of the forces in an optical tweezers system, for metallic nanoparticles in the
Rayleigh regime. Initially a Gaussian beam is described in the scalar approximation, and the forces it can apply on Rayleigh
dielectric and metallic particles are computed within the point-dipole approach. The method is then extended to dielectric
and metallic Rayleigh particles in a Laguerre-Gaussian beam, i.e. a higher order beam that is increasingly used for optical
trapping experiments. We discuss the limits of the approximation for the beam intensity by comparing the numerical results
with the experimental measurements that can be found in literature.
One of the main problems in using oil immersed objectives for optical trapping is due to the reflective index mismatch. While in water immersed lens or in "classical" ones the index mismatch is, at most, because of the propagation from a low-index to a high-index medium so no big aberrations appear, in the oil-immersed ones the aberrations cannot be neglected. Due to the propagation of a focalized beam from a high refraction index medium to a low one, spherical aberrations appear. But the numerical aperture an oil immersed objective can reach is usually above 1.2, so theoretically, the Q-factor can reach higher values. In this paper we confront two different methods of force calculations. The first one is based on Ashkin's (Biophys. J. - Feb. 1992) formulas and uses a ray-tracing approach. Using this approach we can observe the asymmetry of the forces along the optical axis and the high Q-factors values on the orthogonal plane. The second approach is a more rigorous one and is based on wave optics. We analyze the Gaussian beam propagation using the scalar version of the diffraction theory and the formulas of the scattering and gradient forces developed by W.H.Wright &al. (Applied Optics - March 1994). This approach, although more precise in defining the focus shape, is not necessarily more precise in determining the force value. This essentially because of the approximations used in the force formulas. While in the ray-tracing approach, the error can be minimized by changing the sampling period of the beam, in the wave-optics approach this cannot be done. In the same time, in the latter approach, the beam shape can be described better so the accuracy of the simulation improved. To our belief the possibility of using both simulations must be taken into account while in the "resonance" regime and, function of the needs one has, decide which one is to be considered reliable.
Since the first demonstrations of optical trapping, both theoretical and experimental parts of this technique evolved. With all this, the main problem when trapping in the Mie regime is due to the limited numerical aperture a microscope objective has. In literature one finds characterizations of “classical” microscope lens or, at most, water immersed ones. In this paper we are analysing the forces generated in an optical tweezers setup using oil immersed microscope objective and having as entrapped particles water-immersed silica beads. Using such a set-up, we can take advantage of the numerical aperture an oil-immersed objective can reach. This numerical aperture can have a value as high as 1.4. From Roosen's 1 and Ashkin's 2 formulas, we calculated the forces involved in our experiment. We observed that the entrapping range on the optical path axis is larger and asymmetric. This generates the possibility to build optical catapult and optical tweezers in the same time, changing only the distance from the sample to the entrapment point. One of the disadvantages of optical trapping in these conditions is that the focus point and the entrapment one can be different. This fact generates the need of using a second microscope for inspecting the entrapped particle so the optical setup is more complicated. To our belief, this set-up for optical tweezers can have big advantages in the field of optical trapping mainly due to the not so strict trapping spatial conditions.
Trapping and manipulation of microparticles using optical tweezers is usually performed within a sample cell formed by two parallel microscope cover slides. In this paper we discuss and demonstrate trapping and manipulation conditions when the cell has more complex configurations like microchannels or capillary tubes. The microchannels are fabricated on the surface of the cover slide by means of lithographic techniques. Experimental results of trapping and micromanipulation for silica microspheres and biological samples immersed in water show the usefulness of our study for microfluidics and biological applications.
Although optical tweezers have been a valuable research tool since their invention in the 1980s, they have remained limited for many years to trapping only one particle per laser beam. One of the most exciting developments in optical tweezers in recent years has been the creation of two- and three-dimensional arrays of optical traps by using diffractive optical elements (DOEs). We have developed our own algorithms and codes to design phase DOEs that can transform a single laser beam into an array of independent traps, each with individually specified characteristics, arranged in various geometrical configurations. The DOEs were fabricated by means of e-beam lithography in PMMA and recently were implemented in computer addressed liquid crystal spatial modulators. This allows us to control the configuration of the optical tweezers almost in real time. Experimental results presented in this paper show trapping and manipulation of multiple silica micro-spheres immersed in water. The trapped particles are moved independently along the x-y-z directions and rotated along circular trajectories with different angular velocities.
In this paper we report results obtained in the design and fabrication of diffractive optical elements (DOEs) with minimum feature size down to tens of nanometers by the use of e-beam and x-ray lithography. The DOEs are patterned using e-beam lithography and replicated by x-ray lithography. Since in our days there is an increased interest for extreme ultraviolet and x-ray microscopy our work has been focused toward the fabrication of DOEs mainly for these applications. Different types of zone plates (ZPs) were fabricated for x-ray beam focusing: high resolution ZPs for high resolution beam focusing, multilevel phase ZPs to increase the diffraction efficiency in the desired order and high aspect ratio ZPs for hard x-rays. Recently we have extended the concept of the ZPs to a more general category of DOEs which beside simple focusing can perform new optical functions in the range of x-rays. In particular, the intensity of the beam after the DOE can be distributed with almost complete freedom. We have designed and fabricated DOEs that focus the beam in an array of spots disposed either in plane or along the optical axis. This type of DOEs has been tested successfully in x-ray differential interference contrast microscopy. The possibility to introduce a specified phase shift between the generated spots is demonstrated in this paper by preliminary results obtained from computer simulations and experiments performed in visible light.
In this paper we compare two techniques to design diffractive gratings with periods close to the wavelength of the illumination beam. The first method, based on the Rayleigh method, is faster and allows good result if grating period is more than 1.2 bigger than the wavelength. The second method, known as Fourier Modal Method, is more precise but also requires a longer calculation time. Results obtained from the computer simulations and experiments are presented for gratings with 0.750μm and 1.5μm period illuminated by a collimated wave with 0.633μm wavelength.