Desenvolupament d'un microscopi òptic de camp proper (SNOM) per a la caracterització de components optoelectrònics integrats

Consultable des del TDXTítol obtingut de la portada digitalitzadaL'objectiu principal d'aquesta tesi és el desenvolupament d'una nova eina de caracterització local, basada en tècniques de microscòpia d'escombrat, que permeti estudiar la propagació de la llum en medis de guiat òptic. Aquest nou microscopi s'anomena microscopi òptic de camp proper (SNOM). L'aplicació d'aquest nou instrument a diferents estructures i dispositius òptics integrats ha permès obtenir informació del comportament de la propagació de la llum en funció dels paràmetres del procés tecnològic. La informació que és pot obternir es pot separar en dues parts: una caracterització topogràfica que permet estudiar l'estructura morfològica del dispositiu (rugositat de la superfície, dimensions geomètriques dels dispositius, etc) amb gran resolució (~nm), i sense malmetre la mostra; per altra banda, capturant les ones evanescents presents a la superfície de l'estructura, s'estudia la propagació modal de la llum a través del dispositiu, amb una resolució per sota del límit de difracció, essent l'única tècnica que permet aquest tipus de caracterització. Addicionalment s'obté la l'índex de refracció efectiu de propagació per la guia, i la mesura del paràmetre de decaïment del camp evanescent. El microscopi s'ha construït al propi laboratori, i és del tipus «stand-alone», perfectament compatible amb un banc òptic de mesura. Els rangs del microscopi són de 300mmx150mm en el pla de la mostra, i de 5mm verticalment. El control de la distància punta-mostra és del tipus «tuning fork shear force control». El tipus de mostres estudiades es basen en la tecnologia de silici. Primer s'han estudiat dispositius basats en guies de nitrur de silici d'estructura en esglaó, amb un nucli de 200nm. S'ha estudiat la distribució modal en funció de l'amplada de la guia, i s'ha determinat l'índex efectiu de propagació. Finalment s'ha caracteritzat l'atac Reactive Ion Etching per obtenir esglaons < 10nm. A continuació s'han estudiat dispositius basats en l'estructura ARROW, del tipus multicapa, amb un nucli ~3mm. S'ha determinat la condició de propagació monomode, s'ha avaluat el camp evanescent vertical i lateral de guies rectes, s'ha estudiat la unió en Y d'un interferòmetre Mach-Zehnder, i finalment s'ha mesurat a través del camp evanescent, la longitud d'acoblament d'un acoblador direccional. Finalment, la última part de la tesi s'ha dedicat a explorar l'ús de diferents tècniques litogràfiques per modificar guies microfabricades de forma estàndard, per tal d'obtenir dispositius nous i de menors dimensions. Les guies modificades són guies de nitrur de silici. La primera tècnica estudiada és una combinació de litografia Làser i atac químic, i s'ha aconseguit visualtizar un nou comportament en aquestes guies, la conversió de mode TE a TM. La segona tècnica estudiada és una combinació de litografia AFM i atac RIE, i s'ha determinat les condicions d'atac per provocar canvis significatius en la propagació de la llumThe main objective of this thesis is to develop a new instrument, a Scanning Near-Field Optical Microscope (SNOM), based on scanning microscopy techniques, for the characteritzation of optical guiding devices. Applied to different integrated optical structures and devices, information on the light propagation as a function of technological parameters will be obtained. The obtained information can be separated in two branches: first, a topographic characterization, which allows to study the morphology of the structure (surface roughness, geometric dimensions, etc) with a resolution below 1nm without any special preparation of the sample; and second, an optical characterization which allows to study the light propagation with a resolution below the classical diffraction limit. By picking up the evanescent waves presents on the surface of the optical guiding devices the modal propagation is visualized, being the unique technique allowing such measurement. Also the decay length of the evanescent field is characterized, as well as the effective refractive index of the structure. The home-made built microscope is a stand-alone design, fully compatible with a standard optical bench for the characterization of optical waveguiding devices. The ranges of the microscope are 300mmx150mm in the plane and 5mm vertical, and the tip-sample distance control is based on tuning fork shear force control. Based on silicon technology, two different waveguide structures has been studied. The first one is a silicon nitride based rib waveguides, with a thin core of about 200nm The modal propagation depending on the width of the structure is characterized, as well as the evanescent field. Finally, the Reactive Ion Etching used to make the rib has been teste to produce steps below 10nm. The second structure is known as ARROW structure, which is a multilayer structure with a core similar to that of a fiber (~3mm). In this case the single mode propagation condition has been determined, and the vertical and lateral evanescent field have been measured. A Y-branch of an interferometer has been also studied, and finally the coupling length of a directional coupler has been experimentally determined. The last part of the thesis is devoted to the modification of standard microfabricated optical waveguides by new lithographic techniques, in order to obtain new and smaller devices. The first technique is a combination of laser lithography and chemical etching, and by modifying a standard rib silicon nitride waveguide is has been observed a polarization conversion (TE-TM conversion). The second technique is a combination of AFM-lithography and RIE, which allows nanometer-scale modifications. The results of this new technique shows whether the modification can significantly changes the propagation conditions or no

Geometric frustration in a hexagonal lattice of plasmonic nanoelements

We introduce the concept of geometric frustration in plasmonic arrays of nanoelements. In particular, we present the case of a hexagonal lattice of Au nanoasterisks arranged so that the gaps between neighboring elements are small and lead to a strong near-field dipolar coupling. Besides, far-field interactions yield higher-order collective modes around the visible region that follow the translational symmetry of the lattice. However, dipolar excitations of the gaps in the hexagonal array are geometrically frustrated for interactions beyond nearest neighbors, yielding the destabilization of the low energy modes in the near infrared. This in turn results in a slow dynamics of the optical response and a complex interplay between localized and collective modes, a behavior that shares features with geometrically frustrated magnetic systems

An inverted Honeycomb plasmonic lattice as an efficient refractive index sensor

We present an efficient refractive index sensor consisting of a heterostructure that contains an Au inverted honeycomb lattice as a main sensing element. Our design aims at maximizing the out-of-plane near-field distributions of the collective modes of the lattice mapping the sensor surroundings. These modes are further enhanced by a patterned SiO2 layer with the same inverted honeycomb lattice, an SiO2 spacer, and an Au mirror underneath the Au sensing layer that contribute to achieving a high performance. The optical response of the heterostructure was studied by numerical simulation. The results corresponding to one of the collective modes showed high sensitivity values ranging from 99 to 395 nm/RIU for relatively thin layers of test materials within 50 and 200 nm. In addition, the figure of merit of the sensor detecting slight changes of the refractive index of a water medium at a fixed wavelength was as high as 199 RIU1. As an experimental proof of concept, the heterostructure was manufactured by a simple method based on electron beam lithography and the measured optical response reproduces the simulations. This work paves the way for improving both the sensitivity of plasmonic sensors and the signal of some enhanced surface spectroscopies

Metamirrors Based on Arrays of Silicon Nanowires with Height Gradients

Arrays of silicon nanowires with height gradients fabricated using metal-assisted chemical etching act as tunable metamirrors enabling light focusing the reflected light in arbitrary shapes. Metamirrors with non-cylindrical nanowires can simultaneously focus the reflected light and induce strong polarization conversion effect

In-plane thermal diffusivity determination using beam-offset frequency-domain thermoreflectance with a one-dimensional optical heat source

We present an innovative contactless method suitable to study in-plane thermal transport based on beam-offset frequency-domain thermoreflectance using a one-dimensional heat source with uniform power distribution. Using a one-dimensional heat source provides a number of advantages as compared to point-like heat sources, as typically used in time- and frequency-domain thermoreflectance experi- ments, just to name a few: (i) it leads to a slower spatial decay of the temperature field in the direction perpendicular to the line-shaped heat source, allowing to probe the temperature field at larger distances from the heater, hence, enhancing the sensitivity to in-plane thermal transport; (ii) the frequency range of interest is typically < 100 kHz. This rather low frequency range is convenient regarding the cost of the required excitation laser system but, most importantly, it allows the study of materials without the presence of a metallic transducer with almost no influence of the finite optical penetration depth of the pump and probe beams on the thermal phase lag, which arises from the large thermal penetration depth imposed by the used frequency range. We also show that for the case of a harmonic thermal excitation source, the phase lag between the thermal excitation and thermal response of the sample exhibits a lin- ear dependence with their spatial offset, where the slope is proportional to the inverse of the thermal diffusivity of the material. We demonstrate the applicability of this method to the cases of: (i) suspended thin films of Si and PDPP4T, (ii) Bi bulk samples, and (iii) Si, glass, and highly-oriented pyrollitic graphite (HOPG) bulk samples with a thin metallic transducer. Finally, we also show that it is possible to study in-plane heat transport on substrates with rather low thermal diffusivity, e.g., glass, even using a metallic transducer. We achieve this by an original approach based on patterning the transducer using focused ion beam, with the key purpose of limiting in-plane heat transport through the thin metallic transducer

Design, fabrication, and characterisation of wire grid polarizers for the deep UV spectral range

In this communication, we show preliminary results on transmissive TiO wire-grid polarizers (WGP) operating in the deep ultraviolet (DUV) range. WGP are devices based on strips of materials with large values of the modulus of the dielectric constant along with high absorption in the operational range. The merit function I is introduced as a new tool to find the optimum material for WGPs in a given spectral range. The experimental dielectric constant of TiO thin films deposited by pulsed laser deposition are obtained through spectroscopic ellipsometry, and the function indicates that TiO is the best candidate for WGP in the DUV range when it is compared with other oxides. Once the material selection for WGP is done, we present and compare two different design approaches for WGP: one using an effective medium theory for the periodic structure, and the second using finite-difference time-domain (FDTD) analysis. A prototype of WGP is fabricated by electron beam (e-beam) lithography followed by lift-off process; the topography of the sample is analyzed by AFM, and we found noticeable deviations in the grating from the designed values. In preliminary characterization work the effective dielectric constant in two perpendicular orientations is obtained by ellipsometry and the contrast is compared with the design

Tailored height gradients in vertical nanowire arrays via mechanical and electronic modulation of metal-assisted chemical etching

In current top-down nanofabrication methodologies the design freedom is generally constrained to the two lateral dimensions, and is only limited by the resolution of the employed nanolithographic technique. However, nanostructure height, which relies on certain mask-dependent material deposition or etching techniques, is usually uniform, and on-chip variation of this parameter is difficult and generally limited to very simple patterns. Herein, a novel nanofabrication methodology is presented, which enables the generation of high aspect-ratio nanostructure arrays with height gradients in arbitrary directions by a single and fast etching process. Based on metal-assisted chemical etching using a catalytic gold layer perforated with nanoholes, it is demonstrated how nanostructure arrays with directional height gradients can be accurately tailored by: (i) the control of the mass transport through the nanohole array, (ii) the mechanical properties of the perforated metal layer, and (iii) the conductive coupling to the surrounding gold film to accelerate the local electrochemical etching process. The proposed technique, enabling 20-fold on-chip variation of nanostructure height in a spatial range of a few micrometers, offers a new tool for the creation of novel types of nano-assemblies and metamaterials with interesting technological applications in fields such as nanophotonics, nanophononics, microfluidics or biomechanics. Based on metal-assisted chemical etching using a catalytic gold layer perforated with nanoholes, it is demonstrated how high aspect-ratio nanostructure arrays with directional height gradients can be accurately tailored by: i) control of mass transport through the nanohole array, ii) mechanical properties of the perforated metal layer, and iii) conductive coupling to the surrounding gold film to accelerate the local electrochemical etching process. The proposed technique, enabling 20-­-fold on-­-chip variation of nanostructure height in a spatial range of a few microns, offers a new tool for the creation of novel types of nano-­-assemblies and metamaterials with interesting technological applications in fields such as nanophotonics, nanophononics, microfluidics or biomechanics

Imaging of Antiferroelectric Dark Modes in an Inverted Plasmonic Lattice [Dataset]

6 pages. -- S1. Transversal electric field distribution for the SLR at 1.57 eV. -- S2. Simulated electric field and charge distributions for a threesome of slits. -- S3. Simulated electric field and charge distributions for the simplest local dark mode of the inverted honeycomb lattice. -- S4. Profiles of the EELS signal and the simulated electric field along the slits for the antiferroelectric dark modes. -- S5. Array of the magnetic dipoles over the structure used to simulate antiferroelectric dark modes.Plasmonic lattice nanostructures are of technological interest because of their capacity to manipulate light below the diffraction limit. Here, we present a detailed study of dark and bright modes in the visible and near-infrared energy regime of an inverted plasmonic honeycomb lattice by a combination of Au+ focused ion beam lithography with nanometric resolution, optical and electron spectroscopy, and finite-difference time-domain simulations. The lattice consists of slits carved in a gold thin film, exhibiting hotspots and a set of bright and dark modes. We proposed that some of the dark modes detected by electron energy-loss spectroscopy are caused by antiferroelectric arrangements of the slit polarizations with two times the size of the hexagonal unit cell. The plasmonic resonances take place within the 0.5–2 eV energy range, indicating that they could be suitable for a synergistic coupling with excitons in two-dimensional transition metal dichalcogenides materials or for designing nanoscale sensing platforms based on near-field enhancement over a metallic surface.Peer reviewe

Manipulation of competing ferromagnetic and antiferromagnetic domains in exchange-biased nanostructures

Using photoemission electron microscopy combined with x-ray magnetic circular dichroism we show that a progressive spatial confinement of a ferromagnet (FM), either through thickness variation or laterally via patterning, actively controls the domains of uncompensated spins in the antiferromagnet (AF) in exchange-biased systems. Direct observations of the spin structure in both sides of the FM/AF interface in a model system, Ni/FeF2, show that the spin structure is determined by the balance between the competing FM and AF magnetic energies. Coexistence of exchange bias domains, with opposite directions, can be established in Ni/FeF2 bilayers for Ni thicknesses below 10 nm. Patterning the Ni/FeF2 heterostructures with antidots destabilizes the FM state, enhancing the formation of opposite exchange bias domains below a critical antidot separation of the order of a few FeF2 crystal domains. The results suggest that dimensional confinement of the FM may be used to manipulate the AF spin structure in spintronic devices and ultrahigh-density information storage media. The underlying mechanism of the uncompensated AF domain formation in Ni/FeF2 may be generic to other magnetic systems with complex noncollinear FM/AF spin structures

Desenvolupament d'un microscopi òptic de camp proper (SNOM) per a la caracterització de components optoelectrònics integrats

L'objectiu principal d'aquesta tesi és el desenvolupament d'una nova eina de caracterització local, basada en tècniques de microscòpia d'escombrat, que permeti estudiar la propagació de la llum en medis de guiat òptic. Aquest nou microscopi s'anomena microscopi òptic de camp proper (SNOM). L'aplicació d'aquest nou instrument a diferents estructures i dispositius òptics integrats ha permès obtenir informació del comportament de la propagació de la llum en funció dels paràmetres del procés tecnològic. La informació que és pot obternir es pot separar en dues parts: una caracterització topogràfica que permet estudiar l'estructura morfològica del dispositiu (rugositat de la superfície, dimensions geomètriques dels dispositius, etc) amb gran resolució (~nm), i sense malmetre la mostra; per altra banda, capturant les ones evanescents presents a la superfície de l'estructura, s'estudia la propagació modal de la llum a través del dispositiu, amb una resolució per sota del límit de difracció, essent l'única tècnica que permet aquest tipus de caracterització. Addicionalment s'obté la l'índex de refracció efectiu de propagació per la guia, i la mesura del paràmetre de decaïment del camp evanescent. El microscopi s'ha construït al propi laboratori, i és del tipus "stand-alone", perfectament compatible amb un banc òptic de mesura. Els rangs del microscopi són de 300mmx150mm en el pla de la mostra, i de 5mm verticalment. El control de la distància punta-mostra és del tipus "tuning fork shear force control".El tipus de mostres estudiades es basen en la tecnologia de silici. Primer s'han estudiat dispositius basats en guies de nitrur de silici d'estructura en esglaó, amb un nucli de 200nm. S'ha estudiat la distribució modal en funció de l'amplada de la guia, i s'ha determinat l'índex efectiu de propagació. Finalment s'ha caracteritzat l'atac Reactive Ion Etching per obtenir esglaons A continuació s'han estudiat dispositius basats en l'estructura ARROW, del tipus multicapa, amb un nucli ~3mm. S'ha determinat la condició de propagació monomode, s'ha avaluat el camp evanescent vertical i lateral de guies rectes, s'ha estudiat la unió en Y d'un interferòmetre Mach-Zehnder, i finalment s'ha mesurat a través del camp evanescent, la longitud d'acoblament d'un acoblador direccional.Finalment, la última part de la tesi s'ha dedicat a explorar l'ús de diferents tècniques litogràfiques per modificar guies microfabricades de forma estàndard, per tal d'obtenir dispositius nous i de menors dimensions. Les guies modificades són guies de nitrur de silici. La primera tècnica estudiada és una combinació de litografia Làser i atac químic, i s'ha aconseguit visualtizar un nou comportament en aquestes guies, la conversió de mode TE a TM. La segona tècnica estudiada és una combinació de litografia AFM i atac RIE, i s'ha determinat les condicions d'atac per provocar canvis significatius en la propagació de la llumThe main objective of this thesis is to develop a new instrument, a Scanning Near-Field Optical Microscope (SNOM), based on scanning microscopy techniques, for the characteritzation of optical guiding devices. Applied to different integrated optical structures and devices, information on the light propagation as a function of technological parameters will be obtained.The obtained information can be separated in two branches: first, a topographic characterization, which allows to study the morphology of the structure (surface roughness, geometric dimensions, etc) with a resolution below 1nm without any special preparation of the sample; and second, an optical characterization which allows to study the light propagation with a resolution below the classical diffraction limit. By picking up the evanescent waves presents on the surface of the optical guiding devices the modal propagation is visualized, being the unique technique allowing such measurement. Also the decay length of the evanescent field is characterized, as well as the effective refractive index of the structure. The home-made built microscope is a stand-alone design, fully compatible with a standard optical bench for the characterization of optical waveguiding devices. The ranges of the microscope are 300mmx150mm in the plane and 5mm vertical, and the tip-sample distance control is based on tuning fork shear force control. Based on silicon technology, two different waveguide structures has been studied. The first one is a silicon nitride based rib waveguides, with a thin core of about 200nm The modal propagation depending on the width of the structure is characterized, as well as the evanescent field. Finally, the Reactive Ion Etching used to make the rib has been teste to produce steps below 10nm. The second structure is known as ARROW structure, which is a multilayer structure with a core similar to that of a fiber (~3mm). In this case the single mode propagation condition has been determined, and the vertical and lateral evanescent field have been measured. A Y-branch of an interferometer has been also studied, and finally the coupling length of a directional coupler has been experimentally determined.The last part of the thesis is devoted to the modification of standard microfabricated optical waveguides by new lithographic techniques, in order to obtain new and smaller devices. The first technique is a combination of laser lithography and chemical etching, and by modifying a standard rib silicon nitride waveguide is has been observed a polarization conversion (TE-TM conversion). The second technique is a combination of AFM-lithography and RIE, which allows nanometer-scale modifications. The results of this new technique shows whether the modification can significantly changes the propagation conditions or no