Influence of phonon confinement on to photoluminescence of porous silicon

Predstavljen je razlog i motivacija proučavanja luminiscencije poroznog silicija: daljnji napredak u mikroelektroničkoj industriji i integracija s optičkim komponentama. Uveden je model kvantnog zatočenja ekscitona u nanokristalićima unutar sloja poroznog silicija kao jedan od glavnih uzroka luminiscencije te su navedeni i ostali uzroci. Uveden je i raspisan model fononskog zatočenja te veza s Ramanovom spektroskopijom kao glavnom karakterizacijskom tehnikom poroznog silicija u radu. U Materijalima i metodama izložena je elektrokemija poroznog silicija i reprezentativna IV karakteristika procesa jetkanja. Predstavljena je komorica za jetkanje poroznog silicija napravljena u okviru rada, a detaljno je opisana u Dodatku. Proučena je tehnika određivanja i zadavanja sastava elektrolita za jetkanje, čiji je izvod raspisan u Dodatku. Opisana je procedura mjerenja otpornosti silicijeve pločice, pripreme uzoraka, mjerenja IV karakteristika i jetkanja uzoraka. Predstavljena je teorija Ramanove spektroskopije i tehnika proučavanja fononskog zatočenja. Pokazano je da otpornost na jednoj pločici i između pločica varira zanemarivo da bi utjecala na dobivene jetkane uzorke. Proučavane su IV karakteristike procesa jetkanja u potrazi za idealnim parametrima jetkanja, no za visoke koncentracije fluorovodične kiseline nije bilo moguće uočiti režime. Analizirane su tri serije uzoraka gdje se prilagodbom modela fononskog zatočenja na Ramanov spektar dobila procjena veličine kristalića. Snimljena fotoluminiscencija je uspoređena s veličinom kristalića i pozicijom fononske vrpce. Dobiveno je da intenzitet fotoluminiscencije raste smanjivanjem kristalića, a između energije fotoluminiscencije i veličine kristalića nije dobivena povezanost. Rezultati su usporedeni s literaturom te su navedeni razlozi poklapanja ili odstupanja.Motivation for studying luminescence in porous silicon is introduced: further microelectronic development and integration with optoelectronics. We presented exciton quantum confinement model in porous silicon as one of the main reasons for photoluminescence. Other causes are also mentioned. Phonon confinement model is presented and its link with Raman spectroscopy which is used as the main characterization technique. Porous silicon electrochemistry is presented along with etching I-V characteristic. New etching cell is presented with detailed description in Appendix. Electrolyte composition is studied with details in Appendix. We described four point probe resistivity measuring technique, samples preparation, I-V measurements and etching procedure. Raman spectroscopy theory is presented. We showed that resistivity variation over the surface of the silicon wafer is negligible to have effect on etching experiments. We studied I-V characteristics in search for ideal etching conditions. It wasn’t possible to see the etching regimes in electrolytes with high hydrofluoric acid concentration. Three series of samples are analyzed. By fitting phonon confinement model on experimental Raman spectra we calculated the size of nano-crystals in porous silicon. The effect of nano-crystal size and phonon peak shift on photoluminescence is studied. Photoluminescence intensity rises as the nano-crystal size is becoming smaller, with no opservable maxima, probably due to porous silicon matrix effect. There was no link between nano-crystal size and photoluminescence energy. We ascribe that to sample oxidation

Raman and Photoluminescence Spectroscopy with a Variable Spectral Resolution

Raman and photoluminescence (PL) spectroscopy are important analytic tools in materials science that yield information on molecules’ and crystals’ vibrational and electronic properties. Here, we show results of a novel approach for Raman and PL spectroscopy to exploit variable spec- tral resolution by using zoom optics in a monochromator in the front of the detector. Our results show that the spectral intervals of interest can be recorded with different zoom factors, significantly reducing the acquisition time and changing the spectral resolution for different zoom factors. The smallest spectral intervals recorded at the maximum zoom factor yield higher spectral resolution suitable for Raman spectra. In contrast, larger spectral intervals recorded at the minimum zoom factor yield the lowest spectral resolution suitable for luminescence spectra. We have demonstrated the change in spectral resolution by zoom objective with a zoom factor of 6, but the perspective of such an approach is up to a zoom factor of 20. We have compared such an approach on the prototype Raman spectrometer with the high quality commercial one. The comparison was made on ZrO2 and TiO2 nanocrystals for Raman scattering and Al2O3 for PL emission recording. Beside demonstrating that Raman spectrometer can be used for PL and Raman spectroscopy without changing of grating, our results show that such a spectrometer could be an efficient and fast tool in searching for Raman and PL bands of unknown materials and, thereafter, spectral recording of the spectral interval of interest at an appropriate spectral resolution

Engineering SERS Properties of Silicon Nanotrees at the Nanoscale

Large specific surface area nanostructures are desirable in a wide range of sensing applications due to their longer light-trapping path and increased absorption. Engineering of the specific nanotree structure which possesses a high branch density turned out to be challenging from the experimental point of view, and certainly not adequately explored. This paper shows how to design substrates with a silicon nanotree structure for surface-enhanced Raman spectroscopy (SERS) applications. Silicon nanotrees were synthesized by a Ag-Au nanocluster-catalyzed low- pressure chemical vapor deposition method (LPCVD). By the presented approaches, it is possible to manipulate branches’ number, length and thickness. The synthesized nanostructures are flexible after immersion in water which improves SERS performance. The amount of sputtered metal played a key role in preserving the flexibility of the nanotree structure. The obtained substrates with highly fractal nanostructure were tested on 4- mercaptophenylboronic acid (MPBA) to match the optimal SERS parameters. The silicon nanotrees fabrication, and particularly obtained SERS substrates plated with Ag and Au nanoparticles, demonstrated good features and a promising approach for further sensor development

Raman scattering enhancement using photonic nanojet of dielectric microspheres

Ramanova spektroskopija metoda je karakterizacije i identifikacije materijala bazirana na neelastičnom Ramanovom raspršenju fotona svjetlosti na materijalu. Primjenjuje se u raznim znanstvenim i industrijskim područjima, od fizike, kemije, biologije, medicine, znanosti o materijalima, do kontrola sigurnosti, detekcije nedozvoljenih tvari i proučavanja umjetničkih dijela. Zbog neelastičnosti procesa, Ramanovo raspršenje daje vrlo slab signal, zbog čega su razvijene različite metode pojačanja. Jedna od novih metoda pojačanja, čije su prednosti jednostavnost i niska cijena primjene, neinvazivnost, reproducibilnost i stabilnost, temelji se na fotonskom nanomlazu koji nastaje obasjavanjem mikroleće svjetlošću. Odabirom pogodnih parametara, fotonski nanomlaz može imati izrazito visok intenzitet, usku širinu, ili veliku duljinu, zbog čega se osim za Ramanovu spektroskopiju, istražuje i za primjene u nanolitografiji, super-rezoluciji, optičkim silama, pohrani podataka i sličnim poljima. Trenutno, njegova primjena u Ramanovoj spektroskopiji nedovoljno je istražena, a i sama svojstva i uvjeti za njegov nastanak nerazjašnjeni su. Istraživanjima u sklopu ovog doktorskog rada unaprijeđena je metoda pojačanja, te su dobivena i nova saznanja o fotonskom nanomlazu općenito. Napravljen je računalni program temeljen na Generaliziranoj-Lorenz Mie teoriji, kojim je izračunat širok raspon različitih konfiguracija za fotonski nanomlaz iz dielektrične mikrosfere. Dobiven je sistematičan uvid u svojstva i ovisnost fotonskog nanomlaza o parametrima te uvid u promjene pojedinih ovisnosti u različitim uvjetima. Pokazana je kritična važnost parametra pozicije upadne zrake. Odabirom određene kombinacije parametara dobiven je izrazito intenzivan, vrlo uzak ili izrazito dug fotonski nanomlaz daleko izvan mikrosfere. Istraživanjem područja visokog indeksa loma mikrosfere, modeliranjem je pokazano da fotonski nanomlaz može nastati izvan mikrosfere i kada je indeks loma viši od 2, što je u dosadašnjoj literaturi bilo označeno kao gornja granica. Varijacijom eksperimentalnih parametara optimizirano je pojačanje. Pomoću vertikalnog ramanskog mapiranja određena je optimalna pozicija upadne zrake za pojačanje te je pokazana njezina važnost. Izmjerena je ovisnost pojačanja o kolekcijskom vlaknu, mikroskopskom objektivu i veličini mikrosfere. Diskutiran je mehanizam pojačanja, koji smo podijelili u dva doprinosa - od fotonskog nanomlaza, te od kolekcijskog sustava. Dobiveno je kombinirano pojačanje mikrosfere i plazmonskog pojačanja. Dizajniran je i testiran novi sustav za mehaničku kontrolu mikrosfere pod mikroskopskim objektivom, kojim je moguće iskoristiti pojačanje za svaku točku ramanskog mapiranja. Dobiveno je pojačanje intenziteta i rezolucije mapiranja.Raman spectroscopy is a method for characterization and identification of materials, which is widely used in a broad range of scientific and industrial fields, like materials science, physics, chemistry, biology, medicine, security control, substance control and art examination. It is based on Raman scattering of light on molecules and crystals. As opposed to its elastic counterpart - Rayleigh scattering, Raman scattering is inelastic, which means that the scattered photon has a different energy than the incident one. Because of this, Raman scattering has low probability of occurrence and the scattered Raman intensity is very low. For this reason, many methods for the enhancement of Raman scattering have been developed through the years. One of the new methods of enhancement is based on photonic nanojet, which is a concentrated beam of light emerging from the shadow side of an illuminated microlens. This method of enhancement is characterized by a low cost and a simple principle of implementation. It is a non-invasive, reproducible and reliable way of enhancement. By careful choice of parameters, photonic nanojet can have very high intensity, very narrow width, or very long length. This makes it suitable not only for Raman scattering enhancement, but also for applications in nanolithography, super resolution, optical forces, data storage and similar fields. Although being a promising technique, the role and usage of photonic nanojet in Raman spectroscopy is currently underexplored. Moreover, the properties and conditions for emergence of the photonic nanojet generally are still not clear. This PhD dissertation is a result of four years of research on photonic nanojet and its usage for Raman spectroscopy. It is based on four published papers [1, 2, 3, 4], one still unpublished body of work, and a patent application [5, 6]. This research has resulted not only in the improvement of the method of Raman enhancement, but also with new findings in the general field of photonic nanojet. The research was performed from two angles, experimental and computational. The series of computer codes were written in order to model the photonic nanojet in various conditions. The codes, based on Generalized Lorenz-Mie theory, calculate the electric field intensity from scattering of a Gaussian beam on a dielectric microsphere, upon which a photonic nanoi Extended abstract jet emerges. A large amount of configurations was calculated which provided a systematic overview of photonic nanojet properties and its dependence on parameters. Also, the change of dependencies is detected and investigated by variation of other parameters. The parameters which are varied are the incident Gaussian beam wavelength, position and waist radius, and the microsphere radius. The microsphere refractive index was taken to correspond to SiO2 material. The investigated properties of a photonic nanojet are its maximum intensity, position, width and length. The incident Gaussian beam position is shown to be of critical importance for photonic nanojet properties. Two types of photonic nanojet are identified: Type 1 has lower intensity, its position is further away from the microsphere and has larger dimensions, while Type 2 has higher intensity, it is positioned close to the microsphere edge, and has smaller dimensions. The size matching between the incident beam waist radius and microsphere radius is shown to improve the intensity of the photonic nanojet, but it is not the main contribution. Proper positioning of the incident beam, small waist radius and short wavelength are shown to be important for high intensity. It is also shown that all parameters are important in their absolute value, and that size parameter from Lorenz-Mie theory cannot be applied. Furthermore, parameter combinations for the photonic nanojet of extremely high intensity, very narrow width, or extremely long length with long working distance are determined. In some regimes, intensity oscillations are also detected, and they are identified as whispering-gallery modes and Mie interferences. The occurrence of the photonic nanojet is also investigated when a high refractive index microsphere is used. The investigation followed three theoretical levels: geometrical optics, ray transfer matrix analysis, and Generalized Lorenz-Mie theory. Geometrical optics show that divergent incident light rays can be focused outside a high refractive index microsphere. Ray transfer matrix analysis show that divergent cone of a Gaussian beam produces output beam with a waist outside a high refractive index microsphere. The mathematical condition for that occurrence is derived. Finally, the Generalized Lorenz-Mie theory calculations show that a photonic nanojet can emerge outside the microsphere even when the refractive index of a microsphere is higher than two, which was up to now considered a limit in literature. The calculations also show the difference in focusing of the incident beam based on the refractive index of the microsphere, which is confirmed by the vertical Raman mapping. The Raman enhancement is optimized by variation of experimental parameters. Optimal position of the incident laser beam is determined by vertical Raman mapping, and explained with ray transfer matrix analysis. Laser beam profiles under the microscope objective are determined by a knife-edge method. Antenna effect of the microsphere for the enhancement is detected. The dependence of the enhancement on the collection fiber diameter, microscope objective and microsphere size is determined. Two microsphere materials were used: SiO2 and barium tiii Extended abstract tanate glass. The dependence on microsphere radius shows different behaviors depending on the objective used. The calculations of a photonic nanojet intensity are compared with experimental values of the Raman enhancement, which suggest that the photonic nanojet is not the only contribution to the enhancement. The enhancement strongly lowers by increasing the numerical aperture of the objective. The highest enhancement of the silicon substrate, of 19.29× is achieved in configuration of barium titanate glass microsphere of radius of 4.5 μm and 10× NA 0.25 microscope objective. The mechanism of the enhancement is discussed, which is separated into two contributions. The first contribution comes from the photonic nanojet, and the second contribution comes from the collection system. The model of the effective numerical aperture of the microsphere-objective system is presented, and compared with the experimental results. The usage of the microsphere for the enhancement was further improved by designing the new system for mechanical control of the microsphere. The system is called two-stemmed microsphere and allows positioning of the microsphere under the laser beam of the microscope objective independently of the substrate position. This way, Raman mapping can be performed in which each point is enhanced. The system is tested on a silicon substrate with domains separated by visible borders. Raman mappings are compared with atomic force microscope measurements. With two-stemmed microsphere, the intensity enhancement is 4× and the estimated resolution enhancement is 3×. Combined enhancement of SERS (surface-enhanced Raman scattering) and microsphere is achieved. The SERS substrates which were used were non-uniform and uniform silver nanoislands. The used analytes were 4-mercaptophenylboronic acid or 4-mercaptobenzoic acid. The non-uniform substrates combined with the microsphere show higher but less reproducible enhancement than the uniform substrates with the microspheres

Raman scattering enhancement using photonic nanojet of dielectric microspheres

Ramanova spektroskopija metoda je karakterizacije i identifikacije materijala bazirana na neelastičnom Ramanovom raspršenju fotona svjetlosti na materijalu. Primjenjuje se u raznim znanstvenim i industrijskim područjima, od fizike, kemije, biologije, medicine, znanosti o materijalima, do kontrola sigurnosti, detekcije nedozvoljenih tvari i proučavanja umjetničkih dijela. Zbog neelastičnosti procesa, Ramanovo raspršenje daje vrlo slab signal, zbog čega su razvijene različite metode pojačanja. Jedna od novih metoda pojačanja, čije su prednosti jednostavnost i niska cijena primjene, neinvazivnost, reproducibilnost i stabilnost, temelji se na fotonskom nanomlazu koji nastaje obasjavanjem mikroleće svjetlošću. Odabirom pogodnih parametara, fotonski nanomlaz može imati izrazito visok intenzitet, usku širinu, ili veliku duljinu, zbog čega se osim za Ramanovu spektroskopiju, istražuje i za primjene u nanolitografiji, super-rezoluciji, optičkim silama, pohrani podataka i sličnim poljima. Trenutno, njegova primjena u Ramanovoj spektroskopiji nedovoljno je istražena, a i sama svojstva i uvjeti za njegov nastanak nerazjašnjeni su. Istraživanjima u sklopu ovog doktorskog rada unaprijeđena je metoda pojačanja, te su dobivena i nova saznanja o fotonskom nanomlazu općenito. Napravljen je računalni program temeljen na Generaliziranoj-Lorenz Mie teoriji, kojim je izračunat širok raspon različitih konfiguracija za fotonski nanomlaz iz dielektrične mikrosfere. Dobiven je sistematičan uvid u svojstva i ovisnost fotonskog nanomlaza o parametrima te uvid u promjene pojedinih ovisnosti u različitim uvjetima. Pokazana je kritična važnost parametra pozicije upadne zrake. Odabirom određene kombinacije parametara dobiven je izrazito intenzivan, vrlo uzak ili izrazito dug fotonski nanomlaz daleko izvan mikrosfere. Istraživanjem područja visokog indeksa loma mikrosfere, modeliranjem je pokazano da fotonski nanomlaz može nastati izvan mikrosfere i kada je indeks loma viši od 2, što je u dosadašnjoj literaturi bilo označeno kao gornja granica. Varijacijom eksperimentalnih parametara optimizirano je pojačanje. Pomoću vertikalnog ramanskog mapiranja određena je optimalna pozicija upadne zrake za pojačanje te je pokazana njezina važnost. Izmjerena je ovisnost pojačanja o kolekcijskom vlaknu, mikroskopskom objektivu i veličini mikrosfere. Diskutiran je mehanizam pojačanja, koji smo podijelili u dva doprinosa - od fotonskog nanomlaza, te od kolekcijskog sustava. Dobiveno je kombinirano pojačanje mikrosfere i plazmonskog pojačanja. Dizajniran je i testiran novi sustav za mehaničku kontrolu mikrosfere pod mikroskopskim objektivom, kojim je moguće iskoristiti pojačanje za svaku točku ramanskog mapiranja. Dobiveno je pojačanje intenziteta i rezolucije mapiranja.Raman spectroscopy is a method for characterization and identification of materials, which is widely used in a broad range of scientific and industrial fields, like materials science, physics, chemistry, biology, medicine, security control, substance control and art examination. It is based on Raman scattering of light on molecules and crystals. As opposed to its elastic counterpart - Rayleigh scattering, Raman scattering is inelastic, which means that the scattered photon has a different energy than the incident one. Because of this, Raman scattering has low probability of occurrence and the scattered Raman intensity is very low. For this reason, many methods for the enhancement of Raman scattering have been developed through the years. One of the new methods of enhancement is based on photonic nanojet, which is a concentrated beam of light emerging from the shadow side of an illuminated microlens. This method of enhancement is characterized by a low cost and a simple principle of implementation. It is a non-invasive, reproducible and reliable way of enhancement. By careful choice of parameters, photonic nanojet can have very high intensity, very narrow width, or very long length. This makes it suitable not only for Raman scattering enhancement, but also for applications in nanolithography, super resolution, optical forces, data storage and similar fields. Although being a promising technique, the role and usage of photonic nanojet in Raman spectroscopy is currently underexplored. Moreover, the properties and conditions for emergence of the photonic nanojet generally are still not clear. This PhD dissertation is a result of four years of research on photonic nanojet and its usage for Raman spectroscopy. It is based on four published papers [1, 2, 3, 4], one still unpublished body of work, and a patent application [5, 6]. This research has resulted not only in the improvement of the method of Raman enhancement, but also with new findings in the general field of photonic nanojet. The research was performed from two angles, experimental and computational. The series of computer codes were written in order to model the photonic nanojet in various conditions. The codes, based on Generalized Lorenz-Mie theory, calculate the electric field intensity from scattering of a Gaussian beam on a dielectric microsphere, upon which a photonic nanoi Extended abstract jet emerges. A large amount of configurations was calculated which provided a systematic overview of photonic nanojet properties and its dependence on parameters. Also, the change of dependencies is detected and investigated by variation of other parameters. The parameters which are varied are the incident Gaussian beam wavelength, position and waist radius, and the microsphere radius. The microsphere refractive index was taken to correspond to SiO2 material. The investigated properties of a photonic nanojet are its maximum intensity, position, width and length. The incident Gaussian beam position is shown to be of critical importance for photonic nanojet properties. Two types of photonic nanojet are identified: Type 1 has lower intensity, its position is further away from the microsphere and has larger dimensions, while Type 2 has higher intensity, it is positioned close to the microsphere edge, and has smaller dimensions. The size matching between the incident beam waist radius and microsphere radius is shown to improve the intensity of the photonic nanojet, but it is not the main contribution. Proper positioning of the incident beam, small waist radius and short wavelength are shown to be important for high intensity. It is also shown that all parameters are important in their absolute value, and that size parameter from Lorenz-Mie theory cannot be applied. Furthermore, parameter combinations for the photonic nanojet of extremely high intensity, very narrow width, or extremely long length with long working distance are determined. In some regimes, intensity oscillations are also detected, and they are identified as whispering-gallery modes and Mie interferences. The occurrence of the photonic nanojet is also investigated when a high refractive index microsphere is used. The investigation followed three theoretical levels: geometrical optics, ray transfer matrix analysis, and Generalized Lorenz-Mie theory. Geometrical optics show that divergent incident light rays can be focused outside a high refractive index microsphere. Ray transfer matrix analysis show that divergent cone of a Gaussian beam produces output beam with a waist outside a high refractive index microsphere. The mathematical condition for that occurrence is derived. Finally, the Generalized Lorenz-Mie theory calculations show that a photonic nanojet can emerge outside the microsphere even when the refractive index of a microsphere is higher than two, which was up to now considered a limit in literature. The calculations also show the difference in focusing of the incident beam based on the refractive index of the microsphere, which is confirmed by the vertical Raman mapping. The Raman enhancement is optimized by variation of experimental parameters. Optimal position of the incident laser beam is determined by vertical Raman mapping, and explained with ray transfer matrix analysis. Laser beam profiles under the microscope objective are determined by a knife-edge method. Antenna effect of the microsphere for the enhancement is detected. The dependence of the enhancement on the collection fiber diameter, microscope objective and microsphere size is determined. Two microsphere materials were used: SiO2 and barium tiii Extended abstract tanate glass. The dependence on microsphere radius shows different behaviors depending on the objective used. The calculations of a photonic nanojet intensity are compared with experimental values of the Raman enhancement, which suggest that the photonic nanojet is not the only contribution to the enhancement. The enhancement strongly lowers by increasing the numerical aperture of the objective. The highest enhancement of the silicon substrate, of 19.29× is achieved in configuration of barium titanate glass microsphere of radius of 4.5 μm and 10× NA 0.25 microscope objective. The mechanism of the enhancement is discussed, which is separated into two contributions. The first contribution comes from the photonic nanojet, and the second contribution comes from the collection system. The model of the effective numerical aperture of the microsphere-objective system is presented, and compared with the experimental results. The usage of the microsphere for the enhancement was further improved by designing the new system for mechanical control of the microsphere. The system is called two-stemmed microsphere and allows positioning of the microsphere under the laser beam of the microscope objective independently of the substrate position. This way, Raman mapping can be performed in which each point is enhanced. The system is tested on a silicon substrate with domains separated by visible borders. Raman mappings are compared with atomic force microscope measurements. With two-stemmed microsphere, the intensity enhancement is 4× and the estimated resolution enhancement is 3×. Combined enhancement of SERS (surface-enhanced Raman scattering) and microsphere is achieved. The SERS substrates which were used were non-uniform and uniform silver nanoislands. The used analytes were 4-mercaptophenylboronic acid or 4-mercaptobenzoic acid. The non-uniform substrates combined with the microsphere show higher but less reproducible enhancement than the uniform substrates with the microspheres

Raman scattering enhancement using photonic nanojet of dielectric microspheres

Ramanova spektroskopija metoda je karakterizacije i identifikacije materijala bazirana na neelastičnom Ramanovom raspršenju fotona svjetlosti na materijalu. Primjenjuje se u raznim znanstvenim i industrijskim područjima, od fizike, kemije, biologije, medicine, znanosti o materijalima, do kontrola sigurnosti, detekcije nedozvoljenih tvari i proučavanja umjetničkih dijela. Zbog neelastičnosti procesa, Ramanovo raspršenje daje vrlo slab signal, zbog čega su razvijene različite metode pojačanja. Jedna od novih metoda pojačanja, čije su prednosti jednostavnost i niska cijena primjene, neinvazivnost, reproducibilnost i stabilnost, temelji se na fotonskom nanomlazu koji nastaje obasjavanjem mikroleće svjetlošću. Odabirom pogodnih parametara, fotonski nanomlaz može imati izrazito visok intenzitet, usku širinu, ili veliku duljinu, zbog čega se osim za Ramanovu spektroskopiju, istražuje i za primjene u nanolitografiji, super-rezoluciji, optičkim silama, pohrani podataka i sličnim poljima. Trenutno, njegova primjena u Ramanovoj spektroskopiji nedovoljno je istražena, a i sama svojstva i uvjeti za njegov nastanak nerazjašnjeni su. Istraživanjima u sklopu ovog doktorskog rada unaprijeđena je metoda pojačanja, te su dobivena i nova saznanja o fotonskom nanomlazu općenito. Napravljen je računalni program temeljen na Generaliziranoj-Lorenz Mie teoriji, kojim je izračunat širok raspon različitih konfiguracija za fotonski nanomlaz iz dielektrične mikrosfere. Dobiven je sistematičan uvid u svojstva i ovisnost fotonskog nanomlaza o parametrima te uvid u promjene pojedinih ovisnosti u različitim uvjetima. Pokazana je kritična važnost parametra pozicije upadne zrake. Odabirom određene kombinacije parametara dobiven je izrazito intenzivan, vrlo uzak ili izrazito dug fotonski nanomlaz daleko izvan mikrosfere. Istraživanjem područja visokog indeksa loma mikrosfere, modeliranjem je pokazano da fotonski nanomlaz može nastati izvan mikrosfere i kada je indeks loma viši od 2, što je u dosadašnjoj literaturi bilo označeno kao gornja granica. Varijacijom eksperimentalnih parametara optimizirano je pojačanje. Pomoću vertikalnog ramanskog mapiranja određena je optimalna pozicija upadne zrake za pojačanje te je pokazana njezina važnost. Izmjerena je ovisnost pojačanja o kolekcijskom vlaknu, mikroskopskom objektivu i veličini mikrosfere. Diskutiran je mehanizam pojačanja, koji smo podijelili u dva doprinosa - od fotonskog nanomlaza, te od kolekcijskog sustava. Dobiveno je kombinirano pojačanje mikrosfere i plazmonskog pojačanja. Dizajniran je i testiran novi sustav za mehaničku kontrolu mikrosfere pod mikroskopskim objektivom, kojim je moguće iskoristiti pojačanje za svaku točku ramanskog mapiranja. Dobiveno je pojačanje intenziteta i rezolucije mapiranja.Raman spectroscopy is a method for characterization and identification of materials, which is widely used in a broad range of scientific and industrial fields, like materials science, physics, chemistry, biology, medicine, security control, substance control and art examination. It is based on Raman scattering of light on molecules and crystals. As opposed to its elastic counterpart - Rayleigh scattering, Raman scattering is inelastic, which means that the scattered photon has a different energy than the incident one. Because of this, Raman scattering has low probability of occurrence and the scattered Raman intensity is very low. For this reason, many methods for the enhancement of Raman scattering have been developed through the years. One of the new methods of enhancement is based on photonic nanojet, which is a concentrated beam of light emerging from the shadow side of an illuminated microlens. This method of enhancement is characterized by a low cost and a simple principle of implementation. It is a non-invasive, reproducible and reliable way of enhancement. By careful choice of parameters, photonic nanojet can have very high intensity, very narrow width, or very long length. This makes it suitable not only for Raman scattering enhancement, but also for applications in nanolithography, super resolution, optical forces, data storage and similar fields. Although being a promising technique, the role and usage of photonic nanojet in Raman spectroscopy is currently underexplored. Moreover, the properties and conditions for emergence of the photonic nanojet generally are still not clear. This PhD dissertation is a result of four years of research on photonic nanojet and its usage for Raman spectroscopy. It is based on four published papers [1, 2, 3, 4], one still unpublished body of work, and a patent application [5, 6]. This research has resulted not only in the improvement of the method of Raman enhancement, but also with new findings in the general field of photonic nanojet. The research was performed from two angles, experimental and computational. The series of computer codes were written in order to model the photonic nanojet in various conditions. The codes, based on Generalized Lorenz-Mie theory, calculate the electric field intensity from scattering of a Gaussian beam on a dielectric microsphere, upon which a photonic nanoi Extended abstract jet emerges. A large amount of configurations was calculated which provided a systematic overview of photonic nanojet properties and its dependence on parameters. Also, the change of dependencies is detected and investigated by variation of other parameters. The parameters which are varied are the incident Gaussian beam wavelength, position and waist radius, and the microsphere radius. The microsphere refractive index was taken to correspond to SiO2 material. The investigated properties of a photonic nanojet are its maximum intensity, position, width and length. The incident Gaussian beam position is shown to be of critical importance for photonic nanojet properties. Two types of photonic nanojet are identified: Type 1 has lower intensity, its position is further away from the microsphere and has larger dimensions, while Type 2 has higher intensity, it is positioned close to the microsphere edge, and has smaller dimensions. The size matching between the incident beam waist radius and microsphere radius is shown to improve the intensity of the photonic nanojet, but it is not the main contribution. Proper positioning of the incident beam, small waist radius and short wavelength are shown to be important for high intensity. It is also shown that all parameters are important in their absolute value, and that size parameter from Lorenz-Mie theory cannot be applied. Furthermore, parameter combinations for the photonic nanojet of extremely high intensity, very narrow width, or extremely long length with long working distance are determined. In some regimes, intensity oscillations are also detected, and they are identified as whispering-gallery modes and Mie interferences. The occurrence of the photonic nanojet is also investigated when a high refractive index microsphere is used. The investigation followed three theoretical levels: geometrical optics, ray transfer matrix analysis, and Generalized Lorenz-Mie theory. Geometrical optics show that divergent incident light rays can be focused outside a high refractive index microsphere. Ray transfer matrix analysis show that divergent cone of a Gaussian beam produces output beam with a waist outside a high refractive index microsphere. The mathematical condition for that occurrence is derived. Finally, the Generalized Lorenz-Mie theory calculations show that a photonic nanojet can emerge outside the microsphere even when the refractive index of a microsphere is higher than two, which was up to now considered a limit in literature. The calculations also show the difference in focusing of the incident beam based on the refractive index of the microsphere, which is confirmed by the vertical Raman mapping. The Raman enhancement is optimized by variation of experimental parameters. Optimal position of the incident laser beam is determined by vertical Raman mapping, and explained with ray transfer matrix analysis. Laser beam profiles under the microscope objective are determined by a knife-edge method. Antenna effect of the microsphere for the enhancement is detected. The dependence of the enhancement on the collection fiber diameter, microscope objective and microsphere size is determined. Two microsphere materials were used: SiO2 and barium tiii Extended abstract tanate glass. The dependence on microsphere radius shows different behaviors depending on the objective used. The calculations of a photonic nanojet intensity are compared with experimental values of the Raman enhancement, which suggest that the photonic nanojet is not the only contribution to the enhancement. The enhancement strongly lowers by increasing the numerical aperture of the objective. The highest enhancement of the silicon substrate, of 19.29× is achieved in configuration of barium titanate glass microsphere of radius of 4.5 μm and 10× NA 0.25 microscope objective. The mechanism of the enhancement is discussed, which is separated into two contributions. The first contribution comes from the photonic nanojet, and the second contribution comes from the collection system. The model of the effective numerical aperture of the microsphere-objective system is presented, and compared with the experimental results. The usage of the microsphere for the enhancement was further improved by designing the new system for mechanical control of the microsphere. The system is called two-stemmed microsphere and allows positioning of the microsphere under the laser beam of the microscope objective independently of the substrate position. This way, Raman mapping can be performed in which each point is enhanced. The system is tested on a silicon substrate with domains separated by visible borders. Raman mappings are compared with atomic force microscope measurements. With two-stemmed microsphere, the intensity enhancement is 4× and the estimated resolution enhancement is 3×. Combined enhancement of SERS (surface-enhanced Raman scattering) and microsphere is achieved. The SERS substrates which were used were non-uniform and uniform silver nanoislands. The used analytes were 4-mercaptophenylboronic acid or 4-mercaptobenzoic acid. The non-uniform substrates combined with the microsphere show higher but less reproducible enhancement than the uniform substrates with the microspheres

Engineering SERS Properties of Silicon Nanotrees at the Nanoscale

Large specific surface area nanostructures are desirable in a wide range of sensing applications due to their longer light-trapping path and increased absorption. Engineering of the specific nanotree structure which possesses a high branch density turned out to be challenging from the experimental point of view, and certainly not adequately explored. This paper shows how to design substrates with a silicon nanotree structure for surface-enhanced Raman spectroscopy (SERS) applications. Silicon nanotrees were synthesized by a Ag-Au nanocluster-catalyzed low-pressure chemical vapor deposition method (LPCVD). By the presented approaches, it is possible to manipulate branches’ number, length and thickness. The synthesized nanostructures are flexible after immersion in water which improves SERS performance. The amount of sputtered metal played a key role in preserving the flexibility of the nanotree structure. The obtained substrates with highly fractal nanostructure were tested on 4-mercaptophenylboronic acid (MPBA) to match the optimal SERS parameters. The silicon nanotrees fabrication, and particularly obtained SERS substrates plated with Ag and Au nanoparticles, demonstrated good features and a promising approach for further sensor development

Features and advantages of flexible silicon nanowires for SERS applications

The paper reports on the features and advantages of horizontally oriented flexible silicon nanowires (SiNWs) substrates for surface-enhanced Raman spectroscopy (SERS) applications. The novel SERS substrates are described in detail considering three main aspects. First, the key synthesis parameters for the flexible nanostructure SERS substrates were optimized. It is shown that fabrication temperature and metal-plating duration significantly influence the flexibility of the SiNWs and, consequently, determine the SERS enhancement. Second, it is demonstrated how the immersion in a liquid followed by drying results in the formation of SiNWs bundles influencing the surface morphology. The morphology changes were described by fractal dimension and lacunar analyses and correlated with the duration of Ag plating and SERS measurements. SERS examination showed the optimal intensity values for SiNWs thickness values of 60–100 nm. That is, when the flexibility of the self-assembly SiNWs allowed hot spots occurrence. Finally, the test with 4-mercaptophenylboronic acid showed excellent SERS performance of the flexible, horizontally oriented SiNWs in comparison with several other commercially available substrates