Плазмонные наноструктуры для оптических метаматериалов тема диссертации и автореферата по ВАК РФ 01.04.05, доктор наук Драчев Владимир Прокопьевич

• Драчев Владимир Прокопьевич
• доктор наук
• 2022

Оглавление диссертации доктор наук Драчев Владимир Прокопьевич

Contents

INTRODUCTION

I. Dielectric function of noble metals in plasmonic nanostructures

1.1 The Ag Dielectric Function in Plasmonic Metamaterials

1.2 Drude Relaxation Rate in Grained Gold Nanoantennae

12.1 Introduction

12.2 Experiment Methods

12.3 Results and Discussion

12.4 Discussion and Conclusions

1.3 Size dependent x(3) for conduction electrons in Ag nanoparticles 35 II. New mechanism of plasmons specific for spin-polarized nanoparticles 43 II.1 Introduction 43 II. 2 Results

11.3 Discussion

11.4 Methods

11.5 Supporting Information

115.1 Magnetic properties of Co nanoparticles

115.2 Dynamic light scattering

115.3 Structural characterization

115.4 Long-term variations of absorption spectra

115.5 Absorption cross-section spectra of Co nanoparticles in comparison with Au and Ag nanoparticles

III. Optical metamaterials with magnetism and negative refractive index

III. Introduction

III.1 Experimental verification of optical negative index materials at 1.5 |im

1111.2 Fabrication of test samples

1111.3 Experiments and refractive index retrieval

1111.4 Simulations

1111.5 Results and discussions

1111.6 Conclusions

III.2 Metamagnetics using grating and NIMs using cross-gratings

ffl.3 Negative Index Metamaterials based on cross-gratings

III. 3.1 Conclusions

III.4 Optical Negative Index Metamaterials with compensated Losses

IV. Sub-wavelength Patterning in a Hyperbolic Metamaterials by Volume Plasmon Polaritons

IV. 1 Introduction

IV.2 Volume plasmon polaritons in a hyperbolic medium

IV.3 Experimental demonstration of sub-wavelength interference in a hyperbolic medium

104

IV.4 Summary

V. Scattering Suppression by Epsilon-Near-Zero Plasmonic Fractal Sh e l l s

V.1 Introduction

V.2 Results and Discussion

V.3 Methods

V.4 Conclusions

120

Conclusive summary and defended provisions

124

Publications and approbations

131

References Introduction

References Chapter I

References Chapter II

References Chapter III

References Chapter IV

References Chapter V

Введение диссертации (часть автореферата) на тему «Плазмонные наноструктуры для оптических метаматериалов»

Introduction

A phenomenon relevant to a surface electromagnetic wave propagating at the interface between two media possessing permittivities with opposite signs, metal-dielectric interface, is a surface plasmon polariton (SPP). An external electromagnetic wave might excite the oscillations of free electrons in a metallic particle called localized surface plasmons, whose resonance frequency is the plasmon frequency dependent on the size and, mainly, the shape of the particle. The electromagnetic field is enhanced in the close vicinity to the surface of the particle or metal-dielectric boundary.

Localized plasmons are observed on rough electrodes ([1] Ritchie, 1973; [2] Fleischmann et al., 1974; [3] Moskovits, 1985), in designed nanostructures ([4] Quinten et al., 1998; [5] Averitt et al., 1999; [6] Brongersma et al., 2000; [7] Mock et al., 2002; [8] Pham et al., 2002), as well as in clusters and aggregates of nanoparticles ([9] Kreibig and Vollmer, 1995; [10] Quinten, 1999; [11] Su et al., 2003). Local fields are concentrated in the spots, where molecules' and atoms' linear and nonlinear optical responses are largely enhanced. This effect results in many important applications, in particular surface-enhanced Raman scattering (SERS) ([2] Fleischmann et al., 1974; [3] Moskovits,1985).

Enhanced local fields due to surface plasmons can dramatically enhance the Raman signal. As a result, SERS makes possible rapid molecular assays to detect biological and chemical substances ([12] Kneipp et al., 2002). The high sensitivity of SERS enables observation of Raman scattering from a single molecule attached to a colloidal metal particle ([13] Kneipp et al., 1997; [14] Nie and Emory, 1997).

Raman scattering sensing techniques can provide a great deal of information on exactly what specific molecules are being detected. SERS enables molecular "fingerprinting" important for organic and bio-molecule sensing. There are some particularly efficient and sensitive SERS substrates. Among them are nanoshells developed by the Halas group ([15] Prodan et al., 2003), the Van Duyne group's substrates fabricated with nanosphere lithography ([16] Hayes and Van Duyne, 2003), and adaptive silver films developed by Drachev et al. (2004) [17]. Metal fractal aggregates of nanoparticles supporting localized SPs can lead to extremely large local field enhancement relative to isolated metallic particles ([18] Markel et al., 1996, [19] Shalaev et al., 1996).

Several nonlinear optical phenomena, such as highly efficient harmonic generation and Kerr effect due to huge field enhancement in hot spots of fractal nanoparticles have been theoretically predicted and experimentally demonstrated by Rautian, Safonov, Markel, Stockman, Shalaev, and co-workers [20].

Brus and Nitzan (1983) [21] proposed to use giant localized fields to influence photochemical reactions. Later this phenomenon was studied in application to photocells, detectors, and other processes, including vision ([22] Hutson, 2005). A response of a p-n junction by localized SPs has been studied ([23] Schaadt et al., 2005). A field enhancement close to metallic tip enables linear

and nonlinear near-field scanning microscopy, spectroscopy, and photo-modification with nanoscale resolution ([24] Stockman, 1989; [25] Ferrell, 1994; [26] Sanchez et al., 1999). The resonant excitation of SPs has explained extraordinary light transmission through metal films with periodic arrays of subwavelength holes by Ghaemi et al. (1998) [27].

Metal structures and nanoparticles can influence not only nonlinear but also linear optical responses. Thus, the dependence of emission spectra and emission lifetimes of luminescent centers on submicron distance from a metal mirror has been studied in detail by Drexhage (1974), [28] and references therein. Dye molecules adsorbed onto metal islands or films emit luminescence with enhanced intensity. If the plasma resonances are coupled to the absorption spectra of the molecules, the photoluminescence enhancement can be observed (Glass et al. 1980, 1981) [29-30] and Ritchie and Burstein (1981) [31]). An enhancement and modification of the emission lifetime for dye molecules or trivalent rare-earth ions in the vicinity of rough metallic surfaces, metallic islands, engineered structures, etc. have been studied more recently ([32] Weitz et al., 1983; [33] Denisenko et al., 1996; [34] Kikteva et al., 1999; [35] Selvan et al., 1999; [36] Lakowicz et al., 2001). Biteen et al. (2004) [37] demonstrated an increase in optical absorption of CdS quantum dots by gold nanospheres.

Surprisingly, the SP-induced enhancement of absorption and emission of dye can significantly improve the laser performance if a mixture of dye solution and metallic nanoparticles is used as a laser medium. For example, a mixture of rhodamine 6G (R6G) dye with aggregated silver nano-particles placed in an optical micro-cavity (capillary glass tube) results in the laser action at the pumping power lower than the lasing threshold in a pure dye solution of the same concentration (Kim et al. (1999) [38] and Drachev et al. (2002) [39]). Note, that the experiments in the studies by Kim et al. (1999) [38] and Drachev et al. (2002) [39] were not highly reproducible, and the mechanisms of the dramatic reduction of the lasing threshold were not well understood. These motivated other groups to carry out a more systematic study of the effects of spontaneous emission, the laser emission in a two-mirror laser cavity, and the stimulated emission in a pump-probe setup (Noginov et al. [40-43] (Noginov et al., 2005a; Noginov et al. 2006a, b, c). First, it was demonstrated an enhancement of localized SP oscillation in the aggregate of Ag nanoparticles by optical gain due to surrounding dye and second, an enhancement of spontaneous and stimulated emission of rhodamine 6G (R6G) laser dye caused by Ag aggregate [40-43]. Most of the applications of nanoplasmonics suffer from damping caused by metal absorption and radiation losses. Several studies have been made on how to rid of plasmon loss. Namely, in 1989 Sudarkin and Demkovich [44] proposed to increase the propagation length of SPP by the population inversion created in the dielectric medium adjacent to the metallic film (Sudarkin and Demkovich, 1989 [44]). The experimental test was designed to observe an increased reflectivity of a metallic film in the frustrated total internal reflection. Sudarkin and Demkovich (1989) [44] also discussed the possibility of operating an SP-based laser. This work cited the observation of super-luminescence and light generation by a dye solution under the condition of internal reflection ([45] Kogan et al., 1972) and gain-enhanced total internal reflection in the presence of metallic film ([46] Plotz et al., 1979).

These earlier ideas above have been further developed later. Gain-assisted propagation of SPPs in metal waveguides has been studied by Nezhad et al. (2004) [47]. SPPs at the interface between metal and material with optical gain have been studied theoretically by Avrutsky (2004) [48]. It has been shown that the properly chosen optical indices of two media can result formally in infinitely large effective wave vectors of the surface waves [48]. Note that the resonant plasmons have very low group velocity and are localized close to the interface. The amplification of SPPs at the interface between the silver film and dielectric medium with optical gain (laser dye) has been recently demonstrated by Seidel et al. (2005) [49]. This observation was done using an experimental setup similar to [44], and the change in the metal reflection was about 0.001 %. Similarly, Lawandy (2004) [50] has predicted the localized SP resonance in metallic nanospheres to exhibit a singularity when the surrounding dielectric medium has a critical value of optical gain. This singularity comes from canceling both, real and imaginary terms in the denominator of the field enhancement factor in metal nanospheres, which is proportional to (ed - em) /(2 ed + em). This effect can be evidenced by an increase of the Rayleigh scattering within the plasmon band and lead to low-threshold random laser action, light localization effects, and enhancement of SERS (Lawandy, 2004) [50] (here ed and em are complex dielectric constants of dielectric and metal, respectively).

This study was continued in work by Lawandy (2005) [51], where a three-component system consisting of (i) metallic nanoparticle, (ii) shell of adsorbed molecules with optical gain, and (iii) surrounding dielectric (solvent) has been considered. In particular, depending on the thickness of the layer of an amplifying shell, the absorption of the complex can be increased or decreased with the increase of the gain in the dye shell (Lawandy, 2005) [51].

A seemingly similar phenomenon has been described in an earlier publication ([52] Bergman and Stockman, 2003) using a completely different set of arguments. Thus, Bergman and Stockman (2003) [52] have proposed a new way to excite localized fields in nanosystems using SP amplification by stimulated emission of radiation (SPASER). SPASER radiation consists of SPs (bosons), which undergo stimulated emission, but, in contrast to photons, can be localized on the nanoscale. SPASER consists of an active medium with population inversion that transfers its excitation energy by radiationless transitions to a resonant nanosystem, which plays a role analogous to the laser cavity ([52] Bergman and Stockman, 2003; [53] Stockman, 2005). Alternatively, the major features of SPASER can be probably described in terms of Forster dipoledipole energy transfer ([54] Forster, 1948; [55] Dexter et al., 1969) from an excited molecule (ion, quantum dot, etc.) to a resonant SP oscillation in a metallic nanostructure. The polarizability (per unit volume) for isolated metallic nano-particles is given by P = (4n) -1 (ed - em) /(ed + p(em- ed)), where p is the depolarization factor ([56] Shalaev, 2000). If the dielectric is an active medium with ed" = -p em"/(1 - p), then at the resonance wavelength Xc both, the real and imaginary parts in the denominator become zero, leading to extremely large local fields limited only by saturation effects ([57] Drachev et al., 2004; [50] Lawandy, 2004). The saturation effect limitations will be discussed in the Chapter 1 of the thesis. We use the Drude formula for the metal permittivity em = eb - rop2/[ro(ro + ir)], with known optical constants from Johnson and Christy

(1972) [58]). The value of the critical gain to compensate losses is about 103 cm-1 as estimated in Ref. (Lawandy, 2005) [51].

Negative-index materials (NIMs) have a negative refractive index so that the phase velocity is pointed against the flow of energy. There are no known naturally occurring NlMs in the optical range. Nevertheless, NIMs can be realized with artifi-cially designed materials (metamaterials). Metamaterials open new avenues to achieve unprecedented functionality unattainable with naturally existing materials, as was first described by Veselago in his seminal paper ([59] Veselago, 1968). Optical NlMs (ONlMs) promise to create entirely new prospects for controlling and manipulating light, optical sensing, nanoscale imaging, and photolithography. The possible design of such materials was repeatedly discussed in the literature mainly for the microwave spectral range ([60] Lagarkov, Sarychev, Vinogradov 1984). In particular, there were derived formulas necessary for developing materials with elongated conducting inclusions (sticks) having s < 0. Later, Lagarkov et al. 1997 [61] experimentally demonstrated materials with both negative | and s in the microwave spectral range and derived corresponding formulas. It seems that the first materials purposefully synthesized to verify a number of effects predicted in [59] were obtained by J.B. Pendry et al. 1999 [62, and references therein].

Extensive literature presents discussion on the form of constitutive equations in electrodynamics Vinogradov et al. 1996 [63], Vinogradov 2002 [64]. In particular, to solve the dispersion equation k2 = k0e(M, k) we must define the square root as a regular single-valued function.

To do this it suffices to take a cut along the negative real axis and calculate the square root for the permittivity and permeability of the medium separately (Lagarkov,Kisel 2001 [65]): k = k0^e(M, k) ^^(m, k) . Defining the square root in this way ensures physically correct solutions both for active and passive media. These works made possible to state that there are both, technologies for fabrication such materials and computational methods for predicting and designing materials having negative real parts of the permittivity s' and permeability It has become possible to predict such materials also in the case of dissipative media. Proof-of-principle experiments ([66] Smith et al., 2000; [67] Shelby et al., 2001) have shown that metamaterials can act as NIMs at microwave wavelengths. A large amount of attention to NIMs has been initiated by Pendry, who revisited Veselago NIM-based superlens [59], which allows an imaging resolution which is limited only by material quality ([68] Pendry, 2000). The near-field version of the superlens has been reported then by the Zhang and Blaikie groups ([69] Fang et al., 2005; [70] Melville and Blackie, 2005).

While negative permittivity s' < 0 (s = s' + is") in the optical range can be attained for metals, there are no naturally occurring materials with a magnetic response at such high frequencies. Terahertz frequency experiments showed that a magnetic response and negative permeability |i' < 0 (^ = |i' + i|" can be accomplished ([71] Linden et al., 2004; [72] Yen et al., 2004; [73] Zhang et al., 2005). An optical metamaterial with the negative refractive index at 1.5 |im, based on paired metal nanorods embedded in a dielectric was designed ([74-75] Podolskiy, Sarychev, Shalaev, 2002, 2003, and [76] Podolskiy et al., 2005). Then it was experimentally demonstrated in our work,

which is included in this thesis ([77] Shalaev et al., 2005; [78] Drachev et al., 2006; [79] Kildishev et al., 2006).

This introduction clearly indicates a great interest to plasmonic nanostructures at the beginning of the century and explains multiple reasons for the interest growing. The main goals of the efforts presented in the thesis were experimental feasibility study on optical NIMs, metamagnetics across the visible spectral range, subwavelength photolithography using hyperbolic metamaterials, scattering suppression with epsilon near-zero fractal shell, and effect of nanostructurering on the metal dielectric function. We have demonstrated, for the first time, feasibility of optical NIMs and losses compensation with gain for NIMs. These studies were based mainly on seminal theoretical predictions for plasmonic metamaterials. We should highlight though our interesting experimental finding of a new type of plasmonics in magnetic nanoparticles with spin-polarization.

The thesis includes five chapters.

First chapter presents consideration important question on the permittivity of metals, which can be different from bulk materials due to the quantum size effect, saturation of the local field enhancement at relatively high intensity. Ag permittivity (dielectric function) in coupled strips is different from bulk and has been presented in section 1.1 for strips of various dimensions and surface roughness. Arrays of such paired strips exhibit the properties of metamagnetics. The surface roughness does not affect the Ag dielectric function, although it does increase the loss at the plasmon resonances of the coupled strips. The size effect is significant for both polarizations of light, parallel and perpendicular to the strips with large A-parameter.

Section 1.2 presents effect of grain boundaries on the electron relaxation rate, which is significant even for large area noble metal films and especially for nanostructures. Optical spectroscopy and X-ray diffraction show substantial improvement of the plasmon resonance quality for the square type nanoantennae due to 1.8 times enlarged grain size after annealing and improved grained boundaries described by the electron reflection coefficient. The electron relaxation rate decreases by factor 3.2.

Section 1.3 presents our theoretical analysis of the third-order susceptibility (x(3)) for Ag dielectric composite reveals a critical role of saturation of optical transitions between discrete states of conduction electrons in metal quantum dots. The calculated size dependence of the x(3) for Ag nanoparticles is in a good agreement with published experimental results, in contrast to the commonly used theoretical approach. Saturation effects are responsible for a decrease of the local field enhancement factor that is of particular importance for surface-enhanced phenomena, such as Raman scattering and nonlinear optical responses.

Second chapter covers peculiarities of localized plasmons in the case of spin polarized magnetic nanoparticles. Our experiments on Co nanoparticles with a single-domain crystal structure show that they support a plasmon resonance at approximately 280 nm with better resonance quality than gold nanoparticle resonance in the visible. Magnetic nature of the nanoparticles suggests a new type of these plasmons. The exchange interaction of electrons splits the energy bands between

spin-up electrons and spin-down electrons. Electron scattering without spin flip makes possible a coexistence of two independent channels of conductivity as well as two independent plasmons in the same nanoparticle with very different electron relaxation. Indeed, the density of empty states in a partially populated d-band is high, resulting in a large relaxation rate of the spin-down conduction electrons and consequently in low quality of the plasmon resonance. In contrast, the majority electrons with a completely filled d-band do not provide final states for the scattering processes of the conduction spin-up electrons, therefore supporting a high quality plasmon resonance. The scattering without spin flip is required to keep these two plasmons independent. Third chapter discusses experimental demonstration of an optical negative index metamaterials. We start with comprehensive studies of a periodic array of gold nanorods pairs in section III.1 to demonstrate unique optical properties of this array near the plasmon resonance, high anisotropy of transmission and refractive index, high reflection below resonance frequency at about 10% of metal coverage, and a negative refractive index in the optical range above the plasmon resonance. Section III2 presents metamaterials with electromagnetic properties in the visible range that do not exist in nature. Optical metamaterials can provide us with artificial magnetic response and negative refractive index at optical frequencies. In this paper we review the recent progress made in the area of optical metamaterials. Optical magnetic metamaterials have been developed to demonstrate artificial magnetic response in the infrared and across the entire visible spectrum. Metamaterials showing negative refractive index, also called negative index materials (NIM) have been demonstrated in the infrared and at the border with the visible spectral range. Here we report the results of a sample that displays NIM behavior for red light at a wavelength of 710 nm. This is the shortest wavelength so far at which NIM behavior has been observed for light (excluding surface plasmon polaritons). We also discuss the fabrication challenges and the impact of fabrication limitations, specifically surface roughness of the fabricated structures on the properties of the metamaterials.

Section III.4 discusses optical negative index metamaterials with compensated losses. Fourth chapter incudes experiments on optical metamaterials with hyperbolic dispersion. Hyperbolic materials enable numerous surprising applications that include far-field subwavelength imaging, nanolithography, and emission engineering. The wavevector of a plane wave in these media follows the surface of a hyperboloid in contrast to an ellipsoid for conventional anisotropic dielectric. The consequences of hyperbolic dispersion were first studied in the 50's pertaining to the problems of electromagnetic wave propagation in the Earth's ionosphere and in the stratified artificial materials of transmission lines. Recent years have brought explosive growth in optics and photonics of hyperbolic media based on metamaterials across the optical spectrum. Inside of a hyperbolic medium, the principal components of the permittivity tensor have opposite signs causing the medium to exhibit a 'metallic' type of response to light waves in one direction, and a 'dielectric' response in the other. Our study shows that inside hyperbolic media, volume plasmon-polaritons (VPPs) propagate along the characteristic interfaces of the constitutive materials, forming distinct, directionally dependent optical responses. This is very similar to the propagation of conventional surface plasmon-polaritons (SPPs) along the planar interfaces separating the

isotropic dielectrics and metallic slabs. E.g., transverse-magnetic (TM) VPPs in a uniaxial medium, e = diag(eo,eo,ee), propagate along the resonance cone. The Young's double-slit scheme

is used to study the spatially-confined diffraction in a hyperbolic slab, made of many thin planar layers of a metal and dielectric, to obtain the sub-wavelength interference pattern of VPPs at the output interface. Proof-of-concept systems for producing such patterns applicable to nanolithography and subwavelength probes are demonstrated in this chapter. Final, fifth chapter covers experimental demonstration of scattering suppression for the core-shell nanoparticles of about micron size. Significant extinction from the visible to mid-infrared makes fractal shells very attractive as aerosolized obscurants. In contrast to the planar fractal films, where the absorption and reflection equally contribute to the extinction, the shells' extinction is caused mainly by the absorption. The Mie scattering resonance at 560 nm of a silica core with 780 nm diameter is suppressed by 75% and only partially substituted by the absorption in the shell so that the total transmission is noticeably increased. The silica vibrational stretching band at 9 p,m in absorption also disappears. Effective medium theory supports our experiments and indicates that light goes mostly through the epsilon-near-zero shell with approximately wavelength independent absorption rate.

Each chapter includes some introductive information for specific topic. Publications summary:

There are 77 published papers by the author with colleagues indexed in Scopus, WoS, and 12 invited chapters related to the plasmonic nanostructures.

Among them, there are 35 papers published on the thesis topics [A1-A35], 45 invited conference talks, and 29 invited lectures for academic instituted in Russia and USA. Acknowledgements:

The author sincerely grateful to all the colleagues co-authored on these papers.

Заключение диссертации по теме «Оптика», Драчев Владимир Прокопьевич

CONCLUSIONS AND DEFENDED PROVISIONS

In the thesis we present experimental study on optical NIMs, metamagnetics across the visible spectral range, subwavelength photolithography using hyperbolic metamaterials, scattering suppression with epsilon near-zero fractal shell, and effect of nanostructurering on the metal dielectric function. We have demonstrated, for the first time, feasibility of optical NIMs and losses compensation with gain for NIMs. We should highlight our finding of a new type of plasmonics in magnetic nanoparticles with spin-polarization.

The significance of the study can be judged based on the number of citations. It's a one of the parameters to justify the importance of work and impact it has on our research field, and how others may benefit from it. Thus we provide a number of citations according to Scopus for each staement below.

Defended provisions

1. The Ag dielectric function for moderately sized, about 100 nm, strips differ from that of bulk Ag and is size-dependent for both polarizations of light. Surprisingly, the geometrical effect of roughness is mostly responsible for increased losses at the plasmon resonances of the nanostructure, while the surface roughness does not affect the Ag permittivity. Anisotropy in c" observed in the experiments indicates a significant contribution from the quantum size effect and the chemical interface effect. The size-dependent terms of e" for two polarizations have relatively large A-parameters. (242 citations for paper A23)

2. Effect of annealing on the nanostructure performance depends on the geometry, and can be positive or negative. In the case of square shape nanoantennas annealing not only increases the grains but also reduces the reflection coefficient of the potential barriers between grains. Both factors result in substantially improved electron relaxation rate for nanostructures and make it comparable with the large area samples. The size of the nanoantenna is around 100 nm, and the annealing temperature is up to 400°C . The measured transmission and reflection spectra of the nanoantenna sample before and after annealing are matched with those simulated spectra by frequency domain finite element method. Loss factor was introduced in the modeling of the gold permittivity. When the annealing temperature increases from the room temperature to 400 °C , the loss factor decreases from 3.54 to 1.35, where factor 1 corresponds to the bulk gold. (146 citations for paper A16; 104 for A19)

3. Available in the literature experimental data on the nonlinear susceptibility were analyzed to compare the existing theoretical models in order to resolve the origin of the optical nonlinearity in plasmonic nanoparticles. Based on the x(3) values and its size dependence it was concluded that the conduction electron intra-band transitions play a major role in contrast to the common belief about negligible role of the conduction electrons.

This conclusion has been made based on the quantum well theory with the Hamiltonian of electron-field interaction taking the form H= -dE, where d is the dipole moment and E is the electrical field. S.G. Rautian have showed that, for nanosized spherical particles, the use of the given

Hamitonian is preferred, and that this Hamiltonian is no longer equivalent to the standard Hamiltonian in terms of a vector potential. Our results re-affirm the Rautian model, and we find good agreement of the size-dependent Xm(3) with the experiment. A result that is not achieved with a theoretical size dependence derived by Hache, Ricard, and Flytzanis, with the Hamiltonian that uses a description in terms of a vector potential and electron momentum.

Furthermore, our studies emphasize the importance of saturation effects for the local field enhancement factor, which strongly affects nonlinear processes and SERS. The results presented here suggest the saturation of optical transitions in metal nanostructure as the probable reason for the decrease in SERS enhancement. Therefore, the saturation will be especially important when using high-intensity laser light typical for pulsed fs and ps lasers. (91 citations for paper A31)

4. Cobalt nanoparticles synthesized by high temperature reduction of cobalt salt show strong plasmon resonance at 280 nm with better quality than that of gold nanoparticles in the visible spectrum. Our experiments with Co nanoparticles clearly show a new type of plasmon excitation, which is specific for spin polarized single domain nanoparticles. This type of plasmon has unusual properties due to existence of two independent groups of electrons with opposite spins providing weak interaction so that all electron scattering processes occur without spin flip. Magnetic response of the nanoparticles enables controlled and reversible aggregation accompanied by the tailoring of optical absorption.

These results show an optical analogous of giant magneto-resistance (GMR). An extensive collection of literature on GMR proof that the materials with spin polarization of d-electrons have two independent channels of the conductivity, mainly caused by the s-p electrons. The values of conductivity are very different for two channels. The two channels are independent only if the electron scattering goes without spin-flip. The scattering with spin flip can be initiated by the domain structure.

5. For an array of pairs of parallel gold rods, we obtained a negative refractive index of n' « -0.3 at the optical communication wavelength of 1.5 |im. This new class of negative-index materials (NIMs) is relatively easy to fabricate on the nanoscale and it opens new opportunities for designing negative refraction in optics. It was the first experimental demonstration of optical NIMs in this competitive field. (1450 citations for paper A28; 103 for A29)

6. Feasibility of metamagnetics in the visible spectral range was demonstrated for a grating the nanostrips pairs. We show strong magnetic resonant behavior in all the samples ranging from 491 nm to 754 nm, covering the majority of the visible spectrum. The position of the resonance moves towards the shorter wavelengths as the width of the nanostrips pairs decreases. The permeability ranges from -1.6 at 750 nm to 0.5 at 500 nm. (246 citations for paper A24; 180 for A26).

7. Metamaterials have the potential of adding another dimension to the set of existing materials by providing us with novel properties that do not exist in nature. Negative refractive index has been demonstrated at the optical wavelengths using various geometries like paired crossed gratings arrays (fishnets). It has been used to demonstrate a double negative-NIM response at wavelengths as short as 813 nm. The sample showed a maximum FOM (figure of merit) of 1.3 with n'« -1.3 .

We have also reported the shortest wavelength at which NIM behavior has been observed. The sample displayed a single negative-NIM response at 710 nm with a maximum FOM of 0.5 with n'« -0.6. We have also showed that the quality of the artificial structures can significantly impact the properties of the metamaterial. (180 citations for paper A25; 142 for paper A17)

8. We have experimentally demonstrated an active negative-index metamaterial. Our results for the first time solve the inherent problem of loss in negative-index metamaterials made from nanostructured metal-dielectric composites.

Another important observation: the effectiveness of the loss compensation in our sample arises from the local-field enhancement of the structure when a gain medium is used as the spacer layer. Since the effective extinction coefficients at 737 nm are a ^ 6.75 x 103cm_1 and a = 1.13x10® cm'1 for the device with and without gain, respectively, the effective amplification is a = —1.07 x 10s cm-1, which is 46 times larger at this wavelength than the "seed" value (without the local-field factor) that was used in simulations. According to our numerical modeling, the high local fields in the fishnet structure result in a total (spatially integrated) energy produced by the gain medium that is about 45 times larger than that produced by a homogeneous gain material of the same volume, in good agreement with the factor of 46 mentioned above. (646 citations for paper A15; 119 for paper A27).

9. We have experimentally shown that diffracted light propagates inside a hyperbolic material made of a planar silver-silica lamellar structure along the resonance cone boundary between the directions with Ree(^c) > 0 and Ree(^c) < 0. Such propagation across the real metal-dielectric

interfaces is a characteristic feature of the volume plasmon-polaritons. The interference of VPPs from a double-slit creates a sub-wavelength interference pattern, which is six times smaller than the free space wavelength at 465 nm. This is in sharp contrast to the double-slit experiment in silica, which results in a diffraction-limited pattern. The hyperbolic material properties tend to be closer to the target effective parameters as the layering-period is decreased, providing potential pattern sizes of 22 nm. Such unique subwavelength interference patterns offered by hyperbolic metamaterials allow for a range of applications in nanophotonics - from photo-lithography demonstrated in our work, to a sub-wavelength optical probe for sensing. Planar structures are preferred due to their ease of fabrication and integration into planar photonic devices and conventional optical systems. (239 citations for paper A7; 140 for paper A9; 130 for paper A8; 73 for paper A14.)

10. Our experiments show that the optical response of the core-shell microsphere with a gold fractal shell is dominated by the shell absorption. Similar to the planar fractal films, the absorption is enhanced in the broad spectral range up to 20 ^m. It is interesting though that the specular reflection and backscattering are relatively small for the fractal shells due to 3D spherical geometry.

What is also counterintuitive is that the resulting transmission cross-section for the core-shell is higher than the bare silica core at the Mie resonance wavelength. This is due to the forward scattering suppression of silica microspheres by adding the plasmonic gold shells. By measuring transmittance, reflectance, and forward- and back- scattering we found that the absorption in the shell contributes the most to the extinction of the whole core-shell microsphere.

Another surprising result is that the Mie scattering resonance at 560 nm of a silica core with 780 nm diameter is suppressed by 75% and partially substituted by the absorption in the shell so that the total transmission is increased by factor of 1.6 due to the gold fractal shell.

The effective permittivity of the gold shell manifests the epsilon-near-zero condition over the whole spectral range under study. Also, in the mid infrared spectral range, one can see that the O-Si-O vibrational stretching band of the core is "hidden" in the spectra of the core-shell extinction. As the gold coverage increases on the silica microspheres, the relative contribution of the vibrational stretching band at 9 ^m in the total extinction is gradually decreasing and finally disappears. Effective medium theory describes the experimental spectra reasonably well and gives an epsilon-near-zero real part of the effective shell permittivity and approximately wavelength independent product of the imaginary part of the permittivity and light frequency over the broad spectral range 0.5-20 ^m (this product is responsible for the energy density dissipation rate of the plane wave).These observations for the visible and mid-IR spectra indicate that the light goes mostly through the epsilon-near-zero shell with approximately wavelength independent absorption rate. Thus the fractal films being synthesized on the microspheres show interesting properties of "guiding" light and could be promising aerosolized obscurants in the visible-infrared spectral range. (38 citations for paper A21)

Author contribution in the dissertation materials and defended statements is significant. Typically, it includes supervising students on their experimental work; a guidance in assembling the set-up; planning the experiments and partly the numerical simulations and fabrication; results analysis; writing manuscripts. Out of cited below papers: 10 - as a first author, 12 - as a leading author, and 8 - as a second after Ph.D. student author.

NOVELTY

We present for first time the experimental realization on optical NIMs, metamagnetics across the visible spectral range, subwavelength photolithography using hyperbolic metamaterials, scattering suppression of dielectric microsphere with epsilon near-zero fractal shell, and effect of nano-structuring on the metal dielectric function. We have demonstrated, for the first time, feasibility of optical NIMs and losses full compensation with gain for NIMs. We should highlight also our finding of a new type of plasmonics in magnetic nanoparticles with spin-polarization.

Список литературы диссертационного исследования доктор наук Драчев Владимир Прокопьевич, 2022 год

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