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Realization of Fano resonance in plasmonic oligomers is often exploited to design efficient plasmonic substrates for surface-enhanced coherent anti-Stokes Raman scattering. Disk-type Fano-resonant plasmonic oligomers are widely used to enhance the Raman signal of the probe material. Generally, hot spots are generated in those oligomers at different spatial locations at different wavelengths and only a few spatially overlapping hot spots at multiple wavelengths can be achieved with oblique incidence of excitation light. In this work, we proposed hexagonal gold nanoparticle based Fano-resonant plasmonic oligomers that can yield higher number of spatially overlapped hot spots compared to the disk type oligomers even with the normal incidence of excitation light. The oligomers were numerically modelled and optimized for surface-enhanced coherent anti-Stokes Raman scattering with 780 nm pumping and 500–1800 cm− 1 Raman signature region. The Fano lineshape was engineered to ensure near-field energy coupling at pump while enhancing the coherent anti-Stokes Raman signal at the far field. Our computational studies explored the purely electric origin of Fano resonance in those oligomers and provided maximum Raman enhancements of 1012–1013 from them to enable single-molecular level applications. Our findings provide a way to realize fabrication-friendly nanostructures with higher number of spatially localized hotspots for improving the Raman detection sensitivity. [ABSTRACT FROM AUTHOR]

We investigate the structure and orientation of water molecules at the water-lipid interface, using vibrational sum-frequency generation in conjunction with a maximum entropy phase retrieval method. We find that interfacial water molecules have an orientation opposite to that predicted by electrostatics and thus are likely localized between the lipid headgroup and its apolar alkyl chain. This type of water molecule is observed for phospholipids but not for structurally simpler surfactants. [ABSTRACT FROM AUTHOR]

We propose a numerical method for the misplacement phase error correction in terahertz time-domain reflection spectroscopy (THz-TDRS). The developed algorithm is based on the maximum entropy principle and can be readily implemented into data processing, allowing one to reveal material parameters of the opaque materials from the THz reflection measurements. The method resolves the phase retrieval problem in the THz-TDRS and dramatically simplifies the experimental procedure. [ABSTRACT FROM AUTHOR]

Phase retrieval is one of the classical problems in various fields of physics including x-ray crystallography, astronomy and spectroscopy. It arises when only an amplitude measurement on electric field can be made while both amplitude and phase of the field are needed for obtaining the desired material properties. In optical and terahertz spectroscopies, in particular, phase retrieval is a one-dimensional problem, which is considered as unsolvable in general. Nevertheless, an approach utilizing the maximum entropy principle has proven to be a feasible tool in various applications of optical, both linear and nonlinear, as well as in terahertz spectroscopies, where the one-dimensional phase retrieval problem arises. In this review, we focus on phase retrieval using the maximum entropy method in various spectroscopic applications. We review the theory behind the method and illustrate through examples why and how the method works, as well as discuss its limitations. [ABSTRACT FROM AUTHOR]

A general method to obtain the complex refractive index of a medium from absorbance, or alternatively from optical path length data, without knowing the sample thickness is proposed. The method can be formulated in any spectral range and it is here applied particularly in the terahertz spectral range to both simulated and experimental data. The key idea is the derivation of nonconventional dispersion relations that partly resemble traditional Kramers-Kronig relations. The method is shown to work well in extracting the complex refractive index of a drug system and a precipitated calcium carbonate. [ABSTRACT FROM AUTHOR]

Abstract: This paper summarizes recent developments in phase retrieval methods from surface plasmon resonance reflectance that are applied in the extraction of the complex effective refractive index of liquids containing nanoparticles. Both classical dispersion theory and the maximum entropy model (MEM) are considered. We show that the MEM is more suitable for a reflectance analysis, and it is applied to a variety of topologies with different volume fractions of nanoparticles in a solution. The MEM is always suitable for the extraction of the effective complex refractive index of the whole solution whereas the complex refractive index of constituent nanoparticles requires extraction using an effective medium theory. Therefore, a reliable model for the effective complex refractive index of the solution is needed. In this paper we give a summary of different effective medium theories that have been applied to describe the effective complex refractive index of nanoparticles in a liquid environment. [Copyright &y& Elsevier]

We show that surface plasmon resonance reflectance measurement can be used to extract the effective complex refractive index of a Bruggeman nanoliquid, i.e. a solution consisting of spherical nanoparticles embedded into a host liquid such as water, using the maximum entropy method. This retrieval can be immediately performed once the optical properties of the host liquid are known. In this paper we investigate the upper limit of the volume fraction of spherical nanoparticles in a solution that still yields a reliable result in the retrieval based on the maximum entropy method. Finally, we show that by squeezing the spectrum we can significantly reduce the errors in the analysis and retrieve the effective complex refractive index for a high volume fraction of nanoparticles. [ABSTRACT FROM AUTHOR]

Nonlinear spectral properties of two different types of conjugated polymers (polythiophene PT 10 and polysilane PDHS) with nanoscale TiO[SUB2] particles forming a Maxwell Garnett or a Bruggeman composite are studied. According to the present simulations, it is possible both to enhance and to tailor the magnitude and the spectral properties, respectively, of the effective degenerate third-order nonlinear susceptibility of the composite. This is done simply by tuning the volume fraction of TiO[SUB2] inclusions and by changing the topology of the composite. [ABSTRACT FROM AUTHOR]

The dispersion theory of nonlinear Bruggeman liquids is investigated by using the effective medium theory of nanocomposites. It is shown that the complex effective degenerate third-order Bruggeman susceptibility, which is a meromorphic function, does not obey conventional Kramers–Kronig relations. A maximum entropy procedure is applied for the first time in a phase-retrieval from nonlinear total reflectance. [Copyright &y& Elsevier]

A method for obtaining the complex permittivity of a Maxwell Garnett liquid with the aid of spectral surface plasmon resonance spectroscopy is presented. The Maxwell Garnett liquid is assumed to consist of small absorbing spherical inclusions in a host liquid. It is shown that the permittivity of the inclusions can be calculated if the optical constants of the host liquid are known. It is believed that such a method will gain increasing importance in the analysis of biological reactions. [ABSTRACT FROM AUTHOR]

The article discusses the 11th Finland-Japan Joint Symposium on Optics in Engineering (OIE'15) that was held September 1-3, 2015, at Carelicum in Joensuu, Finland.

Palladium nanoparticles are widely used in catalysis, sensors, coatings and hydrogen storage devices. This paper reports the one-pot photochemical reduction of Pd(II) alcohol solutions into Pd0 nanoparticle suspensions. The synthesis involves the reduction of [PdCl4] dissolved in EtOH/2-PrOH mixtures by the ketyl radicals (Me)2(HO)C generated from 254nm irradiation, in the absence of other chemical reducing agents. Clusters of nm Pd0 particles thus produced could be separated from their suspensions by microfiltration. Preliminary tests are reported on the catalytic potential of such particles for the thermal dehydrogenation of alcohols. Photochemical production of palladium nanoparticles for catalytic purposes is presented using common laboratory chemicals. The simple and easily-to-replicate process is also monitored with a low-cost OEM spectrometer. The as-prepared particles exhibit narrow size distribution and are prone to agglomerating in microclusters that allow convenient separation with common filter media. The particles serve as active catalysts for example alcohol dehydrogenation. [ABSTRACT FROM AUTHOR]

Highlights • Determination of complex refractive index of SU-8 epoxy as a function of wavelength. • Fourier-transform infrared (FT-IR) spectrometer is the relatively rapid and wide IR spectral range by non-contact technique. • The data analysis is based on the elegant Singly Subtractive Kramers–Kronig (SSKK) equations. • Kramers–Kronig method is faster and more reliable in a broad spectrum range. Abstract Accurate determination of the complex refractive index of SU-8 epoxy has significant for the wide variety of applications in optical sensor technology at IR range. The complex refractive index of SU-8 is determined by recording the transmission of light spectra for the wavelength range of 2.5–22.0 µm. The data analysis is based on the Kramers–Kronig dispersion relation method. The method has several merits, such as ease of operation, non-contact technique, measurement accuracy, and rapid measurement. The present method is not restricted to the case of SU-8 but it is also proposed to be applicable across a broad range of applications, such as assessment of the optical properties of paints and biomedical samples. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]

In recent decades, the use of nanomaterials has become very common. Different nanomaterials are being used in over 1600 consumer products. Nanomaterials have been defined as having at least one dimension in the range of 1−100 nm. Such materials often have unique properties. Despite some warnings of applying bulk optical constants for nano size materials, stated already in 1980s, bulk constants are still commonly used in the light scattering measurements of nano size particles. Titanium dioxide is one of the materials that is manufactured and used as an engineered nanomaterial in increasing quantities. Due to the aforementioned facts, it is quite crucial for successful research and production of nanoparticles to find out the dependence of the refractive index function (RIF) of the material on its crystal size. We have earlier performed several ab initio computations for obtaining the dependence of the RIF of TiO 2 on the crystal or on the cluster size, for particles of size up to ca. 2 nm. Extending the calculations to greater sizes has turned out to be infeasible due to the unbearable increase in computational time. However, in this study we show how the crystal-size-dependent-RIF (CS-RIF), for both rutile and anatase can be modeled from measured extinction or turbidity spectra of samples with varying crystal and particle sizes. For computing the turbidity spectrum, we constructed a model including primary crystals whose distributions were parameterized by mean and standard deviation, and also including aggregates consisting of mean sized primary particles, parameterized just by mean aggregate size. Mainly because of the long computing times Mie calculation was used in the computation of extinction spectra. However, in practical process applications, the obtained RIF will be used together with the T-matrix method. We constructed the RIFs used in the model using generalized oscillator model (GOM) as expanded to crystal size dependence. The unknown parameters of the model were solved using nonlinear least squares estimation. When the crystal size becomes smaller than the bulk size the shape of the estimated CS-RIFs reveal two distinct regions for both rutile and anatase. In the first region, starting apparently already from ca. 200 nm, the height of both the real part and the imaginary part of CS-RIF decreases on crystal diameter. However, the band gap remains constant. In the second region, starting when the crystal diameter is decreased to ca. 3 nm, a blue shift starts to increase the band gap. The band gap dependence on crystal size is quite consistent with the existing experimental values. Consequently, it is of great importance to use CS-RIF in light scattering measurements for nanoparticle size determination. Neglecting this, the smaller particles in the size distribution will have too small values, already for sub-micrometer particles, naturally distorting also the mean value. To our knowledge, this is the first time ever that a CS-RIF from bulk to 1 nm size is determined for any material. [ABSTRACT FROM AUTHOR]

The ability to observe samples qualitatively at the microscopic scale has greatly enhanced our understanding of the physical and biological world throughout the 400 year history of microscopic imaging, but there are relatively few techniques that can truly claim the ability to quantify the local concentration and composition of a sample. We review coherent anti-Stokes Raman scattering (CARS) as a quantitative, chemically specific, and label-free microscopy. We discuss the complicating influence of the nonresonant response on the CARS signal and the various experimental and mathematical approaches that can be adopted to extract quantitative information from CARS. We also review the uses to which CARS has been employed as a quantitative microscopy to solve challenges in material and biological science. [ABSTRACT FROM AUTHOR]

Abstract: Monitoring systems providing fast and reliable, even on-line data, from a distinct process stage or final product are needed in drug development, from the early stages of drug discovery until the drug product manufacturing procedures. This includes also processes involving solid particles, such as drug dissolution. However, the existing in vitro drug dissolution test methods suffer limitations, such as long sampling times of 30–60s and thus the inability to be adapted to continuous monitoring, time consuming sample preparation and consumption of large amounts of reagents. In this study, an optical method for monitoring the dissolution rate of pharmaceutical powders was evaluated with model drugs having different dissolution rates. The measuring system consisted of a laser source, light detector, oscilloscope, magnetic stirrer and sample vessel. The intensity of laser light transmitted through the dissolution medium was recorded and displayed by the oscilloscope. Dissolution curves were produced by fitting the raw data with mathematical functions. The optical method was found to be resource-saving, reliable and capable of detecting differences in even rapid dissolution rates of drug compounds. This technique might have targets of application in real-time monitoring of processes in many different sectors, including the pharmaceutical industry. [Copyright &y& Elsevier]