Jarkko Mutanen

Fabrication of terahertz wire-grid polarizers

Wire-grid polarizers for terahertz region were fabricated by manufacturing triangular grating using a ruling-based, ultraprecision diamond machining process and replicating the pattern into polymethylpentene (TPX) and cyklo-olefin copolymer (COC) sheets using hot embossing. On top of the imprinted structures, aluminum was evaporated in an oblique angle, forming an aluminum wire grid. The achieved extinction rate was over 150 for TPX polarizers and near 1000 for COC polarizers.

Weak intermolecular interactions promote blue luminescence of protonated 2,2′-dipyridylamine salts

In this work we demonstrate that protonation and π-stacking can be exploited to convert non-luminescent 2,2′-dipyridylamine into blue-emitting derivatives. We have synthesized a series of luminescent 2,2′-dipyridylamine (dpa) salts, i.e., (dpaH)X·nSolv (dpa = 2,2′-dipyridylamine, X = HF2, n = 0.5, Solv = H2O 1; X = Cl, n = 2, Solv = H2O 2; X = Br, n = 2, Solv = H2O 3; X = I n = 1, Solv = H2O 4a; X = I n = 1, Solv = CHCl34b), (dpaH)2[SiF6]·H2O 5 and (dpaH)X (X = I36; SbF67; BF48) and characterized their emission properties, both in the solid-state and in solution. To rationalize our observations and relate the luminescence properties to the structure in the solid state and in solution, we have performed ab initio computations and showed that π-stacking interactions facilitate emission by sterically hindering radiation-less deactivation via a conical intersection. Our results thus not only provide a synthetic approach to cheap blue-emitting materials, but also revealed the essential role of weak intra- and intermolecular interactions, in particular hydrogen bonding and π-stacking, in the emission process. Our insights therefore provide new concepts for designing dpa-based systems with improved luminescence properties.

Micropatterned luminescent optical epoxies.

We study the fabrication and optical properties of micropatterned luminescent optical epoxy samples. Five different photoluminescent materials were added to epoxy resin to form luminescent epoxies of different colors and micropatterned gratings were imprinted on the surface of the samples. The absorbance spectra of the unpatterned epoxy samples were measured with spectrometer and the luminescence intensities of all samples were measured using custom made bispectrometer. The methods used in this work offer an efficient and straightforward way to produce micro- or nanopatterned luminescent optical epoxies for various applications, such as LED coatings and solar concentrators.

Manufacturing of cylindrical diffractive lens by ruling

A Moore 350FG 5-axis ultra precision machine tool platform with diamond tooling was used to generate 10 mm x 10 mm cylindrical diffractive lens on electroless nickel insert. An electroless nickel insert was machined by diamond turning and a diffractive collimating lens was cut by ruling with a sharp diamond tool. The path from optical designing to machining of the lens was proven. Selection of the tool type, stability of the cutting process and tool path programming play an important role in the outcome of the lens. This method will produce high quality diffractive lenses for further replication processes although great care must be taken in controlling the machining process stability.

Fabrication of hybrid optical structure by direct machining

A Moore 350FG 5-axis ultra precision machine tool platform with diamond tooling was used to generate v-shaped diffractive pattern on a refractive sinusoidal shaped structure. The refractive surface behaves like a cylindrical lens and the diffraction grating stretches the line focuses generated by the refractive surface. The structure was machined on an electroless nickel insert. The design and the simulation of the hybrid structure were performed using ray tracing and exact electromagnetic theory. The response of the grating was calculated by using Fourier modal method and the data was used in ZEMAX to calculate the amplitude and the phase of the diffracted rays.

Manufacturing of freeform mirror by milling and altering its optical characteristics by Ns-laser polishing and ALD coatings

In this study we tested ns-laser and an atomic layer deposition (ALD) for polishing and coating CNC-machined aluminum freeform mirrors that are used in a compact multipoint fiber optical probe. Two types of ALD coatings, aluminum oxide and silicon dioxide were tested. The surface roughness of mirrors was analyzed prior to and after nanosecond-laser polishing and coating them on a Beneq TFS 200 ALD device. The freeform aluminum mirrors with and without coatings were then measured with optical profiler. The results show that improvement in the surface roughness can be seen with ns-laser polished and ALD coated aluminum surfaces.

Luminescent optical epoxies for solar concentrators

In this work luminescent optical epoxy samples with micropatterning were fabricated. Five different photoluminescent materials were used as substance in the curing of epoxy resin to form luminescent epoxies of different colors. The absorbance spectra of the unpatterned epoxy samples were measured with spectrometer and the luminescence intensities of all fabricated luminescent epoxy samples were measured using custom made bispectrometer. The micropatterns were copied adequately in the epoxy surface, which would also suggest that nanopatterning would also be possible. Furthermore, the ability to pattern the surface allows fabrication of for example anti-reflective, hydrophobic and selfcleaning structures. Further investigation in different luminescent materials suitable for solar concentrator applications is required in order to develop current systems.

Thermochromism in color measurement

Accurate color measurements have become more and more important during the past few decades. This is valid not only in physical research but also in industrial production, where the importance of accurate measurements is mainly due to increased quality requirements set by the customers of various goods. The development of technology enables more and more accurate measuring systems. While the accuracy has improved one has noticed, that many unexpected factors affect the color of an object. One of these factors is the temperature of the sample. It is known that for example the reflectance of the ceramic reference tiles used for calibration of colorimeters and spectrophotometers is temperature dependent. This phenomenon is called thermochromism, which is a reversible change of color of the sample as a function of temperature. It may be noticed already at room temperature if the temperature varies few centigrades. Red and orange samples are especially sensitive to temperature variation and may cause difficulties in precise color measurements. We show, how the phenomenon is based on physical processes and not only reflects the instability of red color pigments. We derive simple formulas, which are shown to explain the experimental data. We also discuss the meaning of thermochromism for color measurements, measure the magnitude of it and propose the experimental conditions to avoid this effect.