Merjenje temperature s holesteričnimi tekoče-kristalnimi 3D laserji na daljavo

Pirnat, Gregor (author), Muševič, Igor (mentor), Humar, Matjaž (co-mentor)

V delu obravnavamo fizikalne lastnosti holesteričnih tekočih kristalov in njihovo uporabo v kapljični geometriji za realizacijo 3D mikrolaserjev. Holesterični tekoči kristali so fotonski material, v katerih obstaja prepovedan pas fotonov v disperzijski enačbi. Tako je za določene valovne dolžine propagacija prepovedana; hkrati pa se na robovih prepovedanega fotonskega pasu poveča fotonska gostota stanj, zaradi česar je v takih resonatorjih možno lasersko delovanje pri nizkih pragih. Pri črpanju z ekscitacijskim pulznim laserjem povzročimo obrnjeno zasedenost aktivnega medija, emisija katerega se optično ojača zaradi optičnih lastnosti holesteričnega tekočega kristala. Resonatorji holesteričnih tekočih kristalov so občutljivi na zunanje motnje zaradi katerih se valovna dolžina laserske črte spremeni. Valovna dolžina laserske emisije se spremeni zvezno v območju fluorescence aktivnega medija kot odziv na spreminjanje strukture resonatorja. To omogoča njihovo uporabo za zaznavanje fizikalnih in kemijskih količin v opazovanem sistemu. Z eksperimentom smo realizirali merjenje temperature z opazovanjem premika laserske črte, pri čemer smo mikrolaserje vzbujali in merili odziv na daljavo. Ta metoda omogoča merjenje temperature v majhnem volumnu, ki ni vezan le na površino opazovanega sistema, na razdaljah več deset metrov, v realnem času in z natančnostjo pod stopinjo celzija v nekaj sekundah. V prvem delu predstavimo osnovne pojme in enačbe s področja fizike laserjev in fizike tekočih kristalov. Pri obravnavanju svetlobnih interakcij s tekočimi kristali se poglobimo v mehanizem ojačanja svetlobe v holesteričnih tekočih kristalih ter zajemanje svetlobnega signala z eksperimentalno napravo. Zaradi geometrije kapljic opazimo več načinov laserskega delovanja. Te načine opišem in analiziram njihove spektre, ter proučim kako jih lahko izkoristimo v različnih sistemih. V osrednjem delu opišem optične sisteme, ki sem jih uporabil pri eksperimentalnih meritvah in izdelavo holesteričnih tekoče-kristalnih kapljic. Pokažem rezultate in analiziram eksperimentalne meritve ter ovrednotim možnost uporabe izdelanega optičnega sistema v kombinaciji s 3D mikrolaserskimi senzorji v industriji, medicini in raziskovanju.

• Language: Slovenian
• Keywords: tekoči kristali, tekoče-kristalne kapljice, fotonika, laserji, fotonski kristali, daljinsko zaznavanje temperature, senzorji
• Work type: Master's thesis/paper
• Typology: 2.09 - Master's Thesis
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2018
• PID: 20.500.12556/RUL-103678
• COBISS.SI-ID: 3239780
• Publication date in RUL: 21.09.2018

Secondary language

• Language: English
• Title: Remote temperature measurements with cholesteric liquid-crystal 3D microlasers

Abstract

In this thesis we present physical properties of cholesteric liquid crystals and their usability in spherical geometry as 3D microlasers. Cholesteric liquid crystals are photonic materials in which a band gap in dispersion relation of propagating wave exist. Therefore the propagation for specific wavelengths of light are forbidden inside the structure; at the same time as a result of the bandgap, there is an increase of density of states at the edges of the forbbiden band, making low-threshold lasing possible at the edges of the bandgap. The excitation of dye molecules by an external pump laser source creates population inversion in these systems, which results in stimulated laser emission. Since the cholesteric liquid crystal structure acts as a resonator cavity and is sensitive to external stimuli, the lasing wavelength changes in response. Lasing wavelength changes continuously in the region of fluorescent emission of the gain medium for sustained/ever-present stimuli. This enables their use as sensors for measuring physical and chemical properties of the observed system. We demonstrate their sensisivity and usability for remote temperature measurements. This method enables temperature measurements in a very small volume, which is not constrained to the surface of the measured system, at distances up to tens of meters, in real time and with the accuracy below a one degree Celsius in less than a secound. In the first part we present basic concepts and equations from the area of laser physics and physics of liquid crystals. We discuss in depth the interaction of light with cholesteric liquid crystals, which enables laser operation in these stuctures. We also discuss the collection of light with the constructed optics system. The spherical geometry of the cholesteric liquid crystal droplets enables multiple modes of lasing. We describe these modes and analyze their spectra in the experiment. In the main part we describe the optical systems, that were used during the experimentation, the preparation of the samples and show the experimental results. We also discuss the possibility of impoving the optical system as well as the sensors and mention why that could be used in industry, medicine or research.

• Keywords: liquid crystals, liquid-crystal droplets, photonics, lasers, photonic crystals, remote temperature sensing, sensors