Alumiininen mikroelektromekaaninen värähtelijä supraneste heliumin tutkimuksessa

In this work, the response of magnetomotively driven MEMS resonators have been measured at low temperatures in vacuum (16 mK – 4 K) and superfluid 4He (18 mK – 1 K). The goal post shaped devices (dimensions ∼ 10 μm) are made from suspended aluminium beams with rectangular cross section 0.15 μm × 1 μm and their operation frequencies range from 300 kHz to 500 kHz. We believe that the force resolution ∆F ∼ 1 fN of the devices is sufficient for studying dynamics of a single vortex pinned to the device. In vacuum, the dissipation follows a power law and the resonance frequency increases logarithmically with temperature. The behaviour is attributed to tunneling two level systems present in the structure. In superfluid 4He, the dissipation increases further as a consequence of momentum transfer in collisions with ballistic phonons and rotons. At large oscillation amplitudes, the devices show nonlinear Duffing-like behaviour with an amplitude dependent quadratic frequency shift. In superfluid 4He at T = 18 mK we observe a collapse of the oscillations at vc = 0.17 m/s independently of the driving force. We interpret this as a sudden increase of the drag force, when the critical velocity for the emission of vortex rings is exceeded.Tässä työssä mitattiin mikroelektromekaanisten värähtelijöiden vastetta magneetti- ja sähkökenttien synnyttämään herätteeseen kylmissä lämpötiloissa sekä tyhjiössä (16 mK – 4 K) että supraneste heliumissa (4He, 18 mK – 1 K). Kyseisten laitteiden koko on luokkaa 10 μm, ja ne on valmistettu alumiinista jalkapallomaalin muotoon (kaksi jalkaa ja puomi). Maali muodostuu palkeista, joiden poikkipinta on neliskulmainen ja kooltaan 0.15 μm × 1 μm. Värähtelijöiden ominaistaajuudet sijaitsevat välillä 300 kHz – 500 kHz. Uskomme että laitteiden osoittama voimien erottelukyky ∆F ∼ 1 fN on riittävän tarkka yksittäisten kvanttipyörteiden dynamiikan tutkimiseen. Tyhjiössä resonaattoreiden häviöt seuraavat potenssilakia ja ominaistaajuudet kasvavat logaritmisesti lämpötilan noustessa. Kyseinen käyttäytyminen liittyy tunneloituviin kaksitasosysteemeihin laitteiden rakenteissa. Supraneste heliumissa häviöt kasvavat lisää, sillä resonaattorit törmäilevät ballistisesti eteneviin fononeihin ja rotoneihin siirtäen liikemääräänsä niille. Suurilla värähtelyamplitudeilla laitteet käyttäytyvät epälineaarisesti Duffing-resonaattorin kaltaisesti, ja resonanssitaajuus siirtyy neliöllisesti amplitudin kasvaessa. Supraneste heliumissa, 18 mK lämpötilassa, havaitaan värähtelyiden yhtäkkinen pysähtyminen kun saavutetaan nopeus vc = 0.17 m/s riippumatta herätteen voimakkuudesta. Tulkitsemme tämän äkillisenä häviöiden kasvuna, kun kriittinen nopeus pyörrerenkaiden emissiolle ylitetään

Výzkum kvantových kapalin a kvantové turbulence pomocí mikro- a nano-rezonátorů

In this Thesis, we present an excessive study of the dynamics of quantum fluids em- ploying the detectors in the form of mechanical resonating structures with characteristic dimensions below 1 mm. We operate the devices in normal and superfluid liquid phases of both helium isotopes scanning the wide range of temperatures between 2.17 K and ≈ 150 µK. We show, that the detectors in the form of quartz tuning forks and supercon- ducting vibrating wires are suitable probes in both hydrodynamic and ballistic regimes of superfluids, described by two-fluid model. These devices can be used to initiate and observe turbulent transition in quantum fluids leading to the generation of quantum tur- bulence. The same devices can work as detectors of externally driven turbulent flows. The phenomenon of quantum turbulence, representing any turbulent flow of quantum fluids, is discussed in more detail. We further report observation of turbulent onset in mechanically and thermally driven oscillatory flows. This transition can have origin in both of the components of superfluid 4 He, leading to either classical-like instability or "quantum" instability connected with the generation of quantized vortices. Finally, we discuss the properties and potential of the MEMS and NEMS devices, advancing from much smaller dimensions,...Táto Práca predstavuje rozsiahlu štúdiu dynamiky kvantových kvapalín prostredníc- tvom ponorených mechanických rezonátorov o rozmeroch menších ako milimeter. Tieto detektory boli použité pre výzkum normálnej aj supratekutej kvapalnej fáze oboch izoto- pov hélia v širokom rozsahu tepl̂ot od 2.17 K po ≈ 150 µK. Rezonátory vo forme kremen- ných ladičiek a supravodivých vibtujúcich dr̂otikov sa ukázali byť vhodným nástrojom ako v hydrodinamickej limite tak aj balistickej limite supratekutého hélia popísaného dvojzložkovým modelom. V našich experimentoch možeme použiť tieto rezonátory nielen k vybudeniu turbulentného prúdenia ale aj k detekcii turbulencie v prúdení generovanom externými zdrojmi. K tomuto účelu prezentujeme charakterizačné merania na lokálnej úrovni s vibrujúcim dr̂otikom v tepelnom protiprúde a porvnávame ich s výsledkami me- tódy tlmenia druhého zvuku. Súčasťou Práce je ďalej charakterizácia prvotnej nestability, ktorá vzniká v oscilačných prúdeniach budených buď mechanicky alebo tepelne a jej pô- vod v normálnej či supratekutej zložke supratekutého hélia m̂ože byť identifikovaný na základe vhodného bezrozmerného parametru. Tieto merania umožnujú vysvetliť nejas- nosti v určovaní kritickej rýchlosti v predchádzajúcich experimentoch pojednávajúcich oscilačný protiprúd. Na záver diskutujeme...Department of Low Temperature PhysicsKatedra fyziky nízkých teplotFaculty of Mathematics and PhysicsMatematicko-fyzikální fakult

Nanophotonic Structures for Coupling to Quantum Emitters in the Visible

This thesis is about the design, fabrication, and characterization of nanophotonic elements in the visible that can enhance light-matter interaction for single quantum emitters. We focus on two material systems: single photon sources based on the nitrogen-vacancy (NV) center in diamond with improved spontaneous emission rates and collection efficiencies, and passive TiO2 devices that comprise a potentially broadband (from the visible to the infrared), low loss photonics platform and that are suitable for probing and manipulating single colloidal quantum dots. We first discuss the requirements for using color center emission in bulk diamond crystals for potential applications in quantum information processing, and provide examples of using nanowire structures and planar resonators made in diamond for engineering the the NV center’s pump and collection efficiencies, and spontaneous emission rates, respectively. We also describe the integration of diamond with plasmonic structures. We have designed and implemented diamond-silver apertures for broadband enhancements of the spontaneous emission rates of NV centers. We show that shallow-implanted NV centers in diamond nanoposts provide a good system for controlling the NV center spontaneous emission rates, allowing for quenched emission with long lifetimes in the bare case, and enhanced emission with fast decay rates (corresponding to a Purcell factor of around 6) when coated with silver. We add plasmonic gratings around the diamond-silver apertures to improve the collection efficiency of the system, and observe over two-fold improvement in collection. We demonstrate the fabrication of chip-scale linear optical elements such as waveguides and racetrack resonators in low-loss $TiO_2$ thin films. The fabricated waveguides operate over a wide bandwidth with propagation losses from from 30 dB/cm in the visible to 4 dB/cm in the IR, while racetrack resonators can critically couple to waveg- uides and have quality factors as high as ~22000 in the red wavelengths. We present the fabrication of dielectric slot waveguides and their integration with colloidal quan- tum dots. Finally, we describe efforts to study and control charge transfer processes between quantum dots and $TiO_2$ on a single emitter level.Engineering and Applied Science