ETD system

Electronic theses and dissertations repository


Tesi etd-05052016-090234

Thesis type
Tesi di laurea magistrale
Design and implementation of hybrid superconducting devices for coherent phonon emission
Corso di studi
relatore Prof. Tredicucci, Alessandro
relatore Dott. Pitanti, Alessandro
Parole chiave
  • superconductivity
  • mechanics
  • nanofabrication
  • phonon
Data inizio appello
Riassunto analitico
Telecommunication is defined as “Any transmission, emission or reception of signs, signals, writings, images and sounds or intelligence of any nature by wire, radio, optical or other electromagnetic systems”. Telecommunication occurs when the exchange of in- formation between two or more entities (communication) includes the use of technology. Communication technology uses channels to transmit information, either over a physical medium, or in the form of electromagnetic waves.
Nowadays, two advanced branches in information transport are electronics and pho- tonics, which exploit electrons and photons, respectively. Both of them are able to carry information over long distances, through electrical circuits or the technology of optical fibers. However, they have some defects: electronics circuits are affected by heat dissi- pation and thermal management and electronic noise; instead, photons
Electronics deals with electrical circuits that involve active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive elec- trical components and interconnection technologies. The nonlinear behaviour of active components and their ability to control electron flows makes amplification of weak signals possible, and electronics is widely used in information processing, telecommunication, and signal processing. The analogous in the field of photonics can be performed using light to carry information. An optical communication system uses a transmitter, which encodes a message into an optical signal, a channel, which carries the signal to its des- tination, and a receiver, which reproduces the message from the received optical signal. The most common type of channel for optical communications is constituted by optical fibers. The transmitters in optical fiber links are generally light-emitting diodes (LEDs) or laser diodes. Infrared light, rather than visible light is used more commonly, because optical fibers transmit infrared wavelengths with less attenuation and dispersion. The signal encoding is typically simple intensity modulation. The need for periodic signal regeneration was largely superseded by the introduction of the erbium-doped fiber am- plifier, which extended link distances at significantly lower cost.
Beyond electrons and photons, an intriguing possibility to transport information is constituted by phonons; they are collective excitations of a periodic, elastic arrangement of atoms or molecules in solids and some liquids. Often designated as quasiparticles, they represent an excited state in the quantization of the vibrational modes of elastic struc- tures of interacting particles. With respect to electrons and photons, phonons have the particularity of coupling with almost everything, for example hybrid quantum devices and spin systems; this is very attractive in order to perform communication between systems which are very different from each other. This kind of communication typically cannot not be done using electrons and photons. However, phonons strongly depend on temperature and they are able to carry information only over small distances. Moreover, their characteristic frequencies (up to few GHz) are much lower than the photon ones (hundreds of THz).
In order to perform on chip a simple communication experiment with phonons, three objects are required: a source, a waveguide and a detector. In this thesis I will focus on the implementation of coherent phonon sources. With respect to the laser working principle in photonics, where stimulated emission is used as the process at the basis of the oscillator, a phonon source is generally considered as an object which injects coherent phonons in the system.
Ways of generating coherent phonons are coherent phonon spectroscopy (CPS) with femtosecond laser pulses, impulsively stimulated Raman scattering (ISRS), displacive excitation of coherent phonons (DECP); otherwise coherent acoustic phonons can be generated in piezoeletric semiconductor heterostructures.
Our idea to realize a coherent phonon emitter combines the field of nanomechanics with the technology of superconducting devices. Superconductors are particularly interesting for three reasons: they work at very low temperatures (few K), as many quantum de- vices do, and this strongly reduces the thermal noise; superconducting devices can be perfectly integrated on chip; finally, the new advanced nanofabrication techniques allow for the realization of superconducting devices (such as single junctions or SQUIDs) with very small dimensions. If a part of the device is suspended, a mechanical oscillator with vibrational frequencies up to few GHz can be obtained.
The best candidate to be the final device is a Superconducting QUantum Interference Device (SQUID), which consists of a superconducting circuit with two superconduct- ing/normal metal/superconducting (SNS) junctions coupled in parallel. The idea is to fabricate a SQUID and then suspend one of the two junctions through an etching pro- cess. If a current is flowing along the wire and an in-plane magnetic field is applied orthogonal to the junction, the field exerts a Lorentz force on the nanowire itself. If the force frequency is resonant with the beam out-of-plane eigenmodes, coherent vibrations can be induced.
The aim of the present thesis is to design, fabricate and investigate the transport and magnetic properties of single SNS junctions and DC SQUIDs, made of Al and Cu. the two classes of devices are fabricated through the combined use of electron-beam lithog- raphy (EBL) and metal deposition with the shadow-mask technique.
The experimental characterization of single SNS junctions allows to investigate their su- perconducting properties and in particular the critical current behaviour as function of the applied in-plane magnetic field for different temperatures; on the other hand, testing non-suspended SQUIDs is a relevant milestone to verify the possibility of fabricating devices with the right geometry for the final experiment.
The analytical model describing the behaviour of a mechanically actuable SQUID is solved in the time domain with the commercial software Mathematica. Using realistic parameters coming from the measurement of the fabricated devices, we demonstrate that it is possible to perform a DC detection of the mechanical vibration (with GHz frequen- cies) of the nanowire.
Finally, an innovative, challenging approach, relying on the use of a silicon nitride membrane, is introduced as a way forward towards the fabrication of the final, suspended SQUID device.