Description of the Research
Idea and background
The fluctuation-dissipation theorem marks the origin of spin noise spectroscopy: as long as displacements from equilibrium are sufficiently small, the relaxation dynamics of a spin system are dictated by it's linear response function, regardless of the source of excitation. This implies two ways of measurement. The common method is pump-probe spectroscopy, where the spin system is driven away from equilibrium by a pump laser, and the subsequent relaxation back to the ground state is observed by a weaker probe laser. Such techniques are widely used at Experimental Physics II.
Spin noise spectroscopy relies on the existence of nonzero temporal fluctuations of spin magnetization. These arise spontaneously in time, and the system responds with the same linear dynamics as in the first case. Thus by carefully listening to the spin noise, one can in principle obtain spin dynamics from fluctuations alone. Such an approach especially offers the advantage to perform studies in or at least close to thermal equilibrium.
Spin noise imparts proportional noise on the Faraday rotation of a linearly polarized probe laser beam, and can therefore be measured and analyzed with optical means. Research at Experimental Physics II covers investigations on a wide range of materials, such as bulk solid state systems, semiconductor nanostructures or classical atomic gases.
Experimental technique and setup
Optical spectroscopy of spin noise requires stable lasers. At Experimental Physics II we basically use a single frequency Ti:Sapphire ring laser. The Faraday rotation noise is measured with a common polarimeter, as it is used for pump probe measurements also. Detection occurs with a wideband balanced photoreceiver. Its output signal is amplified and then digitized. The spectra are gained from FFT of this digitized signal. As the spin noise contribution to the overall detector shot noise is usually very small, fast and efficient averaging is desirable to achieve suitable signal to noise-ratios (SNR): overall bandwidths between 200 MHz and 1 GHz can be efficiently covered with a digitizer that incorporates FPGA technology. For even smaller bandwidths, a conventional high-speed digitizer is used in combination with multicore FFT computation.
Completed projects and recent results
Polarimetric sensitivity enhancement, and spin noise spectroscopy at high probe laser intensities
In this project attention was attracted to the fact that the ultimate (shot-noise-limited) polarimetric sensitivity can be enhanced by orders of magnitude leaving the photon flux incident onto the photodetector on the same low level. This opportunity is of crucial importance for present-day spin noise spectroscopy, where a direct increase of sensitivity by increasing the probe beam power is strongly restricted by the admissible input power of the broadband photodetectors. The gain in sensitivity was achieved by replacing the commonly used polarimetric detection schemes with balanced detectors by geometries with stronger polarization extinction. The efficiency of these high-extinction polarization geometries with enhancement of the detected signal by more than an order of magnitude was demonstrated by measurements of the spin noise spectra of bulk n-GaAs in the spectral range 835–918 nm. It could be shown that the inevitable growth of the probe beam power with the sensitivity gain makes spin noise spectroscopy much more perturbative, but, at the same time, opens up fresh opportunities for studying nonlinear interactions of strong light fields with spin ensembles.
Two-colour correlation spin noise spectroscopy in quantum dot ensembles
Spin noise spectroscopy usually measures intrinsic spin fluctuations. This study showed that correlations in these fluctuations can be further exploited in multi-probe noise studies to reveal information that in general cannot be accessed by conventional linear optical spectroscopy, such as the underlying homogeneous linewidths of individual constituents within inhomogeneously broadened systems. This is demonstrated in singly charged (In,Ga)As quantum-dot ensembles using two weak probe lasers: When the lasers have similar wavelengths, they probe the same quantum dots in the ensemble and show correlated spin fluctuations. In contrast, mutually detuned probe lasers measure different subsets of quantum dots, giving uncorrelated fluctuations. The noise correlation versus laser detuning directly reveals the quantum dot homogeneous linewidth even in the presence of a strong inhomogeneous broadening. Such noise-based correlation techniques are not limited to semiconductor spin systems, but are applicable to any system with measurable intrinsic fluctuations.
Spin noise spectroscopy involving RF magnetic fields to go beyond the restrictions of thermal equilibrium and linear response
Per the fluctuation-dissipation theorem, the information obtained from spin noise studies in thermal equilibrium is necessarily constrained by the system’s linear response functions. However, by including weak radio frequency magnetic fields, it could be demonstrated that intrinsic and random spin fluctuations even in strictly unpolarized ensembles can reveal underlying patterns of correlation and coupling beyond linear response, and can be used to study nonequilibrium and even multiphoton coherent spin phenomena. This capability was demonstrated in a study on a classical vapor of potassium atoms, where spin fluctuations alone directly revealed Rabi splittings, the formation of Mollow triplets and Autler-Townes doublets, AC Zeeman shifts, and even nonlinear multiphoton coherences.
Influence of the nuclear quadrupole interaction on electron and hole spin dephasing in quantum dot ensembles
This project focused on theory: the real-time spin dynamics and the spin noise spectra are calculated for p and n-charged quantum dot ensembles within an anisotropic central spin model, that is extended by additional nuclear electric quadrupolar interactions (QC) and augmented by experimental data. Using realistic estimates for the distribution of hyperfine coupling constants including an anisotropy parameter, it is shown that the characteristic long time scale of the central spin dephasing is of the same order for electron and hole spins. In particular, the dephasing is strongly determined by the QC, even though the analytical form of the spin decay differs significantly - which is consistent with experimental measurements. The low frequency part of the electron spin noise spectrum is approximately 1/3 smaller than that of hole spins as a consequence of the spectral sum rule and the different spectral shapes. This is confirmed by experimental spectra measured on both types of quantum dot ensembles in the low power limit of the probe laser.
Current Offers For Bachelor-, Master- or PhD-Theses
Bachelor-, Master- and PhD candidates are highly welcome to join our team. Please don't hesitate to contact us!
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Location & approach
The campus of TU Dortmund University is located close to interstate junction Dortmund West, where the Sauerlandlinie A 45 (Frankfurt-Dortmund) crosses the Ruhrschnellweg B 1 / A 40. The best interstate exit to take from A 45 is "Dortmund-Eichlinghofen" (closer to Campus Süd), and from B 1 / A 40 "Dortmund-Dorstfeld" (closer to Campus Nord). Signs for the university are located at both exits. Also, there is a new exit before you pass over the B 1-bridge leading into Dortmund.
To get from Campus Nord to Campus Süd by car, there is the connection via Vogelpothsweg/Baroper Straße. We recommend you leave your car on one of the parking lots at Campus Nord and use the H-Bahn (suspended monorail system), which conveniently connects the two campuses.
TU Dortmund University has its own train station ("Dortmund Universität"). From there, suburban trains (S-Bahn) leave for Dortmund main station ("Dortmund Hauptbahnhof") and Düsseldorf main station via the "Düsseldorf Airport Train Station" (take S-Bahn number 1, which leaves every 20 or 30 minutes). The university is easily reached from Bochum, Essen, Mülheim an der Ruhr and Duisburg.
You can also take the bus or subway train from Dortmund city to the university: From Dortmund main station, you can take any train bound for the Station "Stadtgarten", usually lines U41, U45, U 47 and U49. At "Stadtgarten" you switch trains and get on line U42 towards "Hombruch". Look out for the Station "An der Palmweide". From the bus stop just across the road, busses bound for TU Dortmund University leave every ten minutes (445, 447 and 462). Another option is to take the subway routes U41, U45, U47 and U49 from Dortmund main station to the stop "Dortmund Kampstraße". From there, take U43 or U44 to the stop "Dortmund Wittener Straße". Switch to bus line 447 and get off at "Dortmund Universität S".
The AirportExpress is a fast and convenient means of transport from Dortmund Airport (DTM) to Dortmund Central Station, taking you there in little more than 20 minutes. From Dortmund Central Station, you can continue to the university campus by interurban railway (S-Bahn). A larger range of international flight connections is offered at Düsseldorf Airport (DUS), which is about 60 kilometres away and can be directly reached by S-Bahn from the university station.
The H-Bahn is one of the hallmarks of TU Dortmund University. There are two stations on Campus Nord. One ("Dortmund Universität S") is directly located at the suburban train stop, which connects the university directly with the city of Dortmund and the rest of the Ruhr Area. Also from this station, there are connections to the "Technologiepark" and (via Campus Süd) Eichlinghofen. The other station is located at the dining hall at Campus Nord and offers a direct connection to Campus Süd every five minutes.
The facilities of TU Dortmund University are spread over two campuses, the larger Campus North and the smaller Campus South. Additionally, some areas of the university are located in the adjacent "Technologiepark".