Spin coherence in nanocrystals

Motivation:

Nanocrystals are a particularly useful platform for studying electron spin behavior in semiconductor materials [1].  Semiconductor nanocrystals act as quantum dots, with energy level spacing large compared to thermal energy at room temperature.  Thus they allow one to confine a controlled number of electrons or holes at room temperature in an engineered, versatile nanostructure.  Our previous work has revealed the power of nanocrystals as a platform for manipulating electron spin states, including the engineering of single and coupled states within individual nanocrystals [2 ,3], and enhancement of the spin/photon interaction by integrating nanocrystals into optical cavities [4]. In our ongoing research, semiconductor nanocrystals provide an ideal platform to introduce electron spins into a controlled, complex environment, and to then observe the effect of that environment on electron spin coherence.

Nanocrystal Fabrication:

Fig. 1: CdSe/CdS nanocrystal heterostructures of varying size luminescing under UV excitation.

Semiconductor nanocrystals are synthesized using standard colloidal chemistry.  We are currently working with samples purchased from NN Labs, and produced by our collaborators in the Burda Group, in the CWRU chemistry department. 

Nanocrystals may be synthesized from a variety of semiconducting materials, and different materials may be layered within a nanocrystal structure to engineer the electronic and optical properties.  Once synthesized, the nanocrystals may be studied in solution, as shown above, or may be introduced into a large number of different environments, such as close-packed solids, self-assembled monolayers, and polymers, to name a few.

Measurement of coherent spin dynamics:

A simplified schematic of a typical measurement of coherent spin dynamics is shown below.

Fig. 2: Time-resolved Faraday rotation measurement of coherent spin dynamics.

In the setup shown here, two lasers produce short, synchronized pulses separated by a fixed time delay.  In practice, the delay between pulses is controlled simply by changing the distance travelled by each pulse.  This method yields excellent temporal resolution -- changing the optical path legnth by 1 mm changes the arrival time of the pulse by about 3 ps. 

Both pulses are focused onto a single spot on the sample, which for example, could be nanocrystals in suspension as shown in Fig. 1.  The first pulse to arrive (the pump pulse) is circularly polarized, and serves to excite electrons into specific higher energy spin states in the nanocrystals.  The electrons in these states then evolve freely until the second pulse arrives (the probe pulse).  As the probe pulse passes through the sample, its polarization is slightly modified proportionally to the projection of the total spin polarization along the probe laser direction, a phenomenon known as the Faraday effect.  Thus, by measuring this change to the probe's polarization, we learn about how the spin states created by the pump have evolved up to the arrival of the probe.  We can then sweep the time delay between pump and probe, and map out the coherent evolution of the electron spins.

 

Results:

A typical measurement of nanocrystal spin dynamics is shown in Fig. 3.

Fig. 3.  Left: Projection of spin polarization along the probe direction as a function of time after the pump pulse. Right: Fourier transform of data on left, showing three spin precession frequencies.

In this case, the sample consisted of layered CdSe/ZnS core/shell structures engineered to have multiple stable states, with different spin dynamics [3].  This can be seen in the Fourier transform of the data that shows three distinct frequencies in the spin dynamics, corresponding to the evolution of electron spins residing in different states within the nanocrystals.

References:

1 . O. Gywat, H. Krenner, and J. Berezovsky. Spins in optically active quantum dots. Wiley-VCH, 2010.

2 . J. Berezovsky, M. Ouyang, F. Meier, D. D. Awschalom, D. Battaglia, and X. Peng. Spin dynamics and level structure of quantum-dot quantum wells. Phys. Rev. B 71, 081309 (2005).

3 . J. Berezovsky, O. Gywat, F. Meier, D. Battaglia, X. Peng, and D. D. Awschalom. Initialization and read-out of spins in coupled core-shell quantum dots. Nature Physics 2, 831–834 (2006).

4 . Y. Q. Li, D. W. Steuerman, J. Berezovsky, D. S. Seferos, G. C. Bazan, and D. D. Awschalom. Cavity enhanced Faraday rotation of semiconductor quantum dots. Appl. Phys. Lett. 88, 193126 (2006).