Awschalom Group

Spin Coherence in Microcavities

We study electron-photon information exchange with a variation – instead of varying the materials aspect of light-matter interactions, we use optical microcavities to confine light and tune the photonic field density. Enhanced light-matter interactions in microcavities have led to the development of a wide range of applications in optical communications and inspired proposals of quantum information processing and computational schemes. Microdisk lasers, such as the structures we study here, have been the subject of particular considerable interest, given their fast response time, scalability and in-plane emission. These features make them attractive components for on-chip integration in optoelectronic devices. In this section, we study the coupling between the microdisk emission and the localized spins in its active region, as a first step towards the realization of using light-matter information exchange for quantum information processing schemes in a solid-state system.

Figure 1 (left) Scanning Electron Microscopic (SEM) image of a 4 micron diameter microdisk. The active area is ~ 110 nm thick and consists of naturally grown Quantum dots embedded in quantum wells. An artistic rendition of the disks with quantum dots is shown in the right. The planar geometry of the cavities confines light by total internal reflection at the circular edge, which is emitted in the plane of the structures at the circumference, as demarcated by the green emission in the schematic. These optical modes are known as Whispering Gallery Modes (WGMs), so named after the Whispering Gallery in St. Paul's Cathedral in London.

The microdisks are fabricated from MBE-grown GaAs/(Al,Ga)As multiple Quantum Well (QW) structures. The QWs are 4.2 nm wide, and are separated by 10 nm wide (Al,Ga)As barriers. During the growth process, 2 minutes of growth interruptions are introduced at the QW-barrier interface, to allow the formation of monolayer-fluctuation induced QDs.

Figure 2 (a) Photoluminescence (PL) emission from a single microdisk. The high Q (Quality factor) lasing mode overwhelms the background spontaneous emission from the Quantum Well (QW) as the incident excitation power is increased, as shown in (b). Time-resolved charge emission, analyzed with a streak camera, shows in (c) the recombination time decreasing as the microdisk is pumped harder, which also results in a decrease in the “delay time” – the time lag between the excitation pulse and the maximum of the disk emission.

Optical characterization of the microdisks, using excitation from a pulsed (150 fs) Ti:Sapphire laser, and including both time-integrated and time-resolved studies (Figure 2), reveal high finesse single mode lasing in the microdisks at a temperature of 5.5 K. The mode shown above has a Q-factor of ~ 5000, and a lasing threshold of 0.5 kW/cm2. Time-resolved traces of this mode show the charge dynamics speeding up with increasing excitation power which is an expected occurrence as the system crosses over from spontaneous emission to stimulated regime.

Figure 3 (a) Schematic of experimental set-up used for time-resolved Kerr rotation studies. A circularly-polarized pump pulse is used to optically inject spin polarization in a single microdisk. The Kerr rotation of a linearly-polarized, time-delayed probe pulse then detects the remnant polarization to map out the spin coherence as a function of pump-probe delay. (b) Kerr rotation signal in the Voigt geometry from a single disk. The transverse spin coherence time (T2*) varies non-monotonically with pump power when the pump-probe wavelength is resonant with the lasing mode In this case, the resonant wavelength is 770 nm.

To look at the spin dynamics in a single microdisk, we use the technique of time-resolved Kerr Rotation (TRKR). In particular, we address the question of how the electron spin dynamics are modified by the stimulated emission in the disks. We observe (Figure 3) that the spin lifetime is enhanced over a certain power range when the optical pump excitation is in resonance with the high quality lasing mode at 770 nm, indicating a possibility of electron-photon coupling. To verify this, we map out the spin coherence time as a function of pump power while continuously varying the pump-probe wavelength.

Figure 4 (a) T2* as a function of pump power and wavelength. There is a “hot-spot” at the lasing mode of 770 nm, where the spin lifetime is enhanced. This effect is absent at non-resonant wavelengths. Further, the pump power where this increase occurs is the same as the optical lasing threshold power. Linecuts through this plot show this effect more clearly (b) where we also see that the spin coherence enhancement is absent in control samples (microdisks designed without QDs) and in unprocessed QWs (not designed into cavities).(c) The spin lifetime variation follows the qualitative changes in the Q-factor of the mode with pump power. (d) An additional indication for the influence of the cavity on the electrons spins is the power dependence of the Larmor precession frequency ωL, which shows a similar resonant enhancement.

We notice (Figure 4) that this resonant enhancement occurs only at low pump powers, and when the pump wavelength is in resonance with the lasing mode. And as the pump power increases further, the spin coherence time decreases. This variation in T2* follows the additional changes in the mode optical Q-factor, which is an indication that the semiconductor carrier spins are indeed coupling to the photons in the optical mode. This leads us to the next part of this project, where we re-design our microcavities to optimize this information exchange.

Figure 5 (a) Redesigned smaller microdisks with diameter of 1.5 micron. (b) Spin lifetime varying with pump wavelength and power showing an increase in coherence times at the optical resonance wavelength again, as observed in the larger disks. The smaller diameter of the disk shifts the resonance wavelength to 776 nm.(c) Linecuts emphasizing (b) while also showing the effect being absent in the bare QW. Additionally, the variation in T2* again follows the Q-factor change with power.(d) Larmor frequency is also enhanced on resonance.

So far, we have seen that while the spin coherence can be influenced by the cavity, this effect is limited to a very narrow range of pump power, occurring only near the lasing threshold. We have also observed that the spin lifetime couples to the quality of the lasing emission. It therefore follows that if the optical mode degradation at high pump powers could be reduced, the enhancement in spin coherence would follow suit. For this purpose, we design smaller microdisks. The decrease in the cavity dimension results in increased spectral mode spacing, which in turn prevents the phenomenon of “mode-hopping” at higher pump powers, preventing the Q-factor from degrading, as seen in Figure 5c. This has the desired result of making the spin coherence more robust (Figure 5b).

These results show that it is possible to enhance the electron spin coherence in semiconductor microdisk lasers through engineering the Q-factor by careful design of the cavity structure.

To learn more about our studies, please refer to "Enhancement of spin coherence using Q-factor engineering in semiconductor microdisc lasers", S. Ghosh, W. H. Wang, F. M. Mendoza, R. C. Myers, X. Li, N. Samarth and D. D. Awschalom, Nature Mat 5, 261 (2006) and the related article "Long live the spin" Nature Mat 5, 255 (2006)