Measurement of Spin Relaxation by Optical Holeburning
IBM/SJSU/NSF SUMMER 2001
NSF GOALI Grant CHE9625628
Susan English1, Roger Macfarlane2
(1) San Francisco State University
(2) IBM Almaden Research Center
What is spin?
Spin is an intrinsic angular momentum of particles such as electrons, and has an associated magnetic field, much like that of a bar magnet.
Electrons have two spin states, spin-up and spin-down. In the presence of a magnetic field, an electron has a different energy depending on whether the spin is aligned or anti-aligned with the field.
Why is spin important?
Spin tends to be isolated from the environment, so that long relaxation and decoherence times are possible. Spin is a key parameter in:
Quantum Computers: using the spin of a particle as a quantum bit or “qbit”
Spintronics (spin-based electronics): using spin states inside semiconductor materials
For these applications it is important to know the time for a spin to flip its orientation (spin relaxation) and to lose its phase coherence (spin decoherence).
In inhomogeneously broadened materials individual atoms and/or molecules each absorb light at very specific frequencies due to slight non-uniformities in their environment.
For our study we looked at Nd:YV04 at a wavelength of 880 nm, which is the wavelength of the optical resonance of the Nd3+ ion. We chose this system because the photon coupling with the f electrons of Nd3+ is strong, and therefore our sample can be quite dilute (120 ppb). This is important because the ions are isolated and so we are able to distinguish between the two relaxation processes: spin-spin and spin-lattice relaxation.
spin-spin relaxation
Mutual spin flips through dipolar coupling don’t change the net magnetic field of the sample, and conserve energy. This process depends on ion concentration, specifically the number of opposite spins within the range of the dipole coupling (1/R3 dependence).
spin-lattice relaxation
Energy goes into the lattice as phonons. This process depends on (1) the density of modes of vibration into which the electrons can get rid of their energy, and (2) the magnetic field mixing other electronic states into the ground state. The mixing of other wave functions means the states are no longer pure. With spin-lattice relaxation we expect a dependence on the magnetic field which goes as Ho4.
The degeneracy of the states is lifted by the application of a magnetic field Ho.
1) A single-frequency "burning" light beam is incident on an inhomogeneously broadened material in a magnetic field.
2) Only those atoms whose ground-to-excited state transition matches the burning beam's frequency are promoted to the excited state.
3) The electrons can then relax down to the spin-up state and the material as a whole becomes less absorptive at the burning laser's frequency.
4) Absorption remains lowered until the spin-up electrons relax to their spin-down state.
spectral hole
When the absorption of the material is measured versus frequency, there is a "hole" or decrease in absorption at the burning beam frequency, and as the spins relax this absorption decreases. The time needed for the hole to recover is the relaxation time.
Acousto-optic modulator controls the intensity of the beam and the length of burning and probe pulses. The burning pulse is ~100 times the intensity of the probe pulse. The laser controller scans the frequency of the probe beam over 500 MHz. The sample is at 1.6 K.
At very low fields we propose mutual spin flips dominate, while for fields above 1000 G, spin-lattice relaxation turns on.
(1) Relaxation times for the electron spin are extremely long.
(2) From Ho dependence the onset of spin-lattice relaxation can be identified.
Another important parameter which needs to be examined is that of the coherence time of the spins. This is expected to be similar to optical coherence times, i.e., tens of ms, but is the subject of further study.
Thanks to Bill Risk, Bob Shelby, Charles Wade, Dolores Miller, Caroline Alexander, IBM, SFSU, SJSU, NSF GOALI Grant CHE962-5628