Side hole structure

The amplitude of the photon echo signal as a function of the pulse separation, in the presence of a 100 G magnetic parallel to the C3 axis, is shown in Fig. 5.11. The laser power used for these measurements was 100 mW. The echo signal was found to have a maximum amplitude for a ti/2 pulse duration of 2 ps. This suggest that the nutation frequency due to the optical driving field is 125 kHz. The signal was observed to decay exponentially as function of the delay time, with a decay time of 7.55±0.1 ps. From Equation 4.23, T2 is twice the observed decay time 15.1± 0.2 ps, and the homogeneous linewidth given by 1 (7cT2) is 21± 0.3 kHz. As discussed in Chapter 3 the narrowest possible width of a spectral hole is twice the homogeneous linewidth, which in the present case means a linewidth of 42 ± 0.6 kHz. This value for T2 is shorter than that of Macfarlane et al. [Macfarlane et al. 1979] who get 23 ps at 80 G. This indicates a sample dependence.

Fig 5.12 shows two spectra of optical holes created with 5 mW pulses 200 ps long, one burnt and probed in the absence of an external magnetic field, the other in the presence of a 53 G field along the C3 axis. The delay between burning the hole, and the sideband scanning through it was approximately 500 ps. The width of the hole burnt with zero applied magnetic field was 220 ± 10 kHz FWHM. On the application of the 53 G field the hole split into three with the holewidth reducing to 80±10 kHz. When the applied magnetic field was increased the frequency of the two side holes was observed to shift at a rate of 4.0±0.2 kHz/G, as shown in Fig 5.13.

As shown in Figs 5.14 and 5.15 the frequency of the side holes were found to be independent of the direction of the applied field, although the frequency of other Zeeman

delay (ps)

Figure 5.11. Photon echo amplitude as a function of the delay time, for the 3H4 => ]Ü2 transition in L a f ^ P r 3* in the presence of a 100 G magnetic field parallel to the C3 axis. The laser intensity was 100 mW and the duration of the 7t/2 pulse was 2 ps. The observed decay rate was 7.55 ± 0.05 ps.

a n sm is si o n ( a rb it ra ry un its ' -0.5 0 0.5

laser offset frequency (MHz)

Figure 5.12 Optical holebuming spectra. Trace (a) was taken in the presence of the earth's magnetic field only. Trace (b) was taken in the presence of a 53 G magnetic field parallel to the C3 axis of the crystal.

a n sm is si o n f ä rb u n magnetic field

67.5 G

135 G

180 G

225 G

laser offset frequency (kHz)

Figure 5.13 Optical holeburing spectra taken in the presence of a magnetc field parallel to the C3 axis. The magnitude of the field was varied from 0 to 225 G.

laser frequency offset (MHz)

F igure 5.14 Optical hole burning spectra taken in the presence of a 86 G magnetic field in a plane including the C3 and C2 axes. For trace (a) the field is parallel to the C3

axis, trace (b) 45° to the C3 axis and for trace (c) the field is parallel to the C2 axis. For all o f the angles sideholes are observed at ± 344 kHz.

a n sm is si o n ( a rb it ra ry u n it

laser frequency offset (MHz)

F igure 5.15. Optical hole burning spectra taken in the presence of a 210 G magnetic field perpendicular to the C3 axis. For trace (a) the field is 15° from one of the C2 axis trace (b) 30°, trace (c) 45° and for trace (d) 53°. For all angles sideholes are observed at ± 840 kHz.

structure was observed to vary. The magnetic field dependence of these side holes, in both the magnitude and direction of the field, is consistent with these side holes being the result of optical transitions where the spin of a N-N 19F nucleus, with its isotropic magnetic moment of 4.0 kHz/G, is flipped along with the excitation of the Pr3+ ion to the

^ 2 state.

To confirm that the origin of these side holes are due to the neighbouring 19F nuclei and not to Zeeman structure in the Pr3+ ion, the total hole structure within 5 MHz of the centre hole as function of the field applied along the C3 axis was investigated. Fig 5.16 shows a typical spectrum. The burn pulse duration was increased to 5 ms to accentuate the side hole structure. In Fig 5.17 the frequency of the side holes is plotted as a function of the applied field strength. Fitted to this data is the theoretical position of the side holes due to the hyperfine structure in the 1Ü 2 (r 1) state (thin lines). The thick line shows the expected position of the side holes due to 19F spin flips with a gradient of 4.0 kHz/G. The gradient of the line fitted to the points lying close to this line was 3.95 ± 0.1 kHz/G.

To obtain an estimate of the relative strength of these F spin flip transitions the saturation of the optical transition of the burn pulse was reduced by reducing the duration of the pulse to 200 (is, reducing the laser power to 100 pW and the diameter of the laser spot on the crystal was doubled to 400 pm. From the nutation frequency estimated from the photon echo measurements the nutation frequency under the conditions just outlined should be 2 kHz. With a nutation frequency of 2 kHz less than 25% of the ions in resonance with the laser will be excited during a 200 ps pulse. Under these low saturation conditions the depths of the holes should reflect the relative strengths of the associated transitions. The relative depth of each side hole to the centre hole for a hole burnt in the presence of a magnetic field of 34 G was 20% as shown in Fig 5.18. The theoretical value of the relative strength of the spin-flip transitions is 7%, calculated using the methods described in Section 5.2.6. The measurement was repeated for two other magnetic fields also shown in Fig 5.18. The depth of the side holes were observed to decrease with increasing applied magnetic field, consistent with the behaviour predicted in Section 5.2.6.