Frequency Dependent Deflections

An investigation was made on the frequency dependence o f the 'islands' observed in the complete parameter search. Frequency scans o f were again recorded at different locations in the reflected beam profile. In addition, deflection angle scans across the reflected beam profile were recorded at differing laser 1 detunings. The results o f this investigation are presented in figure 4.16.

c e n c e S ig n a l (A rb . U n its ) F lu o re sc e n c e Sig na l (A rb . U n it s)

Chapter 4 Results and Discussion

(a)

Laser 1 Detuning in Atom Frame (GHz)

Deflection Angle (mrad)

Figure 4.16 : (a) Frequency scans of laser 1 recorded at deflection angles X and Y from figure 4.15. (b) Deflection angle scans recorded at two different laser 1 frequencies corresponding to points E and C in

Chapter 4 Results and Discussion

Figure 4.16 presents two series o f traces which were recorded using the same

experimental parameters. The velocity o f detected atoms was 1088ms’1 (A2=+825MHz), and the incidence angle o f the atomic mirror was 1.79mrad. The saturation parameter o f both evanescent waves was G =3.8xl04.

Figure 4.16(a) presents two frequency scans which were recorded at different deflection angles on the reflected beam profile. The light trace is the familiar frequency scan taken at the peak o f the reflection profile (at position X in figure 4.15 or 4.16 (b)). The heavier trace is a frequency scan recorded for a shallower deflection angle (at point Y in figure 4.15 or 4.16(b)). Note that the heavy trace is 2.5x more sensitive than that for the light trace.

There are significant differences between the two traces in figure 4.16(a). The shape o f light trace has been explained in section 4.3. The heavy trace, however, reaches a maximum at the position marked C on figures 4.15 and 4.16(a). This position corresponds to the detuning at which the grating velocity is equal to the atomic velocity.

Figure 4.16(b) presents two reflected beam profiles taken at two different laser 1 detunings. The light trace is the familiar reflection profile (from figure 4.1 and 4.2) taken while laser 1 was tuned to peak reflection flux (at point E in figure 4.16(a)). The heavy trace is a reflection peak profile taken when laser 1 was tuned so that the grating velocity was equal to the atom velocity (at point C in figure 4.15 and 4.16(a)). The scales for the two traces X and Y are identical.

The heavy trace in figure 4.16(b) is a spatial profile of the reflected atomic beam. The plateau at the low deflection angle side o f the reflected beam is the feature o f interest, and corresponds to the 'islands' in the parameter search (figure 4.15). This plateau occurs at some particular frequencies o f laser 1, but comparison with the light trace

Chapter 4 Results and Discussion

shows that at other frequencies of laser 1 it is not present at all. This plateau at point Y is therefore a frequency dependent deflection.

To examine the dependence of the plateau on frequency, the detection system was positioned at point Y and a laser 1 detuning scan was recorded. This frequency scan is the heavy trace in figure 4.16(a). This trace peaks at the point where the grating is moving with the same velocity as the detected atoms. At this point, there are the greatest number of quasipotential avoided crossings energetically accessible to the atoms.

Although there is insufficient data to make any quantitative assessment of the competing process, one possible explanation is proposed here.

Consider the case where the quasipotential positions are similar to those in figure 4.14 (a condition which occurs near the point C). At these conditions, there is a possible output channel (diffraction order) which lies above the input channel. This channel corresponds to lower atomic velocity in the y direction for emerging atoms. Thus, atoms which emerge from the evanescent field in such a channel would be detected at lower deflection angles than specular reflection.

Alternatively, the presence of atoms at deflection angles lower than the specular reflection angle may be explained by the presence of one avoided crossing. As discussed in section 4.3.3 and using figure 4.9, an atom may encounter an avoided crossing during the reflection process as it leaves the evanescent field. If the atom remains in the higher adiabatic potential at the avoided crossing, then it will emerge from the evanescent field in an attractive potential. The time spent in the attractive potential will reduce the kinetic energy of the atom, thus reducing the final deflection angle.

While these explanations are consistent with the features of the heavy traces in figure 4.16, the only firm conclusion which may be drawn at this point is that a frequency

Chapter 4 Results and Discussion

dependent deflection of the atomic beam has been observed for a moving evanescent grating. This deflection has the greatest flux when the velocity of the atomic beam in the moving grating frame is small.

The next stage in the project may have been to model the experimental parameters using the quasipotential model to predict the theoretical population of the diffraction orders. At this stage, however, limitations of the two-level quasipotential model were becoming apparent. Furthermore, the frequency dependent deflections occurred in a region where the quasipotentials were very closely spaced (as in Fig. 4.14).

The modelling of this process was therefore deferred and the development of the multi­ level theory is described in section 5.2.