5.3 Experimental setups and results
5.3.1 Beam distortion
Pyroelectrically-induced photorefractive damage in LiNbO3:MgO
Pyroelectrically-induced photorefractive damage in LiNbO3:MgO
z-axis
l/2 wave plate
Polarizer Lens Crystal on 3-axis translation stage
and heated aluminum block
Screen Laser
Figure 5.5: Setup for the observation of beam distortion. The laser is a 532-nm frequency-doubled continuous-wave Nd:YAG laser.
and propagating along the x-direction of the crystal. The temperature of the crystal is controlled with a temperature accuracy of±0.1◦C at the surface of the crystal in contact with the heating block; the temperature is also measured at the opposite (air) surface of the crystal. During illumination the crystal is heated until the top and bottom of the crystal reaches stable Tf. Experiments are also carried out with the illumination applied after the crystal has been thermally equilibrated at the final temperatureTf.
The setup is shown schematically in Fig. 5.5. Before the beam enters the crystal, it passes through a half-wave plate and a polarizer which, together with neutral density filters, enables continuous adjustment of the powerP incident on the crystal from 20µW to 2 W.
The beam is focused at the center of the crystal to a 1/e2 intensity diameter2w, which is 100 µm in all experiments unless otherwise noted. The incident beam is extraordinarily polarized, i.e. polarized along thez-axis of the crystal. The transmitted beam is observed on a screen placed approximately 50 cm beyond the output face of the crystal. Various temperature differencesTf−Tiare applied in several experiments, however,Tfis always kept below 50 ◦C. After illumination is stopped, the c-faces of the crystal are short-circuited to discharge any pyroelectric surface charges while the crystal cools down to the initial temperature Ti. Note that any internal space charge fields that may have been created during illumination would not be erased by this process [114].
5.3.1.2 Results
Before studying the effects of changing the temperature of the MgO:LN crystals, we first carry out measurements on samples CTIMgOLN1, CTIMgOLN2, and YamMgOLN1 at constant temperature in order to examine their conventional photovoltaic PRD behavior.
For these measurements, the crystals are illuminated along thex-axis, first withP = 2W and then withP = 20µW. No measurable PRD is observed in any of the MgO:LN crystals (less than 1 %change in diameter on the screen in the far field).
Pyroelectrically-induced photorefractive damage in LiNbO3:MgO
The MgO:LN samples are then illuminated at a constant power of 20 µW, and the tem-perature is ramped from Ti = 40 ◦C to Tf = 50 ◦C in order to investigate the effects of varying temperature. A picture of the shape of the beam transmitted through sample YamMgOLN1 at different stages of the experiment is shown in Fig. 5.6.
Before heating During heating After 60 min at T
f
a) b) c)
Figure 5.6: Shape of a laser beam after passing a MgO:LiNbO3 crystal (sample YamMgOLN1) that is heated from 40 ◦C to 50◦C.
All MgO:LN samples show similar behavior. Less than one second after initiation of the temperature ramp, PRD is observed with typical beam distortion and far-field pattern formation. After several seconds the original beam shape cannot be recognized anymore (Fig. 5.6b); however, the beam shape continues to evolve. Beam distortion remains even after the crystal equilibrates to Tf. Finally, after several minutes in this stage, the beam shape begins to restore slowly towards its original shape, but it does not fully recover.
Scattered light is still observed in the region outside the original beam diameter even after one hour elapses (Fig. 5.6c).
In another experiment the samples are heated first and then, after thermal equilibration atTf, are subject to subsequent illumination with the same beam parameters as described above in order to create optical damage. Illumination is started once the selected temper-ature has been reached. Note between heating and illumination the crystal c-facets are not short-circuited again. In all the samples the beam shows similar distortion as seen in Fig. 5.6b.
In order to investigate the persistence of these photorefractive effects, the MgO:LN crystals with PRD are stored at room temperature in the dark for times up to several weeks, and then probed optically with the same 532-nm beam as it was used in the previous experiments. Scanning the beam along they-axis of the crystal on a path that intersected the previously damaged spot results in scattered light when the beam intersects the regions displaced slightly to the+y and −y side of the previously illuminated and damaged spot.
In contrast, in areas where no PRD was created earlier, no PRD is observed. For this experiment the crystal is not heated in order to make sure that the transmitted beam does not create any additional optical damage.
Pyroelectrically-induced photorefractive damage in LiNbO3:MgO
We also observe that the PRD can be completely erased by homogeneously illuminating the crystal with an incandescent lamp for 30 minutes. After that exposure the beam diameter in the far field is within 1%of its original value. For applications, it is important to note that after erasing the optical damage with white light, the MgO:LN crystals can be used for subsequent experiments without any apparent change in properties.
Similar experiments are also conducted with different starting temperatures and temper-ature differences. All experiments reveal the same qualitative behavior. However, when the incident power is increased to P = 100 mW rather than the P = 20µW used in the previous experiments, the strong distortion shown in Fig 5.6b is not observed, but the weak scattering in the wings of the beam (as in Fig. 5.6c) is still observed. The effect again persists for weeks in crystals stored in the dark.
It is very important to note that the beam is not distorted and no patterning occurred in any experiment when the z-faces of the MgO:LN crystal are short-circuited during heating, e.g. by painting silver conducting paste on thez-faces and connecting them with each other. Furthermore, conducting the experiment with sample CLN2, photorefractive damage is not observed with 20µW optical power and 100-µm beam diameter, whether or not a temperature step is applied during illumination. However, at higher laser powers (on the order of milliwatts) at room temperature the conventional bulk-photovoltaic PRD [77]
is observed in CLN independent of heating whereas, as already noted, bulk-photovoltaic PRD is not observed in MgO:LN under these conditions.