6.3 fs-Pulsed LAP
6.3.2 Domain Direct-Write using fs-LAP
The phenomenon of fs-pulsed light-induced reduction of the nucleation field can be applied to directly writing domain structures in the surface and bulk of undoped and doped LN and LT. As mentioned in Section 6.3.1, the domain nucleation process has an additional dependence not present in the cw case. As a result, fs-pulsed LAP depends upon the applied electric field, peak intensity, and illumination time. Therefore, any direct-writing of domain structures must take into account an incubation effect that may be present, particularly at low E-field and intensity conditions. Nonetheless, fs- pulsed light provided a very simple and reliable method of engineering a domain structure within the target material.
The majority of experiments conducted made use of λ = 400 nm light in Mg:CLN. The 400 nm wavelength was produced by an SHG crystal by doubling the fundamental frequency of the laser, thus enabling a larger output and more stable power than provided by the OPA. As evidenced from Section 6.3.1, λ = 400 nm also showed similar results as the other wavelengths tested, and therefore provided a simple and reliable operating point. Mg:CLN was an ideal material for primary investigations due to its very large intensity-dependent reduction of the nucleation field as well as the ease with which reproducible domain inversion could be formed. Additionally, apart from undoped CLN
0 200 400 600 800 1000 1200 1400 1600 1800 0 1 2 3 4 5 6x 10 5
E−field [V/mm]
Domain Area [
µ
m
2]
0.5 s 5 s 15 s 30 s 60 s 120 sFigure 6.15: Inverted domain area on the−z face of Mg:CLN vs. E-field induced by fs-pulsed LAP for various illumination times and an intensity I = 1.2 GW/cm2 using
λ= 400 nm light. The solid lines are guides for the eye.
and CLT, Mg:CLN is the most studied, reliable, and high-quality material from the LN and LT family commercially available.
The size of the inverted domain area was studied first. By setting the bias voltage applied across the 5-mol% Mg:CLN crystal, then focussing theλ= 400 nm light onto the−z face, individual domain spots could be written in succession. These inverted domains followed the shape of the illumination pattern, and grew in size following increases in any of the three exposure conditions of E-field, intensity, and illumination time. Following HF etching, these domains were measured via SEM and plotted in Figure 6.15 for various
E-fields and exposure times, showing an increasing domain area following both these parameters while illuminating with a constant intensity of I = 1.2 GW/cm2 and a spot size of ∼5×104 µm2.
The inverted domain area increased as a function of the illumination time, as shown in Figure 6.16. The curves with intensity of 0.06 and 0.30 GW/cm2 conform well to a log- arithmic fit. However, increasing the intensity by a factor of four to 1.2 GW/cm2 shows the beginnings of a departure from this logarithmic fit and, for exposures>20 s, a trend towards a linear increase was observed. For higher intensity and longer exposures, the domain area becomes linearly dependent upon exposure time, as shown in Figure 6.17.
0 10 20 30 40 50 60 70 0 1 2 3 4 5 6 7 8x 10 4
Illumination Time [s]
Domain Area [
µ
m
2]
1.20 GW/cm2 0.30 GW/cm2 0.06 GW/cm2Figure 6.16: Logarithmic fits of inverted domain area on the−z face of Mg:CLN vs. illumination time induced by fs-pulsed LAP for various intensities usingλ = 400 nm light, biased to E = 0.5 kV/mm. The solid lines are least-squared fits to an equation of the formy =c1log10(x) +c2 or y=c1x+c2. The horizontal dashed line indicates
the beam area.
Interestingly, this transition from a logarithmic to linear fit occurs as the inverted do- main area approaches the spot area, in this case ∼5×104 µm2 indicated by the dashed line in Figure 6.16. This means that under the conditions of low exposures, the inverted domain area will only increase slowly with greater exposure time, allowing controlled areas to be inverted. For higher exposures, once the domain has spread beyond the spot size, the domain increases in area more quickly.
This control of the domain growth can be applied to the direct write of domain structures more complicated than simple spots. By translating the beam over the crystal, arbitrary illumination patterns can be written in the crystal, and result in light-assisted domain- inversion patterns. One such example is shown in Figure 6.18, where the manually- controlled beam path spelled out the letters “ORC” in undoped CLN usingλ= 400 nm fs-pulsed light. The structure, limited by the large spot size, was composed of straight and curved line segments measuring approximately 120 µm in width. Importantly, the inverted regions formed bulk domains reaching 300µm from the−z face [Figure 6.18(a)] to the opposite +z face [Figure 6.18(b)], as revealed by HF etching. Additionally, even when viewed on high magnification by the SEM, curved domain walls were observed on both faces of the crystal [inset of Figure 6.18(a)]. Straight line segments were also
0 20 40 60 80 100 120 140 0 1 2 3 4 5 6x 10 5
Illumination Time [s]
Domain Area [
µ
m
2]
1.20 GW/cm2 1.6 kV/mm 1.20 GW/cm2 1.0 kV/mm 1.20 GW/cm2 0.5 kV/mm 0.30 GW/cm2 0.5 kV/mm 0.06 GW/cm2 0.5 kV/mmFigure 6.17: Inverted domain area on the−z face of Mg:CLN vs. illumination time induced by fs-pulsed LAP for variousE-fields and intensities usingλ= 400 nm light.
The solid lines are least-squared fits to an equation of the formy=c1x+c2.
formed along directions not parallel to any of the x or y axes. Both of these capabilities are potentially very useful in obtaining truly arbitrary domain patterns.
Rounded domain shapes have also been observed using regular EFP, but have been shown to extend no further than ∼10 µm from the −z face of undoped CLN [Jungk05]. On the contrary, the LAP-induced curved and arbitrarily-angled straight domain walls were observed on both the +z and−z faces of the crystal after HF etching. Because of this and the presence of light across the entire crystal, these arbitrary domain shapes are suspected to exist throughout the bulk, although this has not yet been verified.
This direct-write method of domain patterning has also been applied to 5-mol% Mg:CLN with a thickness of 5 mm, a tenfold increase over typical wafer thicknesses. Direct-write via fs-pulsed LAP was also successful in achieving bulk domains in this material, first nucleating at the −z face and extending the entire 5 mm to the opposite +z face. These domains were very large (several hundred micrometres wide), but this was a function of the large beam size and high exposure. Additionally, these bulk domains had internal structures on the +z face, as described further in Section 6.5.4. Under improved focussing and exposure conditions, smaller domains will be possible throughout this large thickness.
Figure 6.18: Domain direct-write using fs-pulsed LAP using λ = 400 nm in 300-
µm thick undoped CLN, showing (a) nucleation on the −z face and (b) extending to the +z face, as revealed by HF etching. An E-field of 16.3 kV/mm and intensity of
∼30 GW/cm2were used. The inset of (a) shows a high magnification image of a curved
domain wall.