Surface SHG from lithium niobate has been shown to be a potentially very useful technique for non-destructively studying poled lithium niobate. It can be used to locate thexand yaxes of the crystal, and distinguish between+zand−zfaces of
virgin crystals.
Experiments were conducted on poled lithium niobate samples. A fundamental pulsed laser beam of wavelength 532 nm was directed on to a lithium niobate surface and the reflected second harmonic signal at 266 nm was recorded as either a function of sample rotation angle,φ, or the angle of polarisation of the incident beam,γ.
The variation of SH signal with sample rotation was used to orientate the sample before further experiments were conducted. For a rotation of 120°two peaks were observed, as expected from theory. Measurements of the SH signal with input polarisation were compared for virgin and repoled+zand−zfaces.
No significant difference was recorded in the SH signal from the+z and -z faces when the SH polarisationΓ =s, hence most results were recorded usingΓ =p. The reason for this difference is clear when the equations for s andp polarised SHG, (using ap-polarised fundamental) are compared, as shown below.
E(2pω) =X
" r
t2 m2
sinθm2c2d31+(cosθt2)(sin(3φ)c2d22
#
E2 (7.1)
E(2sω) =Ycos(3φ)c2d22E2 (7.2)
Es(2ω)is dependent only ond22, whileEp(2ω)is dependent ond22andd31. Asd31has a
component along thezaxis, whereasd22does not, it is likely that the measurement
ofd31 is sensitive to changes in the orientation of the crystal, and that the changes
in SHG from a poled area also arise due to a change ind31.
Surface SHG was shown to be able to distinguish between repoled and virgin areas, as a difference in SH intensity of 40 % occurred between a virgin −z face
and a recently repoled +z face. It seems likely that this difference will recover over time, in a similar way to the shifted Raman peaks returned to their original wavenumber positions, as was discussed in Chapter 5. However, the recovery of SHG intensity appears to be slower than that of the Raman peaks, with only small increases in intensity occurring over six hours after poling. Measurements made after longer periods of time are inconclusive, as a three month-old sample shows complete recovery, while a four month-old sample has not fully recovered. The time dynamics of this process are thus suitable for further study, but due to time limitations could not be fully investigated as part of this PhD thesis.
7.4.1 Future Work
Further work could also be conducted to use surface SHG to measure the nonlinear coefficients, which would be of interest when using doped or modified lithium niobate, in waveguide applications for instance. The surface SHG technique could also be used to provide useful information on the period length in periodically poled lithium niobate, which is difficult to measure non-destructively with current techniques (for instance selective etching is routinely used to visualise PPLN, but this necessarily damages the crystal surface). This would work by selecting a suitable sample orientation and fundamental beam polarisation, and then scanning the beam across the PPLN (or moving the sample relative to the beam) and measuring the SH intensity. The alternating domains would affect the intensity and hence a plot of intensity versus position would reveal the period of the PPLN.
7.5
References
Crouch, C. H., J. E. Carey, J. M. Warrender, M. J. Aziz, E. Mazur and F. Y. Genin. “Comparison of structure and properties of femtosecond and nanosecond laser- structured silicon.” Applied Physics Letters,84(11), 1850 (2004).
Mailis, S., C. L. Sones, J. G. Scott and R. W. Eason. “UV laser-induced ordered surface nanostructures in congruent lithium niobate single crystals.” Applied Surface Science,247(1-4), 497 (2005).
Sones, C. L., C. E. Valdivia, J. G. Scott, S. Mailis, R. W. Eason, D. A. Scrymgeour, V. Gopalan, T. Jungk and E. Soergel. “Ultraviolet laser-induced sub-micron periodic domain formation in congruent undoped lithium niobate crystals.”
Applied Physics B,80, 341 (2005a).
Sones, C. L., M. C. Wengler, C. E. Valdivia, S. Mailis, R. W. Eason and K. Buse. “Light- induced order-of-magnitude decrease in the electric field for domain nucleation in MgO-doped lithium niobate crystals.” Applied Physics Letters,Accepted(2005b). Valdivia, C. E., C. L. Sones, J. G. Scott, S. Mailis, R. W. Eason, D. A. Scrymgeour, V. Gopalan, T. Jungk, E. Soergel and I. Clark. “Nanoscale surface domain formation on the +z face of lithium niobate by pulsed ultraviolet laser illumination.” Applied Physics Letters,86(2), 022906 (2005).
A
Diagnostic Equipment
A.1
Scanning Electron Microscope (SEM)
Scanning Electron Microscope (SEM) images have been used extensively through- out this work, to visualise the surface structures that have been created on lithium niobate. An SEM was chosen over an optical microscope due to the much higher resolution images that an SEM can achieve. As its name implies, an SEM uses a beam of electrons to create the image. The electrons are controlled using magnetic fields, which focus and scan the electron beams. Figure A.1 shows a schematic illustration of an SEM.
A beam of electrons are emitted from a tungsten filament in the electron gun and are then accelerated along the SEM tube by the anode. The electron beam is then collimated by condensing coils. Scanning coils are used to deflect the beam, typically to raster-scan over an area of the sample surface. A final magnetic objective then focusses the beam on to the sample surface. The SEM beam path and sample chamber requires a vacuum to operate, as otherwise gas molecules would hinder the electron beam and cause the filament to rapidly burn out.
When the primary electrons strike the surface of the sample, a number of interactions take place leading to the production of:
Electron Gun Anode Condensing Lenses Scanning Coils Objective Lens Secondary Electron Detector Sample Stage Electron Beam
Figure A.1:Schematic diagram showing the components of a typical scanning electron microscope.
• Backscattered electrons: Elastically scattered primary electrons, of high energy.
• Secondary electrons: Lower energy electrons ejected from the valence-band of the material by the primary electrons.
• Auger electrons: The primary electrons may cause an electron to be ejected from inner shell orbitals, leaving a vacancy which is then filled by an electron from an outer shell. This can cause an Auger electron to be emitted to conserve energy.
• X-rays: The interaction of the primary electrons with the atoms of the sample can also result in X-ray emission.
The backscattered and secondary electrons are most commonly used in SEMs. Backscattered electrons can give topographic and compositional information about the material being examined, but because they can be scattered from within a relatively deep area, the topographic contrast is not as good as that obtained from secondary electrons, which originate from within a few Angstroms of the surface. For this reason, the secondary electrons are normally used to produce topographic
images. The electrons are detected by a scintillator-photomultiplier which passes the data to a computer for processing.
Sample preparation is relatively simple for use in an SEM, with the two requirements being that the sample must be able to resist the vacuum inside the sample chamber (which is usually only an issue for biological samples) and that the sample must be electrically conductive. When imaging non-conductive samples, a thin coating
∼10 nm of gold can be applied using a sputter coater, to prevent charge build-up.