Direct Laser Writing (DLW) by two-photon photopolymerization (TPP) enables the fabrication of micron-scale polymeric structures in soft matter systems. The technique has implications in a broad range of optics and photonics; in particular fast-switching liquid crystal (LC) modes for the development of next generation display technologies. In this paper, we report two different methodologies using our TPP-based fabrication technique. Two explicit examples are provided of voltage-dependent LC director profiles that are inherently unstable, but which appear to be promising candidates for fast-switching photonics applications. In the first instance, ~1 μm-thick periodic walls of polymer network are written into a planar aligned (parallel rubbed) nematic pi-cell device containing a nematic LC-monomer mixture. The structures are fabricated when the device is electrically driven into a fast-switching nematic LC state and aberrations induced by the device substrates are corrected for by virtue of the adaptive optics elements included within the DLW setup. Optical polarizing microscopy images taken post-fabrication reveal that polymer walls oriented perpendicular to the rubbing direction promote the stability of the so-called optically compensated bend mode upon removal of the externally applied field. In the second case, polymer walls are written in a nematic LC-optically adhesive glue mixture. A polymer- LCs-polymer-slices or ‘POLICRYPS’ template is formed by immersing the device in acetone post-fabrication to remove any remaining non-crosslinked material. Injecting the resultant series of polymer microchannels (~1 μm-thick) with a short-pitch, chiral nematic LC mixture leads to the spontaneous alignment of a fast-switching chiral nematic mode, where the helical axis lies parallel to the glass substrates. Optimal contrast between the bright and dark states of the uniform lying helix alignment is achieved when the structures are spaced at the order of the device thickness, which was also found to be the case for the achiral system. The high resolution DLW technique limits structures to the focal spot size of the beam, ~1 μm in diameter, such that the transmittance is expected to be significantly enhanced relative to other stabilization techniques. Moreover, both devices remain stable under electrical and thermal cycling.
The accurate focusing of ultrashort laser pulses is extremely important in multiphoton microscopy. Using adaptive optics to manipulate the incident ultrafast beam in either the spectral or spatial domain can introduce significant benefits when imaging. Here we introduce pulse front adaptive optics: manipulating an ultrashort pulse in both the spatial and temporal domains. A deformable mirror and a spatial light modulator are operated in concert to modify contours of constant intensity in space and time within an ultrashort pulse. Through adaptive control of the pulse front, we demonstrate an enhancement in the measured fluorescence from a two photon microscope.
The focusing of ultrashort laser pulses is extremely important for processes including microscopy, laser fabrication and fundamental science. Adaptive optic elements, such as liquid crystal spatial light modulators or membrane deformable mirrors, are routinely used for the correction of aberrations in these systems, leading to improved resolution and efficiency. Here, we demonstrate that adaptive elements used with ultrashort pulses should not be considered simply in terms of wavefront modification, but that changes to the incident pulse front can also occur. We experimentally show how adaptive elements may be used to engineer pulse fronts with spatial resolution.
Graphitic wires embedded beneath the surface of single crystal diamond are promising for a variety of applications. Through a combination of ultra short (femtosecond) pulsed fabrication, high numerical aperture focusing and adaptive optics, graphitic wires can be written along any 3D path. Here, we demonstrate a non-reciprocal directional dependence to the graphitization process: the features are distinct when the fabrication direction is reversed. The non-reciprocal effects are significantly determined by the laser power, the fabrication speed, the light polarization and pulse front tilt. The influences of these factors are studied.
Graphitic wires embedded beneath the surface of single crystal diamond are demonstrated. Through a combination of ultrashort (femtosecond) pulsed fabrication, high numerical aperture focusing and adaptive optics, wires are created with sub micrometre dimensions that can follow any three dimensional path within the diamond. The increased level of focal control available through the use of adaptive optics appears particularly important in the generation of high quality wires, with measured conductivities over an order of magnitude greater than previous laser-induced graphitic wires in diamond. Applications for the embedded wires are discussed.
Ultrashort pulsed lasers are used to fabricate 3D structures in single crystal CVD diamond. The interaction of the laser with diamond lattice leads to a permanent structural modification, which is highly localized at the focus. Severe spherical aberrations compromise fabrication precision below the diamond surface. We implement adaptive aberration compensation to ensure optimum fabrication performance. The nature of the structural modification is analysed for both surface and subsurface laser fabrications.
We perform structural characterisation of direct laser write (DLW) waveguides. Quantitative phase microscopy, based on solution of the transfer of intensity equation, is used to measure the cumulative refractive index change through a waveguide perpendicular to its axis. Results are compared with interferometry, cross-sectional measurements using third harmonic microscopy, and analysis of the near-field image of the mode propagating in the waveguide. We show that in many situations, notably in the presence of depth dependent spherical aberrations, the cross-section for DLW waveguides may not be assumed symmetric about the waveguide axis. This is particularly important when fabricating at depths greater than 2 mm in fused silica. Therefore additional measurements are required to fully characterise the refractive index profile.
The “quill effect" describes a directional phenomenon encountered during ultrafast laser fabrication. Even in homogeneous and isotropic materials, fabrication effects can depend on the direction of focus translation. The directionality has been attributed to pulse front tilt, leading to a spatiotemporal asymmetry in the focus. We use adaptive optics to control pulse front tilt and demonstrate controllable quill effect writing in fused silica using a femtosecond laser. Through adaptive control of the intensity profile, we also confirm that inhomogeneous pupil illumination causes similar directional effects. We show dynamic control of ultrashort pulses and directional effects during fabrication.
Direct laser writing is widely used to fabricate 3D waveguide devices by modi cation of a materials refractive index. The fabrication delity depends strongly on focal spot quality, which in many cases is impaired by aberrations, particularly spherical aberration caused by refractive index mismatch. We use adaptive optics to correct aberration and maintain fabrication performance at a range of depths. Adaptive multifocus methods are also shown for increasing the fabrication speed for single waveguides.
We outline recent research into the application of adaptive optical techniques to the laser fabrication of threedimensional
structures with sub-micrometer precision. Aberration correction can be implemented using deformable
mirrors or liquid crystal spatial light modulators (LCSLMs). The correction ensures that the quality
of the laser focus is maintained when focussing at depth into a material with high refractive index. Flexible
parallel fabrication methods have been implemented using a LCSLM through both holographic beam shaping
and an addressable microlens array. Applications have been shown in a range of high index materials, including
diamond, lithium niobate and glasses.