Using new GPC-modalities
Generalized Phase Contrast (GPC) is an extension of Zernike’s Nobel Prize winning phase contrast microscopy primarily designed for beam shaping, but also for use in quantitative phase imaging, optical encryption and in novel fields such as neuroscience and atomtronics. GPC can optically project laser light without absorbing photons into arbitrary distributions.
Zernike phase contrast is typically used to visualise naturally occurring, thin and unknown phase variations. With its primary use on beam shaping instead of sample imaging, GPC emphasises the efficient conversion of the user-defined phase patterns into output intensity excitations. Subsequent to the initial proposal of using GPC in lossless image projection and its experimental demonstration for the efficient projection of binary images, GPC has been successfully shown to be a viable dynamic projection technology, especially for real-time interactive micro-manipulation.
GPC can project grayscale lattices and is suitable for efficient laser projection of grayscale non-periodic patterns. It can also accommodate non-uniform profiles such as Gaussian beams. GPC’s use of a direct mapping geometry avoids dispersion effects which makes it advantageous for use with multiple wavelengths or temporal focusing which can effectively confine light along the axial direction. Since GPC directly maps phase patterns into intensity patterns through a 4f configuration, computational requirements are significantly lower requiring only the direct re-positioning of mapped phase patterns instead of iterative Fourier transformations. This enables real-time reconfigurability even on modest computing hardware. Patterns are thus easily updated in real time giving more control when manipulating complex 3D microstructures or allowing use in conjunction with other high-speed techniques.
GPC’s use with rapid galvanometric scanning mirrors, for example, allows trapping of massive arrays. Unlike speckled or discontinuous patterns, GPC-encoded light distributions with contiguous intensity and phase remain localised while propagating, enabling extended optical manipulation. The flat phase profile of the output also makes GPC convenient for certain volume-oriented applications such as counter-propagating optical traps that can catapult particles to a height of ~100µm. With its contiguous, speckle-free patterns and computationally simple encoding, GPC therefore finds increasing use in contemporary applications beyond optical trapping and manipulation such as in scanless two-photon optogenetics.
The synergy between nanotechnology, biotechnology and photonics has spawned the emerging field of nanobiophotonics. Optics and photonics already hurdle the diffraction barrier for imaging with nanoscopic resolutions, as celebrated by the Nobel Prize 2014 in Chemistry. However, scientific hypothesis testing demands tools, not only for passively observing nanoscopic phenomena, but also for actively reaching into and handling constituents in this tiny size domain.
We have promoted the idea of fabricating a new class of light-actuated microstructures via two-photon polymerisation and pioneered their use for so-called light robotics (light-driven micro-robotics). A strong scientific desire is to be able to combine advanced topology optimisation, microfabrication, surface functionalisation and spatio-temporal light manipulation to demonstrate the structure-mediated micro-to-nano coupling paradigm for controlled operation of robotic tools overcoming the diffraction limit while still being optically manoeuvrable and observable.
3D printing based on two-photon fabrication can already today create intricate features merged into larger structures that, in turn, are steerable by dynamic light trapping beams. Applying multiple independently controllable laser beam traps on these structures can enable real-time light-driven microrobotics with six degrees of freedom. This is already today pointing to new great discoveries by using calibrated steering of optimally shaped and functionalised tools at the sub-cellular level and in full 3D.