Fiber propagation of vector modes

Here we employ both dynamic and geometric phase control of light to produce radially modulated vector-vortex modes, the natural modes of optical fibers. We then measure these modes using a vector modal decomposition set-up as well as a tomography measurement, the latter providing a degree of the non-separability of the vector states, akin to an entanglement measure for quantum states. We demonstrate the versatility of the approach by creating the natural modes of a step-index fiber, which are known to exhibit strong mode coupling, and measure the modal cross-talk and non-separability decay during propagation. Our approach will be useful in mode division multiplexing schemes for transport of classical and quantum states.Comment: 6 pages, 4 figures, 1 tabl

Two-photon optics of Bessel-Gaussian modes

In this paper we consider geometrical two-photon optics of Bessel-Gaussian modes generated in spontaneous parametric down-conversion of a Gaussian pump beam. We provide a general theoretical expression for the orbital angular momentum (OAM) spectrum and Schmidt number in this basis and show how this may be varied by control over the radial degree of freedom, a continuous parameter in Bessel-Gaussian modes. As a test we first implement a back-projection technique to classically predict, by experiment, the quantum correlations for Bessel-Gaussian modes produced by three holographic masks, a blazed axicon, binary axicon and a binary Bessel function. We then proceed to test the theory on the down-converted photons using the binary Bessel mask. We experimentally quantify the number of usable OAM modes and confirm the theoretical prediction of a flattening in the OAM spectrum and a concomitant increase in the OAM bandwidth. The results have implications for the control of dimensionality in quantum states.Comment: 8 pages, 10 figure

Optical tweezing and micromanipulation.

Over the last 30 years, the ability to perform controlled manipulation of microscopic particles using only light, has become a prevalent technique. Many new and existing fields have benefited from the versatility that is offered by optical tweezers. This dissertation provides a theoretical description of the interaction of light with microscopic matter within an optical tweezer. An experimental investigation into the construction of an optical tweezer is described in detail, along with experimental results verifying the assembly of a fully functional and calibrated optical tweezer. The use of novel beam shapes including Bessel-Gauss, Laguerre-Gauss and super-Gaussian beams are applied to the tweezing setup, demonstrating additional manipulation methods. The procedures for generating each beam shape are discussed, together with the specific properties of the beam that provide further value to optical tweezing

Detection of Bessel beams with digital axicons

We propose a simple method for the detection of Bessel beams with arbitrary radial and azimuthal indices, and then demonstrate it in an all-digital setup with a spatial light modulator. We confirm that the fidelity of the detection method is very high, with modal cross-talk below 5%, even for high orbital angular momentum carrying fields with long propagation ranges. To illustrate the versatility of the approach we use it to observe the modal spectrum changes during the self-reconstruction process of Bessel beams after encountering an obstruction, as well as to characterize modal distortions of Bessel beams propagating through atmospheric turbulence

The importance of active-learning, student support, and peer teaching networks: A case study from the world’s longest COVID-19 lockdown in Melbourne, Australia

In the Australian state of Victoria, the city of Melbourne endured the world’s longest number of lockdown days, with severe government health orders and travel restrictions in place for extended periods of 2020 and 2021. In common with others, we found the provision of field teaching in introductory geology, structural geology, and volcanology, and the online replacement of practical instruction in petrology and petrography to be the greatest pedagogical challenges. We developed and used a range of different virtual field excursions that, given time and travel constraints imposed on us, were necessarily “low-tech” and non-immersive. Despite this, our students largely engaged enthusiastically with the virtual excursions, met many preexisting learning goals, and gained additional skills, particularly in regional-scale geological synthesis. In teaching petrology and petrography online, curated resources improved student understanding of some fundamental concepts, and it was advantageous that students were all assessed using identical imagery, rather than one sample from a non-identical class-set. On the other hand, we found we were less able to train students in the advanced skills of thin section interpretation. Assessment changes associated with online teaching have resulted in a permanent shift from low-level recall-style assessments to instead emphasizing higher-level synthesis and “geological thinking” skills. Our efforts throughout the pandemic demonstrated the value of instructor-student engagement and yielded teaching resources that have subsequently enhanced our face-to-face teaching and increased flexibility for students. Moreover, the COVID-19 pandemic has highlighted the importance of collaborative teaching practice and we have increasingly seen the benefits of local and national-scale teaching networks for peer support and for resource sharing

Vector Quality Measure for Vector Beams

Vector beams are spatial modes of light with spatially variant polarization states in the transverse profile. Over the years, vector beams have found their way into plenty of applications ranging from material processing and lithography to electron acceleration and particle trapping. Though qualitative measurements are routinely used to analyse vector beams, there is currently no quantitative measure for vector beam purity. Here, we introduce a new measure, the vector quality factor (VQF), that maps the purity of vector beams to a scale ranging from 0 to 1. We demonstrate a simple optical setup to generate and detect vector beams using a birefringent phase plate known as a q-plate. Tomographic measurements are performed by decomposing the vector beam into its circular basis states, and measuring the expectation values of the Pauli matrices as intensity measurements which, are used to evaluate the VQF of vector beams