Two different experimental techniques for preparing and analysing superpositions of Gaussian and Laguerre-Gaussian modes are presented. These involve exploiting an interferometric method in one case and using computer-generated holograms in the other. It is shown that by shifting a hologram with respect to an incoming Gaussian beam, different superpositions of the Gaussian and the Laguerre-Gaussian beam can be produced. An analytical expression connecting the relative phase, the amplitudes of the modes and the displacement of the hologram is given. The application of such orbital angular momenta superpositions in quantum experiments such as quantum cryptography is discussed.

Superpositions of the orbital angular momentum for applications in quantum experiments : Special issue on atoms and angular momentum of light

Optical communication is an integral part of the modern economy, having all but replaced electronic communication systems. Future growth in bandwidth appears to be on the horizon using structured light, encoding information into the spatial modes of light, and transmitting them down fibre and free-space, the latter crucial for addressing last mile and digitally disconnected communities. Unfortunately, patterns of light are easily distorted, and in the case of free-space optical communication, turbulence is a significant barrier. Here we review recent progress in structured light in turbulence, first with a tutorial style summary of the core concepts, before highlighting the present state-of-the-art in the field. We support our review with new experimental studies that reveal which types of structured light are best in turbulence, the behaviour of vector versus scalar light in turbulence, the trade-off of diversity and multiplexing, and how turbulence models can be exploited for enhanced optical signal processing protocols. This comprehensive treatise will be invaluable to the large communities interested in free-space optical communication with spatial modes of light.

Structured Light in Turbulence

On‐chip integrated micro‐ and nanoscale vortex lasers are key elements for addressing the exponentially growing demand for information capacity. Although tunable vortex microlasers have been reported, each laser pulse still possesses one particular topological charge and requires additional multiplexing. In this study, the simultaneous generation of coherent laser arrays with different topological charges by combining metalenses with semiconductor microlasers is demonstrated. A TiO2 vortex metalens converts two orbital angular momentum beams into the same diffraction‐limit spot with opposite circular polarizations through spin‐to‐orbital conversion. Consequently, the microlaser emission at the focal spot, which can be decomposed into two circularly polarized beams, is collimated into vortex microlaser beams with two different topological charges through a time‐reversal process. This concept is extended to a 2 × 2 metalens array by introducing off‐axis terms, and eight topological charges are produced simultaneously. This research is a significant step toward the on‐chip integration of micro‐ and nanolasers.

All‐Dielectric Metasurface‐Enabled Multiple Vortex Emissions

Structured light has become topical of late, where controlling light in all its degrees of freedom has offered novel states of light long predicted, enhanced functionality in applications, and a modern toolbox for probing fundamental science. Structuring light as single photons and entangled states allows the spatial modes of light to be used to encode a large alphabet, accessing high dimensional Hilbert spaces for fundamental tests of quantum mechanics and improved quantum information processing tasks. In this tutorial, we outline the basic concepts of high dimensional quantum states expressed in a basis of spatial modes (structured light) and explain how to create, control, and detect such quantum states in the laboratory with a focus on transverse spatial modes such as the orbital angular momentum and pixel (position) modes. Finally, we highlight some example applications of such quantum structured light, from communications to imaging.

/pdf/quantum-structured-light-in-high-dimensions-14706z9t.pdf

Quantum structured light in high dimensions

High-power vortex light generated directly from lasers will help drive their applications in material processing, optical manipulation, levitation, particle acceleration, and communications, but limited power has been achieved to date. In this work, we demonstrate record vortex average power of 31.3 W directly from a laser, to the best of our knowledge, using an interferometric mode transforming output coupler to convert a fundamental mode Nd:YVO4 laser into a LG01 vortex output. The vortex laser was Q-switched with up to 600 kHz pulse rate with a high slope efficiency of 62.5% and an excellent LG01 modal purity of 95.2%. We further demonstrate > 30W laser power in a high quality HG10 mode by simple adjustment of the output coupler. Experimental investigations of varying output coupling transmission are compared with theory. This successful implementation of the interferometric output coupler in a high power system demonstrates the suitability of the mode transforming method for robust turn-key vortex lasers with high efficiency and high modal purity, with scalable power and pulse rate.

30 W vortex LG01 or HG10 laser using a mode transforming output coupler.

Controlling light with subwavelength-designed metasurfaces (MSs) has allowed for the arbitrary creation of structured light by precisely engineered matter. We report on the purity and conversion efficiency of hybrid orbital angular momentum (OAM)-generating MSs. We use a recently reported method to design and fabricate meta-surfaces that exploit generalized spin-orbit coupling to produce vector OAM states with asymmetric OAM superpositions, e.g., 1 and 5, coupled to linear and circular polarization states, fractional vector OAM states with OAM values of 3.5 and 6.5, and also the common conjugate spin and OAM of ±1 as reported in previous spin-orbit coupling devices. The OAM and radial modes in the resulting beams are quantitatively studied by implementing a modal decomposition approach, establishing both purity and conversion efficiency. We find conversion efficiencies exceeding 75% and purities in excess of 95%. A phase-flattening approach reveals that the OAM purity can be very low due to the presence of undesired radial components. We characterize the effect and illustrate how to suppress the undesired radial modes.

Purity and efficiency of hybrid orbital angular momentum-generating metasurfaces

The mid-infrared and far-infrared regions of the spectrum (3–300 μm) for a long time did not realize its full potential, scientifically and technologically, notwithstanding its paramount importance for chemistry, spectroscopy and a myriad of potential applications, mainly because of the lack of a compact, room temperature, widely tunable and eye-safe laser. This situation drastically changed with the advent of the Quantum Cascade Laser (QCL) in 1994 and its subsequent momentous development in the following two decades [1–4]. I will highlight the fundamentally different design principles and underlying physics of QCLs, which showcase a rich variety of optical and transport phenomena. I will illustrate the impact of QCLs in spectroscopy, chemical physics and atmospheric chemistry and technology, present and future, ranging from IR countermeasures to a wide range of sensor applications including health care, the environment, security and, ultimately, the “internet of everything”.

https://www.osapublishing.org/viewmedia.cfm?uri=ASSL-2016-JM1A.1&seq=0

High Performance Quantum Cascade Lasers from the Mid-IR to the Far-IR and their impact on Science and Technology

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Purity and efficiency of hybrid orbital angular momentum-generating metasurfaces

High Performance Quantum Cascade Lasers from the Mid-IR to the Far-IR and their impact on Science and Technology