Free-space Beam Research Articles

Holographically fabricated out-of-plane blazed gratings and channel waveguides in silica for integrated free-space beam delivery

Grating couplers are widely used in integrated optics to generate free-space beams and facilitate localized interactions with systems such as atom or ion traps. However, etched devices often exhibit small-scale inconsistencies; exacerbated by the high index contrast of the devices, this can lead to phase errors, limiting devices to a sub-millimeter scale. Here we present the first demonstration, to our knowledge, of tilted, out-of-plane blazed gratings in planar silica fabricated by UV inscription using a 213 nm laser. Our devices deliver collimated and focusing beams into free space from a waveguide input without the need for additional optics such as beam expanders.

Optimal Design of Small-Aperture Optical Terminals for Free-Space Links

We present the generalised design of low-complexity, small-aperture optical terminals intended for kilometre-scale, terrestrial, free-space laser links between fixed and dynamic targets. The design features single-mode fibre coupling of the free-space beam, assisted by a fast-steering, tip/tilt mirror that enables first-order turbulence suppression and fine target tracking. The total power throughput over the free-space link and the scintillation index in fibre are optimised. The optimal tip/tilt correction bandwidth and range, aperture size, and focal length for a given link are derived using analytical atmospheric turbulence modelling and numerical simulations.

Reconfigurable unitary transformations of optical beam arrays

Spatial transformations of light are ubiquitous in optics, with examples ranging from simple imaging with a lens to quantum and classical information processing in waveguide meshes. Multi-plane light converter (MPLC) systems have emerged as a platform that promises completely general spatial transformations, i.e., a universal unitary. However, until now, MPLC systems have demonstrated transformations that are far from general, e.g., converting from a Gaussian to Laguerre-Gauss mode. Here, we demonstrate the promise of an MLPC, the ability to impose an arbitrary unitary transformation that can be reconfigured dynamically. Specifically, we consider transformations on superpositions of parallel free-space beams arranged in an array, which is a common information encoding in photonics. We experimentally test the full gamut of unitary transformations for a system of two parallel beams and make a map of their fidelity. We obtain an average transformation fidelity of 0.85 ± 0.03. This high-fidelity suggests that MPLCs are a useful tool for implementing the unitary transformations that comprise quantum and classical information processing.

Gain-enhanced suspended optomechanical system with tunable dissipative coupling strength

Active cavity optomechanical system provides an invaluable physical platform for cavity optomechanics research, particularly those involving dissipative coupling, which holds significant potential for advancing the field of quantum physics. In our previous work, an active levitated optomechanical system was established for the first time [Nat. Phys 19, 414 (2023)10.1038/s41567-022-01857-9]. Here we report a gain-enhanced suspended optomechanical system based on the dissipative coupling between the SiN membrane and the intracavity laser. This system has a high dissipative coupling strength which is widely tunable through simple mechanical adjustments. Moreover, the influence of pumping power and the propagation distance of the free-space beam on the maximum effective dissipative coupling strength is comprehensively investigated. Based on the numerical discussion, we propose effective methods to enhance the dissipative coupling experimentally. The active suspended cavity optomechanical system has great potential in realizing the cooling of the membrane to the quantum ground state or heating the membrane to produce phonon lasers, which can be applied to such cutting-edge fields as quantum precision measurements, macroscopic quantum state, and information transmission and processing.

Traceable characterisation of fibre-coupled single-photon detectors

Abstract The detection of single photons plays an essential role in advancing single-photon science and technologies. Yet, within the visible/near-infrared spectral region, accurate fibre-based optical power measurements at the few-photon level are not yet well-established. In this study, we report on a fibre-based setup, enabling traceable optical power measurements at the few-photon level in this spectral region. The setup was used to calibrate the detection efficiency (DE) of four single-photon avalanche diode (SPAD) detectors. The relative standard uncertainties on the mean DE values obtained from repeat fibre-to-detector couplings ranged from 0.67% to 0.81% (k = 2). However, the relative standard deviation of DE values, which ranged from 1.38% to 3.20% (k = 2), poses a challenge for the metrology of these devices and applications that require high accuracy and repeatability. We investigated the source of these variations by spatially mapping the response of a detector’s fibre connector port, using a focused free-space beam, allowing us to estimate the detector’s spatial non-uniformity. In addition, we realise a novel calibration approach for fibre-coupled SPADs in a free-space configuration, enabling a direct comparison between the fibre-based setup and the National Physical Laboratory’s established free-space facility using a single SPAD. Finally, we investigated alternative coupling methods, testing the repeatability of different fibre-to-fibre connectors in addition to direct fibre-to-detector couplings: SPADs from three manufacturers were tested, with both single-mode and multi-mode fibre.

Fluorescence collection efficiency of atoms in dipole traps.

The fluorescence collection from single atoms and emitters has been extensively utilized in quantum information and quantum optics research. Here, we investigated the collection efficiency of an objective lens by drawing an analogy between the free-space beam (FSB) and a waveguide mode. We explored how efficiency is influenced by their thermal motion within a dipole trap. Furthermore, we introduce an effective energy fraction ratio to quantify potential imperfections in the focusing of the objective lens. Our results provide valuable insights for optimizing the fluorescence collection in single-atom experiments and highlight the importance of considering realistic experimental conditions when estimating achievable efficiencies.

Topological Structures of Energy Flow: Poynting Vector Skyrmions.

Topological properties of energy flow of light are fundamentally interesting and may introduce novel physical phenomena associated with directional light scattering and optical trapping. In this Letter, skyrmionlike structures formed by Poynting vectors are unveiled in the focal region of two pairs of counterpropagating cylindrical vector vortex beams in free space. The appearance of local phase singularities, and the distinct traveling and standing wave modes of different field components passing through the focal spot lead to a Néel-Bloch-Néel transition of Poynting vector skyrmion textures along the light propagating direction. By shaping the wave front of the incident beams with patterned amplitudes, the topological invariant of the Poynting vector skyrmions can be further tuned in a prospective area. This work expands the family of optical skyrmions and holds great potential in energy flow associated applications.

Morphology-independent general-purpose optical surface tractor beam

Optical tractor beams capable of pulling particles backward have garnered significant and increasing interest. Traditional optical tractor beams are limited to free space beams with small forward wavevectors, enabling them to pull selected particles. Here, we present a comprehensive theory for the optical force exerted by a surface wave using analytical and numerical calculations, revealing the relationship between the canonical momentum and optical forces. Based on this theory, we demonstrate a general purpose optical surface tractor beam that can pull any passive particle, regardless of size, composition, or geometry. The tractor beam utilizes a surface wave with negative canonical momentum characterized by a single well-defined negative Bloch k vector. The tractor beam relies on a mechanism where the negative incident force always surpasses the recoil force. As such, the tractor beam, when excited on the surface of a double-negative index metamaterial, can pull particles with different morphologies.

Free-Space Beam Steering with Twisted Bilayer Photonic Crystal Slabs

Free-Space Beam Steering with Twisted Bilayer Photonic Crystal Slabs

Trapped Atoms and Superradiance on an Integrated Nanophotonic Microring Circuit

Interfacing cold atoms with integrated nanophotonic devices could offer new paradigms for engineering atom-light interactions and provide a potentially scalable route for quantum sensing, metrology, and quantum information processing. However, it remains a challenging task to efficiently trap a large ensemble of cold atoms on an integrated nanophotonic circuit. Here, we demonstrate direct loading of an ensemble of up to 70 atoms into an optical microtrap on a nanophotonic microring circuit. Efficient trap loading is achieved by employing degenerate Raman-sideband cooling in the microtrap, where a built-in spin-motion coupling arises directly from the vector light shift of the evanescent-field potential on a microring. Atoms are cooled into the trap via optical pumping with a single free space beam. We have achieved a trap lifetime approaching 700 ms under continuous cooling. We show that the trapped atoms display large cooperative coupling and superradiant decay into a whispering-gallery mode of the microring resonator, holding promise for explorations of new collective effects. Our technique can be extended to trapping a large ensemble of cold atoms on nanophotonic circuits for various quantum applications. Published by the American Physical Society 2024

Propagation behaviors of two-dimensional chirped finite-energy Pearcey beams in free space

We introduce two-dimensional chirped finite energy Pearcey beams (FEPBs) for the first time and investigate the propagation dynamics. First, we applied the Huygens–Fresnel integral to derive an explicit analytical expression which is suitable for describing FEPB propagation in free space. It is interesting to find that FEPBs will experience three typical propagation patterns, i.e. the single-autofocusing case, dual-autofocusing case and non-autofocusing diffraction case, only depending on the value of the input asymmetric chirp. We further arrive at the critical condition of these three patterns analytically. However, by changing the sign, another input symmetric chirp acts to strengthen or weaken the autofocusing intensity but does not affect the focal distance. Our findings suggest that two-dimensional chirped FEPBs have more potential in controlling linear self-focusing and optical particle manipulation, when compared with the corresponding Airy field or conventional Gaussian field.

Strong diffraction effects accompany the transmission of a laser beam through inhomogeneous plasma microstructures.

In the study we thoroughly analyze diffraction effects accompanying the laser beam transmission through inhomogeneous plasma microstructures and simulate their diffraction patterns at the object output and in the near field. For this we solve the scalar Helmholtz wave equationin the first Rytov approximation and compute the diffraction spreading of the transmitted beam in free space. Diffraction effects are found to arise within the beam passage through inhomogeneous plasma microstructures even in the simplest approximations of the laser beam interaction with plasma. These effects become strong in the near-field region and significantly distort the patterns of plasma formations, as well as facilitate the appearance of various optical artifacts in the plasma images. By performing numerical simulations, we characterize in detail the features of the visualization of plasma formations in the field of a coherent laser beam registered by a lens system. The calculations are in good agreement with the experimental data. The study can find broad applications in the processing of the laser images of plasma microstructures registered by lens systems in the presence of strong diffraction effects.

Integrated optical phased array with on-chip amplification enabling programmable beam shaping

We present an integrated optical phased array (OPA) which embeds in-line optical amplifiers and phase modulators to provide beam-forming capability with gain and beam steering in the 1465–1590 nm wavelength range. We demonstrate up to 21.5 dB net on-chip gain and up to 35.5 mW optical output power. The OPA circuit is based on an InP photonic integration platform and features the highest measured on-chip gain and output power level recorded in an active OPA (i.e., with amplification), to the best of our knowledge. Furthermore, the OPA enables the independent control of both amplitude and phase in its arms and through this we demonstrate programmable beam shaping for two cases. First, we carried out a Gaussian apodization of the power distribution profile in the OPA emitter waveguides, leading to 19.8 dB sidelobe suppression in the far-field beam, which is the highest value recorded for active OPAs, and then we demonstrated beam forming of 0th, 1st, and 2nd order 1D Hermite–Gaussian beams in free-space.

Multiplication of the orbital angular momentum of phonon polaritons via sublinear dispersion

Optical vortices (OVs) promise to greatly enhance optical information capacity via orbital angular momentum multiplexing. The need for the on-chip integration of orbital angular momentum technologies has prompted research into subwavelength-confined polaritonic OVs. However, the topological order imprinted by the structure used for transduction from free-space beams to surface polaritons is inherently fixed after fabrication. Here we overcome this limitation via dispersion-driven topological charge multiplication. We switch the OV topological charge within a small frequency range (~3%) by leveraging the strong sublinear dispersion of low-loss surface phonon polaritons on silicon carbide membranes. Applying the Huygens principle, we quantitatively evaluate the topological order of experimental OVs detected by near-field imaging. We further explore the deuterogenic effect, which predicts the coexistence of multiple topological charges in higher-order polaritonic OVs. Our work demonstrates a viable method to manipulate the topological charge of polaritonic OVs, paving the way for the exploration of novel orbital-angular-momentum-enabled light–matter interactions at mid-infrared frequencies.

Improving fiber coupling efficiency by shaping the transmission trajectory of the vortex beam

Vortex beams with orbital angular momentum (OAM) have the potential to significantly increase the communication capacity in long-distance free-space optical communication systems. In order to obtain optimal coupling efficiency, few-mode fibers (FMFs) are employed to receive spatial light. This paper presents a computer model that describes the process of coupling vortex beams into FMFs after they have been transmitted via free space. We propose to improve the coupling efficiency by controlling the transmission trajectory of the vortex beam in free space. The coupling efficiency of a vortex beam into FMFs can be enhanced at a specific transmission distance by pre-designing the transmission trajectory of the optical field and tweaking the parameters of the coupling lens and fiber. This paper also examines the coupling correlation between higher-order OAM modes vortex beams and FMF modes. The numerical results of this study show that controlling the radial phase distribution to manage the transmission trajectory of the optical field is an effective and flexible method for improving the efficiency of coupling light between space and fiber. The results of this study can be applied in the domain of free-space optical communications.

Airy‐Like Hyperbolic Shear Polaritons in High Symmetry van der Waals Crystals

AbstractControlling light at the nanoscale by exploiting ultra‐confined polaritons—hybrid light and matter waves—in various van der Waals (vdW) materials empowers unique opportunities for many nanophotonic on‐chip technologies. So far, mainstream approaches have relied on interfacial techniques (e.g., refractive optics, meta‐optics, and moire engineering) to manipulate the polariton wavefront. Here, it is proposed that orbital angular momentum (OAM) of incident light can offer a new degree of freedom to structure vdW polaritons. With vortex excitations, a new class of accelerating polariton waves is observed—Airy‐like hyperbolic phonon polaritons (PhPs) in high‐symmetry orthorhombic vdW crystal α‐MoO3. In analogous to the well‐known Airy beams in free space, such Airy‐like PhPs also exhibit self‐accelerating, nonspreading, and self‐healing characteristics. Interestingly, the helical phase gradient of the vortex beam leads to asymmetry excitation of polaritons, as a result, the Airy‐like PhPs possess asymmetric propagation features even with a symmetric mode, analogous to the asymmetry hyperbolic shear polaritons in low‐symmetry crystals. The finding highlights the potential of OAM to manipulate polaritons in vdW materials, which can be further extended into a variety of applications such as active structured polaritonic devices.

Sub-wavelength scale characterization of on-chip coupling mirrors.

Miniature free-space optical beams, originating from on-chip microstructures, are usually measured and quoted without reference to a particular polarization state. We develop an automated platform to characterize tightly focused free-space optical beams in three dimensions. We present a detailed description of each subsystem including the calibration and test procedure. We demonstrate how amplitude and phase are measured at sub-wavelength resolution using a cleaved fiber with a heterodyne reference. Further analysis provides information about the phase and intensity profile of the beam with regards to its polarization content and spatial confinement. We perform a proof-of-concept experiment for a custom waveguide-coupled micro-mirror. The work opens new possibilities for rapid analysis of micro-mirrors in prototyping and optimization of integrated optical systems.

Generating free-space structured light with programmable integrated photonics

Structured light is a key component of many modern applications, ranging from super-resolution microscopy to imaging, sensing and quantum information processing. As the utilization of these powerful tools continues to spread, the demand for technologies that enable the spatial manipulation of fundamental properties of light, such as amplitude, phase and polarization grows further. The rapidly advancing field of reconfigurable integrated photonics allows entirely new routes towards beam shaping that not only outperform existing devices in terms of speed but also have substantial potential with respect to their footprints, robustness and conversion efficiencies. In this study, we demonstrate how a multipurpose programmable integrated photonic processor can generate and control a wide range of higher-order free-space structured light beams, all starting from only a single injection waveguide. Our method controls the distribution of light’s amplitude and phase within sub-milliseconds, and it is fully reconfigurable and has no moving parts. These advancements broaden the spectrum of potential methods, applications and devices that utilize spatially tailored light by providing a pathway to combine the strengths and versatility of integrated photonics and free-space structured light.

Propagation properties of specular and antispecular twisted Gaussian Schell-model beams

We introduce a class of specular and antispecular twisted Gaussian Schell-model beams, which are generated by inserting a twisted Gaussian Schell-model beam into a wavefront folding interferometer (WFI). The analytical expression for the cross-spectral density function of the beam propagating in free space is derived, and the statistical properties of the optical field are investigated in detail. The results show that the twisted effect is still maintained after the transformation, and the spectral density of the light field always rotates to 90 degrees around the axis during propagation. Furthermore, with appropriate optical field adjustment, the twist effect of the spectral degree of coherence (DOC) can be observed, but in opposite directions to the irradiance profile. We also find that the twisted phase not only controls the rotation of the field, but also effectively modulates the overall spot contour. For the far-field spectral density distribution, a larger twist effect will induce a smaller ellipticity of the beam spot. However, the intensity pattern in the central area is mainly determined by the phase difference of WFI. To be specific, the specular twisted field always has a sharp central peak during propagation, and in the antispecular case it has a central dip. Besides, the DOC distribution can be flexibly adjusted by the source coherence, the twisted phase and the phase difference of the WFI. The results of our work have important applications in the fields of free-space beam communication and particle trapping.

Learning-enabled recognition of LG beams from multimode fiber specklegrams

The use of orbital angular momentum (OAM) beams, like Laguerre Gaussian (LG) beams is a significant advancement in optical communications, enhancing versatility in data encoding. In optical communications, especially OAM multiplexing, previous studies have successfully recognized LG beams in free space. However, free space propagation presents challenges including power loss, beam distortion, wandering, and physical obstacles. To overcome these hurdles, we introduce a novel method using speckle patterns (specklegrams) within a multimode fiber (MMF) for LG beam recognition through deep learning. Our study focuses on recognizing 15 LGp|l| beams (0≤p,|l|≤3; excluding LG00) generated with a spatial light modulator (SLM) and launched into a standard silica MMF with a 200 µm core diameter, producing corresponding speckle patterns. We utilize the widely-used deep convolutional neural network (CNN) model, AlexNet, to train these specklegram images, achieving an impressive 91 % recognition accuracy. Furthermore, we assess the recognition accuracy of 15 LGp(-l) modes, obtaining an 86.5 % accuracy rate. We also explore the impact of external perturbations, such as transverse weights (ranging from 0 to 3 kg in 1 kg increments) and temperature variations (ranging from 50 °C to 150 °C in 10 °C steps) on a section of MMF. In both scenarios, recognition accuracies are > 80 %. These findings highlight the effectiveness of our proposed approach, promising high-capacity data transmission in optical communications.