Building hybridized 28-baseline pupil-remapping photonic interferometers for future high-resolution imaging.

One key advantage of single-mode photonic technologies for interferometric use is their ability to easily scale to an ever-increasing number of inputs without a major increase in the overall device size, compared to traditional bulk optics. This is particularly important for the upcoming extremely large telescope (ELT) generation of telescopes currently under construction. We demonstrate the fabrication and characterization of a hybridized photonic interferometer, with eight simultaneous inputs, forming 28 baselines, which is the largest amount to date, to the best of our knowledge. Using different photonic fabrication technologies, we combine a 3D pupil remapper with a planar eight-port ABCD pairwise beam combiner, along with the injection optics necessary for telescope use, into a single integrated monolithic device. We successfully realized a combined device called Dragonfly, which demonstrates a raw instrumental closure-phase stability down to 0.9° over $8\pi$ phase piston error, relating to a detection contrast of ${\sim}6.5 \times {10^{- 4}}$ on an adaptive-optics-corrected 8 m telescope. This prototype successfully demonstrates advanced hybridization and packaging techniques necessary for on-sky use for high-contrast detection at small inner working angles, ideally complementing what can currently be achieved using coronagraphs.

Divide and conquer: an efficient solution to highly multimoded photonic lanterns from multicore fibres.

Photonic lanterns typically allow for single-mode action in a multimode fibre. Since their invention over a decade ago for applications in astrophotonics, they have found important uses in diverse fields of applied science. To date, large aperture highly-mulitmoded to single-mode lanterns have been difficult as fabrication techniques are not practical for mass replication. Here as a proof of concept, we demonstrate three different devices based on multicore fibre photonic lanterns with: 100µm core diameters; NAs = 0.16 and 0.15; and requiring 259 single-mode core system, specifically 7 multicore fibres each with 37 cores, instead of 259 individual single-mode fibres. The average insertion loss excluding coupling efficiencies is only 0.4dB (>91% transmission). This concept has numerous advantages, in particular, (i) it is a direct scaleable solution, (ii) eases imprinting of photonic functions, e.g. fibre Bragg gratings; and (iii) new approach for large-area optical fibre slicers for future large-aperture telescopes.

Measurement and limitations of optical orbital angular momentum through corrected atmospheric turbulence.

In recent years, there have been a series of proposals to exploit the orbital angular momentum (OAM) of light for astronomical applications. The OAM of light potentially represents a new way in which to probe the universe. The study of this property of light entails the development of new instrumentation and problems which must be addressed. One of the key issues is whether we can overcome the loss of the information carried by OAM due to atmospheric turbulence. We experimentally analyze the effect of atmospheric turbulence on the OAM content of a signal over a range of realistic turbulence strengths typical for astronomical observations. With an adaptive optics system we are able to recover up to 89% power in an initial non-zero OAM mode (ℓ = 1) at low turbulence strengths (0.30" FWHM seeing). However, for poorer seeing conditions (1.1" FWHM seeing), the amount of power recovered is significantly lower (5%), showing that for the terrestrial detection of astronomical OAM, a careful design of the adaptive optics system is needed.

Low loss mid-infrared ZBLAN waveguides for future astronomical applications.

Photonic technologies will be at the heart of future terrestrial planet hunting interferometers. In particular the mid-infrared spectral region between 3.5 - 4.2 µm is the ideal window for hunting for young extra-solar planets, since the planet is still hot from its formation and thus offers a favorable contrast with respect to the parent star compared to other spectral regions. This paper demonstrates two basic photonic building blocks of such an instrument, namely single-mode waveguides with propagation losses as low as 0.29±0.03 dB/cm at a wavelength of 4 µm as well as directional couplers with a constant splitting ratio across a broad wavelength band of 500 nm. The devices are based on depressed cladding waveguides inscribed into ZBLAN glass using the femtosecond laser direct-write technique. This demonstration is the first stepping stone towards the realization of a high transmission mid-infrared nulling interferometer.

Demonstration of uniform multicore fiber Bragg gratings.

Fiber Bragg gratings in multicore fibers have significant potential as compact and robust filters for research and commercial applications. With the aid of an innovative, flat-fielded Mach-Zehnder interferometer, we demonstrate deep (>30 dB) narrow (100 pm at 3 dB; 90 pm at 10 dB) notches in the outer 6 cores of a 7-core fiber at a constant wavelength ( ± 15 pm). This is a crucial step in the development of FBGs operating within multimode fibers that carry an arbitrary number of spatial modes.

High-performance 3D waveguide architecture for astronomical pupil-remapping interferometry.

The detection and characterization of extra-solar planets is a major theme driving modern astronomy. Direct imaging of exoplanets allows access to a parameter space complementary to other detection methods, and potentially the characterization of exoplanetary atmospheres and surfaces. However achieving the required levels of performance with direct imaging from ground-based telescopes (subject to Earth's turbulent atmosphere) has been extremely challenging. Here we demonstrate a new generation of photonic pupil-remapping devices which build upon the Dragonfly instrument, a high contrast waveguide-based interferometer. This new generation overcomes problems caused by interference from unguided light and low throughput. Closure phase measurement scatter of only ∼ 0.2° has been achieved, with waveguide throughputs of > 70%. This translates to a maximum contrast-ratio sensitivity between star and planet at 1λ/D (1σ detection) of 5.3 × 10(-4) (with a conventional adaptive-optics system) or 1.8 × 10(-4) (with 'extreme-AO'), improving even further when random error is minimized by averaging over multiple exposures. This is an order of magnitude beyond conventional pupil-segmenting interferometry techniques (such as aperture masking), allowing a previously inaccessible part of the star to planet contrast-separation parameter space to be explored.

Integrated photonic building blocks for next-generation astronomical instrumentation II: the multimode to single mode transition.

There are numerous advantages to exploiting diffraction-limited instrumentation at astronomical observatories, which include smaller footprints, less mechanical and thermal instabilities and high levels of performance. To realize such instrumentation it is imperative to convert the atmospheric seeing-limited signal that is captured by the telescope into a diffraction-limited signal. This process can be achieved photonically by using a mode reformatting device known as a photonic lantern that performs a multimode to single-mode transition. With the aim of developing an optimized integrated photonic lantern, we undertook a systematic parameter scan of devices fabricated by the femtosecond laser direct-write technique. The devices were designed for operation around 1.55 µm. The devices showed (coupling and transition) losses of less than 5% for F/# ≥ 12 injection and the total device throughput (including substrate absorption) as high as 75-80%. Such devices show great promise for future use in astronomy.

Developing arrayed waveguide grating spectrographs for multi-object astronomical spectroscopy.

With the aim of utilizing arrayed waveguide gratings for multi-object spectroscopy in the field of astronomy, we outline several ways in which standard telecommunications grade chips should be modified. In particular, by removing the parabolic-horn taper or multimode interference coupler, and injecting with an optical fiber directly, the resolving power was increased threefold from 2400 ± 200 (spectral resolution of 0.63 ± 0.2 nm) to 7000 ± 700 (0.22 ± 0.02 nm) while attaining a throughput of 77 ± 5%. More importantly, the removal of the taper enabled simultaneous off-axis injection from multiple fibers, significantly increasing the number of spectra that can be obtained at once (i.e. the observing efficiency). Here we report that ~12 fibers can be injected simultaneously within the free spectral range of our device, with a 20% reduction in resolving power for fibers placed at 0.8 mm off-centre.

Design of optically path-length-matched, three-dimensional photonic circuits comprising uniquely routed waveguides.

A method for designing physically path-length-matched, three-dimensional photonic circuits is described. We focus specifically on the case in which all the waveguides are uniquely routed from the input to output-a problem that has not been addressed to date and that allows for the waveguides to be used in interferometric measurements. Circuit elements were fabricated via the femtosecond laser direct-write technique. We demonstrate via interferometric methods that the fabricated circuits were indeed optically path-length matched to within 45 µm, which is within the coherence length required for many applications.

Scalable photonic-based nulling interferometry with the dispersed multi-baseline GLINT instrument.

Characterisation of exoplanets is key to understanding their formation, composition and potential for life. Nulling interferometry, combined with extreme adaptive optics, is among the most promising techniques to advance this goal. We present an integrated-optic nuller whose design is directly scalable to future science-ready interferometric nullers: the Guided-Light Interferometric Nulling Technology, deployed at the Subaru Telescope. It combines four beams and delivers spatial and spectral information. We demonstrate the capability of the instrument, achieving a null depth better than 10-3 with a precision of 10-4 for all baselines, in laboratory conditions with simulated seeing applied. On sky, the instrument delivered angular diameter measurements of stars that were 2.5 times smaller than the diffraction limit of the telescope. These successes pave the way for future design enhancements: scaling to more baselines, improved photonic component and handling low-order atmospheric aberration within the instrument, all of which will contribute to enhance sensitivity and precision.

Exceptional astronomical seeing conditions above Dome C in Antarctica.

One of the most important considerations when planning the next generation of ground-based optical astronomical telescopes is to choose a site that has excellent 'seeing'--the jitter in the apparent position of a star that is caused by light bending as it passes through regions of differing refractive index in the Earth's atmosphere. The best mid-latitude sites have a median seeing ranging from 0.5 to 1.0 arcsec (refs 1-5). Sites on the Antarctic plateau have unique atmospheric properties that make them worth investigating as potential observatory locations. Previous testing at the US Amundsen-Scott South Pole Station has, however, demonstrated poor seeing, averaging 1.8 arcsec (refs 6, 7). Here we report observations of the wintertime seeing from Dome C (ref. 8), a high point on the Antarctic plateau at a latitude of 75 degrees S. The results are remarkable: the median seeing is 0.27 arcsec, and below 0.15 arcsec 25 per cent of the time. A telescope placed at Dome C would compete with one that is 2 to 3 times larger at the best mid-latitude observatories, and an interferometer based at this site could work on projects that would otherwise require a space mission.

Coherent band excitations in CePd3: A comparison of neutron scattering and ab initio theory.

In common with many strongly correlated electron systems, intermediate valence compounds are believed to display a crossover from a high-temperature regime of incoherently fluctuating local moments to a low-temperature regime of coherent hybridized bands. We show that inelastic neutron scattering measurements of the dynamic magnetic susceptibility of CePd3 provides a benchmark for ab initio calculations based on dynamical mean field theory. The magnetic response is strongly momentum dependent thanks to the formation of coherent f-electron bands at low temperature, with an amplitude that is strongly enhanced by local particle-hole interactions. The agreement between experiment and theory shows that we have a robust first-principles understanding of the temperature dependence of f-electron coherence.

Writing Bragg Gratings in Multicore Fibers.

Fiber Bragg gratings in multicore fibers can be used as compact and robust filters in astronomical and other research and commercial applications. Strong suppression at a single wavelength requires that all cores have matching transmission profiles. These gratings cannot be inscribed using the same method as for single-core fibers because the curved surface of the cladding acts as a lens, focusing the incoming UV laser beam and causing variations in exposure between cores. Therefore we use an additional optical element to ensure that the beam shape does not change while passing through the cross-section of the multicore fiber. This consists of a glass capillary tube which has been polished flat on one side, which is then placed over the section of the fiber to be inscribed. The laser beam enters the fiber through the flat surface of the capillary tube and hence maintains its original dimensions. This paper demonstrates the improvements in core-to-core uniformity for a 7-core fiber using this method. The technique can be generalized to larger multicore fibers.

Net primary productivity estimates and environmental variables in the Arctic Ocean: An assessment of coupled physical-biogeochemical models.

The relative skill of 21 regional and global biogeochemical models was assessed in terms of how well the models reproduced observed net primary productivity (NPP) and environmental variables such as nitrate concentration (NO3), mixed layer depth (MLD), euphotic layer depth (Zeu), and sea ice concentration, by comparing results against a newly updated, quality-controlled in situ NPP database for the Arctic Ocean (1959-2011). The models broadly captured the spatial features of integrated NPP (iNPP) on a pan-Arctic scale. Most models underestimated iNPP by varying degrees in spite of overestimating surface NO3, MLD, and Zeu throughout the regions. Among the models, iNPP exhibited little difference over sea ice condition (ice-free versus ice-influenced) and bottom depth (shelf versus deep ocean). The models performed relatively well for the most recent decade and toward the end of Arctic summer. In the Barents and Greenland Seas, regional model skill of surface NO3 was best associated with how well MLD was reproduced. Regionally, iNPP was relatively well simulated in the Beaufort Sea and the central Arctic Basin, where in situ NPP is low and nutrients are mostly depleted. Models performed less well at simulating iNPP in the Greenland and Chukchi Seas, despite the higher model skill in MLD and sea ice concentration, respectively. iNPP model skill was constrained by different factors in different Arctic Ocean regions. Our study suggests that better parameterization of biological and ecological microbial rates (phytoplankton growth and zooplankton grazing) are needed for improved Arctic Ocean biogeochemical modeling.

Disorder and intermolecular interactions in a family of tetranuclear Ni(II) complexes probed by high-frequency electron paramagnetic resonance.

High-frequency electron paramagnetic resonance (HFEPR) data are presented for four closely related tetranuclear Ni(II) complexes, [Ni(hmp)(MeOH)Cl]4.H2O (1a), [Ni(hmp)(MeOH)Br]4.H2O (1b), [Ni(hmp)(EtOH)Cl]4.H2O (2), and [Ni(hmp)(dmb)Cl]4 (3) (where hmp(-) is the anion of 2-hydroxymethylpyridine and dmb is 3,3'-dimethyl-1-butanol), which exhibit magnetic bistability (hysteresis) and fast magnetization tunneling at low temperatures, properties which suggest they are single-molecule magnets (SMMs). The HFEPR spectra confirm spin S = 4 ground states and dominant uniaxial anisotropy (DSz(2), D < 0) for all four complexes, which are the essential ingredients for a SMM. The individual fine structure peaks (due to zero-field splitting) for complexes 1a, 1b, and 2 are rather broad. They also exhibit further (significant) splitting, which can be explained by the fact that there exists two crystallographically distinct Ni 4 sites in the lattices for these complexes, with associated differences in metal-ligand bond lengths and different zero-field splitting (ZFS) parameters. The broad EPR lines, meanwhile, may be attributed to ligand and solvent disorder, which results in additional distributions of microenvironments. In the case of complex 3, there are no solvate molecules in the structure, and only one distinct Ni 4 molecule in the lattice. Consequently, the HFEPR data for complex 3 are extremely sharp. As the temperature of a crystal of complex 3 is decreased, the HFEPR spectrum splits abruptly at approximately 46 K into two patterns with very slightly different ZFS parameters. Heat capacity data suggest that this is caused by a structural transition at 46.6 K. A single-crystal X-ray structure at 12(2) K indicates large thermal parameters on the terminal methyl groups of the dmb (3,3-dimethyl-1-butanol) ligand. Most likely there exists dynamic disorder of parts of the dmb ligand above 46.6 K; an order-disorder structural phase transition at 46.6 K then removes some of the motion. A further decrease in temperature (<6 K) leads to further fine structure splittings for complex 3. This behavior is thought to be due to the onset of short-range magnetic correlations/coherences between molecules caused by weak intermolecular magnetic exchange interactions.

Synthesis and characterisation of a Ni4 single-molecule magnet with S4 symmetry.

[Ni4Cl4(HL)4] () {H2L=HN(CH2CH2OH)2} has S4 symmetry and crystallises in the tetragonal space group I4(1)/a. Two exchange couplings are observed between the four Ni(II) centres, with J1=7.29 cm(-1) and J2=-2.08 cm(-1), leading to an S=4 ground state. The Ni4 complex shows the onset of frequency dependent signals in the out-of-phase ac susceptibility below 3 K. In single-crystal measurements carried out using a micro-SQUID, hysteresis loops are observed below 0.5 K, confirming that shows slow relaxation of magnetisation. The loops are temperature dependent but only weakly sweep rate dependent due to the presence of small intermolecular interactions, which hinder quantum tunnelling. This exchange bias between Ni4 molecules is also seen in high-frequency high-field EPR measurements, which give the parameters D=-0.75 cm(-1), B4 degrees=-6.7x10(-5) cm(-1) and gz=2.275.

Single-molecule magnets: high-field electron paramagnetic resonance evaluation of the single-ion zero-field interaction in a Zn(II)3Ni(II) complex.

High-field electron paramagnetic resonance spectra were collected at several frequencies for a single crystal of [Zn3.91Ni0.09(hmp)4(dmb)4Cl4] (1), where dmb is 3,3-dimethyl-1-butanol and hmp- is the monoanion of 2-hydroxymethylpyridine. This crystal is isostructural to [Ni4(hmp)4(dmb)4Cl4] (2), which has been characterized to be a single-molecule magnet (SMM) with fast quantum tunneling of its magnetization (QTM). The single Ni(II) ion zero-field-splitting (zfs) parameters Di [= -5.30(5) cm(-1)] and Ei [= +/-1.20(2) cm(-1)] in the doped complex 1 were evaluated by rotation of a crystal in three planes. The easy-axes of magnetization associated with the single-ion zfs interactions were also found to be tilted 15 degrees away from the crystallographic c direction. This inclination provides a possible explanation for the fast QTM observed for complex 2. The single-ion zfs parameters are then related to the zfs parameters for the Ni4 molecule by irreducible tensor methods to give D = -0.69 cm(-1) for the S = 4 ground state of the SMM, where the axial zfs interaction is given by DS(Z)2.

Adaptive-optics performance of Antarctic telescopes.

The performance of natural guide star adaptive-optics systems for telescopes located on the Antarctic plateau is evaluated and compared with adaptive-optics systems operated with the characteristic mid-latitude atmosphere found at Mauna Kea. A 2-m telescope with tip-tilt correction and an 8-m telescope equipped with a high-order adaptive-optics system are considered. Because of the large isoplanatic angle of the South Pole atmosphere, the anisoplanatic error associated with an adaptive-optics correction is negligible, and the achievable resolution is determined only by the fitting error associated with the number of corrected wave-front modes, which depends on the number of actuators on the deformable mirror. The usable field of view of an adaptive-optics equipped Antarctic telescope is thus orders of magnitude larger than for a similar telescope located at a mid-latitude site; this large field of view obviates the necessity for multiconjugate adaptive-optics systems that use multiple laser guide stars. These results, combined with the low infrared sky backgrounds, indicate that the Antarctic plateau is the best site on Earth at which to perform high-resolution imaging with large telescopes, either over large fields of view or with appreciable sky coverage. Preliminary site-testing results obtained recently from the Dome Concordia station indicate that this site is far superior to even the South Pole.