Optical Vectorial Vortex Coronagraphs using Liquid Crystal Polymers: theory, manufacturing and laboratory demonstration

Optimal design of the annular groove phase mask central region

Vortex phase masks have been shown to be an efficient means to reduce the blinding stellar light in high-contrast imaging instruments. Once placed at the focal plane of the telescope, the helical phase ramp of a vortex phase mask diffracts the light of a bright on-axis source outside the re-imaged telescope pupil, while transmitting the light of a faint off-axis companion nearly unaffected. The Annular Groove Phase Mask (AGPM) is a broadband metasurface implementation of a vector vortex phase mask using the artificial birefringence of a circular subwavelength grating etched onto a diamond substrate. To date, the AGPM design has been optimized using rigorous coupled-wave analysis (RCWA), which is a valid tool to simulate periodic straight gratings. However, we have now reached a performance level where the curvature of the grating lines at the center becomes a limiting factor. Here, we use a finite-difference time-domain (FDTD) method to correctly describe the AGPM performance, including the effect of the curved grating close to its center. We confirm the validity of this simulation framework by comparing its predictions with experimental results obtained on our infrared coronagraphic test bench, and we show that RCWA fails at reproducing correctly the central AGPM performance, confirming the need for a full 3d simulation tool such as FDTD. Finally, we use FDTD to optimize the grating parameters at the AGPM center, and conclude with a new optimal design.

Mueller matrix imaging microscope using dual continuously rotating anisotropic mirrors

We demonstrate calibration and operation of a Mueller matrix imaging microscope using dual continuously rotating anisotropic mirrors for polarization state generation and analysis. The mirrors contain highly spatially coherent nanostructure slanted columnar titanium thin films deposited onto optically thick titanium layers on quartz substrates. The first mirror acts as polarization state image generator and the second mirror acts as polarization state image detector. The instrument is calibrated using samples consisting of laterally homogeneous properties such as straight-through-air, a clear aperture linear polarizer, and a clear aperture linear retarder waveplate. Mueller matrix images are determined for spatially varying anisotropic samples consisting of a commercially available (Thorlabs) birefringent resolution target and a spatially patterned titanium slanted columnar thin film deposited onto a glass substrate. Calibration and operation are demonstrated at a single wavelength (530 nm) only, while, in principle, the instrument can operate regardless of wavelength. We refer to this imaging ellipsometry configuration as rotating-anisotropic-mirror-sample-rotating-anisotropic-mirror ellipsometry (RAM-S-RAM-E).

Equal-intensity beam splitter fabricated by segmented half-wave plate for passive laser speckle reduction

An equal-intensity beam splitter (EIBS) for passive laser speckle reduction is reported. The EIBS consists of a segmented half-wave plate (SHWP) with the designed orientation of the fast axis of each segment, a polarization beam splitter, and a mirror. The SHWP is fabricated using patterned photoalignment material and liquid crystal polymer. Ten laser sub-beams are generated by the twenty-one-pixelated EIBS, where the largest intensity ratio among them is 7.6. Laser temporal and spatial coherences are destroyed because of the optical path delays among the laser sub-beams. The EIBS is used to reduce laser speckle passively, and objective speckle contrast is reduced from 0.82 to 0.33 when all 10 laser sub-beams are used.

Improved two-step optimization procedure used for designing an apodizer and Lyot stop in the Lyot coronagraph

The Lyot coronagraph is a widely known astronomical instrument used to realize direct imaging of exoplanets, and designing transmittance of an apodizer and Lyot stop is the key to obtaining high-contrast imaging. In this paper a new (to the best of our knowledge) optimization procedure used to design the apodizer and Lyot stop in the Lyot coronagraph is proposed. A two-step optimization program is established to obtain the optimum transmittance of an apodizer and Lyot stop in a sequential way. By using the optimized apodizer and Lyot stop obtained through the proposed optimization procedure, both the stellar light and its diffraction light could be strongly suppressed. Numerical results indicate that such an optimized Lyot coronagraph can produce a 1e-10 extinction of the stellar light near the diffraction limit (1.59λ/D), and a high contrast imaging of 1e-07 could still be obtained even with the influence of light intensity of planets themselves. In addition, the two-step optimization procedure brings in two benefits. First, the two-step optimization is approximately 1000 times faster than the joint optimization method [J. Astron. Telesc. Instrum. Syst.2, 011012 (2016)2329-412410.1117/1.JATIS.2.1.011012]. Second, the optimum transmittance of the Lyot stop is binary, and therefore, the requirements of the production process are reduced, resulting in a greatly reduced cost. At the same time, the performance of the optimized Lyot coronagraph is also analyzed in the case of a monochromatic light incident and bandwidth light incident, and the effect of the diameter of the Lyot stop on the results is also discussed in this paper, which makes sense when designing a coronagraph.

Bragg-Berry flat reflectors for transparent computer-generated holograms and waveguide holography with visible color playback capability

Various approaches are being pursued to realize compact optical elements with the ability to manipulate light, but it is difficult to simultaneously achieve high reflectivity and the ability to see through the element. Here, we present a reflective computer-generated hologram that is completely transparent in the visible, based on the Berry (geometric) phase in a self-organizing Bragg reflector. The Bragg reflector has a helical dielectric tensor distribution with the phase information imprinted in the distribution of the optic axis on the substrate. The structure possesses only a single Fourier component and high-order reflections are suppressed; thus, the device appears completely transparent by setting the main reflection band outside the visible range for all angles of incidence accessible by ambient light. On the other hand, the encoded phase information can be played back using visible light by increasing the accessible incidence angle, which we demonstrate experimentally by (i) attaching a coupling prism, and (ii) integrating the device in a waveguide. Bragg-Berry reflectors thus enable a new route to realize advanced optical elements with no apparent reflection in the visible region.

Nonlinear optical vortex coronagraph

A nonlinear optical vortex coronagraph (n-OVC) based on sum-frequency generation (SFG) in a periodically poled lithium niobate (PPLN) crystal is presented. We demonstrate an n-OVC by mixing the image of an on-axis point source ($ {\lambda _s} = 1.6\;{\unicode{x00B5}{\rm m}} $λ_s=1.6µm) inside the PPLN crystal with a pump beam (${\lambda _p} = 1064\;{\rm nm}$λ_p=1064nm) imprinted with a helical phase profile from a vector vortex mask (topological charge ${l} = 2$l=2). Due to quasi-phase matching and orbital angular momentum conservation, a coronagraphic image is produced at the SFG wavelength (${\lambda _{\rm up}}\sim 630\;{\rm nm}$λ_up∼630nm). We validate that the n-OVC is tunable to signal wavelength but only requires a vortex mask operating at the pump wavelength. The acceptance bandwidth of the SFG process provides the n-OVC a degree of achromaticity even with a monochromatic vortex mask. The n-OVC exhibits an inner working angle of $ \sim {\lambda _s}/{D}$∼λ_s/D and an experimental contrast of ${{10}^{ - 4}}$10^-4 at $3{\lambda _s}/{D}$3λ_s/D.

Flat cholesteric liquid crystal polymeric lens with low f-number

We use a simple photoalignment method to fabricate four reflective cholesteric liquid crystal (CLC) polymeric lenses with diameter D=2.45 cm and low f-numbers (f/2, f/0.9, f/0.45, f/0.33) at 550 nm. Such a flat CLC lens can be converging or diverging, depending on the handedness and direction of the incident light. Our CLC lenses can achieve ∼85% diffraction efficiency for a designated polarization state and manifest decent imaging ability.

Vortex fiber nulling for exoplanet observations. I. Experimental demonstration in monochromatic light

Vortex fiber nulling is a method for spectroscopically characterizing exoplanets at small angular separations, ≲λ/D, from their host star. The starlight is suppressed by creating an optical vortex in the system point spread function, which prevents the stellar field from coupling into the fundamental mode of a single-mode optical fiber. Light from the planet, on the other hand, couples into the fiber and is routed to a spectrograph. Using a prototype vortex fiber nuller (VFN) designed for monochromatic light, we demonstrate coupling fractions of 6×10^-5 and >0.1 for the star and planet, respectively.

Novel liquid crystal photonic devices enabled by two-photon polymerization [Invited]

In addition to displays, liquid crystals (LCs) have also found widespread applications in photonic devices, such as adaptive lens, adaptive optics, and sensors, because of their responses to electric field, temperature, and light. As the fabrication technique advances, more sophisticated devices can be designed and created. In this review, we report recent advances of two-photon polymerization-based direct-laser writing enabled LC devices. Firstly, we describe the basic working principle of two-photon polymerization. With this powerful fabrication technique, we can generate anchoring energy by surface morphology to align LC directors on different form factors. To prove this concept, we demonstrate LC alignment on planar, curvilinear surfaces as well as in three-dimensional volumes. Based on the results, we further propose a novel, ultra-broadband, twisted-nematic diffractive waveplate that can potentially be fulfilled by this technique. Next, we briefly discuss the current status of direct-laser writing on LC reactive mesogens and its potential applications. Finally, we present two design challenges: fabrication yield and polymer relaxation/deformation, remaining to be overcome.

Reconstructing a plasmonic metasurface for a broadband high-efficiency optical vortex in the visible frequency

Metasurfaces consisting of a two-dimensional metallic nano-antenna array are capable of transferring a Gaussian beam into an optical vortex with a helical phase front and a phase singularity by manipulating the polarization/phase status of light. This miniaturizes a laboratory scaled optical system into a wafer scale component, opening up a new area for broad applications in optics. However, the low conversion efficiency to generate a vortex beam from circularly polarized light hinders further development. This paper reports our recent success in improving the efficiency over a broad waveband at the visible frequency compared with the existing work. The choice of material, the geometry and the spatial organization of meta-atoms, and the fabrication fidelity are theoretically investigated by the Jones matrix method. The theoretical conversion efficiency over 40% in the visible wavelength range is worked out by systematic calculation using the finite difference time domain (FDTD) method. The fabricated metasurface based on the parameters by theoretical optimization demonstrates a high quality vortex in optical frequencies with a significantly enhanced efficiency of over 20% in a broad waveband.

High-charge and multiple-star vortex coronagraphy from stacked vector vortex phase masks

Optical vortex phase masks are now installed at many ground-based large telescopes for high-contrast astronomical imaging. To date, such instrumental advances have been restricted to the use of helical phase masks of the lowest even order, while future giant telescopes will require high-order masks. Here we propose a single-stage on-axis scheme to create high-order vortex coronagraphs based on second-order vortex phase masks. By extending our approach to an off-axis design, we also explore the implementation of multiple-star vortex coronagraphy. An experimental laboratory demonstration is reported and supported by numerical simulations. These results offer a practical roadmap to the development of future coronagraphic tools with enhanced performances.

Multiple-Star System Adaptive Vortex Coronagraphy Using a Liquid Crystal Light Valve

We propose the development of a high-contrast imaging technique enabling the simultaneous and selective nulling of several light sources. This is done by realizing a reconfigurable multiple-vortex phase mask made of a liquid crystal thin film on which local topological features can be addressed electro-optically. The method is illustrated by reporting on a triple-star optical vortex coronagraphy laboratory demonstration, which can be easily extended to higher multiplicity. These results allow considering the direct observation and analysis of worlds with multiple suns and more complex extrasolar planetary systems.

Vortex coronagraphy from self-engineered liquid crystal spin-orbit masks

We report on a soft route toward optical vortex coronagraphy based on self-engineered electrically tunable vortex masks made of liquid crystal topological defects. These results suggest that a nature-assisted technological approach to the fabrication of complex phase masks could be useful in optical imaging whenever optical phase singularities are at play.

Independent Manipulation of Topological Charges and Polarization Patterns of Optical Vortices

We present a simple and flexible method to generate various vectorial vortex beams (VVBs) with a Pancharatnam phase based on the scheme of double reflections from a single liquid crystal spatial light modulator (SLM). In this configuration, VVBs are constructed by the superposition of two orthogonally polarized orbital angular momentum (OAM) eigenstates. To verify the optical properties of the generated beams, Stokes polarimetry is developed to measure the states of polarization (SOP) over the transverse plane, while a Shack–Hartmann wavefront sensor is used to measure the OAM charge of beams. It is shown that both the simulated and the experimental results are in good qualitative agreement. In addition, polarization patterns and OAM charges of generated beams can be controlled independently using the proposed method.

Interferometric Sensor of Wavelength Detuning Using a Liquid Crystalline Polymer Waveplate

Operation of a polarization interferometer for measurement of the wavelength changes of a tunable semiconductor laser was investigated. A λ/8 waveplate made from liquid crystalline polymer is placed in one of interferometers’ arms in order to generate two output signals in quadrature. Wavelength was measured with resolution of 2 pm in the wavelength range 628–635 nm. Drift of the interferometer, measured in the period of 500 s, was 8 nm, which corresponded to the change in the wavelength of 1.3 pm. If needed, wavelength-dependent Heydemann correction can be used to expand the range of operation of such interferometer.

Three-dimensional vector recording in polarization sensitive liquid crystal composites by using axisymmetrically polarized beam

Three-dimensional anisotropic structures were fabricated by a recording axisymmetrically polarized beam in azobenzene (azo)-dye doped liquid crystal polymer composites. Polarization and wavefront modulation properties of fabricated anisotropic structures are investigated by experimentally and theoretically analyzing the diffraction properties. Photo-induced anisotropic structures would be utilized to generate singular light waves, such as optical and polarization vortices.

Pattern formation of drying lyotropic liquid crystalline droplet

We present a study of pattern formation in drying sessile droplets of aqueous solutions of cetyltrimethylammonium bromide (CTAB)–water system using polarising optical microscopy (POM) and computer simulation.

Three-dimensional positioning and control of colloidal objects utilizing engineered liquid crystalline defect networks

Topological defects in liquid crystals not only affect the optical and rheological properties of the host, but can also act as scaffolds in which to trap nano or micro-sized colloidal objects. The creation of complex defect shapes, however, often involves confining the liquid crystals in curved geometries or adds complex-shaped colloidal objects, which are unsuitable for device applications. Using topologically patterned substrates, here we demonstrate the controlled generation of three-dimensional defect lines with non-trivial shapes and even chirality, in a flat slab of nematic liquid crystal. By using the defect lines as templates and the electric response of the liquid crystals, colloidal superstructures are constructed, which can be reversibly reconfigured at a voltage as low as 1.3 V. Three-dimensional engineering of the defect shapes in liquid crystals is potentially useful in the fabrication of self-healing composites and in stabilizing artificial frustrated phases.

Topological defects can be used not only to modify the properties of liquid crystals but also as scaffolds to build new structures by trapping particles. Here, Yoshida et al. construct three-dimensional colloidal superstructures in a nematic host, which are reconfigurable in an electric field.

Polarization holography for vortex retarders recording: laboratory demonstration

This paper will present a prototype of the first set of vortex retarders made of liquid crystal polymers recorded by polarization holography. Vortex retarders are birefringent plates characterized by a rotation of their fast axis. Liquid crystals possess birefringent properties and they are locally orientable. Their orientation is defined by the perpendicular to the local orientation of the recording field. Polarization holography is a purely optical recording method. It is based on the superimposition of coherent and differently polarized beams. It is used to shape the electric field pattern to enable the recording of vortex retarders. The paper details the mathematical model of the superimposition process. The recording setup is exposed; it is characterized by a nearly common path interferometer. Two sets of measurements allowing the prediction of the retarder's features are presented and compared. Finally, the experimentally recorded retarder is shown, its characteristics are investigated and compared to the predicted ones.

Reducing the risk of laser damage in a focal plane array using linear pupil-plane phase elements

A compact imaging system with reduced risk of damage owing to intense laser radiation is presented. We find that a pupil phase element may reduce the peak image plane irradiance from an undesirable laser source by two orders of magnitude, thereby protecting the detector from damage. The desired scene is reconstructed in postprocessing. The general image quality equation (GIQE) [Appl. Opt.36, 8322 (1997)] is used to estimate the interpretability of the resulting images. A localized loss of information caused by laser light is also described. This system may be advantageous over other radiation protection approaches because accurate pointing and nonlinear materials are not required.

Performance characterization of a broadband vector Apodizing Phase Plate coronagraph

One of the main challenges for the direct imaging of planets around nearby stars is the suppression of the diffracted halo from the primary star. Coronagraphs are angular filters that suppress this diffracted halo. The Apodizing Phase Plate coronagraph modifies the pupil-plane phase with an anti-symmetric pattern to suppress diffraction over a 180 degree region from 2 to 7 λ/D and achieves a mean raw contrast of 10(-4) in this area, independent of the tip-tilt stability of the system. Current APP coronagraphs implemented using classical phase techniques are limited in bandwidth and suppression region geometry (i.e. only on one side of the star). In this paper, we introduce the vector-APP (vAPP) whose phase pattern is implemented through the vector phase imposed by the orientation of patterned liquid crystals. Beam-splitting according to circular polarization states produces two, complementary PSFs with dark holes on either side. We have developed a prototype vAPP that consists of a stack of three twisting liquid crystal layers to yield a bandwidth of 500 to 900 nm. We characterize the properties of this device using reconstructions of the pupil-plane pattern, and of the ensuing PSF structures. By imaging the pupil between crossed and parallel polarizers we reconstruct the fast axis pattern, transmission, and retardance of the vAPP, and use this as input for a PSF model. This model includes aberrations of the laboratory set-up, and matches the measured PSF, which shows a raw contrast of 10(-3.8) between 2 and 7 λ/D in a 135 degree wedge. The vAPP coronagraph is relatively easy to manufacture and can be implemented together with a broadband quarter-wave plate and Wollaston prism in a pupil wheel in high-contrast imaging instruments. The liquid crystal patterning technique permits the application of extreme phase patterns with deeper contrasts inside the dark holes, and the multilayer liquid crystal achromatization technique enables unprecedented spectral bandwidths for phase-manipulation coronagraphy.

Vortex-phase filtering technique for extracting spatial information from unresolved sources

A white light vortex coronagraph was used to experimentally achieve sub-resolution detection. The angular location of the centroid γ, and the angular extent of circular pinhole sources Θ, were measured to within errors of δγ=±0.015λ/D and δΘ=±0.026λ/D. This technique has two advantages over conventional imaging: simple power measurements are made and shorter exposure times may be required to achieve a sufficient signal-to-noise ratio.

Active stabilization of a Michelson interferometer at an arbitrary phase with subnanometer resolution

We report on the active stabilization of a Michelson interferometer at an arbitrary phase angle with a precision better than 1° at λ=632.8  nm, which corresponds to a precision in the optical path difference between the two arms of less than 1 nm. The stabilization method is ditherless, and the error signal is computed from the spatial shift of the interference pattern of a reference laser, measured in real-time with a CCD array detector. We discuss the usefulness of this method for nanopositioning, optical interferometry, and quantum optical experiments.

Optical vortex arrays from smectic liquid crystals

We demonstrate large-area, closely-packed optical vortex arrays using self-assembled defects in smectic liquid crystals. Self-assembled smectic liquid crystals in a three-dimensional torus structure are called focal conic domains. Each FCD, having a micro-scale feature size, produces an optical vortex with consistent topological charge of 2. The spiral profile in the interferometry confirms the formation of an optical vortex, which is predicted by Jones matrix calculations.

Wide-band six-region phase mask coronagraph

An achromatic six-region phase mask coronagraph, used for the detection of exoplanets, is proposed. The mask has six regions in angular direction and could work in wideband. Furthermore, a six-level phase mask, as an example of the six-region phase mask, is theoretically investigated. According to numerical simulations, this specific mask has a deep elimination of starlight, good performance of achromatism and small inner working angle. As a single phase mask, the ratio of the remaining starlight of the six-level phase mask to the total incident starlight is less than 0.001 when the wavelength is between 500 nm and 600 nm.

Polarization holography for vortex retarders recording

We present an original static recording method for vortex retarders (VRs) made from liquid crystal polymers (LCPs) using the superimposition of several polarized beams. VRs are birefringent plates characterized by a rotation of their fast axis about their center. The new method is based on polarization holography and photo-orientable LCP. Combining several polarized beams induces the polarization patterns required for the recording process of VRs without mechanical action. A mathematical description of the method, the outcomes of the numerical simulations, and the first experimental results are presented.

Manipulating the orbital angular momentum of light at the micron scale with nematic disclinations in a liquid crystal film

We report on the experimental manipulation of the orbital angular momentum of light by exploiting a kind of topological defects that spontaneously appear in nematics-disclinations-as microscopic optical spin-orbit interfaces whose operating wavelength can be controlled electrically. Using six different kinds of disclinations, we demonstrate the efficient generation of both scalar and vectorial singular light beams with a broad topological diversity from a fundamental Gaussian beam.

Design and laboratory demonstration of an achromatic vector vortex coronagraph

A vector vortex coronagraph (VVC) is one of promising means for imaging extremely faint objects around bright stars such as exoplanets. We present a design of an achromatic VVC, in which an axially-symmetric half-wave plate (AHP) is placed between crossed polarization filters (circular polarizer and analyzer). The circular polarizer and the analyzer are both composed of a polarizer and a quarter-wave plate (QWP). We demonstrate, via Jones calculus and Fourier analysis, that the achromatic stellar elimination can theoretically be realized by optimal polarization filters, even when chromatic AHP and QWPs are used. We carried out laboratory demonstrations of the designed VVC using a photonic-crystal AHP. As a result, we observed achromatic coronagraphic performance, a light suppression level of 7 × 10(-5), over a wavelength from 543 nm to 633 nm.

Optical vortex coronagraphy with an elliptical aperture

An optical vortex coronagraph that makes efficient use of a larger fraction of the clear aperture of a Cassegrain-type telescope is described. This design incorporates an elliptical subaperture rather than the conventional circular subaperture. We derive a new vortex phase mask that maintains the same theoretical contrast of a circularly symmetric vortex coronagraph.

Design, manufacturing, and performance analysis of mid-infrared achromatic half-wave plates with diamond subwavelength gratings

In this paper, we present a solution for creating robust monolithic achromatic half-wave plates (HWPs) for the infrared, based on the form birefringence of subwavelength gratings (SWGs) made out of diamond. We use the rigorous coupled wave analysis to design the gratings. Our analysis shows that diamond, besides its outstanding physical and mechanical properties, is a suitable substrate to manufacture mid-infrared HWPs, thanks to its high refractive index, which allows etching SWGs with lower aspect ratio. Based on our optimized design, we manufactured a diamond HWP for the 11-13.2 μm region, with an estimated mean retardance ~3.143±0.061 rad (180.08±3.51°). In addition, an antireflective grating was etched on the backside of the wave plate, allowing a total transmittance between 89% and 95% over the band.

Electrical control of shape of laser beam using axially symmetric liquid crystal cells

This work demonstrates the electrical tuning of laser beam shape using an axially symmetric dye-dope liquid crystal (ASDDLC) device that is fabricated using a photo-alignment method. Various beam shapes can be obtained by linearly polarized Gaussian laser beams through an ASDDLC device under various applied voltages. The far-field intensity patterns generated by laser beams of selected shapes under various applied voltages are simulated, and the results are consistent with experiment. A rotatable petal-shaped beam is obtained by controlling the polarization of the output donut-shaped beam. The tenability of beam shape of light with a wavelength of 1064 nm, which is commonly used in biomedical applications, is also demonstrated.

Tunable optical vortex arrays from a single nematic topological defect

We report on the generation of tunable structured light fields endowed with various sets of phase singularities from a single topological defect in a nematic liquid crystal mesophase. The experimental demonstration relies on the use of electric field-induced nonsingular topological defects called "umbilics."

Improved high-contrast imaging with on-axis telescopes using a multistage vortex coronagraph

The vortex coronagraph is one of the most promising coronagraphs for high-contrast imaging because of its simplicity, small inner working angle, high throughput, and clear off-axis discovery space. However, as with most coronagraphs, centrally obscured on-axis telescopes degrade contrast. Based on the remarkable ability of vortex coronagraphs to move light between the interior and exterior of pupils, we propose a method based on multiple vortices, that without sacrificing throughput, reduces the residual light leakage to (a/A)(n), with n ≥ 4, and a and A being the radii of the central obscuration and primary mirror, respectively. This method thus enables high contrasts to be reached even with an on-axis telescope.

Propagation of aberrations through phase-induced amplitude apodization coronagraph

The specification of polishing requirements for the optics in coronagraphs dedicated to exoplanet detection requires careful and accurate optical modeling. Numerical representations of propagated aberrations through the system as well as simulations of the broadband wavefront compensation system using multiple DMs are critical when one devises an error budget for such a class of instruments. In this communication, we introduce an analytical tool that serves this purpose for phase-induced amplitude apodization (PIAA) coronagraphs. We first start by deriving the analytical form of the propagation of a harmonic ripple through a PIAA unit. Using this result, we derive the chromaticity of the field at any plane in the optical train of a telescope equipped with such a coronagraph. Finally, we study the chromatic response of a two-sequential-DM wavefront actuator correcting such a corrugated field and thus quantify the requirements on the manufacturing of PIAA mirrors.

An image of an exoplanet separated by two diffraction beamwidths from a star

Three exoplanets around the star HR 8799 have recently been discovered by means of differential imaging with large telescopes. Bright scattered starlight limits high-contrast imaging to large angular offsets, currently of the order of ten diffraction beamwidths, 10lambda/D, of the star (where lambda is the wavelength and D is the aperture diameter). Imaging faint planets at smaller angles calls for reducing the starlight and associated photon and speckle noise before detection, while efficiently transmitting nearby planet light. To carry out initial demonstrations of reduced-angle high-contrast coronagraphy, we installed a vortex coronagraph capable of reaching small angles behind a small, well-corrected telescope subaperture that provides low levels of scattered starlight. Here we report the detection of all three HR 8799 planets with the resultant small-aperture (1.5 m) system, for which only 2lambda/D separate the innermost planet from the star, with a final noise level within a factor of two of that given by photon statistics. Similar well-corrected small-angle coronagraphs should thus be able to detect exoplanets located even closer to their host stars with larger ground-based telescopes, and also allow a reduction in the size of potential space telescopes aimed at the imaging of very faint terrestrial planets.