Photonic waveguide mode to free-space Gaussian beam extreme mode converter

Integration of photonic chips with atomic, micromechanical, chemical and biological systems can advance science and open many possibilities in chip-scale devices and technology. Compact photonic structures for direct coupling of light between high-index single-mode waveguides and arbitrary free-space modes spanning hundreds of waves in cross-section would eliminate bulky optical components and enable integration of photonics into many new applications requiring wide beams, structured light and centimeter-scale propagation distances with low diffraction-limited losses. Conventional fiber-coupling approaches do not scale well for accurate, low-loss coupling across the extremely large mode scale mismatch ($\approx10^6$ times in modal area). Here we present an extreme mode converter that can transform the photonic waveguide mode to the diffraction-limited, free-space Gaussian beam, with a beam waist of about $160~\mu$m. Using two identical converters, we demonstrate a grating-to-grating coupling that couples the radiating beam back to the chip through a mirror reflection in free-space. Operating at 780~nm for integration with chip-scale atomic vapor cell cavities, our design can be adapted for visible, telecommunication or other wavelengths. Furthermore, other types of beams can be implemented by using the 2-stage expansion approach presented in this paper

Surface-Normal Free-Space Beam Projection via Slow-Light Standing Wave Resonances in Extra-Large Near-Zero Index Grating Couplers

On-chip grating couplers directly connect photonic circuits to free-space light. The commonly used photonic gratings have been specialized for small areas, specific intensity profiles and non-vertical beam projection. This falls short of the precise and flexible wavefront control over large beam areas needed to empower emerging integrated miniaturized optical systems that leverage volumetric light matter interactions, including trapping, cooling, and interrogation of atoms, bio- and chemi- sensing and complex free-space interconnect. The large coupler size challenges general inverse design techniques, and solutions obtained by them are often difficult to physically understand and generalize. Here by posing the problem to a carefully constrained computational inverse design algorithm capable of large area structures, we discover a qualitatively new class of grating couplers. The numerically found solutions can be understood as coupling an incident photonic slab mode to a spatially extended slow-light (near-zero refractive index) region, backed by a Bragg reflector. The structure forms a spectrally broad standing wave resonance at the target wavelength, radiating vertically into free space. A reflection-less adiabatic transition critically couples the incident photonic mode to the resonance, and the numerically optimized lower cladding provides 70 $\%$ overall theoretical conversion efficiency. We have experimentally validated efficient surface normal collimated emission of $\approx$ 90 $\mu$m full width at half maximum Gaussian at the thermally tunable operating wavelength of $\approx$ 780 nm. The variable-mesh-deformation inverse design approach scales to extra-large photonic devices, while directly implementing the fabrication constraints. The deliberate choice of smooth parametrization resulted in a novel type of solution, which is both efficient and physically comprehensible

Surface-Normal Free-Space Beam Projection via Slow-Light Standing-Wave Resonance Photonic Gratings

On-chip grating couplers directly connect photonic circuits to free-space light. The commonly used photonic gratings have been specialized for small areas, specific intensity profiles, and nonvertical beam projection. This falls short of the precise and flexible wavefront control over large beam areas needed to empower emerging integrated miniaturized optical systems that leverage volumetric light–matter interactions, including trapping, cooling, and interrogation of atoms, bio- and chemi- sensing, and complex free-space interconnect. The large coupler size challenges general inverse design techniques, and solutions obtained by them are often difficult to physically understand and generalize. Here, by posing the problem to a carefully constrained computational inverse-design algorithm capable of large area structures, we discover a qualitatively new class of grating couplers. The numerically found solutions can be understood as coupling an incident photonic slab mode to a spatially extended slow-light (near-zero refractive index) region, backed by a reflector. The structure forms a spectrally broad standing wave resonance at the target wavelength, radiating vertically into free space. A reflectionless adiabatic transition critically couples the incident photonic mode to the resonance, and the numerically optimized lower cladding provides 70% overall theoretical conversion efficiency. We have experimentally validated an efficient surface normal collimated emission of ≈90 μm full width at half-maximum Gaussian at the thermally tunable operating wavelength of ≈780 nm. The variable-mesh-deformation inverse design approach scales to extra large photonic devices, while directly implementing the fabrication constraints. The deliberate choice of smooth parametrization resulted in a novel type of solution, which is both efficient and physically comprehensible

Exceptional points in photonic grating band diagrams lead to decay-free radiation

We demonstrate that exceptional points in a photonic band diagram lead to constant-intensity free-space beams projected from grating couplers. Our findings pave the way for projecting spatially broad free-space beams with decay-free profiles

Collimating a Free-Space Gaussian Beam by Means of a Chip-Scale Photonic Extreme Mode Converter

Integration of nanophotonics with micromechanical, biological, and chemical systems can miniaturize the reference samples for SI traceable calibration. However, an efficient conversion between photonic modes and free-space beams remains a main hurdle due to a gigantic mode mismatch (10(6) times in modal area). Here, we report an extreme mode converter realized on a photonic chip, which enables projecting a 160-micrometer wide Gaussian beam in free space by a sequential mode expansion from a waveguide to a slab mode and then to free space using an apodised grating. The design can be easily adapted for visible, telecom, and UV wavelengths