(3+1)D printing towards the scalable and efficient integration of high-performance hybrid platforms

. We employ one-and two-photon polymerizatin, i


Introduction
Combining the strength of multiple photonic and electronics concepts in one hybrid and multi-chip platform is a promising solution for the diversification of chips for specific computing tasks to boost performance [1].We demonstrated flash-TPP [2], a novel CMOS compatible lithographic methodology that enables ultra-fast fabrication of high-performance 3D photonic circuits based on additive fabrication via one-(OPP) and two-photon polymerization (TPP) combined with direct-laser writing (DLW), see Fig. 1 (a).In flash-TPP printing, we adapt the resolution of the different sections of a photonic circuit in order to accelerate the printing time by a 90 % required to fabricate the same volume compared to using only TPP [2].
The waveguides cores, which are the basic tool to propagate optical signals, are fabricated via DLW-TPP with a precisely optimized laser power and fine resolution in the (x, y)-plane.Mechanical supports, i.e. the surfaces defining the outer boundaries, are rapidly fabricated to assure the stability of the volumetric circuit.After development, where the unexposed photoresist outside the limits defined by the mechanical supports is removed, we expose the 3D photonic chip under UV light, see Fig. 1 (a).This process polymerizes the cladding of the waveguides via OPP, resulting in a low refractive index contrast needed for optical propagation.

Experimental results
We obtained a refractive index contrast between core and cladding in the order of ∆n ≈ 5 × 10 −3 , enabling singlemode propagation over large (6 mm) distances, and with only 1.3 dB/mm (0.26 dB) propagation (injection) losses, see Fig. 1 (b-c).Via flash-TPP, we fabricated single-mode * e-mail: adria.grabulosa@femto-st.fr optical splitters leveraging adiabatic transfer from one input to up to 4 outputs in a single component [3].We showed a general tapering strategy that can be applied to higher-order 1 to M couplers, see the output intensity profiles of 1 to 2, 1 to 3 and 1 to 4 adiabatic couplers in Fig. 2 (a).Furthermore, we arranged a double-layer of 1 to 4 couplers, and the resulting 1 to 16 single-mode outputs, with only 1 dB global losses, are shown in the last output intensity from Fig. 2 (a).Finally, we tested the broadband functionality of the 1 to 2 optical splitters by injecting different wavelenghts ranging from λ = 520 nm to λ = 980 nm during which global losses remain below 2 dB, see Fig. 2 (b).
In a last investigation, we showed the concept's CMOS compatibility by successfully printing our cascaded 1 to 16 adiabatic couplers on top of GaAs quantum dot micropillar laser array [4], as shown in the SEM micrograph from Fig. 2 (c).Preliminary optical characterization of photonic waveguides printed on top of such semiconductor device showed good performance in terms of optical losses and stability over time.

Conclusion
We developed flash-TPP [2], a simple lithographic configuration that combines DLW-TPP and OPP for the ultra-fast fabrication of polymer-cladded single-mode photonic waveguides and adiabatic splitters.We obtained low 1.3 dB/mm (0.26 dB) propagation (injection) losses and record optical coupling losses of 0.06 dB with very symmetric (3.4 %) splitting ratios for adiabatic couplers [3].
For the latter, we demonstrated almost octave-spanning broadband functionality from λ = 520 nm to λ = 980 nm during which losses remain below 2 dB.Finally, we have demonstrated the reliability of flash-TPP with CMOS technology by printing a cascaded 1 to 16 adiabatic couplers on top of micro-laser arrays [4].With this, we lay a promising foundation for scalable integration of hybrid photonic and electronic platforms [1].Such scalability is essential for efficient parallel communication throughout a densely-connected network, which is challenging in 2D.

Figure 1 .
Figure 1.3D printed photonic circuits and optical performance.(a) Flash-TPP concept for photonic integration: (i) the waveguides cores (mechanical supports) are printed with high(low)resolution via DLW-TPP; (ii) the cladding is polymerized under UV blanket exposure via OPP.(iii) SEM micrograph of a crosssection where the red (blue) region represents the sections polymerized via TPP (OPP).(b) Numerical aperture NA (black) and cladding refractive index n 2 (blue) versus OPP dose.(c) Propagation and injection losses versus waveguide's length.Inset: 6 mm length waveguide.

Figure 2 .
Figure 2. 3D printed adiabatic 1 to M splitters and CMOS compatibility.(a) Output intensities profiles of 1 to 2, 3, 4 and (cascaded) 16 adiabatic couplers.(b) Global losses versus injection light wavelenght λ of the 1 to 2 optical splitters.(c) SEM micrograph of a 3D cuboid integrating a 1 to 16 adiabatic coupler printed on top of a GaAs quantum dot micropillar laser array [4].