In-situ fabricated 3D micro-lenses for photonic integrated circuits

: Aspheric astigmatic polymer micro-lenses were fabricated directly onto photonic integrated circuits using two-photon lithography. We observed a 12.6 dB improvement in the free space coupling efficiency between integrated ridge laser pairs with micro-lenses to those without. applications in analytical chemistry. We compare the power coupling efficiency of integrated pairs of lasers, with and without micro-lenses, using


Introduction
The integration of multiple optical components, on a monolithic substrate to form photonic integrated circuits (PICs) has long been seen as a logical progression following developments in electronics [1]. Recent progress includes the development of generic foundry processes, with Multi Project Wafer users in the healthcare and sensing application areas, as well as in the more conventional, tele-and data-communications [2]. The scale of photonic integration has currently reached a position, where 100's of optical functions, in a system on chip for data communications applications, are commercially deployed, and greater than 1700 optical functions have been demonstrated in the research laboratory [3]. Integration has proceeded in 2-dimensional (2D) structures, and also in 3-dimensional (3D) structures, which consist of stacks of 2D structures [4]. Increasingly large numbers of optical interconnects are required between structures and for general input / output. This demand is soon expected to exceed what is currently feasible with optical fiber coupling technology, due to the limitations imposed by both, the relatively large size of single mode fibers (< 250 m), and, the cost and complexity associated with their alignment [5]. A solution may be the use of two-photon lithography to produce so called photonic wire bonding [5]. An alternative method, and in some cases a requirement, is for light to be coupled between elements on the same chip in free space and not enclosed in a waveguide. For example, in particle sensing applications [6-9], light passes from the high refractive index waveguide of the PIC, into a low index medium where it interacts with a particle. Any divergence of the light on leaving the waveguide reduces the efficiency and range over which optical power and information, can be coupled back to in-plane sensing elements on the chip. Furthermore, the rates of divergence in the direction normal to the plane of the device (fast axis) to that parallel to the plane (slow axis), can differ substantially when using active optical elements such as a diode laser. This results in non-ideal, elliptical beam shapes, that are not collimated, and which, for particle sensing applications, demand non-trivial analysis [9].
The same two-photon lithography technique used for photonic wire bonding can be used to create free-form components in a variety of polymer materials and by subsequent processing, metallic materials [10]. By utilizing a combination of a high numerical aperture objective lense and the non-linear two-photon absorption process, two-photon lithography is unique amongst additive manufacturing techniques, in its ability to exceed the diffraction limit of the system, and produce spatial resolution of sub-100 nm [11]. This capability has enabled fabrication of optical grade, micro-scaled components, such as, micro-lens arrays [12], refracting lenses [13] in-situ on multi-core fibers, and on vertically emitting, single photon sources [14,15]. Two-photon, lithographically fabricated optics, have also been applied indirectly to edge emitting lasers [16]. This was achieved by fabricating the lenses on a planar sacrificial substrate, separating the lenses from the substrate, and finally positioning them with high precision, relative to the edge-emitting laser [16]. To develop a more manufacturable, wafer-scale solution, the lenses should be formed in situ. However, for any in-plane PIC application, where the required additive micro-optics are in close proximity to existing structures, shadowing of the source light required for exposure of the resist, is a significant problem and, a priori, it is not clear whether such structures can be produced.
In this paper we demonstrate how two-photon, lithographically fabricated micro-lenses can be formed in-situ, over deep-etched substrates, for the purpose of enhancing the coupling between monolithically integrated, edge-emitting light sources. These sources should be geometrically separated by some 200 m for the future provision of a fluid-conducting trench, for use in microfluidic, lab-on-a-chip applications in analytical chemistry. We compare the power coupling efficiency of integrated pairs of lasers, with and without micro-lenses, using photo-voltage measurements, and discuss current fabrication limitations of the technique for this application, and the constraints that these place on lens/device design.

Materials and methods
The optically active semiconductor material used for the PIC substrate was chosen as it has previously been used for particle detection [9] and is a suitable wavelength to examine blood cells in a fluid flow. The material is comprised of a single, quantum well Ga 0.51 In 0.49 P, p-i-n diode structure, emitting at a wavelength of 633 nm. 1 mm long, 10 m wide ridge lasers were patterned into the upper surface of the substrate, using a thermally evaporated Ni etch mask, defined with a PMMA lift-off mask, patterned using electron beam lithography. The ridge lasers were formed by vertically etching 4.3 m into the substrate using an Ar/Cl 2 Inductively Coupled Plasma (ICP). The ridge structures were planarized with a 5 m thick layer of Microchem SU-8 3005, and back etched using O 2 /CF 4 Reactive Ion Etching plasma, to expose the p-doped capping layer. 10/300 nm Cr/Au layers were thermally evaporated over the ridges to form p-side electrical contacts. The backside of the sample was thinned to a thickness of 100 m, and 100/28/300 nm thick AuGe/Ni/Au layers were thermally evaporated, to form n-side electrical contacts.
To minimize the footprint of the micro-lenses, an aspheric astigmatic refracting lens design, capable of shaping the laser beam in the both the fast and slow axes, with a single refracting surface, was chosen, Figs. 1(a) and 1(b).   Fig. 1(c). The icle sensing is examining h ]. Here we hav process but, of application laser to match A 20 m deep ections, Fig. 2( s coated in IPlaser facets, us efit of two-pho Fig. 2(b), thus sing the transm ocusing the Na ch lens pattern aligning the s hed into the GaAs the 3D micro-le buried inside mico -photon lithog h the exposure raphy, the ligh posing radiatio adowed", by t ults, can cause he desired lens al qualitative d ens in Fig. 3 ximity of s possible Fig. 3(b). zed, when ization of lymerized ize of the rface. An wo-photon or the lens s. Optical ltiphysics ken from profile as ed for the hape and calculated aser pairs, alignment fabricated nm Au) as ughness is hich could that twough these ng one of d on the measured ugh these photo-voltage signals are measured in arbitrary units, by taking the ratio of the gradients of the above threshold regions, of the L-I curves for the lensed and non-lensed lasers, a relative collection efficiency of the two pairs of lasers can be calculated. From this comparison, we calculate that the coupling efficiency of the lensed pair shows a 12.6 dB improvement. A typical signal to noise value for one of the integrated laser -detectors pairs (without lenses), and with the laser operating at 140 mA and zero bias on the detector, was 10.6. A 12.6 dB improvement in optical coupling due to the lenses therefore results in an SNR of 181. Using the laser beam divergence data, we undertook COMSOL optical simulations of the coupling efficiencies of lasers across a 200 m trench. Without lenses, the coupling efficiency was 0.90% (-20.5 dB), whereas with the lenses, it was 30.1% (-5.28 dB). By extrapolating the above threshold region of the L-I curves to zero on the y-axis, threshold current values were estimated for the lasers and the lasers without lenses. The threshold current values agree within error, which suggests that the addition of the lens may have a negligible effect on the optical feedback to the laser and that the lens surface quality may be such, that any scattered light will be insufficient to change the laser operation. However, spectral linewidth, or relative intensity noise measurements, could provide a more accurate assessment of such potential optical feedback issues, and should be investigated in further studies.This exploratory study demonstrates certain, potential advantages of employing the in-situ fabrication of lenses in photonic integrated circuits, namely regarding the issues of alignment, handling and geometrical complexity. However, this potential, needs to be examined much more closely, including issues associated with the economy of the process, for wafer scale processing. The fabrication of a single lens (volume 1.2×10 -4 mm 3 ) in this study, using the Nanoscribe two-photon lithography instrument, took 25 minutes, at a write speed of 10 mm/s and 63.8 mW laser power. This is suitable for some prototyping, but clearly insufficient for direct adoption to wafer-scale production. Other issues also arise, such as the addition of non-reflective coatings. Furthermore, the alignment of the coupled lenses and the lasers were undertaken manually within the control software of the Nanoscribe, but would require automation in a production scenario. The qualities of the microstructures, such as lenses as produced here, not only depend upon the laser power and writing speed, but also upon the material properties of the photo-resin, which again, needs further research, to develop a diversity of optical qualities, optimised for given applications.

Conclusions
Astigmatic aspheric beam shaping micro-lenses have been fabricated in-situ on photonic integrated circuits using two-photon lithography. By comparing the light -current characteristics of integrated laser pairs, with and without micro-lenses, we conclude that the free space coupling efficiency is improved by 12.6 dB with our micro-lens design. When performing two-photon lithography over pre-structured PIC substrates, shadowing of the exposing radiation by substrate structures, leads to under exposure of the resulting pattern, which is an effect that must be considered and corrected for at the design stage.
The monolithic integration of such in-situ lenses with chip based lasers could play a part in very large scale integration, were a fabrication process available which could overcome the extreme limitations, that are currently inherent in the sequential processing methodology of the direct write technique currently used, and demonstrated here. If such a process were possible, it could possibly be extended to create both sophisticated lens arrays, and actuatable lenses that could be dynamically driven to cater for a diversity of different applications. Such actuation could employ structured piezoelectric nanocomposite resists that have already been demonstrated [18].

Funding
This work was supported by EPSRC grant EP/L005409/1, the National Research Network Wales for advanced engineering and materials (NRN097), funded by the Higher Education Funding Council for Wales and the Welsh Government and funding from the EPSRC (EP/L006669/1 and EP/R009147/1).