Characterization, Selection, and Microassembly of Nanowire Laser Systems

Semiconductor nanowire (NW) lasers are a promising technology for the realization of coherent optical sources with ultrasmall footprint. To fully realize their potential in on-chip photonic systems, scalable methods are required for dealing with large populations of inhomogeneous devices that are typically randomly distributed on host substrates. In this work two complementary, high-throughput techniques are combined: the characterization of nanowire laser populations using automated optical microscopy, and a high-accuracy transfer-printing process with automatic device spatial registration and transfer. Here, a population of NW lasers is characterized, binned by threshold energy density, and subsequently printed in arrays onto a secondary substrate. Statistical analysis of the transferred and control devices shows that the transfer process does not incur measurable laser damage, and the threshold binning can be maintained. Analysis on the threshold and mode spectra of the device populations proves the potential for using NW lasers for integrated systems fabrication.

produced using a laser lithography technique: an epoxy-based negative photoresist (SU-8) was spin-coated onto a quartz disk substrate with a film thickness of 4 µm. A custom-built maskless laser lithography tool (λ = 370 nm) was used to expose the marker patterns that were then developed using EC solvent, as depicted in figures S1(b-c). Prior to the development those were soft baked at 95 • C for 1 min. After the marker patterns were fabricated, a thin layer of photopolymer (NOA-65) was spin-coated onto quartz disk samples. The spincoated layer was cured using a UV lamp. The specific recipe enabled to produce a coating thickness of 4 µm, this was verified with the profilometer measurements. A schematic of the final disk structure is shown in figure S1(c).

Large-area Integration, Mapping and Printing of NW devices
A slab of PDMS was used to transfer as-grown GaAs-AlGaAs core-shell NW lasers 1 from their growth substrate onto a host quartz disk, as depicted in figures S2(a-d). The PDMS sample was fabricated following the standard recipe and then cut using a scalpel knife forming a square (∼ 2 mm 2 ) piece. Next, it was mounted as a stamp into the MTP rig and aligned with the GaAs-AlGaAs NW growth sample, as shown in figure S2(a). The NW capture process was done in two stages shown in figure S2   Each identified and characterized NW device was assigned a unique identification number.

Figures S4(a-f) show an example of typical characterization information for a NW device.
A plot in figure S4(a) shows the device (red dot) location on the disk substrate relatively to the alignment markers (black grids). Processed bright-field images, like that in figure   S4(b), were used to estimate the NWs' lengths and their relative orientation to that of the markers. The 'orientation parameter' became crucial when verifying if a specific device was correctly identified, prior to its further integration onto the host disk substrate. Figure S4(c) plots a dark-field micrograph of the lasing emission of the NW when optically pumped laser above its threshold. Figures S4(d-e) depict collected emission spectra of the NW laser at various excitation powers and processed the LILO curve, respectively. The later was used to calculate the device lasing threshold level. Figure S4(f) shows the measured PL spectrum measurement of the device below its lasing emission threshold. By using the processed data, the NWs were pre-binned and selected for their further transfer printing onto the host substrate. The NW devices used in this project are multi-quantum well core-shell GaAs-AlGaAs NW lasers. These devices can emit both from its core (E core ) and quantum wells (E M QW ) at room temperature. Hence, using low power PL measurements (see 2 for a reference) we were able to assess if the NW lasers were damaged during the printing process. This was achieved by evaluating PL emission peaks pre-and post-processing. An example in figures S5(a-b) show two normalized PL emission spectra of a pre-and post-processed device. Analyzed PL plots showed no significant mode shifts or mode suppression between pre-and post-processed devices. Based on that, we conclude that the emission parameters of the MTP lasers were not affected by the described printing processes. Case 'A' group, shown in figure S6(a), includes NW devices whose FP modes remained the same after the post-processing. Moreover, their peak power corresponding to the wavelength of emission was retained. Effectively, in these devices the lasing spectra was not significantly altered between 1st and 2nd measurement rounds. However, it should be noted that due to the radial asymmetry found in NW lasers 4 , their emission radiation is strongly related to their position on a substrate. As a result, in these cases NW lasers are assessed as 'NWon-substrate' devices, rather than NW behavior independently. Hence, that could partially explain why there are more case 'A' devices in un-printed (45 out of 74) than printed devices (8 out of 24). When re-printing those we change their geometry relative to the substrate orientation, directly affecting device performance. Figure S6(b) shows case 'B' devices, these contained a group of wires where FP modes overlap with those identified in the pre-processing, however with a changed peak power.

Definition of NW Laser Cases
Again, this could be related to the fact that devices/substrate has changed the conditions or the change in the pumping conditions was a significant enough to affect the lasing modes of the devices.
With Case 'C' devices, shown in figure S6(c), the FP modes were not identified with the previously characterized ones, although the PL measurements were comparable with those measured before. It should be noted however, that although it is difficult to explain exactly the nature of these changes, the devices still retained their lasing emission at room temperature.