Direct Coupling of Coherent Emission from Site-Selectively Grown III

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Direct coupling of coherent emission from site-selectively grown III-V nanowire lasers into proximal silicon waveguides Thomas Stettner, Tobias Kostenbader, Daniel Ruhstorfer, Jochen Bissinger, Hubert Riedl, Michael Kaniber, Gregor Koblmueller, and Jonathan J. Finley ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00805 • Publication Date (Web): 02 Sep 2017 Downloaded from http://pubs.acs.org on September 3, 2017

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Direct coupling of coherent emission from site-selectively grown IIIV nanowire lasers into proximal silicon waveguides T. Stettner1, T. Kostenbader1, D. Ruhstorfer1, J. Bissinger1, H. Riedl1, M. Kaniber1,2, G. Koblmüller1,2, and J. J. Finley1,2 1

Walter Schottky Institut and Physik Department, Technische Universität München, 85748 Garching,

Germany 2

Nanosystems Initiative Munich (NIM), Schellingstr. 4, 80799 München, Germany

KEYWORDS. Nanowire lasers, monolithic III/V integration on Si, waveguide coupling, photoluminescence spectroscopy, Si photonics

Abstract Semiconductor nanowire (NW) lasers are nanoscale coherent light sources that exhibit a small footprint, low-threshold lasing characteristics, and properties suitable for monolithic and siteselective integration onto Si photonic circuits. An important milestone on the way towards novel on-chip photonic functionalities, such as injection locking of laser emission and alloptical switching mediated by coherent optical coupling and feedback, is the integration of individual, deterministically addressable NW lasers on Si waveguides with efficient coupling and mode propagation in the underlying photonic circuit. Here, we demonstrate the monolithic integration of single GaAs-based NW lasers directly onto lithographically defined Si ridge waveguides (WG) with low threshold power densities of 19.8 µJ/cm² when optically excited. The lasing mode of individual NW lasers is shown to couple efficiently into propagating modes of the underlying orthogonal Si WG, preserving the lasing characteristics during mode propagation in the WG in good agreement with Finite-Difference Time-Domain (FDTD) simulations. Using a WG structure with a series of mask openings along the central mode propagation axis, we further illustrate the out-coupling properties of both spontaneous

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and stimulated emission and demonstrate propagation of the lasing mode over distances > 60 µm, despite absorption in the silicon dominating the propagation losses.

Silicon photonics promises to provide a platform for low-cost, compact photonic and optoelectronic circuits that integrate photonic and microelectronic elements onto a single chip1. In particular, the integration of optical interconnects onto silicon (Si) chips is expected to open the way toward new functionalities, with key applications including ultra-high bandwidth inter-and on-chip communication2,3. Indeed, the most basic building blocks required for on-chip Si-based photonic technology have been already realized, including highspeed modulators and photodetectors4,5, as well as passive devices such as multiplexers and demultiplexers6. In contrast, practical Si-based on-chip light sources, which are key for interand intra-chip optical interconnects, are still missing due to the comparatively poor optical gain of indirect-gap semiconductors7. This situation has fueled intense research into the heterogeneous integration of optically efficient III-V semiconductor lasers onto the silicon-on-insulator (SOI) platform. The ability to realize low-loss photonic circuitry using SOI technologies, combined with the potential to directly integrate III-V coherent light sources that couple light into the underlying photonic circuitry8,9 would be a key technological step towards SOI photonics. Extensive work has been carried out to integrate III-V lasers on SOI by wafer bonding or flip-chip integration techniques9-13. However, these approaches suffer from low yield and difficulties associated with large-scale integration, as well as costs due to growth of the active gain medium on expensive III-V substrates10. A more appealing approach is to monolithically integrate III-V laser sources onto Si using direct epitaxial growth processes. While this approach is commonly plagued by high lattice and thermal mismatch constraints and, hence, high densities of threading dislocation (TD) defects propagating into the active gain


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medium14, the implementation of thick buffer layers or active regions containing quantum-dot (QD) layers that are less sensitive to TD defects have recently provided impressive results10,1517

. To circumvent many of the heterogeneous III-V-on-Si integration issues, the site-

selective growth of freestanding nanowire (NW) based resonators is an unique concept for integrating compact nanoscale coherent light sources on silicon without the problems related to extended TD defects18. Recently, a few examples have highlighted that NW-based cavities monolithically grown on Si exhibit low-threshold lasing at wavelengths in the infrared spectral region18-22. Here, a major challenge for the realization of such vertical-cavity NW lasers is to overcome the poor modal reflectivity between the III-V based NW and the underlying Si substrate. This problem can be obviated either by using tapered nanopillars that support whispering gallery modes18 or by integrating dielectric back mirrors providing high refractive index contrast for fundamental modes propagating inside the NW cavity19. Verticalcavity NW-based lasers on Si have also been shown to host high spontaneous emission factors (β-factor > 0.2) due to the low spectral mode density and small mode volume19,23, contributing to reduced laser thresholds. A further attractive feature of III-V NWs is that they can be grown site-selectively on Si19,21, which enables direct monolithic integration of NWresonators on SOI photonic circuits, such as Si waveguides (WG) and gratings at precise positions24. Concepts have been explored to integrate NW arrays onto Si-based WGs and functionalize them as grating couplers to enhance mode propagation inside Si WGs25,26. In addition, NW arrays have been functionalized recently as photonic crystal nanobeam cavities to demonstrate coupling of infrared lasing emission into propagating Si WG modes27. Beyond these concepts it is highly desirable to achieve coupling of coherent emission from individual III-V NW resonators into SOI photonic circuits. Such demonstration would open-up unique opportunities in manipulating interactions of individually addressable


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nanolasers via Si WGs and, thereby, provide routes for injection locking, frequency stabilization, and all optical switching on a single photonic chip28. In this work, we demonstrate ultralow-threshold lasing from an individual monolithically grown GaAs-based NW laser on a Si-based WG realized on an SOI platform. Lasing output from the single NW laser is shown to efficiently couple coherent emission into the Si-based WG modes, which propagate over several tens of micrometers, while preserving their lasing characteristics. The high quality lasing properties are achieved by carefully engineering the WG geometry to achieve high modal reflectivity at the NW/Si-WG interface, while maintaining reasonable coupling properties as supported by FDTD simulations. To prepare the underlying Si photonic interface, Si ridge WGs were fabricated from a [111]-oriented SOI wafer using standard electron beam lithography (EBL) processes. Note that we chose the [111]-orientation simply because it coincides with the natural growth orientation of III-V NWs29. This allows for perfectly vertically aligned NW growth on the top Si (111) surface and, thus provides a model system for light coupling from the NW optical resonator into the propagating modes of the orthogonal Si WG. In a first step, we thinned down the top Si device layer of the SOI to the desired WG height of ~100 – 200 nm using reactive ion etching (RIE). After a first thermal oxidation step, the desired WG geometry (width of ~ 700 nm) was realized by EBL patterning and RIE. In order to prevent parasitic growth at the bare Si WG sidewalls, an additional thermal oxidation step was introduced creating a final SiO2 mask layer on the Si ridge WG with a thickness of ~50 nm. Afterwards, 200 nm diameter circular openings are defined in the SiO2 layer along the center axis of the Si WG using EBL and RIE to precisely define the nucleation sites for the NW growth. Different sets of Si ridge WGs were fabricated in this way, creating different pattern periodicities with spacings between individual mask openings varying between 3 µm and 10 µm. For further details regarding Si WG fabrication we refer to the Supporting Information section.


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GaAs-AlGaAs core-shell NW laser structures were then grown on top of the asprepared Si ridge WGs using solid source molecular beam epitaxy (MBE)19,30. In a first growth step, a nominally ~8-µm long and ~50 – 60 nm thick GaAs NW core was grown along the vertical [111] direction employing a self-catalyzed vapor-liquid-solid (VLS) growth mechanism. During the core growth, the substrate temperature was set to 650 °C and the gallium

(Ga)

flux to 0.07 nm s-1, as calibrated for nominal planar GaAs growth31. During the first 15 min of growth, the arsenic (As) flux was set to 0.11 nm s-1 and then increased afterwards to 0.22 nm s-1 for the remaining 45 min of growth. This two-step procedure results in a relatively straight NW core with minimum tapering as found from growth optimization on reference SiO2masked planar SOI wafers. After the core growth was completed the growth conditions were modified to crystallize the Ga droplet at the NW tip and to induce radial growth around the {110} NW sidewall facets by increasing the Ga and As flux to 0.17 nm s-1 and 1.89 nm s-1, respectively. Using a growth temperature of 650 °C, the GaAs resonator cavity was grown under these conditions to a total cavity width of ~ 400 nm by radial growth of a GaAs shell (nominal shell thickness of 170 nm). Finally, the GaAs resonator cavity was passivated in-situ using a thin 5-nm AlGaAs shell/5-nm GaAs cap layer to provide high optical efficiency and low-loss waveguiding32. During the passivation layer growth, the temperature was further increased to 680 °C to reduce parasitic material deposition on top of the WG. All growth steps are performed under constant substrate rotation of 5 rpm. The scanning electron microscopy (SEM) image shown in Fig. 1(a) shows an endsection of a Si ridge WG, where a single GaAs-AlGaAs core-shell NW laser was successfully grown from a predefined mask opening. The length of this particular NW is ~5.5 µm and the diameter is ~370 nm. As expected from the underlying Si (111) orientation, the NW is aligned perpendicular to the WG axis. In addition, crystalline GaAs clusters with a periodic spacing of


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5 µm are observed at the other mask openings, which exhibit sizes on the order of ~600 – 800 nm. The fact that no NWs formed in these openings might be due to the general difficulties in obtaining high-yield selective-area growth of GaAs NWs on SiO2–templated Si(111) under self-catalyzed growth processes33,34. Recent work demonstrated that optimization of growth yield depends sensitively on mask opening size, SiO2 mask thickness, pre-wetting conditions, and kinetic growth parameters such as V/III ratio and growth temperature33,35-37. Below we show that the specific mask openings, where GaAs clusters formed, can be used to probe the in- and out-coupling behavior of the lasing emission from proximal single WG-integrated NW lasers. To obtain a closer look at the interface between the NW and Si WG, focused ion beam (FIB) milling was used to prepare a cross-section close to the base of the nanowire. Hereby, the sample was milled under an angle of 30° with respect to the surface normal. After milling, an SEM image recorded under a viewing angle of 45° is shown in Fig. 1(b). The image verifies that the NW is connected to the Si WG and allows to further analyze the WG and top SiO2 mask layer dimensions. From the images we obtain a WG width and height of 730±35 nm and 105±5 nm, respectively, whereas the top SiO2 layer has a maximum thickness of 46±3 nm at the perimeter and exhibits the form of a truncated cone towards the center of the WG where the NW nucleated.

Fig. 1: (a) SEM image of a single GaAs-AlGaAs NW laser monolithically integrated onto a planar Si ridge waveguide. Other mask openings resulted in the formation of crystalline GaAs clusters rather than NWs; (b) Cross-sectional SEM image (recorded under a 45° view) of the NW-waveguide interface as prepared by FIB milling (milling angle of 30°). This close-up


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shows a waveguide width and height of 730 nm and 105 nm, respectively, and a top-SiO2 layer in the shape of a truncated cone with a maximum thickness of 46 nm.

To optically investigate the NW lasers, low-temperature micro-photoluminescence (µPL) measurements were performed on individual NWs in their freestanding geometry on top of the Si WG. Hereby, we used an excitation geometry in which the NW is optically pumped perpendicular to its axis in order to create a homogeneously distributed population inversion inside the NW laser. Detection of the resulting emission was recorded on-axis along the NW using a two-axis confocal microscope with independent control of the detection position on the WG surface and excitation channel on the NW (see setup schematic in the Supporting Information, Fig. S2). For optical excitation, a 400 ps duration pulsed laser diode emitting at 655 nm with a repetition frequency of 40 MHz was used. All measurements were performed at a nominal temperature of 10 K. Fig. 2(a) shows typical pump power dependent PL spectra as a function of the photon energy for excitation pump pulse fluences ranging from 24.1 µJ/cm² to 101.3±5 µJ/cm². For low excitation power of 24.1±1 µJ/cm², a broad spectrum attributed to spontaneous emission (SE) is observed at an energy around 1.505 eV, close to the low-temperature band gap of GaAs (1.515 eV at T=0 K)38. With increasing excitation level, the SE spectrum grows in intensity, until at a pump pulse fluence of 59.9±3 µJ/cm² a peak with an intensity that increases super-linearly with pump level, emerges at ~1.49 eV. To further illustrate the lasing characteristics of this particular NW, Fig. 2(b) plots the peak intensity at 1.49 eV (blue data) as a function of the pump pulse fluence on a doublelogarithmic scale. The data exhibits a distinct “s-shape”-dependence, supporting the expected transition from SE to amplified spontaneous emission (ASE) and ultimately to lasing for highest excitation levels. These three regions can be further characterized by using the allometric intensity-power law given by I ∝ Pk. The values obtained from best fits to the data for the SE regime (k ~2.0), ASE regime (k ~6.8) and the lasing regime (k ~1.8) confirm the ACS Paragon Plus Environment

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strongly non-linear characteristic and lasing of the optically pumped GaAs-AlGaAs NW lasers integrated on the Si WGs39 and lie within the theoretically expected range for nearband-edge SE regime40. The transition from SE to ASE and finally to the lasing regime is also reflected by the decrease of the full width at half maximum (FWHM) value of the lasing peak down to 3.2 meV (red data in Fig. 2(b)). The inset in Fig. 2(b) further shows the linear plot of the input-output characteristic allowing the extraction of the lasing threshold of Pth = 59.9±3 µJ/cm². In total, four WG-integrated NW lasers were investigated which exhibit an average lasing threshold of 59.2±27.7 µJ/cm². We attribute this spread to inhomogeneities in the size and shape of the mask opening affecting the end facet reflectivity, as well as NW size fluctuations and deviations from the perfect hexagonal NW cross-section which modify the optical confinement properties30. The peak energy in Fig. 2(a) is about ~25 meV below the expected band gap energy of GaAs, which can be attributed to several reasons. First, the lattice temperature is expected to be increased due to the non-resonant excitation conditions, leading to heating and band gap narrowing. Secondly, with increasing excitation powers one also expects a carrier-induced change in refractive index41,42.

Fig. 2: (a) Pump power dependent PL spectra of a single waveguide-integrated GaAs-AlGaAs NW laser excited from the side and detected along the NW-axis with excitation pump pulse fluences ranging from 24.1±1 µJ/cm² to 101.3±5 µJ/cm² at 10 K; above 59.9±3 µJ/cm² a nonACS Paragon Plus Environment

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linear lasing peak arises at ~1.49 eV. (b) Lasing peak intensity (blue data) and corresponding line width (red data) plotted as a function of the pump pulse fluence on a double-logarithmic scale. The inset shows the threshold level (intercept with x-axis) and linear increase in laser output power above threshold on a linear scale.

After demonstrating lasing operation from the single vertical-cavity NW resonators, we performed spatially resolved, two-dimensional PL scans to examine the light coupling behaviour between the NW laser and the Si ridge WG. During these scans care was taken that the excitation laser homogeneously pumps the NW (constant in power and excitation spot position), whereas the detection spot is raster scanned over the surface along the WG axis. An optical microscope image of the scanned area is shown in Fig. 3 (upper image), which allows a clear identification of the NW, the clusters and the ~40-µm-long Si ridge WG. The nucleation site periodicity, i.e., spacing between SiO2 mask openings on the WG, is 5 µm in line with the data shown in Fig.1(a). In the Supporting Information (Fig. S3), corresponding data is also shown for WGs with different spacings of 3 µm and 10 µm, respectively. For the specific Si WG studied here, NW growth occurred only in a single mask opening in the left region of the WG, while the remaining openings resulted in crystalline GaAs clusters. In addition, this WG structure contains also a simple out-coupling structure on the right-hand side, consisting of three 100 nm wide cylindrical holes inside the WG (in line and perpendicular to the WG axis) where no GaAs cluster formed. The middle section of Fig. 3 shows the spatially resolved PL scan (area 49 µm x 7 µm, with scan step sizes of ∆x = ∆y = 1 µm), at the lasing energy of 1.49 eV plotted on logarithmic intensity scale. The NW laser is located at x = 0 µm and shows a strong PL signal when pumped with a pump pulse fluence of 101±5 µJ/cm², well above threshold. In addition, we observe clearly enhanced intensities at detection spots along the WG that correspond to the 5-µm spaced mask openings. This means that the proximal mask openings, where only GaAs clusters have formed, can be used to study the in- and out-coupling of emission via the WG. We note that the out-coupling via the GaAs


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clusters is more efficient compared to the WG-integrated out-coupler. Moreover, out-coupling at both WG terminations is observed. The bottom part of Fig. 3 shows a 1D line scan along the maximum peak intensities of the NW laser and clusters again on logarithmic scale, unambiguously demonstrating coupling and propagation of light inside the WG over a distance of >30 µm. Even longer propagation in excess of 60 µm is found for similar WG structures with 10-µm spaced mask openings (see Supporting Information, Fig. S3). Comparison of the out-coupled emission intensity from the different cluster sites (1D line scan of Fig. 3), shows that emission intensity decays with the distance away from the NW laser. This is a first indication that the detected signals from the equidistant 5-µm spacings originate from the NW laser rather than light emitted by the clusters. Indeed, the emission from GaAs clusters is spectrally much broader and occurs at higher energy (~1.51 eV) with a much weaker intensity as confirmed in the Supporting Information (Fig. S4).

Fig. 3: Spatially resolved identification of the WG-integrated NW laser. The upper part shows an optical microscope image that allows a clear identification of the NW, clusters (labeled as C) and the WG with a total length of ~40 µm. The middle section shows a spatially resolved, two dimensional PL scan across the Si WG recorded in normal incidence to the WG for a pump fluence of 101±5 µJ/cm² well above lasing threshold. The PL intensity is integrated around the lasing peak at 1.49 eV, normalized and plotted on a logarithmic scale. The bottom


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part shows a 1D line scan along the maximum peak intensities of NW and clusters, clearly confirming increased light emission at the 5-µm spaced nucleation sites.

We conducted a systematic investigation of the coupling behavior between the NW laser and the out-coupled emission along the WG openings. Hereby, we performed pump power dependent PL measurements of the same NW laser excited from the side, while the resultant PL signal is recorded from the top of the NW and compared to the PL signal outcoupled from the openings C2 and C4 as depicted in Fig. 4(a). For each position, three spectra are presented for threshold-normalized excitation powers of P/Pth=0.5 (blue), P/Pth=1.3 (green) and P/Pth=2.2 (red), which correspond to the SE, ASE and lasing regimes, respectively. For excitation well above threshold (P/Pth=2.2) we observe two distinct nonlinear lasing peaks emerging at 1.50 eV and at 1.49 eV. While this particular NW laser exhibits, therefore, bimodal lasing we note that most of the studied WG-integrated NW lasers show single mode lasing under the investigated pump conditions (see also Supporting Information). Most importantly, the bimodal lasing characteristics are well preserved in the out-coupled signal at the two respective openings C2 and C4. Also, it is obvious that the broad SE-background from the NW is strongly reduced during the coupling procedure. A quantitative analysis of the coupling efficiency of the SE background shows that it is by a factor of ~2x smaller as compared to the lasing mode (see Supporting Information Fig. S5). Hence, the lasing mode has a higher coupling efficiency to the WG, as also confirmed by simulations below, providing a useful route for coupling and steering coherent light across NW/WG interfaces. Fig. 4(b) further compares the input-output characteristics of the laser peak at higher energy (~1.50 eV, indicated by the blue shaded guide to the eye) on a double logarithmic scale. Here, the PL intensities stemming from the openings C2 and C4 were multiplied by factors of 10x for better comparison. From this representation, we find that the intensity of the ACS Paragon Plus Environment

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lasing emission is gradually reduced when it is out-coupled further away from the NW laser source. Such attenuated mode propagation along the Si WG is expected, given the relatively strong absorption of coherent light at central wavelengths of ~820 nm within Si ridge WGs as further discussed below. While the high-energy lasing peak has a threshold of Pth = 19.8±1 µJ/cm² (CW equivalent power of 246 µW ≙ 794 W/cm²) as measured at the NW position, the lasing emission out-coupled from C2 and C4 show comparable thresholds (Pth, C2 = 25.4±1 µJ/cm² and Pth, C4 = 23.4±1 µJ/cm², respectively). We emphasize that these threshold values are about 3.5× lower than that reported in Ref. 19, and, thus, the lowest reported value so far achieved at cryogenic temperatures for an individual monolithically integrated III-V NWbased laser on Si. Threshold values for stimulated emission have recently been also reported for InP nanopillars on Si WGs (~223 µJ/ cm²)43 as well as for InP-InGaAs quantum-well nanopillars on Si (36 kW/cm²)21, which are more than one order of magnitude larger. On the other hand, NW-array lasers, as most recently reported27, exhibit comparable thresholds (15 µJ/cm²) as the present individual WG-integrated GaAs-NW laser. We attribute the very low threshold of our WG-integrated NW laser to the high quality of the NW-Si WG interface as well as an increased bottom facet reflectivity due to the very thick (~4 µm) SiO2 layer below the WG.


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Fig. 4: (a) Pump power dependent PL measurements for three different excitation powers of P/Pth=0.5 (blue), P/Pth=1.3 (green) and P/Pth=2.2 (red) showing bimodal lasing behavior. The excitation spot is fixed at the NW and the emission is detected at three different positions on the WG (on-axis emission from the NW as well as the emission out-coupled from the openings C2 and C4 at distances of 10 µm and 20 µm away from the NW, respectively. (b) Input-output characteristics of the lasing peak at 1.50 eV for the three detection spots under highest pump power (P/Pth=2.2). The intensities for data points C2 and C4 are multiplied by a factor of 10x for better visibility.

To further describe the effective mode coupling from the NW laser into the Si ridge WG and the propagation therein, numerical simulations were performed. Fig. 5(a) presents the intensity distribution of the electromagnetic field in the longitudinal cross-section across the NW/Si-WG heterointerface, as obtained from 3D optical FDTD simulations (Lumerical). The top inset shows the transversal cross-section of the electric field intensity distribution of the mode TE01, which was used as the source mode. Indeed, for the given NW cavity dimensions this mode is expected to be one of the dominant lasing modes as shown by Saxena et al.44. We also emphasize that for the FDTD simulations we adapted directly the geometrical dimensions of the NW and WG from the quantitative SEM analysis in Fig. 1. For this configuration, we obtain a value for the modal reflectivity at the NW/WG interface of ~29 %. This


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comparatively low value can be attributed to the cone-shaped mask opening from which the NW laser nucleated, and which thereby reduces the reflectivity of the SiO2 mask layer. Higher modal reflectivities of close to 60% may be obtained if the mask opening has a well-defined cylindrical shape with small opening diameter19, as shown in the Supporting Information (Fig. S6). Nevertheless, modal reflectivity and in-coupling efficiency come at a tradeoff, whereby the cone-shaped opening results in larger mode coupling efficiency compared to the regularly shaped opening. The calculated mode coupling efficiency of the selected TE01 mode is ~16% for the investigated NW laser with cone-shaped opening, which is significantly higher compared to 5% for the regularly shaped opening. The good mode coupling efficiency of the TE01 mode is also directly shown in the bottom inset. We attribute the higher mode coupling efficiency to a change of the total effective refractive index below the NW bottom facet. By adjusting the thickness of the oxide layer between NW and WG, the overlap of the NW and WG mode can be tuned. To quantitatively assess the attenuated mode propagation along the Si WG, we performed additional spatially-resolved PL measurements along the WG and compared data to a generic model describing the expected absorption losses of the propagating mode. Hereby, the NW was excited into the lasing regime while the emission from each mask opening was detected for individually adjusted focus and, thus, optimized intensity collection. Fig. 5(b) shows the normalized PL intensity extracted at the lasing peak energy of 1.49 eV that is plotted against the spatial separation with respect to the NW laser for a Si WG with mask openings every 10 µm. Similar plots were also generated for other waveguides with mask opening spacings of 3 µm and 5 µm, and are shown in the Supporting Information (Fig. S7).


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Fig. 5: (a) 3D FDTD simulations of the WG-coupled NW laser under a geometrical configuration adapted from Fig. 1(b). The image shows the intensity distribution of the electric field of the TE01 mode in the longitudinal cross-section and verifies efficient coupling into the Si-WG (also shown in transverse sectional view, see bottom inset). (b) Attenuation of mode propagation inside the Si-WG as illustrated by the out-coupled lasing emission intensity as a function of the distance of mask openings along the Si-WG (equidistant spacings of 10 µm). Data is obtained under a pump pulse fluence of ~100 µJ/cm² and normalized to the peak intensity of the first out-coupling mask opening at a distance x = 10 µm away from the NW laser source. The red dashed line represents an exponential fit based on Beer-Lambert’s law.

The data clearly shows an exponential attenuation behavior of the out-coupled lasing emission intensity with the distance from the NW laser source. To describe this characteristic behavior we define the energy dependent absorption coefficient αSi (E) based on BeerLambert’s law via I(x) = I0.e-αx, where I(x) is the intensity over the propagation distance x. Besides absorption, also a certain fraction of light, which couples out through the mask openings (GaAs clusters), contributes to the attenuation of the intensity of the propagating wave. The following equation combines both effects, i.e., Iout,n(x) = I0.e-αx(1-f)n-1f, where Iout,n(x) gives the intensity that is coupled out of the WG after n number of mask openings with an out-coupling coefficient f for the openings. For simplicity, the out-coupling coefficient is assumed to be equal for all openings (fn = f). In a first approximation, the outcoupling coefficient f is set to zero, taking only absorption into account. By fitting the simple


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Beer-Lambert law to the data (red dashed line in Fig. 5(b)), we thereby obtain an absorption coefficient of α = 784±83 cm−1. Within the tolerance of the fit, the obtained value is in good agreement with the theoretical value for a planar wave at a wavelength of 830 nm propagating in bulk Si (αtheoretical = 647 cm−1)

45

. When applying the modified equation taking the out-

coupling coefficient f into account, we find that the out-coupling efficiency at the sites of the first three GaAs clusters C1 to C3 behave very similar (f1,2 = 0.44 and f2,3 = 0.51), which allows us to assume f1,2 = f2,3 for a linear system of equations. This system can be solved selfconsistently, giving a value of α = 610 cm−1, which fits even more accurately to the theoretical value. In contrast, for WGs with higher mask opening periodicity, i.e., spacings of 3 µm and 5 µm, the contribution from the out-coupling coefficient associated with the openings becomes more significant. This is illustrated in the Supporting Information (Fig. S7), where the extracted absorption coefficients for WGs with 3 µm and 5 µm spacings are 2415±517 cm−1 and 1588±222 cm−1, respectively, and deviate significantly from the theoretical value. In fact, for these WGs the fitting procedure to the data is hampered by the fact that the GaAs clusters residing in the out-coupling openings are less homogeneous and, thus, a fixed out-coupling coefficient cannot be applied. Overall, the presented data illustrates that for WGs with low mask opening periodicity (spacing of 10 µm) absorption dominates over the intensity losses stemming from the out-coupling openings, whereas this situation inverts for higher mask opening periodicities. In order to evaluate the coupling mechanism further, specific structures with e.g. grating couplers at the end facets of the WGs could be employed in future experiments. Moreover, additional efforts need to aim at reducing absorption losses by developing single WG-integrated NW lasers with emission wavelengths in the Si-based telecommunication bandwidth. In conclusion, we demonstrated lasing from individual GaAs-AlGaAs NWs integrated on Si ridge waveguides (WG) on a SOI platform under optical excitation. The observed lasing


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behavior shows clear “s-shape”-characteristics, linewidth narrowing and threshold values down to 19.8±1 µJ/cm², which is the lowest value reported to date for this kind of integrated lasing structure. In addition, proof-of-principle coupling of both spontaneous and stimulated emission to the Si WG was shown from a single NW, with excellent preservation of the lasing characteristics inside the WG and propagation distances exceeding > 60 µm. These results pave the way for future on-chip optical interconnects and exploration of injection locking and optical switching schemes exploiting individually addressable, monolithically integrated III-V NW lasers.

ASSOCIATED CONTENT Supporting

Information.

The

following

files

are

available

free

of

charge.

Details on Si waveguide fabrication, two-axis confocal microscopy setup, experimental and


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simulated data on coupling between NW and Si waveguide, and absorption properties of Si waveguide. (PDF)

AUTHOR INFORMATION Corresponding Author * [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was supported financially by the Deutsche Forschungsgemeinschaft (DFG) via project FI 947/4-1 KO 4005/7-1, the Nanosystems Initiative Munich (NIM), the International Graduate School of Science and Engineering (TUM-IGSSE) and the IBM International Ph.D. Fellowship Program. (1) R. Soref, “The past, present, and future of silicon photonics,” IEEE J. Sel. Topics Quantum Electron., 2006, vol. 6, pp. 1678-1687. (2) D. A. B. Miller, “Device requirements for optical interconnects to silicon chips,“ Proc. IEEE, 2009, vol. 97, pp. 1166-1185. (3) K. Ohashi, et al., 2009, “On-chip optical interconnects”, Proc. IEEE, vol. 97, pp. 11861198.


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For Table of Contents Use Only Direct coupling of coherent emission from site-selectively grown IIIV nanowire lasers into proximal silicon waveguides T. Stettner, T. Kostenbader, D. Ruhstorfer, J. Bissinger, H. Riedl, M. Kaniber, G. Koblmüller, and J. J. Finley


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Direct Coupling of Coherent Emission from Site-Selectively Grown III

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