Letter pubs.acs.org/journal/apchd5
Printed Nanolaser on Silicon Jungmin Lee,† Indra Karnadi,†,‡ Jin Tae Kim,§ Yong-Hee Lee,*,† and Myung-Ki Kim*,⊥ †
Department of Physics, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, South Korea Department of Electrical Engineering, Universitas Kristen Krida Wacana, Jakarta Barat 11470, Indonesia § Creative Future Research Laboratory, Electronics and Telecommunications Research Institute (ETRI), Daejeon 34129, South Korea ⊥ KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 02841, South Korea ‡
S Supporting Information *
ABSTRACT: We propose and demonstrate a direct integration of a wavelength-scale III−V nanolaser onto a silicon-on-insulator (SOI) waveguide. By employing high-precision microtransfer printing techniques, with an optimally designed photonic crystal nanolaser structure, we experimentally achieved a coupling efficiency of 83% between the InGaAsP nanobeam laser and the SOI waveguide. Our III−V nanobeam laser is designed as an asymmetric one-dimensional photonic crystal cavity, which allows unidirectional coupling to the combined III−V nanobeam waveguide with high efficiency. Through the compact vertical coupler in the region where the III−V and SOI waveguides overlap at the optimal length of 3.2 μm, 88% of the light from the printed III−V nanolaser can theoretically be coupled to a vertically integrated SOI waveguide. KEYWORDS: silicon photonics, III−V/Si integration, nanolasers, photonic crystals, silicon waveguides, photonic integrated circuits
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by using the transfer printing method.16,17 In spite of these, the size of lasers has still not reached the nanometer scale. In this work, we propose and demonstrate an ultracompact and efficient integration of a wavelength-scale unidirectional III−V nanobeam laser onto a conventional SOI waveguide. By optimally designing the asymmetric one-dimensional III−V photonic crystal nanobeam laser and printing it precisely on a SOI waveguide using a microtransfer printing method, we experimentally achieved a coupling efficiency of 83% between the InGaAsP nanobeam laser (Vm = 0.26 (λ/n)3) and an integrated SOI waveguide, within a coupling length of 3.2 μm. We strongly believe our nanoscale hybrid III−V/Si laser platform will open the possibilities for compact, fast, and efficient silicon-based optical communications in the future.
ilicon photonics is one of the most powerful means of replacing electronics-based interconnects by optical interconnects and the most promising platform for future photonic integrated circuits.1−5 One key requirement is the seamless integration of high-performance lasers on silicon, because silicon cannot produce light directly due to its natural indirect band gap.6 Recently, silicon-based hybrid laser platforms using high-performance III−V lasers attached to silicon have attracted much attention.7−14 Several III−V/Si integration techniques, such as wafer bonding7−10 and direct growth methods,11−14 have been developed and are considered as the most common methods to date. Lasers used in III−V/Si hybrid platforms have evolved to smaller sizes, thus minimizing the overall circuit dimensions, increasing the communication speed, and reducing the energy consumption in optical communications.5 Micrometer-size hybrid III−V/Si microring and microdisk lasers have been successfully demonstrated with low power consumption and high coupling efficiency with a silicon-on-insulator (SOI) waveguide.6,8,15 In 2016, D. Liang et al. demonstrated an electrically driven III−V-on-silicon microring laser, with a diameter of ∼40 μm, by using wafer bonding techniques.15 For smaller, faster, and more efficient hybrid III−V/Si photonics, the laser size should be reduced. However, reducing the size to the nanometer scale leads to inevitable problems in integration with silicon photonics, such as a significant reduction of coupling efficiency due to the large beam divergence of small lasers and challenges of nanofabrication. Recently, a new method, the so-called “pick-up and placement”, has been introduced to selectively integrate small III−V lasers on silicon © XXXX American Chemical Society
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DEVICE CONCEPT The schematic of the proposed III−V/Si nanolaser on the SOI waveguide is illustrated in Figure 1a. The small footprint photonic crystal (PhC) InGaAsP nanobeam laser, with a width, length, and thickness of 580 nm, 8 μm, and 280 nm, respectively, is placed on top of the SOI waveguide. The width and thickness of the SOI waveguide are 600 and 220 nm, respectively, and it is designed to operate in a single transverse electric (TE) propagation mode. The structure of the III−V nanolaser is based on an asymmetric one-dimensional photonic crystal cavity, which allows light propagation to the III−V nanobeam waveguide with high efficiency in one direction only Received: May 15, 2017 Published: August 7, 2017 A
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Figure 1. (a) Schematic of the proposed hybrid III−V/Si nanolaser on a silicon-on-insulator (SOI) waveguide. (b) Principle of efficient coupling from the III−V nanolaser to the SOI waveguide.
2a) in the cavity of 310, 340, 370, and 400 nm, respectively. The Q-factor is calculated to be 4.9 × 106, and the resonant wavelength is 1562 nm. The nanobeam without the air holes (or the nanobeam waveguide) can be seamlessly combined with the photonic crystal nanobeam laser, as shown in Figure 2b. Here, the asymmetric PhC cavity delivers the laser emission to the nanobeam waveguide (−x direction), preventing the propagation to the right direction (+x direction). Moreover, reducing the number (N) of air holes between the cavity and the waveguide increases the coupling efficiency (ηlaser to III−V) from the III−V laser to the III−V nanobeam waveguide, as shown in Figure 2b, while the Q-factor is degraded due to the increase of coupling loss to the waveguide. We can see here that an efficiency of over 95% can be obtained with N < 8. The remaining less than 5% light is emitted into the air. The Qfactor starts to degrade when N < 10. In the actual experiment, we chose N = 5 to compensate the inevitable degradation of coupling attributable to fabrication imperfection. For the structure with N = 5, the cavity Q-factor and coupling efficiency ηlaser to III−V are calculated to be 1100 and 98%, respectively. Along the right direction (+x direction) from the PhC nanobeam cavity, a sufficiently long (>4 μm) PhC structure is designed with more than 10 air holes, to completely prevent the propagation loss along the +x direction. Figure 2c shows the calculated |E|2 profile in the xy-plane, when N = 5. It is clearly shown that most of the energy from the laser propagates to the left side of the III−V nanobeam waveguide (−x direction). On the basis of this optimal design, we fabricated the freestanding InGaAsP nanobeam laser with N = 5, as shown in Figure 2d (see Methods). The fabricated InGaAsP nanobeam structure contains three active quantum wells in the middle of the nanobeam. We intentionally introduced thin and short tether structures at both ends of
(−x direction). The coupling strength is controllable by simply adjusting the number of air holes between the cavity and the waveguide.18 A 7-μm-long air gap is introduced under the photonic crystal nanobeam, in order to maintain the original high quality factor (Q-factor), and the resonant wavelength of the nanobeam laser, as shown in Figure 1b. The light from the III−V laser is initially coupled to the III−V waveguide, attached to one end of the nanobeam laser. Then, through the directional coupler in the region where the III−V and silicon waveguides overlap, the light is vertically coupled from the upper III−V waveguide to the lower silicon waveguide. Finally, the light from the nanolaser propagates along the low-loss silicon waveguide.
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DESIGN OPTIMIZATION First, we designed a standalone, freestanding InGaAsP nanobeam laser, by using a three-dimensional (3D) finitedifference time-domain (FDTD) simulation method (see Methods). As shown in Figure 2a, the laser cavity is designed as a one-dimensional PhC nanobeam structure. The periodic lattice constant (a0) is 430 nm, the thickness and width of the nanobeam are 280 and 580 nm, respectively, the radius of each air hole (ri) is 0.34ai, and the refractive index of InGaAsP is 3.4. To increase the Q-factor of the cavity, we gradually modified the size of four air holes with different lattice constants (ai) to form a Gaussian-like photonic well (circles in Figure 2a).19 The circle in Figure 2a represents the cutoff frequencies of the PhC dielectric bands, showing the Gaussian-like profile along the x direction. The resonant mode (red line in Figure 2a) generated by this photonic well has a Gaussian-like field profile, which strongly suppresses the radiation losses because those wavevector components (k-vectors) inside the light cone are negligible in the Fourier k-domain space.20 We found the optimal design with lattice constants (a1, a2, a3, and a4 in Figure B
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Figure 2. (a) Profiles of the photonic crystal cutoff frequency (circle) and electric-field amplitude of the resonance mode (red line) along the xposition. Here, a0 = 430 nm, a1 = 310 nm, a2 = 340 nm, a3 = 370 nm, and a4 = 400 nm. (b) Quality factor of the laser cavity and coupling efficiency between laser and III−V waveguide (ηlaser to III−V) as a function of N. (c) |E|2 profile along the xy-plane, in logarithm scale, when N = 5. (d) SEM image of the fabricated freestanding InGaAsP nanobeam laser/waveguide, with N = 5.
calculated to be 1.428 μm−1 (= π/ΔLc). When Lc = 2.0 and 4.0 μm, ηIII−V to SOI (or ηlaser to SOI) has a minimum of 33% and 36% (or 28% and 30%), respectively. In the experiments, we selected Lc = 3.2 μm, to increase the accuracy of the position of the III−V nanobeam laser on the SOI waveguide in the printing step. Figure 3c shows the calculated |E|2 profiles along the xzplane in logarithm scale, with different Lc values of 0.2, 1.0, 2.0, and 3.2 μm. Note that, when Lc is 1.0 and 3.2 μm, most of the emission from the laser is coupled to the SOI waveguide through the directional coupler in the region where the two waveguides overlap. However, when Lc is 0.2 and 2.0 μm, a small fraction of the laser emission is coupled to the SOI waveguide.
the nanobeam, to allow easy separation of the nanobeam laser from the InGaAsP wafer in the transferring step. In the next step, we investigated the vertical coupling between the III−V waveguide and the silicon waveguide. Figure 3a illustrates the InGaAsP nanobeam waveguide vertically stacked on top of the SOI waveguide. Here, the overlapped region functions as a directional coupler. The fact that the effective refractive indices of the two waveguides are similar ensures smooth transition between the two different waveguides. Figure 3b represents the calculated coupling efficiencies (1) from the III−V laser to the III−V waveguide (black curve; ηlaser to III−V), (2) from the III−V waveguide to the SOI waveguide (blue curve; ηIII−V to SOI), and (3) from the III−V laser to the SOI waveguide (red curve; ηlaser to SOI), as a function of the overlap length Lc. Here, ηlaser to SOI is expressed as ηlaser to SOI = ηlaser to III−V × ηIII−V to SOI. As discussed in Figure 2b, the dependence of ηlaser to III−V on N is stronger than on Lc, so it is almost constant at ∼90%, regardless of the value of Lc, as shown in Figure 3b. On the other hand, ηIII−V to SOI shows the strong sinusoidal dependency on Lc, which is a typical characteristic of a directional coupler (see the Supporting Information). When Lc = 1.0 and 3.2 μm, ηIII−V to SOI (or ηlaser to SOI) has a maximum value of 95% and 97% (or 85% and 88%), respectively. From the periodicity ΔLc of 2.2 μm, the coupling strength (kIII−V/Si) between two waveguides is
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FABRICATION On the basis of the optimal design studied in Figures 2 and 3, we fabricated the hybrid InGaAsP PhC nanobeam laser printed on a SOI waveguide. As described in Figure 2d, we prepared the standalone, freestanding nanobeam laser on an InGaAsP/ InP wafer that contains three InGaAsP quantum wells and an InP sacrificial layer under a 280-nm-thick InGaAsP slab, using conventional nanofabrication processes (see Methods). To selectively transfer the fabricated freestanding InGaAsP nanolaser onto a SOI waveguide, we employed microtransfer printing techniques21 (see the Supporting Information pdf C
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Figure 3. (a) Schematic of the vertically stacked III−V and SOI waveguides, with an overlap length of Lc. Here, the region where two waveguides overlap can be used as a good directional coupler. (b) Coupling efficiencies from III−V laser to III−V waveguide (black; ηlaser to III−V), from III−V waveguide to SOI waveguide (blue; ηIII−V to SOI), and from III−V laser to SOI waveguide (red; ηlaser to SOI), as a function of Lc. (c) The |E|2 profiles along the xz-plane in logarithm scale, when Lc is 0.2, 1.0, 2.0, and 3.2 μm.
Figure 4. Schematic of the transfer printing steps and corresponding microscope images: (a) the target InGaAsP nanobeam laser is selectively picked up from the InGaAsP/InP wafer by using a polydimethylsiloxane (PDMS) stamp of 15 μm × 15 μm × 40 μm, (b) the InGaAsP nanobeam laser attached to the PDMS stamp is lifted and transferred to the preprocessed SOI waveguide, (c) the InGaAsP nanobeam laser is printed precisely on top of the SOI waveguide.
and movie). The schematic of the transfer printing steps and the corresponding microscope images are shown in Figure 4. First, the target InGaAsP nanobeam laser is selectively picked up from the InGaAsP/InP wafer, by using a polydimethylsiloxane (PDMS) stamp of 15 μm × 15 μm × 40 μm (Figure 4a). In this process, physical force is applied to the PDMS stamp to break the thin tethers located at both ends of the InGaAsP nanobeam structure. Then, the InGaAsP nanobeam laser attached to the PDMS stamp is lifted and transferred to the prepared SOI waveguide (Figure 4b). Here, the SOI waveguide
is preprocessed by oxygen plasma to produce a thin reactive oxide layer on the upper surface of the silicon, which further enhances adhesion between the InGaAsP laser and the SOI waveguide. By using high-resolution optical imaging and carefully aligning the III−V and SOI waveguides, the InGaAsP nanobeam laser is printed on top of the SOI waveguide with alignment errors of |Δx| < 300 nm, |Δy| < 300 nm, and |Δθ| < 1.3°, where θ is the rotation angle between the InGaAsP nanobeam and the SOI waveguide (Figure 4c; see the Supporting Information). Once the InGaAsP laser is in contact D
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Figure 5. (a) Fabricated hybrid III−V/Si nanobeam laser printed on the SOI waveguide. Here, Lc is measured to be 3.2 μm. (b) Emission powers measured near the end of the SOI waveguide (black) and near the InGaAsP nanobeam (red) as a function of peak pump power. The inset shows the spectrum of the laser emission. (c, d) NIR charge-coupled device (CCD) images of the laser emissions for the structures with an Lc of 3.2 and 2.0 μm, respectively. Here, the peak pump power is fixed at 0.5 mW. (e, f) Calculated normal Poynting vector profiles in the plane 1.0 μm above the nanobeam laser, when Lc is 3.2 and 2.0 μm, respectively.
Figures 5c and d clearly indicate that the coupling is different, depending on Lc, and the structure with Lc = 3.2 μm shows a large coupling between the InGaAsP nanolaser and the SOI waveguide. To quantitatively estimate the coupling efficiency for the structure with Lc = 3.2 μm, we measured and compared the emission power near both the end of the SOI waveguide (PSOI, black curve in Figure 5b) and the InGaAsP nanobeam (PIII−V, red curve in Figure 5b). At the above threshold power, the ratio of the value of the black curve to that of the red curve (PSOI/PIII−V) is measured to be 2.5, from which the coupling efficiency is estimated to be 83% (see the Supporting Information). Figures 5e and f show the calculated normal Poynting vector profiles in the plane 1.0 μm above the nanobeam laser, when Lc = 3.2 and 2.0 μm, respectively. Note that the emission profiles calculated from the simulation agree well with experimentally obtained CCD images in Figures 5c and d. In summary, we have proposed and demonstrated an efficient integration of a wavelength-scale photonic crystal nanolaser and a conventional silicon waveguide. By optimally designing the hybrid III−V/Si laser structure with an asymmetric one-dimensional photonic crystal nanobeam cavity and a compact vertical coupler and employing the highprecision microtransfer printing techniques, we experimentally achieved a coupling efficiency of 83% between a III−V nanolaser and an integrated silicon waveguide, within a coupling length of 3.2 μm. We believe that our nanoscale
with the SOI waveguide, the PDMS stamp separates the laser away, with a relatively low speed of ∼10 μm/s. The final structure with an InGaAsP nanobeam laser printed on a SOI waveguide is shown in Figure 5a, where the coupling length Lc is measured to be 3.2 μm.
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CHARACTERIZATION AND DISCUSSION The coupling from the fabricated InGaAsP nanolaser to the SOI waveguide was characterized by using a near-infrared (NIR) confocal setup (see Methods). By using an objective lens (50×) with an NA of 0.85, we focused the 980 nm pump beam onto the nanobeam laser and simultaneously collected and imaged the photoluminescent emission from the sample. First, we confirmed the lasing action of our hybrid III−V/Si nanolaser by analyzing the L−L curves (Figure 5b) and the emission spectrum (the inset of Figure 5b). The lasing starts at a threshold pump power of ∼0.2 mW, and the wavelength is 1556 nm. To compare the emission profiles for the samples with different Lc values of 3.2 and 2.0 μm, we took the NIR charge-coupled device (CCD) images in Figures 5c and d, respectively, at the above threshold pump power of 0.5 mW. For the structure with Lc = 3.2 μm, a strong emission is observed at the end of the SOI waveguide, while the structure with Lc = 2.0 μm shows a strong scattering at the interface between the InGaAsP nanobeam and the SOI waveguide. Note that the power of the scattered light at the end of the SOI waveguide is around 13% of the actual power propagating inside the SOI waveguide. The different emission profiles in E
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ORCID
hybrid III−V/Si laser platform will open the possibilities for compact, fast, and efficient silicon nanophotonics in the future.
Jin Tae Kim: 0000-0002-1739-9885 Myung-Ki Kim: 0000-0002-8896-6912
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METHODS A. Fabrication of the III−V Nanolaser. The freestanding nanobeam laser on an InGaAsP/InP wafer that contains three InGaAsP quantum wells and an InP sacrificial layer under a 280-nm-thick InGaAsP slab was fabricated by the following nanofabrication processes. A 150 nm thick poly(methyl methacrylate) (PMMA) mask was patterned by electron beam lithography, and the InGaAsP slab was etched through the patterned PMMA mask, by using a Cl2 gas assisted accelerated Ar ion beam. An O2 plasma ashing process was then performed, to remove the remaining PMMA mask. A final wet etching process (HCl/H2O = 3:1) created the freestanding PhC nanobeam structure, by removing the InP sacrificial layer under the InGaAsP slab. B. Optical Measurements. The fabricated III−V/Si nanolaser was tested for efficient coupling, by imaging and measuring the laser emissions in a confocal setup. By using a 50× objective lens (NA = 0.85), a pump beam was focused on the sample with a spot size of ∼3.2 μm, and the emission was collected at the same time. Here, an InGaAs laser at 980 nm was used under pulse pumping conditions, with a repetition rate of 1 MHz and pulse width of 50 ns. Direct imaging of the lasing emission was performed by using a NIR CCD camera. Spectral measurements were collected by using a monochromator (Digikrom-480). The results of Figure 5b were measured at two different points of the sample, by adjusting the position of the image formed on the entrance slit (slit width = 300 μm) of the monochromator. C. Simulation Details. The simulations were conducted numerically by using a commercially available FDTD software package (Lumerical Solutions, Inc.). In the FDTD simulation, the simulation domain around the devices was divided by a spatially uniform, 20 nm grid. The refractive indices of the InGaAsP, silicon, and SiO2 were 3.4, 3.45, and 1.45, respectively. The coupling efficiency of the III−V laser to each waveguide was calculated as the ratio of the Poynting vectors integrated over the square plane (1.5 μm × 1.5 μm) through the waveguides and over the surrounding enclosure of the device. The cutting of the waveguide leads to degradation of the coupling efficiency, due to the back reflection from the end of the waveguides, which is identified in Figure 5f.
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grants (MSIP) (NRF2017R1D1A1B03036010, NRF-2014M3C1A3052567), the KIST Institutional Program (2E26680-16-P024), and the KUKIST School Project. J.T.K. acknowledges support received from the Internal Research Program of the Electronics and Telecommunications Research Institute (17ZS1310).
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.7b00488. Coupling strength between the III−V and SOI waveguides; microtransfer printing setup; alignment errors in transfer printing; misalignment tolerances for coupling efficiency; interface state between InGaAsP nanobeam and SOI waveguide; measurement setup; estimation of the coupling efficiency (PDF) Movie (AVI)
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REFERENCES
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AUTHOR INFORMATION
Corresponding Authors
*E-mail (Y.-H. Lee):
[email protected]. *E-mail (M.-K. Kim):
[email protected]. F
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