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Mid-infrared all-optical modulation in low-loss germanium-on-silicon waveguides Li Shen, Noel Healy, Colin J. Mitchell, Jordi Soler Penades, Milos Nedeljkovic, Goran Z. Mashanovich, and Anna C. Peacock* Optoelectronics Research Center, University of Southampton, Southampton SO17 1BJ, UK *Corresponding author: [email protected] Received October 15, 2014; accepted November 29, 2014; posted December 8, 2014 (Doc. ID 225027); published January 14, 2015 All-optical modulation has been demonstrated in a germanium-on-silicon rib waveguide over the mid-infrared wavelength range of 2–3 μm using a free-carrier absorption scheme. Transmission measurements have shown the waveguides to have low propagation losses that are relatively independent of wavelength out to 3.8 μm, indicating that the modulation could be extended further into the mid-infrared region for applications in sensing and spectroscopy. By monitoring the material recovery, the free-carrier lifetime of the micron-sized waveguides has been estimated to be ∼18 ns, allowing for modulation speeds within the megahertz regime. © 2015 Optical Society of America OCIS codes: (160.6000) Semiconductor materials; (230.7370) Waveguides; (230.4110) Modulators. http://dx.doi.org/10.1364/OL.40.000268
Spanning the wavelength range λ 2–20 μm, the midinfrared (mid-IR) region is of tremendous spectroscopic interest as it contains strong vibrational signatures for a number of gases and molecules. Photonic components that operate in this long-wavelength region can thus be applied for a host of applications in environmental and bio-chemical sensing [1]. Over the years, a number of mid-IR material platforms have been investigated, including chalcogenides glasses [2], silver halides [3], and various unary and compound semiconductors [4–6]. Of late, there has been growing interest in the area of group IV semiconductor platforms, from which it is possible to leverage the well-established fabrication processes and CMOS compatibility to develop integrated mid-IR optoelectronic systems [6]. In this regard, much of the focus has been on silicon-based platforms, with low-loss waveguides having been demonstrated in silicon-on-insulator [7], silicon-on-sapphire [8], and suspended silicon [9]. However, a drawback of using silicon is that its transparency is limited to wavelengths below 8 μm, thus excluding its application in the important molecular ‘fingerprint’ region. As a result, some of the attention in group IV photonics has been turning to germanium-based platforms, which offer extended mid-IR transmission out to 14 μm [6]. Moreover, compared to silicon, germanium has several other desirable properties such as higher nonlinear coefficients [10] and superior electronic properties [11] that can be exploited for use in a wide range of active and passive devices. Although germanium-on-silicon (Ge-on-Si) platforms have long been proposed as potential candidates for mid-IR applications [12], it is only fairly recently that the first Ge-on-Si waveguides have emerged [13]. Prior to this, efforts to realize germanium-based waveguides have included germanium core fibers [14] and germanium slabs on zinc sulfide substrates [15]. However, a key advantage of the Ge-on-Si platforms is that they facilitate integration with other group IV photonic chip-based technologies [16]. So far these first-generation Ge-on-Si waveguides have exhibited low transmission losses for selected wavelengths across the 2–6 μm range, and are 0146-9592/15/020268-04$15.00/0
already showing great promise for the development of chip-based spectrometers, with evanescent waveguide sensors [17] and wavelength (de)multiplexers [18] having been demonstrated. To maintain progress in this area, efforts must continue to build on these basic elements by demonstrating more complex optical and electronic functionality within the germanium platforms. In this Letter, we present what we believe to be the first demonstration of all-optical modulation in a Ge-on-Si rib waveguide using a simple free-carrier absorption scheme [19]. The experimental setup is based on a pump-probe geometry where a below bandgap λ 1.54 μm pulsed source was used to modulate various continuous wave (CW) signals across the mid-IR wavelength range of 2–3.2 μm. Modulation extinctions of up to 5 dB were obtained for pulse energies of a few picojoules. By studying the material recovery, we have also obtained an estimate for the free-carrier lifetime of τ ∼ 18 ns within the micron-sized waveguides. We expect that by further reducing the waveguide dimensions to decrease the lifetime and improve the pump/probe overlap, it will be possible to develop high-speed and high-extinction ratio all-optical modulators capable of operating across much of the important mid-IR regime. The starting Ge-on-Si wafers were fabricated by epitaxially depositing a 2-μm-thick germanium layer onto a silicon substrate using a chemical vapor deposition (CVD) method [20]. To fabricate the rib waveguides, a SiO2 hard mask was defined, which was deposited via plasmaenhanced CVD, then patterned via contact lithography and reactive-ion etching (RIE) using an Ar∕CHF3 plasma. The photoresist was subsequently stripped and a further RIE step performed (SF6 ∕CHF3 plasma) to form the waveguides. Finally, the SiO2 mask was removed using a HF bath, and the samples were cleaned and cleaved. The waveguides used in this work were designed to have an etch depth of 1.2 μm and a core width of 2.25 μm, so that they only supported the fundamental (TE and TM) modes for wavelengths >3 μm. We note that although for shorter wavelengths the core does support the second-order modes, with optimal coupling, it is © 2015 Optical Society of America
January 15, 2015 / Vol. 40, No. 2 / OPTICS LETTERS
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Fig. 1. (a) SEM micrograph of Ge-on-Si rib waveguides. The straight sections correspond to the 10 μm-wide coupling tapers, and the curved section is the smaller 2.25 μm wide core. (b) Cross-sectional image of the input taper and (c) the core.
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straightforward to launch only into the fundamental. To help with the coupling, input and output tapers were fabricated at each end with a maximum width of 10 μm and a flare angle that was carefully chosen to minimize coupling losses across the investigated wavelength range (in all cases the transmission from taper to waveguide core was >96%). Waveguides of different lengths were incorporated on the chip by introducing four identical bends, of varying separation, in each core section. Figure 1(a) shows a scanning electron microscope (SEM) image of the patterned waveguides, from which both the wide tapered input and waveguide core (with bends) can be identified. SEM cross-sectional images of the input facet and waveguide core are then displayed in Figs. 1(b) and 1(c), respectively, showing that the rib structure is well formed. To characterize the optical transmission properties of our Ge-on-Si waveguides, linear loss measurements were performed using various mid-IR sources over the wavelength range 1.9–3.8 μm. An effective cut-back method was applied by comparing the transmitted power through waveguides of different lengths, where any influence from the bends was removed by ensuring the bend number and radius were fixed for the entire waveguide set. Furthermore, the bends were designed to have a relatively large radius of 100 μm [see Fig. 1(a)], which ensured that their contribution to the overall losses was minimal. Our initial measurements were conducted over the short wavelength range of 1.9–2.5 μm using a tunable CW Cr2 :ZnSe laser source. For these wavelengths, the light could be launched into the core using a 63× silica microscope objective (0.85 NA), which allowed for optimal coupling into the fundamental TE mode of the taper, and hence the waveguide core. Confirmation that the mid-IR light was predominantly guided in the fundamental mode can be seen through the highquality output profile displayed in the inset of Fig. 2(b). The output was then collected via a ZnSe objective with a 6 mm focal length, and measured using a PbSe pre-amplified photoconductive detector. A plot of the normalized transmission for an input wavelength of 2.3 μm over several waveguide lengths (L 1.4–2.4 cm) is shown in Fig. 2(a), revealing a loss of 3.0 0.3 dB∕cm. Subsequent measurements were performed for longer wavelengths, first using an Aculight CW optical parametric oscillator (OPO) over the range 2.6–3.2 μm, then a
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quantum cascade laser (QCL) to push out as far as 3.8 μm. As it was not possible to use the silica coupling objective for these wavelengths, an aspheric chalcogenide lens with a focal length of 1.875 mm was used to couple light from the OPO, while a mid-IR fiber buttcoupled to the waveguide core was used to launch light from the QCL [7]. A summary of the loss measurements taken over the entire wavelength range is provided in Fig. 2(b). The high loss value recorded at 1.95 μm is expected as it is close to germanium’s band edge, and beyond 2 μm the losses flatten out to a consistently low value of ∼3 dB∕cm. The results indicate that the Ge-on-Si waveguides should exhibit a broad transmission window across much of the mid-IR regime provided the waveguide design is optimized to minimize the modal overlap with the Si substrate, which becomes lossy for λ > 8 μm. Compared to previous measurements in Geon-Si waveguides, our lowest loss of 2.5 0.2 dB∕cm taken at λ 3.8 μm is commensurate with the losses reported for wavelengths ∼5 μm [13,18]. However, the fast drop-off in the losses near the band edge does represent a significant improvement over our previous measurements in this regime [16], which we attribute to a reduction of strain-induced defects near the core/cladding interface [21]. Importantly, the low losses near ∼2 μm open up the possibility to make use of the highperformance telecoms diagnostic tools that operate up to this short wavelength edge of the mid-IR regime.
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Fig. 3. Experimental setup to demonstrate all-optical modulation in the Ge-on-Si waveguides. ATT, attenuator; BS, beamsplitter; O1 & O2, microscope objective lenses.
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To demonstrate all-optical modulation in the Ge-on-Si waveguides, a series of pump-probe experiments were undertaken across the 2–3.2 μm wavelength region. A schematic of the experimental setup is provided in Fig. 3, where the pump must be positioned at a wavelength where the photon energy is much larger than the bandgap of germanium (λ < 1.9 μm) to efficiently excite the free-carriers that modulate the propagating signal. For the pump, we chose a high-power mode-locked fiber laser operating at the telecommunications wavelength of 1.54 μm, with a pulse duration of 720 fs and a 40 MHz repetition rate. The low-power signal was then provided by either the Cr2 :ZnSe laser or the OPO so that modulation could be investigated over the selected wavelengths of λ 2.01, 2.6, 3.0, and 3.2 μm. In all cases, the pump and probe beams were combined with a pellicle beam-splitter (BS) before coupling into the waveguide using an appropriate objective. In this co-propagating configuration, the absorption of the pump occurred in the tapered region near the waveguide input, and not in the small core. As a result, the only output from the waveguide was the modulated mid-IR probe light, which was collected by the ZnSe objective for detection. Although the λ 2.01 μm signal was positioned just on the edge of the high loss region of the material, at this short wavelength we could make use of our fast, 10 GHz bandwidth InGaAs photodetector (peak responsivity at 1.9 μm) to study the temporal dynamics of the modulation. For the longer wavelengths, we simply made use of the PbSe detector to monitor the modulated power change. The inset of Fig. 4 shows the free-carrier modulated probe amplitude of the λ 2.01 μm signal as recorded on a 12 GHz real-time oscilloscope. The measurement was conducted with a coupled signal power of ∼1 mW and a pump energy of only 44 pJ. Unfortunately, owing to the limited bandwidth of our commercially available InGaAs detector, it was not possible to fully resolve the fast build-up of free-carriers that are generated on the timescale of the femtosecond pump pulses [22]. However, the slower dynamics associated with the freecarrier recombination can be clearly resolved, and through an exponential fit to the recovery (dashed lines), we can estimate the free-carrier lifetime to be τ ∼ 18 ns. This lifetime is considerably faster than the microsecond timescales reported for bulk germanium samples [23], as we would expect for a confined waveguide geometry.
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Furthermore, if we recall that the recombination is taking place in the tapered input, this lifetime is also faster than what has been measured for silicon waveguides with similar micron-sized core dimensions [24], which we attribute to the increased carrier mobility of germanium. As the two main mechanisms for removing carriers from the core of a rib waveguide are diffusion into the slab [24], and recombination via surface and interface states [25], we expect that the lifetime would be even lower in the smaller core section. Thus a route to improving the speed of our system would be to pump the 2.25 μm core from the top, although this would come at the expense of a reduced pump/probe overlap, and would also greatly complicate the experimental setup [19,22]. Finally, although it is not possible to directly measure the modulation depth, we can consider the pump induced carrier generation to be instantaneous when compared to the slow recovery time, which allows us to use the exponential fit to place a lower bound on the extinction ratio of ∼4 dB [22]. The remaining measurements were then conducted within the lower loss, longer wavelength region using the OPO. In these experiments the carrier lifetime was used to fit the temporal dynamics of the average power changes measured on the PbSe detector, from which it was possible to estimate the modulation depth. Figure 4 shows a plot of the modulation depth as a function of the pump pulse energy for the longest signal wavelength of 3.2 μm, showing the expected trend of increasing modulation for increasing pump power. We note that the highest extinction ratio of 5.1 dB was only limited by the available pump energy. Repeating these measurements for λ 2.6 μm and 3 μm revealed maximum extinctions of 4.6 and 4.8 dB, respectively. We attribute the trend of increasing extinction for increasing wavelength to the larger free-carrier absorption coefficient and mode size in the longer wavelength region [19], which is promising for applications extending even further into the mid-IR. In summary, we have demonstrated all-optical modulation out to 3.2 μm in a Ge-on-Si rib waveguide using a
January 15, 2015 / Vol. 40, No. 2 / OPTICS LETTERS
simple free-carrier absorption scheme. These waveguides have been shown to exhibit low losses that are relatively independent of wavelength over the range 2–3.8 μm, which suggests that the transmission region should extend well into the mid-IR. Thus we expect that it will be straightforward to apply this approach to longer wavelengths to develop integrated devices for a range of sensing and spectroscopy applications. Although the speeds of this modulation scheme will always be limited by the free-carrier lifetime, these could be increased by simply reducing the waveguide dimensions or by applying free-carrier sweep-out [26]. We expect that all-optical modulation systems will play a key role in future mid-IR infrastructures, and these results provide further evidence that Ge-on-Si platforms are a promising candidate to replace silicon in this important regime.
10. 11. 12. 13. 14. 15. 16.
The authors acknowledge EPSRC for financial support under the project MIGRATION (EP/L01162X/1). G. Z. Mashanovich would like to acknowledge support from the Royal Society through his University Research Fellowship. References 1. J. Hu, J. Meyer, K. Richardson, and L. Shah, Opt. Mater. Express 3, 1571 (2013). 2. P. Ma, D.-Y. Choi, Y. Yu, X. Gai, Z. Yang, S. Debbarma, S. Madden, and B. Luther-Davies, Opt. Express 21, 29927 (2013). 3. R. Grille, G. Martin, L. Labadie, B. Arezki, P. Kern, T. Lewi, A. Tsun, and A. Katzir, Opt. Express 17, 12516 (2009). 4. C. Charlton, M. Giovannini, J. Faist, and B. Mizaikoff, Anal. Chem. 78, 4224 (2006). 5. S. B. Mirov, V. V. Fedorov, I. S. Moskalev, and D. V. Martyshkin, IEEE J. Sel. Top. Quantum Electron. 13, 810 (2007). 6. R. Soref, Nat. Photonics 4, 495 (2010). 7. M. Nedeljkovic, A. Z. Khokhar, Y. Hu, X. Chen, J. Soler Penades, S. Stankovic, D. J. Thomson, F. Y. Gardes, H. M. H. Chong, G. T. Reed, and G. Z. Mashanovich, Opt. Mater. Express 3, 1205 (2013). 8. F. Li, S. D. Jackson, C. Grillet, E. Magi, D. Hudson, S. J. Madden, Y. Moghe, C. O’Brien, A. Read, S. G. Duvall, P. Atanackovic, B. J. Eggleton, and D. J. Moss, Opt. Express 19, 15212 (2011). 9. J. Soler Penades, C. Alonso-Ramos, A. Z. Khokhar, M. Nedeljkovic, L. A. Boodhoo, A. Ortega-Monux, I.
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OPTICS LETTERS / Vol. 40, No. 2 / January 15, 2015
Mid-infrared all-optical modulation in low-loss germanium-on-silicon waveguides Li Shen, Noel Healy, Colin J. Mitchell, Jordi Soler Penades, Milos Nedeljkovic, Goran Z. Mashanovich, and Anna C. Peacock* Optoelectronics Research Center, University of Southampton, Southampton SO17 1BJ, UK *Corresponding author: [email protected] Received October 15, 2014; accepted November 29, 2014; posted December 8, 2014 (Doc. ID 225027); published January 14, 2015 All-optical modulation has been demonstrated in a germanium-on-silicon rib waveguide over the mid-infrared wavelength range of 2–3 μm using a free-carrier absorption scheme. Transmission measurements have shown the waveguides to have low propagation losses that are relatively independent of wavelength out to 3.8 μm, indicating that the modulation could be extended further into the mid-infrared region for applications in sensing and spectroscopy. By monitoring the material recovery, the free-carrier lifetime of the micron-sized waveguides has been estimated to be ∼18 ns, allowing for modulation speeds within the megahertz regime. © 2015 Optical Society of America OCIS codes: (160.6000) Semiconductor materials; (230.7370) Waveguides; (230.4110) Modulators. http://dx.doi.org/10.1364/OL.40.000268
Spanning the wavelength range λ 2–20 μm, the midinfrared (mid-IR) region is of tremendous spectroscopic interest as it contains strong vibrational signatures for a number of gases and molecules. Photonic components that operate in this long-wavelength region can thus be applied for a host of applications in environmental and bio-chemical sensing [1]. Over the years, a number of mid-IR material platforms have been investigated, including chalcogenides glasses [2], silver halides [3], and various unary and compound semiconductors [4–6]. Of late, there has been growing interest in the area of group IV semiconductor platforms, from which it is possible to leverage the well-established fabrication processes and CMOS compatibility to develop integrated mid-IR optoelectronic systems [6]. In this regard, much of the focus has been on silicon-based platforms, with low-loss waveguides having been demonstrated in silicon-on-insulator [7], silicon-on-sapphire [8], and suspended silicon [9]. However, a drawback of using silicon is that its transparency is limited to wavelengths below 8 μm, thus excluding its application in the important molecular ‘fingerprint’ region. As a result, some of the attention in group IV photonics has been turning to germanium-based platforms, which offer extended mid-IR transmission out to 14 μm [6]. Moreover, compared to silicon, germanium has several other desirable properties such as higher nonlinear coefficients [10] and superior electronic properties [11] that can be exploited for use in a wide range of active and passive devices. Although germanium-on-silicon (Ge-on-Si) platforms have long been proposed as potential candidates for mid-IR applications [12], it is only fairly recently that the first Ge-on-Si waveguides have emerged [13]. Prior to this, efforts to realize germanium-based waveguides have included germanium core fibers [14] and germanium slabs on zinc sulfide substrates [15]. However, a key advantage of the Ge-on-Si platforms is that they facilitate integration with other group IV photonic chip-based technologies [16]. So far these first-generation Ge-on-Si waveguides have exhibited low transmission losses for selected wavelengths across the 2–6 μm range, and are 0146-9592/15/020268-04$15.00/0
already showing great promise for the development of chip-based spectrometers, with evanescent waveguide sensors [17] and wavelength (de)multiplexers [18] having been demonstrated. To maintain progress in this area, efforts must continue to build on these basic elements by demonstrating more complex optical and electronic functionality within the germanium platforms. In this Letter, we present what we believe to be the first demonstration of all-optical modulation in a Ge-on-Si rib waveguide using a simple free-carrier absorption scheme [19]. The experimental setup is based on a pump-probe geometry where a below bandgap λ 1.54 μm pulsed source was used to modulate various continuous wave (CW) signals across the mid-IR wavelength range of 2–3.2 μm. Modulation extinctions of up to 5 dB were obtained for pulse energies of a few picojoules. By studying the material recovery, we have also obtained an estimate for the free-carrier lifetime of τ ∼ 18 ns within the micron-sized waveguides. We expect that by further reducing the waveguide dimensions to decrease the lifetime and improve the pump/probe overlap, it will be possible to develop high-speed and high-extinction ratio all-optical modulators capable of operating across much of the important mid-IR regime. The starting Ge-on-Si wafers were fabricated by epitaxially depositing a 2-μm-thick germanium layer onto a silicon substrate using a chemical vapor deposition (CVD) method [20]. To fabricate the rib waveguides, a SiO2 hard mask was defined, which was deposited via plasmaenhanced CVD, then patterned via contact lithography and reactive-ion etching (RIE) using an Ar∕CHF3 plasma. The photoresist was subsequently stripped and a further RIE step performed (SF6 ∕CHF3 plasma) to form the waveguides. Finally, the SiO2 mask was removed using a HF bath, and the samples were cleaned and cleaved. The waveguides used in this work were designed to have an etch depth of 1.2 μm and a core width of 2.25 μm, so that they only supported the fundamental (TE and TM) modes for wavelengths >3 μm. We note that although for shorter wavelengths the core does support the second-order modes, with optimal coupling, it is © 2015 Optical Society of America
January 15, 2015 / Vol. 40, No. 2 / OPTICS LETTERS
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Fig. 1. (a) SEM micrograph of Ge-on-Si rib waveguides. The straight sections correspond to the 10 μm-wide coupling tapers, and the curved section is the smaller 2.25 μm wide core. (b) Cross-sectional image of the input taper and (c) the core.
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straightforward to launch only into the fundamental. To help with the coupling, input and output tapers were fabricated at each end with a maximum width of 10 μm and a flare angle that was carefully chosen to minimize coupling losses across the investigated wavelength range (in all cases the transmission from taper to waveguide core was >96%). Waveguides of different lengths were incorporated on the chip by introducing four identical bends, of varying separation, in each core section. Figure 1(a) shows a scanning electron microscope (SEM) image of the patterned waveguides, from which both the wide tapered input and waveguide core (with bends) can be identified. SEM cross-sectional images of the input facet and waveguide core are then displayed in Figs. 1(b) and 1(c), respectively, showing that the rib structure is well formed. To characterize the optical transmission properties of our Ge-on-Si waveguides, linear loss measurements were performed using various mid-IR sources over the wavelength range 1.9–3.8 μm. An effective cut-back method was applied by comparing the transmitted power through waveguides of different lengths, where any influence from the bends was removed by ensuring the bend number and radius were fixed for the entire waveguide set. Furthermore, the bends were designed to have a relatively large radius of 100 μm [see Fig. 1(a)], which ensured that their contribution to the overall losses was minimal. Our initial measurements were conducted over the short wavelength range of 1.9–2.5 μm using a tunable CW Cr2 :ZnSe laser source. For these wavelengths, the light could be launched into the core using a 63× silica microscope objective (0.85 NA), which allowed for optimal coupling into the fundamental TE mode of the taper, and hence the waveguide core. Confirmation that the mid-IR light was predominantly guided in the fundamental mode can be seen through the highquality output profile displayed in the inset of Fig. 2(b). The output was then collected via a ZnSe objective with a 6 mm focal length, and measured using a PbSe pre-amplified photoconductive detector. A plot of the normalized transmission for an input wavelength of 2.3 μm over several waveguide lengths (L 1.4–2.4 cm) is shown in Fig. 2(a), revealing a loss of 3.0 0.3 dB∕cm. Subsequent measurements were performed for longer wavelengths, first using an Aculight CW optical parametric oscillator (OPO) over the range 2.6–3.2 μm, then a
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quantum cascade laser (QCL) to push out as far as 3.8 μm. As it was not possible to use the silica coupling objective for these wavelengths, an aspheric chalcogenide lens with a focal length of 1.875 mm was used to couple light from the OPO, while a mid-IR fiber buttcoupled to the waveguide core was used to launch light from the QCL [7]. A summary of the loss measurements taken over the entire wavelength range is provided in Fig. 2(b). The high loss value recorded at 1.95 μm is expected as it is close to germanium’s band edge, and beyond 2 μm the losses flatten out to a consistently low value of ∼3 dB∕cm. The results indicate that the Ge-on-Si waveguides should exhibit a broad transmission window across much of the mid-IR regime provided the waveguide design is optimized to minimize the modal overlap with the Si substrate, which becomes lossy for λ > 8 μm. Compared to previous measurements in Geon-Si waveguides, our lowest loss of 2.5 0.2 dB∕cm taken at λ 3.8 μm is commensurate with the losses reported for wavelengths ∼5 μm [13,18]. However, the fast drop-off in the losses near the band edge does represent a significant improvement over our previous measurements in this regime [16], which we attribute to a reduction of strain-induced defects near the core/cladding interface [21]. Importantly, the low losses near ∼2 μm open up the possibility to make use of the highperformance telecoms diagnostic tools that operate up to this short wavelength edge of the mid-IR regime.
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Fig. 3. Experimental setup to demonstrate all-optical modulation in the Ge-on-Si waveguides. ATT, attenuator; BS, beamsplitter; O1 & O2, microscope objective lenses.
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To demonstrate all-optical modulation in the Ge-on-Si waveguides, a series of pump-probe experiments were undertaken across the 2–3.2 μm wavelength region. A schematic of the experimental setup is provided in Fig. 3, where the pump must be positioned at a wavelength where the photon energy is much larger than the bandgap of germanium (λ < 1.9 μm) to efficiently excite the free-carriers that modulate the propagating signal. For the pump, we chose a high-power mode-locked fiber laser operating at the telecommunications wavelength of 1.54 μm, with a pulse duration of 720 fs and a 40 MHz repetition rate. The low-power signal was then provided by either the Cr2 :ZnSe laser or the OPO so that modulation could be investigated over the selected wavelengths of λ 2.01, 2.6, 3.0, and 3.2 μm. In all cases, the pump and probe beams were combined with a pellicle beam-splitter (BS) before coupling into the waveguide using an appropriate objective. In this co-propagating configuration, the absorption of the pump occurred in the tapered region near the waveguide input, and not in the small core. As a result, the only output from the waveguide was the modulated mid-IR probe light, which was collected by the ZnSe objective for detection. Although the λ 2.01 μm signal was positioned just on the edge of the high loss region of the material, at this short wavelength we could make use of our fast, 10 GHz bandwidth InGaAs photodetector (peak responsivity at 1.9 μm) to study the temporal dynamics of the modulation. For the longer wavelengths, we simply made use of the PbSe detector to monitor the modulated power change. The inset of Fig. 4 shows the free-carrier modulated probe amplitude of the λ 2.01 μm signal as recorded on a 12 GHz real-time oscilloscope. The measurement was conducted with a coupled signal power of ∼1 mW and a pump energy of only 44 pJ. Unfortunately, owing to the limited bandwidth of our commercially available InGaAs detector, it was not possible to fully resolve the fast build-up of free-carriers that are generated on the timescale of the femtosecond pump pulses [22]. However, the slower dynamics associated with the freecarrier recombination can be clearly resolved, and through an exponential fit to the recovery (dashed lines), we can estimate the free-carrier lifetime to be τ ∼ 18 ns. This lifetime is considerably faster than the microsecond timescales reported for bulk germanium samples [23], as we would expect for a confined waveguide geometry.
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Fig. 4. Modulation depth as a function of pump pulse energy for a signal wavelength of λ 3.2 μm. Inset: temporal dynamics of the all-optical modulation recorded for λ 2.01 μm. The dashed red line is an exponential fit to determine the freecarrier lifetime.
Furthermore, if we recall that the recombination is taking place in the tapered input, this lifetime is also faster than what has been measured for silicon waveguides with similar micron-sized core dimensions [24], which we attribute to the increased carrier mobility of germanium. As the two main mechanisms for removing carriers from the core of a rib waveguide are diffusion into the slab [24], and recombination via surface and interface states [25], we expect that the lifetime would be even lower in the smaller core section. Thus a route to improving the speed of our system would be to pump the 2.25 μm core from the top, although this would come at the expense of a reduced pump/probe overlap, and would also greatly complicate the experimental setup [19,22]. Finally, although it is not possible to directly measure the modulation depth, we can consider the pump induced carrier generation to be instantaneous when compared to the slow recovery time, which allows us to use the exponential fit to place a lower bound on the extinction ratio of ∼4 dB [22]. The remaining measurements were then conducted within the lower loss, longer wavelength region using the OPO. In these experiments the carrier lifetime was used to fit the temporal dynamics of the average power changes measured on the PbSe detector, from which it was possible to estimate the modulation depth. Figure 4 shows a plot of the modulation depth as a function of the pump pulse energy for the longest signal wavelength of 3.2 μm, showing the expected trend of increasing modulation for increasing pump power. We note that the highest extinction ratio of 5.1 dB was only limited by the available pump energy. Repeating these measurements for λ 2.6 μm and 3 μm revealed maximum extinctions of 4.6 and 4.8 dB, respectively. We attribute the trend of increasing extinction for increasing wavelength to the larger free-carrier absorption coefficient and mode size in the longer wavelength region [19], which is promising for applications extending even further into the mid-IR. In summary, we have demonstrated all-optical modulation out to 3.2 μm in a Ge-on-Si rib waveguide using a
January 15, 2015 / Vol. 40, No. 2 / OPTICS LETTERS
simple free-carrier absorption scheme. These waveguides have been shown to exhibit low losses that are relatively independent of wavelength over the range 2–3.8 μm, which suggests that the transmission region should extend well into the mid-IR. Thus we expect that it will be straightforward to apply this approach to longer wavelengths to develop integrated devices for a range of sensing and spectroscopy applications. Although the speeds of this modulation scheme will always be limited by the free-carrier lifetime, these could be increased by simply reducing the waveguide dimensions or by applying free-carrier sweep-out [26]. We expect that all-optical modulation systems will play a key role in future mid-IR infrastructures, and these results provide further evidence that Ge-on-Si platforms are a promising candidate to replace silicon in this important regime.
10. 11. 12. 13. 14. 15. 16.
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