Group IV mid-infrared photonics [Invited]

In this paper we review our recent results on group IV mid-infrared photonic devices. In particular, passive structures suitable for long wavelength operation, such as suspended Si, Ge-on-Si and suspended Ge, are analyzed. In addition, Ge-on-insulator waveguides have been characterized at 3.8 μm. Several active devices have been also realized: optical modulators in silicon and germanium, and silicon and graphene detectors operating at shorter mid-IR wavelengths. © 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement OCIS codes: (130.0130) Integrated optics; (040.3060) Infrared; (130.5990) Semiconductors. References and links 1. R. Soref, “Mid-infrared photonics in silicon and germanium,” Nat. Photonics 4(8), 495–497 (2010). 2. G. Z. Mashanovich, M. M. Milošević, M. Nedeljkovic, N. Owens, B. Xiong, E.-J. Teo, and Y. Hu, “Low loss silicon waveguides for the mid-infrared,” Opt. Express 19(8), 7112–7119 (2011). 3. 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, “Silicon photonic devices and platforms for the midinfrared,” Opt. Mater. Express 3(9), 1205–1214 (2013). 4. G. Z. Mashanovich, F. Y. Gardes, D. J. Thomson, H. Youfang, L. Ke, M. Nedeljkovic, J. Soler Penades, A. Z. Khokhar, C. J. Mitchell, S. Stankovic, R. Topley, S. A. Reynolds, W. Yun, B. Troia, V. M. N. Passaro, C. G. Littlejohns, T. Dominguez Bucio, P. R. Wilson, and G. T. Reed, “Silicon photonic waveguides and devices for nearand mid-IR applications,” IEEE J. Sel. Top. Quantum Electron. 21(4), 1–12 (2015). 5. T. Baehr-Jones, A. Spott, R. Ilic, A. Spott, B. Penkov, W. Asher, and M. Hochberg, “Silicon-on-sapphire integrated waveguides for the mid-infrared,” Opt. Express 18(12), 12127–12135 (2010). 6. 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, “Low propagation loss silicon-on-sapphire waveguides for the mid-infrared,” Opt. Express 19(16), 15212–15220 (2011). 7. R. Shankar, I. Bulu, and M. Lončar, “Integrated high-quality factor silicon-on-sapphire ring resonators for the mid-infrared,” Appl. Phys. Lett. 102(5), 051108 (2013). 8. S. Khan, J. Chiles, and S. Fathpour, “Silicon-on-nitride waveguides for midand near-infrared integrated photonics,” Appl. Phys. Lett. 102(12), 121104 (2013). 9. Y.-C. Chang, V. Paeder, L. Hvozdara, J.-M. Hartmann, and H. P. Herzig, “Low-loss germanium strip waveguides on silicon for the mid-infrared,” Opt. Lett. 37(14), 2883–2885 (2012). 10. G. Roelkens, U. Dave, A. Gassenq, N. Hattasan, C. Hu, B. Kuyken, F. Leo, A. Malik, M. Muneeb, E. Ryckeboer, Z. Hens, R. Baets, Y. Shimura, F. Gencarelli, B. Vincent, R. Loo, J. Van Campenhout, L. Cerutti, J.B. Rodriguez, E. Tournié, X. Chen, M. Nedeljkovic, G. Z. Mashanovich, S. Li, N. Healy, A. C. Peacock, X. Liu, R. Osgood, and W. J. Green, “Silicon-based heterogeneous photonic integrated circuits for the mid-infrared,” Opt. Mater. Express 3(9), 1523–1536 (2013). 11. M. Nedeljkovic, J. S. Penades, C. J. Mitchell, A. Z. Khokhar, S. Stankovic, T. D. Bucio, C. G. Littlejohns, F. Y. Gardes, and G. Z. Mashanovich, “Surface-grating-coupled low-loss Ge-on-Si rib waveguides and multimode interferometers,” IEEE Photonics Technol. Lett. 27(10), 1040–1043 (2015). Vol. 8, No. 8 | 1 Aug 2018 | OPTICAL MATERIALS EXPRESS 2276 #323442 https://doi.org/10.1364/OME.8.002276 Journal © 2018 Received 16 Feb 2018; revised 1 Apr 2018; accepted 11 Apr 2018; published 19 Jul 2018 12. M. Brun, P. Labeye, G. Grand, J. M. Hartmann, F. Boulila, M. Carras, and S. Nicoletti, “Low loss SiGe graded index waveguides for mid-IR applications,” Opt. Express 22(1), 508–518 (2014). 13. J. M. Ramirez, Q. Liu, V. Vakarin, J. Frigerio, A. Ballabio, X. Le Roux, D. Bouville, L. Vivien, G. Isella, and D. Marris-Morini, “Graded SiGe waveguides with broadband low-loss propagation in the mid infrared,” Opt. Express 26(2), 870–877 (2018). 14. W. Li, P. Anantha, S. Bao, K. H. Lee, X. Guo, T. Hu, L. Zhang, H. Wang, R. Soref, and C. S. Tan, “Germaniumon-silicon nitride waveguides for mid-infrared integrated photonics,” Appl. Phys. Lett. 109(24), 241101 (2016). 15. A. Spott, J. Peters, M. L. Davenport, E. J. Stanton, C. D. Merritt, W. W. Bewley, I. Vurgaftman, C. S. Kim, J. R. Meyer, J. Kirch, L. J. Mawst, D. Botez, and J. E. Bowers, “Quantum cascade laser on silicon,” Optica 3(5), 545 (2016). 16. M. Muneeb, A. Vasiliev, A. Ruocco, A. Malik, H. Chen, M. Nedeljkovic, J. S. Penades, L. Cerutti, J. B. Rodriguez, G. Z. Mashanovich, M. K. Smit, E. Tourni, and G. Roelkens, “III-V-on-silicon integrated micro spectrometer for the 3 μm wavelength range,” Opt. Express 24(9), 9465–9472 (2016). 17. Y.-C. Chang, P. Wägli, V. Paeder, A. Homsy, L. Hvozdara, P. van der Wal, J. Di Francesco, N. F. de Rooij, and H. Peter Herzig, “Cocaine detection by a mid-infrared waveguide integrated with a microfluidic chip,” Lab Chip 12(17), 3020–3023 (2012). 18. P. Tai Lin, H.-Y. Greg Lin, Z. Han, T. Jin, R. Millender, L. C. Kimerling, and A. Agarwal, “Label-free glucose sensing using chip-scale mid-infrared integrated photonics,” Adv. Mater. 4, 1755–1775 (2016). 19. Y. Hu, T. Li, D. J. Thomson, X. Chen, J. Soler Penades, A. Z. Khokhar, C. J. Mitchell, G. T. Reed, and G. Z. Mashanovich, “Wavelength division (de)multiplexing in mid-infrared wavelength range using interleaved angled multimode interferometer on the silicon-on-insulator platform,” Opt. Lett. 39, 1406–1409 (2014). 20. J. Soler Penades, A. Z. Khokhar, M. Nedeljkovic, and G. Z. Mashanovich, “Low loss mid-infrared SOI slot waveguides,” IEEE Photonics Technol. Lett. 27, 1197–1199 (2015). 21. M. Nedeljkovic, A. V. Velasco, A. Z. Khokhar, A. Delâge, P. Cheben, and G. Z. Mashanovich, “Mid-infrared silicon-on-insulator Fourier-transform spectrometer chip,” IEEE Photonics Technol. Lett. 28(4), 528–531 (2016). 22. A. G. Griffith, R. K. W. Lau, J. Cardenas, Y. Okawachi, A. Mohanty, R. Fain, Y. H. D. Lee, M. Yu, C. T. Phare, C. B. Poitras, A. L. Gaeta, and M. Lipson, “Silicon-chip mid-infrared frequency comb generation,” Nat. Commun. 6(1), 6299 (2015). 23. M. Muneeb, X. Chen, P. Verheyen, G. Lepage, S. Pathak, E. Ryckeboer, A. Malik, B. Kuyken, M. Nedeljkovic, J. Van Campenhout, G. Z. Mashanovich, and G. Roelkens, “Demonstration of Silicon-on-insulator mid-infrared spectrometers operating at 3.8 μm,” Opt. Express 21(10), 11659–11669 (2013). 24. S. A. Miller, M. Yu, X. Ji, A. G. Griffith, J. Cardenas, A. L. Gaeta, and M. Lipson, “Low-loss silicon platform for broadband mid-infrared photonics,” Optica 4(7), 707–712 (2017). 25. Z. Cheng, X. Chen, C. Y. Wong, K. Xu, C. K. Fung, Y. M. Chen, and H. K. Tsang, “Focusing subwavelength grating coupler for mid-infrared suspended membrane waveguide,” Opt. Lett. 37(7), 1217–1219 (2012). 26. J. Soler Penadés, C. Alonso-Ramos, A. Z. Khokhar, M. Nedeljkovic, L. A. Boodhoo, A. Ortega-Moñux, I. Molina-Fernández, P. Cheben, and G. Z. Mashanovich, “Suspended SOI waveguide with sub-wavelength grating cladding for mid-infrared,” Opt. Lett. 39(19), 5661–5664 (2014). 27. J. Chiles, S. Khan, J. Ma, and S. Fathpour, “High-contrast, all-silicon waveguiding platform for ultra-broadband mid-infrared photonics,” Appl. Phys. Lett. 103(15), 151106 (2013). 28. J. S. Penades, A. Ortega-Moñux, M. Nedeljkovic, J. G. Wangüemert-Pérez, R. Halir, A. Z. Khokhar, C. AlonsoRamos, Z. Qu, I. Molina-Fernández, P. Cheben, and G. Z. Mashanovich, “Suspended silicon mid-infrared waveguide devices with subwavelength grating metamaterial cladding,” Opt. Express 24(20), 22908–22916 (2016). 29. J. S. Penadés, A. Sánchez-Postigo, M. Nedeljkovic, A. Ortega-Moñux, J. G. Wangüemert-Pérez, Y. Xu, R. Halir, Z. Qu, A. Z. Khokhar, A. Osman, W. Cao, C. G. Littlejohns, P. Cheben, I. Molina-Fernández, and G. Z. Mashanovich, “Suspended silicon waveguides for long-wave infrared wavelengths,” Opt. Lett. 43(4), 795–798 (2018). 30. A. Gutierrez-Arroyo, E. Baudet, L. Bodiou, J. Lemaitre, I. Hardy, F. Faijan, B. Bureau, V. Nazabal, and J. Charrier, “Optical characterization at 7.7 μm of an integrated platform based on chalcogenide waveguides for sensing applications in the mid-infrared,” Opt. Express 24(20), 23109–23117 (2016). 31. A. Malik, S. Dwivedi, L. Van Landschoot, M. Muneeb, Y. Shimura, G. Lepage, J. Van Campenhout, W. Vanherle, T. Van Opstal, R. Loo, and G. Roelkens, “Ge-on-Si and Ge-on-SOI thermo-optic phase shifters for the mid-infrared,” Opt. Express 22(23), 28479–28488 (2014). 32. B. Troia, J. S. Penades, A. Z. Khokhar, M. Nedeljkovic, C. Alonso-Ramos, V. M. N. Passaro, and G. Z. Mashanovich, “Germanium-on-silicon Vernier-effect photonic microcavities for the mid-infrared,” Opt. Lett. 41(3), 610–613 (2016). 33. C. Alonso-Ramos, M. Nedeljkovic, D. Benedikovic, J. S. Penadés, C. G. Littlejohns, A. Z. Khokhar, D. PérezGalacho, L. Vivien, P. Cheben, and G. Z. Mashanovich, “Germanium-on-silicon mid-infrared grating couplers with low-reflectivity inverse taper excitation,” Opt. Lett. 41(18), 4324–4327 (2016). 34. L. Shen, N. Healy, C. J. Mitchell, J. S. Penades, M. Nedeljkovic, G. Z. Mashanovich, and A. C. Peacock, “Midinfrared all-optical modulation in low-loss germanium-on-silicon waveguides,” Opt. Lett. 40(2), 268–271 (2015). Vol. 8, No. 8 | 1 Aug 2018 | OPTICAL MATERIALS EXPRESS 2277


Introduction
Group IV materials, such as Si, Ge, SiN, GeSn and graphene all have attractive properties for applications in the mid-IR.Si and Ge in particular are transparent up to 8 and 15 micrometers, respectively, and offer realization of compact and potentially low cost integrated circuits for a range of applications [1].GeSn alloys and graphene complement the two by enabling integrated mid-IR sources and detectors.A number of mid-IR devices have been reported in the last few years, based on several material platforms: silicon-on-insulator (SOI) [e.g 2-4.], silicon-on-sapphire (SOS) [e.g 5-7.], silicon-on-nitride (SON) [e.g 8], germanium-on-silicon (GOS) [e.g 9-11]), SiGe-on-Si [12,13] and Ge-on-silicon nitride [14].Integration with III-V sources and detectors has also been successfully realized [e.g 15,16], as has demonstration of simple sensing experiments [e.g 17,18].
In this paper we report our recent results on Si and Ge based mid-IR devices.In the first section, we briefly review our recently published results on SOI, suspended Si and GOS devices.We t for suspended waveguides a modulators in the SOI platf mediated Si d

Si and Ge mi
developed fabr NIR photonic d SOI.A series ating up to a of other passiv R (NIR) [e.g 2 as been recentl 4], alternative p ded Si solution for lo nd Si.By doing ches to achieve n the side thro ess points for H th approach w [26].It is also   n the core enable HF esigns, in f Canada, including in Fig. 1.
The waveguide propagation loss was 0.8 dB/cm and the 90° bend loss was only 0.01 dB/bend [28].This demonstration was very promising and by adapting it for longer wavelengths we have recently fabricated suspended Si waveguides and bends operating at 7.7 μm using a thicker SOI platform (1.4 μm compared to 500 nm SOI from [28]).This time, the propagation loss was higher, 3.1 dB/cm [29], however we estimated that 2.1 dB/cm came from the intrinsic material absorption of Si at 7.7 μm, meaning that the loss related to scattering was ~1 dB/cm.This loss is very similar to the loss of chalcogenide [30] and GOS waveguides at the same wavelength.After this first demonstration of a low loss Si waveguide operating at such a long wavelength, our future work will involve development of other passive devices for ~8 μm and implementation of the platform for sensing as it can enable higher interaction between the evanescent optical mode and an analyte.

Ge-on-Si
The second platform suitable for longer wavelengths is GOS.Germanium can be grown on Si substrates by CVD techniques and although Si has higher material loss beyond 8 μm, based on bulk material loss data from literature, GOS waveguides should have low loss up to 11-12 μm.There have been several demonstrations of waveguides [9][10][11], MUXs [10] and thermooptic modulators [31] at shorter wavelengths.SiGe waveguides operating at 7.4 μm were reported in [12], followed by a recent report on relatively thick SiGe on Si waveguides with loss of ~3 dB/cm at 8.5 μm, whilst thinner SiGe waveguides showed losses of >8 dB/cm beyond 7 μm for TE and >16 dB/cm beyond 8 μm for TM polarization [13].
Our work on this platform includes 0.6 dB/cm GOS waveguides [11], low loss MMIs [11], Vernier rings [32], and grating couplers [33] at 3.8 μm, all optical modulation [34], and TPA measurements [35] at 2-3.9 μm. S. Radosaljevic et al. have very recently demonstrated Vernier racetrack resonator tunable filters on a Ge-on-SOI waveguide platform operating in the 5 μm wavelength range [36].We have recently investigated passive GOS devices at 7.5-9 μm [37,38] and found that the 3 μm GOS platform showed 2.5 dB/cm at 7.5 μm but became very lossy beyond 8 μm (>15 dB/cm) [38].There are several potential reasons that can contribute to higher losses such as Si substrate absorption, defects/dislocations at Ge-Si interface, time dependent haze (TDH) formation, free carrier absorption or stress effects.However, our estimates did not predict that any of these should contribute significantly to the loss.Therefore, further investigation is required to find the reasons for high GOS losses beyond 8 μm.

Suspended Ge waveguides
Together with further investigation of the GOS platform and its suitability for long wavelength operation, we have recently initiated work on suspended Ge.This platform should enable operation up to 15 μm and can benefit from well-developed techniques already demonstrated in the suspended Si platform (section 2.1).
The waveguides were fabricated using 6" Ge-on-SOI wafers with a 400 nm Ge layer grown by RPCVD on 220 nm SOI.Rib waveguides were designed for single mode propagation at λ = 3.8 μm.The dimensions were: height (H) = 400 nm, width (W) = 1.1 μm and etch depth (D) = 250 nm.The SOI substrate consisted of a 220 nm thick layer of Si on a 3 μm thick layer of SiO 2 .The waveguides were patterned using e-beam lithography.They were then defined by dry etching using ICP (Fig. 2(b)).A second e-beam lithography step followed in order to define the holes, which were etched down to the BOX (Fig. 2(c)).The sample then underwent two wet etch steps.First, the sample was immersed in 1:7 HF for 10 minutes, which removed the BOX (Fig. 2(d)).Then it was immersed in a 25% aqueous solution of Tetramethylammonium Hydroxide (TMAH) at room temperature for 2 hours, which resulted in a partial removal of the Si layer (Fig. 2(e)).Out of 220 nm of the initial thickness, approximately 70 nm were left.An SEM of the fabricated waveguides is shown in Fig. 3.The waveguides were measured using the effective cut-back method.Waveguides of different lengths were fabricated for the propagation loss measurement.The measured value was 2.9 dB/cm at λ = 3.8 μm.The lab setup used for the characterisation is described in more detail in [2].Our future work will involve improvement of the fabrication process such that Ge is grown on thinner SOI wafers resulting in a shorter TMAH etch, and development of other passive devices in this platform and its implementation for sensing at longer wavelengths.

Ge-on-insulator waveguides
The Ge-on-insulator (GOI) platform has been suggested for MIR photonics due to a large difference between the refractive indices of Ge and SiO 2 (∼4.0 and ∼1.4 respectively) resulting in strong confinement of light in the Ge waveguide, which is necessary to achieve compact photonic devices.Either crystalline Ge or a-Ge on SiO 2 can be used [37].
The GOI samples reported here have been fabricated as described in [39]: a SiO 2 capping layer was deposited on a bulk Ge wafer for protection; H + ions were implanted under the Ge surface and the SiO 2 layer removed; a 10 nm Al 2 O 3 layer was deposited on the Ge surface; the Ge wafer was bonded to a Si wafer with 2 μm SiO 2 layer on the Si surface; the wafer was annealed to cause splitting along the implanted H + ions; the wafer underwent a chemical mechanical polishing (CMP) process to reduce the Ge surface roughness (see Fig. 4).The resulting Ge layer was 515 nm thick.
For single dimensions in 1.1 μm waveg cut-back meth previously rep be used for suspending G For longer w platform on S [40].

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Modulati
To date there 41,42].Howe bandwidth av injection mod modulators op In the nea junction (e.g convert the p junction modu 2 μm, and we were placed a power splitter and at 1950 n bias.We mea at 1950 nm.T OOK signal w able to measu nm we measu

5(a)
), but at that wavelength we were prevented from measuring higher speed modulation by the external InGaAs detector bandwidth.At longer wavelengths there may be applications for modulation related to free-space communications, signal processing, and switching.Semi-empirical equations for the freecarrier effect for Si were presented in [46] and for Ge in [47], predicting that the strength of the effect would increase approximately proportionally to the square of wavelength in both materials.They also predict that the effect is generally stronger in Ge than in Si.
We have designed and fabricated modulators in SOI and GOS material platforms for 3.8 μm , in which PIN diodes are integrated with a waveguide, and carrier injection into the waveguide increases the absorption of the waveguide core.The cross-sections of the PIN diodes of the SOI and GOS devices (and their dimensions) are shown in Figs.5(b) and 5(d), respectively.Because of the larger dimensions of the GOS waveguide it has lower lateral mode confinement, and therefore the highly doped Ohmic contact regions are placed further away to minimize the excess loss from free carrier absorption.The SOI PIN diode was 2 mm long, while the GOS diode was 1 mm long.In both devices grating couplers were used to couple light in and out of the waveguide.The transmission of each device was measured while applying varying forward bias across the PIN diode using a DC power supply.The modulation depths of both devices under varying DC forward bias are shown in Fig. 5(c), where it can be seen that both achieve a modulation depth > 30 dB.It can be seen from Fig. 5(c) that a significantly lower voltage was required to produce the same modulation depth in the SOI device, which can be attributed to a combination of the shorter diode length in the GOS modulator, the much larger waveguide dimensions and Ohmic contact separation, and potentially the shorter carrier lifetime from Ge crystal growth defects near the Ge/Si interface.The SOI PIN modulator had an insertion loss of 2.9 dB at 3779 nm, but we were unable to reliably measure the insertion loss of the GOS modulator, because the normalization waveguides on the chip were damaged during fabrication.

Group IV mid-IR detectors
One of the most important devices in group IV MIR integrated circuits is a photodetector.Several approaches have been demonstrated so far.Following a similar approach already demonstrated in the NIR, InP detectors were bonded on SOI waveguides using BCB achieving responsivity of 1.6 A/W at 2.35 μm and dark current of 5 nA at −0.5 V [48].In the 3-4 μm wavelength range InAsSb PIN photodiodes have been realized, again in SOI, with a responsivity of 0.3 A/W at room temperature [16].GeSn photodetectors are group IV alternatives that offer considerable potential to extend the sensitivity of germanium technologies into the MIR.Such photodetectors have achieved responsivities of 0.1 A/W for surface illumination [49].
Silicon is traditionally limited in terms of spectral coverage (<1 μm) by its bandgap energy.However, it has been shown that Si detection can be extended towards the NIR by using three main sub-bandgap absorption mechanisms: the internal photoemission (IPE) effect, two-photon absorption (TPA) and defect-mediated absorption.The most useful monolithic approach is via introduction of lattice defects and associated deep level charge states in the silicon bandgap, which provide sub-bandgap photon absorption, and these defects can be introduced by ion implantation [e.g 50,51].These photodiodes (PDs) are usually created in a standard SOI rib waveguide by implanting doped p + and n + regions on either side of a waveguide to form a lateral PIN diode, and then introducing defects in the waveguide core by implantation.These two regions should be placed such that they enable fast operation whilst introducing negligible excess loss due to free-carrier absorption.This defect-mediated absorption does not require heterogeneous integration, making it a robust and low-cost technology.It also allows for operation at room temperature.In collaboration with McMaster University, we have achieved significantly large bandwidth (>15 GHz) and good responsivity of 0.3 A/W at 2 μm for a photodetector based on the 220nm SOI platform and implanted with boron [52].However, the responsivity was obtained in avalanche mode and was rather modest compared to what we have measured at 1550 nm (3 A/W), and dropped by an order of magnitude at 2.5 μm, showing that the PD structure needs significant further optimization.
By combining graphene's superior electronic and optical properties and Si and Ge platforms, photodetection can be achieved in the MIR wavelength region.A graphene on silicon waveguide based detector operating at a wavelength of 2.75 μm has previously achieved 0.13 A/W responsivity [53].Here we present the first graphene photodetector operating at a wavelength of 3.8 µm, based on the coplanar integration method with SOI waveguides.The graphene layer was grown by CVD and transferred to the SOI waveguide.
A schematic of the device cross-section is shown in Fig. 6.The contacts to the graphene were arranged in an asymmetric metal-graphene-metal (MGM) configuration.The core of the waveguide was 1.3 µm wide and 500 nm high with a 50 nm thick slab region.A 90 nm thick PECVD SiO 2 layer was deposited for passivation and decreasing charging effects during graphene deposition.The CVD grown graphene was then transferred on the top of the chip, and patterned by reactive ion etching (RIE).Finally, 100 nm thick Au contacts were fabricated on top of the graphene on either side of the Si waveguide with a separation of 1.5 µm and 5 µm, respectively (Fig. 6).The interaction length between the SOI waveguide and the graphene layer was 500 µm.
The device was characterized with bias voltages from −1 to 1 V. Generated photocurrent was measured by a picoammeter (Keithley 6487).The photocurrent is plotted as a function of the increased optical power that was coupled into the graphene photodetector in Fig. 7.The gradient of the linear fitting gives the photoresponsivity of the device as 2.2 mA/W at 3.8 µm under a −1 V bias voltage.The optical power incident on the photodetector was calculated by taking into account the absorption from the input fiber, the coupling loss of the input grating coupler, and the loss from access waveguides.rk will be used up to zed at 3.8 tly higher ovements atform an group IV We have 2 μm.We have also fabricated injection type Si and Ge modulators at 3.8 μm and measured >30 dB extinction ratios.Defect mediated detection in Si is a promising route for the realization of monolithic MIR detectors.Graphene is another promising candidate for detection and in our first attempt we have fabricated a graphene-SOI waveguide integrated detector and measured 2.2 mA/W responsivity at 3.8 μm.Further improvements are to be investigated to increase the responsivity and to demonstrate graphene detectors at longer wavelengths.

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Fig. 2 .
Fig. 2. Fabrication process flow for suspended Ge waveguides (green, yellow and red represent Ge, Si and BOX layers, respectively).

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Fig. 5 .
Fig. 5. (a) Experimental eye diagram showing transmission of SOI PN junction carrier depletion modulator at 1.95 µm, driven with a 20 Gb/s PRBS signal.(b) Schematic diagram of SOI PIN junction carrier injection modulator, designed for 3.8 µm wavelength.(c) Measured modulation depths of a 2 mm long SOI and a 1 mm long GOS carrier injection modulator with varying DC forward bias, both at 3.8 µm.(d) Schematic diagram of GOS PIN junction carrier injection modulator, designed for 3.8 µm wavelength.

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