Subscriber access provided by UNIV OF SCIENCES PHILADELPHIA
The SiGeC waveguide for all-optical data switching Bo-Ji Huang, Cheng-Ting Tsai, Yung-Hsiang Lin, Chih-Hsien Cheng, HuaiYung Wang, Yu-Chieh Chi, Po-Han Chang, Chih-I Wu, and Gong-Ru Lin ACS Photonics, Just Accepted Manuscript • Publication Date (Web): 22 Mar 2018 Downloaded from http://pubs.acs.org on March 22, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Photonics
The SiGeC waveguide for all-optical data switching Bo-Ji Huang, Cheng-Ting Tsai, Yung-Hsiang Lin, Chih-Hsien Cheng, Huai-Yung Wang, Yu-Chieh Chi, Po-Han Chang, Chih-I Wu, and Gong-Ru Lin* Graduate Institute of Photonics and Optoelectronics, and Department of Electrical Engineering, National Taiwan University (NTU), No. 1, Roosevelt Road Sec. 4, Taipei 10617, Taiwan R.O.C.
E-mail:
[email protected] RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to) Title Running Head : The SiGeC waveguide for all-optical data switching. *To whom correspondence should be addressed. Address: 1, Roosevelt Road Sec. 4, Taipei 10617, Taiwan R.O.C. Phone: +886-2-33663700#6519; Fax: +886-2-33669598; E-mail:
[email protected] ACS Paragon Plus Environment
1
ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 33
ABSTRACT
The SiGeC waveguide based ultrafast all-optical switching by using cross-absorption modulation (XAM) effect is demonstrated to perform the wavelength conversion and format inversion of a pulsed return-to-zero on-off-keying (PRZ-OOK) data stream. Under the intensively optical data-bit illumination as the pump, the two-photon absorption (TPA) effect is observed to induce free carriers in the SiGeC for absorbing the power of the probe beam. This free-carrier absorption (FCA) procedure inversely modulates the probe beam to cause the XAM switching via the intensively pumping data-bit induced TPA/FCA process. With employing the Ge content into the SiC matrix, the picosecond freecarrier lifetime of the SiGeC can be obtained by further degrading the injected pumping energy under all-optical switching operation. With reducing the pumping energy from 0.5 nJ to 22 pJ, the effective response time of the all-optical XAM switching successfully shortens from 120 to 2 ps. Experiments declare that even a relatively low pumping energy can result in a sufficiently large XAM to perform the ultrafast all-optical switching in the SiGeC waveguide based all-optical modulator, providing the alloptical data format inversion and broadband wavelength conversion of the PRZ-OOK data stream at 1.2-6.0 Gbit/s.
KEYWORDS SiGeC waveguide, Optical data switching, All-optical cross-absorption modulation.
ACS Paragon Plus Environment
2
Page 3 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Photonics
Table of Contents/Abstract Graphic.
ACS Paragon Plus Environment
3
ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 33
During the past decades, the Si-based ultrafast optical modulator has been rapidly developed for its comprehensive application in the field of Si photonics, which is expected to handle high-speed data processing within the photonic integrated circuits.1 In previous works, the crystalline and polycrystalline Si-based optical modulators have been demonstrated by utilizing the free-carrier dispersion (FCD) or free-carrier absorption (FCA) effects, which achieve several kinds of functionalities such as all-optical switching,2-4 thermo-optic switching,5 electro-optic modulation,6-8 phase shifting,9 and optical logic grating,10 etc.. However, the long lifetime of carriers existed in Si inevitably limits the modulation bandwidth of some devices based on electro-absorption (EA) or free-carrier absorption modulation (FCA) effect. Such lifetime dependent effects even affect the device based on electro-optic effect as they are coupled together. Therefore, several methods have been proposed to increase the modulation bandwidth of Si-based modulators, including the carrier sweep-out through external bias and the carrier trapping via ion implantation, etc.11-16 In 2005, Rong et al. shortened the carrier lifetime in Si waveguide to 1.2 ns with an integrated p-i-n structure under a negative bias condition.11 In 2008, Waldow et al. further reduced the carrier lifetime to 15 ps by implanting the oxygen ion into silicon waveguide.15 Notably, the p-n junction was added to the Si modulator for accelerating the carrier sweep-out speed at negative bias condition, which efficiently helps to form the EA or FCA modulator with the modulation bandwidth increasing beyond the carrier-lifetime limitation under the assistance of fast carrier depletion effect.17 The main limitation for the depletion-based modulators is originated from the RC time constant. In 2013, Tu et al. has already employed the p-n junction to demonstrate the electro-optic modulator, which rapidly sweeps out the carriers to exclude the carrier lifetime dependence such that the data rate of 50 Gbit/s and the extinction ratio of 5.56 dB can be achieved.17 Due to the ion implantation, these waveguides also exhibit an additional propagation loss of up to 68 dB/cm. Some studies have been applied to reduce the free-carrier lifetime by shrinking the device dimension to enhance surface recombination.18-20 Dimitropoulos et al. elucidated the correlation between the carrier lifetime and geometrical feature of Si waveguide, and declared that the effective carrier lifetime can be shortened to
ACS Paragon Plus Environment
4
Page 5 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Photonics
1 ns within the submicron waveguide.18 In view of recent demonstrations, the most convenient way to speed up the device is either seeking for short-lifetime material or shrinking the waveguide size. Hence, the nonstoichiometric SiOx based slot waveguide was employed to demonstrate the nonlinear optical devices, including the four-wave-mixing (FWM) process,21-23 and the ultrafast optical modulation by using the nonlinear Kerr effect.24 To decrease the propagation and coupling losses within the longer interaction length, the silicon nitride (Si3N4) based rib and channel waveguides with nonlinear optical mechanism have been considered.25-27 Although two-photon absorption (TPA) induced FCA effect was also considered as a modulation mechanism, both SiO2 and Si3N4 with a large bandgap hardly facilitate a TPA induced FCA process at telecommunication wavelengths. Therefore, the Si quantum dot (Si-QD) doped Si-rich SiOx and SiNx were utilized as the waveguide core in some studies, which not only increase the effective refractive index but also enhance the TPA induced FCA effect.28-31 In 2009, Spano et al. studied the TPA effect of Si nanoscrytal embedded in SiOx film to report a nonlinear absorption coefficient of 6.7×10-9 cm/W.28 In 2014, Wu et al. demonstrated the Si-QD doped SiOx waveguide for all-optical data inversion at 200 kbit/s.29 The data rate of 2 Mbit/s was achieved with reducing the Si-QD size to 1.7 nm for suppressing its carrier lifetime.30 In 2016, the Si-QD doped Si-rich SiNx was demonstrated by using the TPA mechanism, which shows a nonlinear absorption coefficient as high as 1.8×10 m/GW.31 Nevertheless, both SiOx and SiNx are insulated dielectrics with low surface recombination rate and small diffusion constant. Alternatively, the non-stoichiometric SiCx material has been discussed extensively due to its thermal stability32 and tunable bandgap with varying C/Si composition ratio.33-35 Moreover, the carrier lifetime of versatile SiCx materials have been studied to develop the SiCx based waveguides and modulators.36-38 In particular, with introducing Ge atoms into the SiC material, the refractive index of SiCx host matrix can be increased to enlarge the refractive index difference between the core and cladding layers. The SiGeC alloy also possesses several advantages, such as the structural stability,39 the predictable bandgap,40 and the high optical confinement. In 1996, Soref et al., fabricated the low-loss the SiGeC planner waveguide at telecommunication wavelengths.41 In 2007, Schubert et al. ACS Paragon Plus Environment
5
ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 6 of 33
demonstrated strain-free the SiGeC waveguides to serve as an electrooptic modulators with short turnon time of 0.2 ns.39 In particular, the carrier lifetime can be shortened by detuning the C content in the SiGeC alloys.42 However, the all-optical switching by using the SiGeC waveguide has been seldom investigated. In this work, the all-optical cross-absorption modulation (XAM) switching in the SiGeC waveguide is demonstrated by using the TPA induced FCA effect. The carrier lifetime of the SiGeC can be measured by using a pump-probe system. In addition, the pumping sources with different pulsewidths and optical energies are introduced into the SiGeC waveguide to compare the carrier lifetime and modulated performance. Finally, the TPA induced FCA effect is employed in the SiGeC modulator to demonstrate the data inversion and wavelength conversion for delivering picosecond pulsed-return-tozero on-off keying (PRZ-OOK) data format.
METHODS Design and fabrication of the SiGeC waveguide The SiGeC film with a thickness of 300 nm was deposited on the SiO2/Si substrate. The reactive gaseous recipe consisted of argon diluted silane (8% SiH4+92% Ar), diluted germane (10% GeH4+90% Ar) and methane (CH4). During the synthesis, the plasma power and chamber pressure were maintained as 120 W and 0.3 torr, respectively. The substrate temperature was set as 550oC. The X-ray photoelectron spectroscopy (XPS) with an Al Kα-line X-ray radiation source was employed to analyze the composition ratio of the SiGeC film. The spectral peaks located at 99.8, 281.9, and 29.2 eV are contributed by the Si2p, C1s, and Ge3d orbital electrons,43, 44 respectively, as shown in Fig. 1(a). By integrating the peak of each elements, the atomic percentages of Si, C and Ge contents are calculated as 26.6%, 31.1% and 25.2%, respectively. During PECVD growth, the oxygen content was added to the SiGeC film for adjusting the refractive index of core waveguide. To perform the single-mode waveguide design, the content of the residual oxygen in the SiGeC film is detuned to 17.2%, which is confirmed by observing the binding energy of O1s orbital electrons at 532 eV in the XPS spectrum, as ACS Paragon Plus Environment
6
Page 7 of 33
shown in Fig. 1(a).
a
O1s
b 2.7
800
SiCGe
10
4
10
3
10
2
10
1
10
0
600 Ge3d
C1s
2.5
Si2p
400
2.4
200
α ( cm-1)
2.6
n
XPS intensity (a.u.)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Photonics
2.3
0 0
100
200
300
400
500
Binding energy (eV)
600
700
2.2 800
1000
1200
1400
1600
Wavelength (nm)
Figure 1. (a) The XPS spectrum of the SiGeC film. (b) The refractive index and absorption spectra of the SiGeC film. In addition, Figure 1(b) provides the refractive index and absorption spectra of the SiGeC film, showing the refractive index of 2.3 and the absorption coefficient of
