Based All-Optical Modulator - American Chemical Society

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CMOS-compatible WS2-based all-optical modulator Shuo Yang, De Chao Liu, Ze Lin Tan, Ken Liu, Zhi Hong Zhu, and Shiqiao Qin ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b01206 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 18, 2017

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CMOS-compatible WS2-based all-optical modulator Shuo Yang, †, § De Chao Liu, †, § Ze Lin Tan, †, § Ken Liu, *, † Zhi Hong Zhu, † and Shi Qiao Qin †, ‡ †

College for Advanced Interdisciplinary Studies, National University of Defense Technology,

Changsha, Hunan 410073, China ‡

State key laboratory of High Performance Computing, National University of Defense

Technology, Changsha, Hunan 410073, China

ABSTRACT: Two-dimensional materials are compatible with silicon complementary metaloxide semiconductor processes. As such, the integration of two-dimensional materials with silicon-based semiconductors could lead to an amalgamation of the beneficial properties of both silicon and the two-dimensional material thereby facilitating improved performance. Single-layer transition metal dichalcogenide materials like MoS2, MoTe2 and WS2 are characterized by direct band gaps and possess high emission efficiencies. These materials are, therefore, perfectly suited for ameliorating the optical emission inadequacies of silicon-based-materials. In this study, we integrate WS2 with a silicon-based silicon-nitride waveguide to modulate a 532-nm pump light source, and successfully modulate and amplify an optical signal at 640 nm. Compared to other modulators based on two-dimensional materials, e.g., graphene, the proposed WS2-based modulator theoretically incurs lower losses and exhibits higher contrast levels and signal-tonoise ratios. As the proposed modulator can compensate for losses and has potential for on-chip integration, it has excellent prospects for application in the field of on-chip optical interconnects.

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KEYWORDS: transition metal dichalcogenide, optical modulator, intra-chip optical connections, Si3N4 waveguide, WS2

Intra- and inter-chip optical connections possess intrinsic advantages over electrical connections on account of their considerably broader bandwidth, higher operating speeds, and lower energy consumption.1-6 Recently, a microprocessor design employing intra-chip optical connections was realized.6 However, on account of the difficulties involved in the integration of an intra-chip light source, an off-chip source of light was used in the above microprocessor design. Silicon, when used as a single material, possesses advantages as well as disadvantages with regard to optical interconnections. A severe disadvantage in this regard is that silicon, being an indirect band gap semiconductor, exhibits very low emission efficiencies. Semiconductors that demonstrate high emission efficiencies tend to be of Type III-IV with direct band gaps.7 However, the fabrication process of such semiconductors is incompatible with the complementary metal-oxide semiconductor (CMOS) process, which limits their utilization in integrated silicon-based on-chip optical interconnects.3 A major advantage of using silicon is that it is a transparent material which is capable of guiding a light beam whilst incurring extremely low losses at communication wavelengths. However, being transparent, it cannot absorb light, and therefore, cannot be used for photodetection.4 Other semiconductor materials, such as germanium, are capable of absorbing the light; however, these materials are difficult to integrate with silicon.4 In contrast, two-dimensional (2D) materials (e.g., graphene) can be readily integrated with silicon via transfer-based methods. Through exploitation of this property of 2D materials, design of CMOScompatible photodetectors has been realized in extant studies.8-10


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In addition to photodetectors, on-chip optical modulators are also key components for optical interconnects. Silicon crystals do not exhibit the Pockels effect and, therefore, possess weak electro-optical modulation abilities.5 Graphene, being a 2D material, does not possess a band gap, and its light-absorbing ability is dependent upon changes in the Fermi energy level. A classical structure for graphene-silicon optical modulators that utilize this particular feature has been reported in,11 wherein a driving voltage is applied to control the Fermi energy levels of the graphene, thereby altering near-infrared light absorption of the silicon-based graphene system. Photonic crystal12 and ring cavity resonators13 have also been used to improve the performance of the above system. All-optical modulators integrated in optical fibers have been recently reported.14 These graphene modulators facilitate operation in the ultra-broadband range of wavelength at high modulation speeds. However, the modulators usually suffer from relatively low modulation depth, which is an important figure of merit in modulator characterization. To overcome this drawback, alternate modulation mechanisms, such as the graphene-assisted phase shifter and optical bistability integrated fiber, which demonstrate extinction ratios greater than 20 dB, have been recently reported.15, 16 In this study, single-layer WS2 was employed as a modulator, which was integrated with a silicon nitride (Si3N4) waveguide on top of an oxide layer on a silicon substrate. Fabrication of a microprocessor6 integrated with photonic devices on a single layer is likely to be hindered by incompatibility problems. This is because electrical devices would require a sub-10-nm process line; photonic devices, however, are capable of utilizing process line of up to 100 nm, since optical wavelengths are considerably longer compared to electronic wavelengths. A possible method for successful intra-connection, may involve construction of three dimensional (3D) blocks with photonic devices placed in the upper layer. Therefore, in the proposed design, the


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Si3N4 waveguide was placed above the silicon substrate. Very few 2D materials, such as black phosphorus and MoTe2, possess the property of optical emissions at communication wavelengths.17,

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However, both black phosphorus and MoTe2 are chemically unstable. The

waveguide structure employed in this study facilitates stable operation of the system at wavelengths exceeding those corresponding to the edge of the silicon band gap, thereby allowing optical emissions from a wider variety of 2D materials to be utilized. Transition metal dichalcogenides (TMDCs) such as WS2, belong to a different category of 2D materials, possess different energy band structures compared to graphene. They behave as indirect band gap materials with two or more layers. However, monolayer MoS2, WSe2, and WS2 are direct band gap materials with high emission efficiencies. Laser structures fabricated using TMDCs have already been reported in the literature.18-21 WS2, in particular, demonstrates a higher luminescence quantum efficiency (2.3%) compared to MoS2 (0.13%),22 and was, therefore, selected as the 2D material of choice for the modulator design reported in this work. RESULTS AND DISCUSSIONS Figure 1a depicts a schematic of the fabricated WS2-based optical modulator structure proposed in this paper. A 2-µm-wide and 300-nm-thick Si3N4 (refractive index: 2.0) waveguide was made on a silica layer (refractive index: 1.46) with silicon substrate. A chemical vapor deposition (CVD) grown WS2 monolayer was then transferred onto the waveguide. A 30-nmthick Al2O3 layer (refractive index: 1.65) was grown on top of WS2 layer through atomic layer deposition (ALD). Deposition of the Al2O3 layer serves two purposes—(i) the main purpose is that it isolated the WS2 monolayer from air, which could degrade its luminescence property.20, 23; and (i) it helped to enhance the optical confinement in the WS2 layer.


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The distribution of the Si3N4–WS2–Al2O3 waveguide transverse electromagnetic fundamental (TEM0) mode is shown in Figure 1b. The optical confinement factor Γ can be expressed as 2

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Γ = ( ∫ ε WS2 E|| dv) /( ∫ ε E dv) ,20 where ε is the dielectric constant, and E ∥ is the in-plane WS2

V

electric field along WS2 surface. Γ≈1.6‰ for the Si3N4–WS2–Al2O3 waveguide and ≈1.2‰ for the Si3N4–WS2 waveguide (without the Al2O3 covering layer). Thus, it can be realized that addition of the Al2O3 covering layer serves to increase the optical confinement by approximately 33%. Tests were conducted in which a tapered fiber was used to couple a 532-nm green light source to the Si3N4 waveguide. Because the energy of the 532-nm light far exceeds the energy level of the WS2 band gap, it acted as an absorbing material for this incident light. As depicted in Figure 1b, the optical intensity on the waveguide surface has a very strong evanescent field. Therefore, when the WS2 monolayer was transferred on top of the Si3N4 waveguide, the evanescent field on the waveguide surface strongly interacts with WS2, thereby causing it to strongly absorb the 532nm light. Consequently, the proposed Si3N4–WS2–Al2O3 waveguide demonstrates very high absorption of the 532-nm light. After absorption of the incident green light, the WS2 emitted radiations of the red light, as shown in Figure 2. To some degree, the radiations emitted by the WS2 layer were absorbed by the layer itself. However, the red light is absorbed to a much less extent compared to the absorption of the incident green light. Being a 2D material, the photoluminescence (PL) of the WS2 monolayer is very sensitive to the presence of other materials and ambient conditions.20, 24 With growth of the Al2O3 layer on top and presence of the Si3N4 waveguide at its bottom, peak emission corresponding to the excitation of the WS2 monolayer occurs at approximately 640 nm at room temperature, as shown


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in Figure 3. In the PL measurement, a 640-nm red light and 532-nm green light were coupled to the waveguide together. Because the WS2 monolayer emitted 640-nm light when pumped by the 532-nm light, 532-nm light pumping yields amplification of the optical signal during transmission through the Si3N4–WS2–Al2O3 waveguide. Without 532-nm light pumping, the 640nm light would be absorbed by the WS2 ( Figure 3). It was expected that modulation of the source of 532-nm light would lead to modulation of the output 640-nm light as well. An experiment was, therefore, conducted to confirm this behavior. A schematic diagram of the measurement setup is depicted in Figure 4. In this measurement, 532-nm blue light and 640-nm red light were used as the pump light source and optical signal, respectively. The 532-nm light (Laser 1 in Figure 4) was filtered using a 532-nm bandpass filter (BPF), transformed into a modulated signal using an acousto-optic modulator (AOM), and then transmitted through an 80/20 beam splitter (BS). The 640-nm red light (Laser 2) was reflected by the BS and transmitted along with the 532-nm light. The two beams of light were subsequently coupled into an optical fiber by a lens, and then coupled into the sample by means of a tapered fiber lens. The 640-nm optical signal received as output from the sample was a modulated signal. Subsequently, a double beam splitter (DBS) at a wavelength of 542 nm was employed to separate the two beams of light by transmitting the 640 nm modulated signal and reflecting the 532-nm excitation signal. The resulting 640-nm optical signal was then passed through a long-pass filter (LPF) at 538 nm. Finally, the signal was coupled into a photodetector (New Focus 2151), which converted the optical signals into electrical signals for display on an oscilloscope (Yokogawa DL6154). Figure 5 shows the time history of the modulated signal displayed by the oscilloscope. From the figure, it is apparent that modulation of the 532-nm pump light source by AOM leads to


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modulation of the output signal received from the proposed WS2–Si3N4–Al2O3 modulator. The measured output power from the tapered fiber lens was approximately 5 mW and 1 µW for the 532-nm and 640-nm light, respectively, and the focus diameter of the tapered lens was approximately 4 µm. Minimum coupling insertion losses between the tapered fiber and waveguide were estimated to be approximately 13 dB and 15 dB for the 532-nm light and 640nm light, respectively. Propagation losses were more than 10 dB for the 532-nm light, which can mainly be attributed to the high absorption of the WS2 layer, and about 2 dB for 640-nm light which mainly resulted from the relatively low absorption of and scattering by the WS2 layer. It is to be noted here that, although the Si3N4 waveguide used in this experiment was approximately 50 µm in length, the overall signal gain achieved was approximately one order of magnitude (Figure 3). This implies an extinction ratio in excess of 10 dB for the proposed modulator structure. If the input signal intensity of the 640-nm light is maintained constant, the intensity of the output optical signal increases linearly upon increase the input power 532-nm excitation light. Thus higher extinction ratios could be achieved by increasing the input power of the 532-nm excitation light. In theory, if the optical gain of WS2 is sufficiently large, the output optical signal can be amplified to any desired amplitude. This implies that the amplitude of the modulated signal may be amplified to a very high value; however, realization of this would require use of higher-energy excitation signals and longer waveguide lengths. The light intensity of the 532-nm and 640-nm light along the waveguide can be written as I P ( L) = I P (0)e −α P L and I s ( L) = I s (0)e ( g ( L )−α s ) L . I P and I s are the 532-nm pump light intensity and 640-nm signal light intensity respectively, α p and α s are the transverse modal loss for 532-nm light, and 640-nm light respectively, g(L) is the transverse modal gain for 640-nm light, and L is the waveguide length. Since the propagation loss α p for the 532-nm light is much larger compared to α s for


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the 640-nm signal, it is inferred that the optical gain supplied by the 532-nm excitation signal decreases quickly along the waveguide thereby, simultaneously, causing an increase in the propagation losses of the 640-nm signal along the waveguide. Since g ( L) ∝ I P ( L) , we can deduce that the growth rate of I s becomes slower, even negative, at longer waveguide length, thus the total length of the waveguide to be used must be optimized. Comparing with graphene modulators,11-16 the modulation processes of the proposed modulator yield gains, which could be used to compensate for losses that occur during optical signal transmission. Therefore, the amplitude of the modulated signal, in theory, can be raised to a large value, and modulator performance can be greatly enhanced. Additionally, the intensity of emission from TMDCs can be considerably increased via chemical treatment,25 or through the use of photonic-crystal structures,26, 27 nano-photonic cavity28, 29 and plasmonic structures.30-33 The modulation rate can also be increased by decreasing the recombination lifetime to tens of picoseconds using nano-resonant structures.26, 27, 33 By employing these methods, the modulation rate of the proposed optical modulator design can be increased to the gigahertz level. It should be noted that because response time of the photodetector, employed in the above experiment, was limited to 0.1 ms and modulation rate of the excitation signal, received from the AOM, was relatively low, and the extinction ratio of the AOM was limited, the performance of the proposed modulator design in the above experiment was far from its theoretical optimum. In conclusion, a WS2-based and silicon CMOS-compatible technique was employed in this study to realize the design and fabrication of an all-optical modulator. The proposed modulator structure can be readily miniaturized and its performance can be enhanced through combination with resonant optical nanostructures. Furthermore, the proposed structure can be modified and employed as an electro-optical modulator through electron-injected photon emissions.28 As the


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proposed structure is fully compatible with the CMOS fabrication technique, it serves as an excellent prospect to be used in the field of on-chip integrated optical interconnects.

MATERIALS AND METHODS Si3N4/WS2/Al2O3 waveguide fabrication. During fabrication, a silicon wafer covered with a 2-µm-thick silica layer was used. Low-pressure chemical vapor deposition was employed to deposit a 300-nm-thick layer of Si3N4 on the silica-on-silicon wafer. A 2-µm-wide Si3N4 waveguide structure was then fabricated on the silica layer through lithography and reactive-ion etching. Next, the WS2 monolayer was grown using the CVD technique, wherein sulfur powder was placed upstream, and the WO3 source material was placed in the tube furnace in the presence of sapphire used as substrates. The growth was performed at 1000 °C under a constant flow of Ar gas at the rate of 20 sccm for the entire duration (approximately 15 min) of the process. During the transfer process, PMMA was spin-coated (at 3000 rpm for 60 s) onto the WS2 monolayer. Subsequently, WS2 was removed from the sapphire substrate in alkaline solution, and finally placed on top of the Si3N4 waveguide. At this point, the PMMA layer was dissolved. Finally, a layer of Al2O3 was grown on top of the WS2 layer through ALD, wherein the Al2O3 covering layer was grown at 200 ℃ over a period of 3 h to a thickness of approximately 30 nm. Based on the authors' experience in the laboratory, the above technique of growing an Al2O3 layer yielded higher quality of the end layer and lesser defects.

Corresponding Author *E-mail: [email protected].

ORCID Ken Liu: 0000-0003-1941-1160


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Author Contributions §

These authors (S. Y., D. L. and Z. T.) contributed equally.

Funding Sources This work was supported by the National Natural Science Foundation of China (61404174, 11674396, 11374367); and National University of Defense Technology Foundation (ZK16-0201).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We thank Prof. Bing Lei and Wei Liu for the supply of the acousto optic modulator.

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Figure 1. (a) Schematic of the WS2-based optical modulator structure. Bottom to top: silicon substrate, silica, Si3N4 waveguide, WS2 monolayer, and Al2O3 layer. (b) Electric field mode distribution of Si3N4–WS2–Al2O3 waveguide transverse electromagnetic fundamental (TEM0) mode. The white lines show the silicon, silica, Si3N4 waveguide, WS2, and Al2O3 interfaces and edges. The waveguide cross-section is 2 µm wide and 300 nm thick, the WS2 layer is 0.65 nm thick and the Al2O3 layer is 30 nm thick.


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Figure 2. (a) Micrograph of Si3N4 waveguide before the WS2 and Al2O3 layers being grown over it. (b) Micrograph of Si3N4– WS2–Al2O3 waveguide at the instant when the waveguide is irradiated with 532-nm light. The tip of the tapered fiber is apparent because of illumination by a weak white light. The incident green light was filtered by the setup.


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Figure 3. Emission spectra of the signal transmitted through Si3N4 waveguide without and with WS2 monolayer, and with 532nm laser excitation.


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Figure 4. Schematic diagram of Si3N4–WS2–Al2O3 modulation structure experiment. Inset: Photograph of part of the experimental setup.


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Figure 5. Time history of the modulated signal acquired by oscilloscope using the setup shown in Figure 4.


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TOC


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Figure 1. (a) Schematic of the WS2-based optical modulator structure. Bottom to top: Silicon substrate, silica, Si3N4 waveguide, WS2 monolayer, and Al2O3 layer. (b) Electric field mode distribution of Si3N4– WS2–Al2O3 waveguide transverse electromagnetic fundamental (TEM0) mode. The white lines show the silicon, silica, Si3N4 waveguide, WS2, and Al2O3 interfaces and edges. The waveguide cross-section is 2 µm wide and 300 nm thick, the WS2 layer is 0.65 nm thick and the Al2O3 layer is 30 nm thick. 83x56mm (300 x 300 DPI)


ACS Photonics

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Figure 1. (a) Schematic of the WS2-based optical modulator structure. Bottom to top: Silicon substrate, silica, Si3N4 waveguide, WS2 monolayer, and Al2O3 layer. (b) Electric field mode distribution of Si3N4– WS2–Al2O3 waveguide transverse electromagnetic fundamental (TEM0) mode. The white lines show the silicon, silica, Si3N4 waveguide, WS2, and Al2O3 interfaces and edges. The waveguide cross-section is 2 µm wide and 300 nm thick, the WS2 layer is 0.65 nm thick and the Al2O3 layer is 30 nm thick. 83x43mm (300 x 300 DPI)


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Figure 2. (a) Micrograph of Si3N4 waveguide before the WS2 and Al2O3 layers being grown over it. (b) Micrograph of Si3N4–WS2–Al2O3 waveguide at the instant when the waveguide is irradiated with 532-nm light. The tip of the tapered fiber is apparent because of illumination by a weak white light. The incident green light was filtered by the setup. 83x31mm (300 x 300 DPI)


ACS Photonics

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Figure 3. Emission spectra of the signal transmitted through Si3N4 waveguide without and with WS2 monolayer, and with 532-nm laser excitation. 83x58mm (300 x 300 DPI)


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Figure 4. Schematic diagram of Si3N4–WS2–Al2O3 modulation structure experiment. Inset: Photograph of part of the experimental setup. 83x40mm (300 x 300 DPI)


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Figure 5. Time history of the modulated signal acquired by oscilloscope using the setup shown in Figure 4. 83x58mm (300 x 300 DPI)


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TOC 79x39mm (300 x 300 DPI)


Based All-Optical Modulator - American Chemical Society

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