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Black Phosphorus based All-Optical-Signal-Processing: towards High Performances and Enhanced Stability Jilin Zheng, Zhenghua Yang, Si Chen, Zhiming Liang, Xing Chen, Rui Cao, Zhinan Guo, Ke Wang, Ying Zhang, Jianhua Ji, Meng Zhang, Dianyuan Fan, and Han Zhang ACS Photonics, Just Accepted Manuscript • Publication Date (Web): 10 May 2017 Downloaded from http://pubs.acs.org on May 13, 2017

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Manuscript type: Article Black Phosphorus based All-Optical-Signal-Processing: towards High Performances and Enhanced Stability Jilin Zheng,†,‡ Zhenghua Yang,† Si Chen,† Zhiming Liang,† Xing Chen,† Rui Cao,† Zhinan Guo,† Ke Wang,† Ying Zhang,† Jianhua Ji,§ Meng Zhang,ǁ Dianyuan Fan,† and Han Zhang*,† †

Shenzhen Engineering Laboratory of Phosphorene and Optoelectronics, Collaborative

Innovation Center for Optoelectronic Science and Technology, and Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China ‡

Institute of Communication Engineering, PLA University of Science and Technology,

Nanjing, 210007, China §

College of Information Engineering&Shenzhen Key Lab of Communication and

Information Processing, Shenzhen University, Shenzhen, 518060, China ǁ

School of Electronic and Information Engineering, Beihang University, Beijing, 100191,

China

ABSTRACT: Two-dimensional (2D) black phosphorus (BP) shows thickness dependent direct energy band-gaps in association with strong light-matter interaction and broadband optical response, rendering it with promising optoelectronic advantages particularly at the telecommunication band. However, intrinsic BP suffers from irreversible oxidization, restricting its competences towards real device applications. As one potential of 2D materials, all-optical signal processing sensitively depends on the strength of light matter interaction. BP can be utilized as a novel optical medium. Herein, few-layer BP is synthesized with metalion-modification against oxidation and degradation, and then the feasibility of BP-coated microfiber as an optical Kerr switcher and a four-wave-mixing-based wavelength converter is 1 ACS Paragon Plus Environment

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demonstrated. The wavelength-tuning, long term stability, wide-band RF frequency and timerepeating measurements confirm that this optical device can operate as a broadband all-optical processor. It is further anticipated that metal-ion-modified BP might provide a new effective option for photonic applications towards high performances and enhanced stability. Keywords: 2D material, black phosphorus, all-optical-signal-processing, optical Kerr switcher, four-wave-mixing.

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n recent decades, 2D materials have undergone remarkable advancements that highlight great potentials of 2D materials for diverse photonic and optoelectronic applications.1-12

Unlike conventional bulk materials, the electronic and optical properties of layered materials can be more effectively engineered over a wide range by chemical or electronic doping, offering a great flexibility in tailoring its optoelectronic properties.13-15 Among all the achievements, all-optical signal processing, including signal regeneration and controlling light by light, remains much less investigated despite that 2D materials might find unique advantages over conventional materials owing to its planar structure, reduced optical scattering, enhanced figure of merit, broadband and ultrafast optical response. All-optical signal processing plays a critical role in ultrahigh bit rate communication systems,16-20 as the current electronic processing speed is gradually approaching the fundamental limits at 60 G-bps.21-22 However, challenges still remain on how to significantly improve the device performances in order to fulfill some emerging applications, for example, reducing footprint size, weakening required switching optical power and boosting figure of merit. As is well known that, the performance of all-optical signal processing heavily depends on optical material and device structure. Although all-optical signal processing is widely regarded as an effective way to overcoming the fundamental “bottle neck” that exists in the current electronic or electro-optic systems. However, the advancements in all-optical signal 2 ACS Paragon Plus Environment

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processing are still impeded by lack of suitable materials. In spite of persistent efforts to pursue nonlinear optical devices with enhanced performances for all-optical processing based on the traditional material systems, the limitations coming from the current materials still trigger strong motivations to search for new advanced optical materials with desired properties. Fortunately, 2D materials with large nonlinear optical susceptibilities open a new pathway of seeking for the coveted optical nonlinear materials. In the family of 2D materials, graphene dominates the pioneering work of all-optical processing, followed by Transition metal dichalcogenides (TMDs), topological insulator (TI). 23-26 Those previous work related to all-optical processing such as optical Kerr switcher, four-wave-mixing (FWM)-based wavelength converter and saturable-absorption (SA)–effect-based optical modulator, have demonstrated feasibilities and features of grapheme and TMDs.9, 27-29 However, on one hand, the relative weak absorption in graphene (only 2.3% of incident light per single layer) would degenerate its light modulation ability while its semi-metallic energy band-gap would lead to very low electronic on/off ratio, unless special device structure was adopted.30 On the other hand, optical response of TMDs mainly occurs in the visible range due to the relatively wide band-gap, delimiting their applications at the optical communication band. Very recently, BP has joined in the family of 2D materials.31-32 The basic structure of BP is similar to that of bulk graphite, in which individual atomic layers were stacked together and held by van der Waals interaction.33 Inside single layer, each phosphorus atom is covalently bonded by three adjacent phosphorus atoms in order to form a unique puckered honeycomb structure. In bulk form, BP has a band-gap of 0.3 eV due to the interlayer interaction while its band-gap increases with the decrease of the thickness.34 Consequently, the emergence of fewlayer BPs can bridge the gap between graphene and TMDs for infrared photonics and optoelectronics, particularly suitable for the optical communication devices. Such a direct and 3 ACS Paragon Plus Environment

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moderate energy gap in few-layer BPs endow it with unique advantage over other 2D materials in the aspect of optical communication device, such as optical detector, modulator and signal processor. Similar to the other 2D materials, such as graphene and TMDs, BP can be utilized as an excellent nonlinear optical material due to its high nonlinear refractive index.35 Recently, BP has been intensively investigated in many nonlinear optical applications, such as the passive mode-locker or Q-switcher for ultrafast photonics applications,5,

36-38

and FWM-based

wavelength converter. 39 However, there remains a fundamental drawback in BP owing to lack of sufficient stability under ambient conditions as oxygen and water could lead to rapid degradation of the electronic and optical properties of BP.40 In order to mitigate this intrinsic obstacle, some strategies including capping layer protection by using oxidation or h-BN layer41 and ligand surface coordination42 have been demonstrated to improve the long term stability of BP. In this work, based on a liquid phase exfoliation approach, we had fabricate few-layer BP nano-flakes, which were then under further modification through metal ion doping (Ag, Au etc). Thus, metal-ion-modified BP with enhanced stability had been synthesized against oxidation and degradation. We further investigated the applicability of metal-ion-modified BP for all-optical signal processing by using its nonlinear Kerr effect. The optical Kerr effect, as one of the non-resonant electronic nonlinear polarizabilities, occurs as a result of the nonlinear response of bound electrons with respect to the external applied optical field.43 BP-coated microfiber had been fabricated through depositing metal-ion-modified BP onto the microfiber by using the optical deposition approach. The mutual light–matter interaction is achieved through the interaction of BP with the evanescent field of propagating light in microfiber. Both the optical Kerr switcher and the FWM-based wavelength converter had been experimentally investigated. As the first prototypical BP-based optical Kerr switching device, our results show that this device could operate as an optical Kerr switcher 4 ACS Paragon Plus Environment

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with an extinction ratio as high as 26 dB and an operation range over 18 nm. In addition, a FWM-based wavelength converter with a maximum conversion efficiency of -59.15 dB and an extinction ratio of 10 dB, is demonstrated in this paper, which could be considered to be an improvement compared to previous work. 39 We also investigate the RF-modulation frequency dependence of FWM-based wavelength conversion under different RF frequencies, at a range from 100 MHz to 10 GHz. Results show that RF information modulated in the signal light can be copied to the newly generated FWM signals under the entire RF range. Therefore, our results verify that this BP-coated microfiber could operate as a broadband nonlinear photonics device for all-optical signal processing. RESULTS AND DISCUSSION Materials characterization and BP-coated microfiber. The liquid phase exfoliation (LPE) is considered as a simple but effective technique to synthesize 2D materials from the layered bulk crystals towards few-layer structures (See Methods for details). The Raman spectrum of the prepared functional modification few-layer BPs with AgNO3 was measured by a Raman microscope, and are presented in Figure 1(d). Three Raman peaks of functional modification few-layer BPs with AgNO3 corresponding to one out-of-plane vibration mode A1g and two inplane vibration modes B2g and A2g are located at 362.7, 439.5 and 466.2 cm-1, respectively. Raman spectrum of BPs doped with HAuCl4 were also measured and are shown in the Supporting Information. In order to study the morphologies of the as-prepared few-layer BPs, the transmission electron microscope (TEM) image and atomic force microscopy (AFM) image were provided. As can be seen in Figure 1(a), functional modification BPs could be clearly identified to be layered structure with a large size of several micrometers. AFM of BP on Si/SiO2 substrates revealed the presence of a range of shapes and thickness of phosphorene, showing a nanosheet with size about 750 nm, as shown in Figure 1(b). Heightprofiling of the nanosheet be marked shown in Figure 1(c), revealed a thickness in the range 5 ACS Paragon Plus Environment

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of 14.4 nm ~18.6 nm. The X-ray photoelectron spectroscopy (XPS) were provided to study the functional modification BPs, which are shown in the Supporting Information. 1.2 μm

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800 1000 1200

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Figure 1. Characterizations of functional modification few-layer BPs: (a) TEM image; (b) AFM images of functional modification few-layer BPs; (c) Height profiles of the section marked in (b); (d) Raman spectrum of functional modification few-layer BPs doped with AgNO3. After deposition process (See Methods for details), the BP-coated microfiber-based is shown in Figure 2. As it can be seen, the taper waist diameter and deposition length of fewlayer BPs is about ~ 7 µm and ~ 242 µm, respectively.

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~ 242 µm

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Figure 2. (a) Schematic diagram of the optical Kerr switcher based on BP-coated microfiber; (b) Microscopic image of the fabricated BP-coated microfiber. BP-Based Optical Kerr Switcher. The optical Kerr switcher enabled by the nonlinear optical effect is an essential component for all-optical networks. Similar to the nonlinear polarization rotation in highly nonlinear optical fiber, 44 the nonlinear polarization rotation can also been realized in the BP-coated microfiber due to the high nonlinear Kerr refractive index of BP and sufficient light matter interaction distance. The operation principle of the optical Kerr switcher based on BP-coated microfiber is schematically shown in Figure 2 (See Methods for details).

Figure 3. Experimental setup of the optical Kerr switcher based on BP-coated microfiber.


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The experimental setup on nonlinear polarization rotation is shown in Figure 3 (See

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Figure 4. Output spectra obtained after the polarizer with (a) pump signal turned off and (b) pump signal turned on, the spectra of the pump light are shown in the inset of (b). The wavelengths of pump and probe light were fixed at 1550.52 nm and 1554.68 nm, respectively. The experimental results show that, through switching on/off the pump light, the output probe signal could operate in two different states as seen in Figure 4. When the pump light was absent, the power of probe signal after polarizer was ~-53.6 dBm due to the blocking of the orthogonal polarizer, although the original probe power before optical coupler was ~ 0 dBm. However, when the pump light with a power of ~ 300 mW was launched into the input port of the optical coupler, the power of probe signal increased to -26.9 dBm immediately. Such a switching on/off state of the probe signal indicates that the high-power pump signal has induced the polarization rotation of the probe signal, and most of the power of probe signal can pass the polarizer. The extinction ratio is ~26 dB between the switching on and off states of the probe signal. It is therefore suggested that we are able to deliberately control the transmission of the probe signal through modulating the intensity of the pump signal, which constitutes a BP-microfiber-based optical Kerr switcher. It should be noted that the power data read from the spectrum recorded by optical spectrum analyzer (OSA) might not represent the true value of power of signal, which is usually attributed to the loss of measurement and the wavelength resolution of OSA. However, it is reasonable that the OSA’s 8 ACS Paragon Plus Environment

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data can be used to measure the relative change of power of signal, such as the extinction ratio. Furthermore, in order to clarify whether or not the cross phase modulation-induced nonlinear polarization rotation occurred due to light interaction with BP particles, we conducted another experiment for the same Kerr switcher using a tapered fiber only without BP particle coating, and compare the results with the case using a tapered fiber coated with BP particles. The final results provide strong experimental evidences that the nonlinear polarization rotation occurred due to BP particles. The detailed result can be found in the Supporting Information. In order to well characterize the dependence of the change of the SOP of probe signal on the incident pump power, the pump power was gradually tuned, and the probe transmittance under different pump powers was correspondingly recorded. At the output port, the phase difference of two polarization components of the probe signal manifests as a change of the SOP. As a result, the transmittance of the probe signal will increase with the increase of the pump power, suggesting that the nonlinear polarization rotation can change the SOP of the probe signal. In the meanwhile, the probe signal can change from the off-state to the on-state once the pump signal is transited from low- to high-power regime. Apparently, the transmittance of probe light is related to the light intensity of pump. In our experiment, the maximum output power of the EDFA that we used can reach up to 25 dBm by increasing the current, and the optical power launched into the sample could be adjusted from 0 mW to ~140 mW. When the power of pump light into the sample is tuned from 1 mW to ~140 mW, an increase of the probe can be observed clearly. The entire evolution process had been well summarized in Figure 5(a). The normalized probe transmittance against input pump power into the sample was plotted in Figure 5(b). The probe transmittance can reach its maxima under a launched pump power of 123 mW, indicating that the SOP of probe signal has been rotated to be in parallel with the polarizer.


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80% 60% 40% Theoretical fitting Experimental results

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Figure 5. (a) The evolution of the output spectra under different pump power from 1 mW to ~140 mW. (b) The relation of the probe transmittance against the incident pump power. The measured transmittance plotted in Figure 5 (b) also shows that, with the increase of the pump signal beyond 123 mW, saturation of the probe transmittance might occur. To some extent, probe transmittance even exhibits a trend of decreasing when the input pump power is tuned to be stronger than 123 mW. Such a phenomenon can be well explained with an analysis of the evolution process of the SOP of probe signal under ever-rising pump power. As to this experiment, when the pump power reaches up to ~123 mW, the probe signal already reaches its maxima. Any exceeded pump power will induce extra rotation of SOP of probe signal relative to the former optimized parallel state, and subsequently, there is a transmittance decrease in probe signal. The experimental results could be well fitted with the theoretical equation.45 However, limited by both the maximum output power of EDFA available in our laboratory and the optical damage problem of BP-coated microfiber, the probe transmittance under higher pump power was not further investigated. In the following, we also investigate the dependence of the performance of BP-coated optical switcher on wavelength spacing between the pump and probe signal. The wavelength spacing is adjusted by tuning the central wavelength of the probe while the central wavelength of pump light is fixed. During the process of wavelength tuning, the power of both pump and probe are fixed, that is 25 dBm and 0 dBm, respectively. In our experiment, the wavelength of pump light is fixed at 1550.52 nm, while the wavelength of probe signal is tuned from 10 ACS Paragon Plus Environment

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1540.68 nm to 1558.68 nm at a wavelength interval of 1 nm. It should be noted that due to the relatively-wide bandwidth of the OBPF (~2 nm @ 3 dB), wavelength ranging from the leftside of the OBPF to the right-side of the OBPF are not used for the probe. Figure 6(a) shows the output spectra versus the wavelength changes of probe signal. As shown in Figure 6(b), when the pump signal wavelength is adjusted from 1540.68 nm to 1558.68 nm, the extinction ratio was always kept at ~26 dB. 0

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Figure 6. (a) The output spectra versus the wavelength of probe light tuned from 1540.68 nm to 1558.68 nm and (b) extinction ratio versus the wavelength of probe light tuned from 1540.68 nm to 1558.68 nm. To investigate the stability of switching-on state, we record the repeated scans of the switching-on state measured at a time interval of 10-minute as shown in Figure 7(a). The extinction ratios at different time are also calculated according to the measured spectrum, as shown in Figure 7(b). Results show that extinction ratio was almost kept nearly constant at ~26 dB. Therefore, both the wavelength-tuning measurement and the time-repeating measurement verify that this as-fabricated BP-coated microfiber could operate as a broadband device with reasonably high on/off ratio and long term stability. The comparison among BPbased, TI-based28 and CNT-based45 optical Kerr switcher can be found in the Supporting Information.


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The Four-Wave Mixing Effect of the BP-Based Optical Nonlinear Kerr Device. When pump light and probe signal simultaneously co-propagate through BP-coated microfiber, FWM can be generated provided that optical power is sufficiently high. The experimental setup is schematically shown in Figure 8 (See Methods for details).

Figure 8. Experimental setup on four-wave mixing in the BP-coated microfiber, the OBPF is optional. In the first experiment while the OBPF is not used, the signal wavelength is fixed at 1550.70 nm, and the tunable ECL is tuned at 1552.35 nm. The powers of the signal light and the ECL light before the OC are measured to be -3 dBm and 2.6 dBm, respectively. In order to generate noticeable FWM spectrum, the output light of the OC is boosted up to 25 dBm by the HP-EDFA. Figure 9 shows a typical output FWM spectrum obtained after the BP-coated 12 ACS Paragon Plus Environment

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microfiber. Two newly converted idlers at 1549.05 nm and 1554.0 nm are generated, respectively. Figure 9(a) and 9(b) are the results of with or without 10 GHz-RF modulation, respectively. From Figure 9(b), we can find the RF information modulated in the signal light has also been copied into the newly generated FWM signals. 0

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Figure 9. Output spectra after the BP-coated microfiber: (a) without 10 GHz-RF modulation; (b) with 10 GHz-RF modulation. In the second experiment, the OBPF is employed. In order to suppress both the FWM spectrum induced by other nonlinear devices along with the whole optical link except the BPcoated microfiber and the massive ASE brought by the HP-EDFA, it is necessary to employ an OBPF to filter out the optical spectrum at the wavelengths of potential FWM before being launched into the BP-coated microfiber. Then the final FWM spectrum obtained after the BPcoated microfiber should be mostly generated by the BP-coated microfiber. However, due to the limitation of experimental condition available, there are no matched OBPFs to filter out the original input optical light. In this experiment, an OBPF with an optical bandwidth of ~ 6.5 nm had been used to filter out the optical spectrum at the wavelengths longer than that of pump light. In other words, both the FWM spectrum induced by other nonlinear devices before the BP-coated microfiber and the massive ASE brought by the HP-EDFA, which are located at the wavelengths longer than that of pump light, can be dramatically suppressed. Through investigating the optical spectrum of newly generated FWM located at the wavelengths longer than that of pump light after the BP-coated microfiber, we can derive the 13 ACS Paragon Plus Environment

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contributions of BP-coated microfiber to FWM generation. Figure 10(a) shows the measured optical spectrum of three cases. The spectrum with black line is the output spectrum after BPcoated microfiber without OBPF, and the red one is the output spectrum after BP-coated microfiber with an OBPF, while the blue one is the output spectrum after bare microfiber with an OBPF. The transmission spectrum of the OBPF is also measured for reference, which is plotted as the green line. Figure 10(b) shows the whole transmission spectrum of the OBPF at a larger wavelength range, which shows out-of-band rejection (OOBR) of such an OBPF is ~24.25 dB. From Figure 10(a), we can derive the quantitative contribution of BP-coated microfiber and BP material only to FWM generation. The inset corresponding to the pump light provides a reference level for evening up the power levels of these three cases in terms of insertion loss of the OBPF, fiber connectors and other uncertain factors, which shows that the power level of the case “without filter, with BP-coated microfiber” is 0.47 dB and 0.92 dB (= 0.47+0.45 dB) higher than that of “with filter, with BP-coated microfiber” and “with filter, with bare microfiber”, respectively. The inset corresponding to the right-side generated FWM shows that different powers of FWM have been produced under such three conditions. The generated FWM of the case without OBPF is ~16.55 dB higher than that of with an OBPF. Then, taking into account the difference of power levels, the generated FWM of the case of with the OBPF is only 15.63 dB lower than that of without OBPF. Finally, in view of the OOBR with such an OBPF (~24.25 dB), the power of newly generated FWM by just the BPcoated microfiber is only observed to be ∼16.25 dB lower than the one by all nonlinear devices contained in the entire experimental system. That is, under the same experimental conditions, the newly FWM could only be generated by BP-coated microfiber with ∼16.25 dB lower than the one with black line in Figure 10(a). We also measure the generated FWM spectrum after a piece of bare microfiber without BP for reference under the same experimental conditions, which is observed to be ∼4.69 dB (=4.24+0.45 dB) lower than the 14 ACS Paragon Plus Environment

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one with BP when taking the difference of power levels into consideration. That is, under the same experimental conditions, the converted idler with BP is ∼294.4% higher than the one without BP, which means that 74.6% of the FWM generated by BP-coated microfiber is attributed to the BP material other than fiber nonlinearity. Hence, the power of newly generated FWM purely contributed by BP material is estimated to be ∼17.52 dB (=16.25+1.27 dB). Hence, the contribution of BP-coated microfiber and BP material only to FWM generation is confirmed quantitatively. For instance, the experimental results shown in Figure 10(a) indicate the conversion efficiencies brought by the BP-coated microfiber and BP material only are estimated as -59.15 dB (= -42.9-16.25 dB) and ~-60.42 dB (= -42.9-17.52 dB), respectively. Here, the conversion efficiency is defined as the power ratio between the


generated signal and the original signal. 0 -15 -30 -45 -60 -75 -90

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Figure 10. (a) Measured optical spectra after the BP-coated microfiber. (b) Transmission spectrum of the OBPF.


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To investigate the stability of FWM process, we record the repeated scans of the output spectrum after the BP-coated microfiber measured at a time interval of 10-minute as shown in Figure 11(a). Both the conversion efficiency and the extinction ratio at different time are also calculated according to the measured spectra as shown in Figure 11(b). Results show that the conversion efficiency and the extinction ratio are almost kept nearly constant at ~ -42.9 dB and 10 dB, respectively. It should be noted that the conversion efficiency mentioned here is the entire nonlinear effect of the whole system, which is 16.25 dB higher than the nonlinear effect induced by BP-coated microfiber.


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Figure 12. (a) Output FWM spectra against wavelength detuning, (b) Conversion efficiency against wavelength detuning. To investigate the wavelength dependence of the conversion efficiency, the wavelength of the pump light is tuned from 1550.876 nm to 1557.126 nm at the step of 0.25 nm, while the 16 ACS Paragon Plus Environment

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signal wavelength is fixed at 1550.70 nm. As shown in Figure 12(a), with the increase of the wavelength spacing, the intensity of the generated FWM wavelength decreases correspondingly. The relation of the conversion efficiency with different wavelength detuning is shown in Figure 12(b). It should also be noted that the conversion efficiency calculated here is the entire nonlinear effect of the whole system, which is 16.25 dB higher than the nonlinear effect induced by BP-coated microfiber. As shown in Figure 12(b), the conversion efficiency decreases with the increase of wavelength detuning, which is attributed to the walk-off effect. There exists strong optical birefringence caused by the fiber bending and material inhomogeneity. It can, therefore, introduce a strong walk-off effect between these interacting laser beams, rendering a phase mismatch between the pump and the signal. It therefore could decrease the conversion efficiency, and the detailed analysis can be found in the Supporting Information. To investigate the RF-modulation frequency dependence of FWM-based wavelength conversion, the RF frequency modulating the signal light is tuned from 100 MHz to 10 GHz at a frequency step of 500 MHz, while all the other experimental conditions are kept unchanged. As shown in Figure 13(a), with an increase of RF-modulation frequency, the sidebands in the generated FWM light become wider and clearer. The extinction ratio and conversion efficiency could be derived from the measured spectrum as shown in Figure 13(b). Results show that the conversion efficiency is kept nearly constant at ~ -42.9 dB, and the extinction ratio experiences a minor decrease from 12 dB to 10 dB. Such a variation trend of the extinction ratio is mainly attributed to the intrinsic effect in the EDFA, because the FWM spectrum become wider with an increase of RF-modulation frequency, while the total optical power remains unchanged. In the meanwhile, the spectral sidebands shown in the newly generated FWM light indicate the RF information carried in the signal light may have been copied into the newly generated FWM light. This inference can be further confirmed by 17 ACS Paragon Plus Environment

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analyzing both the signal light spectrum and the corresponding FWM spectrum in Figure 13(c) and (d), respectively. We can find that, except for high-order sidebands, FWM spectra agree well with the signal spectra. The absence of high-order sidebands in FWM spectra is mainly attributed to the fact that their intensity is too weak to be detected by the optical characterization system. Another reason is that the low-intensity high-order sidebands are blocked by the massive noise of ASE. The values of first sideband separation are also figured out through the measured spectrum of generated FWM, showing a linear increase ranging from ~0.0008 nm to ~0.08 nm. When RF-modulation frequency is tuned at 10 GHz, the detailed profiles of both the original signal and the corresponding generated FWM are shown

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Figure 13. (a) Measured spectrum under different RF-modulation frequency. (b) Extinction ratio and conversion efficiency under different RF-modulation frequency. (c) Detailed profiles of signal light under different RF-modulation frequency. (d) Corresponding detailed profiles of FWM light under different RF-modulation frequency, the inset is the calculated first sideband separation. (e) Detailed profiles of signal light under 10 GHz RFmodulation frequency. (f) Corresponding detailed profiles of FWM light under 10 GHz RFmodulation frequency. It should be noted that unlike ref.,27, 46 we haven’t evaluated the bit-error-rate (BER) performance of FWM signal in this work because its power level is too low to be effectively detected or amplified. This work is merely an demonstration of proof of principle that RF information carried in the signal light can been copied into the newly generated FWM light via the BP-coated microfiber. In the future work, optimization measures should be taken into consideration to further increase the FWM conversion efficiency. Conclusion In order to investigate the applicability of metal-ion-doped few-layer BP for all-optical signal processing by using the nonlinear Kerr effect, we employ an evanescent field interaction scheme of the propagating light with BP deposited onto a microfiber. Experimental results show that such a BP-coated microfiber device could operate both as an optical Kerr switcher with an extinction ratio as high as 26 dB and an operation range over 18 nm, and a FWM-based wavelength converter with a maximum conversion efficiency of -59.15 dB. All the measurements demonstrate that this novel nonlinear optical device could operate as a broadband device for all-optical processing with reasonably high stability and durability, which should be credited to the relatively high nonlinear optical index and protection induced by metal-ion-modification in BP. Our contributions might constitute the first prototypical BPbased optical Kerr switching device. It is therefore envisaged that 2D BP, which possesses a 19 ACS Paragon Plus Environment

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natural direct energy band-gap within the optical communication band, may be utilized as an excellent nonlinear optical medium for all-optical processing towards practical applications with high performance and good stability, opening a new door towards the development of photonics based on 2D materials as well as the advancement of optical communication and microwave photonics. METHODS Materials preparation and characterization. The bulk black phosphorus is firstly immersed into a liquid, typically N-methyl-2-pyrrolidone (NMP) and then the 2D layer materials could be obtained through ultrasonic exfoliation. To form few-layer BP nanosheets, bulk BP was exfoliated in NMP (1 mg/mL in NMP) by using bath ultra-sonication (operating at 40 kHz frequency and 100% power) for four hours to well undertake the liquid exfoliation process of the bulk BP. The temperature of the bath was maintained below 30℃ throughout the entire exfoliation by using water-cooling. After exfoliation, solution was centrifuged at 3000 rpm for 10 minutes in order to remove any non-exfoliated bulk BP. To functionalize few-layer BP flakes, we prepared AgNO3 solution (10-5 M) mixed in the NMP solution. By adding the NMP solution in BP suspension (1 ml BP suspension and 1ml AgNO3), the mixture was put in a sonicator operated for 30 min. To better characterize the prepared functional modification few-layer BPs with AgNO3, Raman spectrum measurement was carried out by a Horiba Jobin-Yvon Lab Ram HR VIS high-resolution con-focal Raman microscope equipped with a 633 nm laser. BP-coated microfiber. In this work, the experimental process of optical deposition for fabrication of BP-coated microfiber is similar to that of literature47. Firstly, a piece of microfiber was fabricated by stretching the standard single mode fiber (SMF) with heat produced by a flame. The fiber was tapered down so that its taper waist diameter became ~ 7


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µm. Secondly, The BP (doped with AgNO3) NMP solution with a concentration of 0.08 mg/ml was dropped onto the microfiber for optical deposition. BP-Based Optical Kerr Switcher. The Kerr switcher consists of a piece of BP-coated microfiber and an optical polarizer. At the input port, both the pump and probe signals are simultaneously injected into the same piece of BP-coated microfiber. At the output port, an optical polarizer is used to convert the polarization rotation to variation of the optical power. The state of polarization (SOP) of the probe light is aligned to be substantially orthogonal with the polarizer at the case of absence of the pump signal, sufficiently suppressing the transmission of the probe signal. Provided that the pump signal is absent or weak, the probe signal will be almost blocked by the optical polarizer because the probe signal is orthogonally polarized with respect to the polarizer. With the gradual increase of the pump intensity, the mutual nonlinear interaction between the pump and probe signals become correspondingly stronger, resulting in an additional nonlinear phase shift. Since BP possesses large nonlinear refractive index and the mutual light interaction is sufficiently strong, the probe signal might encounter a cross phase modulation, leading to the polarization rotation of the probe signal after propagating along BP-coated microfiber. Consequently, owing to the cross phase modulation (XPM) caused by the co-propagating pump signal, the SOP of the probe signal will be under significant evolution and deliberately controlled by the strength of the pump signal. When the SOP of the probe light becomes parallel with the polarizer under proper injection power of the pump, the transmittance of probe light reaches its maximum. Therefore, by tuning the intensity of pump light, the probe signal could exhibit different on/off states.46 The detailed analysis can be found in the Supporting Information. In our experiment, the distributed feedback (DFB) semiconductor laser operates as the pump light while another continuous wave (CW) laser emitted from the external cavity laser (ECL), operates as the probe signal. Two different pieces of polarization controllers (PCs) were employed to adjust 21 ACS Paragon Plus Environment

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the SOP of the pump and probe signals, respectively. The pump light was firstly amplified by a commercial erbium-doped fiber amplifier (EDFA) followed by an optical band-pass filter (OBPF) in order that the amplified spontaneous emission (ASE) from EDFA could be sufficiently suppressed. The bandwidth of the OBPF available in our laboratory is about ~2 nm. The pump and probe signals were then combined through a 3 dB coupler, and copropagate along the BP-coated microfiber. Before turning on the pump light, the SOP of the probe light is aligned to be substantially orthogonal with respect to the polarizer, which can allow for the maximum conversion of the Kerr switcher. The pump light is aligned with a SOP of 45° with respect to that of the probe light. The Four-Wave Mixing Effect of the BP-Based Optical Nonlinear Kerr Device. The CW from the DFB semiconductor laser serves as the signal light for the FWM and is fed into a 10-GHz LiNbO3 intensity modulator (LN-IM). Before the LN-IM, the PC1 is employed to adjust the SOP of signal light to maximize the output power after the LN-IM. A tunable radio frequency (RF) resource, which can be adjusted from 0 GHz to 10 GHz, is used to modulate the signal light. Such a modulated signal is also aligned by another PC2 before being combined by a 3 dB optical coupler (OC). Another tunable CW from the ECL, which serves as the pump light, is combined with the modulated signal after being aligned by PC3. It should be noted that both the PC2 and PC3 must be well adjusted in order to ensure that the SOP of modulated signal is parallel with that of pump light because the FWM is sensitive to SOP. Similar as the operation in ref.,27, 46 the light from the output of OC is amplified by a high-power EDFA (HP-EDFA), and launched into the BP-coated microfiber. The amplified pump and probe signal take part in the FWM process when passing through the BP sample and a newly converted idler is generated. The OBPF with bandwidth ~ 6.5 nm is optional during the following experiments. Finally, the optical spectrum after the sample is measured with an optical spectrum analyzer (OSA). 22 ACS Paragon Plus Environment

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AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected]

ORCID Han Zhang: 0000-0002-2197-7270 Author Contributions All authors have given approval to the final version of the manuscript. Jilin Zheng and Zhenghua Yang contributed equally to this work. Acknowledgements The research is partially supported by the National Natural Science Fund (Grant Nos. 61435010, 61575089, 61504170 and 61671306), Science and Technology Innovation Commission of Shenzhen (Grant Nos. KQTD2015032416270385, JCYJ20150625103619275, JCYJ20160422171348663 and JCYJ20160328145357990), China Postdoctoral Science Foundation (Grant Nos. 2016M602517 and 2016M592528). Received: ((will be filled in by the editorial staff)) Revised: ((will be filled in by the editorial staff)) Published online: ((will be filled in by the editorial staff)) Supporting Information Supporting Information Available: Nonlinear polarization rotation in BP-coated microfiber, comparison of nonlinear polarization rotation between bare microfiber and BPcoated microfiber, FWM conversion efficiency deduction, optical Kerr Switcher parameters of different three types of nano-materials based nonlinear fiber devices, Raman spectrum of BPs doped with HAuCl4 and XPS of the functional modification BPs. This material is available free of charge via the Internet at http://pubs.acs.org. References 23 ACS Paragon Plus Environment

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TOC Graphic (For Table of Contents Use Only) Black Phosphorus based All-Optical-Signal-Processing: towards High Performances and Enhanced Stability Jilin Zheng, Zhenghua Yang, Si Chen, Zhiming Liang, Xing Chen, Rui Cao, Zhinan Guo, Ke Wang, Ying Zhang, Jianhua Ji, Meng Zhang, Dianyuan Fan, and Han Zhang

In this work, few-layer BP is synthesized with metal-ion-modification against oxidation and degradation, and then the BP-coated microfiber is demonstrated as an optical Kerr switcher and a four-wave-mixing-based wavelength converter. The experimental results confirm that this optical device could function as a broadband all-optical processor. It is further anticipated that metal-ion-modified BP might provide a new effective option for photonic applications towards high performances and enhanced stability.

S p e ctral In ten sity (d B m )

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