Stimulated Emission Controlled Photonic Transistor on a Single

Publication Date (Web): October 1, 2018 ... provide a useful enlightenment for the rational design of novel all-optical photoswitches with desired per...
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Stimulated Emission Controlled Photonic Transistor on a Single Organic Triblock Nanowire Kang Wang, Wenqing Zhang, Zhenhua Gao, Yongli Yan, Xianqing Lin, Haiyun Dong, Chunhuan Zhang, Wei Zhang, Jiannian Yao, and Yong Sheng Zhao J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b04699 • Publication Date (Web): 01 Oct 2018 Downloaded from http://pubs.acs.org on October 1, 2018

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Journal of the American Chemical Society

Stimulated Emission Controlled Photonic Transistor on a Single Organic Triblock Nanowire Kang Wang,†,‡ Wenqing Zhang,† Zhenhua Gao,†,‡ Yongli Yan,*,† Xianqing Lin,†,‡ Haiyun Dong,† Chunhuan Zhang,† Wei Zhang,† Jiannian Yao,†,‡ Yong Sheng Zhao*,†,‡ †

Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China

‡

University of Chinese Academy of Sciences, Beijing 100049, China

Supporting Information Placeholder ABSTRACT: In this work, we demonstrate a stimulated emission controlled photonic transistor on organic triblock nanowires composed of alternate energy donor and acceptor. The population of acceptor excitons was engineered by energy transfer to achieve enhanced fluorescence, which was further amplified by the stimulated emission of the donor and the optical feedback in the nanowire microcavities, yielding a remarkable nonlinear amplification of the acceptor emission. On this basis, a prototype of photonic transistor with high nonlinear gain at very low pump energy was achieved. The results will provide a useful enlightenment for the rational design of novel all-optical photoswitches with desired performances.

Photonic transistor is the optical analog of electronic transistor that forms the basis of modern electronic technology.1-6 Since the input signal intensity is, in most cases, weaker than that of the source in an integrated photonic device, a photonic transistor is sought to switch or amplify the optical signal for the applications in optical computing and fiber-optic communication networks.7-10 Till now, all-optical light manipulations at micro/nanoscale have been demonstrated with various schemes, such as photoinduced refractive-index changes,11 photochromic reactions,12 energy transfer,3 etc., which permit to achieve linear switching with either attenuation or enhancement functionalities. However, as a key requirement of photonic transistors, nonlinear amplification of light signals still remains as a big challenge, which restricts their practical applications.13,14 Stimulated emission, where incident photons induce exponentially growth of radiative photons,15-18 is a possible physical mechanism of nonlinear light amplification for the operation of photonic transistors.9 In comparison with their inorganic counterparts, organic materials are promising for stimulated emission because of their excellent optical gain effect.19-24 Benefiting from outstanding doping flexibilities and weak intermolecular interactions,25,26 different kinds of organic materials can self-assemble into regular one-dimensional (1D) multiblock heterostructures,27-29 affording an opportunity to integrate the input, output, and control units in a single nanostructure.30 Moreover, the large optical cross-sections in organic materials are beneficial for efficient energy transfer in donor-acceptor heterostructures.31,32 The energy transfer offers an effective scheme to enhance the acceptor emission, which could be further amplified by the stimulated emission of the donor,

yielding a nonlinear amplification of the light signal.33 To this end, organic materials are ideal candidates to materialize nanoscale photonic transistors.34 Herein, we report a strategy to achieve stimulated emission controlled photonic transistor on a single 1D organic triblock nanowire (TNW). The nanowire heterostructures were fabricated from a pair of energy donor-acceptor compounds through a liquidphase co-assembly method. The population of acceptor excitons was regulated by energy transfer to realize enhanced fluorescence, which was further amplified by the stimulated emission of the donor and the optical feedback in the nanowire Fabry-Pérot (FP) cavities, resulting in nonlinear amplification of the acceptor emission. On this basis, we experimentally demonstrated a prototype of photonic transistor with significant nonlinear signal enhancement at very low pump energy. Our results open up a new way to achieving miniaturized photonic amplification devices. Nanowire structures with both ends as natural input and output units are adopted to manipulate light signals due to their strong optical confinement and on-chip interconnect applications.2,35-37 Inserting a nonlinear control unit into the nanowire structures would enable effectual light modulation, facilitating the realization of all-optical transistors in multiblock nanowires.30 Figure 1A presents the working principle for the nonlinear amplification of light signal on a single organic TNW, where the signals input from “Source” propagate to “Drain” through “Gate”. The source and drain ports, made up of energy acceptor (A), are utilized to inject and export light signals, respectively. The gate unit (composed of energy donor, D) is capable of lasing and converting donor excitons to A. During the transition from spontaneous to stimulated emission, the photons produced in D will be significantly amplified, and then transfer the energy to A, leading to a nonlinear enhancement of the source signal. The energy donor, 1,4-bis(α-cyano-4-diphenyl)-2,5diphenylbenzene (OPV-D, Figure S1) and the energy acceptor, 1,4-bis(α-cyano-4-diphenylaminostyryl)-2,5-diphenylbenzene (OPV-A, Figure S2), were selected as the model compounds for the following reasons. (i) There is a large spectroscopic overlap between the emission of OPV-D and the absorption of OPV-A (Figure S3), which is indispensable for efficient energy transfer. (ii) The stimulated emission in OPV-D opens up the opportunity for nonlinearly amplifying the light signal. (iii) Both compounds with similar molecular structures prefer one-dimensional growth,38 facilitating us to synthesize 1D multiblock nanowires.

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the absence of acceptor (area b in Figure 2D, Figure S7), while it decreased to 0.30 ns in area a (tDA). The efficiency of energy transfer was estimated to be 74.1% (Table S1), which is pretty high compared with the ever reported heterostructures.40,41 The high efficiency together with sharp shortening of the donor lifetime at the ends of TNWs validate that Förster resonance mechanism behaves as the major way of energy transfer,41 which was further verified by a longer rise time in acceptor’s PL decay curves (Figure S8). Such an efficient energy transfer is favorable for effective amplification of light signals.

Figure 1. (A) Diagram for the working principle of the organic TNW based photonic transistor. (B) Illustration for the coassembly of OPV-D and OPV-A into TNWs. (C) PL image of discrete TNWs under UV excitation. Scale bar: 10 μm. (D, E) TEM images of the central part (D) and the tip (E) of a single TNW. Scale bars: 500 nm. Insets: SAED patterns collected from the marked areas. (F) Bright-field (left) and PL (right) images of an isolated TNW. Scale bar: 5 μm. (G) Spatially resolved PL spectra collected from different locations marked in (F). Through a liquid-phase co-assembly method (Figure 1B), we prepared 1D organic TNWs in a controllable way (see Supporting Information for experimental details). Well-defined 1D nanowires with triblock color distribution were obtained, as verified by the PL image (Figure 1C). The composition of the heterostructures can be tuned by varying the preparation conditions (Figure S4). The central part of the TNWs are of high crystallinity growing along the [100] direction of OPV-D, as evidenced by the transmission electron microscopy (TEM) image and selected area electron diffraction (SAED) pattern in Figure 1D and Figure S5. The SAED of the end part in Figure 1E exhibits an identical pattern as that in Figure 1D, implying that the entire TNWs are almost made up of OPV-D. The spatially resolved PL spectra of an individual TNW (Figure 1F, G) show that the emission from location 2 is in good consistence with that of OPV-D (Figure S6). The spectra from the ends (locations 1 and 3) are dominated by the green emission, which suggests that OPV-A is doped into the OPV-D matrices. The absence of OPV-D emission in locations 1 and 3 should be attributed to the efficient energy transfer. Energy transfer is an effective way to manipulating light signal through exciton conversion. Figure 2A depicted the exciton conversion between OPV-D and OPV-A according to their intrinsic energy levels. Upon excitation, OPV-D molecules would be pumped to the excited states. Thereby, OPV-D excitons transfer the energy to OPV-A, giving rise to enhanced acceptor emission at the expense of the donor emission. As illustrated in the confocal image in Figure 2B, both ends of TNWs are much darker than the central part, suggesting that the fluorescence of OPV-D is severely quenched by the doped OPV-A. For a better understanding, fluorescence lifetime imaging microscopy (FLIM) was conducted to evaluate the efficiency of energy transfer.39 The FLIM image in Figure 2C show that the PL decay becomes obviously faster from the central part to the tips, which is consistent with the distribution of the OPV-A molecules. The average lifetime of donor (tD) was determined to be 1.16 ns in

Figure 2. (A) Schematic of energy transfer from OPV-D to OPVA. (B) Confocal image of some typical TNWs in the donor channel (450 ± 35 nm). Scale bar: 10 μm. (C) Corresponding FLIM image of the TNWs in the same channel. (D) Instrumental response function (black) and PL decay curves (colored) collected from the selected areas (a, b) in (C). Light amplification mechanisms in TNW were investigated with a home-built far-field micro-photoluminescence system (Figure S9). Upon excitation of a femtosecond laser (fs-laser, Figure 3A), the outcouplings from TNW’s tips are a mixture of PL from both the donor and acceptor, which is confirmed by the PL image (Figure 3A inset) and the two peaks centered at 465 nm and 540 nm in Figure 3B. The plot of PL intensity at 465 nm versus pump fluence exhibits a nonlinear behavior with a threshold of 11.5 μJ/cm2 (Figure 3C), accompanied by the sharp decrease in full width at half maximum (FWHM, Figure S10), which demonstrates the transition from spontaneous to stimulated emission in the donor. The acceptor shows broad PL spectra peaked at 540 nm (Figure 3B inset) and the FWHM remains unchanged with increasing pump power (Figure S10), which indicates the spontaneous emission from acceptor. By plotting the PL intensities at 540 nm against the pump fluence, we observed a superlinear growth with the same threshold as that of donor (Figure 3C), suggesting that the nonlinear amplification of acceptor emission should be ascribed to the cooperative effect of stimulated emission of the donor, and the energy transfer process between them (Figure S11). The underlying mechanism behind this nonlinear enhancement of green light is schemed in Figure 3D. The fs-laser irradiation produces OPV-D excitons, which diffuse along the TNWs and transfer their energy to OPV-A, resulting in the green emission.

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With increasing pump intensity, the population of OPV-D excitons grows linearly, and accordingly, the acceptor emission intensity is also linearly increased. Once the pump fluence exceeds the lasing threshold of OPV-D, the photons generated in OPV-D are significantly amplified by the selective feedback of FP microcavities (Figure S12).17 These confined photons propagate along the nanowire back and forth via facet reflection, rendering nonlinear enhanced emission from acceptor through energy transfer. Hence, the combination of energy transfer and stimulated emission in the TNWs is essential for nonlinearly amplifying the light signal.

reversibly tuned by switching the fs-laser irradiations, and the signal gain remained nearly unchanged after ten cycles (Figure 4E) with the frequency reaching as high as the order of GHz (Figure S13), which demonstrates good reproducibility and low device fatigue.

Figure 4. (A) PL images of a typical TNW irradiated with UV beam (top), and two focused laser beams (bottom). Scale bar: 5 μm. (B) PL spectra around 540 nm from the drain with varied gate power. (C) Gate pump fluence dependent profiles of the PL intensity and FWHM around 540 nm. (D) Plot of signal gain at 540 nm versus gate pump fluence. (E) Plot of the signal gain against the switching cycles. Figure 3. (A) Illustration for the optical excitation and collection on a TNW. Inset: PL image of a single TNW excited with fs-laser. Scale bar: 5 μm. (B) PL spectra taken from an isolated TNW with adjustable pump fluences. Inset: PL spectra around 540 nm. (C) Power-dependent profiles of the PL intensities at 465 nm and 540 nm. (D) Schematic for the nonlinear amplification of acceptor emission. Based on the above discussions, we designed a prototype of stimulated emission controlled photonic transistor. As shown in Figure 4A, a consistent continuous-wave (CW) laser (405 nm, 0.4 W/cm2) was adopted to perform fluorescence excitation of acceptor at the left end, which served as the source. A pulsed laser (400 nm, 100 fs) was applied on the central part to populate the excited states of donor and function as the gate. As displayed in Figure 4B, the PL spectra of outcoupled lights from the right drain tip were significantly amplified by increasing the power of the gate light. Figure 4C plots the peak intensity around 540 nm as a function of gate power. At low pump energy, the intensity grew steadily. When the pump fluence exceeded 4.09 μJ/cm2, the peak intensity increased rapidly and superlinearly, while the FWHM at 540 nm was essentially unchanged, revealing a nonlinear enhancement of spontaneous emission. The signal amplification factor of the photonic transistor can be defined as gain (dB)= 10×lg(Ion/Ioff),8,14 where Ion and Ioff are the intensities of green emission output from drain with and without gate excitation, respectively. Figure 4D shows the relationship between signal gain and the gate power. Obviously, the gain continuously changes from 0 dB (OFF) to 9.7 dB (ON) with the increase of gate pump fluence from 0 to 4.09 μJ/cm2 to open the transistor, and saturates at 4.51 μJ/cm2, which is rather necessary for photonic transistors to work among cutoff, amplification and saturation regions, respectively. This significant amplification was

In summary, we report the design and realization of stimulated emission controlled photonic transistors with 1D organic donoracceptor triblock nanowire heterostructures. The highly efficient energy transfer from the donor to the acceptor and the lowthreshold stimulated emission of the donor in TNWs were experimentally illustrated. Based on the combination of energy transfer and stimulated emission, we demonstrated remarkable nonlinear enhancement of light signal at low pump power, which made TNWs function as photonic transistors with good reproducibility and reversibility. The results gain us a deep insight into the mechanism of nonlinear modulation of light signal, and provide the guidance for nanowire-based photonic components.

ASSOCIATED CONTENT Supporting Information Experimental details and additional data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected]; [email protected];

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported financially by the Ministry of Science and Technology of China (Nos. 2017YFA0204502 and 2015CB932404), the National Natural Science Foundation of China (Nos. 21773265, 21533013 and 21790364), and the Youth Innovation Promotion Association CAS (2014028).

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