Universal Near-Infrared and Mid-Infrared Optical Modulation for

Universal Near-Infrared and Mid-Infrared Optical Modulation for Ultrafast Pulse Generation Enabled by Colloidal Plasmonic Semiconductor Nanocrystals Qiangbing Guo,† Yunhua Yao,‡ Zhi-Chao Luo,⊥ Zhipeng Qin,§ Guoqiang Xie,§ Meng Liu,⊥ Jia Kang,∇ Shian Zhang,‡ Gang Bi,∇ Xiaofeng Liu,*,† and Jianrong Qiu*,∥ †

State Key Laboratory of Modern Optical Instrumentation, School of Materials Science and Engineering and ∥College of Optical Science and Engineering, Zhejiang University, Hangzhou 310027, China ‡ State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200062, China ⊥ Guangdong Provincial Key Laboratory of Nanophotonic Functional Materials and Devices, School of Information and Optoelectronic Science and Engineering, South China Normal University, Guangzhou, Guangdong 510006, China § Key Laboratory for Laser Plasmas (Ministry of Education), IFSA Collaborative Innovation Center, Department of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China ∇ School of Information and Electrical Engineering, Zhejiang University, City College, Hangzhou 310015, China S Supporting Information *

ABSTRACT: Field effect relies on the nonlinear current−voltage relation in semiconductors; analogously, materials that respond nonlinearly to an optical field can be utilized for optical modulation. For instance, nonlinear optical (NLO) materials bearing a saturable absorption (SA) feature an on−off switching behavior at the critical pumping power, thus enabling ultrafast laser pulse generation with high peak power. SA has been observed in diverse materials preferably in its nanoscale form, including both gaped semiconductor nanostructures and gapless materials like graphene; while the presence of optical bandgap and small carrier density have limited the active spectral range and intensity. We show here that solution-processed plasmonic semiconductor nanocrystals exhibit superbroadband (over 400 THz) SA, meanwhile with large modulation depth (∼7 dB) and ultrafast recovery (∼315 fs). Optical modulators fabricated using these plasmonic nanocrystals enable mode-locking and Q-switching operation across the near-infrared and mid-infrared spectral region, as exemplified here by the pulsed lasers realized at 1.0, 1.5, and 2.8 μm bands with minimal pulse duration down to a few hundreds of femtoseconds. The facile accessibility and superbroadband optical nonlinearity offered by these nonconventional plasmonic nanocrystals may stimulate a growing interest in the exploiting of relevant NLO and photonic applications. KEYWORDS: localized surface plasmon resonance, semiconductor nanocrystals, nonlinear optics, optical modulation, ultrafast photonics, superbroadband nonlinearity.1,6−8 Up to the present, nonlinear saturable absorption has been observed in diverse materials arising from the fast electronic transitions involving conducting or valence electrons in both metallic and semiconducting materials.9−17

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ltrafast lasers offer extremely short temporal resolution, high peak intensity, and high pulse repetition rate, which are indispensable in the exploration of femtochemistry, high-capacity optical communication, high precision material processing, bioimaging, and laser microsurgery.1−5 The generation of ultrafast pulse in commercial laser systems is all based on passively Q-switching and modelocking techniques, the core of which is the nonlinear optical (NLO) modulator, based on, e.g., saturable absorbers (SAs), which demonstrate absorption saturation due to optical © 2016 American Chemical Society

Received: July 8, 2016 Accepted: September 13, 2016 Published: September 13, 2016 9463

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Figure 1. Characterizations of Cu2−xS nanocrystals. (a) Transmission electron micrographs (TEM) and electron diffraction patterns (inset) of the synthesized nanocrystals. (b,c) HRTEM images showing the top- and side-view of the nanocrystals. (d) Size distribution and (e) powder XRD of the nanocrystals (together with the major peaks of roxbyite phase, PDF#00-064-0278). Inset is the crystal structure of roxbyite, and yellow and blue balls represent S and Cu atoms, respectively. (f,g) XPS analysis of Cu (f) and S (g) in the as-synthesized nanocrystals, yielding a Cu/S ratio of ∼1.8 by analyzing the integrated areas of Cu and S in the XPS spectra.

crystals as SA for ultrafast pulse generation, which is characterized of superbroad and tunable spectral response, large modulation depth, and ultrafast recovery speed.

To strengthen light−matter interaction that plays a key role in optical nonlinearity, nanoscale materials are much favored due to the modulation of electronic structure and the relevant properties by the structural low dimensionality. For instance, semiconductor quantum wells have been used to construct saturable absorption mirrors (SESAMs) for mode-locking operation with an operation bandwidth of no more than 100 nm.8,18 Similarly, natural quantum wells, i.e., two-dimensional (2D) semiconductors such as transition-metal dichalcogenides (TMDCs)13,14 and black phosphorus (BP),17 recently have demonstrated saturable absorption and enabled pulse generation in the near-infrared (NIR). In 2D TMDCs, the presence of bandgap, which further expands with decreased thickness, strongly limits the examination of optical nonlinearity at longer wavelengths in the mid-infrared (mid-IR) region. Gapless materials, such as single-walled carbon nanotubes (SWNTs)19 and their 2D, Dirac counterpart graphene12 as well as other Dirac materials (topological insulators like Bi2Se3),15 also exhibit saturable absorption, and they indeed have been used for Q-switch or mode-locked pulse generation in the IR region. These materials also feature high carrier mobility which may facilitate fast carrier cooling following pulsed laser excitation, while the small density of state near the Dirac point (or Fermi level) presents as a bottleneck for both linear and nonlinear absorption especially in the mid-IR range.6 In addition, the inefficient light−matter interaction due to the intrinsic subnanometer thickness of 2D materials with a low modulation depth is an issue that needs to be carefully circumvented before fully releasing its potential. Therefore, an optical modulator for ultrafast lasers with broad operation bandwidth, ultrafast recovery dynamics, large modulation depth, and cost-effective fabrication is highly desirable and yet to be developed. Here, we report optical modulation by using the solution-processed plasmonic semiconductor nano-

RESULTS AND DISCUSSION Plasmonic materials are intensively exploited by the optics community for applications ranging from subwavelength waveguides,20 optical nanoantennas,21 and optical invisibility cloaks22 to light concentrators.23 In their nanoscale counterpart, the localized surface plasma resonance (LSPR) arising from the collective oscillation of conducting electrons features a strong and broad absorption band, whose bandwidth and location depends on the carrier density, dielectric environment, and geometric factors (size and shape).24,25 Compared with noble metal nanomaterials, heavily doped semiconductors usually demonstrate a plasmonic resonance peak in the IR due to lower carrier density, therefore offering new possibilities for modern photonic applications in the IR range that are not easily accessible by most metal-based plasmonics.26 As a typical platform for the exploiting of semiconductor-based plasmonic materials, we here choose off-stoichiometric semiconductor copper(I) sulfide (Cu2−xS) which is degenerately self-doped (ptype) with a composition-dependent LSPR covering a very broad range from NIR to mid-IR.25−29 The Cu2−xS nanocrystals were synthesized through the hot-injection method,30 yielding monodispersed platelet-shape nanocrystals with an average diameter of ∼10 nm (Figure 1a). The crystallinity was confirmed by high-resolution transmission electron micrograph (HRTEM) characterizations (Figure 1b,c) and further corroborated by X-ray diffraction pattern (XRD) (Figure 1d). The diffraction peaks are best indexed with a copper-deficient phase known as roxbyite, which can be regarded as a distorted hexagonal close-packing of the sulfur atoms filled with ordered Cu atoms and Cu vacancies (inset of Figure 1d). Elemental 9464

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Figure 2. Linear optical absorption of the synthesized nanocrystals. (a) Absorption spectrum of the synthesized nanocrystals spin-coated on high-purity quartz (absorption test at longer wavelengths is limited by our spectrometer). (b) Calculated band structure and density of states (DOS) of the synthesized roxbyite phase with the composition of Cu58S32, see Supporting Information Part 4 for details. The red lines denote the Fermi level. (c) Tunability of the plasmonic resonance peak by doping/alloying with different ions (Te, Se, In) and (d) corresponding plasmonic resonance band position as a function of free carrier density. Note that sharp fluctuations at ∼1700 nm in (c) arise from absorption of the solvent chloroform in which the nanocrystals were dispersed.

Figure 3. Nonlinear optical absorption of the plasmonic semiconductor nanocrystals. (a) Typical Z-scan curves of Cu2−xS and Cu2S nanocrystals recorded at 1300 nm, and (b) corresponding input power-dependent transmission, which can be well fitted by α α(I ) = 1 + Is/ I + αns , where αs = 63.28%, αns = 20.72%, and Is = 56.12 GW/cm2) are the saturable absorption, nonsaturable loss, and s

saturable intensity, respectively. A slightly high nonsaturable loss of ∼20.7% should be mainly from the scattering of inhomogeneously casted nanocrystals on the quartz slide and could be significantly reduced by further optimization of the homogeneity of the sample. (c) Modulation depth as a function of pump wavelength (recorded at 800, 1200, 1300, 1400, and 1550 nm) with power density of ∼28 mJ/cm2. Linear absorption, shown as a dotted line, is for comparison. It can be clearly seen that the modulation depth closely follows the plasmonic resonance intensity. (d) Transient absorption of the plasmonic nanocrystals at 1300 nm wavelength. Inset is the decay curve recorded up to 200 ps.

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Figure 4. Ultrafast pulse generation near 1.5 μm. (a) Typical mode-locking optical spectrum; (b) mode-locking pulse train; (c) autocorrelation trace; (d) the RF optical spectrum at the fundamental frequency.

analysis via SEM-EDS (Supporting Information Figure S1) together with X-ray photoelectron spectroscopy (XPS) (Figure 1f,g) yields a Cu:S ratio close to 1.8:1, in agreement with the stoichiometry of roxbyite.31 The colloidal copper(I) sulfide nanocrystals show a characteristic broadband absorption from 600 nm to over 3300 nm (corresponding to a bandwidth larger than 400 THz) with a peak at around 1300 nm (Figure 2a). This sub-bandgap absorption is ascribed to the LSPR of the nanocrystals containing a high concentration of delocalized free carriers arising from nonstoichiometry. The plasmonic origin of the absorption is further substantiated by its dependence on the refractive index of the dielectric solvent (Supporting Information Figure S2). By fitting with the Drude model,25,32 we find a vacancy density of ∼3.6 × 1021 cm−3 (see Supporting Information Part 3), which is in close agreement with the number of Cu deficiencies in the roxbyite phase. The presence of Cu deficiencies, as can be seen from density functional theory (DFT) calculation, moves the Fermi level into the valence band, where the strong s−p hybridization dominates the electronic states, as shown in Figure 2b. To confirm the tunability of plasmonic absorption, we performed a series of control experiments by judicious modulation of the free carrier concentrations through doping. As shown in Figure 2c,d, doping and alloying effect contribute to the adjusting of plasmonic resonance peak between ∼800 and ∼1500 nm, corresponding to a carrier density change from ∼9.7 × 1021 to 3.18 × 1021 cm−3. Furthermore, it is noteworthy that the spectral location and width can be further controlled by carefully regulating the doping concentration, which is not amenable to metal-based plasmonics.33 High optical nonlinearity is indispensable to enable efficient optical modulation.6 We then examined the NLO properties of our synthesized nanocrystals using well-developed openaperture Z-scan technique with femtosecond laser as the excitation source. Shown in Figure 3a is a typical Z-scan curve recorded at around the plasmon resonance peak (∼1300 nm),

which clearly characterizes a saturable absorption feature. This is in sharp contrast to the stoichiometric Cu2S (Supporting Information Figures S3 and S4), in which the absence of plasmonic absorption leads to complete disappearance of the saturable absorption (Figure 3a). By fitting the Z-scan data based on the nonlinear absorption model,13,34 the nonlinear saturable absorption coefficient β, the imaginary part of thirdorder , meanwhile with susceptibility Imχ(3), and the figure of merit (FOM, defined as |lmχ(3)/α|) were estimated to be ∼ − 161 cm/GW, −1.39 × 10−10 esu, and 3.79 × 10−15 esu cm, respectively (see details in Supporting Information Part 5). The input power-dependent transmission at this wavelength (Figure 2b) gives a modulation depth of 63.3% (corresponding to ∼7 dB), which is much higher than that of other low dimensional materials such as noble metal nanocrystals, graphene, and MoS2. The high modulation depth can be ascribed to the large optical nonlinearity of these plasmonic semiconductor nanocrystals compared with other SA materials,12,35 as shown in Supporting Information Table 1. To correlate the broadband NLO responsiveness with the broadband LSPR, we performed Z-scan measurements at longer and shorter wavelength sides of resonance peak (i.e., 800, 1200, 1400, 1550 nm). Saturable absorption can be found at all these wavelengths using the same measurement setup; further examination at mid-IR bands is only limited by the available mid-IR pumping sources. Interestingly, we find a very close match between the modulation depth obtained at different wavelengths and the plasmonic absorption spectrum (Figure 3c), which again confirms the obvious connection between optical nonlinearity and linear absorption and distinctly figures out a plasmon resonance-dependent optical modulation. Another critical factor for ultrafast optical modulation by SA is the high carrier cooling rate that ensures instant recovery.6 In the present work, the temporal dynamical characteristics of the NLO response were analyzed by a pump−probe experiment (see Supporting Information Part 1 and Figure S5 for details) at 9466

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Figure 5. Ultrafast pulse generation near 2.8 μm. (a) Typical pulse spectrum; (b) pulse trains at the output powers of 126, 167, and 214 mW, respectively; and (c) single pulse profile at output power of 214 mW, inset is the corresponding RF spectrum.

modulator. Pulsed lasers operating near this spectral region are of particular interest for applications in molecular spectroscopy, remote sensing, defense technology, and laser microsurgery, while the lack of a reliable optical modulator has restricted its development.39 In our experiment, an Er-doped ZBLAN fluoride fiber laser was built with a 976 nm laser diode as pump source and a plasmonic nanocrystal-sprayed reflective mirror as end mirror (Supporting Information Part 1 and Figure S9). Once the pump power exceeds a threshold of 2.7 W, a stable Q-switched pulse train centered at 2769 nm with an average output power of 126 mW and repetition rate of 66.4 kHz can be generated. Further increasing of input power results in higher average output power and repetition rate, as presented in Figure 5b. At the maximum pumping power of 5.11 W, the shortest pulse was generated with a pulse duration of ∼0.75 μs (Figure 5c), an average output power of 214 mW, a repetition rate of 90.7 kHz (Figure 5b), and a signal-to-noise of ∼30 dB (inset of Figure 5c), indicating stable Q-switching operation. These results demonstrate a successful performance of pulse generation near 2.8 μm spectral region, which we believe could be extended to even longer wavelengths covered by the plasmonic resonance region of our nanocrystals but only limited by our available pumping sources.

around the plasmonic resonance peak (1300 nm) of the Cu2−xS NCs. As shown in Figure 3d, the transient spectral response decay follows a biexponential behavior, with a faster decay component characterized by a short lifetime of ∼315 fs followed by a slower part of ∼34 ps. The decay rate is much faster than that of noble metal-based plasmonic materials (∼1 and 100 ps, respectively, for noble metals)36,37 and can also be interpreted based on a two-temperature model similar to that in noble metals.38 The fast decay part corresponds to electron− phonon interaction, while the second corresponds to phonon− phonon interaction, which is discussed in more details in Supporting Information Part 6. The plasmonic semiconductor nanocrystals with superbroadband and ultrafast NLO response were employed for the fabrication of optical modulators for short pulse generation over a broad spectral range. In a proof-of-concept demonstration, we first fabricate a fiber optical modulator (using the plasmonic nanocrystals as SA) for self-starting stable femtosecond mode-locked pulse generation centered at the telecommunication band of 1562.6 nm (see Supporting Information Part 1 and Figure S6 for details), where both linear and nonlinear absorptions reach maxima. As shown in Figure 4, the fiber laser delivered the pulse train with a repetition rate of 7.28 MHz, which matches with the cavity round trip time and indicates that the laser was mode-locked in fundamental mode operation. Importantly, femtosecond pulses with temporal width of 295 fs (assuming a Sech2 pulse shape) are generated, as can be seen from the autocorrelation trace in Figure 4c. The ratio-frequency (RF) spectrum in Figure 4d indicates that stable pulse generation with a high signal-to-noise ratio of over 50 dB (corresponding to ∼105 contrast) was achieved. In addition, for an observation period of over 2 h the laser pulses remained very stable (Supporting Information Figure S7), implying the superior robustness against photon damage of the chalcogenide plasmonic nanocrystals and the developed fiber modulator. We have also achieved optical modulation based on the plasmonic nanocrystals at shorter wavelength region where linear absorption and modulation depth are both much smaller, which, however, is especially suitable for mode locking of bulk lasers due to the generally lower gain coefficient of bulk lasers. Consequently, a stable continuous-wave mode-locked operation at 1030 nm with a pulse duration of 7.8 ps, a repetition rate of 84.17 MHz, and a high signal-to-noise of 60 dB was achieved (see Supporting Information Part 1 and Figure S8). To further test the capability of pulse generation at longer mid-IR spectra region, we performed pulsed laser experiment near 2.8 μm with the plasmonic nanocrystal-based optical

CONCLUSION In conclusion, we have demonstrated optical modulation for ultrafast pulse generation based on solution-processed plasmonic semiconductor nanocrystals, which are characterized by terahertz operation bandwidth (over 400 THz), ultrafast recover speed (∼315 fs at 1.3 μm), and large modulation depth (∼7 dB at 1.3 μm). High-performance optical modulators fabricated using these plasmonic nanocrystals as a SA can efficiently modulate the loss and Q-factor of laser cavities, thus allowing for the generation of ultrafast pulse in the entire NIR and part of mid-IR spectral region, as exemplified here by the ultrafast pulse generation at 1.0, 1.5, and 2.8 μm bands. Besides, the facile spectral tunability of the plasmonic resonance of the semiconductor nanocrystals can enable convenient access to IR-active materials at even a broader spectral range, meanwhile with high optical nonlinearity. We believe that the plasmonic semiconductor nanocrystals with adjustable carrier concentration could provide practically commendable material solutions to nonlinear optic and ultrafast photonic devices operating in the IR region. 9467

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METHODS

G.B. thanks the support of National Natural Science Foundation of China (grant no. 61275108). The authors thank Zhi Chen and Hang Zhang in South China University of Technology for help in Z-scan measurement. The authors also thank Prof. Frank Wise in Cornell University for useful discussions and valuable suggestions.

Sample Preparation and Characterizations. A common air-free Schlenk line technique was adopted to synthesize the nanocrystals according to the literature30 with some modifications. Please refer to the Supporting Information for details of the processes. Besides, characterizations of the synthesized nanocrystals, including TEM, XRD, XPS, EDS and linear optical absorption spectra, are also detailed in the Supporting Information. Nonlinear Optical Properties. The NLO properties of the plasmonic nanocrystals were studied with well-developed openaperture Z-scan technique with excitation of a femtosecond laser equipped with an optical parametric amplifier to tune the output wavelengths (see the Supporting Information for details). Besides, the temporal dynamics of the NLO response was measured by the pump− probe test. The transmittance of a weak probe as a function of delay between the probe and a strong pump of the same wavelength was recorded, please refer to the Supporting Information for details. Ultrafast Pulse Generation. Three pulsed lasers at 1.0, 1.5, and 2.8 μm bands, respectively, were constructed with the plasmonic nanocrystals as SA. The details of the laser setups and general operating conditions are thoroughly illustrated in the Supporting Information.

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b04536. Synthesis and characterization of the nanocrystals, measurement of linear and NLO properties, pulsed laser setups, supplementary figures, theory and simulation methods, and other supporting information (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: xfl[email protected]. *E-mail: [email protected]. Author Contributions

X.L. and J.Q. supervised the project. Q.G. and X.L. designed experiments. Q.G. fabricated the samples, performed characterizations of linear and NLO properties with the help of Y.Y. and S.Z. Z.L and M.L. performed pulsed laser experiment at 1.5 μm. Z.Q. and G.X. performed pulsed laser experiments at 1.0 and 2.8 μm. J.K. and G.B. performed the calculation of electronic structure. Q.G., X.L., and J.Q. analyzed the experimental data and wrote the manuscript. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (grants nos. 51132004, 61475047, 11504323), Science and Technology department of Zhejiang Province (grant no. 2015C31045), Natural Science Foundation of Zhejiang Province (grant no. Q14E020009), Guangdong Natural Science Foundation (grant no. S2011030001349), open fund of State Key Laboratory of Precision Spectroscopy (East China Normal University). This work was also supported by the Open Fund of the State Key Laboratory of High Field Laser Physics (Shanghai Institute of Optics and Fine Mechanics.). Z.L. thanks the support of Guangdong Natural Science Funds for Distinguished Young Scholar (2014A030306019). G.X. thanks the support of National Basic Research Program of China (2013CBA01505). 9468

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