Broadly Tunable Plasmons in Doped Oxide Nanoparticles for Ultrafast

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Broadly Tunable Plasmons in Doped Oxide Nanoparticles for Ultrafast and Broadband Mid-Infrared All-Optical Switching Qiangbing Guo, Zhipeng Qin, Zhuan Wang, Yu-Xiang Weng, Xiaofeng Liu, Guoqiang Xie, and Jianrong Qiu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b07866 • Publication Date (Web): 29 Nov 2018 Downloaded from http://pubs.acs.org on November 30, 2018

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Broadly Tunable Plasmons in Doped Oxide Nanoparticles for Ultrafast and Broadband MidInfrared All-Optical Switching Qiangbing Guo,†,# Zhipeng Qin,‡,# Zhuan Wang,△ Yu-Xiang Weng, △,§,* Xiaofeng Liu,◇ Guoqiang Xie,‡,* and Jianrong Qiu†,* †State

Key Laboratory of Modern Optical Instrumentation, College of Optical Science

and Engineering, Zhejiang University, Hangzhou 310027, China.

‡Key

Laboratory for Laser Plasmas (Ministry of Education), Collaborative Innovation

Center of IFSA (CICIFSA), School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China.

△Beijing

National Laboratory for Condensed Matter Physics, CAS Key Laboratory of

Soft Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China.

ACS Paragon Plus Environment

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§School

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of Physical Sciences, University of Chinese Academy of Sciences, Beijing

100049, China.

◇ School

of Materials Science and Engineering, Zhejiang University, Hangzhou 310027,

China.

*Correspondence

should

be

addressed

to

J.Q.

([email protected]),

G.X.

([email protected]) or Y.W. ([email protected]).

ABSTRACT: Plasmons in conducting nanostructures offer the means to efficiently manipulate light at the nanoscale with subpicosecond speed in an all-optical operation fashion, thus allowing for constructing high performance all-optical signal-processing devices. Here, by exploiting the ultrafast nonlinear optical properties of broadly tunable mid-infrared (MIR) plasmons in solution-processed, degenerately doped oxide nanoparticles, we demonstrate ultrafast all-optical switching in the MIR region, which features with subpicosecond response speed (with recovery time constant of ‹400 fs) as well as


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ultrabroadband response spectral range (covering 3.0-5.0 μm). Furthermore, with the degenerately doped nanoparticles as Q-switch, pulsed fiber lasers covering 2.0-3.5 μm were constructed, of which a watt-level fiber laser at 3.0 μm band show superior overall performance among ever-reported passively Q-switched fiber lasers at the same band. Notably, the degenerately doped nanoparticles show great potential to work in the spectral range over 3.0 μm that is beyond the accessibility of commercially available but expensive semiconducting saturable absorber mirror (SESAM). Our work demonstrates a versatile while cost-effective material solution to ultrafast photonics in the technologically important MIR region.

KEYWORDS

mid-infrared, tunable plasmons, doped oxide nanoparticles, ultrafast photonics, alloptical switching


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Mid-infrared (MIR) region, especially the so-called atmospheric transmission window of 3.0-5.0 μm spectral range, has great technological relevance for optical communication, both for civilian and military.1 Although significant progress has been made in MIR light sources, waveguides and detectors in recent years,2-4 the development of MIR optical communication technologies still remains stagnant due to the underbalanced development of optical modulating materials and devices in the MIR region which are largely lagged behind compared with their counterparts in the nearinfrared (NIR) region.1,5-9 On the other hand, the past decades have witnessed a boom of plasmonic materialsbased nanophotonics, especially its great potential for signal processing.10-14 Plasmons, the collective oscillations of free carriers in conducting materials, offer the ability to manipulate the optical field and energy flow of light at the nanoscale with subpicosecond timescale response speed, thus being recognized as an excellent platform for realizing ultrafast and ultracompact signal-processing devices for modern information technologies as well as quantum information computing schemes.15-20 However, the operation spectral range of current widely available plasmonic noble


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metals is mainly limited to the visible and NIR range by its non-tunable high carrier density (on the order of 1022 cm-3), and it also suffers a lot from high ohmic losses.20-24 Besides, graphene plasmons have recently emerged as a promising alternative for MIR to terahertz applications, with relatively low losses and high spatial confinement as well as wide optical tunability by gating/doping and geometry pattern control,23,25-28 however, fabrication of high-quality and precise patterned graphene nano/microstructures is still a challenge that stands in the way for widely practical applications.27,29 Degenerately doped semiconductors, such as transparent conducting oxides, silicon and III–V semiconductors, with relatively lower carrier density (1019-20 cm-3) compared with metals, have been considered as another alternative infrared plasmonic media with even faster responsivity and higher flexibility.19,21,22,30-32 As the plasma frequency 𝜔𝑝 of these Drude-like materials follows 𝜔𝑝 =

𝑁𝑒2 𝜀0𝑚 ∗

(where N is the electron density, e the

electron charge, 𝜀0 the free space permittivity and 𝑚 ∗ the electron effective mass), it can be easily and broadly tuned in the infrared region (no access by metals) by chemical doping, electronic gating and optical pumping.30-32 Besides, the CMOS-


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compatibility as well as rich material choices and easy availability also make this plasmonic media closer to practical applications.33,34 Here, we explore ultrafast all-optical switching application in the MIR region based on the MIR plasmons in degenerately doped oxide nanoparticles (NPs), by exploiting the plasmon-related ultrafast nonlinear optical response. Specifically, taking doped zinc oxide NPs as an example, which have been demonstrated to be a low-loss infrared plasmonic material,22,35,36 we systematically studied the ultrafast nonlinear optical properties by infrared degenerate pump-probe technique and demonstrated broadband all-optical switching in the 3.0-5.0 μm region with subpicosecond response speed (fully recovered within ~1.0 ps with time constant of ‹400 fs). As a proof-of-concept demonstration, we further constructed MIR pulsed lasers by employing degenerately doped zinc oxide NPs as Q-switch (so-called saturable absorber, SA) to modulate continuous wave into laser pulses. Importantly, a watt-level Q-switched fiber laser at 3.0 μm was constructed, of which the overall performance is among the best levels of ever reported passively Q-switched fiber lasers at 3.0 μm band. What’s more impressive, the working range of this SA can be extended from 2.0 μm to 3.5 μm, with the potential to


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be extended to even longer wavelengths but only limited by the available laser sources in our lab. Definitely, this is far beyond what one can expect from commercially available but expensive semiconductor saturable absorber mirror (SESAM), because its working range is below 3.0 μm with a working bandwidth of ~100 nm, meaning different structural parameters thus complicated fabrication processes should be used for SESAMs working at different bands.37 Besides, working in the longer MIR region of most low-dimensional semiconducting materials-based SAs are also limited by their large bandgaps.38-41

RESULTS AND DISCUSSION Firstly, we synthesized degenerately doped zinc oxide NPs by colloidal chemistry method (details can be referred to Method in the Supporting Information), which has been widely explored to develop cost-effective optoelectronic devices and provides much convenience for doping control. As shown in Figure 1a, typical indium-doped NPs with size of ~16 nm (Figure 1d) were obtained. X-ray diffraction (Figure 1e) indicates


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that the as-synthesized NPs correspond to a wurtzite phase, which can be further collaboratively confirmed by high-resolved transmission electron microscope (HRTEM) image (Figure 1b) and electron diffraction pattern (Figure 1c).

Figure 1. Characterizations of degenerately doped zinc oxide NPs. Typical TEM (a), HRTEM (b) images, corresponding electron diffraction pattern (c) and size distribution (d) of synthesized indium-doped zinc oxide (IZO) NPs. Corresponding characterizations of synthesized undoped zinc oxide (ZnO) NPs can be found in Supporting Information Figure S1. (e) X-ray diffraction pattern of doped and undoped zinc oxide NPs. PDF card (#36-1451) corresponding to the standard wurtzite phase zinc oxide is also shown as a reference.


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The localized plasmon absorption of the synthesized degenerately doped zinc oxide NPs is shown in Figure 2a, which covers a broad range from 1.0 μm to over 5.0 μm with an absorption peak at ~2.9 μm, while no apparent infrared absorption can be observed in undoped sample. It should be noted that it is rather difficult for noble metal nanostructures to demonstrate plasmon absorption in this region. The indium doping slightly increases the optical bandgap from 3.35 eV to 3.47 eV as a result of BursteinMoss effect (inset of Figure 2a), which is consistent with previous report.42 Importantly, the plasmon absorption peak can be easily tuned from 2.9 μm to 4.5 μm simply by controlled doping (dopant species and concentration) (Figure 2b), with the potential to be extended to even longer wavelengths.43,44 The easy accessibility to MIR response and wide tunability enable degenerately doped zinc oxide a promising platform to construct broadband responsive photonic and optoelectronic devices in the MIR region.


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Figure 2. Broadband and tunable absorption of doped zinc oxide NPs. (a) Linear absorption spectroscopies of indium- (IZO, 5% In-doped) and undoped zinc oxide NPs. The wide range spectra were obtained by combining a spectrum from 300 nm to 2500 nm recorded by UV/vis/IR spectrophotometer with a spectrum from 2500 nm to 5000 nm recorded by FTIR. The symbol (*) indicates the junction point. The sharp peaks at ~3435 nm and ~3513 nm results from the stretching vibrations of C-C and C-H bonds of surface ligands. Inset shows plot of (αhν)2 vs photon energy (eV) of corresponding samples, which indicates that the optical bandgap increases from 3.35 eV to 3.47 eV due to indium doping. (b) Linear absorption spectra of samples with controlled doping


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(In or Al doping with different levels). The corresponding characterizations of synthesized Al-doped samples can be found in Supporting Information Figure S2.

In order to explore the ultrafast all-optical switching behavior in the MIR, infrared pump-probe technique was adopted to study the ultrafast nonlinear optical properties. Details of the optical setup can be found in the Supporting Information Figure S3. As seen from Figure 3a, upon degenerately pumping the doped zinc oxide NPs at 3.0 μm around the plasmon resonant peak (intraband pump), a transient bleach occurs (no signal was observed in undoped sample without plasmon absorption, see Supporting Information Figure S4), which results from the transient redshift of plasmon resonance absorption induced by intraband pumping that is similar in nature to noble metals and other degenerately doped semiconductors.19,31,45,46 Specifically, under intraband pumping, the high electron temperature significantly changes the electron effective mass and thus dielectric function due to the conduction band nonparabolicity, giving rise to a transient redshift of the plasmon absorption.31,32,35 As shown in Figure 3b, the change amplitude of transmittance increases with pump power, resulting in a


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modulation depth of 4.12% at 3.05 GW cm-2, which is comparable to that of highperformance ITO nanorods at similar pumping intensity.31 The properties at higher pumping intensity is limited by the available 3.0 μm pulse energy with our system. Notably, the transient bleach can be almost fully recovered within ~1.0 ps, with the recovery time constant as short as ~370 fs (Figure 3b), which is of about one order faster than that of noble metals because of the relatively lower electron density and thus smaller electron heat capacity in degenerately doped semiconductors.45-49 From the broadband transient transmission spectra at different delay time (Figure 3c, intraband pump), it is clear that a bleach occurs on the blue side of the plasmon feature, while a pump induced absorption occurs on the red side. On the bleach side, when pumped at 3.2 μm (also near the plasmon resonant region) with a fixed intensity, the modulation depth at different wavelengths is closely related with linear absorption intensity, as presented in Figure 3d, which has also been observed in copper chalcogenides and may refer to plasmon resonance-enhanced nonlinear optical processes.46,47,49 To further demonstrate the pump induced absorption on the red side, we degenerately pumped the sample at 5.0 μm. As shown in Figure 3e, opposite


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modulation signal is observed, of which the modulation depth increases with pump intensity and reaches ~18% at 17.5 GW cm-2 (Figure 3f). And the recovery time constant is ~130 fs that is even slightly faster than the bleach recovery. Therefore, the superior all-optical modulation performance and subpicosecond response, together with the broadband responsivity and wide tunability, endow degenerately doped zinc oxide NPs with great potential for ultrafast all-optical switching.

Figure 3. Ultrafast all-optical switching in the MIR region. Pump power-dependent transient dynamics (a,e) and modulation depth and decay time constant (b,f) of doped zinc oxide NPs (IZO) by degenerate pump-probe at 3.0 μm (a,b) and 5.0 μm (e,f). (c) Transient transmission spectra at different delay time with fixed pump power of 1.4 GW


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cm-2 at 3.0 μm. (d) Transient dynamics probed at different wavelengths with fixed pump power of 12 GW cm-2 at 3.2 μm.

Inspired by the transient optical bleach performance in degenerately doped zinc oxide NPs, we reason that it could enable a superior optical switch for laser pulse generation in the MIR region, which, combined with its wide tunability, could even work broadly in the MIR. Considering that the commercially available SESAM can’t work in the MIR region for wavelengths beyond 3.0 μm,48 a suitable optical switch for MIR pulse generation is in urgent demand, to which a promising solution could be expected here, based on the degenerately doped zinc oxide NPs. Firstly, as a proof-of-concept demonstration, we constructed a 6.0 mol.% Er-doped fluoride fiber laser that works at 3.0 μm band (Figure 4a, and see Method for more details). As shown in Figure 4b, with a thin film of degenerately doped zinc oxide NPs spin-coated on the gold mirror as the optical switch (denoted as DZO-SAM below, see Figure 4a) in the cavity, a continuouswave laser with central wavelength at 2789 nm (Figure 4c) can be switched into a stable train of pulses (with signal-to-noise of ~37 dB, inset of Figure 4c) under proper pumping


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conditions, while no pulse was generated with only blank gold mirror or un-doped zinc oxide NPs coated gold mirror under the same conditions. Besides, the pulse duration, repetition rate, average output power and pulse energy can be tuned by changing the pump power (see Supporting Information Figure S5), which is a typical signature of Qswitched lasers. Notably, under a launched pump power of 4.6 W, a watt-level passively Q-switched fiber laser is obtained with an average output power of ~1.01 W (corresponding to peak power of 11.25 W, see Supporting Information Figure S5b) and pulse duration of 560 ns (Figure 4d), which are among the best levels in ever reported passively Q-switched fiber lasers at the 3.0 μm band (see comparison in Figure 5 and overall comparison in Supporting Information Table S1) and comparable to that of commercially available but expensive SESAM, indicating our solution-processed, degenerately doped zinc oxide NPs can enable high-performance while cost-effective short-pulse generation in the MIR region.


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Figure 4. Q-switching at 3.0 μm band. (a) Schematic illustration of laser setup. LD, laser diode; L1, collimating lens; L2, focusing lens; DM, dichroic mirror; CMS, cladding mode stripper; DZO-SAM, doped zinc oxide saturable absorber mirror. Here a 6.0 mol.% Erdoped ZBLAN fiber was used. Inset shows the simplified energy levels involved in the pumping and lasing action. Degenerately doped zinc oxide NPs were spin-coated on the gold mirror (DZO-SAM), serving as the optical switch for Q-switching operation. (b) Typical Q-switched pulse train. (c) Optical spectrum. Inset is the radio frequency spectrum, indicating a signal-to-noise ratio of 37 dB. (d) Single pulse profile.


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Figure 5. Comparison of ever reported passively Q-switched fiber lasers at 3.0 μm band. It can be clearly seen that average output power and pulse duration of the degenerately doped zinc oxide NPs-based fiber laser are superior compared with other nanomaterials-based ones, and comparable to the best level of commercial SESAMbased. Besides, a more detailed comparison can be found in the Supporting Information Table S1, which further demonstrates an overall superior performance of the fiber laser constructed here.

Considering the broadband response of transient optical bleach, it’s reasonable to further extend the working range to over 3.0 μm that is beyond the accessibility of SESAM. As shown in Figure 6a, another 1.0 mol.% Er-doped fluoride fiber laser with


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dual-wavelength pumping scheme was constructed (see Method for more details). Two pump lasers, i.e. 970 and 1973 nm, were simultaneously used to induce a population inversion between energy levels 4F9/2 and 4I9/2 in Er ions (inset of Figure 6a), which results in lasing action with the central wavelength at 3462 nm. By inserting the same DZO-SAM used above for 3.0 μm band into the 3.5 μm laser cavity (Figure 6a), Qswitching operation was also successfully achieved. As presented in Figure 6b, a typical Q-switched pulse train (signal-to-noise ratio of ~30 dB, see inset of Figure 6c) with pulse duration of ~1.78 μs (Figure 6d) was obtained, indicating that the working range can be successfully extended to 3.5 μm band. It should be noted that higher output power is limited by the intrinsic low gain of Er:ZBLAN fiber at 3.5 μm band, which could be alleviated by using other media with relatively larger optical gain. As for other laser gain media beyond 3.0 μm, Dy:BLAN at 3.1 μm and Ho:InF3 at 3.9 μm are candidates that could potentially be used, combined with the DZO-SAM, to demonstrate Q-switching operations.50


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Figure 6. Q-switching at 3.5 μm band. (a) Schematic illustration of laser setup. DM, dichroic mirror; TM1 and TM2, trichroic mirrors, L1-L3, ZnSe aspheric lenses; OC, output coupler; DZO-SAM, the same one as used in Q-switching at 3.0 μm band. Here a 1.0 mol.% Er-doped ZBLAN fiber was used. Inset shows the simplified energy levels involved in the pumping and lasing action. (b) Typical Q-switched pulse train. (c) Optical spectrum. Inset is the radio frequency spectrum, indicating a signal-to-noise ratio of ~30 dB. (d) Single pulse profile.

Furthermore, by employing just the same DZO-SAM as optical switch, we also succeed in constructing a Q-switched laser at 2.0 μm, as demonstrated in the


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Supporting Information Figure S6 and S7. Impressively, we have simultaneously achieved Q-switched fiber lasers ranging from 2.0 μm to 3.5 μm spectral region with only one optical switch that based on solution-processed, degenerately doped zinc oxide NPs. To the best of our knowledge, this is the broadest SA in the MIR that have ever been simultaneously achieved in one device and also among the earliest two reported SAs worked beyond 3.0 μm band at present, with the other one being black phosphorus-based.41 Given that the working range of black phosphorus is limited to ~4130 nm by its bulk bandgap (~0.3 eV), one could expect much more with the degenerately doped zinc oxide NPs, for its widely tunable plasmon absorption in the MIR region as well as higher chemical stability and solution processability.

CONCLUSION In summary, by exploiting the broadband and ultrafast nonlinear optical response of MIR plasmons in degenerately doped zinc oxide NPs, we successfully achieved subpicosecond all-optical switching in the MIR region covering 3.0-5.0 μm. Furthermore, by employing the solution-processed, degenerately doped NPs as Q-switch, we


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successfully constructed Q-switched fiber lasers, of which a watt-level fiber laser at 3.0 μm band was demonstrated with superior overall performance among other ever reported passively Q-switched fiber lasers at this band. Besides, the working spectral range can cover 2.0-3.5 μm, with potential to be extended to even longer wavelengths due to its broadband and widely tunable optical response in the MIR region. Considering that commercially available but highly expensive SESAM and most of recently developed low-dimensional materials-based saturable absorbers can hardly work beyond 3.0 μm,48 our work demonstrates a versatile while cost-effective solution to MIR pulse generation, especially for the spectral region beyond 3.0 μm that is currently still almost a blank area. With the rapid rising up of longer wavelength MIR laser sources,50 we believe that the degenerately doped oxide NPs could fully fulfil its great potential in constructing MIR pulsed lasers in the near future. As for its broadband and ultrafast all-optical switching behaviors in the MIR region, other applications such as free-space and on-chip signal processing also leave much space to be expected.1,5,7,13


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METHODs Synthesis of Doped Oxide Nanoparticles. In a typical synthesis of In-doped zinc oxide NPs, 1.89 g Zinc stearate (ZnSt2) was mixed with 5.76 mL 1-dodecanol (DDOL, 99%), 3 mL Oleic acid (OA, 90%), 21 mL 1-octadecene (ODE, 90%), and 43.75 mg indium acetate (In(ac)3) (corresponding to 5% In-doping, while no addition of In(ac)3 for undoped sample) in a three-neck round-bottom flask. Then the flask was kept under nitrogen flow during the whole reaction. The temperature was increased to ~120 °C and kept for 30 min to make sure all the reagents were homogeneously dissolved, followed by increasing the temperature to 250 °C in 60 min under rapid stirring. After kept at 250 °C for 3 hours, the heating mantle was removed and cooled to room temperature naturally, followed by washing with acetone for three times. Finally, the synthesized NPs were stored in toluene for further use. As for Al-doped zinc oxide NPs, hot-injection method was adopted, which, according to our experiments, have a higher doping efficiency than above non-injection method for Al. Therefore, we adopted hot-injection method here for Al-doped zinc oxide NPs. In a


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typical synthesis, one three-necked round-bottom flask (A) containing a mixture of 1 mmol ZnSt2, 0.03 (3 mol.% Al)/0.06(6 mol.% Al) mmol aluminum acetylacetonate (Al(AcAc)3), 3 mmol OA, and 4 mL ODE, and the other flask (B) containing a mixture of 10 mmol 1,2-hexadecanediol (HDDIOL, 98%) and 11 mL ODE both were kept under nitrogen flow during the whole reaction. The flask A was heated to 140 °C under stirring in about 20 min, and then kept at this temperature for ~1 hour with continuous stirring to ensure all the chemicals are dissolved. Then the flask B was increased to ~250 °C, followed by rapid injection of solution in flask A into B. After being kept for 5 hours, the reaction mixture was cooled down to room temperature, followed by washing three times with ethanol. Finally, the synthesized NPs were stored in toluene for further use.

Characterizations. The samples for TEM were prepared by dropping dilute solutions of nanocrystals onto carbon-coated 200 mesh copper grids, and a JEOL JEM-2100F microscope equipped with a field-emission gun working at 200 kV was used for the characterizations. Powder XRD (RIGAKU D/MAX 2550/PC with Cu Kα X-rays) was used to confirm the crystal phases of the synthesized nanocrystals. Linear optical


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absorption spectra ranging from 300 to 2500 nm were recorded using a Hitachi 4100 UV-vis-NIR scanning spectrophotometer. The absorption in the MIR ranging from 2500 to 6000 nm were measured by a FTIR spectrometer (Tensor 27, Bruker, Germany).

MIR Ultrafast All-Optical Switching. The infrared pump-probe measurements were performed with a 35 fs amplified Ti:sapphire laser (Spitfire Ace, Spectra Physics) operating at 800 nm with a repetition rate of 1 kHz, of which the output beam is split into two beams, one for generating pump light, the other for probe light (optical setup see Supporting Information Figure S3). MIR pump pulses were generated by feeding one beam into an optical parametric amplifier (OPA, TOPAS, Spectra Physics) and the repetition rate were reduced to 500 Hz by a chopper (MC2000B-EC). As for the MIR probe pulses, a portion of the other beam passed through a BBO crystal is transformed to 400 nm (second harmonic) and then ultrabroadband super-continuum covering almost the whole MIR region was produced by simultaneously focusing the 400 and 800 nm light on air. The resulting MIR super-continuum is used as the probe light. The pump-probe time delay was controlled by a viable path delay stage and retroreflector


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that was set on the pump line. Both beams were focused onto the sample. The sample was prepared by spin-coating the NPs on a 2 mm thick CaF2 plate. The spectrum for the MIR probe light was detected by a liquid-nitrogen-cooled mercury-cadmium-telluride array detector after frequency resolved by a spectrograph (iHR 320, HORIBA Jobin Yvon).

Q-switching at 3.0 μm Band. The Er:ZBLAN fiber laser was pumped by a commercial 970 nm laser diode (BWT Beijing, Ltd.), which delivered a maximum power of 30 W via a pigtail fiber with a core diameter of 105 μm and a numerical aperture (NA) of 0.22. The pump light was collimated and focused by two plano-convex lenses L1 (f1=50 mm) and L2 (f2=75 mm), respectively. A 45°-placed dichroic mirror (DM, T=90% @ 970 nm and R=99.2% @ 2.8 μm) was employed to split the pump and output laser. The used Er:ZBLAN gain fiber (FiberLabs, Inc.) has an Er-doping concentration of 6 mol.%, a core diameter of 16.5 μm, and a NA of 0.12. Lasing at 2.8 μm occurs based on the 4I9/2→4I13/2 transition of Er ions. With a cladding-pump scheme, the pump light could be easily coupled into inner cladding due to its large diameter of 250 μm and NA of 0.5. Through


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a cutback method, the cladding absorption at 970 nm and the pump coupling efficiency were measured to be 3.04 dB/m and 89%, respectively. The used fiber with a length of 3 m resulted in a pump absorption efficiency of 88%. Laser cavity was formed between the perpendicularly cleaved fiber end facet (with Fresnel reflectivity of 4%) and DZOSAM (with reflectivity of ~80% at 2.8 μm), which was fabricated by twice spin-coating (five minutes kept in ambient between each for evaporation of residual solvent) of assynthesized degenerately doped zinc oxide NP dispersion (in toluene, ~40 mg/mL) onto gold mirror at 1000 rpm for 45 seconds. The fiber end close to the DZO-SAM was 8°cleaved to avoid the parasitic oscillation. In order to alleviate the effect of pump light on the DZO-SAM, a cladding mode stripper (CMS) was used to eliminate the residual pump light. Under proper pumping conditions, stable Q-switched pulse trains at 2.8 μm could be generated from the Er:ZBLAN fiber laser. The Q-switched pulses were captured by a MIR photoelectronic detector (VigoSystem, PCI-9) and displayed on a digital oscilloscope (Tektronis, DPO03054). The radio frequency spectrum and optical spectrum were measured by a radio frequency spectrum analyzer (Agilent, E4402B)


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and a MIR optical spectrum analyzer (Ocean Optics, SIR5000) with a resolution of 0.22 nm, respectively.

Q-switching at 3.5 μm Band. The 3.5 μm Er:ZBLAN fiber laser was simultaneously pumped by a 970 nm laser diode and a 1973 nm fiber laser. The 970 nm pump excites the electrons to metastable level of 4I11/2 of Er ions, which acts as a virtual ground state. Under further pumping of 1973 nm, the electrons are excited from 4I11/2 to 4F9/2 level and lasing at 3.5 μm occurs based on the 4F9/2 → 4I9/2 transition. In the dual-wavelength pumping scheme, 970 nm pump was coupled into the inner cladding and 1973 nm pump was required to couple into fiber core due to a small absorption coefficient of Er:ZBLAN fiber at 1973 nm. The same 970 nm LD laser as in 3.0 μm band laser was used here. The used 1973 nm pump source was a home-made Tm-doped fiber laser with a maximum output power of 10 W, delivered from a double-clad silica fiber with a core diameter of 10 μm and a NA of 0.15. Two pump beams were combined with a DM (T=87% @ 970 nm and R=96% @ 1973 nm) and then focused into a Er:ZBLAN gain fiber (FiberLabs Inc.) by a ZnSe aspheric lens (L1, f1=12.7 mm). The double-clad


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Er:ZBLAN single-mode fiber with a core diameter of 16.9 μm and a NA of 0.15 has a length of 2.6 m and an Er-doping concentration of 1.0 mol.%. A perpendicularly-cleaved fiber end facet was butted against the output coupler (OC), which had a transmission of 30% at 3.5 μm. The other end of the gain fiber was 8o-cleaved for avoiding parasitic oscillation. After collimating lens L2 (f2=12.7 mm), a 45o-placed trichroic mirror (TM2, T=87% @ 970 nm, T=97% @1973, and R=99.2% @ 3500 nm) was employed to filter residual 970 nm and 1973 nm pump light for avoiding DZO-SAM damage. A ZnSe aspheric lens L3 (f3=12.7 mm) was used to focus the laser beam onto the DZO-SAM (with reflectivity of ~70% at 3.5 μm). With the same DZO-SAM as used in the 3.0 μm band laser, Q-switched pulses at 3.5 μm were generated from the dual-wavelength pumped Er:ZBLAN fiber laser. The same detection and analysis systems were used as in the 3.0 μm band laser.

ASSOCIATED CONTENT


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Supporting Information. Other supplementary materials including Figures, Table and details of Q-switching experiment at 2.0 μm. This material is available free of charge via the Internet at http://pubs.acs.org.

The authors declare no competing financial interest.

AUTHOR INFORMATION

Corresponding Authors

*Email: [email protected] (Y.W.)

*Email: [email protected] (G. X.)

*Email: [email protected] (J.Q.)

Author Contributions

#Q.

G. and Z. Q. contribute equally to this work. Q. G. designed and supervised the

project with support of J. Q.. Q. G. performed the synthesis and structural


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characterizations of samples and the absorption spectra. The ultrafast spectra were measured with assistance from Z. W. and Y. W.. Pulsed laser experiments were performed by Z. Q. and G. X.. Q. G. wrote the manuscript with suggestions from all the other authors.

ACKNOWLEDGMENTS

This work is financially supported by the Postdoctoral Science Foundation of China (NO. 2017M620242), Postdoctoral Special Science Foundation of China (NO. 2018T110588), National Key R&D Program of China (NO. 2018YFB1107200), National Natural Science Foundation of China (NO. 51472091, 51772270), open funds of State Key Laboratory of Precision Spectroscopy (East China Normal University) and State Key Laboratory of High Field Laser Physics (Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences). G. Q. Xie acknowledges the support of National Natural Science Foundation of China (11721091). Z. P. Qin acknowledges the support of National Postdoctoral Program for Innovative Talents (BX20170149). Y. X.


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Weng acknowledges support of National Natural Science Foundation of China (21633015) and MOST (2018YFA0208700).

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TOC Graphic:


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Broadly Tunable Plasmons in Doped Oxide Nanoparticles for Ultrafast

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