Largely Enhanced Optical Nonlinear Response of Heavily Doped Ag

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Largely Enhanced Optical Nonlinear Response of Heavily Doped Ag:CdTe Nanocrystals around the Excitonic Band Edge Si-Jing Ding, Fan Nan, Xiaona Liu, Xiao-Li Liu, Ya-Fang Zhang, Shan Liang, Duanzheng Yao, Xin-Hui Zhang, and Qu-Quan Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b06408 • Publication Date (Web): 12 Oct 2015 Downloaded from http://pubs.acs.org on October 13, 2015

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The Journal of Physical Chemistry

Largely Enhanced Optical Nonlinear Response of Heavily Doped Ag:CdTe Nanocrystals around the Excitonic Band Edge Si-Jing Ding,† Fan Nan,† Xiao-Na Liu,‡ Xiao-Li Liu,† Ya-Fang Zhang,† Shan Liang,†,§ DuanZheng Yao,† Xin-Hui Zhang,*,‡ and Qu-Quan Wang*,†,|| †

Key Laboratory of Artificial Micro- and Nano-structures of the Ministry of Education, and

School of Physics and Technology, Wuhan University, Wuhan 430072, P. R. China. ‡

State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese

Academy of Sciences, Beijing 100083, P. R. China. §

Department of Physics, Hunan Normal University, Changsha 410081, P. R. China.

||

The Institute for Advanced Study, Wuhan University, Wuhan 430072, P. R. China.

ACS Paragon Plus Environment

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ABSTRACT: Semiconductor quantum dots (SQDs) have good nonlinear figures of merit (FOMs) but relatively poor nonlinear refraction. The linear and nonlinear optical processes of SQDs in the proximity of metallic nanostructures can be enhanced by the surface plasmon resonance, but the nonlinear FOMs are limited by the enhanced linear absorption. Here, we investigate optical third-order nonlinearity and the corresponding FOMs of the CdTe quantum dots heavily doped with Ag. The excitonic resonant absorption is enhanced and the 1S peak is red-shifted and broadened after doping silver. Intriguingly, the nonlinear refraction near the band edge is enhanced more than 35 times, while the nonlinear absorption keeps very small at the crossover of the one-photon saturation absorption and two-photon excitation near the band edge, leading to the desired one- and two-photon FOMs (W and T) for the demands of all-optical waveguide switching. Our observations offer a strategy to prepare doped semiconductor quantum dots with large third-order susceptibility and good nonlinear FOMs, thus show prospective applications in optical information processing, switching, and modulating.


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INTRODUCTION Semiconductor quantum dots (SQDs), characterized with size-tunable exciton resonances and large nonlinear susceptibilities with ultrafast response time, have become a promising class of nonlinear materials and provided building blocks of photonic nanodevices.1-7 The nonlinear refraction (NLR) and the nonlinear absorption (NLA) are corresponding to the real and imaginary parts of the third-order susceptibility χ(3), respectively. Most SQDs have very large NLA coefficient but relatively poor NLR index,8-17 which could be applied in multi-photon bioimaging and bio-labeling. Preferentially enhancing NLR and suppressing NLA of the SQDs is highly desired for practical applications in ultrafast all-optical switching, modulating, and information processing, yet it remains a big challenge. The third-order susceptibility χ(3) of the colloidal SQDs were found to be several orders of magnitude larger than those of the bulk materials owing to the quantum confinement effect in low-dimensional nanostructures.18-20 Extensive studies have been devoted to further increase nonlinear responses of the SQDs for the practical applications in nanodevices, the following three approaches have been reported in recent years. (i) Exciton resonance enhancement in SQDs. The enhanced NLR near the excitonic band edge and the enhancement of revised saturation absorption (RSA) caused by the photoinduced excited state absorption is observed in PbS SQDs.21 Resonant two-photon absorption in CdS and CdTe nanocrystals is investigated,22,23 large resonant third-order optical nonlinearity of CdSe SQDs is also revealed.24 (ii) The interaction between excitons and doped metal ions in SQDs. Mn, Cu, and Ag are the most commonly used metal ion dopants in II-VI semiconductor hosts to improve fluorescence


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quantum efficiency as well as nonlinear responses.25-44 Three- and four-photon absorption is observed in Cu- and Mn-doped ZnS SQDs,45,46 different NLA behaviors in the Cu- and Mndoped ZnS SQDs are observed,47,48 and the size-dependent off-resonant NLA and NLR are observed in Mn-doped ZnSe multi-core/shell SQDs.49 Ultrafast third order nonlinear enhancement of donor and acceptor codoped silicon QDs has also been reported.50 (iii) Plasmon-exciton resonance interaction. The local electromagnetic (EM) field around the metallic nanostructures is strongly enhanced by a factor of f(ω) owing to the surface plasmon resonance, which induces a much larger enhancement of the third-order susceptibility by a factor of f 2(ω)|f(ω)|2. Extensive studies on the nonlinear enhancements of a diverse of SQDs by various plasmonic metal nanostructures have been reported.51-59 However, the nonlinear figures of merit (FOMs) of the nanosystems consisting of metal and SQDs is usually limited by the strong resonance absorption of surface plasmon.56 Efficiently combining the advantages of three enhancement mechanisms mentioned above and exploring a new approach have great significance for the design and preparation of SQDs with desired nonlinear responses. In this letter, we synthesize Ag-doped CdTe (Ag:CdTe) SQDs and investigate their linear and nonlinear optical responses around the excitonic band edge. As the increase of the silver dopant concentration, the excitonic resonant absorption of the SQDs is increased, the 1S peak is red-shifted and broadened, the resonant NLR is enhanced more than 35 times while the NLA keeps very small near the wavelength of 590 nm owing to the changeover from one-photon SA to two-photon RSA. Consequently, the excellent one- and two-photon nonlinear FOMs of these SQDs are also obtained near the band edge. The physical mechanism of the largely enhanced NLR with the desired nonlinear FOMs in doped SQDs is also discussed.


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EXPERIMENTAL SECTION Synthesis of bare CdTe, Ag:CdTe, and CdTe/AgTe core/shell SQDs. Three samples of SQDs are prepared by using hydrothermal methods reported by Rogach and Ding.60-63 Silver nitrate (AgNO3, 99.8%), sodium hydroxide (NaOH, 96.0%), sodium borohydride (NaBH4, 96.0%), Cadmium nitrate (Cd(NO3)2·4H2O, 99.0%), and tellurium power (Te, 99.99%) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). 3-mercaptopropionic acid (MPA) was obtained from Sigma-Aldrich (America). Ultrapure water with a resistivity of about 18.25 MΩ·cm was used as the solvent in all experiments. The bare CdTe SQDs and Ag:CdTe SQDs were synthesized following the established methods. In a typical synthesis, 3.5 mL of 0.1 mol/L Cd(NO3)2·4H2O and 20 mL of 0.07 mol/L fresh MPA were stirred with 45.0 mL of water. After adjusting the pH value of the solution to approximate 11.0 by 2.0 mol/L NaOH, a newly prepared NaHTe solution was added, and the mixture was heated under reflux at 100 ℃. Ag:CdTe SQDs were synthesized using the same method described above, except adding 0.2 mL AgNO3 with different concentration (5×10-4, 5×10-3, 1×10-2, 2×10-2, 5×10-2, 1×10-1, 2×10-1, and 5×10-1 mol/L)

before adjusting pH value by NaOH. The volume fraction of

synthesized Ag:CdTe SQDs in the solution is calculated to be approximately 0.01%. The CdTe/AgTe SQDs were obtained by growth of AgTe shells on CdTe cores with similar method. Characterizations and Femtosecond Z-scan Measurements. TEM observations were performed with a JEOL 2010 FET transmission electron microscope operated at 200 KV. The absorption spectra were recorded by UV-Vis-NIR spectrophotometry (Cary 5000, Varian). The fluorescence spectrum was recorded by a Hitachi F-4500 fluorescence spectrophotometer with a Xe lamp as the excitation source. The nonlinear absorption and nonlinear refraction of the


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samples were measured by the open- and close-aperture Z-scan technique. The ultrafast laser pulses were generated using a mode-locked Ti/Sapphire laser (Mira 900, Coherent) and a synchronously pumped OPO (APE, GmbH), the pulse width is 120 fs and the repetition rate is 76 MHz. The optical length of the sample is 1 mm, the focal length of the lens used in Z-scan setup is 100 mm.

RESULTS AND DISCUSSION Absorption and Fluorescence Spectrum of Ag:CdTe SQDs. The transmission electron microscopy (TEM) image reveals that the Ag:CdTe SQDs have an average size of ~3.6 nm (see Figure 1a). The average size of the bare CdTe SQDs is approximately 3.1 nm, which is slightly smaller than the doped ones. The structural and optical characterizations of the Ag:CdTe SQDs are reported in the previous work.63 The excitonic resonant behavior of the CdTe SQDs is efficiently tuned by increasing doping concentration of silver as shown in Figure 1b. The resonant wavelength of 1S peak red-shifts from ~500 to ~550 nm, the absorption strength of 1S peak increases ~30%, and the width of 1S band is significantly broadened after doping silver in CdTe SQDs. The red-shift of the excitonic band edge is mainly caused by the size effect of the doped colloidal SQDs. The prominent spectral broadening and absorption enhancement indicate that the local density of states and the excitonic transition dipole moment of the SQDs are efficiently increased by doping silver. The corresponding normalized fluorescence spectra of the Ag:CdTe SQDs with silver concentration ρAg = 0, 0.0005, 0.005, 0.01, 0.02, 0.05, 0.1, 0.2, and 0.5 M are presented in Figure 1c, it clearly shows that the fluorescence peak red-shifts from 550 to 620 nm and the spectral


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width increases ~50%, but the fluorescence intensity is extremely weak owing to quenching effect caused by the heavy dopants.

Figure 1. TEM image, absorption and fluorescence spectra of the Ag:CdTe SQDs. (a) A TEM image of Ag:CdTe SQDs (ρAg = 0.5 M). (b) Absorption spectra of the Ag:CdTe SQDs with silver concentration ρAg = 0, 0.0005, 0.05, 0.2, and 0.5 M. The resonant absorption is increased, 1S peak is broadened and red-shifted. (c) Fluorescence spectra of the Ag:CdTe SQDs with ρAg = 0, 0.0005, 0.005, 0.01, 0.02, 0.05, 0.1, 0.2, and 0.5 M. Fluorescence peak is red-shifted and the spectral width is broadened by doping silver.


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Nonlinear Refraction of Ag:CdTe SQDs. The refractive index (n) of the optical nonlinear materials is dependent on the laser power density (I) with the relationship of n = n0 + n2I, where n2 is the nonlinear refractive index. The Ag:CdTe SQDs exhibit largely enhanced NLR around the excitonic band edge. Figure 2a presents the normalized close-aperture (CA) Z-scan nonlinear transmittance (TCA /TOA) of the undoped CdTe and Ag:CdTe SQDs with dopant concentrations of 0.1 and 0.5 M at the wavelength of 590 nm. The nonlinear transmittance TCA / TOA has a relationship with n2,64,65 n kI L TCA = 1 +( 4 z / z0 ) 2 2 2 0 eff2 2 TOA ( z / z0 + 9)( z / z0 + 1)

(1)

where I0 is the peak irradiance at the focus (z = 0), k = 2π/λ is the wave vector of the laser radiation and z0 is the effective Rayleigh length of the incident Gaussian beam for typical TPA process. Leff = [1 – exp(−α0L)]/α0 is the effective thickness of the samples, with L being the thickness of the sample. From Equation (1), the n2 values of the Ag:CdTe SQDs (ρAg = 0, 0.1, and 0.5 M) are calculated to be about -0.016×10-4, -0.12×10-4, and -0.60×10-4 cm2/GW, which indicates that the nonlinear refraction of SQDs is enhanced more than 35 times by doping silver. To reveal the physical mechanism of the NLR enhancement, the wavelength-dependent NLR is measured and presented in Figure 2b. The value of n2 keeps negative in the whole wavelength range, and dramatically increases from -0.18×10-4 to -0.56×10-4 cm2/GW as the wavelength decreases from 630 to 600 nm, then slowly increases to -0.7×10-4 cm2/GW when further tuning the wavelength to ~550 nm. This indicates that the nonlinear refractive index is enhanced by exciton resonance at the band edge. It is found that the Ag:CdTe SQDs exhibit much larger NLR enhancement than the bare SQDs near the excitonic band edge.


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Figure 2. NLR of the bare CdTe and Ag:CdTe SQDs (ρAg = 0.1 and 0.5 M). (a) Normalized closed-aperture Z-scan nonlinear transmittance (TCA/TOA) of the CdTe and Ag:CdTe SQDs at the wavelength of 590 nm. (b) Wavelength-dependent NLR index n2 of the CdTe and Ag:CdTe SQDs. The n2 value above the excitonic band edge of the SQDs is enhanced more than 30 times by doping silver. Nonlinear Absorption of Ag:CdTe SQDs. The absorption coefficient (α) of the optical nonlinear materials is dependent on the laser power density (I) with the relationship of α = α0 +

βI, where β is the nonlinear absorption coefficient. The Ag:CdTe SQDs exhibit intriguing NLA around the excitonic band edge. Figure 3a-b presents the open-aperture (OA) Z-scan nonlinear transmittance of the undoped CdTe and Ag:CdTe SQDs with dopant concentrations of 0.1 and 0.5 M at the wavelength of 590 and 600 nm. The undoped SQDs exhibit almost a powerindependent transmittance, a constant TOA in Z-scan trance (see Figures S1 and S2). Interestingly,


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the Ag:CdTe SQDs with concentrations of 0.1 and 0.5 M respectively exhibit weak RSA (a valley in TOA) at 610 nm and SA (a peak in TOA) at 590 nm.

Figure 3. NLA of the bare CdTe and Ag:CdTe SQDs (ρAg = 0.1 and 0.5 M). (a) (b) Normalized open-aperture Z-scan nonlinear transmittance (TOA) of the CdTe and Ag:CdTe SQDs at the wavelength of 590 and 600 nm. (c) Wavelength-dependent NLA coefficient β of the CdTe and Ag:CdTe SQDs. The Ag:CdTe SQDs exhibit RSA at λ > 590 nm and SA at λ < 590 nm. The dopant silver enhances both two-photon RSA below the exciton band and one-photon SA above the band of the SQDs.


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The nonlinear transmittance TOA has a relationship with β,64,65 (− β I 0 Leff ) m

∞

TOA = ∑

m =0

(1 + z 2 / z02 )m (1 + m)

3

(2) 2

From Equation (2), the β values of the Ag:CdTe SQDs with dopant concentration ρAg = 0, 0.1, and 0.5 M at the wavelength of 590 nm are calculated to be about -0.02, -0.07, and -0.12 cm/GW. Figure 3c indicates that both sign and magnitude of the NLA of the Ag:CdTe SQDs can be tuned by adjusting the dopant concentrations. The wavelength-dependent NLA is also measured and presented in Figure 3c. The nonlinear absorption β of Ag:CdTe SQDs (ρAg = 0.5 M) decreases from -0.54 to -0.12 cm/GW when tuning wavelength from 560 to 590 nm, while the β value becomes to be positive with wavelength tuned to be longer than 600 nm. The positive β is attributed to the two-photon absorption with the photon energy below the excitonic band edge. At the wavelength of 610 nm, the β value is increased from 0.008 to 0.18 cm/GW by doping silver. The negative β is caused by the saturation effect of one-photon absorption with the photon energy above the excitonic band edge. At the wavelength of 560 nm, the β value is enhanced from -0.12 to -0.54 cm/GW by doping silver. Notice that the one-photon SA in this study excited with relatively weak intensity is significantly different from the two-photon saturation excited with extremely strong intensity.66,67

One- and Two-photon FOMs of Ag:CdTe SQDs. To qualitatively evaluate the performance of the optical nonlinear materials, one- and two-photon FOMs are defined as,68 W = |n2|I/αλ,

(3)

T = |βλ/n2|

(4)


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where I is laser power density. The Ag:CdTe SQDs also exhibit improved nonlinear FOMs around the excitonic band edge. The W value increases from 0.38 to 1.08 as λ decreases from 630 to 610 nm, and remains as high as around 1.15 in the wavelength range of 550 – 600 nm (see Figure 4a), which is about 3 times larger than that of the bare SQDs. The T value reaches the minimum 0 (Figure 4b), which is caused by the negligible NLA at the crossover wavelength from the two-photon RSA below the band-edge to the one-photon SA above the band-edge. Therefore, the Ag:CdTe SQDs with ρAg= 0.5 M satisfy the demands of W > 1 and T < 1 for the all-optical wavelength switching.

Figure 4. Wavelength-dependent nonlinear FOMs of the bare CdTe and Ag:CdTe (ρAg = 0.1 and 0.5 M) SQDs near the excitonic band edge. (a) Wavelength-dependent one-photon FOM W. The W value dramatically increases to be larger than 1 near the excitonic band edge as the wavelength decreases. (b) Wavelength-dependent two-photon FOM T. The T value of Ag:CdTe SQDs is close to 0 at 590 nm owing to the changeover from two-photon RSA to one-photon SA.


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Ag-Concentration-Dependence of Nonlinear Responses of Ag:CdTe SQDs. To further reveal the influence of Ag-dopant on the nonlinear responses, the Ag-concentration-dependence of four nonlinear parameters n2, β, W, and T at the wavelength of 590 nm is presented in Figure 5 (see also Figure S3). As silver concentration ρAg increases from 0 to 0.5 M, n2 increases from 0.016×10-4 to -0.60×10-4 cm2/GW (Figure 5a), W increases from 0.22 to 1.15 (Figure 5c), β increases from -0.01 to -0.12 GW/cm (Figure 5b), T decreases from 0.59 to 0.12 (Figure 5d). It clearly demonstrates that the heavily-doped Ag:CdTe SQDs have very large n2, small β, and desired FOMs W and T around the excitonic band edge. The value of n2 and W of Ag:CdTe SQDs can be further increased by increasing silver dopant concentration.

Figure 5. Ag-concentration dependences of the nonlinear parameters (a) n2, (b) β, (c) W, and (d) T of the Ag:CdTe SQDs at the wavelength of 590 nm. The laser power density is set at 25 mW/cm2, the laser wavelength is tuned to be 590 nm. As Ag-concentration increases, n2 and W increases, β and T decreases. The nonlinear FOMs W and T of the heavily doped Ag:CdTe SQDs satisfy the demand for all-optical switching.


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Comparison of Ag:CdTe, CdS/Ag, and CdTe/AgTe Core/Shell SQDs and Mechanism of NLR Enhancement. Finally, we compare the nonlinear responses of the Ag:CdTe SQDs and CdS/Ag core/shell SQDs, the nonlinear parameters n2, β, W, and T of four kinds of SQDs (CdTe, Ag:CdTe, CdS, and CdS/Ag) are summarized in Table 1. The silver shell grown on CdS SQDs leads to ~200 times enhancement of n2, and ~430 times enhancement of β owing to the enhanced two-photon resonance process by silver shells, therefore, the nonlinear FOMs W and T are still poor and do not satisfy the demand for switching applications.56 Similar to CdS/Ag core/shell SQDs, very large enhancements on both NLR and NLA could also be obtained in CdTe/Ag core/shell SQDs. The dopant silver in Ag:CdTe SQDs leads to ~35 times enhancement of n2, but keeps very small β around the excitonic band edge owing to the crossover from one-photon SA to two-photon RSA, therefore, both one- and two-photon FOMs W and T are significantly improved and satisfy the demand for practical applications. This indicates that the silver-doped SQDs show great advantage to the applications in optical information processing over the semiconductor/metal core/shell SQDs.

Table 1. Nonlinear parameters of Ag:CdTe SQDs and CdS/Ag QDs. n2 (cm2/GW) CdTe Ag:Cd CdS* CdS/Ag

W

T

-0.02

0.22

0.36

-0.12

1.15

0.12

-4

0.039

0.05

3.0

16.8

0.16

5.8

0.011×10 -2.3×10

β (cm/GW)

-4

-0.016×10 -0.60×10

-4

-4

* from Ref. [56]

Finally, we discuss the possible physical mechanism of the enhanced NLR with improved FOMs of Ag:CdTe SQDs near the band edge. On the one hand, the exciton resonance strength is largely enhanced and the corresponding 1S resonance peak is also significantly broadened by the


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heavily doped silver ions acting as charge centers within the CdTe SQDs,69 which indicates larger probability of resonant transitions from the ground state to the excited states and leads to stronger third-order susceptibility (χ(3)).70 Since the NLA (the imaginary part of χ(3)) is very small around the changeover from one-photon to two-photon transitions near the band edge, therefore, the NLR (the real part of χ(3)) is largely enhanced and the FOMs are also improved. On the other hand, both linear and nonlinear enhancements of the heavily-doped Ag:CdTe SQDs and CdTe/AgTe core/shell SQDs are very similar (see Figure S4), which strongly suggests that the heavily doped silver is favorably located near the surface of the CdTe SQDs. The linear refractive index of AgTe is much larger than that of CdTe, therefore, the gradient distribution of silver in the heavily-doped SQDs have a large local field enhancement within the SQDs and results in a higher nonlinear susceptibility. This field enhancement mechanism for NLR has been first observed in Mn-doped ZnSe SQDs and discussed by used effective medium theory by Gan and coworkers.49 These two physical mechanism also well explain that the largely enhanced NLR is only observed in the Ag:CdTe SQDs with heavy dopant concentrations.

CONCLUSIONS In summary, we synthesized the Ag:CdTe SQDs with heavy dopant concentration. The absorption around the excitonic band edge is increased 45%, 1S peak is red-shifted and broadened, the nonlinear refraction near the band edge is enhanced more than 35 times by doping silver, and the nonlinear absorption of Ag:CdTe SQDs is very small near the excitonic band edge owing to the crossover from one-photon SA to two-photon RSA. Therefore, both the one- and two-photon FOMs W and T are improved and satisfy the demand for all-optical waveguide switching. The enhanced NLR is attributed to the enhancements of excitonic absorption and gradient local field in the heavy-doped Ag:CdTe and CdTe/AgTe core/shell SQDs, much large


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enhancements on both NLR and NLA could be obtained in CdTe/Ag core/shell SQDs. Our observations offer a strategy to prepare ion-doped semiconductor quantum dots with large nonlinear refraction and good FOMs and have prospective applications in photonic nanodevices.

SUPPORTING INFORMATION 1, Wavelength-dependent open-aperture Z-scan nonlinear transmittance (TOA) of the Ag:CdTe SQDs with silver concentration ρAg = 0 , 0.1, and 0.5 M (Figure S1); 2, Wavelength-dependent normalized closed-aperture Z-scan nonlinear transmittance (TCA / TOA) of the Ag:CdTe SQDs with silver concentration ρAg = 0, 0.1, and 0.5 M (Figure S2); 3, Silver-concentration-dependent open- and closed-aperture Z-scan nonlinear transmittance (TOA and TCA / TOA) of the Ag:CdTe SQDs at the wavelength of 590 nm (Figure S3). 4, Absorption spectrum and closed-aperture Z-scan nonlinear transmittance (TCA / TOA) of the bare CdTe and CdTe/AgCdTe core/shell SQDs (Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *Email: [email protected]. *Email: [email protected].

Author Contributions S.-J. Ding and F. Nan contributed equally to this work.

ACKNOWLEDGMENTS


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This work was partially supported by the National Program on Key Science Research of China (2011CB922201) and the Natural Science Foundation of China (11174229).

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Largely Enhanced Optical Nonlinear Response of Heavily Doped Ag

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