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Enhanced All-Optical Modulation of Terahertz Waves Based on Manganese Ferrite Nanoparticles Weien Lai, Peng Huang, Beatriz Pelaz, Pablo del Pino, and Qian Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07756 • Publication Date (Web): 13 Sep 2017 Downloaded from http://pubs.acs.org on September 18, 2017

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The Journal of Physical Chemistry

Enhanced All-Optical Modulation of Terahertz Waves Based on Manganese Ferrite Nanoparticles Weien Lai,a Peng Huang,b Beatriz Pelaz,c Pablo del Pino*c and Qian Zhang*b a

Key Laboratory of Special Display Technology of the Ministry of Education, National Engineering Laboratory of Special Display Technology, National Key Laboratory of Advanced Display Technology, Academy of Photoelectric Technology, HeFei University of Technology, HeFei, 230009, China b Institute of Nano Biomedicine and Engineering, Shanghai Engineering Research Center for Intelligent diagnosis and treatment instrument, Key Laboratory for Thin Film and Microfabrication Technology of the Ministry of Education, Department of Instrument Science and Engineering, School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, 800 Dongchuan RD, Shanghai, 200240, China c Centro Singular de Investigación en Química Biolóxica e Materiais Moleculares (CIQUS), and Departamento de Física de Partículas, Universidade de Santiago de Compostela, Santiago de Compostela, 15782, Spain

ABSTRACT We present an all-optical modulator based on manganese ferrite nanoparticles (MnFe2O4 NPs), which provides an enhanced attenuation of broadband terahertz waves. A wide-band modulation of THz transmission was observed in a frequency range from 0.15 to 1.2 THz. The experimental results were assessed by simulations in the context of a band structure model of semiconductors. Our work demonstrated that coatings of MnFe2O4 NPs can be efficiently used to improve the performance of THz modulators based on optical modulation. This paper describes a new route to increase the surface photoconductivity of semiconductors by coating of MnFe2O4 NPs. This work demonstrates that the THz modulator based on MnFe2O4 NPs can significantly boost the overall performance of THz communication systems, and MnFe2O4 NPs may offer some useful solutions for future THz devices.

1. INTRODUCTION Over the last two decades, many nanomaterials have attracted a great interest by scientists from several interdisciplinary research areas involving chemistry, physics, biology, materials science and medicine, etc.12 Technology based on nanomaterials (i.e., nanotechnology) are routinely extending to diverse applications which are closely related to human well-being, including air purification3, cosmetic products4, light generation5, accurate diagnosis6, solar cells7, disease treatment8-9, electrochemical devices10-11 and so forth. Those diversified applications of nanotechnology are all mainly governed by the unique physico-chemical properties of nanomaterials12-14, such as

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nanoparticles (NPs) and 2D-nanomaterials, which typically arise from the nanoscale size (high surface-to-volume ratio compared to bulk materials)15, surface and interface effects16. The development of interdisciplinary science is facilitating the integration of nanotechnology with terahertz (THz) technology, which makes the nanomaterials play a key role in the field of THz technology17-19. Furthermore, THz technology has become increasingly attractive for its potential applications20-23, such as THz imaging24 and THz communication25-26. With the development of high-speed telecommunications, THz communication has become an increasingly attractive research area27-29. In THz communication systems, there are desired demands for THz modulators with excellent performance, which include some important parameters such as modulation bandwidth, modulation depth and modulation speed. In the past, various THz modulators, which based on metamaterials30-31,

graphene32-33,

superconductors34-35,

and

vanadium

dioxide36-37,

were

demonstrated. However, these modulators have complex fabrication requirements and are limited by the properties of materials, which restrict their practical applications. In this paper, we propose a new approach to obtain a broadband THz modulator by using coating of MnFe2O4 NPs. The THz modulator based on coating of MnFe2O4 NPs has excellent performance. In addition, the apparent advantages of coating of MnFe2O4 NPs, such as low-cost, present high-stability and environmental compatibility, make this type of coatings a good candidate for industrial applications. Hence, our concept is suitable for other THz device, which opens a new window for future THz devices.

2. METHODS 2.1 Materials. Iron(III) acetylacetonate (Fe(acac)3, 98%), manganese(II) acetylacetonate (Mn(acac)2, 97%)，Dglucose (99%),oleic acid (OLA, 90%), oleylamine (OLAM, 80−90%),1-octadecene (1-ODE, ˃90%) were purchased from Aladdin. 1,2-hexadecanediol (90%) was order from Sigma-Aldrich. Chloroform (99%), acetone (99%) and ethanol (99.9%) were purchased from Sinopharm Chemical Reagent Co., Ltd. high-resistivity silicon substrates (resistivity >2000 Ω·cm) were cut from a silicon wafer. 2.2 Synthesis of MnFe2O4 NPs with core diameter (dc) of 16 nm Monodisperse MnFe2O4 NPs were produced by a thermal decomposition process as previously described 38-39. Briefly, 2 mmol of Fe(acac)3, 1 mmol of Mn(acac)2, 10 mmol of 1,2-hexadecanediol and 4 mmol of D-glucose were weighted into a flask, with following addition of a mixture containing 6 mmol of OLA, 6 mmol of OLAM and 10 mL of 1-ODE. After heating the mixture up to 100 °C, vacuum was applied during 30 min in order to remove residual water and O2. Then under magnetic stirring with a flow of N2, the solution was heated up to 200 °C and aged for 2 h. Then, the


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temperature was increased into reflux (320 °C) and aged for another 1 h. The obtained product was cooled down to the R.T. by removing the heating mantle. Upon addition of 30 mL of ethanol and 5 mL of chloroform, the black precipitates were separated by centrifugation (3000 rcf, 15 min), and the precipitate was recovered by adding 15 mL of chloroform containing a mixture of OLA and OLAM (~10 µL). Few seconds of sonication speeded up the dispersion of the NPs. The cleaning step was then repeated at least twice to get the final product. At last, the acquired monodisperse MnFe2O4 NPs were redispersed in chloroform (concentration: 2 mg/mL) for further use. 2.3 Fabrication of NPs@Si based on MnFe2O4 NPs The NPs@Si system was prepared by coating of monodisperse MnFe2O4 NPs on a silicon substrate by a spin coating process, as described previously40. In detail, a silicon substrate was prewashed by acetone solution with sonication 3 times, 50 µL of MnFe2O4 NPs solution obtained above was dropped on the substrate surface, the followed rotation (100 rpm, 10 s) was applied to cast a layer of MnFe2O4 NPs on the substrate to fabricate the NPs@Si system.

3. RESULTS AND DISCUSSION 3.1 Synthesis and characterization of MnFe2O4 NPs Monodisperse MnFe2O4 NPs were produced by the one-pot thermal decomposition method following slightly modified published protocols (details are described in Materials and Methods Section)38-39. The obtained MnFe2O4 NPs dispersed in organic solvents exhibit black color with longterm stability (the product solution can be stored for at least 1 year). Images of transmission electron microscopy (TEM) and high resolution-TEM (HRTEM) were recorded to analyze the morphology and size of MnFe2O4 NPs using transmission electron microscopy (JEM-2100F, Japan), as shown in Figure 1a and 1b, respectively. The MnFe2O4 NPs on the silicon (NPOS) were characterized by scanning electron microscopy. The MnFe2O4 NPs layer was obvious, as shown in Figure 1c. The corresponding size distribution of NPs was obtained by using the software ImageJ to measure the average diameter of inorganic core (dc) of 16.5 ± 1.7 nm (Figure 1d). The magnetization versus magnetic field M (H) curve of MnFe2O4 NPs were analyzed by a physical property measurement system (PPMS-9T, USA) under an applied field of 30 kOe at 300 K. Figure 1e shows a smooth loop curve without obvious hysteresis loss (superparamagnetic behavior), with a saturated magnetization value (Ms) of 75.8 emu/g. UV-Vis spectrum of MnFe2O4 NPs from 300 nm to 1000 nm were recorded by UV-Vis absorption spectrophotometry (Varian Cary 50 spectrophotometer, USA) (Figure1f).



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Figure 1. (a) TEM image of MnFe2O4 NPs evaporated from chloroform (scale bar is 100 nm). (b) HRTEM image of MnFe2O4 NPs (scale bar is 10 nm). (c) SEM image of MnFe2O4 NPs coated on a silicon substrate. (d) The corresponding histogram of MnFe2O4 NPs produced by counting the core diameter (dc) from the TEM image by the software ImageJ (counting number> 300 NPs) (e) M(H) curve of MnFe2O4 NPs powder recorded by using PPMS at 300 K. (f) UV-Vis absorption spectra of MnFe2O4 NPs in chloroform measured by UV-Vis absorption spectrophotometry.

3.2 Characterization of THz transmissions of NPs@Si under CW laser irradiation


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Substrates of silicon coated with MnFe2O4 NPs (NPs@Si) were designed by casting one layer of hydrophobic MnFe2O4 NPs on the high resistivity silicon by spin coating process, as described previously (the fabrication details are shown in Materials and Methods Section)40, and the SEM image of NPs@Si in Figure 1c exhibit an uniform layer of NPs on the silicon substrate. In order to analyze the effect of MnFe2O4 NPs on the substrate, optical properties of NPs@Si substrate and bare silicon substrate (Si) were measured under CW laser irradiation (continuous wave (CW), λex: 808 nm). Figure 2 shows the experimental schematic of examining THz transmission characterization of NPs@Si and Si under CW laser irradiation. The spot diameter of the CW laser was about 3 mm, fully overlapping with the THz beam on the surface of the samples. The THz beam was directed to the samples at normal incidence by a focusing lens, as shown in Figure 2a and 2b. The optical properties of NPs@Si and Si were investigated by changing the irradiation power of the CW laser. Under the laser irradiation with different powers, the waveforms of THz pulses transmitted through the NPs@Si and Si were collected, as shown in Figure 2c. The amplitudes of the THz pulses transmitted through the NPs@Si, as well as the ones transmitted through the Si substrate, decreased as expected upon increasing the power of the laser irradiation. Conversely, under no laser irradiation, the waveform of THz pulses transmitted through NPs@Si was identical to the one transmitted through Si, as shown in Figure 2c. This demonstrates that the absorption of MnFe2O4 NPs in the THz frequency range is negligible. Further, to characterize the changes of amplitudes of THz pulses transmitted through NPs@Si and Si under laser irradiation, the amplitudes of the measured pulses were normalized with respect to the amplitude of the THz pulse transmitted through bare Si under no laser irradiation. At laser powers less than about 420 mW, the normalized transmission of THz pulses through NPs@Si decreased more dramatically with increasing the power of the laser irradiation as compared with bare Si (shown in Figure 2d). This demonstrated the NPs@Si has a significantly enhanced attenuation of THz radiation in comparison with the bare Si under laser irradiation. Figure 2d shows that, when the power of laser irradiation was about P ≈ 500 mW, the transmission change ( ∆TNPs @ Si / TSi0 ⋅ 100% ≈ 91% ) of THz pulses through NPs@Si was significantly higher than the transmission change ( ∆TSi / TSi0 ⋅ 100% ≈ 49% ) of THz pulses through Si. However, at the laser power of about 596 mW, the transmission of THz pulses through NPs@Si almost achieved minimum transmission (shown in Figure 2d).



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Figure 2. Schematic of a THz pulse transmitted through Si (a) and NPs@Si (b) under CW laser irradiation. (c) Waveforms of THz pulses transmitted through Si and NPs@Si under the laser irradiation with different powers (0 mW and 596 mW). (d) Normalized amplitude transmission of the THz pulses transmitted through Si and NPs@Si under laser irradiation with different powers.

Figure 3 shows the frequency-domain transmission of THz pulses transmitted through Si and NPs@Si under laser irradiation with different powers. Comparison between experimental and simulated THz transmissions through Si and NPs@Si showed similar trends. Simulations were obtained from a theoretical model based on band theory32, 41-42, which is described in the Supporting Information. However, the discrepancies between experiment and simulation may arise from the deviation of the conducting layer (optical doping layer) in samples. In contrast to Si under laser irradiation, the frequency-domain transmission of the THz pulses transmitted through NPs@Si dramatically varies with increasing laser powers. This demonstrates the significant attenuation effect caused by MnFe2O4 NPs for the broadband terahertz waves (0.1 THz-1.2 THz).


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Figure 3. Experiment and simulation for frequency-domain transmission of the THz pulses transmitted through Si and NPs@Si under laser irradiation with different powers.

Since the amplitude of THz waves transmitted through a sample is inherently correlated with the conductivity of conducting layer of the sample, under laser irradiation with the same power, the surface conductivity in NPs@Si is significantly higher than that in bare Si. This is due to higher density of photo-excited carriers in NPs@Si as compared to Si. The manganese ferrite NPs used in our experiment present quite different optical properties compared to the plasmonic NPs. Plasmonic NPs (e.g. Au NPs and Ag NPs) exhibit localized surface plasmon resonance (LSPR) behaviors under irradiation of incident radiation at a particular frequency, which can further enhance the surface conductivity of semiconductors43-45. Hereby, LSPR of plasmonic NPs are mainly dependent on their sizes, shapes and compositions. In order to obtain optimal LSPR of plasmonic NPs, the experiments need to optimize the size and shape of plasmonic NPs, which are not easy to be achieved. Instead of plasmonic NPs, we propose MnFe2O4 NPs, which although lack plasmonic properties (see Figure 1f), they still have the capability of enhancing the surface conductivity of semiconductors. Under laser irradiation, THz radiation responses of NPs@Si mainly attributes to the properties of MnFe2O4 NPs (cf. the Supporting Information), which have an effect on the surface conductivity of silicon. Under the same irradiation power, due to the effect of MnFe2O4 NPs, the conductivity of the conducting layer (optical doping layer) in the NPs@Si is much higher than that of the conducting layer in bare Si. Although it is worth to point out that the hot electron injection from plasmonic NPs into



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semiconductors obviously changes the surface conductivity of semiconductors, MnFe2O4 NPs don’t have LSPR, and thus the phenomenon of hot electron injection from MnFe2O4 NPs into semiconductors can be ruled out. 3.3 Performance of the THz modulator based on NPs@Si

Figure 4. The experimental schematic of Si (a) and NPs@Si (b) under modulated laser irradiation. Dynamic modulation of the Si (c) and NPs@Si (d) as THz modulators under modulated laser irradiation.

Figure 4 shows the schematic of Si and NPs@Si under modulated laser irradiation. The modulated laser produces square laser pulses with 50% duty cycle and frequency of 50Hz. Both Si and NPs@Si as THz modulators were used to examine the performance of the dynamic modulation of CW THz wave at 0.33 THz in THz communication region. The dynamic modulation experiment was performed under modulated laser power from 112 mW to 562 mW. In the experiment of dynamic modulation of CW THz wave, the changes of the detected intensity of transmitted CW THz wave were controlled by the modulated laser. Under modulated laser irradiation with the same power, the intensity variation of transmitted CW THz wave modulated by NPs@Si was significantly higher than that of transmitted CW THz wave modulated by Si. The modulation depth of the NPs@Si modulator, as well as the Si modulator, increased with increasing modulated laser power. However, at powers higher than about 460 mW, the modulation depth of NPs@Si and Si modulator only had a small variation with increasing laser powers due to the saturation of photo-excited carriers of the modulators. Notice that the modulation depth of modulators is inherently correlated with the conductivity of their conducting layer, in which the conductivity is mainly dependent on the photo-


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excited carriers. Therefore, the conductivity of the conducting layer in the NPs@Si is much higher than that of the conducting layer in the Si under equivalent power of laser irradiation. The performance of the dynamic modulation of the NPs@Si modulator significantly exceeded that of the Si modulator under modulated laser irradiation at the equivalent irradiation powers.

Figure 5. The modulation depth of transmitted CW THz wave for the NPs@Si modulator and the Si modulator under modulated laser irradiation with different powers, respectively.

Figure 5 shows the modulation depth of transmitted CW THz wave for the NPs@Si modulator and the Si modulator under modulated laser irradiation with different powers, respectively. The modulation depth of the THz modulator was defined as Tp − Tp0 / Tp0 ⋅ 100% , where Tp and Tp0 are the transmittance of CW THz wave through the sample under laser irradiation and under no laser irradiation, respectively. The modulation depth of the NPs@Si modulator is significantly much higher than that of the Si modulator under modulated laser irradiation with the same power. Meanwhile, the enhanced modulation depth of the NPs@Si modulator is greatly notable under low power irradiation. The enhancement of modulation depth of THz waves by MnFe2O4 NPs is significantly achieved at low power of modulated laser. However, the enhancement effect observably decreased with increasing the laser power. At the laser power (P ≈ 562 mW), the modulation depth of the NPs@Si modulator approximately reached 80%, which greatly exceeded modulation depth (about 33%) of the Si modulator. Hence, the modulation performance of NPs@Si modulator is significantly improved in contrast to that of Si modulator. As a result, the MnFe2O4 NPs can be efficiently used to improve the modulation depth of all-optical THz Modulators, which can be efficiently driven by CW laser with low powers.

4. CONCLUSIONS



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In conclusion, we demonstrated that monodisperse MnFe2O4 NPs can be efficiently used to improve the performance of all-optical THz modulators. MnFe2O4 NPs play a key role in increasing photocarrier density on the surface of the silicon under laser irradiation. Coating of MnFe2O4 NPs provides an enhanced attenuation of broadband terahertz waves, so that NPs@Si can be used as an all-optical THz modulator over a broad frequency band. This may lead to a cost-efficient component for THz communication systems operating in transmission mode. Hence, coatings based on MnFe2O4 NPs have the potential to increase the overall performance of THz systems.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. including: Numerical Methods

AUTHOR INFORMATION Corresponding Authors *(P.D. P.) E-mail: [email protected] *(Q.Z.) E-mail: [email protected] Tel: +86-21-34206375. Fax: +86-21-34206886

ORCID Beatriz Pelaz: 0000-0002-4626-4576 Pablo del Pino: 0000-0003-1318-6839 Qian Zhang: 0000-0002-5242-7861

Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS We thank the Fund from Hefei University of Technology to Weien Lai. We thank the financial support from Chinese Scholarship Council and Shanghai Engineering Research Center for Intelligent diagnosis and treatment instrument (15DZ2252000) to Q. Z., MINECO (MAT2015–74381-JIN to B.P., RYC-2014–16962 to P.dP.), the Consellería de Cultura, Educación e Ordenación Universitaria


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(Centro singular de investigación de Galicia accreditation 2016-2019, ED431G/09), and the European Regional Development Fund (ERDF) are acknowledged.

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Figure 1. (a) TEM image of MnFe2O4 NPs evaporated from chloroform (scale bar is 100 nm). (b) HRTEM image of MnFe2O4 NPs (scale bar is 10 nm). (c) SEM image of MnFe2O4 NPs coated on a silicon substrate. (d) The corresponding histogram of MnFe2O4 NPs produced by counting the core diameter (dc) from the TEM image by the software ImageJ (counting number> 300 NPs) (e) M(H) curve of MnFe2O4 NPs powder recorded by using PPMS at 300 K. (f) UV-Vis absorption spectra of MnFe2O4 NPs in chloroform measured by UV-Vis absorption spectrophotometry. 180x285mm (600 x 600 DPI)


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Figure 2. Schematic of a THz pulse transmitted through Si (a) and NPs@Si (b) under CW laser irradiation. (c) Waveforms of THz pulses transmitted through Si and NPs@Si under the laser irradiation with different powers (0 mW and 596 mW). (d) Normalized amplitude transmission of the THz pulses transmitted through Si and NPs@Si under laser irradiation with different powers. 89x69mm (600 x 600 DPI)



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Figure 3. Experiment and simulation for frequency-domain transmission of the terahertz pulses transmitted through Si and NPs@Si under laser irradiation with different powers. 89x69mm (600 x 600 DPI)


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Figure 4. The experimental schematic of Si (a) and NPs@Si (b) under modulated laser irradiation. Dynamic modulation of the Si (c) and NPs@Si (d) as THz modulators under modulated laser irradiation. 90x70mm (600 x 600 DPI)



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Figure 5. the modulation depth of transmitted CW THz wave for the NPs@Si modulator and the Si modulator under modulated laser irradiation with different powers, respectively. 71x57mm (600 x 600 DPI)


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Enhanced All-Optical Modulation of Terahertz ... - ACS Publications

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