Highly Efficient Active All-Dielectric Metasurfaces Based on Hybrid

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Functional Nanostructured Materials (including low-D carbon)

Highly Efficient Active All-Dielectric Metasurfaces Based on Hybrid Structures Integrated with PhaseChange Materials: From Terahertz to Optical Ranges Chuwen Lan, He Ma, Manting Wang, Zehua Gao, Kai Liu, Ke Bi, Ji Zhou, and Xiangjun Xin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22466 • Publication Date (Web): 21 Mar 2019 Downloaded from http://pubs.acs.org on March 21, 2019

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ACS Applied Materials & Interfaces

Highly Efficient Active All-Dielectric Metasurfaces Based on Hybrid Structures Integrated with Phase-Change Materials: From Terahertz to Optical Ranges Chuwen Lan1,2#, He Ma3#, Manting Wang

1#,

Zehua Gao1*, Kai Liu4, Ke Bi1*, Ji

Zhou4, Xiangjun Xin2* 1

State Key Laboratory of Information Photonics and Optical Communications,

School of Information and Communication Engineering & School of Science, Beijing University of Posts and Telecommunications, Beijing 100876, China 2

School

of

Electronic

Engineering,

Beijing

University

of

Posts

and

Telecommunications, Beijing 100876, China 3

College of Applied Sciences, Beijing University of Technology, Beijing, 100124

4

State Key Laboratory of New Ceramics and Fine Processing, School of Materials

Science and Engineering, Tsinghua University, Beijing 100084, China

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Recently, all-dielectric metasurfaces (AMs) have emerged as a promising platform for high-efficiency devices ranging from the terahertz to optical ranges. However, active and fast tuning of their properties, such as amplitude, phase and operating frequency, remains challenging. Here, a generic method is proposed for obtaining high-efficiency active AMs from the terahertz to optical ranges by using “hybrid structures” integrated with phase-change materials. Various phase-change mechanisms including metal–insulator phase change, nonvolatile phase change, and ferroelectric phase change are investigated. We first experimentally demonstrate several high-efficiency active AMs operating in the terahertz range based on hybrid structures composed of free standing silicon microstructures covered with ultrathin phase-change nanofilms (thickness d 10000 /cm2) with a thickness of 100 µm. As a comparison, the case for the waveforms passing through air was also investigated. This data reveals a time difference of approximately 0.6 ps between the sample and air (reference). In addition, the pulse passing through the sample exhibited a significant change as the temperature increased from 25 to 80 °C: the amplitude of the pulse decreased from 1.067 to 0.567 a.u. (approximately 46.86 % modulator depth) and its position shifted by approximately 0.02 ps. These experimental results suggest that the prepared VO2 experiences a remarkable phase transition followed by significant changes in both its optical properties and electric properties. To confirm its potential for application in tunable SAMs, a 200 nm thick VO2 film was deposited on the fabricated SAM. As shown in the terahertz transmission spectrum of the SAM covered with the as-deposited VO2 film (Figure 3d), the presence of the VO2 film led to a slight redshift of the original resonance frequencies. Quantitatively, the first resonance shifted from 0.667 to 0.666 THz, while the second resonance shifted from 0.802 to 0.772 THz. These redshifts can be attributed to the increased optical thickness of the SAM (namely, effective permittivity and physical dimensions). To confirm the active properties of the VO2/SAM, the terahertz transmission spectrums were obtained at various temperatures from room temperature (RT) to 80 °C. As shown in Figure 3d, remarkable tunability was observed for the first, second, and third magnetic resonances. For the first magnetic mode, the resonance frequency shifted from 0.666 to 0.630 THz (0.036 THz frequency tunability). For the second magnetic mode, the resonance frequency shifted from 0.772 to 0.688 THz (0.084 THz frequency tunability), but the resonance strength was seriously degenerated. For the third magnetic mode, the resonance frequency shift from 1.058 to 0.991 THz (0.067 THz frequency tunability) was also accompanied by serious degeneration of the resonance strength. The redshifts of these resonances are due to the increase in the VO2 permittivity and the degeneration of the resonance


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strength occurs since the loss of VO2 increases.

36

We also calculated the

frequency-dependent absolute transmission modulation (T2 - T1), as shown in Figure 3e. Nearly 60 % absolute transmission modulation was achieved for the third magnetic resonance, while those for the first and second resonances were 8 % and 51 %, respectively. This result indicates that the higher modes exhibit higher absolute transmission modulation. This is because the lower mode concentrates more field energy inside the silicon resonator. For example, for the first mode, almost all the field energy is concentrated in the center of the silicon resonator. On the contrary, for the third mode, while some field energy is concentrated in the center, the remaining field energy is located at the boundary between the silicon resonator and air. This property makes this mode sensitive to the background medium. Furthermore, the frequency-dependent tuning contrast (T2 - T1)/T1 was also calculated (Figure 3e). A huge tuning contrast of up to 3800 % was observed at the second resonance mode (0.772 THz). Meanwhile, a high tuning contrast of up to 1400 % was obtained at the third resonance mode. These unprecedented tunabilities can be attributed to the large transmittance change and low transmission at resonance. In addition, remarkable thermal memory for the resonance frequencies was also observed for these three magnetic modes (see the measured hysteresis loops for the third resonance frequency in Figure 3f), which suggests the potential of this system for applications in memory devices. Thus, we have demonstrated that a tunable SAM can be obtained by incorporating an MIPC-based material (VO2). By covering the SAM with an ultrathin VO2 film, the dielectric resonances, especially those for the high modes, can be efficiently tuned when VO2 undergoes a phase change. It is worth mentioning that the phase change can also be triggered by an electric field 37 or an optical pulse, 38 which would be useful for developing high-speed or ultrafast terahertz all-dielectric resonance-based photonic devices. Moreover, the phase transition temperature of VO2 can be effectively decreased through doping with donor impurity atoms. For example, by doping the V sublattice with W, the phase transition temperature can be tuned to near RT, which would be beneficial for various applications, including low-power optical switches and modulators.39 It should be noted that in addition to VO2, numerous other MIPC materials are available for tunable all-dielectric metasurfaces. Table 1 shows a number of correlated MIPC materials and their corresponding MIPC temperatures. 40 The use of such materials would aid in the production of MIPC-based



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tunable all-dielectric metasurfaces with different phase-change temperatures and responses to stimuli. 1. 4 NPC-Based Tunable SAMs The second phase-change mechanism employed was based on the NPC that occurs in some chalcogenide materials, such as GST.

41-43

As a typical PCM with

amorphous and crystalline phases, GST has been widely used in rewriteable DVDs and random access memories (PCRAM). It is well known that remarkable changes in the optical properties of GST accompany the phase change from the amorphous phase to the crystalline phase (Figure 4a). The phase change can be easily triggered by thermal annealing, an electrical pulse, or an optical pulse. Owing to these properties, GST has emerged as a promising medium for achieving active photonics, such as active plasmonics and active metamaterials. Using a sputtering process, we deposited a high-quality GST film with a thickness of 1 µm on a 500 µm thick high-resistivity silicon substrate. The details of the GST deposition process are described in the Supporting Information. To confirm the active optical properties of GST, terahertz time-domain waveforms were passed through the sample under different conditions. As plotted in Figure 4b, the time difference between the sample (silicon substrate with as-deposited GST film) and air (reference) was approximately 2.3 ps. To demonstrate the tunability of the optical properties, the sample was annealed at 160 °C for 0.5 h, which caused a phase change in the GST film from the amorphous phase to the crystalline phase. As shown in Figure 4b, the pulse passing through the sample changed significantly when GST transitioned from the amorphous phase to the crystalline phase, with a shift in position by approximately 0.06 ps and a decrease of the amplitude of the pulse from 0.97 to 0.10 a.u. (approximately 90.7 % modulator depth). It is worth noting that, although both the GST and VO2 films are phase change and memory materials. However, the GST film is a non-volatile film, while the VO2 is a volatile film. As a result, their electric and optical properties are much different with the temperatures. We deposited a GST film with a thickness of 1 µm on top of the fabricated SAM to obtain an active and nonvolatile terahertz AM. The terahertz transmission spectrum of the SAM covered with the as-deposited GST film (Figure 4c) shows that the deposition of a GST film led to a redshift of the original resonance frequencies. Quantitatively, the first resonance shifted from 0.667 to 0.660 THz, while the second resonance shifted from 0.802 to 0.771 THz. These redshifts can be attributed to an


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increase in the optical thickness of the silicon rods. To demonstrate the tunability of this active metamaterial, it was annealed at 160 °C for 0.5 h to cause the amorphous to crystalline phase change for GST film. As shown by the transmission spectrum of the annealed sample (Figure 4c), large redshifts and remarkable degeneration was observed for all the resonances. Quantitatively, the first resonance shifted from 0.660 to 0.540 THz (redshift of 0.12 THz) and the transmission changed from 1.1 % to 8.7 %. For the second resonance, a redshift of 0.091 THz was observed (from 0.771 to 0.680 THz) with a change in the transmission from 0.47 % to 16.0 %. For the third resonance, a redshift of 0.09 THz was observed (from 1.058 to 0.966 THz) accompanied by a change in the transmission from 11.6 % to 54.7 %. Clearly, compared with the VO2/SAM structure investigated above, the phase change GST/SAM structure causes larger redshifts. This is because the permittivity of GST changes by nearly 20 times, whereas that of VO2 only changes by several folds. In addition, the increased thickness of the phase-change medium may also enhance the redshift. To verify the nonvolatile properties of this active metamaterial, rapid annealing at 600 °C was conducted to trigger the crystalline to amorphous phase change of GST, and the corresponding transmission spectrum is shown in this figure. It was found that the measured terahertz response returns to the original state (amorphous phase), which directly confirms the nonvolatile properties of this material. The frequency-dependent absolute transmission modulation (T2 - T1) for these resonances is provided in Figure 4d. Several impressive absolute transmission modulation peaks can be observed at 0.538, 0.768, and 1.576 THz, with corresponding absolute transmission modulations of 78 %, 80 %, and 85 %, respectively. These huge modulations can be attributed to the large redshifts of the resonances

and

their

high-quality

resonance

properties.

In

addition,

the

frequency-dependent tuning contrast (T2 - T1)/T1 is also provided (Figure 4e). A huge tuning contrast of up to 16000 % was observed at the second resonance mode (0.771 THz). This unprecedented modulation depth can be attributed to the strong transmittance change (80 %) and ultralow transmission (0.47 %) at this resonance. For practical applications, the NPC can be easily triggered by an ultrafast stimulus, such as electrical or optical pulse ( 800 nm) and the dielectric loss increases considerably. As a typical NPC medium, GST has also been widely studied in the optical range, with practical applications in commercial memory devices. The complex permittivity of GST has also been reported, with amorphous GST having a high permittivity and low dielectric loss value when λ > 1000 nm.

46

In contrast,

crystalline GST exhibits remarkable increases in permittivity (up to 18) and dielectric loss. Interestingly, ferroelectric perovskites usually have high refractive indices and low loss in the optical range. For example, the refractive indices of BTO and PbTiO3 are approximately 2.5 and 2.85, respectively. More importantly, these materials also show thermally, electrically, and optically tunable properties. Consequently, it is interesting to investigate the possibility of developing active AMs in the optical range based on these three kinds of phase-change mediums. PCM-based nanoresonators, which are the most straightforward structure for tunable AMs in the optical range, have been reported by several groups recently. 18,19Although

good tunable properties can be obtained in PCM nanoresonators, they

are usually not compatible with CMOS processes, which might hinder their applications. To address this limitation, we proposed SAM/PCM hybridized structures, as shown in Figure 6, and considered three configurations. The first structure is composed of silicon nanoresonators placed on the PCM layer. The second structure consists of silicon nanoresonators covered with a PCM disk, which has the same diameter. The third structure is composed of silicon nanoresonators covered with a PCM film. These structures were selected because they can be obtained easily by using conventional fabrication processes. Simulations were performed to predict and evaluate their active optical properties. In the simulations, silicon nanoresonators with a height, diameter, and lattice constant of 250 nm, 500 nm and 1000 nm placed on quartz were considered. The refractive indices of silicon and quartz are approximately 3.5 and 1.5, respectively. In our study, GST was used as the PCM, although other PCMs are also available. The complex permittivity of the different phases of GST can be seen in reference. 46 The simulations for the first structure, with a PCM layer thickness of 30–60 nm, are shown in Figure 6d. Notably, a remarkable transmission dip is observed near the


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optical communication window (1550 nm), which can be assigned to the magnetic-electric hybrid resonance of the silicon nanodisk (The detail can be seen in the Supporting Information). When the thickness of the amorphous GST layer increased, the resonance shifted to lower frequencies. Meanwhile, the resonance band became broader and its Q-factor degenerated. When the amorphous GST layer was changed to a crystalline GST layer, the transmission changed significantly, with a distinct redshift and broadening of the resonance band. To investigate the impact of the thickness of the GST layer on the tunability, a tuning figure of merit (FOM) was calculated according to the following equation: swithing range  [nm] FOM 

FWHM [nm]

(3) A larger FOM corresponds to higher tunability. FOM values of 0.364, 0.317, and 0.179 were calculated for GST layer thicknesses of 30, 45, and 60 nm, respectively. This result indicates that the FOM decreases as the thickness increases in such structures. Simulations were also performed to investigate the impact of the thickness of the GST layer on the tunability of the second structure (Figure 6e). The presence of an amorphous GST layer led to a slight redshift of the magnetic resonance in the silicon disks, but the resonance quality remained nearly unchanged as the thickness of the amorphous GST layer increased. However, when the amorphous GST layer was changed to a crystalline GST layer, a distinct change was observed in the transmission spectrums, with the resonance shifting toward lower frequencies. FOM values of 0.074, 0.136, and 0.173 were obtained for GST layer thicknesses of 30, 45, and 60 nm, respectively. This phenomenon indicates that the tunability increases with the thickness of the GST layer while maintaining the resonance quality. These thickness-dependent optical properties are different from those of the first structure. In fact, this structure is similar to the PCM/SAM in the terahertz range investigated above, where the GST layer was deposited on the top of a silicon resonator. The simulations for the third structure with GST layer thicknesses of 20, 30, 40, 50, and 60 nm are shown in Figure 6f. A similar redshift of the magnetic resonance in silicon disk was observed owing to the presence of the amorphous GST layer. When the amorphous GST layer was changes to a crystalline GST layer, the resonance shifted to a lower frequency and became broader. Further, the redshift increased when the thickness of the GST layer increased. Interestingly, the FOM values were found to increase with the thickness of the GST layer from 20 to 40 nm but decrease with the



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thickness from 40 to 60 nm. The maximum FOM value (0.35) was obtained for a thickness of 40 nm. This property is much different from those of the first and second structures, whose FOM values decreased and increased, respectively, with an increase in the thickness of the PCM. This property can be explained as follow: When the thickness of the PCM film increased, the PCM film on the top of resonator would enhance the FOM; however, the PCM deposited on the substrate would broaden the resonance, which would decrease the FOM. As a result, the FOM values were first found to increase with the thickness of the GST layer but then decrease with the thickness.

PERSPECTIVE AND CONCLUSION For a clearer comparison, we have summarized the performance of recently reported tunable all-dielectric metasurfaces based on different methods, as shown in Supporting Information (Table S1). It can be seen that most of the tunable all-dielectric metasurfaces have difficulties in obtaining a good balance between the tunability and tuning speed. Although some of them have the advantages of excellent tunability and good tuning speed, they still suffer from difficulties in integration because they are not CMOS compatible. On the basis of this table, the proposed active all-dielectric metasurfaces based on hybrid structures integrated with phase-change materials have overcome these drawbacks, revealing a generic and promising method for highly efficient, fast speed and easy-integrated tunable all-dielectric metasurfaces from the THz to optical ranges. It is also worth noting that the proposed structures can also be extended to other active medium, such as 2D materials

17

and active

composites.47 In addition, they can also be extended to other active devices.5-16, 27-32, 48 In conclusion, we have proposed and demonstrated a general method for obtaining active AMs from terahertz to optical ranges based on hybrid structures integrated with PCMs. Using three different phase-change mechanism (MIPC, NPC, and FPC), several high-efficiency active AMs operating in the terahertz range were demonstrated. These systems were based on hybrid structures composed of freestanding SAMs covered with ultrathin phase-change nanofilms (thickness d

Highly Efficient Active All-Dielectric Metasurfaces Based on Hybrid

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