Accelerating terahertz all-optical modulation by hot carriers effects of silver nanorods in PVA film

Nowadays, terahertz (THz) technology plays more and more important roles in our modern life: security apparatus based on millimeter waves in airports,¹ imaging or spectrum system for physical,² chemical and biomedical detection,^3–7 THz communication system for big-data transmission.^8,9 All the applications depend on the vivid development of the THz components, where tremendous progress has been made on sources and detectors^10–13 and there is an urgent need for functional devices, such as modulators,^14–17 lens,¹⁸ isolators¹⁹ and polarizers^20,21 for building complex THz apparatus system. Until these days, weaving functional devices in THz range is still a challenge due to the lack of suitable material and effective strategy. Amplitude modulators which can manipulate THz waves temporally are in high demand for big-data communication and high-resolution imaging system. Tuning the electronic conductivity of the semiconductor has been proven to be an effective approach for wide-band THz wave modulation:²² the transmitted THz waves will be blocked with the increase of the conductivity. However, with the purpose to achieve both low insertion loss and large modulation depth, the conductivity needs be a lower value at the primary state and higher value at the modulation state, which requires a huge dynamic electronic response. Intrinsic semiconductors, such as silicon and GaAs, have low electronic conductivity and play an important role in THz artificial microstructure devices.^23–25 However, its semiconductor property determines its small response to the electronic or photonic excitation which means that it is hard to make intrinsic-semiconductor based device active. Therefore, plasmonics²⁶ and metamaterials²⁷ have been introduced to enhance the interaction between the electromagnetic waves and materials.^28,29 Metamaterial-based modulator^30–32 was reported to achieve a large modulation and quick response due to its ability on slowing down THz waves near its resonance’s frequency, but the working band is narrow. Moreover, it heavily relies on the costly top-down fabrication processes. Recently, two dimensions’ (2D) materials^33–36 have been introduced to form the heterojunction which could assemble more electronics on the interface between 2D material and silicon depending on the modulation of the Fermi level and band gap. Perovskite and organic photoelectric materials have also been studied for modulation with the similar principle.³⁷ On the other hand, Wen et al³⁸ used plasmonic gold nanoparticles to enhance the optical modulation depth of THz wave, which inspires us a new way to manipulate the light to enhance the modulation of THz waves. The optical-pump modulation of terahertz spectroscopy shows very promising terahertz applications for ultrafast and functional devices.^39,40 Cong et al⁴¹ introduce an active hybrid metasurface integrated with patterned semiconductor inclusions for all-optical active control of terahertz waves. Ultrafast modulation of polarization states and the beam splitting ratio are experimentally demonstrated on a time scale of 667 ps. They also develop active flexible photonic devices using the solution-processed perovskite with an intrinsic ultrafast response. A thin perovskite layer on a flexible metadevice enables an ultrafast all-optical switching of Fano resonance with a time Constant of 500 Ps.⁴² In a recent report, a subwavelength active supercavities with extremely high-Q resonances is demonstrate could be reconfigured at an ultrafast time scale. Such supercavities enable all-optical switching and modulation of extremely sharp resonances.⁴³ 

Hot carries have attracted more interests recently as the excitation efficiency can be improved drastically by surface plasmon resonance^44–47 and the quick relaxation from the intraband transition improves the photoelectric and photothermal conversions.⁴⁸ Many photonic, photocatalysis and other light harvesting devices have been reported based on the hot carriers’ excitation.^49,50 Atwater’s group has utilized the hot carriers to enhance the response speed of the thermoelectric detector.⁵¹ Halas’ group have proposed many hot carriers related devices,⁵² such as subwavelength metallic grating for photodetection⁵³ and aluminum nanocrystals photocatalyst⁵⁴ for hydrogen dissociation.

Growing silver nanoparticles in polyvinyl alcohol (PVA) has been gradually applied in health field for fighting against with bacteria for many years.^55–57 Here, we developed the method to grow silver nanorods in the PVA film and utilized its localized surface plasmon resonance (LSPR) and hot carriers’ effect to speed-up and enhance the all-optical THz modulation. The working principle of the modulation enhancement and acceleration have been analyzed and discussed. This hybrid plasmonic film is fabricated by low-cost and convenient bottom-up method, providing a new approach to enhance the efficient wideband modulation of THz waves, which could be further applied in many fields of THz industry or all-optical related fields.

In the experiment, the silver nano-rods were synthetized in the PVA film on top of the silicon substrate in the following steps. 2g PVA powders were added into 25 ml deionized water and the mixed solution was heated at 60 degrees centigrade in a water bath with magnetic stirring. The rotation speed of magnetic rotor was set to be 20 r/min for one hour. Then, the silver nitrate solution (2g AgNO₃ powders dissolved in 10 ml deionized water) was added into the PVA solution. In order to synthesize silver nanorods, 0.3 g glucose powders, as the catalyzer, was dissolved into silver nitrate solution. The hybrid solution was heated for half an hour, and then spin-coated on the silicon substrate. The maximum speed was set to 2000 r/min. At last, the sample were baked at 150 °C for 10∼30 minutes. Figure 1(a) is the image of the hybrid film under scanning electronic microscope. A standard THz time-domain spectroscopy (THz-TDS) system was used to measure the transmission spectra of the samples under 532nm/803 nm CW laser pump with different power densities, and Figure 1(b) shows the scheme of the all-optical modulation experiment setup. A (110) ZnTe crystal excited by a Ti:sapphire femtosecond laser with 75 fs duration and 80 MHz repletion rate at 800 nm is utilized to generated THz pulse. The main power of the THz pulse locates at 1THz. A standard four parabolic-mirror system is used for beam alignment. The Quasi-Optical Detector (Virginia Diodes, Inc) connected with an oscilloscope (Tektronix, Inc. MDO3052), is used to detect the amplitude. The wavelength of the pump laser is 532nm and 808nm and the intensity power on the samples is calculated by the output power and illumination area. In order to detect the modulation speed, a chopper is used to modulate the laser illumination. The experiments are performed at 25 °C.

We firstly characterized the amplitude modulation performance of the hybrid plasmonic film. We use 532nm continue-wave laser light as the pump light obliquely illuminating the sample. In the experiment, the irradiated area of the pump beam is a circular area with a diameter of 1 cm, and the THz spot is a circle with a diameter of 3mm, where the area of the illumination on the sample is a little larger than the THz spot. Figure 2(a) shows the time domain signal when the green light illuminates the samples. Comparing with the bare silicon substrate in Figure 2(b), the transmission signal of the sample is nearly at the same value, which indicates that the coating film does not influence the transmission at original state. When the illumination power increased, the pulse signal dropped down. The signal peak of the sample decreased from 5.05 μV to 0.25 μV when the pump power density increased from 0 to 3.03 W/cm². In Figure 2(b), the signal peak of the silicon substrate only dropped to 2.5 μV at the same illumination power density. It could be concluded that the hybrid plasmonic film can modulate the THz waves deeply by optical illumination.

The modulation depth shown in Figure 2(c) is obtained by comparing the peak of time domain signals under different illumination power densities with the peak of time domain signal at 0W/cm^2. It shows that the modulation depth increased with the green light power density. There was a significant increase when the green light power density was less than 0.60W/cm². At 0.6W/cm², the modulation had reached 80%. When the light power was above 0.60W/cm² growth slowed down. With the green light power density increased, the modulation depth was gradually stabilized and approached almost 100%. The modulated transmission spectrum in frequency-domain was shown in Figure 2(d) through Fourier transform of the time-domain signals. It can be seen that the transmission of the THz waves above 0.3 THz decreased from 70% to 5% when the green light power density increased from 0 to 3.03W/cm², which means the modulation depth of 92.8%.

Then, the modulation speed is characterized in Figure 3, when the pump light was modulated by a chopper. In order to fairly evaluate the modulation speed, we take our previous Laser Processed PVA (LP-PVA, Pure PVA) sample⁵⁸ as the reference. It can be seen that the plasmonic PVA film and LP-PVA film have the same performance when the frequency of the chopper is 1 kHz. However, the LP-PVA sample failed to keep a good responding waveform when the choppers’ frequency increased to 3 kHz. There was a significant drop of the amplitude. The ascending edge failed to reach a high value in 0.33 ms. On the other hand, the modulation of the plasmonic PVA film was well maintained when the chopper’s frequency is 3 kHz. Although the ascending edge and descending edge of the responding waveform are not steeper, the amplitude of the signal can still keep 90% corresponding to that at 1 kHz, which means that the photon-electrons assemble more quickly in the same time. Obviously, with the incorporation of the silver nano-rods into the PVA film, the modulation speed is increased by at least 3 times.

In our opinion, the modulation mechanism generally comes from the photo-excited carriers’ effect. Finite-difference time-domain (FDTD) simulation is used to estimate the conductivity of the film. In the FDTD model, the thickness of the thin film is 1.5 μm, which is much smaller than the wavelength. Therefore, the 2D layer model was used in the simulation, where the refractive index of the thin film is based on the Drude model. The Figure 4 shows the simulated transmission spectrum of the film under different conductivity. Taken the thickness of the film into consideration, the density of the assembling carriers are relatively large. Compare with the bare silicon, this result indicates that more photo-generated carriers are produced at the interface between the film and silicon substrate. Since the amplitude modulation is similar with the LP-PVA film, the carriers assembled is comparable. In the synthesis process of the silver nanorods, the PVA long-chain molecules decomposes into new polymerize with conjugate double-bonds because of the redox reaction process on silver, which formed defect and heterojunction with the silicon surface, leading more non-equilibrium carriers assembled at the interface. Therefore, the modulation depth is greatly enhanced in the hybrid plasmonic film. Although the silver nanorods’ localized surface plasmonic resonance could trap the light and enhanced the interaction between the light and matters, limited by inner quantum efficiency of junction, there is no apparent enhancement in this case. However, the silver nanorods and PVA forms Schottky contact and the localized plasmon resonance would induce hot carrier excitations. The synthesis of the silver nanoparticles is the reason for the decomposition of PVA long-chain molecules. The silver nanoparticles locate at the defect where the hot carriers were generated when the film is illuminated. Therefore, the hot electrons easily jumped out of the barrier in the defect, which contributes to the high assembling speed of the photo-generated carriers. Therefore, the modulation speed in this case is 3 times faster than the LP-PVA sample.

To prove this view, we use UV-VIS spectrometer to measure the absorption of the hybrid film on the transparent silica substrate. It can be seen from the Figure 5(a) that the absorption of the hybrid film from 400 to 600 nm reaches nearly 100% which means that the silver nanorods’ localized surface plasmon resonance trapped the light at this range. Meanwhile, we have characterized the optoelectronic properties of the plasmonics film by the Lake Shore probe station with the semiconductor parameter analyzer. As shown in Figure 5(b), under 45mW power laser light (at 632nm) illumination, the photo generated current of the hybrid film is nearly 5 times larger than (or 5 times of) that in the PVA-only film, which shows that the hot carriers in the hybrid plasmonic films could be collected by electrodes and the optoelectronic response was actually enhanced by silver nanoparticles plasmonics. This result proves that the hot carriers are generated when the light illuminates the sample. Moreover, according to the absorption of the hybrid plasmonic film in Figure 5(a), the absorption near 500 nm is the highest which indicates the plasmonics effect at 500 nm is much stronger than that at other wavelengths.

Therefore, it give us a hint that if the pumping laser lights in different wavelengths, depending on the ‘hot carriers’ effect’, the opto-electronic response of the sample should be different at 3kHz modulation frequency. Therefore, we did another modulation experiment by using 808 nm pump light. The result in Figure 6 shows that the modulation under 808nm illumination did not perform well as that at 532 nm laser light illumination when the chopper’s frequency is 3 kHz. It is obvious that the ascending edge did not reach the highest value. This result indicates that the plasmonic hot carriers’ effect did influence the speed of the modulation and the plasmonic absorption dominates the degree of the acceleration. Based on this principle, the hybrid film could play as a color selective modulator. When the 532nm laser illuminates the sample, the detecting voltage is 0.46V, which we could treat the modulation state as ‘2’. When the 808nm laser illuminates the sample, the detecting voltage is 0.28V, which we could treat the modulation state as ‘1’.

In summary, we produced a silver nanorods in PVA film to realize a high-speed and sensitive photo-induced THz wave modulation, which can be applied as a color-selective all-optical modulator. The modulation could be excited with the modulation depth of 80% under 532 nm laser illumination (0.6 W/cm²) and modulation depth of 40% under 808 nm laser illumination (same power density) at modulation speed of 3 kHz. The modulation enhancement and color selective mechanism has been analyzed and discussed. On the one hand, the conjugate double-bonds play the critical role for photo-induced carrier enhancement at the interface between PVA and silicon structure. On the other hand, thanks to the plasmonic hot carriers’ effect of the silver nanorods, the photo-generated electrons can assemble more quickly, which accelerates the THz wave modulation directly. This plasmonic mechanism dominated the photoelectronic response and influence the THz modulation under the illumination at different wavelengths. The experimental results shows that it can be used as a color-selective modulator. This the easy and cheap bottom-up synthetic method can be introduced into many related organic polymer-silicon chips and has a good application in THz communication/imaging, due to its broadband, efficient and fast modulation on THz waves.

See supplementary material for the scheme of the self-built THz-TDS system, comparison of the modulation depth of the hybrid plasmonic PVA film and the LP-PVA film, influence of the concentration of the AgNO₃ on modulation, thickness of the thin hybrid plasmonic PVA film and Original data picture of Figure 6 respectively.

This work was supported National Key Research and Development Program of China (2017YFA0701000); National Natural Science Foundation of China (61831012, 61671491); Young Elite Scientists Sponsorship Program by Tianjin (TJSQNTJ-2017-12).