1. Introduction

Terahertz (THz) wave technology is promising for biological imaging, security inspection, and communication due to the low photon energy, high transparency, and broad band properties of THz radiation [1–4]. Recent progress, e.g., quantum-cascade lasers [5], THz wave generation through a nonlinear crystal [6], and THz time-domain spectroscopy [7], is promoting THz technology, making it a rapidly growing field. Modulation of THz waves for practical applications has been demonstrated with filters, absorbers, polarizers, frequency selective surfaces, metamaterials, plasmonic devices, and photonic crystals [8–14]. A variety of modulation schemes have been demonstrated, e.g., electronic, all-optical, thermal, magnetic, and nonlinear modulations. In particular, all-optical switching is advantageous because of the possibility of low power and extremely fast switching using ultrafast optical pulses [15, 16]. Here, the modulation speed is mainly determined by the lifetime of the photogenerated carriers.

All-optical modulation of THz waves has been largely demonstrated by using conventional semiconducting materials [9, 17]. The modulation depth can be boosted when combined with micro- and nano-structures, such as metamaterials and plasmonic structures. In general, the addition of a modulator in the path of the optical beams induces inevitable transmission loss. In addition, the presence of substrate materials tends to cause modification of the spectral amplitude and the phase of the transmitted light. Novel functional materials may help increase the possibility of fabricating highly effective optical filters. For instance, network films consisting of single-walled nanotubes have been suggested as a strong candidate for various optical filters, including polarizers, plasmonics, and metamaterial devices operating in the THz frequency range, possibly replacing conventional metal films [13, 18, 19].

In recent years, wide-gap semiconductors, e.g., ZnO, ZnS, and SiC, have attracted increased attention due to their UV photoresponse [20–23]. Among these alternatives, ZnO-based materials have a large bandgap and high mobilities, and ZnO thin films have recently been commercialized for use in transparent conducting electrodes [24–26]. In particular, ZnO nanowires (NWs) are expected to have a good UV response due to their large surface area to volume ratio, enhancing the performance of UV-based devices; UV-selective photoconduction is one potential application of these NWs.

In this work, we present virtually transparent THz modulators that minimize transmission loss, and hence, exhibit a good extinction ratio in terms of absorption. We characterized the films by time-domain THz spectroscopy with a 355 nm continuous-wave (CW) laser and investigated the change in the complex refractive indices in the THz range. In addition, the UV intensity dependence helps us characterize the surface trap states of ZnO NWs.

2. Experimental results and discussion

ZnO NWs were synthesized within a horizontal quartz tube furnace (inner diameter 5 cm) at atmospheric pressure without using any catalyst. A mixture of ZnO and graphite powders (2 ~3 g), with a weight ratio of 1:1, was heated to 1100-1200 °C and the vaporized growth species was transported by a gas flow of 1000 sccm N₂ and 30 sccm O₂. Cotton-like white products consisting of ZnO NWs were obtained in the low-temperature region of the tube (between ~200 °C and room temperature). The typical growth time was 30 min. To fabricate free-standing ZnO network films from the cotton-like NWs, we used a simple filtration method as follows. First, a ZnO NW suspension solution at a concentration of 1 mg/mL was prepared by ultrasonically dispersing the NWs in isopropanol. Second, the NWs were filtered onto a nano-porous anodic aluminum oxide (AAO) membrane with a diameter of 4.3 cm and pore size of 200 nm. Third, a network film of ZnO NWs on an AAO membrane was dried in air at 100 °C for 1 h. Finally, a thin sheet of ZnO NW film was detached from the membrane filter and dried. Figure 1(a) shows a picture of a free-standing ZnO NW film. The range of film thickness is 10-50 μm, depending on the amount of solution used. Such a free-standing film is beneficial in various optoelectronics applications, including applications in the THz frequency region, as it can eliminate the substrate’s multiple reflection effects that frequently obscure the transmittance spectra. Furthermore, the film can be transferred to various substrates, if necessary. Figure 1(b) shows a scanning electron microscopy image for the free-standing ZnO NW film. The film exhibits a continuous network structure with randomly oriented ZnO NWs.

 

Fig. 1 (a) Photograph of a free-standing ZnO NW network film. (b) Scanning electron microscopy (SEM) image of a ZnO NW network film. (c) Schematic of THz time-domain spectroscopy set-up.

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THz transmission characteristics of the ZnO NW films were measured using THz time-domain spectroscopy (TDS) with an electro-optic sampling technique [13, 27]. We used a photoconductive antenna as an emitter and a ZnTe crystal as a detector, as shown in Fig. 1(c). A femtosecond laser pulse of λ = 800 nm was used to generate and detect the transmitted pulses. The ZnO NW film was placed at the focus of the THz beam (focal spot size ~1 mm). In the switching experiments, we illuminated the film with a CW UV laser at 355 nm (diode pumped lasers Zouk, Cobolt), whose spot of 1 mm was placed at the focus of the THz beam. The energy of the UV laser is higher than the ZnO bandgap (3.37 eV), and hence, the UV laser can induce significant amount of the carriers in the ZnO NWs with relatively low light intensity.

The free-standing ZnO NW films were characterized at room temperature in a nitrogen-purged environment. We measured the time-dependent electric fields of the THz pulses transmitted though the ZnO NW film, as shown in Fig. 2(a). The transmission amplitude measured through free-space is used as a reference (gray line). The transmission amplitude decreases only by about 4% even in the presence of a 15-μm-thick ZnO NW film (blue line), with respect to the free-space transmission amplitude. In other words, the ZnO NW network film is virtually transparent against the THz fields without causing a significant phase delay or field attenuation. This is primarily because of the low packing density of the film, which is typically ~7–8%. By contrast, the transmission amplitude decreases noticeably with UV laser illumination of the film with low-intensity of 10 mW (red line). With respect to the reference beam, the transmission amplitude and intensity yield ~85% and 72%, respectively, where the intensity is proportional to the square of the amplitude. This indicates that the free-standing ZnO NW network films can be used as a platform for all-optical, active THz switching materials with negligible transmission loss when switched on.

 

Fig. 2 (a) Time trace of THz transmission through a 15-μm-thick ZnO NW film. (b) THz transmittance spectra with (red) and without (blue) UV laser illumination of the ZnO NW film. (c) Modulation depth induced by the UV laser.

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Figure 2(b) shows the transmission intensity spectra for the ZnO NW film with (red line) and without (blue line) UV laser illumination. The transmission spectra were obtained by applying a fast Fourier transform (FFT) to the time traces of the THz transmission and then normalizing with the reference (free-space) spectrum. Without UV illumination, the transmission through the ZnO NW film is close to unity, without a distinct spectral fingerprint except for the water absorption peaks. With UV laser illumination, the transmission decreases over the entire frequency range studied. This indicates that ZnO-based switching devices can be operational over a broad frequency range. In Fig. 2(c), the transmission intensity ratio is shown between the on (without UV laser) and off (with UV laser) conditions extracted from Fig. 2(b). The modulation depth of the transmission intensity reaches more than 20% and is uniform over a broad frequency range.

We note that UV penetration depth (4 μm) is smaller than the thickness of the film (~15 μm), limiting the active switching volume of the film. A modulation depth of 60% has been reported in an optically controllable THz modulator based on semiconducting materials, but with a much higher intensity of light [28]. More recently, more than 90% modulation depth has been reported in graphene-Ge hybrid systems (with 1.55 μm wavelength and ~4 W/cm²) [29]. In our case, the modulation depth saturated at 35% with a very low light intensity of ~30 mW/cm² (as will be shown later). This light intensity is more than an order of magnitude lower than that in the aforementioned cases, even if we consider the differences in the modulation depth. This is an unexpectedly large response that cannot be achieved in bulk or thin-film semiconductors. In our case, the modulation depth could be further enhanced by increasing the effective switching volume in the films with a novel design, e.g., a design in which the UV laser is illuminated on both sides of the film.

Figure 3 shows the frequency-dependent complex refractive indices ($\tilde{n}=n+i\kappa$) for a ZnO NW film with and without UV laser illumination. $\tilde{n}$ was extracted from the amplitude and phase information in Fig. 2(a) and the relations $n=(c/\omega d)\Delta \varphi +1$ and $\kappa =-(c/\omega d)\ln \rho$, where ω is the angular frequency, c is the speed of light in a vacuum, d is the thickness of the film, ρ is the transmission amplitude, and $\Delta \varphi$ is the phase shift. The value of the real part, n, is close to unity, yielding ~1.25 at 1 THz, as shown in Fig. 3(a). In addition, there is no significant change for n even with UV laser illumination. The real part of ac conductivity ($\sigma$) can be extracted from the relations $\sigma ={\epsilon }_0\omega {\epsilon }_i$ and $\tilde{\epsilon }={\tilde{n}}^2$, where ${\epsilon }_0$ is the free-space permittivity and ${\epsilon }_i$ is the imaginary part of the complex dielectric constants ($\tilde{\epsilon }={\epsilon }_r+i{\epsilon }_i$). The conductivity exhibits large enhancement upon UV laser illumination, increasing from 0.17 S/cm to 0.76 S/cm as shown in Fig. 3(b). Therefore, the transmission decrease can be explained by the increased absorption originating from the enhanced conductivity associated with carrier generation. Note that the optical and ac electrical constants in Fig. 3 are effective values of the entire film, and if we consider the UV penetration depth smaller than the thickness of film, the UV-induced conductivity reaches 2.38 S/cm.

 

Fig. 3 (a) Frequency-dependent real part of the refractive indices for a ZnO NW film with (red) and without (blue) UV laser illumination. (b) Conductivity with (red) and without (blue) UV laser illumination.

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The time response of the UV-induced switching behavior has been demonstrated in Fig. 4. We first recorded in situ THz transmission spectra as a function of time with a step of ~9 s. We extracted the absorptance measured at 1 THz as a function of time (black solid line in Fig. 4(a)) from the THz transmission spectra while switching the UV laser between on and off repeatedly, as indicated by the dashed blue line. Figure 4(b) is a plot of THz absorptance as a function of time when the UV light is turned on at t = 0 s, whereas Fig. 4(c) shows the decrease in THz absorptance as a function of time right after the light is turned off at t = 0 s. The time response is characterized by the relatively fast rise time of 6.5 s and the slow decay time of ~90 s. The slow response time is a strong indication that the UV-induced change is closely linked to the trap states distributed on the ZnO NW surfaces whose carrier lifetime extends over tens of seconds, which has been reported in UV photoresponse experiments on ZnO NWs [21].

 

Fig. 4 (a) THz absorptance as a function of time with UV laser turned on/off repeatedly (black line). (b) THz absorptance as a function of time. The UV laser is turned on at t = 0 s. Solid line is fit to the data (Inset) Schematic of trapping mechanisms of the photogenerated hole carriers under UV illumination. (c) THz absorptance as a function of time with the UV laser turned off at t = 0 s. Solid line is fit to the data (Inset) Schematic of recovery of surface trap states when the UV laser is turned off.

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When a photon with energy higher than the bandgap is incident on the ZnO NW, an electron–hole pair is generated. The positively charged hole neutralizes the chemisorbed oxygen and defects states (such as the oxygen vacancies), thereby releasing the electron back to the conduction band, increasing the conductivity of the sample, as schematically illustrated in the inset of Fig. 4(b) [30]. Specifically, before illumination by UV light, oxygen molecules are adsorbed on the oxide surface; these molecules capture the free electrons present in the n-type oxide semiconductor, i.e.$O_2(g)+e^{-}\to O_2^{-}$, and a low-conductivity depletion layer is formed near the surface. After illumination, electron-hole pairs are photogenerated, i.e. $h\nu \to e^{-}+h^{+}$; holes migrate to the surface along the potential slope produced by band bending and discharge the negatively charged adsorbed oxygen ions, i.e. $h^{+}+O_2^{-}\to O_2(g)$; consequently, oxygen is photodesorbed from the surface. Therefore, the large enhancement in UV-induced THz absorption is possible until the surface trap states are saturated, as in the case of photoconductive sensors with extremely high internal gain [21, 31]. Conversely, when the UV laser is turned off, the holes in the trap states start to recombine with the electrons radiatively or nonradiatively; hence, the transient ZnO conductivity returns very slowly toward the initial states. Therefore, the slow decay in the THz absorption reflects the lifetime of the trap states associated with the physical adsorption of O₂ molecules.

Finally, we studied the dependence of THz absorptance on UV intensity (I_UV). Since the transmission change is primarily due to the surface states, saturation behavior is expected as the surface states are filled due to continuous UV illumination. We plotted the absorptance as a function of the laser intensity in Fig. 5, which shows the saturation behaviors at low UV intensity. The THz absorptance, A_THz, will be proportional to the UV-induced carrier density, N_UV, and hence, can be expressed by A_THz ∝ ηI_UV, where η is the intensity-dependent response function, which is linked to saturation behaviors. η has been described by the following expression:

 

Fig. 5 (a) The dependence of THz absorptance on UV intensity. (Inset) Intensity-dependent response function, η (black boxes). The red solid line is fit to the data.

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3. Conclusion

In conclusion, we performed THz-TDS experiments on free-standing ZnO NW network films, which exhibit a large absorption modulation of ~35% when illuminated by a low-intensity UV laser (~30 mW/cm²). This increase in modulation is more than an order of magnitude higher than that of conventional THz modulators. In addition, because the ZnO NW films were virtually transparent to the THz waves without UV illumination, they may be used as a promising platform for low-loss, all-optical THz modulators. UV-induced changes in the complex refractive index show the large enhancement in ac conductivity upon UV laser illumination without a significant change in the real part of the refractive index. The time response was characterized by a relatively fast rise time and slow decay time that extends over one minute. This is a strong indication that the conductivity enhancement is associated with the filling of trap states on the ZnO NW surfaces. The THz attenuations showed clear saturation behavior with respect to UV intensity. From the saturation intensity, we extracted the surface trap density on the ZnO NW surface (2.5 × 10¹¹ /cm²), which is responsible for the UV-induced THz attenuations. Consequently, we believe that ZnO NW network films will provide an efficient platform for fabricating flexible optoelectronic devices, such as all-optical switching devices, plasmonic filters, and metamaterials.