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Abstract
Optically tunable, strong polarization-dependent transmission of terahertz pulses through aligned Ag nanowires on a Si substrate is demonstrated. Terahertz pulses primarily pass through the Ag nanowires and the transmittance is weakly dependent on the angle between the direction of polarization of the terahertz pulse and the direction of nanowire alignment. However, the transmission of a terahertz pulse through optically excited materials strongly depends on the polarization direction. The extinction ratio increases as the power of the pumping laser increases. The enhanced polarization dependency is explained by the redistribution of photocarriers, which accelerates the sintering effect along the direction of alignment of the Ag nanowires. The photocarrier redistribution effect is examined by the enhancement of terahertz emission from the sample. Oblique metal nanowires on Si could be utilized for designing optically tunable terahertz polarization modulators.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The manipulation of terahertz radiation is essential for realizing promising applications, such as nondestructive evaluation, home security, biological and medical imaging, and THz communication [1–4]. The modulation of THz radiation has been demonstrated by means of two-dimensional electron-gas structures [5,6], birefringent liquid crystals [7,8], and static THz polarizers [9–12]. More recently, THz modulation has also been realized with metamaterials [13–17], plasmonic structures [18–20], organic materials [21–23], and graphene [24–27].
GaAs nanowires and carbon nanotubes have been employed for realizing THz polarizers and polarization modulators [28–32]. It has been also reported recently that tilted Ag nanowires of approximately 1 μm length and 100 nm diameter exhibit polarizer-like THz transmission. Since the skin depth of THz radiation into Ag is about 100 nm [33], the long length Ag nanowires exhibit an extremely low transmittance of approximately 6% [34]. The polarization-dependent THz transmittance was explained by the sintering effect of the Ag nanowires along the direction of alignment. The THz transmittance through Ag nanowires is expected to increase significantly as the length and the diameter of the Ag nanowire decrease. However, the polarization dependence could become weak as the length is reduced. Thus, it is worthwhile to explore other approaches that would enhance the THz transmittance and polarization modulation depth of tilted Ag nanowires simultaneously.
It is well known that a metallic property of a material on a substrate becomes weak as the substrate becomes conductive. For example, a one-dimensional metal grating on a Si substrate can be utilized as a THz polarizer. However, photocarrier excitation in a Si substrate can cause the metal grating to lose the THz polarizer property. Ag nanowires grown using oblique angle deposition on a Si substrate could exhibit a different behavior from that of the metal grating structure under optical excitation because the bottom of the Ag nanowire is grounded on the substrate only.
It is generally known that, under optical excitation, photogenerated carriers (electrons and holes) diffuse into the absorber material [35]. In a p-type Si substrate embedded with Ag nanowires, however, some electrons move to the Ag nanowires because the Fermi energy of the Ag nanowires is lower than the conduction band energy of the p-type Si substrate, causing the Ag nanowire to become negatively charged [36]. Through Coulombic attraction, these negatively charged Ag nanowires allow some holes to adhere, rather than diffuse away. The spatial redistribution of holes in the vicinity of the Ag nanowires is expected to enhance the sintering effect along the direction of alignment. The enhancement of THz emission from the Ag nanowires is also expected because the redistribution of the photocarriers can increase the electric field on the substrate surface.
In this study, the polarization-dependent transmission of THz pulses by means of tilted Ag nanowires on a Si substrate under optical excitation has been investigated. THz pulses primarily pass through nanowires without optical excitation and the transmittance is weakly dependent on the polarization. However, under optical illumination, the polarization dependence of THz transmission is stronger. The extinction ratio increases as the flux and power density of the incident light increase. The enhancement of THz emission from the sample due to the photocarrier redistribution is observed. Optically excited metal nanostructures on semiconductors can be used to control the polarization-dependent transmittance.
2. Experiments
Figure 1 provides a schematic of the experiment for the effects of optical excitation on THz transmission through aligned Ag nanowires on a Si substrate. The scanning electron microscopy (SEM) images in Fig. 1(c) show that the Ag nanowires grown using oblique angle deposition on a Si substrate have an approximate length of 500 nm, a diameter of 50 nm, and tilt angle of 70°. The Si substrate is p-type with a doping density of 1014–1016 atoms/cm3. Ag nanowires were grown on 525-µm-thick silicon substrates using the oblique angle deposition method employing a thermal evaporator [37]. The incidence angle of Ag atoms was 86°. The substrate temperature was maintained at ~40°C by a water circulation system. Ag wires of 99.99% purity were deposited on a tungsten boat. The deposition process was monitored by a quartz microbalance. Ag nanowires grow automatically primarily by the shadow effect from the seeds of Ag islands formed in the early stage of the oblique angle deposition process. The diameter, density, tilting angle, and other morphological properties depend primarily on deposition angle, substrate temperature, and seed structure, where the diffusion effect, as well as the shadow effect, play important roles. The length of the Ag nanowires is almost proportional to the deposition amount. The deposition speed, monitored by the quartz microbalance thickness monitor, was ~0.5 nm/sec. The pressure in the evaporator chamber was < 5 × 10−6 Torr during the entire deposition process.
Fig. 1 (a) Schematic of the optical excitation and polarization-dependent THz transmission through aligned Ag nanowires on a Si substrate. (b) Schematic side view of the sample. (c) SEM image (side view) of Ag nanowires deposited by oblique angle deposition technique on Si substrate. Wire length ~500 nm, wire diameter ~50 nm, wire tilt angle ~70°.
A standard THz time-domain spectroscopy system was modified to enable optical pumping to the sample [38,39]. A Ti:sapphire laser with a center wavelength of 800 nm, pulse duration of 100 fs, and repetition rate of 80 MHz was employed for generation and detection of THz pulses. A continuous-wave laser with a wavelength of 735 nm was utilized for optical pumping. Photoconductive antennas were used for emission and detection of THz pulses. The beam sizes of the THz pulse and the pumping laser on the sample are approximately 3 mm. The experimental arrangement was enclosed in a chamber which was continuously purged with dry air to avoid THz absorption by water vapor in the air.
3. Results and discussion
Figures 2(a) and 2(b) show the features of transmitted THz pulses through the Ag nanowire/Si sample, without illumination, as a function of time and frequency, respectively, when the angle, θ, between the direction of polarization of a THz pulse and the direction of alignment of the Ag nanowires is varied from 0 to 90° at intervals of 15°. The amplitudes of the transmitted THz pulse are normalized to the peak amplitude through a bare Si substrate. The sample with Ag nanowires of length 500 nm exhibits an extremely high THz transmission of up to 95% when θ = 90°. The diameter of the Ag nanowires (~50 nm) corresponds to a half skin depth of THz radiation into Ag (~100 nm). The SEM image, shown in Fig. 1, indicates that THz radiation passes through the oblique Ag nanowires nearly one time and the density of the Ag nanowires is less than 16% (an SEM image is not shown).
Fig. 2 Features of THz pulses transmitted through the Ag nanowire/Si sample without and with illumination. The amplitudes of the transmitted THz pulse were normalized to the peak amplitude of that through a bare Si substrate. (a) and (d) Variation of signal amplitude as a function of time. (b) and (e) Variation of signal amplitude as a function of frequency. Azimuthal angle (θ) is varied from 0 to 90° at intervals of 15°. (c) and (f) Dependence of the transmission (T) at the peak frequency on θ (open circles).
The amplitude of the transmitted THz pulse increases as θ increases from 0 (where the polarization of the electric field of a THz pulse is parallel to the direction of alignment of the Ag nanowires (ETHz // Ag)) to 90° (where the polarization of the electric field of a THz pulse is perpendicular to the direction of alignment of the Ag nanowires (ETHz ⊥ Ag)). Figure 2(c) represents the dependence of transmission (T) at the peak frequency on θ (open circles). The measured data were fit to the expression, T(θ) = 0.5[(T⊥ + T//)−(T⊥ − T//) cos(πθ/90)], where T⊥ and T// are the transmissions at the peak frequency when ETHz ⊥ Ag and ETHz // Ag, respectively. Here T⊥ and T// were deduced as 0.957 and 0.871, respectively.
Figures 2(d) and 2(e) show the normalized amplitude of the transmitted THz pulse through the sample versus time and frequency, respectively, under optical pumping (power = 50 mW). The dependence of T on θ and the fitted curve are shown in Fig. 2(f), where T⊥ and T// are 0.813 and 0.549, respectively. The optical pumping reduces the amplitude of the transmitted THz pulse because the photocarriers in the Si substrate increase the reflectance of THz radiation. It is worth noting that the optical pumping increases the difference between T⊥ and T// and thus, enhances the polarization dependency of the sample. As mentioned above, this is probably because the spatial redistribution of holes concentrated around the Ag nanowires enhances the sintering effect of the Ag nanowires along the direction of alignment.
Figure 3 shows the dependence of T on θ and the fitted curves from θ = −15 – 210° at intervals of 15° when the optical pumping power is varied from 0 to 100 mW at intervals of 25 mW. The dotted lines indicate T of a bare Si substrate. The fluctuation is caused by the mismatch between the centers of the pumping laser and THz pulse. The fit parameters are summarized in Table 1. As the pumping power increases, (T⊥ − T//) increases and the polarizer-like property prevails even though the amplitude of the transmitted THz pulse is determined to decrease.
Fig. 3 Dependence of T on θ and the fitted curves from θ = −15 to 210° at an interval of 15° when the power of the optical pumping changes from 0 to 100 mW at an interval of 25 mW. The parameters of the curves are summarized in Table 1. The dotted lines indicate T of a bare Si substrate.
Table 1. T⊥ and T// for the fitting curve of T(θ) = 0.5[(T⊥ + T//) - (T⊥ - T//) cos(πθ/90)] as a function of the pumping power.
The extinction ratio (ER), defined by (T⊥ − T//) / T⊥, is a typical characteristic by which the polarizer performance can be evaluated. Figure 4 shows the variation of the ER of the sample as a function of pumping power. The ER increases as the pumping power increases and becomes saturated in the vicinity of 100 mW. The value of ER is 0.4 at the pumping power of 80 mW. This indicates that T can be tuned continuously to an arbitrary value between T⊥ and 60% of T⊥ by varying θ.
Thus, THz transmission through tilted Ag nanowires on a Si substrate can be controlled by rotation of the sample as well as by varying the power of the optically pumped light. The rotation sensitivity of THz transmission can be estimated by T(θ)/dθ. From the fitted curve of T(θ), T(θ)/dθ can be deduced as, T(θ)/dθ = π(T⊥ − T//) sin(πθ/90)/180. The rotation sensitivity has a maximum value of π(T⊥ − T//)/180 at θ = 45°. The maximum value increases with the power of the pumping beam.
The strong polarization-dependent THz modulation under optical excitation can be explained by the spatial redistribution of holes in the vicinity of the Ag nanowires to enhance the sintering effect along the direction of alignment. The holes concentrated around the Ag nanowires can increase the electric field normal to the substrate surface and the distancebetween the centers of mass of the electrons and holes. Thus, the redistributed photocarriers are expected to enhance the THz emission from the sample.
The THz emission from the sample was investigated as a function of θ to examine this conjecture. A standard THz time-domain spectroscopy system was modified for the THz emission experiments. A Ti:sapphire laser with a center wavelength of 800 nm, pulse duration of 100 fs, and repetition rate of 80 MHz was employed for generation and detection of THz radiation. A photoconductive antenna was used for detection of THz radiation. Figures 5(a) [Figs. 5(c) and 2(b)-2(d)] show the amplitudes of THz pulses emitted from the sample versus time and frequency, respectively, when θ varies from −30 (90) to 90° (210°) at intervals of 15°. The input power of the laser beam is 300 mW. The spectral amplitudes of the THz pulses were normalized to the peak amplitude of that at θ = 90°. Figure 5(e) shows the dependence of the peak amplitudes, P, of THz pulses on θ (circles). The curve was fitted to a sinusoidal curve with the expression, P(θ) = 0.5[(P⊥ + P//) − (P⊥ − P//) cos(πθ/90)], where P⊥ = 1.0 and P// = 0.38, respectively. The black dotted line indicates the peak amplitudes of THz pulses emitted from a bare Si substrate.
Fig. 5 THz pulses emitted from the sample. (a) and (c) Variation of signal amplitude as a function of time. (b) and (d) Variation of signal amplitude as a function of frequency. (e) Dependence of the peak amplitude P of THz pulses on θ (circles The black dotted line indicates the peak amplitude of THz pulses emitted from a bare Si substrate. (f) Dependence of the reflectance, R, of the laser beam with a center wavelength of 800 nm from the sample on θ.
Figure 5(f) shows the dependence of the reflectance, R, of the laser beam with a center wavelength of 800 nm from the sample on θ. The reflectance of the laser beam from a bare Si substrate (dotted line) is approximately 34%. The reflectance R(θ) exhibits sinusoidal behavior because the aligned Ag nanowires with lengths of 500 nm can produce polarization effects on the laser beam (R ~63% at θ = 0° and ~5% at θ = 90°). The amplitude of the THz pulse emitted from the sample is proportional to the absorption of the laser beam, A(θ) = 1-R(θ). The dependence of the peak amplitude on θ is easily understood from that of A on θ.
It should be emphasized that the peak amplitude of the THz pulse at θ = 0° is larger than that of the THz pulse from a bare Si substrate, even though the absorption of the former at a wavelength of 800 nm is approximately half that of the latter. Moreover, the peak amplitude of the THz pulse at θ = 90° is almost five times that of the THz pulse from the substrate. The observed enhancement of THz emission from the sample implies that our conjecture is reasonable.
It has been recently reported that the transmission of a THz pulse through metamaterial [13,14,17,40] and plasmonic structures [18–20] can be modulated by optical pumping around the resonance frequencies of these structures. Optically excitable organic materials on Si have also been used to modulate THz transmission [21–23]. Tilted Ag nanowires on Si would be potentially useful in designing optically tunable THz polarization-dependent modulators which offer high performance and multi-functional properties in THz devices.
4. Summary
In summary, strong polarization-dependent THz transmission through Ag nanowires on a Si substrate was investigated under optical illumination. This manifests in a significantly polarizer-like property. Enhanced THz emission from the sample is also observed. The enhanced polarization-dependence of THz transmission and THz emission are a consequence of the redistribution of photocarriers, which enhances sintering along the tilt-aligned direction of Ag nanowires and the surface field. The study provides a design feasibility by which tilted metal nanowires on a suitable semiconductor substrate can function as an optically tunable THz polarization modulator.
Funding
GIST Research Institute (GRI) grant funded by the GIST in 2018 and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2015R1D1A1A01059958 and 2016R1D1A1B03933438).
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