Spectral Shape Control of Laser-Induced Terahertz Waves from Micro Split-Ring Resonators Made of Metallic Nanostructures

Abstract

Efficient terahertz sources with controllable characteristics such as frequency range and polarization state are being rapidly researched and developed to suit various practical applications. To address this need, we realized the idea of combining micro- and nano-sized materials by fabricating micrometer-scale split-ring resonators made of a metal nanostructured film. We found that the peak frequencies of the emitted terahertz waves are in good correspondence with the terahertz resonance frequencies of the split-ring resonators. A possible mechanism is that a surge current was induced inside the split-ring resonators as a result of photoexcitation with the help of plasmon resonance around nanostructures, and the induced current emitted terahertz waves reflecting the resonance properties of the split-ring resonators. Although the silver nanostructures constituting the rings are random and homogeneous, the induction of the current parallel to the sample surface is enabled by the oblique incidence excitation, which breaks the symmetry along the sample surface. The present study shows the possibility of making compact terahertz emitters with flexibly tunable spectral shape, potentially leading to the development of terahertz sources optimized for specific spectroscopic uses.

1. Introduction

Terahertz technology has been actively employed in a wide range of fields including basic science, information communication, and safety inspection [1,2,3,4,5,6,7,8]. Among them, terahertz spectroscopy is a powerful tool for material characterization because it can quantitively evaluate the density of free carriers and detect various resonances in materials such as the lattice vibration, spin precession, molecular rotation, and intermolecular vibrations [9,10,11,12,13,14]. To make a terahertz spectroscopy have better performance or lower cost, various types of terahertz wave sources have been developed, such as photoconductive antennas, nonlinear crystals, and spintronic terahertz emitters [15,16,17,18,19,20,21]. Among them, terahertz emitters based on a metallic nanostructure have been studied by several groups [22,23,24,25,26,27,28,29,30]. These metallic nanostructures emit terahertz waves with the help of local field enhancement induced by surface plasmon resonance, and are expected to serve as a thin and large-area terahertz source. The use of metal nanostructure avoids the phonon bands in the terahertz region that are often encountered in nonlinear semiconductor or substrates of photoconductive antenna insulators. In this regard, a terahertz device fabricated with metallic nanoparticles has advantages over traditional photoconductive antennas and nonlinear crystals. One of the important characteristics of a terahertz emitter is the spectral shape of the emitted wave. Developing a method to control the emitted spectral shape will be useful for terahertz spectroscopy because it enables us to make the reference terahertz spectrum strong in the frequency region of interest to improve the signal-to-noise ratio while making it weak in other frequency regions to avoid detector saturation. Several techniques to control the emitted spectral shape have been demonstrated so far, such as shaping excitation light pulses in advance of exciting terahertz emitters [31,32] and filtering or synthesizing terahertz waves after generation [33,34]. However, these methods require apparatuses for spectral shaping in addition to the THz emitters, which makes the system complicated. As a strategy providing more compact and monolithic systems, it has also been proposed [35] and demonstrated to fabricate laser-induced THz emitters specially designed to generate aimed spectral shapes [30,31]. For example, Lee et al. [36] made poled lithium niobate crystals whose domain structures were designed to modify the spectral shapes of emitted terahertz waves. More recently, Polyushkin et al. [37] demonstrated the control of terahertz spectra emitted from grating structures with metallic nanostructures. Both of these previous studies [36,37] utilized the interference of terahertz waves originating from different areas of the source to control the spectra. Our idea is that the use of resonance will enable us to realize more flexible spectral control of emitted terahertz spectra. In this work, we report on terahertz emission from split-ring resonators (SRRs) made of metallic nanostructures under laser pulse excitation. The advantage of SRRs over wire and dot patterns is the versatile tunability of uneven resonant frequencies, which can match absorption peaks of the chemicals under investigation. We confirmed that the spectral peaks of emitted terahertz waves appear near the dip frequencies of the transmittance of the SRRs. The mechanism of the observed results is discussed based on the two types resonances: one is plasmon resonance in optical region around the metal nanostructures, and the other is the resonance of the surge currents inside the SRRs.

2. Materials and Methods

We used metal nanoparticle ink to make metallic nanostructures. Metal nanoparticle ink has been developed for printed electronics [38,39,40] and utilized to make metamaterials [41,42], photoconductive antennas [43,44], and quantum point contacts [45]. It was found that metal nanoparticle ink spontaneously forms random nanostructures by being baked, and so with this ink we can easily make metallic nanostructures in large area that can emit terahertz waves by laser excitation [25]. The use of silver nanoparticle ink has the advantage that there is no need for expensive and sophisticated nanofabrication techniques. Following the detailed process in [25], we spin-coated silver nanoparticle ink (ULVAC, Ag1TeH, Chigasaki, Japan) on a 0.5 mm thick fused silica substrate and baked it at 220 °C for 1 h to prepare a silver nanostructured film. Arrays of SRRs were fabricated by laser machining as follows: the film was moved by the programmatically controlled stage (Aerotech, ANT95-XY) and an output from a Ti:sapphire regenerative amplifier (Spectra-Physics, Solstice, Santa Clara, CA, USA; center wavelength 800 nm, pulse width ~50 fs) was focused onto it to blow away the unnecessary part of the film by laser ablation. Figure 1a,b show the image of one of our samples and the enlarged image of one SRR inside it, respectively. This sample, which we call S80 in the following, has an array of square-shaped SRRs with the outer length of each side L_o of 80 µm, the arm width w of 20 µm, and the gap width g of 20 µm. In addition to this sample, we fabricated samples containing SRRs with outer lengths L_o of 55 µm and 40 µm, which we call, respectively, S55 and S40 in the following. The period p of the SRR array of all three types of samples was identically 100 µm. The parameters of our sample are listed in

Table 1. List of the samples. L_o: outer length of the side of the SRR. w: arm width. g: gap width.

Name	Lo (µm)	w (µm)	g (µm)
S80	80	20	20
S55	55	15	25
S40	40	10	20

. By using a scanning electron microscope (SEM), we confirmed that nanostructures were successfully formed on the silver film constituting SRRs (Figure 1c).

In the terahertz emission experiment described in detail in [25], the sample was excited by femtosecond laser pulses from a Ti:sapphire regenerative amplifier (Spectra-Physics, Solstice, Santa Clara, CA, USA; center wavelength: 800 nm; pulse width: ~50 fs; repetition rate: 1000 Hz) at an incidence angle of 45°. A chopper was used to modulate the repetition frequency of the optical excitation pulses to 500 Hz, and its signal serves as the reference for lock-in amplifier detection to reduce noise significantly. The gap direction of the SRRs was perpendicular (Figure 1d) or parallel (Figure 1e) to the incident plane of the excitation laser pulses. The excitation laser beam had a diameter of ~3.5 mm and was much larger than the size of one SRR in our samples. The excitation laser pulse is p-polarized and has a pulse energy of ~100 µJ, which corresponds to the peak intensity of ~28 GW/cm². The terahertz wave emitted toward the specular reflection direction of the laser beam was detected by using the electro-optic sampling method with a 1 mm thick (110) ZnTe crystal. To remove the absorption by water vapor, the path of the terahertz waves was purged with dry air. To evaluate the resonance frequencies of our samples, we also measured their terahertz transmission measurement by using a terahertz time-domain spectroscopy (THz-TDS) system (Advantest, TAS7500-TS, Chiyoda-ku, Japan), in which the incident angle and polarization of the terahertz wave was 45° and parallel to the incident plane (p-polarization), respectively. To remove the effect of multiple reflection, a Tukey window [46] with the width of 12.8 ps and rise and fall time of 2 ps was applied at the maximum peak of the signal before calculating the spectra.

3. Results and Discussion

Figure 2a shows the results of the THz-TDS measurement for the S80 sample when the gap direction of the SRRs was perpendicular to the terahertz wave polarization, using a bare fused silica substrate as the reference. Around 0.50 THz and 1.1 THz, the transmission spectrum (red solid curve, left axis) had strong dips and the phase shift (purple dotted curve, right axis) showed a jump. The dip and phase jump around 0.5 THz originate from the lowest order of resonance of the SRR ascribed to the current circulating in the ring, whereas those around 1.1 THz are from the second-lowest resonance due to the half wave resonance in a sideline of the SRR [42,47]. The oscillation of the transmission spectrum above 1.1 THz is not only due to the higher-order resonances but also due to the scattering occurring above the diffraction frequency $f_{\mathrm{d}}=c/\left[p\left(n_{\mathrm{s}}+\sin \alpha \right)\right]=$ 1.1 THz, where c is the speed of light in vacuum, p = 100 $\mu \mathrm{m}$ is the period of the SRR array, n_s = 1.97 is the refractive index of the fused silica substrate, and α = 45° is the incident angle [48]. When the gap of the SRR is parallel to the terahertz wave polarization, the terahertz transmittance and phase shift spectra of the S80 sample became as shown in Figure 2b. The transmission spectrum had a wide dip around 1.1 THz originating from the half-wave resonance of two sidelines of the SRR [47]. The oscillation in the transmission spectrum above 1.1 THz is again due to diffraction and higher-order resonances. The solid curve in Figure 2c shows the power spectrum of the terahertz wave emitted from the S80 sample when the gap direction of the SRRs was perpendicular to the incident plane of the excitation laser beam. The dynamic range was 40 dB at 1.1 THz. The signal-to-noise ratio at 1 THz, calculated by the emission power divided by its standard error of the seven iterative measurement results, was as high as 51, which demonstrates the reliability of the observed spectral shape. For comparison, the spectra emitted from the film without being etched by laser machining are also shown by the dashed curve. While the spectrum from the unworked film was broadly extended to ~3 THz with a peak around 1.0 THz, that emitted from S80 had two sharp peaks around 0.49 and 1.1 THz. These peak positions are close to the dip frequencies of the transmittance shown in Figure 2a, corresponding to the circulating current and half-wave resonance in the SRR as described above. On the other hand, when the SRR gap direction of S80 was parallel to the laser incident plane, the emitted terahertz spectral shape was changed to the solid curve in Figure 2d: the peak around 0.50 THz observed in Figure 2c disappeared and the peak around 1.1 THz became broader. These characteristics correspond to that of the transmittance spectrum in case the gap was parallel to the electric field (Figure 2b), which had a broad resonance dip around 1.1 THz.

The observed correspondence between the resonant frequencies of the SRRs and the peak frequencies of the emitted terahertz spectra suggests that the terahertz waves are emitted through the following steps: First, through second-order or high-order nonlinear processes, the femtosecond laser pulse induces a terahertz nonlinear surge current on the surface of the silver nanoparticles inside the SRR. Then, this time-varying surge current radiates terahertz waves [49], and its power spectra will have a peak at the SRR resonance frequencies because the frequency component of the induced current is large at the SRR resonant frequencies. Since this mechanism needs a current flow inside the SRR, a driving force of electrons parallel to the SRR surface is required to be imparted by laser pulse irradiation for the first step. According to the preceding reports, the photoexcited metal nanostructures support the plasmon resonance and have an enhanced local field around them, which results in strong polarization and even electron emission from the surface [22,27,50]. The silver nanostructures constituting the SRRs are random and homogeneous, and therefore are symmetric toward the direction parallel to its surface. If the optical beam is irradiated normally to the sample surface, this symmetry forbids THz generation from even-order processes, and only weak THz signals from higher odd-order processes can be generated, if any. In our experiment, the excitation laser beam impinged upon the sample at an incidence angle of 45°, which broke the symmetry along the sample surface (Figure 2e).

One possible origin of the electron driving force inside the SRRs is the coupling between the radiated terahertz wave and electrons inside SRRs: even when the terahertz wave is first generated by a current due to electron emission or by a nonlinear polarization perpendicular to the surface, the wavefront of the radiated terahertz waves is at an angle of 45° to the sample surface due to the oblique-incidence excitation (right part in Figure 2e). It indicates that there is an electric field component parallel to the sample surface, which can be resonantly coupled with the SRRs. Another possibility is the electron driving force parallel to the sample surface directly from the laser pulse excitation. This may be caused by the plasmon-drag effect [51], in which surface plasmon drags carriers to its propagation direction.

To further confirm the relationship between the resonant properties of the SRRs and the emitted terahertz spectral shape, in Figure 3, we compare the results of S80 (green dotted curve) with the samples with a different SRR size: S55 (blue dashed curve) and S40 (red solid curve). In the transmittance spectra (Figure 3a), the lowest-frequency dip originating from the circulating current resonance was around 0.50, 0.72, and 0.94 THz for S80, S55, and S40, respectively, showing that the frequency of the circulating current resonance became higher as the SRR size became smaller. The second lowest-frequency dip due to the half-wave resonance of the SRR sideline also became higher for the smaller SRRs (around 1.2, 1.5, and 2.1 THz for S80, S55, and S40, respectively) but became broader because the ratio of the SRR sideline length to its width is lower for the smaller SRRs, resulting in less sharp resonance. The corresponding characteristics to these trends in the transmittance spectra were clearly found in the emitted terahertz spectra (Figure 3b): for the samples with smaller SRRs, the peaks shifted to higher frequencies and the second-lowest-frequency peaks became broader. These results demonstrate the controllability of the emitted terahertz spectral shape by forming a metal nanostructured film so that it has resonance in the terahertz region. The peak frequencies in our samples can be used for further tuning and to more accurately serve the security purpose of the detection of dangerous substances, such as the C-4 explosive, which has several distinct absorption peaks at 0.87 THz, 1.5 THz, and 2 THz [4], or to detect trace amount of antibiotics abused in husbandry and food, such as kanamycin [52].

4. Conclusions

In summary, we studied terahertz emission from SRRs with metallic nanostructures irradiated by femtosecond laser pulses. It was observed that the peaks of the emitted terahertz spectra have good correspondence with the resonant frequencies of SRRs. The results show that both the resonance inside the microstructures in the terahertz frequency region and the plasmon resonance around the nanostructures in the optical region contribute to the terahertz emission in our samples. It is also suggested that, due to the laser pulse irradiation of the metallic nanostructured film, the surge current in the direction parallel to the film is induced to emit terahertz waves, which are attributed to the oblique-incidence excitation breaking the symmetry along the sample surface. Our study proved that terahertz spectral shape can be flexibly controlled by changing the form of the microstructures. This technology will be potentially useful as a basis for designing a compact terahertz emitter with a spectral shape suitable for specific applications.