Terahertz radiation enhancement in photoconductive antennas with embedded split-ring resonators Download: 784次
In recent years, the most popular mechanisms for generating broadband terahertz (THz) radiation are based upon optical rectification[1], surface generation[2], and photoconductivity[3]. The latter approach is common, but it has a lower radiation power than the others[4]. Therefore, improving the radiation power of the photoconductive mechanism is an important capability that has significant practical ramifications[5,6]. To achieve this enhancement in photoconductive antennas (PCA), researchers have proposed full-wave numerical techniques based on a coupled solution to the hydrodynamic transport equations, Poisson’s drift diffusion, and Maxwell’s equations[7]. These can simulate PCA with various active region geometries[8]. Researchers have also discussed the appropriate numerical values for impedance matching conditions and experiments to improve the efficiency of PCA[9,10]. The latter include a thin semiconductor layer sandwiched between a dielectric hyper hemispherical lens and a metal substrate[11], a quasi-three-dimensional (3D) post-array[12], and a dielectric structure with periodic GaAs strips grown at low temperatures[13]. A high efficiency THz pulse can be generated by pumping near-infrared light on large-area PCAs[14], nano-plasmonic PCAs with metal nano-islands[15], or interdigitated PCAs based on plasmon electrodes[16]. A lot of semiconductor materials—such as single crystal and polycrystalline ZnSe substrates[17], III-V semiconductors and graphene[18], epitaxial embedded rare-earth arsenide (ErAs and LuAs) nanoparticles in superlattice structures[19]—were researched to enhance THz radiation. PCAs using InGaAs-InAlAs multilayer heterostructures[20] and SI-GaAs and -GaAs wafer surfaces grown with aluminium nitride (AlN) thin films[21] were proposed to achieve this enhancement. The structures of the THz PCA studied include optical nano-antennas[22], fractal geometries[23], tapered helix monopole[24], array hexagonal metal nanostructures[25], large area interdigitated[26], 3D plasmon contact electrodes[27], nano-structured electrodes[28], thin-film plasmon electrodes[29], and plasmon contact electrodes[30]. In this Letter, we propose novel microstructure PCAs (MSPCA) that consist of split-ring resonators (SRRs) and dipole PCAs (D-PCAs). These MSPCAs provide a new way to robustly enhance the radiation power; we obtain phase shift and raise the maximum bias voltage by changing the pump position of the femtosecond laser.
The D-PCA with SI-GaAs as the photoconductive material is illustrated in Fig.
Fig. 1. D-PCA and MSPCA. (a) Micrograph of D-PCA. (b) Structure and parameters of SRRs. (c) Micrograph of MSPCA-A. (d) Micrograph of MSPCA-B.
The D-PCA, MSPCA-A, and MSPCA-B are tested with a THz time domain spectroscopy system, as illustrated in Fig.
The test steps of the PCA are listed as follows. System.Xml.XmlElementSystem.Xml.XmlElement
With the femtosecond laser pumped onto the inter-electrode gap of the MSPCA and the inter-electrode gap and electrode edge of the D-PCA, there are slight differences in the THz time domain spectroscopy, as shown as Fig.
Fig. 4. THz characteristics of femtosecond laser pumping on the D-PCA, MSPCA-A, and MSPCA-B. Comparison of (a) amplitude and normalized amplitude of THz time domain spectra, and (b) power and normalized power of THz frequency domain spectra.
Compared to the femtosecond laser pumping on the electrode edge of the D-PCA, the THz radiation is noticeably enhanced. The absolute value and normalized value of THz amplitude and power are illustrated in Fig.
Fig. 5. Comparison of the characteristics of the THz radiation for femtosecond laser pumping on the split positions of SRR of MSPCA and on the electrode edge of the D-PCA. (a) THz amplitude and normalized THz amplitude of MSPCA-A and D-PCA, (b) THz power and normalized THz power of MSPCA-A and D-PCA, (c) THz amplitude and normalized THz amplitude of MSPCA-B and D-PCA, and (d) THz power and normalized THz power of MSPCA-B and D-PCA.
Compared to the case where the laser is pumped onto the left 5 μm of the center of split positions of the SRR on MSPCA-A or MSPCA-B, the THz time domain spectrum has an opposite phase when the pumping laser is incident on the right 5 μm of the center of split positions of the SRR on MSPCA-A [Fig.
Fig. 6. Comparison of THz time domain spectrum for femtosecond laser pumping at different positions. THz amplitude and normalized THz amplitude of (a) MSPCA-A when the pumping light is incident on −90 μm and −100 μm and (b) MSPCA-B when the pumping light is incident on −110 μm and −100 μm. Pattern of THz time domain spectral peak-to-peak value of (c) MSPCA-A and (d) MSPCA-B.
Figure
Fig. 7. Patterns at the maximum bias voltage when the femtosecond laser is pumped onto different positions of (a) MSPCA-A and (b) MSPCA-B. The electrostatic field of the (c) positive electrode and (d) negative electrode of the MSPCA.
By comparing Fig.
The experimental results reveal that the maximum bias voltage rises when the pumping laser is on the positive or negative electrode, while the maximum bias voltage remains at lower levels for laser pumping at the inter-electrode gap. Moreover, when the pumping laser is on the negative electrode, the bias voltage is higher than when the pumping laser is on the positive electrode. The THz electric intensity increases in the MSPCA, since the bias voltage increase is much greater than in D-PCA; this is proportional to the bias voltage[31]. Meanwhile, the electrostatic field between the positive and negative electrodes of the MSPCA is complex with a two-dimensional non-uniform electric field, as shown in Figs.
In conclusion, our method is novel and significant because of the production of THz radiation that differs in character to traditional D-PCAs. The analysis reveals that only the direction of motion of the photoinduced carrier charges can lead to the phase shift of THz radiation waveform. The electrostatic field of the traditional D-PCA is approximately a one-dimensional uniform electric field with one direct motion of photoinduced carriers, which will not change dramatically with the position of a pumping laser. Unlike D-PCA, the electrostatic field between the positive and negative electrode of the MSPCA is a two-dimensional non-uniform electric field. Accordingly, the photoinduced carriers move within the split position of the SRR rather than the inter-electric gap of the MSPCA. Importantly, our MSPCA outputs enhanced THz radiation power. We have analyzed the reasons for its THz enhancement. First, the SRR has localized the THz energy in the low frequency band, which has caused a sharp increase in the low frequency radiation power. Second, the photogenerated carriers have moved within the split position of the SRR. Third, the bias voltage of the MSPCA has been increased, making the THz radiation power significantly improved. The enhanced THz radiation power of the proposed method could be a solution for space charge shielding and the breakdown between electrodes. This method also has extensive potential for applications relating to biomedicine, material analysis, and other THz uses.
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
Hu Deng, Weiwei Qu, Quancheng Liu, Zhixiang Wu, Liping Shang. Terahertz radiation enhancement in photoconductive antennas with embedded split-ring resonators[J]. Chinese Optics Letters, 2020, 18(11): 113701.