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Abstract
Terahertz (THz) photoconductive antenna (PCA) emitters having one-dimensional (1D) and two-dimensional (2D) micron-size metal line arrays (MLA's) at the transmission side of the semi-insulating GaAs substrate were fabricated via UV photolithography and electron beam deposition. At a fluence of ∼1.2 mJ/cm2 and 20 VPP bias, the enhancement in the THz signal peak-to-peak amplitudes are ∼6 times for 1D MLA and ∼11 times for 2D MLA, compared to the reference PCA, respectively. An all-optical effect via THz extraordinary transmission is conjectured for the enhancement mechanism. This metamaterial PCA design presents a feasible, yet more cost-effective alternative to photo-conducting gap nanostructure fabrication using e-beam lithography.
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1. Introduction
The terahertz (THz) photoconductive antenna (PCA) is a widely-used device for the generation and detection of THz radiation intended for spectroscopy and imaging applications [1–3]. It has the advantage of being compact; requiring relatively low optical power to generate sub-picosecond THz pulses [1–3]. A PCA consists of a semiconductor substrate with short carrier lifetime, high carrier mobility, and high breakdown voltage characteristics [2,4,5]. In a PCA THz emitter, femtosecond optical pulses generate electron-hole pairs in the micron-size gap between the electrodes, which are accelerated by a bias field. The transient current due to the acceleration and decay of photogenerated carriers in the PCA is the origin of the THz frequency pulses [2,4–7].
Diverse approaches to improve the THz performance of PCA emitters are actively being explored. Aside from developments on the photoconductive material itself, large aperture dipoles and interdigitated electrodes have been fabricated mainly to curb the saturation at high optical pump power [2,3,8–11]. Increasing the THz radiation power of PCA’s with the aid of surface plasmon effect have also been investigated [12–18]. Plasmonically-enhanced PCA’s have been demonstrated by fabricating metallic nanostructures such as gratings [12–14], islands [15,16], and nano-patterned arrays [17,18]. Metallic nanostructures are fabricated within the PCA gap with the aim of increasing the optical-to-THz conversion efficiency via mechanisms of local field enhancement to increase photocarrier generation [16], reduction of heat generation to increase the bias field and carrier density [18], and reduction of the carrier transport distance to the contact electrodes to increase the number of collected photocarriers [12,14].
In this work, a technique to increase the THz generation efficiency of a PCA emitter is proposed by integrating micron-sized metal structures at the PCA’s emission side, in contrast to the more popular approaches involving fabrication of metallic nanostructures in the PCA gap area. The PCA design takes advantage of THz transmission enhancement through periodic metal structures, as previously observed from bulk SI-GaAs patterned with micron-width metal line arrays [19,20], which is attributed to extraordinary transmission phenomena in subwavelength apertures [21–23]. This work demonstrates a more feasible and cost-effective PCA fabrication process as compared to the fabrication of nanostructures within the PCA gap area involving cost-prohibitive e-beam lithography.
2. Materials and methods
A semi-insulating (100) GaAs wafer (AXT Inc., carrier concentration ≈ 2 × 1016 cm−3) with both sides polished and with thickness of ∼625 um was used as the PCA substrate. Three 6 mm × 10 mm chips were cleaved from the wafer and each were patterned with a log-spiral PCA with 10 µm antenna gap using a MIDAS MDA-400 mask aligner. Layers of Ni/Au with thicknesses of 25/85 nm were deposited onto the patterns simultaneously under identical electron beam deposition conditions, and were lifted off. A spiral PCA design was chosen so as to generate circularly-polarized THz pulses [24] as the contributions of the p- and s-polarized components of the THz emission are relevant to the geometry of the MLA structure. A representative microscope image of the log-spiral pattern electrodes used for the PCA’s is shown in Fig. 1(a). At the transmission side on one of these PCA’s, metal lines parallel to the length of the PCA with width of 10 µm and periodicity of 410 µm were patterned via photolithography. This sample was designated as the one-dimensional metal line array (1D MLA). At the transmission side of the second PCA, the same line dimensions were used to pattern lines both parallel and perpendicular to the length of the PCA, forming a square array. This sample was designated as the two-dimensional metal line array (2D MLA). Similarly, Ni/Au with thicknesses of 25/85 nm were deposited onto the 1D and 2D metal line patterns and lifted off. Microscope images of the 1D and 2D MLA’s are shown in Fig. 1(b) and (c), respectively. The third PCA was kept without patterns at the emission side to serve as the reference sample. Figures 1(d) and (e) shows the schematic diagram of the fabricated PCA’s integrated with 1D and 2D MLA’s, respectively.
Fig. 1. Microscope images of the (a) log-spiral pattern PCA, (b) one-dimensional and (c) two-dimensional metal line arrays deposited on SI-GaAs substrates. Schematic diagram of the spiral-pattern PCA’s integrated with (d) 1D and (e) 2D metal line arrays. The silicon lens (not illustrated) is placed on the side of the metal line array structure for all devices.
The PCA samples were mounted on an aluminum casing and a silicon lens was mounted on the transmission side of the PCA substrate. The THz emission of the fabricated PCA’s were measured using standard THz time-domain with a femtosecond fiber laser as the photoexcitation source (emission wavelength λ = 780 nm, pulse width τ = 100 fs, pulse repetition rate f = 100 MHz, p- polarized). With the ultrafast optical excitation of the spiral PCA gap, THz pulses were generated and propagated through the cross-section of the SI-GaAs substrate, then transmitted through the 1D or 2D MLA’s (Fig. 1(d),(e)). The THz beam was collimated by the Si lens, collected by off-axis paraboloid mirrors and directed towards a commercial LT-GaAs log-spiral PCA detector which is optically gated by the probe beam. The THz emission intensities of the PCA’s were measured as a function of varying optical pump power from 4 to 16 mW, and the emission enhancement characteristics of the PCA’s with 1D/2D MLA were compared with the PCA without MLA as reference. A wire grid polarizer between the two off-axis paraboloid mirrors was used to measure the relative intensities of the THz p- and s-polarized components. The PCA detector used was equally sensitive to both p- and s- polarizations of the THz radiation [20]. Lastly, a standard knife-edge method was carried out to measure the THz beam diameters at the tip of the silicon lens.
3. Results and discussion
Representative THz emission time-domain and frequency spectra of the reference PCA (without MLA) and PCA’s integrated with 1D and 2D MLA’s are shown in Fig. 2. Compared to the THz signal amplitude of the reference PCA, the THz peak-to-peak intensities are enhanced ∼6x for the PCA with 1D MLA and ∼11x for the PCA with 2D MLA, at a fluence of ∼1.2 mJ/cm2 and peak-to-peak bias voltage of 20 V (Fig. 2(a)). The frequency spectra of the PCA’s integrated with MLA’s showed an improvement in the equivalent signal-to-noise ratio, even with an increase in the noise floor (Fig. 2(b)). Although the noise floor increased by ∼1 order of magnitude, the signal levels increased by ∼1.5 orders, for the 1D MLA device and almost 2 orders, for the 2D MLA device. This corresponds to an improvement in the equivalent SNR of ∼0.3 order of magnitude for the 1D MLA and almost 1 order of magnitude for the 2D MLA device, over a range of ∼0.5 THz. The bandwidths also increased from ∼0.45 THz (reference PCA) to ∼0.8 THz for the 1D MLA sample and ∼1 THz for the 2D MLA sample. In the inset of Fig. 2(b), the normalized plot of the FFT power in linear scale shows the shift of the peak frequencies of the PCA’s with MLA toward higher frequencies and the enhancement of the higher frequency components in the FFT amplitudes. The proposed reason for the THz intensity enhancement is the extraordinary transmission [21–23] through the MLA.
Fig. 2. (a) Terahertz time-domain spectra of the reference spiral PCA and spiral PCA’s integrated with 1D and 2D metal line arrays. (b) The corresponding frequency spectra of the PCA’s, with the FFT power plotted in linear scale in the inset, to show the central frequencies.
Moreover, the THz emission amplitudes were enhanced over a range of optical pump fluences, as shown in Fig. 3, with the 2D MLA sample consistently exhibiting higher THz enhancement than the 1D MLA sample. The saturation fluence (Fsat) of the PCA’s were calculated by fitting the fluence dependence of the THz emission amplitude to the equation ETHz(F) = A(F/Fsat)/(1 + F/Fsat), where F is the incident pump fluence and A is the THz amplitude [4]. Although the emission intensities of the PCA’s with MLA’s are much stronger, the Fsat of the PCA with 1D MLA and the reference PCA are comparable (Fsat ∼1.6 mJ/cm2). However, the Fsat of the PCA with 2D MLA is exceptionally high. Different fluence saturation characteristics are usually related to varying photocarrier screening effects and carrier mobilities [25]. However, in this case, the PCA’s were simultaneously fabricated using the same SI-GaAs substrate. A probable reason for the higher Fsat value of the PCA with 2D MLA will be discussed in the section on the THz emission polarization characteristics of the MLA PCA devices.
Fig. 3. Switching optical pulse fluence dependence of the THz currents of the fabricated PCA emitters and fitted saturation fluence values.
The resistances of the PCA’s are also compared (Fig. 4), to investigate if the increased THz emission is due to lower PCA resistance. At higher optical fluence values wherein the PCA’s are typically expected to be in operation (fluence range of 0.8 to 1.6 mJ/cm2), the PCA resistances have close values. The differences in the resistances at 0.4 mJ/cm2 fluence are not important since low optical fluence values are not normally used in actual PCA operation. Overall, the PCA resistances are not correlated with the THz emission intensities of the PCA’s over the observed fluence range. With all the PCA’s exhibiting similar photoconductance characteristics, an all-optical effect may be considered to explain the observed THz enhancement. For the 1D MLA, the area of the apertures formed by the Ni/Au metal line arrays is ∼2.5% less than the total bare GaAs substrate surface area (Fig. 1(b)), and ∼5% less for the 2D MLA, (Fig. 1(c)). Despite this, a significant enhancement in the THz intensities of the PCA’s with 1D and 2D MLA were obtained. At a fluence of ∼1.2 mJ/cm2 and peak-to-peak bias voltage of 20 V, the area-normalized enhancement of the integrated THz power with respect to GaAs aperture area is ∼42 times with 1D MLA and ∼138 times with 2D MLA (Fig. 2(b)). The increase in transmitted THz intensities is similar to extraordinary optical transmission, a phenomenon of enhanced transmittance through apertures of dimensions much smaller than the wavelength of light [26–28]. This occurs via surface plasmon polaritons (SPP), or the coupling of light with free electrons in conductors such that the surface charges oscillate collectively in resonance with the light wave [21,29]. In a metal with a periodic lattice of sub-wavelength apertures, light incident on the metal can be channeled through the apertures via SPP’s, making the light transmitted through an aperture greater than that incident on its area. This occurs when the grating’s phase-matching condition is satisfied, and manifests as peaks in the transmission spectra at the wavelengths where SPP excitations occur [30]. Originally discovered at visible to UV wavelengths, extraordinary transmission has similarly been achieved in the THz range [31–40].
Fig. 4. Resistance of the PCA’s with varying optical pump fluence showing similar illuminated resistances of the fabricated PCA’s at higher fluence values.
For a subwavelength hole array, the resonant interaction of incident light with surface plasmons obeys the conservation of momentum [41–43]
where ksp is the surface plasmon wavevector, k0 is a free-space wavevector, Gx and Gy are the reciprocal lattice vectors, $\theta $ is the angle of incidence, and i and j are integers. With a lattice periodicity p, Gx = Gy = 2π/p. The dispersion relation of surface plasmons is given byFig. 5. FFT amplitude spectra of the PCA’s integrated with MLA normalized to the reference PCA showing a broad enhancement peak close to the calculated [±1,0] resonance frequency. The region above 0.35 THz is shaded to indicate the uncertainty in the spectral features due to poor SNR.
The polarization characteristics of the THz emission were investigated by comparing the relative intensities of the p- and s- polarization components. Figure 6(a, b) shows the FFT power spectra of the p- and s- polarizations, respectively, with the y-axes set to the same scale to facilitate comparison of the relative intensities. Enhancement in the intensities and broadening of the bandwidths for PCA’s with MLA’s are evident for both polarizations. The integrated FFT powers of the p- and s-polarized components, as well as the intensity ratio of the p- and s- polarizations, are shown in the inset table in Fig. 6(b). The reference PCA has almost equal p- and s- polarized components, consistent with the circularly-polarized THz emission characteristics of spiral PCA’s [24]. For the PCA with 1D MLA, the p- polarized component is nearly twice that of the s-polarized component. In contrast, the p- and s-polarized components are nearly equal for the 2D MLA, similar to the reference PCA.
Fig. 6. Comparison of the FFT power spectra of the (a) p- polarized and (b) s-polarized components of the THz emission of the reference PCA and PCA’s integrated with metal line arrays. The integrated THz powers and ratio of p/s-polarization intensities are shown in the inset table (b).
In the context of extraordinary transmission phenomenon, the efficiency of the coupling of an incident plane wave with the surface EM modes is greater when the incoming E-field has a non-zero component in the direction of a reciprocal lattice vector associated with the geometry of the apertures [44]. Hence, for a lattice of apertures having an asymmetric shape (such as for slits or rectangular holes), the polarization of the incident light with a larger enhancement and stronger transmission efficiency is the polarization perpendicular to the long axis of the apertures [44]. Theoretical [45,46] and experimental [47] studies have shown such polarization-dependent transmission or polarization anisotropy in holes with high aspect ratios. The fabricated 1D MLA is analogous to an array of slit apertures, and exhibited a larger p- polarized transmission, consistent with an expected enhanced transmission perpendicular to the vertical metal lines. We note that the wire grid polarizer does not completely filter the s- polarized THz signal. This leads to polarization leakage in the measured 1D MLA data (Fig. 6(b)). For the 2D MLA, both p- and s- polarized components of the incident THz wave are almost equally enhanced due to the presence of the orthogonal metal lines forming square apertures. Thus, the p/s ratio of the PCA with 2D MLA is almost equal to that of the reference PCA. This may also explain the larger THz intensity and fluence saturation (Fsat) of the PCA with 2D MLA compared with 1D MLA (Fig. 3) since both orthogonal polarization directions are efficiently enhanced. These results demonstrate the possibility of controlling the polarization of THz waves by designing the geometry of the MLA. The observed polarization characteristics further suggest the extraordinary transmission of THz radiation to explain the enhanced THz emission of the PCA’s integrated with MLA’s.
In addition, the horizonal and vertical beam profiles of the radiated THz emissions were obtained using knife-edge measurements. The shape of the THz beam around the focal point is elongated in the same direction as that of the THz beam’s polarization direction [48–50]. Hence, p- polarized beams are elongated along the x- direction, whereas circularly polarized beams have symmetric x- and y- dimensions. The THz beam FWHM and aspect ratios are given in Table 1. For the 1D MLA sample, the transmitted beam profile has a larger width in the p- polarized direction than in the s- polarized direction. In contrast, the beam profile of the 2D MLA sample have closer FWHM values for the p- and s- polarized directions. The aspect ratios of the THz beam for the MLA devices are consistent with the measured polarization characteristics wherein the 1D MLA sample showed a greater p- polarization component, while the 2D MLA sample showed equal p- and s- polarization components, similar to the reference PCA.
Table 1. THz beam FWHM and aspect ratios from knife-edge measurements, for the reference, 1D MLA and 2D MLA PCA emitters
4. Conclusion
Spiral PCA emitters integrated with 1D and 2D MLA’s on SI-GaAs substrates were fabricated via UV photolithography and Ni/Au electron beam deposition. Using standard THz time-domain spectroscopy, the THz emission characteristics of the PCA’s were investigated in the context of switching optical pulse power dependence. At a fluence of ∼1.2 mJ/cm2 and peak-to-peak bias voltage of 20 V, the enhancement in the integrated THz power compared to the reference PCA is ∼42 times with 1D MLA and ∼138 times with 2D MLA, normalized to the GaAs aperture area. The photoconductance characteristics of the PCA’s were similar, from the fluence dependence of THz emission intensities and PCA resistances. This suggests an all-optical effect as the mechanism for the THz enhancement. This is further supported by the relationship between the experimental and calculated resonance frequencies, and the polarization dependence of transmission enhancement consistent with extraordinary transmission phenomenon. The integration of micron-size MLA’s to PCA’s is proposed as a possible design to improve the THz PCA emission, involving a more cost-effective fabrication process, capitalizing on extraordinary transmission in the THz regime.
Funding
University of the Philippines, Diliman - Office of the Vice Chancellor for Research and Development (OVCRD Project No. 202012 ORG).
Acknowledgments
The authors are grateful to Prof. Masahiko Tani from the Research Center for Development of Far Infrared Region, University of Fukui, for the equipment loan. The authors also thank the Department of Science and Technology - Philippine Council for Industry, Energy, and Emerging Technology Research and Development (DOST-PCIEERD) for their support.
Disclosures
The authors declare no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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