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Abstract
Flexible materials for applications in terahertz (THz) range imaging systems are investigated in this study. THz time-domain spectroscopy and THz imaging at 0.6 THz frequency are used to analyze optical properties of zone plates (TZP) with integrated cross-shaped filters, which are fabricated using direct laser writing on thin graphite, HB pencil-shaded graphite on paper, as well as reference metal-based and pure paper zone plates. Spectral features and focusing power comparable to the best metal-based TZP is achieved with graphite-based TZP. The pure paper and paper with pencil-shaded graphite TZPs showed increase in focusing power by a factor of ∼1.5, supporting numerical 3D finite-difference time-domain simulations. The findings show that graphite-based TZPs can serve as a flexible, compact, and inexpensive optics elements for emerging THz imaging systems.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Rapid evolution in terahertz technology [1], in compact solid-state THz emitters [2] and detectors [3,4] necessitates the search for ways to reduce the size of passive optical components. It is also desirable that such components are easy to integrate and align, while maintaining the systems mechanical reliability as well as reasonable costs. Metamaterials are a promising avenue as they allow to redesign bulky optical elements into thin and planar components with a wide range of optical properties [5]. Many applications of THz imaging require that packaged materials can be resolved and recognised without performing a full spectroscopic imaging [6,7], which entails recording of images at fixed THz frequencies that are known or selected a priori [8,9].
Terahertz zone plates (TZP) designed with cross-shaped filters, which permit shaping and focusing of the THz beam for imaging at selected frequencies, were proposed by several groups earlier [10,11,12,13]. The TZPs are compact focusing elements for handling of THz radiation beams and can be used in THz imaging systems with broadband source in case of imaging when only one particular frequency related to the material’s fingerprints is needed to record the image. Innovative structures adding tunability of optical properties into compact optical elements can be provided by microelectromechanical systems (MEMS). One such example is a MEMS switch placed across a slit of a split ring-resonator (SRR), which acts as a switchable metamaterial with tubable frequency [14]. Another proposed design is based on origami containing periodic patterns of SRRs placed on different surfaces [15]. When the folding parameter is varied in these systems, the gap between the rings and hence the capacitance of the resonators is altered inducing a shift of the resonance frequency. Mechanically tunable components can be also achieved via programmability platform based on nonflat-foldable origami by employing its intrinsic self-locking and reconfiguration capabilities [16] or by combining nonlinear mechanical elements with a multimodal architecture enabling a sequence of topological reconfigurations [17].
In this work, we extend the family of mechanically tunable materials by adding optical metamaterials based on flexible thin films. The approach can be found amenable for on-chip designs in integrated photonics and useful in development of origami-principle based optical solutions for compact THz imaging systems. Its particular role can be attributed in biophotonic THz applications, where the mechanical flexibility can reduce strongly the elastic mismatch between biological tissues and photonic components, enabling their conformal integration on curvilinear tissue surfaces.
More specifically, we explore the optical properties of flexible materials shaped in zone plates with integrated filters and produced from three different materials: 10 µm thick graphite foil, thin graphite layer deposited by HB graphite pencil on 100 µm paper sheet, and a pure paper serving as a reference for the TZP fabrication technology. THz time-domain spectroscopy and THz imaging techniques performed at 0.6 THz frequency revealed distinctive spectral features and an excellent focusing performance indicating advantages of the graphite-based TZP. This provides an inexpensive and flexible compact THz focusing element alternative to the metal foil or metal evaporation-based technological approaches.
2. Designs of flexible terahertz zone plates and experimental set-ups
In order to investigate new flexible diffractive optical components, TZPs made of three different materials with integrated cross-shaped filters were tested and compared to a reference metal zone plate. The metal zone plate had an identical design as the other materials used for the 0.6 THz frequency imaging with focal length of 10 mm. The TZP geometry is displayed in Fig. 1(e). The metallic reference zone plate was made from 30 µm thick steel foil. The second zone plate was made from 10 µm thick graphite foil (ɛr=12), which was deposited on a 75 µm thick plastic polyester film (ɛr=2.6), since thin graphite is a soft and non-free-standing material. The graphite film was acquired from “Shenzhen Zhenxing Technology Co., Ltd”. The third zone plate was made from a thin graphite layer produced by shading HB graphite pencil on a 100 µm thick paper sheet (ɛr=2.31) [18]. The fourth zone plate was made from a 100 µm thick paper sheet and also served as reference.
Fig. 1. Photos of the quarter of TZP made from different materials: a) metal; b) graphite foil; c) graphite on paper and d) paper. Inset depicts the shape of one cross-shaped aperture element. Photos were taken using “Hyrox KH-7700” digital microscope. e) Design of the graphite foil terahertz zone plate for 0.6 THz and geometry of cross-shaped filters (M = 40 µm, K = 260 µm, L = 290 µm). f) Raman spectroscopy of 10 µm graphite film and 75 µm polymer film. Most pronounced spectral signatures in plastic: -C = O (1721cm−1), C = C (1608, 3075 cm−1) and CH3 (2959 cm−1). The N-H amine vibration line around 3300-3400 cm−1, it is not prominent, likely due to overlapping with characteristic bands of absorbed water in the same region.
All the TZPs, shown in Fig. 1(a)–(d), were produced using a laser direct writing (LDW) system equipped with high precision polygon scanner (LSE170 from Next Scan Technology) and translation stage (PR0115 from Aerotech).
Zone plates on the pure paper and paper with a graphite layer were fabricated using the 3rd harmonic (355 nm) of picosecond laser Atlantic-60 (1 MHz, 1064 nm, 10 ps, Ekspla). Laser beam was scanned at 1 mm/s, with 0.2 µm pulse pitch using 10 µm diameter laser spot, and 12 J/cm2 laser irradiation fluence. Each scan was repeated 5 times. Paper samples were placed on another paper sheet and held on the sample holder by a vacuum suction. Paper sheet below the sample helped to avoid excessive charring of the paper cut due to the ablation of the metal sample holder. The zone plate on a graphite layer on a plastic substrate was fabricated using 1 mm/s, 0.2 µm pulse pitch, 10 µm diameter laser spot, 12 J/cm2 fluence, and 3 scans.
Figure 1(f) shows Raman spectroscopy of 10 µm thick graphite film (black line) and 75 µm plastic film (red line), which served for fabrication of the graphite TZP. In the Raman spectrum of the graphite film, the line at 1744cm−1 can be attributed to –C = O vibration, while 3239 cm−1 presents –N-H line. One can also see distinct lines in Raman spectrum of the polyester plastic film ascribed to –C = O (1721cm−1), C = C (1608, 3075 cm−1) and CH3 (2959 cm−1) vibrations.
Two different set-ups were employed to investigate spectral properties of all four different zone plates. The THz time-domain spectroscopic measurement was performed by the Teravil-Ekspla “T-SPEC” THz Time Domain Spectrometer (THz-TDS) (set-up presented in Fig. 2(a). Femtosecond laser (Toptica, Femtofiber Pro) providing pulses of 780 nm wavelength, 90 fs pulse duration and 150 mW output power at 80 MHz pulse repetition rate was used for photoconductive antennas made from LT-GaAs excitation. The delay line was based on 10 times per second moving hollow retro-reflector with 120 ps time window what corresponds to ∼8 GHz spectral resolution. A set of two parabolic mirrors was used to focus end effective collection of THz radiation. In the focus point diameter of THz beam was ∼ 2 mm. THz signal was detected by the digital signal processing (DSP) card integrated into electronic module with analog-digital converter (ADC) [19].
Fig. 2. a) THz-TDS set-up for evaluation of spectral properties of investigated zone plates [20], b) THz-CW set-up for evaluation of TZP focusing performance and THz beam profile measurements in the focal (xy) plane and along the z-axis, (zy) plane.
The second set-up was used to evaluate the focusing performance of zone plates and was based on a THz continuous wave (THz-CW) 0.6 THz electronic source (Virginia Diodes, VDI). It consisted of high-density polyethylene (HDPE) lens and a THz detector (titanium microbolometer [3]). The beam profile was recorded in both the focal plane and along the z-axis (set-up presented in Fig. 2(b)), modulating the source electronically at 1 kHz frequency and detecting the microbolometer-induced signal with a lock-in amplifier.
3. Results and discussion
Zone plates transmission spectra were obtained using THz-TDS (Fig. 3). All the spectra given in Fig. 3 are normalized to the unfocused beam values [21]. As the studied TZPs are based on very different materials, we introduced the following procedure to quantify the quality of their focusing performance. The transmittance of the radiation is divided into two zones: the first one (center zone) includes transmittance, which was calculated by integrating area of 2 mm radius from the TZP center. The second zone (peripheral zone) was defined from the average of the transmittance where it decreases two times, and was evaluated by integrating the remaining part of the TZP area. These zones are indicated by the red line (center zones, where the signal intensity is the highest), and the black line (peripheral zone). The choice of this approach is based on the consideration of the maximum contrast in the metal foil-based TZP.
Fig. 3. THz-TDS transmittance spectra and insets depict THz-CW two-dimensional THz beam profiles focused with a) metal TZP; b) graphite foil TZP; c) graphite on paper TZP; d) pure paper TZP. The beam cross sections at the maximum intensity are presented in a linear scale as a solid black line for each case. Maximum signal amplitude of the TZPs is normalized to the maximum amplitude of the unfocused beam.
As one can see, spectra of metallic and graphite foil are quite similar in terms of the spectral shape, although the maximum amplitude in the latter is 18% smaller than the reference. A closer look at the resonance frequency shows that the graphite foil spectra is red-shifted to 0.48 THz, in comparison to the metal one (Fig. 3(b)). Although the design of zone plates was the same, the dimensions of cross shaped filters were slightly smaller than designed (Table 1) due to the fact that the laser cutting parameters were kept constant in both cases, whilst the materials being processed were different. The smaller crosses induced blue-shift of the resonant frequency, ${f_r} = c/(1.8 \cdot K - 1.35 \cdot M + 0.2 \cdot L)$, which for graphite foil TZP amounts to 0.64 THz. However, the plastic film under the graphite layer red-shifts the resonant frequency due to its lower refractive index (n) of dielectric $r = \sqrt {2/({{n^2} + 1} )} $, which is in contact with the zone plate [22]. Taking into account that for plastic n = 1.61, the frequency shifting factor r = 0.75, and, as a consequence, the resonant frequency of graphite foil TZP is red-shifted to 0.48 THz. Spectra of the graphite on paper TZP and the pure paper TZP (Fig. 3(c, d)) reveal no resonances around expected 0.6 THz. Moreover, they are relatively smooth over the whole investigated range. The central part and peripheral parts do not keep constant transmission ratio due to the non-uniform coverage of the graphite by pencil. More details on the transmission properties can be found from the results of THz-TDS and THz-CW given in Table 2.
Table 1. Dimensions of cross-shaped filters
Table 2. Transmittance of different zone plates measured using THz-TDS and THz-CW systems
Insets of Fig. 3 present experimental two-dimensional THz beam profiles in the focal plane of THz-CW imaging system at 0.6 THz frequency. The intensities are calibrated to the unfocused beam intensity. Solid black lines represent the beam cross sections at the maximum intensity. As expected, the best operation is manifested by the metallic TZP. The flexible graphite foil TZP demonstrates focusing by increasing the signal by a factor of ∼8, as compared to the unfocused one. Graphite on paper and pure paper zone plates exhibit poor focusing properties with enhancement factors of 1.5 and 1.3, respectively.
To complete the physical picture and evaluate operational performance, the beam profiles along z-axis were studied by comparing experimental beam profiles (symbol line) with theoretical simulations of the electric field Ez/E0 ratio distribution (straight line), where E0 is incident electric field taken to be equal to 1. Results are depicted in Fig. 4. All investigated TZPs reveal well expressed resonances at the focal point, i. e. at 10 cm from the zone plate, followed by Fabry–Perot oscillations due to interferences between the collimating lens and the studied TZPs. It is also evident that the experimental results are well reproduced by the simulation data. The most pronounced resonance is achieved with the metal TZP, while very similar performance can be attributed to the graphite foil-based TZP. To further analyse the profiles, a pure graphite sheet of 10 µm thickness (without plastic foil as support) was modelled. It shows that the graphite foil operates more effectively than the graphite foil with plastic - the focusing effect is more than twice higher in the former.
Fig. 4. a) Experimental results (symbols + lines) of the beam profile (left scale) and theoretical calculations (straight lines) of the normalized squared electric field (right scale) distribution along z-axes. Inset depicts FWHM variation along the z-axis for the beam focused with all investigated TZPs. (b-d) The beam profile along z-axes (zx-plane) focused with: b) graphite foil TZP, c) graphite on paper TZP, d) pure paper TZP.
It is interesting to note that the pure paper TZP also manifests some focusing performance. As it can be seen from Fig. 4(a), the signal in pure paper TZP increases by a factor of 1.3 in the focal plane, as compared to the unfocused beam, and the experimental data fits well with the modelling results. Graphite added on the paper by the HB pencil shading increases the focusing effect by about 15% with respect to the pure paper TZP. Thus, by adding the focusing TZP with integrated filters one can extend the range of paper applications to design and production of large scale inexpensive THz photonics components, such as paper THz wave plates [23] and aberration corrected paper-based lenses for low-frequency THz radiation [24,25].
Since the spatial beam quality is of particular importance [26], its Gaussian profile was checked recording 2D plot of the emitted beam. To estimate the focusing parameters, full width at half maximum (FWHM) of the Gaussian beam along the z-axis was measured for all TZPs by approximating them as Gaussians. The results are shown in inset of Fig. 4. It can be observed that FWHMs of the graphite foil and metal TZPs are relatively narrow and varies negligibly along the z-axis. In contrast, the FWHM of pure paper and graphite on paper TZPs suffer drastic spreading when moving away from the focal point.
Using experimentally obtained data, focusing parameters such as beam waist (ω0), Rayleigh range (zR), focal depth (b), beam angular spread (θ), and numerical aperture (NA) were evaluated. Estimates are given in Table 3. It is seen that the THz beam, focused with graphite foil zone plate displays only a 7% wider beam waist than the reference metal TZP. One can note that other parameters of both these TZPs also look quite similar. However, comparison of pure paper and graphite on paper based TZPs implies the important fact that the addition of only few microns of graphite strongly affects the parameters of the TZP: ω0, θ and NA were estimated to be more than twice smaller for graphite on paper-based TZPs than the pure paper one. Concerning the Rayleigh range (zR) and the focal depth (b), the difference is more than four times. Therefore, one can infer that even a few microns thick layer of graphite can significantly affect the TZPs focusing performance. This is also illustrated in Fig. 4(b–d), where the beam profiles of graphite foil TZP, graphite on paper TZP, and pure paper TZP are presented.
Table 3. Gaussian beam parameters
To complete the picture, numerical 3D simulations were performed to elucidate the graphite foil, graphite on paper, pure paper-based TZPs, and the metal TZP (Fig. 5). Steel has been chosen as a perfect electrical conductor, paper as a material with permittivity of ɛr=2.31, graphite with electrical conductivity of 1·105 S/m and ɛr=12 and loss free polyester plastic with ɛr=3.5. To model the wave propagation through the cross-shaped apertures, 3D finite-difference time-domain (3D FDTD) method was employed. Simulation grid, corresponding to the 16.5 × 16.5 × 12 mm3 volume in x, y, z directions respectively, was set-up with grid sizes $\varDelta$x = $\varDelta$y = $\varDelta$z = 38 µm. In order to reduce the computational load, symmetry conditions of the structure were used to simulate only quarter of the zone plate, and absorbing boundary conditions were applied.
Fig. 5. Simulated distribution of electric field amplitude behind the a) metal TZP, b) graphite foil TZP, c) graphite on paper and paper TZPs – both focal points overlap; the increase in intensity of graphite on paper TZP is 15% higher than that of the paper alone.
Figure 5 illustrates the calculated distribution of the electric field near the zone plates made of different materials. The transmitted plane wave is found focused in the focal spot of the TZP located at 10 mm.
Comparison of the plots in panels (a) and (b) in Fig. 5 indicates similar focusing abilities of graphite foil TZP and the metal one. Figure 5(c) displays the focusing performance of pure paper zone plate and the graphite on paper TZP. The focal points of both TZP overlap, thus, the focusing intensity is estimated theoretically. For pure paper and graphite on paper TZP the focusing intensity is increased by 1.3 and 1.5 times, respectively, as compared with the unfocused beam, and these values agree well with experimentally obtained data.
Therefore, one can presume that graphite-based THz zone plates can provide rational flexible and compact solutions for design of passive optical elements in THz imaging systems. Graphite is a unique material manifesting metal and non-metal properties, it is also disposable, eco-friendly and cheap, and these advantages forms a new dimension for production of diffractive optical components [27].
4. Conclusions
Flexible graphite-based THz zone plates with integrated cross-shaped filters were demonstrated for compact optics in THz imaging systems. 3D finite-difference time domain method was employed to design the zone plates which were fabricated using direct laser writing technique. Flexible zone plates were produced from 10 µm thick graphite foil, thin graphite layer made by HB graphite pencil on a 100 µm thick paper sheet, and a pure paper sheet. The performance of zone plates was investigated using THz time-domain spectroscopy and THz imaging at 0.6 THz frequency. It was found that the graphite-based zone plate displays frequency features and focusing operation with a quality that nearly matches the metal-based zone plate. The graphite on paper and pure paper zone plates can increase the focusing by a factor of 1.5 and 1.3, respectively, as compared with the unfocused beam. The observed experimental data was supported by 3D finite-difference time domain method numerical calculations. The findings suggest that graphite-based THz zone plates can provide an inexpensive alternative to metal-based elements for design of passive optical elements in THz imaging systems.
Acknowledgements
Authors are greatly indebted to Irmantas Kašalynas, Polina Kuzhir, Alesia Paddubskaya and Tadas Paulauskas for enlightening discussions, to Martynas Talaikis for Raman spectra measurements and to Jonas Zinkevičius for technical assistance in the manuscript preparation.
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