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Abstract
Direct laser writing (DLW) is widely used to fabricate complex metamaterials (MMs) and photonic devices for nanoscale applications across the electromagnetic frequency spectrum. While the optical properties of conventional photoresists used in DLW are well studied in the visible and infrared range, it is still unclear how they behave at lower frequencies. Here, we measure the refractive index and absorption of IP-S and IP-Dip photoresists within the THz range of the electromagnetic spectrum. Further, we utilize THz time-domain spectroscopy (THz-TDS) to experimentally measure the laser-processed three-dimensional (3D) MM structures. We conduct full-wave electromagnetic simulations using the measured refractive index values to validate our experiments. The THz-TDS measurements are in excellent agreement with the theoretical predictions verifying the validity of our refractive index measurements. This study aims to support and lead future investigations utilizing standard DLW photoresists for photonic applications in the THz range.
Published by Optica Publishing Group under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
1. Introduction
The need for fast, large-area micro- and nano-fabrication techniques is ever-increasing and associated with a broad range of applications. Direct writing using ultrashort laser pulses is widely used for fabricating complex 3D multiscale structures for applications ranging from nano-optics to tissue engineering and catalysis [1–4]. This versatile, high-resolution fabrication process delivers structures with feature sizes from tens of nanometers to several millimeters [5,6].
Metamaterial (MM) and metasurface-based devices have demonstrated immense potential for photonic applications due to their unique and unusual electromagnetic properties. Importantly, by varying the size of the constituent meta-atom (unit-cell), MM structures can operate across a vast range of frequencies, from microwaves up to THz and the visible spectrum [7–10]. While the dispersive refractive index $n(\omega )$ and absorption $a(\omega )$ of widely used photoresists in direct laser writing (DLW) are well-known and extensively characterized in the visible and the near-infrared range, it is not yet clear how they behave within the THz frequency range [11,12]. In the last decades, a rapid development of the infrared and THz science and technology has been witnessed [13,14]; thus, it is vital to carefully examine the properties of frequently used photoresists for nanofabrication within the THz range. As the refractive index of photosensitive materials is relatively low, photonic/optoelectronic devices usually require coating the polymeric scaffolds with different materials (metallic, metallic nanoparticles, semiconducting or denser dielectric materials) to reach the desired response [15–18]. In some cases, the coatings can be very thin, making the accurate simulation of the electromagnetic response of the composite structures a challenging task. Thus, there is a critical need for experimental characterisation of frequently used materials for nanofabrication to assist theoretical investigations of complex metamaterial designs.
Here, for the first time to our knowledge, we experimentally study the $n(\omega )$ and $a(\omega )$ of frequently used photoresists in the THz range. Subsequently, we utilize high-resolution, large-scale DLW (section 2.1) to fabricate 3D MMs, coat with atomic layer deposition (ALD, section 2.2) and measure their optical properties with THz time-domain spectroscopy (THz-TDS, section 2.3). Two important aspects distinguish this work from previous works on photoresists: firstly, their optical properties are fully-measured via THz-TDS for uncured, cured, and resists measured as thin films, and secondly as validation of our experiments on 3D printed dielectric metamaterials, we use the experimentally measured $n(\omega )$ and $a(\omega )$ to perform exact full-wave calculations in the THz range (section 2.4). The good agreement between simulations and experiment confirms the validity of our refractive index measurements. The reported optical properties of photosensitive materials can find extensive use by the photonics and MMs community.
In previous work, we consider two-layered structure, covered with metallic nanoparticles via electro-less plating for refractive index sensing applications at the mid-infrared transparency window [15]. By contrast, this work considers a multi-layer 3D structure (12 layers), coated with ZnO via atomic layer deposition (ALD) and is focused on the low THz frequency range.
2. Results and discussion
2.1 Direct laser writing (DLW)
3D THz structures were fabricated via DLW (Fig. 1(a)). Utilizing two-photon polymerization, DLW allows fabrication of high-resolution features far below the diffraction-limited laser spot size. Briefly, the beam of a near-infrared (NIR) pulsed laser is focused inside the volume of a negative tone photoresist (transparent in NIR range) tightly. Strong spatial confinement of the laser beam in combination with the ultrashort laser pulses (fs) causing the photo-initiator of the photoresist to absorb two or more photons and start the polymerisation locally resulting in structure resolution <100 nm. For our experiments, we use a conventional nanoprinter (Nanoscribe) with a Ti:Sapphire femtosecond (fs) laser at a center wavelength of 780 nm, 25 kW peak power, 100 fs pulse duration, and 80 MHz repetition rate. For achieving high-resolution features over large-size structures, essential for THz MMs measurements, we use a 25x magnification (0.8 NA, 3D MF) oil-immersion objective and IP-S photoresist (Nanoscribe) in dip-in laser lithography mode on standard THz transparent high resistivity Si wafers. With the aid of 3D sample stages and galvo mirrors, we fabricate 3D split cube resonator (SCR) MMs of about 3 × 3 mm2 lateral size (∼ 17 × 17 unit cells) and height of 700 µm. Scanning electron microscopy (SEM) images of the structure are depicted in Fig. 1(b), (d). Additional SEM images can be found in the Supplement 1 (Fig. S2). The MMs smallest feature is approximately 13 µm. Following the fabrication, we apply a developing protocol consisting of PGMEA (30 min), isopropanol (5 min), and H2O (single immersion) to wash off the unexposed photoresist.
Fig. 1. Laser-processed THz mm-scale metamaterials (MMs). a) Direct laser writing (DLW) of 3D MMs. b-d) Scanning electron microscopy images of the split cube resonator (SCR) structure coated with 500 nm of ZnO via atomic layer deposition (ALD). In all cases, we use a $\lambda = $ 785 nm fs laser, IP-S resist and Si wafer as a substrate.
2.2 Atomic-layered deposition (ALD)
Since the laser-processed polymeric scaffolds feature a relatively low $n(\omega )$, the sub-wavelength SCR meta-atom cannot support strong Mie-type resonances. As a result, as will become evident in the following section, the structure operates as a photonic crystal, and the spectral response is dominated by the corresponding bandgap. An approach to further enhance the light confinement properties of the SCR meta-atom is to apply a coating of denser dielectric material. We investigate this approach by depositing 500 nm of ZnO in a custom-built ALD system using diethyl zinc (DEZ) precursor and water (H2O) co-reactant at 50 degrees. Such lowering of the temperature (compared to standard ALD deposition temperatures of a few hundred of degrees) is achievable due to the high reactivity of DEZ. Moreover, the deposition parameters were 0.1 s for the precursor pulse, 1 s for exposure, and 90 s for the purge in both cases (DEZ and H2O). The precursor pulse and the purge were executed with 15 sccm and 100 sccm argon flows, respectively. Using SEM, we determine the thickness of the ZnO layer (500 nm, Fig. S3) and calculate the growth per cycle (GPC) of about 0.12 nm/cycle, allowing coating with extreme sub-nm resolution. The conformal coated structures show excellent mechanical and chemical stability, and dimensions are quite close to the desired ones.
2.3 Refractive index of the resists and MMs measurements with THz time-domain spectroscopy (THz-TDS)
For the optical characterization, we utilize a THz-TDS setup, which fully covers the frequency range of interest (0.45-1.50 THz). Briefly, the sample is placed on the rotation stage of the time-resolved THz spectrometer to allow normal light incidence (Tera Flash TF-1503, Toptica Photonics AG). The setup comprises a pair (before and after the sample position) of wire grid polarizers (G30X10-S, SEMIC RF) that allow polarization-resolved spectroscopy. Here THz pulses were collected in a data interval of about 70 ps with a resolution as high as 0.05 ps. For our measurements, a commercial system was used in ambient conditions. The beam size in the focus is 2 mm.
For the refractive index measurements, we used our previously suggested methodology considering thick sample approximation [19–21]. The THz transmission of a sample is calculated using a measurement of the sample (Esample(t)) and a reference (Eref (t)). The THz transmission
The quantities measured and depicted in Fig. 2 are the real part of the refractive index $n(\omega )$ and the intensity attenuation coefficient $a(\omega )$, connected with the extinction coefficient ($\kappa (\omega )$, the imaginary part of the complex refractive index) through
Fig. 2. Measured dispersive refractive index $n(\omega )$ and absorption $a(\omega )$ of IP-S and IP-Dip at the THz range. a-b) $n(\omega )$ and c-d) $a(\omega )$ measurements of IP-S and IP-Dip photoresists. In all cases, blue, red, and yellow stand for uncured, cured, and resists measured as cured thin films.
IP-S shows a relatively spectrally-constant $n(\omega )$ (blue, Fig. 2(a)) that changes slightly upon curing and when measured as a cured film. IP-S $a(\omega )$ exhibits a fairly linear increase in all cases from 10 to 40 cm−1 for the frequency range between 0.40-1.50 THz (Fig. 2(c)). Further, uncured IP-Dip shows higher $n(\omega )$ of about 1.75 that is reduced to 1.63 after the resist is cured (0.40 THz, Fig. 2(b)). Similarly, the absorption of the IP-Dip reaches 60 cm−1 at around 1.50 THz. This drops to 40 cm−1 for the cured and film samples at 1.50 THz (Fig. 2(d)). Recent studies focused on commercially available photoresists indicate similar refractive index of the cured IP-S highlighting the validity of our measurements [23,24]. Further detailed measurements of such photoresists show now that absorption can be decreased by 5x times at the THz range as well as fabrication directly on thin films leading to highly conformable metasurfaces.
As mentioned earlier, for validating the $n(\omega )$ and $a(\omega )$ measurements, we fabricate a 3D SCR MM structure via DLW and using IP-S as photoresist, which is ideal for the fabrication of such structure sizes. Due to the low $n(\omega )$ of the polymer scaffold and the relatively thin ZnO coating (compared to the excitation wavelength), the structure is anticipated to act as a photonic crystal exhibiting a bandgap. In other words, any resonance of the sub-wavelength SCR meta-atom would manifest at higher frequencies compared to the bandgap. The dimensions of the structure are selected in such a way that the bandgap resides at the desired THz range. The aim is to reproduce with full-wave simulations the optical response of the THz structure using the experimentally measured $n(\omega )$ and $a(\omega )$ (see next section). To ensure that incoming radiation only excites the MM structure, we added an iris in the excitation path resulting in a spot size of approximately 2 mm. The linear polarization direction is controlled by either the rotation of the sample stage or the polarizer’s relative angle. Linearly polarized light along the y-axis is exciting the free-standing structure (Si is removed) and collected with a THz detector in the back. The iris helps to minimize measurement noise due to air.
The measurements are depicted in Fig. 3. A clear bandgap is found near 1 THz that partially blocks (due to the limited number of layers along the z-axis) transmission through the structure. In addition, a trend for decreasing transmission through the structure is witnessed as the frequency increases.
Fig. 3. Experimental transmission measurements of 3D THz MM. Normalized transmission of the free-standing SCR MM structure, see inset (twelve layers of SCRs are present in the sample, for simplicity we show only four). The IP-S polymer scaffold is conformally coated with 500 nm of ZnO via ALD.
2.4 Electromagnetic simulations
To verify the experimental results, we conduct full-wave calculations for the SCR MMs using the measured $n(\omega )$ and $a(\omega )$ of IP-S. From the $a(\omega )\; $ coefficient, we can extract the imaginary part of the refractive index ($\kappa $) using the formula
where ${\lambda _0}$ is the free-space wavelength.We use the frequency-domain solver of the commercial software CST Studio based on the finite element method considering an unstructured mesh comprised of tetrahedra, as is typical for this method. We initially specify that the largest dimension of any tetrahedron of the mesh should not exceed λ/10. Subsequently, the mesh is automatically refined through the “adaptive mesh refinement” option, until the desired accuracy (convergence) is achieved. This typically means that in the final mesh smaller tetrahedra are found in domains requiring finer meshing, such as the 80-nm-thick ZnO coating, whereas, in domains such as top and bottom air domains, a sparser mesh is used. The entire procedure is automatic and results in an optimal mesh saving computer time and guaranteeing convergence of the result. More specifically, the final mesh will be selected after the “S-parameter termination criterion” is met, meaning that the computed S-parameters (reflection/transmission coefficients) have converged (the error among consecutive calculations with different meshes drops below a predefined limit, which is set to 1%). Further, simulations are performed on a structure composed of twelve layers of SCRs (z-axis) with periodic boundary conditions at the x and y axes, as depicted in Fig. 4(a), with the in-plane lattice constants $\; {a_x}\; = \; {a_y}\; = \; 186\; \mu m$. The bandgap of the photonic crystal is formed due to the periodic repetition of the SCR layers along the z-axis. As we increase the number of layers along the z direction, the bandgap deepens (see Fig. S1 in Supplement 1).
Fig. 4. Structure of split-cube-resonator (SCR) metamaterial. (a) Structure used for simulations, composed of twelve layers of SCRs. (b) Unit-cell of the periodic structure with annotated geometrical parameters.
A single SCR has the following dimensions: $h\; = \; 60\; \mu \textrm{m}$, $w\; = \; 33\; \mu \textrm{m}$, ${t_1}\; = \; 16.5\; \mu \textrm{m}$ and ${t_2}\; = \; 12\; \mu \textrm{m}$ with reference to Fig. 4(b). The SCRs are placed in pairs of two facing each other at a distance $d\; = \; 75\; \mu m$. Then, the next SCR layer is rotated by 90° in a woodpile-style fashion (Fig. 4(b)). This two-layer structure constitutes the unit cell of the periodic system (Fig. 4(b)), which upon periodic repetition results in the full structure (Fig. 4(a)). The structure is excited by a linearly polarized plane wave that propagates along the z axis, and we obtain both reflected and transmitted power coefficients.
First, we focus on the uncoated SCR. Reflection and transmission (power coefficients) are depicted in Fig. 5(a) with black solid and dashed curves, respectively. The bandgap appears near 1 THz. Characteristic field distributions at the bandgap frequency and at a frequency below the bandgap are depicted in Fig. 5(b). A sharp difference in the degree of light localisation in the structure is clearly visible, as anticipated. Next, the coated structure is investigated by progressively considering 150-, 300-, 500- and 1000-nm thick ZnO coatings. As the coating thickens, the bandgap deepens due to the stronger refractive index contrast. In addition, we observe a redshift of the bandgap center frequency, coming from the increased average refractive index of the SCR “particles”. In all cases, though, the bandgap is found near 1 THz, in agreement with the experimental results (Fig. 3).
However, unlike the experimental data, our simulations show a relatively constant transmission level outside the forbidden band (compare Fig. 3 and 5(a)). To investigate the possible nature of this discrepancy, we repeat the simulations, but this time with different values of the imaginary part of the ZnO electric permittivity (to account for losses of the coating). More specifically, the electric permittivity
is assumed (based on previous experimental measurements) equal to 7.29 (black lines, Fig. 6), 7.29 + 0.5i (red), and 7.29 + 1.0i (green) [25]. Here, the ZnO coating thickness has been set to 500 nm as in the experiment. As the imaginary part increases, a fairly linear drop in the overall transmission level appears, which seems to match better our experimental measurements (see blue line in Fig. 3 and green dashed line in Fig. 6).Fig. 5. Simulation results for bare and ZnO-coated THz MMs. a) Reflection (solid) and transmission (dashed) power coefficients of uncoated (black) and 150 nm (red), 300 nm (blue), 500 nm (grey) and 1000 nm (green) ZnO-coated SCR MMs. In all cases, we consider ZnO as lossless. (b) Field distributions for the bare MM structure at around the bandgap frequency (right) and at a frequency below the bandgap (below). A sharp difference in the degree of light localisation in the structure is clearly visible.
Fig. 6. Simulation results for ZnO-coated THz MMs with a coating thickness of 500 nm, as in the experiment (Fig. 3). Reflection (solid) and transmission (dashed) power coefficients for 7.29 (black), 7.29 + 0.5i (red), and 7.29 + 1.0i (green) ZnO permittivity.
3. Summary
We have measured, for the first time to the best of our knowledge, the dispersive refractive index and absorption of frequently used IP-S and IP-Dip photoresists at the THz frequency range. This provides new possibilities for predicting the behavior of THz photonic devices. Two important features distinguish this work from previous works on photoresists: we have fabricated dielectric 3D MM structures with DLW, and compared the experimentally measured response (THz-TDS) against full-wave simulations, adopting the measured refractive index and absorption data. Our simulation results verify the THz-TDs data, highlighting the validity of the refractive index measurements. Further investigations may concern assembling a refractive index and absorption library of frequently used photoresists or other materials used for 3D printing at the THz range. The acquired properties of widely used photoresists should be effective in providing guidance for predicting and accurately simulating the electromagnetic response of complex 3D printed MM and photonic structures at THz frequencies.
Funding
Hellenic Foundation for Research and Innovation (916); Eidgenössische Materialprüfungs- und Forschungsanstalt (IRC 2021).
Acknowledgments
The authors would like to acknowledge the support of the company TOPAS Advanced Polymers GmbH for providing TOPAS substrate.
Disclosures
The authors declare no conflicts of interest.
Data availability
All data used for the figures are available upon reasonable request from the corresponding author.
Supplemental document
See Supplement 1 for supporting content. Regarding the dependence of number of layers along the z direction and bandgap as well as SEM images of the ZnO coating thickness.
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