Invited Article: Narrowband terahertz bandpass filters employing stacked bilayer metasurface antireflection structures Open Access

The field of terahertz (THz) science and technology has attracted a great deal of attention over the past few decades for a wide range of potential applications including spectroscopy, imaging, chemical and biological sensing, medicine, security screening, material characterizations, and high-speed wireless communications, to name a few.^1–7 Numerous THz active photonic devices have been developed, such as sources,⁸ detectors,⁹ and spatial light modulators,¹⁰ along with desirable passive THz components,¹¹ all of which have witnessed the rapid improvement of device performance to efficiently generate, detect, and manipulate THz radiation. Important breakthroughs and inventions have been made; however, many opportunities and challenges in the THz frequency region remain to be identified and overcome.¹² One of these is the development of high-performance THz filters to select specific frequencies of interest, with a view of eliminating undesirable background radiation and enhancing the signal-to-noise ratio as well as spectral resolution in practical applications such as THz spectroscopy and imaging. An ideal narrowband THz bandpass filter should allow perfect transmission over a very narrow frequency interval while completely block any radiation (i.e., zero transmission) outside the passband. Moreover, the transition should be infinitely sharp (i.e., fast roll-off) from the peak to zero transmission. As compared to matured microwave technologies, the development of such a high-performance and cost-effective narrowband THz filter remains to be elusive.

THz narrow bandpass filters are typically realized using frequency selective surfaces based on freestanding resonant metal mesh or inverse metal grid array structures.^13,14 The suspended metal mesh bandpass filters can provide high transmission in the passband but entail nontrivial thin-film release fabrication steps and are very fragile. A mesh structure fabricated on a supporting dielectric substrate becomes mechanically more robust but introduces higher insertion loss owing to the absorption and Fresnel reflection of the substrate.¹⁵ Recently, alternative approaches to achieving THz bandpass filtering have also been intensively investigated. Subwavelength periodic structures such as one-dimensional metallic gratings¹⁶ or two-dimensional metal hole-arrays^17,18 have been exploited to facilitate THz bandpass filtering enabled by plasmonic enhanced optical transmission.^19,20 While the relative transmission normalized to the fraction of open area can be much enhanced, the absolute transmission through subwavelength holes at plasmonic resonances could still be quite low and is sensitive to the angle of incidence. The transmission passband resulting from the fundamental resonance is usually accompanied by a series of other resonant transmission peaks at higher-order²⁰ or fractional-order²¹ resonances with a fairly broad transmission background, thus severely degrading the bandpass filtering performance. For metal hole-array structures fabricated on a thick dielectric substrate, several non-resonant transmission peaks can also emerge due to the interference of multiple reflections within the substrate.¹⁸ Photonic crystals (PhCs) have been proposed theoretically to achieve bandpass filtering for THz waves, through the incorporation of lattice defects to create a high quality factor transmission passband in the photonic bandgap.^22,23 However, the proposed photonic crystal structures either require very complex fabrication processes or rely on a low-loss multilayer dielectric stack with a precise refractive index profile, which is essentially challenging to realize in the THz frequency region.

Metamaterials,^24,25 a class of artificial effective media composed of periodic arrays of subwavelength metal/dielectric inclusions, exhibit exotic optical properties unattainable by naturally occurring materials. Due to their design flexibility, the electromagnetic properties of metamaterials can be readily tailored to achieve desirable functionalities. Specifically, planar metamaterials, or metasurfaces,^26,27 have recently emerged as an attractive alternative to traditional bulk metamaterials, as they allow for unprecedented capability to manipulate the amplitude, phase, and polarization states of the incident electromagnetic fields, while can be easily fabricated with conventional processing technologies and suffer less losses. In the THz spectral region, few-layer metasurfaces have demonstrated various interesting phenomena and novel functionalities such as perfect absorption,^28–30 polarization conversion,^31–33 beam focusing,³⁴ and optical antireflection.³⁵ Previously, metamaterials have also been employed for THz bandpass filtering.^36–38 

Here, we demonstrate narrowband THz bandpass filtering by cascading multiple bilayer antireflection metasurface structures. Individual bilayer metasurfaces consist of a square array of silicon pillars with a self-aligned top gold resonator-array and a complementary bottom gold slot-array, enabling near-zero reflection and close-to-unity transmission over a narrow THz frequency interval, which is validated by both full-wave numerical simulations and experimental measurements with excellent agreement. We further show that, by simply stacking multiple identical bilayer metasurfaces, the transmission passband of the resulting filters significantly narrows as compared to that of a single bilayer metasurface, while maintaining ∼50% peak transmission power intensity in the passband with an extremely clean background (i.e., zero transmission) in the stop band. The demonstrated narrowband THz bandpass filters are easy to fabricate and mechanically robust and can be employed in a wide variety of applications where spectrally clean, and high THz transmission over a narrow bandwidth is desired.

The scanning electron microscopy (SEM) images in Figs. 1(a) and 1(b) show two bilayer metasurface structures for single-band antireflection operating at THz frequencies. The structures consist of a square array of silicon cylinders/pillars etched directly on a high-resistivity silicon substrate, where the cylinder/pillar top surface and the etched bottom surface between the cylinders/pillars, but not sidewall or underneath, are coated with a thin gold film. We note that anisotropic nanopillars covered with similar metalic structures can enhance the efficiency of polarization conversion (and thereby anomalous refraction),³⁹ and when covered with a continuous metal film, it is possible to eliminate the reflection over a narrow bandwidth (i.e., narrowband perfect absorption)⁴⁰ but not for the purpose of transmission enhancement. We first carried out full-wave numerical simulations using CST Microwave Studio, where a frequency-independent refractive index of 3.42 for silicon² and a 200 nm thick gold film with Drude conductivity⁴¹ were used. All geometric parameters, including the cylinder diameter d (or the side length of square pillar L), height h, and period p (p = 1.2 × d or p = 1.2 × L), have been tuned to optimize the antireflection performance. In Figs. 1(c) and 1(d), we plot the simulated reflection and transmission spectra, respectively, for the metasurface structure illustrated in Fig. 1(a) with diameter d varying from 32 to 64 μm but height fixed h = 8 μm, demonstrating a dramatic reduction of Fresnel reflection and significant enhancement of transmission at frequencies depending on the cylinder diameter. For cylinder diameter d = 48 μm, the reflectance is as low as R = 0.12% at 0.911 THz, and the transmittance is enhanced to T = 97.5%, in comparison to the Fresnel reflectance of 30% and transmittance of 70% for a bare silicon surface owing to the high refractive index. For other cylinder diameters, similar reflection reduction and transmission enhancement are observed, with a red-shifted (blue-shifted) operational frequency as the diameter increases (decreases), covering more than an octave frequency range when the diameter d varies from 32 to 64 μm. We also obtained similar behaviors (not shown) in the bilayer metasurface structure illustrated in Fig. 1(b), in light of the design degrees of freedom offered by metasurfaces. These simulation results indicate that it is possible to eliminate the Fresnel reflection and realize near-unity transmission using our bilayer metasurface structures, operating at a targeted THz frequency through tailoring the metasurface structural parameters. It is also worthwhile mentioning that other unit-cell structures such as cross-resonators may provide even more degrees of freedom in tailoring the spectral response as we have shown recently for dual and broadband optical antireflection.⁴² 

The designed bilayer metasurface structures were fabricated on a 1 mm thick high-resistivity (ρ > 10 000 Ω-cm), double-side-polished silicon substrate. The metasurface structures were defined using conventional contact photolithography in a photoresist layer, which was then served as an etching mask to etch the underlying silicon by 8 μm using a standard Bosch etching process. The remaining photoresist was then removed by oxygen plasma and a piranha wet clean process, forming a large-area (1 cm × 1 cm), square array of silicon cylinders/pillars. Finally, 10 nm/200 nm thick Ti/Au films were deposited using e-beam evaporation, where samples were perpendicular to the evaporation vapor flux to avoid the deposition on sidewall. SEM images of the fabricated bilayer metasurface structures are shown in Figs. 1(a) and 1(b), with the metal coating indicated by the false-colored unit cell in the insets. The fabricated metasurfaces were characterized under normal incidence using a standard terahertz time-domain spectroscopy (THz-TDS) setup schematically shown in Fig. 2, with THz transmission through air and reflection from a gold mirror serving as references.

The experimentally measured reflection and transmission spectra for the fabricated bilayer metasurface structure shown in Fig. 1(a) are plotted in Figs. 1(e) and 1(f), respectively, revealing excellent agreement with numerical simulations shown in Figs. 1(c) and 1(d). For the sample with a designed cylinder diameter d = 48 μm, the measured reflectance is R = 0.19%, and the transmittance reaches T = 94.3% at 0.983 THz. For the samples with other cylinder diameters, the measured reflectance is R < 2.5% and the transmittance is T > 91% at their respective operational frequencies spanning from 0.72 to 1.53 THz. By replacing cylinders with square pillars [see Fig. 1(b)] of the same height (i.e., h = 8 μm), the experimental reflection and transmission spectra are shown in Figs. 1(g) and 1(h), respectively, for a series of fabricated samples. For the sample with a designed square side length L = 40 μm, the measured reflectance is R = 0.27%, and the transmittance reaches T = 95.8% at 0.924 THz. For the samples with other square side lengths, the measured reflectance is R < 2.9%, and the transmittance is T > 90% at their respective operational frequencies spanning from 0.64 to 1.56 THz.

The dependence of antireflection frequency on geometric dimensions can facilitate polarization-dependent metasurface antireflection by breaking the four-fold rotation symmetry of the square pillars or circular cylinders, for example, by using rectangular or elliptical pillars as the unit-cell structure instead. We also observed in experiments small transmission dips and reflection peaks, for instance, at 1.541, 1.321, and 1.159 THz for the samples with cylinder diameters d = 48, 56, and 64 μm, respectively, reproducing those obtained at 1.521, 1.304, and 1.141 THz in numerical simulations. These spectral features are associated with surface plasmon polariton resonances excited in the subwavelength periodic gold hole arrays on the silicon substrate. The resonance frequencies for the gold hole arrays with corresponding structure periods and hole diameters are estimated⁴³ to be 1.523, 1.305, and 1.142 THz, consistent with both measured and simulated values.

The quality factor (Q-factor) of the single-band antireflection metasurfaces demonstrated in Sec. II is less than 2, which is rather low and unsuitable for narrow bandpass filtering, although it is possible to narrow the spectral bandwidth by employing other more complex resonant structures. The high-transmission we have achieved, however, provides an alternative avenue to realize narrow bandpass filters simply by cascading multiple antireflection metasurfaces. Assuming no interference (which is certainly untrue and will be discussed in detail in Sec. IV) among metasurfaces, the overall spectral response of the cascaded assembly can be understood as the multiplication of individual metasurface transmittance, which will significantly narrow the bandwidth when increasing the number of metasurface layers. Although the peak transmittance will decrease due to the less-than-unity transmittance of individual metasurfaces, it remains to be reasonably high for practical applications.

Identical bilayer antireflection metasurfaces based on square pillars [L = 40 μm, h = 8 μm, shown in Fig. 1(b)] were created on both the front and back sides of the silicon substrate through the same fabrication process described earlier. We followed the standard lithographic back-to-front side alignment procedure when patterning metasurfaces on the substrate back surface, although precise registration is not crucial. Therefore, each stack contains two bilayer metasurface structures, i.e., on front and back silicon surfaces. We then stacked multiple fabricated samples, as schematically shown in Fig. 3(a), by using double-sided tape (∼100 μm thick) at the sample edge to hold together the metasurface structures and create air spacers. Note that, with the ∼100 μm thick air spacer, the coupling between adjacent metasurfaces is negligible as we have shown before.⁴⁴ The overall transmission and reflection of the whole assembly is characterized using THz-TDS under normal incidence. The measured transmission and reflection spectra of the resulting bandpass filters are shown in Figs. 3(b) and 3(c), respectively. Note that in the THz-TDS measurements time-windowing has been employed to eliminate the echoes resulting from the multiple reflections within the 1 mm thick silicon substrate (corresponding to an echo time delay of ∼20 ps), but those multiple reflections caused by the much thinner air spacer were included when performing the Fourier transform to obtain the frequency-domain spectra.

The transmission spectra in Fig. 3(b) do show the narrow bandpass filtering features that we have expected. The transmission passband significantly narrows with increasing number of stacks, thereby greatly enhancing the frequency selectivity. The 3 dB bandwidth decreases from 0.54 THz (Q = 1.7) for a single bilayer metasurface [see Fig. 1(h)] to 0.18 THz (Q = 5.3) for a 4-stack assembly, improved by a factor of 3. We expect a metasurface design with narrower antireflection bandwidth would result in THz bandpass filters with an even higher Q-factor. The peak power transmission for the single-stack (i.e., 2 metasurfaces) filter is 85.4%, indicating that the transmittance of an individual bilayer metasurface is about 92.4%, which is slightly lower than the value of 95.8% shown in Fig. 1(h). This is most likely due to the fabrication error, as the measured etching depth h ≈ 9 μm here. For the 4-stack bandpass filter, we will expect 53% transmittance if simply considering the cascading. In experiments, a peak power transmission T = 45% at 0.95 THz [field transmission amplitude 67%, see the inset of Fig. 3(b)] is obtained within the narrow transmission band, despite the use of 8 metasurfaces in cascade, demonstrating the low-loss in individual metasurface layers and showing plenty of room for further improvement. The few percent excess loss is possibly due to the multiple reflections within the air spacer. We expect that, with reasonable further efforts, 70% power transmission (84% field amplitude) is achievable using such a 4-stack THz metasurface filter. The measured reflection spectra for the 1–4-stack THz metasurface filters are shown in Fig. 3(c), revealing near zero reflection at the operational frequency of 0.95 THz. It also shows that the reflection spectrum does not depend on the stack number, which is not surprising because of the time-windowing–only the first metasurface contributes to the reflection signal while the reflection from the other cascaded metasurfaces experiences a time delay and is therefore excluded from the Fourier transform. The results from the reflection measurements indicate that the reduction of the transmittance in the stacked metasurface assembly is mainly caused by the absorption of individual metasurface layers.

The demonstrated narrow bandpass THz filters based on stacked bilayer metasurfaces exhibit an ultra-clean background, indicating that the incident radiation is completely blocked outside of the desirable narrow passband. It is worth mentioning that the metasurface filters are expected to be fairly angle-independent, as it has been shown that the individual bilayer antireflection metasurfaces can operate over a wide angle range,³⁵ and varying the thickness of the subwavelength air spacer in the stacked-assembly has little effect on the filtering functionality. These are in remarkable contrast to many plasmonic filters with background transmission bands resulting from high-order resonances^20,21,45 or suffering from strong incidence angle dependence due to the nature of the surface plasmon polariton resonances.⁴³ The bandpass filters fabricated in this work are insensitive to the incident polarization due to the rotation symmetry of the square pillars, although polarization-dependent response can also be realized by using metasurfaces with other anisotropic unit-cell structures. It should be noted that the dipolar resonance in circular or square resonators is non-ideal for narrowband operation, and it is possible to further narrow the bandwidth by using other metasurface structures (e.g., split-ring resonators) with a narrower antireflection bandwidth.³⁵ We may also achieve multiple narrow passbands based on the bilayer antireflection metasurfaces we have demonstrated recently.⁴² The efficiency of our narrowband THz bandpass filters can be further enhanced by improving the transmission through a single bilayer metasurface structure. In experiments, the peak transmittance of an individual bilayer metasurface is a few percent lower than that obtained from our best fabricated metasurfaces or from numerical simulations where the metallic loss has been included. This ohmic loss represents the only intrinsic loss mechanism in our antireflection metasurfaces, and it can be as low as 1%–2%. The excess losses are mainly due to imperfect fabrication process, for instance, a slightly deviated etching depth and increased gold film roughness resulting from the reactive ion etching (RIE) of the silicon substrate. Improving the fabrication process (e.g., optimizing the etching depth) and performing a hydrogen annealing to eliminate the silicon roughness⁴⁶ will increase the overall efficiency of the stacked metasurface THz bandpass filters.

It should be noted that the present stacked metasurface narrow bandpass THz filters are suitable for pulsed THz applications, which allow us to take advantage of time-windowing in the Fourier transform to obtain the desirable narrow frequency response. For continuous wave (CW) THz radiation, the interference among individual metasurfaces will result in many narrow-spaced Fabry-Pérot-like sharp transmission peaks on top of those clean spectra shown in Fig. 3(b), even at stopped frequencies, therefore setting the limit of the application scope. Even for pulsed THz radiation, the substrate needs to be thick enough (in this work, we use 1 mm thick silicon) so to permit sufficient scanning time before the arrival of the first echo pulse. An alternate solution is using substrates that are much thinner than the wavelengths of interest so to avoid the interference peaks. Similarly, the air spacer needs to be either much larger or smaller than the wavelengths. In our case, we use the air spacer of 100 μm thickness, which contains about 30 round-trips within the air spacer if we consider the 20 ps time delay after the main pulse in the THz-TDS measurements. Taking all these factors into account and use the simulated S-parameters of the bilayer metasurface (slightly reduced S₂₁ and S₂₁ accounted for non-ideal performance in real device), we calculated the spectral response of our stacked metasurface THz filters using a simple multireflection model. The results are shown in Fig. 3(d), exhibiting excellent agreement with the measurements.

In order to further elucidate the impact of interference among metasurface layers on the performance of the narrow bandpass THz filters, we carried out additional theoretical calculations and experimental measurements by changing the air spacer thickness. In Fig. 4(a), we plot the calculated transmission spectra for 1–4-stack metasurface filters with air spacer thickness 740 μm, which corresponds to inserting one layer silicon bar (540 μm thick) in addition to two double-sided tapes (100 μm thick each). It is noted that there are sidelobes (i.e., local transmission maxima) observed in the transmission passband for multi-stack assemblies. The fact that a single-stack metasurface filter does not show any sidelobes suggests that the observed sidelobes are due to the interference between the metasurfaces that sandwich the air spacer, which contains 3 round-trips (i.e., echo pulses) when considering the 20 ps time-windowing. Using the same stacking configuration, the experimental results are plotted in Fig. 4(b), reproducing all the main features obtained in the calculations. These sidelobes were also observed with other air spacer thicknesses that are larger than the wavelengths but smaller than 3 mm, with the latter corresponding to the ∼20 ps time-windowing.

To summarize, we have shown single-band THz antireflection employing self-aligned bilayer metasurface structures. By tailoring the geometry and dimensions of the unit-cell structure, we realize numerically and experimentally near-unity transmission and simultaneously close-to-zero reflection over a narrow THz frequency region. The formation of these self-aligned bilayer antireflection metasurfaces relies mainly on the standard silicon etching and directional metal deposition steps, without involving any deposition of thick dielectric films required for THz wavelengths. Although the quality factor of a single metasurface is relatively low, taking advantage of the high transmission, we have further demonstrated high-performance THz bandpass filters by simply stacking multiple identical bilayer antireflection metasurfaces, achieving a reasonably high transmission in a very narrow passband, and a fast roll-off to an extremely clean background in the stop band. We also discuss possible improvements that may further enhance the efficiency of our narrowband THz bandpass filters. The absorption loss in our bilayer metasurfaces can apparently be managed and certainly is not an intrinsic property that fundamentally limits the use of our stacked bilayer metasurfaces as THz narrowband bandpass filters. We also discussed the impact of the substrate and spacer thicknesses on the filtering performance, particularly how to avoid the sidelobes for pulsed THz applications. Compared to conventional THz bandpass filters using suspended metal mesh structures or subwavelength metal hole-arrays, the demonstrated narrowband THz bandpass filters are easy to fabricate and mechanically robust and are potentially suitable for a wide variety of applications in the THz frequency region.

The authors acknowledge the partial financial support from Los Alamos National Laboratory LDRD program and Fundamental Research Funds for the Central Universities and Program for Innovation Research of Science in Harbin Institute of Technology (PIRS OF HIT) under Grant No. T201408. This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. Department of Energy, Office of Basic Energy Sciences, Nanoscale Science Research Center, operated jointly by Los Alamos and Sandia National Laboratories. Los Alamos National Laboratory, an affirmative action/equal opportunity employer, is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of the U.S. Department of Energy under Contract No. DE-AC52-06NA25396. C.-C.C. gratefully acknowledges Willard Ross for his assistance in device fabrication.