Squeezing Millimeter Waves through a Single, Nanometer-wide, Centimeter-long Slit

Chen, **aoshu; Park, Hyeong-Ryeol; Lindquist, Nathan C.; Shaver, Jonah; Pelton, Matthew; Oh, Sang-Hyun

doi:10.1038/srep06722

Squeezing Millimeter Waves through a Single, Nanometer-wide, Centimeter-long Slit

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Published: 24 October 2014

Volume 4, article number 6722, (2014)
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Squeezing Millimeter Waves through a Single, Nanometer-wide, Centimeter-long Slit

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**aoshu Chen¹^na1,
Hyeong-Ryeol Park¹^na1,
Nathan C. Lindquist²^na1,
Jonah Shaver¹^na1,
Matthew Pelton³^na1 &
…
Sang-Hyun Oh¹^na1

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Abstract

We demonstrate broadband non-resonant squeezing of terahertz (THz) waves through an isolated 2-nm-wide, 2-cm-long slit (aspect ratio of 10⁷), representing a maximum intensity enhancement factor of one million. Unlike resonant nanogap structures, a single, effectively infinitely-long slit passes incident electromagnetic waves with no cutoff, enhances the electric field within the gap with a broad 1/f spectral response and eliminates interference effects due to finite sample boundaries and adjacent elements. To construct such a uniform, isolated slit that is much longer than the millimeter-scale spot of a THz beam, we use atomic layer lithography to pattern vertical nanogaps in a metal film over an entire 4-inch wafer. We observe an increasing field enhancement as the slit width decreases from 20 nm to 2 nm, in agreement with numerical calculations.

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Introduction

Subwavelength apertures and gaps in metal surfaces can tightly localize electromagnetic waves well below the diffraction limit and lead to strong field enhancements^{1,2,Full size image}

Results

Using atomic layer lithography (Fig. 1b), we fabricated centimeter-long nanoslits. An array of rectangular patterns (0.8 cm × 0.1 cm; center-to-center spacing = 1 cm, as shown in Fig. 1c) in a gold film (150 nm in thickness with a 3-nm Ti adhesion layer) was first made on a 4-inch Pyrex glass wafer using photolithography, metal deposition and lift-off. ALD, at 250°C, was then used to conformally deposit a thin layer of alumina (Al₂O₃) over the pattern. The alumina film thickness was precisely defined, with Ångstrom-scale resolution, by the number of ALD cycles (measured deposition rate of 1.1 Å per cycle). A second layer of gold (150 nm) was deposited by directional electron-beam evaporation to fill the pattern and form the nanogap between the first and second metal layer.

After depositing the second metal layer, excess metal must be removed to expose the nanogaps. This type of critical step is typically done through a series of complicated planarization techniques; however, the poor adhesion between noble metals and oxides led us to the simple and effective solution of merely peeling off the excess metal with adhesive tape (Fig. 1b). Many groups have utilized peeling or strip** techniques for patterned metal fabrication. For example, the PEEL process²⁶ combines soft lithography and lift-off to produce free-standing metal films, while template strip**^27,28 from silicon molds produces ultra-smooth patterned metals. In the case of atomic layer lithography, wafer-scale peeling with adhesive tape (e.g. 3M Scotch™ tape) planarizes the vertically oriented nanogap patterns, dramatically simplifying the process flow. Considering that the second metal layer is only 100–200 nm thick and the width of the trench is on the millimeter scale, it would appear that planarization would be difficult. However, as long as we ensure that the tape does not contact the metal inside the trench, by gently applying it without any extra pressure, only the raised portion of the metal layer is removed. We tested structures with various sizes and shapes and this tape-based planarization process works well for structures with lateral dimensions as large as 1 cm.

One part of a long slit with a gap size of 20 nm is shown in the scanning electron microscope image (SEM) in Fig. 1d. For a 2 nm gap, a transmission electron microscope (TEM) was used to confirm the gap size, as shown in Fig. 1e. To confirm that uniform nanogaps are formed through the entire film thickness, we measured optical transmission spectra through the nanoslits. Using near-infrared spectroscopy (see Methods), we observed the first and second order Fabry–Pérot (FP) resonances for different gap sizes (Fig. 2)^5,22,29,30. As the gap size decreases, the resonances of the same order shift to longer wavelengths. In Fig. 2, we show that, when light with a vacuum wavelength of 1550 nm is confined to a 2 nm gap, it takes on a plasmon wavelength of λ_SP = 149 nm^5,22,30, corresponding to an effective refractive index of n_eff = 10.4.

We performed THz time-domain spectroscopy (THz-TDS)^18,31 (see Methods for details) to observe non-resonant transmission of THz waves through single nanoslits in a 150 nm-thick gold film with gap sizes of w = 2, 5, 10 and 20 nm. To illuminate only one side of the loop (that is, a single, straight slit) with the incident THz wave, we use a 1 mm-by-1 mm aperture in a stainless steel plate. A small but non-negligible direct transmission through the unpatterned gold film is subtracted when estimating the field enhancement in the nanogap¹⁸. Figure 3a shows the transmitted electric field amplitudes through the nanoslit samples and the unpatterned gold film on the glass substrate.

Transmitted electric field amplitude spectra were obtained by Fourier-transforming time-domain signals, which were then normalized to the reference signal collected for transmission through the glass substrate. We infer near-field enhancement factors by normalizing the transmitted amplitudes by the slit coverage ratio, according to Kirchhoff diffraction theory^18,32. This can be done because the minimum slit width of 2 nm is larger than the charge screening length (< 1 nm). Figure 3b shows that the enhancement varies approximately as 1/f, where f is frequency, over the entire measured spectral range. Previous work showed that the field enhancement increases as the slit width decreases down to tens of nanometers, due to capacitive charge accumulation from light-induced currents at the sidewalls^18,19,20,33. Reducing the slit width to 2 nm allowed us to achieve a non-resonant electric field (|E|) enhancement of 1300 at f = 0.3 THz; this corresponds to a maximum intensity (|E|²) enhancement of 10⁶, the highest yet obtained in single-slit studies. This enhancement is entirely due to the large ratio between the free-space wavelength of the THz wave and the narrow slit width and does not rely on any resonant effects. While the enhancement is smaller than that obtained for resonant ring apertures^22,34; it is highly broadband without any cut-off or interference effects and is limited only by the frequency response of the metal.

We also carried out two-dimensional finite-element modeling (FEM) using COMSOL to estimate the electric field enhancement as a function of the gap size, w. (For details of the simulations, see Methods.) In Fig. 4a, the simulated field enhancement factors are in good qualitative agreement with the measured factors, showing capacitor-like 1/w dependence^18,33. Figure 4b shows a cross-section of the electric field distribution around a 2-nm-wide slit. The enhanced electric field within the gap is nearly constant along its entire thickness. Strongly confined electric fields with enhancement factors over 1,000 near the entrance and exit corners are also observed.

Conclusions

We produced a wafer-scale array of 2-cm-long and 2-nm-wide gaps through optically thick metal films by combining photolithography, ALD and tape-based planarization. To demonstrate the utility of these extremely high-aspect-ratio (>10⁷) nanostructures, single-slit NIR transmission and THz-TDS were performed. Using these measurements we inferred a non-resonant field enhancement factor of 1300 when a millimeter wave passes through a single 2-nm-wide metal slit. As transmission through the gap and the resulting high field enhancements are non-resonant, they will be effective over a broad range of frequencies. The nanogaps could thus be used to enhance wave-mixing processes that involve extremely different frequencies, such as frequency conversion from the visible or NIR to the THz. Placing molecules and nanoparticles inside the gaps will lead to plasmon-enhanced luminescence; the extended slit length will make it possible to gather statistical data from a large number of single quantum emitters simultaneously. For sensing applications, cm-long nanogaps facilitate integration with a large array of microfluidic channels. Furthermore, by scaling up the loop length and packing density, it may be possible to study resonant transmission of microwaves through nanogaps².

Methods

Nanogap fabrication via atomic layer lithography

The nanogap is fabricated using the atomic layer lithography method developed in our previous work²². First, an array of 0.8 cm × 0.1 cm rectangular patterns in metal films are fabricated on a Pyrex glass substrate using negative resist (NR71-1500P) and photolithography, followed by metal deposition (150 nm Au with Ti adhesion layer) using electron-beam evaporation and lift-off in a solvent (1165 remover). After cleaning with acetone, methanol and IPA and blowing dry with N₂ gas, the patterned metal film is conformally coated with a thin Al₂O₃ layer by ALD at 250°C (Cambridge NanoTech Inc., Savannah^TM). Trimethylaluminum and H₂O vapor were sequentially pulsed through the ALD chamber and N₂ gas was used to purge the chamber after each injection. The thickness of the Al₂O₃ film is calibrated using ellipsometry on a Si reference water placed in the same chamber and the measured deposition rate for Al₂O₃ is 1.1 Å/cycle. After ALD, the trenches are filled with second metal by directional metal evaporation (Temescal). The metal deposited outside of the trench is easily removed using adhesive tape (single-sided 3M Scotch Magic Tape^TM) without leaving any residue.

Electron microscopy sample preparation

TEM samples are prepared by FIB milling. A thin film of Au/Al₂O₃/Au is cut across a nanogap at one side of the rectangular pattern. The lamella is then picked up by an Omniprobe and was attached to a TEM copper grid. The lamella is further thinned with ion beam milling to less than 100 nm and is imaged in a TEM (FEG-TEM, FEI Tecnai G2 30). For preparation of SEM samples, the top alumina layers on the metal surface are etched using reactive ion etching (RIE) with an inductively coupled plasma (Plasmalab System100 ICP180, Oxford Ltd.), to avoid charging effects. The samples are then imaged in an SEM (FEG-SEM, JEOL 6700).

Near-infrared spectroscopy

A broadband fiber-coupled, laser driven light source (Energetiq, EQ-99FC) is used to illuminate single nanogaps through a 600 µm fiber, reflective collimator, polarizer and condenser on top of an inverted microscope (Nikon Ti-S). Transmitted light is collected with a 50×, 0.9 NA objective and imaged onto the entrance slit of a 300 mm focal length imaging spectrometer (Acton SP2300i) equipped with a thermoelectrically cooled, 256-element InGaAs diode array (BaySpec Nunavut). The imaged nanogap length is 5 µm and 5 measurements are obtained, for a total length of 25 µm. The incident light polarization is adjusted for maximum throughput (perpendicular to the nanogap contour). Spectra are background subtracted and normalized using the spectrum for direct transmission through the glass substrate.

Terahertz time-domain spectroscopy

A terahertz pulse with a few picoseconds pulse width is generated from a GaAs-based photoconductive antenna (Tera-SED, Gigaoptics, GmbH) illuminated by a femtosecond Ti:sapphire laser pulse train with a center wavelength of 780 nm, 80 MHz repetition rate and 90 fs pulse width (Mai Tai XF, Newport Corporation). The p-polarized terahertz light passes through the nanogap samples at normal incidence and is detected by electro-optic sampling using a 1-mm-thick ZnTe crystal (INGCRYS Laser System Ltd.).

Two-dimensional Finite-Element Method simulations using COMSOL

We numerically simulated infinitely long metallic slits with various gap sizes between 2 nm and 20 nm using COMSOL. The Drude model is used to fit the optical parameters of the gold film at THz frequencies (plasma frequency 1.37 × 10⁴ THz, dam** parameter 40.7 THz)³⁵. The gold film has a thickness of 150 nm and a size-dependent dielectric constant for the Al₂O₃ layer inside the gap is used^22,36. Due to the extreme length-scale differences between the millimeter waves and nanometer gaps, care is taken while meshing the simulation area. The largest element size in free space is 20 µm whereas the smallest element size in the gap is 0.1 nm. Perfectly matched layers (PML) are used to minimize boundary reflections with plane-wave THz illumination. The simulation area is 9 mm high and 8 mm wide. Field enhancements within the gap are calculated by comparing the average amplitude within the entire gap thickness to a reference amplitude of the transmitted electric field through a glass/air interface without the nanogap or metal layer. After normalization, there is no difference between field enhancement factors with different incident light directions, i.e. from air to glass or from glass to air.

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Acknowledgements

This work was supported by the U.S. Department of Defense (DARPA Young Faculty Award N66001-11-1-4152; X.S.C., H.R.P., J.S., S.-H.O.). Device fabrication was performed at the University of Minnesota, Nanofabrication Center, which receives support from the National Science Foundation (NSF) through the National Nanotechnology Infrastructure Network program and the Characterization Facility, which has received capital equipment funding from NSF through the Materials Research Science and Engineering Center. S.-H.O. also acknowledges support from the Office of Naval Research Young Investigator Award. X.S.C. acknowledges support from the 3M Science and Technology Fellowship and the University of Minnesota Doctoral Dissertation Fellowship.

Author information

Chen **aoshu and Park Hyeong-Ryeol contributed equally to this work.

Authors and Affiliations

Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, MN, 55455, USA
**aoshu Chen, Hyeong-Ryeol Park, Jonah Shaver & Sang-Hyun Oh
Physics Department, Bethel University, Saint Paul, MN, 55112, USA
Nathan C. Lindquist
Department of Physics, University of Maryland, Baltimore, Baltimore County, MD, 21250, USA
Matthew Pelton

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**aoshu Chen
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Hyeong-Ryeol Park
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Nathan C. Lindquist
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Jonah Shaver
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Matthew Pelton
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Sang-Hyun Oh
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Contributions

X.S.C., H.R.P. and S.-H.O. conceived and designed the experiments. X.S.C. fabricated all of the samples and performed SEM and TEM analysis. H.R.P. constructed terahertz time-domain spectroscopy setup, measured terahertz transmission spectra and produced SEM and photographs of devices. N.C.L. performed finite-element modeling. J.S. performed near-infrared spectroscopic measurements and data analysis. All authors analyzed the data and wrote the manuscript together.

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Chen, X., Park, HR., Lindquist, N. et al. Squeezing Millimeter Waves through a Single, Nanometer-wide, Centimeter-long Slit. Sci Rep 4, 6722 (2014). https://doi.org/10.1038/srep06722

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Received: 07 August 2014
Accepted: 03 October 2014
Published: 24 October 2014
DOI: https://doi.org/10.1038/srep06722
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Squeezing Millimeter Waves through a Single, Nanometer-wide, Centimeter-long Slit

Abstract

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Amplification of resonant field enhancement by plasmonic lattice coupling in metallic slit arrays

Nonresonant 104 Terahertz Field Enhancement with 5-nm Slits

High-throughput fabrication of infinitely long 10 nm slit arrays for terahertz applications

Introduction

Results

Conclusions

Methods

Nanogap fabrication via atomic layer lithography

Electron microscopy sample preparation

Near-infrared spectroscopy

Terahertz time-domain spectroscopy

Two-dimensional Finite-Element Method simulations using COMSOL

References

Acknowledgements

Author information

Authors and Affiliations

Contributions

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Competing interests

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About this article

Cite this article

This article is cited by

Polaritonic nonlocality in light–matter interaction

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Squeezing Millimeter Waves through a Single, Nanometer-wide, Centimeter-long Slit

Abstract

Similar content being viewed by others

Amplification of resonant field enhancement by plasmonic lattice coupling in metallic slit arrays

Nonresonant 104 Terahertz Field Enhancement with 5-nm Slits

High-throughput fabrication of infinitely long 10 nm slit arrays for terahertz applications

Introduction

Results

Conclusions

Methods

Nanogap fabrication via atomic layer lithography

Electron microscopy sample preparation

Near-infrared spectroscopy

Terahertz time-domain spectroscopy

Two-dimensional Finite-Element Method simulations using COMSOL

References

Acknowledgements

Author information

Authors and Affiliations

Contributions

Ethics declarations

Competing interests

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About this article

Cite this article

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Polaritonic nonlocality in light–matter interaction

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