Maximum Field Enhancement Research Articles

Fast On-Site Speciation and High Spatial Resolution Imaging of Labile Arsenic in Freshwater and Sediment Using the DGT-SERS Sensor.

Diffusive gradients in thin films (DGT) technique is renowned for in situ passive sampling but not for rapid on-site analysis, whereas surface-enhanced Raman spectroscopy (SERS) excels in ultrasensitive on-site detection but is limited by substrate contamination from complex matrices. Here, a hierarchical nanostructure of silver (Ag) mirror-supported large Ag nanoparticles (∼120 nm) was grown in situ in polyacrylamide hydrogel with a restricted pore size (PAM/Ag mirror/AgNPs) to serve as both the DGT binding phase and the SERS substrate. The substrate exhibited a maximum electric field enhancement factor of 9.9 × 108 and a signal relative standard error of 4.8%. Using the DGT-SERS sensor, As(III) and As(V) in freshwater were simultaneously detected at limits of 0.9 and 0.8 μg L-1, respectively, applicable across a wide range of environmental conditions. The DGT-SERS effectively mitigated the interfacial reduction of As(V) caused by humic acid by excluding it from plasmonic hotspots through size exclusion of the diffusive layer. The Raman analysis of a DGT sample in the field requires only 2 s using a portable spectrometer without DGT device disassembly. More importantly, the DGT-SERS captured the first two-dimensional image of As(III) and As(V) in one DGT at the micron scale resolution, revealing their spatially supplementary distribution patterns at the sediment-water interface. This study paves the way for next-generation speciation imaging DGT and the application of SERS in complex environments.

Polarization-insensitive terahertz third-harmonic generation from degenerate pairs of mirror-coupled super-BICs

Resonant nanostructures have emerged as versatile photonic platforms for boosting optical nonlinear responses on a subwavelength scale for their ability to confine intense electromagnetic fields while relaxing the phase-matching requirements. Recent significant advances in this field are associated with the utilization of non-radiative eigenmodes above the light cone, termed bound states in the continuum (BICs), which provide a unique mechanism for light trapping to realize excitation of ultrahigh quality (Q) factor resonances. Nevertheless, the current studies on BICs predominantly focus on symmetry-protected BICs (SP-BICs), whose excitation requires symmetry breaking, and Q factors are limited by fabrication imperfections. Here, we demonstrate a simple and feasible scheme for creating degenerate pairs of mirror-coupled super-BICs by harnessing magnetic dipole resonances coupled to their mirror images in adjacent metal films. Unlike trivial SP-BICs, mirror-coupled BICs showcases the huge enhancement of Q factors and are resilient against fabrication imperfections. By combining mirror-coupled resonance with the engineered radiative loss, we obtain a perfect absorber with near-unity absorption and ultra-narrow bandwidth at a critical coupling condition. Finally, we numerically demonstrate the terahertz (THz) regime, polarization-insensitive highly efficient third-harmonic generation benefiting from the maximum field enhancement localized within the perfect absorber. Our work not only paves the way toward unlocking the full potential of BIC resonance but also promise valuable insights for developing efficient THz optoelectronic devices and metadevices across a wide range of fields.

Enhanced field emission performance of holey expanded graphite by heat treatment

Expanded graphite (EG) is a promising material in field emission due to its high conductivity, thermal stability, and low cost. In this paper, the field emission performance of EG is enhanced by introducing hole defects through partial air oxidation. First, EG was prepared using electrochemical intercalation and microwave-assisted expansion methods. Subsequently, holey expanded graphite (HEG) was obtained by air-heating treatment of EG at different temperatures. Finally, bulk HEG cathodes were prepared by the cold pressing method, and the holes' effect on field emission performance was investigated. The results showed that EG's conductivity and tensile properties significantly improved after heat treatment. Optimal conditions of heat treatment were established at 400 °C providing material with the best field emission performance. The turn-on field (at 1 mA/cm2) and threshold field (at 10 mA/cm2) are 3.01 V/μm and 3.42 V/μm, respectively, with a maximum current density of 1359.17 mA/cm2 (at 5.83 V/μm) and a maximum field enhancement factor of 2407. The voltage fluctuation is only 1.1 % over 24 h at 3 mA @ 440.33 mA/cm2, indicating good emission stability and operation life.

Plasmonic trimers designed as SERS-active chemical traps for subtyping of lung tumors

Plasmonic materials can generate strong electromagnetic fields to boost the Raman scattering of surrounding molecules, known as surface-enhanced Raman scattering. However, these electromagnetic fields are heterogeneous, with only molecules located at the ‘hotspots’, which account for ≈ 1% of the surface area, experiencing efficient enhancement. Herein, we propose patterned plasmonic trimers, consisting of a pair of plasmonic dimers at the bilateral sides and a trap particle positioned in between, to address this challenge. The trimer configuration selectively directs probe molecules to the central traps where ‘hotspots’ are located through chemical affinity, ensuring a precise spatial overlap between the probes and the location of maximum field enhancement. We investigate the Raman enhancement of the Au@Al2O3-Au-Au@Al2O3 trimers, achieving a detection limit of 10−14 M of 4-methylbenzenethiol, 4-mercaptopyridine, and 4-aminothiophenol. Moreover, single-molecule SERS sensitivity is demonstrated by a bi-analyte method. Benefiting from this sensitivity, our approach is employed for the early detection of lung tumors using fresh tissues. Our findings suggest that this approach is sensitive to adenocarcinoma but not to squamous carcinoma or benign cases, offering insights into the differentiation between lung tumor subtypes.

Corn-inspired high-density plasmonic metal-organic frameworks microneedles for enhanced SERS detection of acetaminophen

Effective monitoring of acetaminophen (APAP) dosage is crucial for preventing antipyretic abuse, ensuring therapeutic efficacy, and minimizing toxic effects. However, existing self-monitoring methods are limited. In this study, we designed a plasmonic microneedle (MN) sensor for real-time nondestructive monitoring of acetaminophen levels in dermal interstitial fluid (ISF) by employing a handheld Raman spectrometer. The fabricated MN sensor incorporated a high-density plasmonic MOFs known as HDPM, which unique structure of large specific surface area, specific pore structure as well as high density gold nanospheres packing enabled the excellent performance of selective ISF drug enrichment and surface-enhanced Raman scattering (SERS). The maximum electric field enhancement factor of the HDPM nanostructure could be calculated as 5.73 × 107. The developed HDPM@MNs was characterized with a core-shell type “soft on the outside and rigid on the inside” structure, which exhibited sufficient hardness and flexibility to penetrate the dermal tissue with little damage, and robust SERS enhancement effect in APAP detection without any interfering peaks. Through a hydrogel drug simulation experiment, the sensor demonstrated robust capabilities for acetaminophen enrichment and monitoring, exhibiting excellent stability and repeatability. The quantitative detection window spanned from 1 to 100 μM, with a low detection limit reaching 0.45 μM. Furthermore, by monitoring the concentration of acetaminophen in the interstitial fluid of rat skin at different doses and for different administration times, the HDPM@MNs can be used to determine the pharmacokinetics of acetaminophen in rats and the physiological characteristics associated with various dosage regimens. This work not only holds promise for drug monitoring but also provides a novel approach for nondestructive monitoring of other crucial low-abundance physiological markers.

Spatially Resolved Near Field Spectroscopy of Vibrational Polaritons at the Small N Limit.

Vibrational polaritons, which have been primarily studied in Fabry-Pérot cavities with a large number of molecules (N ∼ 106-1010) coupled to the resonator mode, exhibit various experimentally observed effects on chemical reactions. However, the exact mechanism is elusively understood from the theoretical side, as the large number of molecules involved in an experimental strong coupling condition cannot be represented completely in simulations. This discrepancy between theory and experiment arises from computational descriptions of polariton systems typically being limited to only a few molecules, thus failing to represent the experimental conditions adequately. To address this mismatch, we used surface phonon polariton (SPhP) resonators as an alternative platform for vibrational strong coupling. SPhPs exhibit strong electromagnetic confinement on the surface and thus allow for coupling to a small number of molecules. As a result, this platform can enhance nonlinearity and slow down relaxation to the dark modes. In this study, we fabricated a pillar-shaped quartz resonator and then coated it with a thin layer of cobalt phthalocyanine (CoPc). By employing scattering-type scanning near-field optical microscopy (s-SNOM), we spatially investigated the dependency of vibrational strong coupling on the spatially varying electromagnetic field strength and demonstrated strong coupling with 38,000 molecules only-reaching to the small N limit. Through s-SNOM analysis, we found that strong coupling was observed primarily on the edge of the quartz pillar and the apex of the s-SNOM tip, where the maximum field enhancement occurs. In contrast, a weak resonance signal and lack of coupling were observed closer to the center of the pillar. This work demonstrates the importance of spatially resolved polariton systems in nanophotonic platforms and lays a foundation to explore polariton chemistry and chemical dynamics at the small N limit-one step closer to reconcile with high-level quantum calculations.

Regulating the aspect ratio of bulk few-layer graphene to improve the field emission performance

Owing to its excellent electrical conductivity and local electric field enhancement capability from abundant electron tunneling edges, graphene is a promising material for field emission. However, graphene thin-film emitters mostly emit in planar structures, and due to the flexibility of the thin film, it is commonly challenging to achieve vertical structure emission with large aspect ratios. Therefore, the emission performance of current graphene field emitters is difficult to meet the requirements of high current applications. In the present investigation, bulk few-layer graphene cathodes are formed by cold pressing to achieve upright structural field emission, and the emission performance is enhanced by appropriately adjusting the aspect ratio. The obtained bulk few-layer graphene emitters exhibit a higher conductivity of 2498 S/cm and tensile strength of 6.02 MPa. The turn-on electric field (at 1 mA/cm2) and threshold electric field (at 10 mA/cm2) in order are determined to be 1.78 V/μm and 2.66 V/μm, with a maximum current density of 986 mA/cm2 (at 6.65 V/μm) and a maximum field enhancement factor of 14,891. Excellent emission stability is achieved over a continuous period of 110 h at 5 mA @ 526 mA/cm2. With the increase of emission current, the bulk few-layer graphene with large aspect ratios exhibits better emission stability. This work provides valuable insights into the application of graphene-based field emitters at high currents.

Near-field Focusing in Ground Penetrating Radar Based on Phase Compensation Algorithm

Underground energy intensity and electromagnetic wave penetration depth are important parameters to characterize the detection performance of Ground Penetrating Radar (GPR). Near-field focusing (NFF) is an effective means to improve the electromagnetic energy fed into the ground by the antennas. In this paper, the phase compensation algorithm considering underground media is derived. In order to verify the effectiveness of the algorithm, the NFF antenna array is proposed. By changing the phase of the NFF array, the focus distance can be adjusted in the range of 400 mm – 800 mm. The maximum field enhancement coefficient in the focal region can achieve 19.2 dB. This shows that NFF array based on phase compensation algorithm has potential application prospects in increasing GPR detection performance.

Realistic prediction and engineering of high-Q modes to implement stable Fano resonances in acoustic devices

Quasi-bound states in the continuum (QBICs) coupling into the propagating spectrum manifest themselves as high-quality factor (Q) modes susceptible to perturbations. This poses a challenge in predicting stable Fano resonances for realistic applications. Besides, where and when the maximum field enhancement occurs in real acoustic devices remains elusive. In this work, we theoretically predict and experimentally demonstrate the existence of a Friedrich-Wintgen BIC in an open acoustic cavity. We provide direct evidence for a QBIC by mapping the pressure field inside the cavity using a Laser Doppler Vibrometer (LDV), which provides the missing field enhancement data. Furthermore, we design a symmetry-reduced BIC and achieve field enhancement by a factor of about three compared to the original cavity. LDV measurements are a promising technique for obtaining high-Q modes’ missing field enhancement data. The presented results facilitate the future applications of BICs in acoustics as high-intensity sound sources, filters, and sensors.

A New 3D Frequency-Selective Structure for 5G Communication

Background: In this paper, a new frequency-selective structure (FSS) for 3 to 4 GHz frequency band of fifth generation (5G) is proposed as a result of an analytical mode-matching method. Methods: A new periodic structure with stepped rods is designed using a closed-form equation derived by the analytical mode-matching method. Performance of the structure is simulated by different numerical packages. Results: The analytical and simulation results demonstrate that the designed structure transmits incident waves in 3.4 to 3.9 GHz frequency range with return loss lower than 10 dB and insertion loss of about 0.5 dB. The structure reflects the frequencies out of this range, especially wireless local area network (WLAN) 5 GHz, which is adjacent to this band. Furthermore, the performance of the proposed structure is independent of the TE and TM polarization of the incident wave and relative to the angle of the incident wave up to 60 degrees from perpendicular to the FSS surface, it has minor variations of about 8% in the transmitted frequency bandwidth. In addition, the average value of maximum field enhancement factor (MFEF) as the ratio of maximum field magnitude on the FSS surface to the magnitude of the incident field, used for assessing power handling capability of the structure, is about 4.5. Conclusion: Therefore, these features make the proposed structure suitable for 5G communication and high power systems.

Miniaturised element frequency selective surface design for high power microwave applications

AbstractA third‐order miniaturised element frequency selective surface (MEFSS) for high‐power microwave (HPM) systems is proposed. The MEFSS design is characterised by the excellent capability to handle high‐power waves and the stable frequency response at large incidence angles. Through combining the particle swarm optimisation algorithm and the equivalent circuit method to improve the efficiency, the design has low resonant temperature while maintaining a low maximum field enhancement factor. A theoretical guide for the selection of substrate parameters in high‐power systems is provided. Instead of being as small as possible, the loss tangent should be properly selected to reach a balance between metal loss and dielectric loss. To the author's best knowledge, it is the first time in‐band low temperature and out‐of‐band high temperature suppression at various incidence angles are realised. The results show that the frequency response remains stable for incident angles up to 60°, and the maximum temperature is reduced from 148°C to 44°C for incident power density of 10 kW/m2. A MEFSS prototype was fabricated, and the transmission coefficients were measured. The full‐wave simulation and measurement results agree well. It has significance to the engineering progress of FSS as radomes for HPM antenna systems.

Numerical Calculation of Magnetic Field Enhancement and Impact of Surface Defects on Premature Entry of Magnetic Field in Thin Nb Films for SRF Cavities

In this work, we have investigated two series of Nb/Cu samples deposited by HiPIMS technique, and differing in Nb deposition conditions, Nb film thickness and Cu substrate polishing techniques. All the films were additionally irradiated by Nd:YAG laser to smooth their surfaces. The impact of the magnetic field enhancement at the surface defects on the premature start of magnetic field penetration into the superconducting film was studied, combining experiments and numerical calculations. Compared to previous study, improved numerical calculations by using the Finite Element Method (FEM) served to calculate the maximum field enhancement factor β <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> , reflecting impact of the most crucial surface defects found in the samples. Magnetization measurements at 4.2 K in DC magnetic field, oriented parallel to the film, were employed to determine the start of the field penetration <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">en</sub> . The SEM and AFM analyses served to investigate the Nb surface morphology. In some samples, deviations from the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">en</sub> (β <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> ) dependence were observed. It was found that the magnetic field penetration could start from the Nb/Cu interface rather than from the free Nb surface due to visibly better quality of the free Nb surface observed by SEM analysis, and that could lead to the deterioration of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">en</sub> (β <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> ) dependence.

The anapole state excited by an oblique incidence

Anapole states supported by high-refractive-index dielectric nanoparticles have mostly been studied under normal incidence, but this work explores the oblique incidence excitation. For a single silicon nanodisk, as the incident angle (θ) increases, the anapole wavelength undergoes a gradual blueshift, while the wavelength of maximum near-field enhancement remains almost unchanged with increasing E-field enhancement factor (|E/E 0 |) due to phase retardation effect caused by oblique incidence, and some unique features in field distributions differed from normal excitation are exhibited. In the case of a silicon nanodisk array, the anapole state and near-field enhancement are affected by near-field coupling and the phase retardation effect is weakened. With increasing θ, the coupling between the units is enhanced, and the anapole wavelength and maximum field enhancement wavelength both blue shift. The field distributions in anapole wavelength and maximum enhancement wavelength have obvious near-field coupling characteristics. Oblique incident excitation gives us a deeper understanding of anapole state and may have potential applications in nanophotonics.

Terahertz asymmetric metallic hole arrays with polarization-independent quasi-bound states in the continuum for membrane sensing.

Resonances with both high-quality factor and polarization-independent characteristics are highly desirable for terahertz (THz) sensing. Here, THz sensors based on asymmetric metallic hole arrays (AMHAs) are experimentally demonstrated. Such sensors consisting of four-hole arrays support polarization-independent quasi-bound states in the continuum (BICs). The induced quasi-BIC presents a quality factor exceeding 2000, which enables enhanced sensing for thin membranes. Results show that the frequency shift is 97.5 GHz for the 25-µm thick polyimide (PI), corresponding to a sensitivity of 147.7 GHz/RIU. The sensing performance strongly relates to the enhanced field originating from sharp quasi-BICs. A maximum field enhancement of 15.88 in contrast to the incident field is achieved. When the PI thickness is large than the decay length of enhanced fields, the interaction strength of field-PI becomes weak, resulting in a saturation effect for the shift of quasi-BICs. The proposed sensor possessing polarization-independent quasi-BICs has great potential for practical sensing applications in real-time chemical and biomolecular.

Enhanced upconversion luminescence using long-range surface plasmon resonance mode with prism coupler

In the study, we present the significant enhancement of the upconversion luminescence by taking advantage of long-range surface plasmon resonance mode. Compared with the reflection enhanced upconversion luminescence of Ag film, the intensity of upconversion luminescence enhanced by long-range surface plasmon resonance mode is improved 100 times. Additionally, the finite-difference time-domain simulation indicates that the incident light energy can concentrate in the upconversion nanoparticles doped dielectric layer at the long-range surface plasmon resonance mode, which the maximum electric field enhancement factor |E|2/|E0|2 can reach to 58. The experimental result shows that the upconversion luminescence enhancement is related to the thickness of doped Polymethylmethacrylate (PMMA) layer. Furthermore, due to the excited upconversion luminescence having high directionality, it is shown that the strong upconversion luminescence could be received at the specific angle.

Dielectric dual-dimer metasurface for enhanced mid-infrared chiral sensing under both excitation modes

Abstract Chirality (C) is a fundamental symmetry property of objects. Detecting and distinguishing molecular chirality in the infrared spectrum is important in life sciences, biology, and chemistry. In this paper, we demonstrate an achiral metasurface based on a gaped dual-germanium-dimer array for enhanced mid-infrared chiral sensing under both circularly polarized light (CPL) and linearly polarized light (LPL) excitations. With the metasurface, strong electric and magnetic dipole resonances with large field enhancement can be generated, resulting in an accessible superchiral hotspot in the dimer gaps under both excitation modes. The maximum electric and magnetic field enhancements exceed 220 and 100 for the bare metasurface, and exceed 70 and 60 for the metasurface coated with a 50 nm chiral biolayer under both excitations, respectively. Importantly, a high volume-averaged C enhancement C E_ave of 241 (444) and C E_ave_bio of 161 (102) under CPL (LPL) excitation can be achieved for the bare metasurface and it coated with the chiral biolayer, respectively. These results may open up new possibilities for ultrasensitive vibrational circular dichroism (VCD) and rotational optical dispersion (ORD) spectroscopy in the mid-infrared range.

Materials for Dielectric Nanotweezers in the Near-Visible Region

Nanotweezers made of dielectric nanoantennas have enabled optical trapping and manipulation of nanoscale objects. Commonly used dielectric materials with a high refractive index, notably silicon (Si), suffer from absorption at visible wavelengths and thus Joule heating of the structure and the trapped object. They are thus normally considered unsuitable for trapping applications in this wavelength range. In this work, dielectric materials with low absorption at 785 nm are studied to minimize Joule heating. We consider a bowtie configuration, which is simulated for four different dielectric materials and optimized for maximum electric field enhancement. The results show a significant electric field enhancement in the bowtie gap, which can act as a “hotspot” for optical trapping and enhancement of Raman scattering of a single nanosphere. We numerically investigate optical trapping of nanospheres and compare the results with an analytical expression. Three nanospheres with different refractive indices are used as examples. We further study the temperature increase and the thermally induced flow around the bowtie nanoantenna to understand if and how the trapping is perturbed by Joule heating. We find that nanoantennas made by Si on a Si substrate, although absorbing, can be used for trapping quantum dots (QDs) and polystyrene (PS) beads. The required input intensities for stable trapping are approximately 10 and 50 mW/μm2, for QDs and PS beads, respectively, inducing temperature increases of ∼1.4 K and ∼7 K, respectively. For transparent materials, the input intensity is the limiting factor, and gallium phosphide (GaP) requires the lowest input intensity due to its high refractive index. However, Si is a far more common and standard material and thus gives a significantly easier fabrication process.

Simultaneous, Label-Free and High-throughput SERS Detection of Multiple Pesticides on Ag@Three-Dimensional Silica Photonic Microsphere Array

Rapid identification and quantitative simultaneous analysis for multiple pesticide in real samples based on surface-enhanced Raman spectroscopy (SERS) is still a challenge because of sample complexity, reproducibility, and stability of SERS substrate. With use of colloidal silver nanoparticles loaded three-dimensional (3D) silica photonic microspheres (SPMs) array as the analytical platform, a SERS-based array assay for multiple pesticides was developed in this work. The silver nanoparticles were fixed into the gaps formed by the self-assembled nanospheres of the 3D SPMs to produce "hot spots", on which the Raman enhanced effect was up to 9.86 × 107 and the maximum electric field enhancement effect reached to 9.75 times, ensuring the target pesticides on the surface of the SERS-substrate integrated SPM can be detected sensitively. Using 2,4-dichlorophenoxyacetic acid (2,4-D), glyphosate, and imidacloprid as the testing pesticides, the label-free and high-throughput SERS assay for simultaneous detection of the pesticides was established, giving good linear detection ranges (0.1-204.8 μg/mL for 2,4-D, 0.3-247.9 μg/mL for glyphosate, and 0.2-204.8 μg/mL for imidacloprid) and low detection limits (3.03 ng/mL for 2,4-D, 3.14 ng/mL for glyphosate, and 8.82 ng/mL for imidacloprid). The spiked recovery rates in the real samples were measured in the range of 82-112%, which was consistent with that of the classical standard methods. The label-free 3D SERS array analytical platform provides a powerful tool for high-throughput and low-cost screening of multiple pesticide residues in real samples.

Nanostructures for Solar Energy Harvesting

Renewable energy sources are becoming more and more essential to energy production as societies evolve toward a fossil-fuel-free world. Solar energy is one of the most abundant sources of green energy. Nanoantennas can be used to improve and enhance the absorption of light into a photovoltaic cell in order to generate more current. In this study, different nanoantenna structures are analysed in tandem with a silicon solar cell in an effort to improve its output. The nanoantennas studied are metallic aperture nanoantennas made up of either silver, aluminium, gold or copper. The three geometries compared are rectangular, circular and triangular. The maximum field enhancement obtained is for an aluminium rectangular nanoantenna of 50 nm thickness. Despite this, the geometry with more improvements compared with a basic silicon cell was the circle geometry with a 100 nm radius.

Thickness-dependent loss-induced failure of an ideal ENZ-enhanced optical response in planar ultrathin transparent conducting oxide films.

Ultrathin planar transparent conducting oxide (TCO) films are commonly used to enhance the optical response of epsilon-near-zero (ENZ) devices; however, our results suggest that thickness-dependent loss renders them ineffective. Here, we investigated the thickness-dependent loss of indium tin oxide (ITO) films and their effect on the ENZ-enhanced optical responses of ITO and ITO/SiO2 multilayer stacks. The experimental and computational results show that the optical loss of ITO films increases from 0.47 to 0.70 as the thickness decreases from 235 to 52 nm, which results in a reduction of 60% and 45% in the maximum field enhancement factor of a 52-nm monolayer ITO and 4-layer ITO/SiO2 multilayer stack, respectively. The experimental results show that the ENZ-enhanced nonlinear absorption coefficient of the 52-nm single-layer ITO film is -1.6 × 103 cm GW-1, which is 81% lower than that of the 235-nm ITO film (-8.6 × 103 cm GW-1), indicating that the thickness-dependent loss makes the ultrathin TCO films unable to obtain greater nonlinear responses. In addition, the increased loss reduces the cascading Berreman transmission valley intensity of the 4-layer ITO/SiO2 multilayer stack, resulting in a 42% reduction in the ENZ-enhanced nonlinear absorption coefficient compared to the 235-nm ITO film and a faster hot electron relaxation time. Our results suggest that the thickness and loss trade-off is an intrinsic property of TCO films and that the low-loss ultrathin TCO films are the key to the robust design and fabrication of novel ENZ devices based on flat ultrathin TCO films.