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Ultrafast all-optical switches based on epsilon-near-zero (ENZ)-enhanced nonlinear refraction in transparent conducting oxides have achieved exciting results in realizing large absolute modulations. However, broad-band, polarization-independent, and wide-angle ultrafast all-optical switches have been challenging to produce, due to the inherent narrow band, polarization-dependent, and angle-dependent characteristics of the ENZ effect. To this end, we propose an ultrafast all-optical switch based on the enhanced nonlinear absorption of corrugated indium tin oxide (ITO) thin films. Taking advantage of the perfect absorption and localized field enhancement of the ENZ and localized surface plasmon resonance modes, we significantly enhanced the nonlinear absorption of the corrugated ITO film in the 1450-1650 nm telecom band. The experimental results show that the nonlinear saturable absorption coefficient of the corrugated ITO film at 1450 nm was as high as -1.5 × 10

The electric field induced by the substrate could adjust the Schottky barrier from a p-type contact (WSe2/Gr) to an n-type contact (Gr/WSe2). Thus, it is more beneficial for the photo-thermionic electrons to transfer from graphene to WSe2 in Gr/WSe2.

We experimentally demonstrated the 3D propagation of laser filament in air by an Fabry-Pérot (F-P) cavity assisted imaging within a single exposure. The F-P cavity was composed of two parallel mirrors with certain reflectivity and transmission at filament laser, so that the beam was reflected and refracted multiple times between the two mirrors. The cross-sectional intensity patterns at different longitudinal positions along filament within a single exposure of CCD (Charge-coupled Device) were recorded. When keeping the incident angle of the F-P cavity as a constant and reducing its spacing distance, a better longitudinally resolved evolution of cross-sectional filament intensity patterns was obtained. The intensity evolution along laser filament by the F-P cavity assisted imaging method was consistent with the filament fluorescence measurement from the side. As an application, the transition of laser propagation from linear to nonlinear was unveiled by the F-P cavity assisted 3D imaging.

A single-shot measurement of electron emittance was experimentally accomplished using a focused transfer line with a dipole. The betatron phase of electrons based on laser wakefield acceleration (LWFA) is energy dependent owing to the coupling of the longitudinal acceleration field and the transverse focusing (defocusing) field in the bubble. The phase space presents slice information after phase compensation relative to the center energy. Fitting the transverse size of the electron beam at different energy slices in the energy spectrum measured 0.27 mm mrad in the experiment. The diagnosis of slice emittance facilitates local electron quality manipulation, which is important for the development of LWFA-based free electron lasers. The quasi-3D particle-in-cell simulations matched the experimental results and analysis well.

We demonstrate an ultra-broadband high temporal contrast infrared laser source based on cascaded optical parametric amplification, hollow-core fiber (HCF) and second harmonic generation processes. In this setup, the spectrum of an approximately 1.8 μm laser pulse has near 1 μm full bandwidth by employing an argon gas-filled HCF. Subsequently, after frequency doubling with cascaded crystals and dispersion compensation by a fused silica wedge pair, 9.6 fs (~3 cycles) and 150 μJ pulses centered at 910 nm with full bandwidth of over 300 nm can be generated. The energy stability of the output laser pulse is excellent with 0.8% (root mean square) over 20 min, and the temporal contrast is >1012 at –10 ps before the main pulse. The excellent temporal and spatial characteristics and stability make this laser able to be used as a good seed source for ultra-intense and ultrafast laser systems.

Temporal contrast directly affects the interaction between ultraintense and ultrashort pulse lasers with matter. Seed laser sources with broad bandwidth and high temporal contrast are significant for overall temporal contrast enhancement. The technique of cascaded nonlinear processes with optical parametric amplification and second-harmonic generation is demonstrated for high temporal contrast seed source generation. Within 40 ps before the main pulse, the temporal contrast reaches over 1011. The pulse energy and duration of the high-contrast pulse are 112 μJ and 70 fs, respectively. Considering its high beam quality and stability, this laser source can serve as a high-quality seed for Nd:glass-based ultraintense and ultrashort pulse laser facilities.

The Dirac fermion, a high-mobility electron in the Dirac cone of monolayer graphene, has significant potential for use in the terahertz probing technique. For undoped graphene, ultrafast terahertz ...

The space-time (ST) wave packet, a type of light source with many anomalous behaviors, has already found application in weak-field optics, however its generation method limits its energy and therefore its application in strong-field optics. Here we show that the type-I collinear optical parametric amplification (OPA) is a natural amplifier for a ST wave packet, because in it, the ST spectrum (i.e., angle-dependent phase-matching spectrum) of the signal and idler has the same analytical expression as that of a ST wave packet under the narrowband approximation. The high gain in a thin-crystal OPA allows for large energy amplification while ensuring unchanged ST correlation. Meanwhile, a double ST wave packet containing two collinear-propagating ones of the amplified signal and the generated idler via the nondegenerate OPA is reported, which with short- and long-wavelengths (relative to the degenerate-wavelength) have superluminal and subluminal velocities, respectively. This study realizes energy amplification, wavelength conversion, and velocity switching of a ST wave packet and will expand its application in many fields.

The upcoming 100 Petawatt (PW) laser is going to provide a possibility to experimentally study vacuum physics. Pulse compression and beam focusing, which can be affected by the spatiotemporal coupling, are two key processes of generating a 100 PW laser and then determine whether its physical objective can be achieved or not. We improved our previous model of the spatiotemporal coupling where only the grating wavefront error and the output optical field of a common compressor configuration were included, and in the improved model, the grating amplitude modulation, the spatio-spectral clipping, and the optical field inside the compressor were added. By using it, we theoretically investigated the spatiotemporal dynamics of an ultra-intense ultrashort laser passing through an imperfect grating compressor for different cases, especially the spatio-temporal/spectral coupling and the on-target intensity variation induced by the phase and amplitude modulation at different grating positions in two different compressor configurations. This study is of importance for both engineering development and physical application of the upcoming Exawatt-class laser.

In this paper, we report that the angular dispersion of the output pulses in a nonlinear process can be efficiently compensated by using a cascaded prism(s) and short hollow-core fiber (HCF) configuration. Here, the prism(s) is used to suppress the angular dispersion and transform it into spatial chirp, while the HCF is used for removing this spatial chirp and the residual angular dispersion, which can also significantly improve the beam quality. The feasibility of this novel method is numerically and experimentally investigated with the ultra-broadband idler pulses centered at 1250 nm wavelength and generated by an LBO crystal based non-collinear optical parametric amplifier. The proof-of-principle experiment shows that the angular dispersion can be effectively removed and ultra-broadband idler pulses with good spectral quality and spatial profile can be obtained. The total transmission efficiency in the experiment is around 67% and the measured M x 2 and M y 2 can reach 1.12 and 1.04, respectively. To the best of our knowledge, this is the first reported ultra-broadband angular dispersion compensation scheme combining prism(s) and HCF, which can remarkably eliminate the angular dispersion while simultaneously possesses high efficiency, good spectral and beam spatial quality.

The group-velocity of the propagation-invariant optical wave-packet generated by the conical superposition can be controlled by introducing well-designed arbitrarily-axisymmetric pulse-front deformation, which permits realizing superluminal, subluminal, accelerating, decelerating, and even nearly-programmable group-velocities. To better understand the tunability of the group-velocity, the generation methods of this propagation-invariant optical wave-packet and the mechanisms of the tunable group-velocity in both the physical and Fourier spaces are investigated. We also have studied the relationship with the recently-reported space–time wave-packet, and this group-velocity-tunable propagation-invariant optical wave-packet should be a subset of the space–time wave-packet.

X-ray free-electron lasers can generate intense and coherent radiation at wavelengths down to the sub-angstrom region1-5, and have become indispensable tools for applications in structural biology and chemistry, among other disciplines6. Several X-ray free-electron laser facilities are in operation2-5; however, their requirement for large, high-cost, state-of-the-art radio-frequency accelerators has led to great interest in the development of compact and economical accelerators. Laser wakefield accelerators can sustain accelerating gradients more than three orders of magnitude higher than those of radio-frequency accelerators7-10, and are regarded as an attractive option for driving compact X-ray free-electron lasers11. However, the realization of such devices remains a challenge owing to the relatively poor quality of electron beams that are based on a laser wakefield accelerator. Here we present an experimental demonstration of undulator radiation amplification in the exponential-gain regime by using electron beams based on a laser wakefield accelerator. The amplified undulator radiation, which is typically centred at 27 nanometres and has a maximum photon number of around 1010 per shot, yields a maximum radiation energy of about 150 nanojoules. In the third of three undulators in the device, the maximum gain of the radiation power is approximately 100-fold, confirming a successful operation in the exponential-gain regime. Our results constitute a proof-of-principle demonstration of free-electron lasing using a laser wakefield accelerator, and pave the way towards the development of compact X-ray free-electron lasers based on this technology with broad applications.

A new near-infrared direct acceleration mechanism driven by Laguerre-Gaussian laser is proposed to stably accelerate and concentrate electron slice both in longitudinal and transversal directions in vacuum. Three-dimensional simulations show that a 2-mu m circularly polarized LG(p)(l) (p = 0, 1 = 1, sigma(z) = -1) laser can directly manipulate attosecond electron slices in additional dimensions (angular directions) and give them annular structures and angular momentums. These annular vortex attosecond electron slices are expected to have some novel applications such as in the collimation of antiprotons in conventional linear accelerators, edge-enhancement electron imaging, structured X-ray generation, and analysis and manipulation of nanomaterials.

Despite the superior optoelectronic properties of quasi-two-dimensional (quasi-2D) Ruddlesden-Popper halide perovskites, the inhomogeneous distribution of mixed phases result in inefficient energy transfer and multiple emission peaks. Herein, the insufficient energy funneling process at the high-energy phase is almost completely suppressed and the excitonic understanding of gain nature is studied in the energy funneling managed quasi-2D perovskite via introducing poly(vinyl pyrrolidone) (PVP) additive. The energy transfer process is facilitated from 0.37 to 0.26 ps after introducing the PVP additive, accelerating the exciton accumulation in the emissive state, and increasing the ratio of the high-dimensional phase for enhancing radiative emission. The gain lifetime is promoted to be as fast as 28 ps to outcompete nonradiative recombination during the build-up of population inversion. Simultaneously, the net gain coefficient is increased by more than twofold that of the pristine perovskite film. Owing to the remarkable gain properties, room-temperature amplified spontaneous emission is realized with a low threshold of 11.3 μJ/cm

The space-time (ST) wave packet, a new type light source with many new features, has already found applications in weak-field optics, however its generation method limits its energy and therefore its application in strong-field optics. We report that the type-I collinear optical parametric amplification (OPA) is a natural amplifier for ST wave packets, because in it, the ST spectrum (i.e., angle-dependent phase-matching spectrum) of the signal and idler has the same analytical expression as that of ST wave packets under the narrowband approximation. The high gain in a thin-crystal OPA allows for large energy amplification while ensuring unchanged ST correlation. Meanwhile, for the first time, a double ST wave packet containing two collinear-propagating ones of the amplified signal and the generated idler via the nondegenerate OPA is reported, which with short- and long-wavelengths (relative to the degenerate-wavelength) have superluminal and subluminal velocities, respectively. This study realizes energy amplification, wavelength conversion, and velocity switching of ST wave packets and will certainly expand the application in many fields.

Emerging evidence shows that modulation of the microbiome can suppress intra-abdominal hypertension (IAH)-induced intestinal barrier damage through the regulation of amino acid (AA) biosynthesis. Here, we investigated the protective effects of orally gavaged Lactobacillus acidophilus L-92 (L92) and a mixture of AA in rats with induced IAH. The results showed that both L92 and AA pretreatments effectively mitigated IAH-induced intestinal damage. Interestingly, L92 but not AA prevented metagenomic changes induced by IAH. Bacteroides fragilis, Bacteroides eggerthii, Bacteroides ovatus, Faecalibacterium prausnitzii

Heterostructures constructed from graphene and transition metal dichalcogenides (TMDs) have established a new platform for optoelectronic applications. After a large number of studies, one intriguing debate is the existence of the interfacial exciton in graphene/TMDs. Hereby, by combined optical pump-terahertz probe spectroscopy and transient absorption spectroscopy, we report the observation of the interfacial exciton in graphene/MoS

Efficient interfacial light-electric interconversion in van der Waals (vdW) heterostructures is crucial for their optoelectronic applications. However, an in-depth understanding of the necessary process for device operation, namely interfacial charge transfer (CT), has thus far remained elusive. In this study, by using photon energy-dependent transient THz spectroscopy, we complementarily investigate the interfacial CT process in heterostructures comprising monolayers of WSe

The efficiency of antimony selenide (Sb2Se3) solar cells has been improved from 10% within only 7 years, but fundamental properties at the heterojunction interface such as the charge carrie...

When approaching the subwavelength or deep subwavelength scale, there is a fundamental trade-off between the ultimate shrinking size and the performance for miniaturized lasers. Herein, to overcome this trade-off, we investigated the excitonic gain nature of quasi-two-dimensional (quasi-2D) perovskites and revealed that both singlet excitons and polarons would make nearly the entire contribution within ∼50 ps to a high net gain of 558 cm-1. Inspired by the gain characteristic, we successfully shrank the quasi-2D perovskites laser to the subwavelength scale using only a layer of ultraviolet glue and a glass substrate in the vertical dimension. In spite of the compact and simple cavity structure, single-mode lasing with a highly linear polarization degree of 81% and a quality factor of 1635 was achieved. The extremely short cavity, excellent lasing performance, and simple structure of the quasi-2D perovskite laser are expected to provide insights into next-generation integrated laser sources.

We propose a method to generate circularly polarized (CP) attosecond pulses by the interactions of a relativistic-intensity right-hand CP laser pulse and magnetized sub-critical plasma. It is theoretically and numerically demonstrated that when an external magnetic field with an appropriate strength is applied to a sub-critical plasma along the laser propagation, the ponderomotive force of a right-hand CP laser at the vacuum-plasma boundary is significantly enhanced. The electrons are then steadily pushed forward until the timely-increasing charge separation field becomes strong enough to pull them back, forming a dense and counter-moving electron sheet. The relativistic-velocity electron sheet works as a flying mirror to compress the tail of the driving laser and efficiently generate a single CP attosecond pulse. The present scheme shows a stable efficiency on different scale lengths of preplasma and thus may provide a robust way to generate bright and CP attosecond pulses.

We demonstrate the stable and flexible light delivery of multi-microjoule, sub-200-fs pulses over a ∼10-m-long vacuumized anti-resonant hollow-core fiber (AR-HCF), which was successfully used for high-performance pulse synchronization. Compared with the pulse train launched into the AR-HCF, the transmitted pulse train out of the fiber exhibits excellent stabilities in pulse power and spectrum, with pointing stability largely improved. The walk-off between the fiber-delivery and the other free-space-propagation pulse trains, in an open loop, was measured to be −7. This walk-off can be further suppressed to ∼2 fs rms simply by using an active control loop, highlighting the great application potentials of this AR-HCF setup in large-scale laser and accelerator facilities.

The subcycle interaction of light and electrons has been one of the key frontiers in free-electron lasers, attosecond science and dynamical investigation of matter. Capturing the underlying subcycle dynamics of electrons with an optical field promises fascinating vistas with unprecedented temporal resolution. Yet the rigorous synchronization requirement has kept its realization out of reach. Here, by direct spatial observation of periodic electron bunch fringes, we demonstrate a laser streaking concept for revealing the dynamics of free electrons emitted from a plasma mirror under sub-relativistic laser intensity. Field-induced electron beam deflection demonstrates subcycle charge dynamics with a streaking speed of ~60 μrad as−1. This provides us with an attosecond-resolution metrology to obtain more direct evidence about the light-field-induced electron dynamics in the plasma mirror. Our results offer unprecedented characterization of attosecond dynamics and open the way to extensive experimental investigations of the interaction of attosecond electrons with intense lasers. By focusing a sub-relativistic infrared laser pulse onto a silica target, a periodic deflection pattern of attosecond electron pulse trains is observed. It reveals these subcycle charge dynamics with a streaking speed of ~60 μrad as−1.

The group-velocity of the Bessel-X optical wave-packet can be controlled by introducing well-designed arbitrarily-axisymmetric pulse-front deformation, which permits realizing superluminal, subluminal, accelerating, decelerating, and even nearly-programmable group-velocities. To better understand the tunability of the group-velocity, the generation methods of this Bessel-X optical wave-packet and the mechanisms of the tunable group-velocity in both the physical and the Fourier spaces are investigated. We also studied the relationship with the recently-reported space-time optical wave-packet, and the group-velocity-tunable Bessel-X optical wave-packet is a subsection of the space-time optical wave-packet.

To improve pulse contrast in chirped pulse amplification petawatt laser systems, the regenerative amplifier is substituted with a multipass amplifier at the Shanghai Superintense Ultrafast Laser Facility (SULF). To reduce the consequent angular dispersion of the broadband spectrum, a double-grating stretcher is established in the SULF front end. A grating compressor is set up for the 10-PW front end to obtain 20-TW output. An accurate adjustment method of grating attitude (angular position) is presented, which references the direction of gravity, improving dispersion management and focusing ability of the beam. After a pulse passes the front end compressor, its duration and phase in the frequency domain are measured, and the duration can be continuously compressed to &lt, 24 fs.

The rapid advancement of perovskite-based optoelectronics devices has caught the world’s attention due to their outstanding properties, such as long carrier lifetime, low defect trap density, large absorption coefficient, narrow linewidth and high optical gain. Herein, the photonic lasing properties of perovskites are reviewed since the first stimulated emission of perovskites observed in 2014. The review is mainly focused on 3D structures based on their inherently active microcavities and externally passive microcavities of the perovskites. First, the fundamental properties in terms of crystal structure and optical characteristics of perovskites are reviewed. Then the perovskite lasers are classified into two sections based on the morphology features: the ability/inability to support lasing behaviors by themselves. Every section is further divided into two kinds of cavities according to the light reflection paths (Standing wave for the Fabry–Pérot cavity and travelling wave for the Whispering-Gallery-Mode cavity). The lasing performance involves fabrication methods, cavity sizes, thresholds, quality factors, pumping sources, etc. Finally, some challenges and prospects for perovskite lasers are given.

Recently, solution-processed conventional I–III–VI2 quantum dots (QDs) demonstrated a potential optical amplification application based on superior optical properties. Nonetheless, exploiting the potential of this type of material as a gain medium for optical amplification devices is still scarce, rendering application prospect restricted. Herein, for the first time, we report the amplified spontaneous emission (ASE) action using AgInS2 (AIS) QDs as the gain medium. ASE with an ultra-low threshold (31.58 μW cm−2) was demonstrated by combining highly reflective distributed Bragg reflectors (DBR) with AIS QDs. Furthermore, the relationship between the number of layers of the DBR mirrors and the ASE performance is systematically discussed, indicating that the seven-layer DBR is a desirable device structure with the best ASE from AIS QDs. These consequences unambiguously uncover the significant feasibility of the traditional AIS QDs as a photonic resource, simultaneously demonstrating the huge potential and promising applications of as-designed devices for optical amplification.

We report dispersion management based on a mismatched-grating compressor for a 100 PW level laser, which utilizes optical parametric chirped pulse amplification and also features large chirped pulse duration and an ultra-broadband spectrum. The numerical calculation indicates that amplified pulses with 4 ns chirped pulse duration and 210 nm spectral bandwidth can be directly compressed to sub-13 fs, which is close to the Fourier-transform limit (FTL). More importantly, the tolerances of the mismatched-grating compressor to the misalignment of the stretcher, the error of the desired grating groove density and the variation of material dispersion are comprehensively analyzed, which is crucially important for its practical application. The results demonstrate that good tolerances and near-FTL compressed pulses can be achieved simultaneously, just by keeping a balance between the residual second-, third- and fourth-order dispersions in the laser system. This work can offer a meaningful guideline for the design and construction of 100 PW level lasers.

High-energy ultraviolet pulse generation in gas-filled hollow capillary fibers (HCFs) through dispersive-wave-emission process, has attracted considerable attentions in recent several years due to its great application potentials in ultraviolet spectroscopy and photochemistry. While the ability of this technique to deliver high-energy, ultrafast ultraviolet pulses is widely recognized, few-fs duration of {\mu}J-level dispersive-wave (DW) pulses has, however, escaped direct measurements. In this letter, we demonstrate that using several chirped mirrors, few-fs ultraviolet DW pulses can be obtained in atmosphere environment without the use of vacuum chambers. The pulse temporal profiles, measured using a self-diffraction frequency-resolved optical gating set-up, exhibit full-width-half-maximum pulse widths of 9.6 fs at 384 nm and 9.4 fs at 430 nm, close to the Fourier-transform limits. Moreover, theoretical and experimental studies reveal the strong influences of driving pulse energy and HCF length on temporal width and shape of the measured DW pulses. The ultraviolet pulses obtained in atmosphere environment with {\mu}J-level pulse energy, few-fs pulse width and broadband wavelength tunability are ready to be used in many applications.

We demonstrate the simultaneous temporal contrast improvement and pulse compression of a Yb-doped femtosecond laser via nonlinear elliptical polarization rotation in a solid state multi-pass cell. The temporal contrast is improved to 109, while the pulse is shortened from 181 to 36 fs, corresponding to a compression factor of 5. The output beam features excellent beam quality with M2 values of 1.18 × 1.16. The total efficiency of the contrast enhancement system exceeds 50%. This technique will have wide applications in high temporal contrast ultra-intense femtosecond lasers.

We report on a vortex laser chirped-pulse amplification (CPA) system that delivers pulses with a peak power of 45 TW. A focused intensity exceeding 1019 W/cm2 has been demonstrated for the first time by the vortex amplification scheme. Compared with other schemes of strong-field vortex generation with high energy flux but narrowband vortex-converting elements at the end of the laser, an important advantage of our scheme is that we can use a broadband but size-limited q-plate to realize broadband mode-converting in the front end of the CPA system, and achieve high-power amplification with a series of amplifiers. This method is low cost and can be easily implemented in an existing laser system. The results have verified the feasibility to obtain terawatt and even petawatt vortex laser amplification by a CPA system, which has important potential applications in strong-field laser physics, for example, generation of vortex particle beams with orbital angular momentum, fast ignition for inertial confinement fusion and simulation of the extreme astrophysical environment.

Infrared femtosecond optical vortices open up many new research fields, such as optical micro–nano manipulation, time-resolved nonlocal spectroscopy in solids, vortex secondary radiation and particle generations. In this article, we demonstrate a femtosecond optical vortex laser system based on a two-stage optical parametric amplifier. In our experiment, 1.45 μm vortex signal pulses with energy of 190 μJ and 1.8 μm vortex idler pulses with energy of 158 μJ have been obtained, and the pulse durations are 51 and 48 fs, respectively. Both the energy fluctuations of the signal and idler pulses are less than 0.5% (root mean square), and the spectral fluctuations are less than 1.5% within 1 hour. This type of highly stable femtosecond optical vortex laser has a wide range of applications for vortex strong-field physics.

Here, we report the recent progress on the front end developed for the 100 PW-class laser facility. Using 3 stages of optical parametric chirped-pulse amplification (OPCPA) based on lithium triborate (LBO) crystals, we realized a 5.26 J/0.1 Hz amplified output with a bandwidth over 200 nm near the center wavelength of 925 nm. After the compressor, we obtained a pulse duration of 13.4 fs. As the compression efficiency reached 67%, this OPCPA front end could potentially support a peak power of 263 TW at a repetition rate of 0.1 Hz. To the best of our knowledge, among all the 100 TW-level OPCPA systems, it shows the widest spectral width, the shortest pulse duration, and it is also the first OPCPA system working at a repetition-rate mode.

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.