It has been a long time for researchers to explore methods to overcome the diffraction limit. Although light can be squeezed through a sub-wavelength hole¹ leading to lateral resolution on the order of the hole size, its transmission is very limited based on Bethe’s law². However, it has been noticed that bowtie antennas (Fig. 1(a)) enable simultaneous high resolution and enhanced transmission³, which is suitable for near-field imaging techniques such as scanning near-field optical microscopy (SNOM) and near field lithography. This phenomenon can be simply explained as follows. The two arms of the bowtie antenna have large surface areas to efficiently collect the incident radiation. Incident light polarized along the gap of the bowtie antenna generates surface currents to carry surface charge to the sharp tips (Fig. 1(c)). The opposite oscillating surface charges at the tips behave like an oscillating electric dipole which radiates light through the aperture. Therefore, light with proper polarization can pass through the bowtie antenna without experiencing much intensity decay. The transmitted light is confined underneath the nanoscale gap region offering an optical resolution far beyond the diffraction limit.

In a more general way, bowtie antennas (hereafter, bowties) acting as optical antennas, receive and emit radiation as they do in the microwave regime^{5, 6}. It means that bowties, regardless of whether it is apertured (Fig. 1(a)) or gaped (Fig. 1(b)), can convert an external electromagnetic field into a confined energy^{7, 8}, and vice versa. Field confinement and enhancement presented in the gap region, thus can be utilized to achieve a variety of optical applications.

Since the early 2000s, bowtie structures have been used as electrodes in molecular engineering⁹ and applied in molecular junctions to investigate electroluminescence¹⁰, and high-conductivity^{11, 12}. Thanks to the development of nanofabrication, bowties in nanometer scale dimensions^13-15 brought about widespread optical applications. In 2006, sub-diffraction nanoscale lithography with bowties^{4, 16} have been demonstrated. Nanolaser based on bowties array was achieved by exciting dye molecules centered in the gap, in 2012¹⁷. In 2018, enhanced absorption via bowtie arrays in 2D black phosphorus was achieved based on the high transmission of the antenna and a perfect polarization selection ratio is observed¹⁸. Recently, nanoscale bowties were reported to be excited by not only external light, but also electricity¹⁹, showing its advantages in high intensity and ultrafast optical response. As we will show, the last decades have witnessed the booming development of bowties under the optical or electrical excitation.

This paper aims to review applications of optically and electrically driven nanoscale bowties. Since rapid advances have been made in this area, it is difficult to cover all the related references in this review. Nonetheless, we hope the present review arouse interest among the researchers in this subject.

Due to the high enhancement and confinement of bowties, strong electric field localizations^20-22 (hot spots) are presented in the gap. These properties promise bowties broad applications ranging from light transmitting, frequency doubling/tripling, focusing, and generation to detection. In this section, we present an overview of the applications of optically excited bowties.

As mentioned before, bowties exhibit highly enhanced fields under the illumination of light. The consequently enhanced light transmission guarantees a set of nanotechnologies such as nanoimaging and trapping.

Near-field imaging techniques, e.g., SNOM, utilizes evanescent waves to generate sub-diffraction-limit resolution. As the evanescent components decay exponentially, SNOM uses a tapered fiber²³ as a probe and needs to work close to the sample surface. Even in this situation, the light coupling efficiency is severely limited because of the poor transmission of the fiber tip. Therefore subwavelength apertures are usually constructed on the tip apex to boost the light collection. Bowtie apetures^{3, 24} are favored over other apertures owing to its giant light field enhancement¹³. Wang et al.³ fabricated metallic bowtie and square apertures on the quartz wafer, and compared their far-field transmission (Fig. 2(a)). Results indicated that the transmission was enhanced exceeding one order of magnitude from bowtie apertures than the squared ones with the same opening areas. Bowtie apertures were then milled on the SNOM probe (Fig. 2(b)), showing a seven times higher near-field measurement counts than the regular aperture probe (Fig. 2(c)).

Strong energy confinement with a high optical gradient provided by bowties also promises applications in optical trapping^25-27. Using a bowtie plasmonic aperture that was patterned on a tapered metal-coated fiber²⁵, 50 nm polystyrene beads were successfully captured, demonstrating the feasibility of nanoparticle trapping. Notably, 4-nm quantum dot was experimentally reported to be trapped in a deep potential well (Fig. 2(e)) using the three-dimensional tapered 5-nm-gap bowtie aperture antenna (Fig. 2(d)).

Typically optical antennas work in the linear regime for weak excitation fields. In other words, the nonlinearity is negligible in most cases. The nonlinear conversion efficiency, however, can be resonantly enhanced by localized surface plasmons (LSPs)²⁸ when appropriate size and shape of the nanoantenna are introduced. As a consequence, new interesting phenomena arise, such as frequency conversion, switching, and modulation²⁹.

Bowties exhibit power enhancement factors larger than 40 dB³⁰, which is beneficial to second or even higher order harmonics generation³¹. As shown in Fig. 3(a), large arrays of antennas are often used to boost nonlinear conversion efficiency^32-34. Nevertheless, Hanke et al.³⁵ observed that third harmonic (visible light) was emitted by using individual bowtie antenna excited resonantly with few-cycle femtosecond laser (~0.6 MW/cm²) in the near infrared. Though the second-order harmonic generation is partially constrained by the symmetry condition, it was also observed in the emission spectra with a small peak (Fig. 3(b)). It has been reported³⁰ that bowties are even capable of lowering the pulsed femtosecond laser intensities (from 10¹³ MW/cm² to 10¹¹ MW/cm² level) that are required to produce coherent extreme-ultraviolet (EUV) light through the nonlinear conversion process. High harmonics up to 17^th were observed.

Two-photon photoluminescence (TPPL) can also occur because of high local fields at LSP resonance and shows Elocal4 dependence on the local E fields¹⁵ (Fig. 3(c)).

The development of modern information technologies makes a request of advanced nano-manufacturing tools. As we will see, great success has been achieved in nanolithography via bowties in the last decades. Bowties are capable of focusing the incident ultra-violet (UV) light into a small spot, showing remarkable advantages over traditional optical lithography, e.g., deep UV and extremely UV lithography, for its low cost and competitive resolution. In 2006 Wang et al.⁴ fabricated apertured bowties by focused ion beam (FIB) on a 150 nm thick aluminum film (Fig. 4(a)). Under the illumination of 355 nm light, 40×50 nm lithography resolution was experimentally reported, as shown in Fig. 4(b). Higher resolution becomes more noticeable by fabricating an ultra-small gap between the antenna tips with the back-side milling method³⁶. Another nice experiment was conducted by Sundaramurthy et al.¹⁶ in the same year. They demonstrated that by utilizing optically resonant metallic bowties, the production of polymer resist nanostructures <30 nm in diameter was achieved at a longer wavelength of 800 nm via the two-photon polymerization (TPP) effect ( Fig. 4(c) and 4(d). Researchers³⁷ from Yonsei University has shown that by using a contact probe made of bowtie aperture antennas (Fig. 4(e)) illuminated by a 405 nm wavelength laser light, high-density line array patterns were recorded with a half pitch up to 22 nm (Fig. 4(f)). Further, the line edge roughness was decreased down to be ~ 17 nm by optimizing the developing process. This work implies bowties-based nanolithography has a great potential for practical applications.

While sub-diffraction resolution (<20 nm)³⁶ can be achieved by using bowties, nearly all the aforementioned results suffer from a small depth of focus (DOF: <10 nm), which poses challenges for industrial production. Such a problem is alleviated by combining bowtie antennas with metamaterials, such as silver superlens/reflectors ^38-40 (Fig. 4(g), DOF: ~30 nm) and hyperbolic metamaterials⁴¹ (Fig. 4(h), DOF: ~100 nm). However, the DOF has to be larger than 100 nm to realize convenient and reliable pattern transferring. This has to be addressed before taking bowtie nanolithography a step closer to widespread applications.

Miniaturized photon sources are essential elements for all-optical circuits and nanoscale biosensors. One way of achieving lasing from nanoscale resonators is to utilize LSPs excited in metal nano-particles. A bowtie-shaped metallic resonator are well suited for this purpose as it can provide high Purcell factor thanks to the high confinement, therefore, the ultra-small effective mode volume⁴². In addition, the threshold condition for lasing can be satisfied at room temperature near LSP resonance wavelengths. Shown in Fig. 5(a), Jae Yong Suh et al.¹⁷ demonstrated that three-dimensional (3D) nano-bowties array coupled with a gain medium (IR-140 dye in polyurethane) can generate coherent and directional light emission (Fig. 5(b) and 5(c)). With a mode volume smaller than 0.001(λ/2n)³ (λ, the wavelength and n, the effective refractive index), room temperature optically pumped lasing at 873 nm with a threshold pump pulse fluence of 0.4 mJ/cm² was observed.

Bowties also show great potential for high-contrast selection of single nanoemitters⁴³. Kinkhabwala et al.⁴³ observed enhancements of a single molecule’s fluorescence up to a factor of 1340 by using fluorescent molecules coated bowties. They attributed this to the result of greatly enhanced absorption and an increased radiative emission rate due to the presence of bowties.

Moving towards the infrared band, silicon carbide bowties (Fig. 5(d)) can serve as a thermal emitter with a narrowband (10 cm⁻¹) spectrum⁴⁴ (Fig. 5(f)), which is suitable for novel applications such as non-dispersive infrared sensing and molecular spectroscopy. The thermal emission spectrum show a clear dependence on the bowtie gap size, indicating that a tunable thermal emitter is achievable. Numerical simulations (Fig. 5(e)) and near-field optical characterization shade light on the physical nature of the resonances in the thermal emission spectra.

The strong plasmonic enhancement between the bowtie tips results in an enhanced in-plane electric fields interaction with two-dimensional (2D) materials, leading to an efficiently absorbed external light. Consequently, bowties have been utilized to enhance the responsivity of photodetectors. For example, Ma et al.⁴⁵ reported that a bowtie-enhanced graphene waveguide photodetector possesses a high external responsivity of 0.5 A/W and a fast frequency response up to at least 110 GHz (Fig. 6(a–c)). As a validation of optical communication, 100 Gbit/s data reception of two-level OOK and four-level PAM-4 intensity encoded signals was successfully demonstrated at C-band (Fig. 6(d)). Venuthurumilli et al.¹⁸ also demonstrated a near infrared photodetector using black phosphorus embellished by bowties, as shown in Fig. 6(e) and 6(g). High photocurrent (an enhancement of 70% in comparison to the device without bowties, Fig. 6(f)) or polarization sensitivity (8.7, Fig. 6(h)) can be achieved with proper bowtie designs and orientations.

Nanoantennas can be excited by not only the external light, but also the bias voltages. They function as an emerging optical nano-source^46-49 via the inelastic electron tunneling (IET) process. Related researches can date back to 1976 in biased metal-insulator-metal junctions⁵⁰, and lately, in scanning tunneling microscope (STM) systems^51-53. Electrically driven antennas bridge electrical and optical circuits at the nanoscale and constitute a vibrant sub-field of nano-photonics.

We also notice that bowtie-shaped metallic structures are widely used in molecule engineering as electrical leads, to exploit charge transport at the level of single molecules⁵⁴. Strictly speaking, however, these devices can hardly be regarded as ‘antennas’ due to the lack of related functions. Therefore in this section, we will devote special attention to light emission from electrically excited bowtie antennas, including the principles, fabrication, characterizations and future directions.

A tunnel junction can be made of a thin insulating layer (usually the air or, here, oxide) sandwiched between the two conducting materials (e.g., here, Al and Au), as illustrated in Fig. 7(a). When voltages are applied, electrons could tunnel through the barrier inelastically and give rise to an energy transfer in the form of (localized or propagated) surface plasmons (Fig. 7(b)). Light can be emitted by the radiative decay or scattering of surface plasmons⁵⁵. Since the tunneling process happens at a time scale of femtoseconds^{56, 57}, it enables electrically driven antennas to be operated as an ultrafast optical source.

A typical signature of IET is that the cut-off frequency ν in electroluminescence spectrum is related to the applied bias voltage Vbias (Fig. 8(c)), also known as the quantum limit hν⩽e|Vbias| in which h the Planck’s constant and e the electron charge. Interestingly, an alternative mechanism that might explain the light emission from tunnel junctions is resulted from hot-electron decay⁵⁸, where the energies of emitted photons are beyond the quantum limit. Our focus is concentrated on the IET process. IET can be theoretically described as an energy-loss model, current fluctuations model and spontaneous emission model. We refer the interested readers to the review⁵⁹ for more mathematic details. Here, we briefly give a physical picture behind light emission from electrically driven antennas, as follows.

The (detectable) electron-to-photon conversion efficiency, or, the external quantum efficiency (EQE) driven by IET can be described as

1EQE=η0IETPtotalP0PradPtotal,

where η0IET is the source efficiency of IET in vacuum (in the order of 10^–6 level at visible frequencies with a modest dependence on the details of the barriers⁵⁹), P0 is the power emitted by a dipole in free space, Ptotal is the total power dissipated (not directly to photon emission but often in the form of other types of optical modes, such as LSP modes) by a dipole in an arbitrary environment and Prad is the radiated power. The key is that, the ratio of P0 and Ptotal is deeply connected to the quantity, the partial local density of the optical states (LDOS) ρp, that is⁵⁹

2PtotalP0=ρpρ0,

where ρ0 is the vacuum LDOS. The radiation efficiency ηrad is defined as Prad/Ptotal. Both the LDOS and radiation efficiency (Fig. 8(d) and 8(e)) are profoundly influenced by the geometries of bowties (say, for example, the bowtie width, height and angle shown in Fig. 8(a)). This demonstrates the practicality of tunable nano-sources based on bowtie junctions. Although the LDOS and ηrad cannot be analytically expressed except a few kinds of geometries, Eq. (2) points out a viable way of estimating EQE by numerical simulations using the Green’s function⁶⁰.

Tunnel junctions can be classified into two categories, namely, the vertical (for example, shown in Fig. 7(a)) and lateral junctions (Fig. 8(a)) , in terms of their orientations relative to the substrate. As for the vertical junctions, they are formed due to the nature of oxidation. When it comes to the lateral counterpart, while the ultra-small (sub 1 nm) gap is hard to fabricate using currently available nanopatterning methods⁶¹, electro-migration^{62, 63} is a good choice to generate bowtie gap-antenna tunnel junctions. This process^{19, 64} starts with the patterning of weakly-bridged (the smallest linewidth is 10 nm) bowties by electron beam lithography, followed by supplying a gradually-increasing bias voltages till the conductance G of the antennas drops well below the quantum conductance⁶⁵G0=2e2/h (shown in Fig. 8(b)). It is at this stage that a tiny gap appears. Our group has demonstrated a 0.6 nm gap bowtie antenna¹⁹ obtained by a well-controlled electro-migration process.

Thanks to the well-designed bowtie gap-antenna tunnel junctions, we have achieved a broadband optical nano-source driven by IET (see Fig. 8(c)) with peak emission power of 1.4 nW¹⁹, which is two orders of magnitude higher than previous results (see Table 1). The EQE is then calculated to be (1.1 ± 0.2) × 10^–4. It is not hard to understand either from the point view of enhanced LDOS (see Fig. 8(d)), or enhanced Purcell factors⁶⁶ due to the small mode volume in bowties. Further, bowtie-shaped structures also function as ‘antennas’, providing a high radiation efficiency (of course, at specific wavelengths). Accordingly, the emission spectrum can be tuned by engineering LDOS and radiation efficiency (Fig. 8(e)), as explained in Underlying Mechanisms. Wang et al. recently have used an approximate model⁶⁴ to analytically calculate the resonant peaks in the emission spectra of electrically driven bowties, showing a good agreement with the experimental results.

Higher efficiencies⁴⁸ are continually pursued by researchers. While EQE roughly shows a dependence on the width d of tunnel gap (d^–1)⁵⁹, however, d cannot be unlimited shortened because quantum mechanics dominates then. The operating speed and footprint may be priority depending on applications. In recent years, two-dimensional materials integrated tunnel devices^{67, 68} attract much attention and remain to be further explored. Directional control of electrically excited sources^69-71 via IET is also of particular interest by using properly designed antenna architectures.

Owing to the field confinement and enhancement, nanoscale bowties can be a good fit for a variety of applications such as near-field imaging, nanotrapping, nanolithography, nano-sources and photodetectors, etc. This review presents the applications of nanoscale bowties excited by photons and electrons. Bowties can be used for bridging electronic and photonic components on a same chip, thus possessing great potential in high-density optical storage and on-chip wireless communication. Nanoscale bowties may meet great challenges but could also lead photonic chip researches to a new step in the future.

This work is supported by National Key Research and Development Program of China (2018YFB2200900), the Key R&D Program of Anhui (Grant No. 202004A05020077) and National Natural Science Foundation of China (61775206). The nanofabrication was carried out at the USTC Center for Micro and Nanoscale Research and Fabrication. We also thank Prof. Xianfan Xu of Purdue University for his warm-hearted discussion.

The authors declare no competing financial interests.