Applied Science and Convergence Technology 2021; 30(1): 6-13
Published online January 31, 2021
Copyright © The Korean Vacuum Society.
Jihye Leea,b and Jong-Souk Yeoa,b,*
aSchool of Integrated Technology, Yonsei University, Incheon 21983, Republic of Korea
bYonsei Institute of Convergence Technology, Yonsei University, Incheon 21983, Republic of Korea
Light sources are ubiquitous in our lives. Common light sources include sunlight and incandescent lamps, which emit visible spectrum to help us to see surrounding objects via blackbody radiation. Further understanding and control on the quanta of light, called photons, have led to numerous advances in nanophotonics and related research fields, but recently their impact has been most notable in quantum computing, communications, biosensing, and imaging technology. Most of these applications have been made possible through the development of ultrasmall and ultrafast light sources based on advanced nanotechnology. In this review, we aim to give a clear picture of the historical achievements regarding light sources and how recent studies have developed the field of ultrasmall light sources, especially in relation to quantum electron tunneling mechanisms. Finally, we discuss the potential applications for emerging quantum devices and sensing technologies.
Keywords: Light source, Quantum electron tunneling, On-chip quantum devices, Biosensors, Chemical sensors
The story of the development of light sources begins before Edison’s invention in 1879 [1, 2]. The Italian physicist, chemist, and pioneer of electricity and power Alessandro Volta invented the voltaic pile in 1799 by using zinc and copper disks with layers of cardboard soaked in salt water and connected to the glowing copper [Fig. 1(a)]. It is considered the earliest battery, as well as an early incandescent lighting concept. Subsequently, Humphry Davy in 1815 and Warren de la Rue in 1840 demonstrated a very bright arc lamp and expensive platinum-filament-based bulb, respectively [Fig. 1(b)], but Joseph Swan and Thomas Edison made efforts to change these concepts to commercialize the electric light in practical use over the candle, gas light, and oil lamp [Fig. 1(c)] [4–7]. Swan first used a carbonized paper filament in a vacuum glass bulb, but it had a short lifetime (15 ). Edison realized that the major problems with Swan’s bulb were the filament and the lack of a good vacuum state; therefore, he changed the filament material to electrically high-resistance tungsten and changed the vacuum state to a gas-filled state . This made a significant difference. Because there was no oxidation, the bulb had a long lifetime, leading to great commercial success. His numerous trials and endeavors have had a profound impact on the modern industrialized world that illuminates our lives. Light-emitting diodes (LEDs), which were developed by Henry Joseph Round (first observation of electrolumi-nescence from solid-state material, 1907), Oleg Losev (first invention of an LED, 1927), James R. Biard (first infrared LED from a tunnel diode, 1961), Nick Holonyak (first visible red LED, 1962), and Shuji Nakamura (high-brightness blue LED, 1993), now provide energy-efficient and long-lasting lighting with small size and fast switching speed [Fig. 1(d)] [8–10]. Thus, they can be used in diverse applications including advanced communication technology and medical devices in addition to general lighting [9, 11]. An LED is a semiconductor light source that emits energy in the form of photons by recombining electron and holes in the semiconducting layer. This means it is categorized in electroluminescence. The wavelength of electroluminescent light is determined by the bandgap of the solid-state material, and the emitted light is spectrally and spatially incoherent compared to a coherent laser light source. These types of LED have become commercialized not only in energy-saving lighting but also in high-contrast-ratio pixels in displays . Cavity structure and optoelectronic considerations are added to the design of LED to enable light amplification by the stimulated emission of radiation (LASER) with a narrow temporal (monochromatic) and spatial (collimated) frequency range. More coherent white light source can be made by combining laser sources of red, green, and blue [Fig. 1(e)] [13–15]. This type of coherent light source can also be used for the applications related to quantum physics.
With the recent development of nanotechnology, light sources have gradually become smaller, and now nanoscale light sources that can provide the potential to integrate subwavelength optics are being developed. In the lateetallictip-based fiberoptic scattering probes were developed that can be applied to near-field imaging [Fig. 1(f)] [16, 17]. In 2007, Nakayama
In the following section, we review the recent advances related to quantum-tunneling-based light source devices based on their structure, materials, and functions. Finally, Section 3 briefly introduces the potential applications based on nanoscale light sources.
Recently, as the development of “Internet of Things” technology has rapidly increased demand for the amount of data to be processed for a hyper-connected world, there has been a corresponding growing demand for advanced materials and devices with information-pro cessing speeds orders of magnitude faster than those of existing technology [36–38]. This demand has reignited interest in inelastic-electron-tunneling-based internal light sources that enable ultrafast transduction [36, 37]. However, for practical applications in data processing, telecommunication, optical interconnects, and optical sensing, the transduction efficiency of electrical-to-optical signal conversion at nanoscale is very low, with the current world record set at approximately 1 %in 2018 . Furthermore, flexible modulation of bandwidth and multi-frequency generation for multi-channel on-chip platforms remains challenging. These challenges also apply to optical biosensing applications, for which efforts have been made to use the compact size of nanoscale sensors for practical point-of-care (POC) devices [40,41]. In an effort to achieve progressively smaller reagent concentrations and sensing volumes, these approaches have experienced a paradigm shift from simple bulk measurements toward engineered nanoscale devices [37, 42]. In this size regime, plasmonic particles and nanostructures provide an ideal toolkit for the realization of novel sensing concepts due to their unique ability to simultaneously focus incident light into subwavelength hotspots and transmit minute changes in the local environment back into the far field as a modulation of their optical response. However, these optical sensors currently still rely on bulky light sources, such as LEDs or lasers, and detectors, limiting their usability in biochemical research and medical diagnostics in which miniaturized and/or integrated sensor devices are crucial for POC applications [43, 44]. In particular, one of the key components missing from current optical sensing approaches is a reliable and nanoscale light source, which can be directly integrated with nanophotonic sensing elements to detect target analytes in a miniaturized package.
In classical mechanics, electrons with insufficient energy to over-come a given potential barrier have zero probability of reaching the other side. However, when taking into account the uncertainty principle of quantum mechanics, there is a small probability for the electron to pass through the barrier with a reduced amplitude of the electron wave function [45, 46]. Regarding this quantum tunneling effect, there are two pathways for the electron to take through the tunneling junction [47,48] [Fig. 2(a)]: elastic or inelastic electron tunneling. When electrons pass through the quantum-tunnel junction elastically, they connect electronic states of the same energy between the two electrodes and therefore do not lose any energy. Most quantum electron tunneling processes involve this elastic behavior. In inelastic electron tunneling, a rarer form of quantum electron tunneling, the transmitting electron loses some of its energy to excite a surface plasmon between the thin insulator tunnel junctions. The excited surface plasmon can either decay into the far field radiatively or decay non-radiatively by excitation of a hot electron. This optical mode can be tuned by 1) the bias voltage that determines the cutoff frequency of the device, 2) the structure design of the MIM geometry, and 3) the antenna geometry and its configuration. In this section, we summarize the state of the art on inelastic-electron-tunneling-based devices.
It has been shown that high-frequency broadband light can be generated based on inelastic electron tunneling. The cutoff frequency of this broadband light is determined by the applied voltage, as described by the quantum relation
Optical antennas can manipulate and control light at subwavelength scales [55,56]. Generally, they can be used for energy transfer between a source (or receiver) and the free-radiation field, that is, transduction into the far field [57, 58]. Recently, optical antennas have been shown to couple to quantum-tunnel junctions and to convert electrons into free-space photons efficiently. In this configuration, the optical antenna plays a crucial role in bridging the size mismatch between far-field radiation and nanoscale volumes, and in strongly enhancing the transduction between electrons and photons [59, 60].
In this section, we first discuss the vertical MIM junction as shown in [Fig. 3(a)–(c)]. In 2015, Parzefall
The configuration of the MIM junction can also form a lateral structure, as shown in [Fig. 3(d)–(f)]. The size of a lateral-type quantum-tunnel junction is controlled and fabricated by several smart methods and engineering of interface or crystallinity control. In 2015, Kern
In 2017, Gurunarayanan
Most research efforts have been focused on the nanoantenna structure. In 2018, Qian
Most studies have attempted to create photon emitters using a standard metal and insulating materials. In this section, we introduce the alternative materials that can replace them in MIM and new MIS structures for light-emitting devices.
In 2016, Du
There have not only been changes to the insulating layer via molecules and polymers, but also several attempts to change the configuration from MIM to MIS [77–79]. Göktas
The new concept of an internal light source can be applied in various fields, such as on-chip photonics, optical communications, bio-and chemical sensing, and optically and electrically operating memory and switching devices [81–83]. In this section, representative applicaapplications based on quantum tunneling devices are explained and summarized.
When high-density metamaterials are used for inelastic-quantum-tunneling devices, they can be used to monitor the chemical reaction in real time with high sensitivity. Wang
Inelastic quantum tunneling can be used as a form of spectroscopy for fingerprinting the chemical vibrational mode from the molecules in the tunnel junction [85, 86]. Fereiro
The quantum-tunnel junction can provide a solution to the interface of photons and electrons for an on-chip wireless optical interconnect, as shown in Fig. 5(d) . It can convert the optical signal directly into an electrical signal by exhibiting a change in conductance depending on the power of the optical signal, and thus enabling a rectenna to rectify the current at the gap antenna structure. It can be further applied to ultrafast information transfer by the antenna and sub-nanometer gap rectenna structure. This ultrafast modulation of tunneling and its light emission pattern has been experimentally monitored by Parzefall
The development of quantum-tunneling-based light sources must overcome some hurdles, such as enhancing the efficiency, providing wide bandwidth tenability from visible to infrared range, ensuring stability of light emission without blinking, and extending the emission time [89–91]. When these problems are solved, it will be possible to achieve highly improved speed communications, data processing, and ultrasensitive sensing technology. By exploring the wide range of material selection and coupling of active materials such as quantum dots, fluorescent materials, diamond, or 2D materials in quantum devices, we can expand the potential of ultrasmall quantum devices in large areas for application in emerging quantum information technology [91–97].
This research was supported by the National Research Foundation (NRF) of Korea under the “Korean-Swiss Science and Technology Program” (2019K1A3A1A1406720011), the Ministry of Trade, Industry and Energy (MOTIE, project number 10080625), the Korea Semiconductor Research Consortium (KSRC) program for the development of future semiconductor devices, Samsung Electronics, and the frame-work of Warm Heart Center program managed by the Institute of Con-vergence Science, Yonsei University.