Eui Su Lee

Bias field tailored plasmonic nano-electrode for high-power terahertz photonic devices.

Photoconductive antennas with nano-structured electrodes and which show significantly improved performances have been proposed to satisfy the demand for compact and efficient terahertz (THz) sources. Plasmonic field enhancement was previously considered the dominant mechanism accounting for the improvements in the underlying physics. However, we discovered that the role of plasmonic field enhancement is limited and near-field distribution of bias field should be considered as well. In this paper, we clearly show that the locally enhanced bias field due to the size effect is much more important than the plasmonic enhanced absorption in the nano-structured electrodes for the THz emitters. Consequently, an improved nano-electrode design is presented by tailoring bias field distribution and plasmonic enhancement. Our findings will pave the way for new perspectives in the design and analysis of plasmonic nano-structures for more efficient THz photonic devices.

Metal-VO2 hybrid grating structure for a terahertz active switchable linear polarizer

An active terahertz (THz) wave hybrid grating structure of Au/Ti metallic grating on VO2/Al2O3 (0001) was fabricated and evaluated. In our structure, it is shown that the metallic gratings on the VO2 layer strengthen the metallic characteristics to enhance the contrast of the metallic and dielectric phases of a VO2-based device. Especially, the metal grating-induced optical conductivity of the device is greatly enhanced, three times more than that of a metallic phase of bare VO2 films in the 0.1-2.0 THz spectral range. As an illustrative example, we fabricated an actively on/off switchable THz linear polarizer. The fabricated device has shown commercially comparable values in degree of polarization (DOP) and extinction ratio (ER). A high value of 0.89 in the modulation depth (MD) for the transmission field amplitude, superior to other THz wave modulators, is achieved. The experimental results show that the fabricated device can be highly useful in many applications, including active THz linear polarizers, THz wave modulators and variable THz attenuators.

Real-time continuous-wave terahertz line scanner based on a compact 1 × 240 InGaAs Schottky barrier diode array detector

We demonstrate real-time continuous-wave terahertz (THz) line-scanned imaging based on a 1 × 240 InGaAs Schottky barrier diode (SBD) array detector with a scan velocity of 25 cm/s, a scan line length of 12 cm, and a pixel size of 0.5 × 0.5 mm². Foreign substances, such as a paper clip with a spatial resolution of approximately 1 mm that is hidden under a cracker, are clearly detected by this THz line-scanning system. The system consists of the SBD array detector, a 200-GHz gyrotron source, a conveyor system, and several optical components such as a high-density polyethylene cylindrical lens, metal cylindrical mirror, and THz wire-grid polarizer. Using the THz polarizer, the signal-to-noise ratio of the SBD array detector improves because the quality of the source beam is enhanced.

Frequency modulation based continuous-wave terahertz homodyne system

In this study, inspired by the frequency-modulated continuous-wave (FMCW) method, an operation scheme of continuous-wave (CW) terahertz (THz) homodyne system is proposed and evaluated. For this purpose, we utilized the fast and stable wavelength tuning characteristics of a dual-mode laser (DML) as a beating source. Using the frequency-modulated THz waves generated by DML, a cost-effective and robust operation of CW THz system to be applicable to the measurements of thickness or refractive index of a sample is demonstrated. We believe that the proposed scheme shows a potential to the implementations of compact and fast CW THz measurement systems that can be useful in many THz applications.

Photonic devices for tunable continuous-wave terahertz generation and detection

A novel type of semiconductor beating source, a monolithically integrated dual-mode laser, and continuous-wave terahertz (THz) system adopting it will be investigated. The combined system of the beating source with broadbandantenna- integrated low-temperature-grown semiconductor photomixers shows the possibility of the realization of the cost-effective and compact continuous-wave THz systems. Such a system is highly-demanded to examine the THz finger prints of specimens without limitations. Since the optimized performance depends not only on the characteristics of functional devices but also module configurations, various approaches such as traveling-wave photomixers, Schottky barrier diodes, and nano-structure contained photomixers have been investigated to implement high-performance THz platforms as the main building blocks of a THz system. Semiconductor-based compact and cost-effective photonics technologies will envisage the bright future of THz systems.

A comparative study of the plasmon effect in nanoelectrode THz emitters: Pulse vs. continuous-wave radiation

Plasmonic field enhancement in terahertz (THz) generation is one of the recently arisen techniques in the THz field that has attracted considerable interest. However, the reported levels of enhancement of THz output power in the literature are significantly different from each other, from less than two times to about two orders of magnitude of enhancement in power, which implies the existence of other major limiting factors yet to be revealed. In this work, the contribution of the plasmonic effect to the power enhancement of THz emitters is revisited. We show that the carrier collection efficiency in a THz emitter with plasmonic nanostructures is more critical to the device performance than the plasmonic field enhancement itself. The strong reverse fields induced by the highly localized plasmonic carriers in the vicinity of the nanoelectrodes screen the carrier collections and seriously limit the power enhancement. This is supported by our experimental observations of the significantly enhanced power in a...

Real-time imaging of moving living objects using a compact terahertz scanner

In this study, we design a compact terahertz (THz) reflection scanner with a scan rate of 20 frames/s. This scanner is based on a benzocyclobutene-embedded InGaAs Schottky barrier diode detector having a maximum responsivity of 300 V/W at 250 GHz and a minimum noise equivalent power of 38 pW/. With this scanner, details such as sharp head and tail and wrinkled segments in a moving caterpillar are observed. The thin and thick parts of the moving caterpillar that are presented in light gray and dark gray, respectively, on the gray scale are also well distinguished.

Terahertz rectifier exploiting electric field-induced hot-carrier effect in asymmetric nano-electrode

Rectifiers have been used to detect electromagnetic waves with very low photon energies. In these rectifying devices, different methods have been utilized, such as adjusting the bandgap and the doping profile, or utilizing the contact potential of the metal-semiconductor junction to produce current flow depending on the direction of the electric field. In this paper, it is shown that the asymmetric application of nano-electrodes to a metal-semiconductor-metal (MSM) structure can produce such rectification characteristics, and a terahertz (THz) wave detector based on the nano-MSM structure is proposed. Integrated with a receiving antenna, the fabricated device detects THz radiation up to a frequency of 1.5 THz with responsivity and noise equivalent power of 10.8 V/W and [Formula: see text] respectively, estimated at 0.3 THz. The unidirectional current flow is attributed to the thermionic emission of hot carriers accelerated by the locally enhanced THz field at the sharp end of the nano-electrode. This work not only demonstrates a new type of THz detector but also proposes a method for manipulating ultrafast charge-carrier dynamics through the field enhancement of the nano-electrode, which can be applied to ultrafast photonic and electronic devices.

Photo-conductive detection of continuous THz waves via manipulated ultrafast process in nanostructures

Time-domain and frequency-domain terahertz (THz) spectroscopy systems often use materials fabricated with exotic and expensive methods that intentionally introduce defects to meet short carrier lifetime requirements. In this study, we demonstrate the development of a nano-photomixer that meets response speed requirements without using defect-incorporated, low-temperature-grown (LTG) semiconductors. Instead, we utilized a thin InGaAs layer grown on a semi-insulating InP substrate by metal-organic chemical vapor deposition (MOCVD) combined with nano-electrodes to manipulate local ultrafast photo-carrier dynamics via a carefully designed field-enhancement and plasmon effect. The developed nano-structured photomixer can detect continuous-wave THz radiation up to a frequency of 2 THz with a peak carrier collection efficiency of 5%, which is approximately 10 times better than the reference efficiency of 0.4%. The better efficiency results from the high carrier mobility of the MOCVD-grown InGaAs thin layer with ...

InGaAs Schottky barrier diode array detectors integrated with broadband antenna (Conference Presentation)

Terahertz (THz) waves have been actively studied for the applications of astronomy, communications, analytical science and bio-technologies due to their low energy and high frequency. For example, THz systems can carry more information with faster rates than GHz systems. Besides, THz waves can be applied to imaging, sensing, and spectroscopy. Furthermore, THz waves can be used for non-destructive and non-harmful tomography of living objects. In this reasons, Schottky barrier diodes (SBD) have been widely used as a THz detector for their ultrafast carrier transport, high responsivity, high sensitivity, and excellent noise equivalent power. Furthermore, SBD detectors envisage developing THz applications at low cost, excellent capability, and high yield. Since the major concerns in the THz detectors for THz imaging systems are the realizations of the real-time image acquisitions via a reduced acquisition time, rather than the conventional raster scans that obtains an image by pixel-by-pixel acquisitions, a line-scan based systems utilizes an array detector with an 1 × n SBD array is preferable. In this study, we fabricated the InGaAs based SBD array detectors with broadband antennas of log-spiral and square-spiral patterns. To optimize leakage current and ideality factor, the dependence to the doping levels of ohmic and Schottky layers have been investigated. In addition, the dependence to the capacitance and resistance to anode size are also examined as well. As a consequence, the real-time THz imaging with our InGaAs SBD array detector have been successfully obtained.