Haewook Han

Pohang University of Science and Technology | POSTECH · Department of Electronic and Electrical Engineering

Modern optical systems are important components of contemporary electronics and communication technologies, and the design of new systems has led to many innovative breakthroughs. This paper introduces a novel application based on deep reinforcement learning, D3QN, which is a combination of the Dueling Architecture and Double Q-Network methods, to...

This paper presents an ultra-thin, ultra-wideband linear-to-circular (LTC) polarization converter based on the crossed-dipole-shaped metasurface. The unit cell of the crossed-dipole-shaped metasurface is made up of two identical T-shaped dipole arms, which are orthogonal to each other and are connected with a diagonal microstrip line. The LTC polar...

In this work, we present a quantitative analysis model based on reference-free self-calibration to analyze scattered fields and approach curves on a dielectric substrate for terahertz scattering-type scanning near-field optical microscopy. The results of our model are compared with experimentally measured data and a fully numerical analysis based o...

The use of filling agents for rubber reinforcement is beneficial in various industrial applications, and several experimental methods have been used to study the effect of fillers on rubber. However, due to the lack of a suitable imaging technique, filler dispersion and distribution in rubber cannot be easily displayed. Thus, we utilize the THz nea...

This letter proposes 50:50 TE terahertz power splitters based on digitized metamaterials. The components are optimized using the particle swarm optimization algorithm. The digitized metamaterials are formed as a silicon slab with circular air holes atop a silicon dioxide substrate. The insertion losses of optimized structures containing air holes w...

The use of filling agents for rubber reinforcement is beneficial in various industrial applications, and several experimentalmethods have been used to study the effect of fillers on rubber. However, due to the lack of a suitable imaging technique, fillerdispersion and distribution in rubber cannot be easily displayed. Thus, we utilize the THz near-...

We designed Si-based all-dielectric 1 × 2 TE and TM power splitters with various splitting ratios by combining the use of the inverse design of adjoint and numerical 3D finite-difference time-domain methods. The structure of the designed Si-based power splitters contains two Si waveguide branches on a SiO2 substrate that is compatible with CMOS fab...

We designed and simulated an ultra-compact 1 × 2 power splitter operating in the terahertz region. A machine learning approach was implemented to design the photonic device. The designed power splitter has a footprint of 500 µm × 500 µm. We calculated the insertion loss using a three-dimensional finite difference time domain method. The calculated...

Highly-efficient broadband polarization beam splitters with small footprint and based on silicon photonics technology are of great importance in optical integrated circuits. In this paper, a crucially applicable optical component of a TE/TM polarization beam splitter has been designed and simulated using the inverse design of adjoint method. The me...

We designed Si-based all-dielectric 1 $\times$ 2 TE and TM power splitters with various splitting ratios and simulated them using the inverse design of adjoint and numerical 3D finite-difference time-domain methods. The proposed devices exhibit ultra-high transmission efficiency above 98 and 99\%, and excess losses below 0.1 and 0.035 dB, for TE an...

This letter proposes a 50:50 TE terahertz (THz) power splitter based on digitized metamaterials. The device is optimized using particle swarm optimization algorithm. The structure of the digitized metamaterials contains circular air holes in silicon slab which are deposited on silicon dioxide substrate. The optimized structure shows insertion loss...

Terahertz near-field microscopy (THz-NFM) could locally probe low-energy molecular vibration dynamics below diffraction limits, showing promise to decipher intermolecular interactions of biomolecules and quantum matters with unique THz vibrational fingerprints. However, its realization has been impeded by low spatial and spectral resolutions and la...

Near-infrared (NIR) responsive nanoparticles (NPs) like gold nanorods (GNRs) are important in biomedical fields because of their transparency for biological tissues. Although GNRs are sought after as contrast agents for theranostics in cancer studies, capping ligands like cetyl trimethyl ammonium bromide (CTAB) used for the GNR synthesis are toxic...

Tris-HCl buffer solution is extensively used in biochemistry and molecular biology to maintain a stable pH for biomolecules such as nucleic acids and proteins. Here we report on the high-precision THz dielectric spectroscopy of a 10 mM Tris-HCl buffer. Using a double Debye model, including conductivity of ionic species, we measured the complex diel...

The papers in this special section on terahertz near-field microscopy (Thz-NFM). THz-NFM is a powerful technique, although still challenging to implement, that is invaluable for probing low-energy light–matter interactions in solid-state and biomolecular nanostructures. In contrast to conventional infrared techniques, owing to the low photon energi...

Cancer nanomedicines are opening new paradigms in cancer management and recent research points to how they can vastly improve imaging and therapy through multimodality and multifunctionality. However, challenges to achieving optimal efficacy are manifold starting from processing materials and evaluating their intended effectiveness on biological ti...

We designed a slit dipole antenna backed by an extended hemispherical silicon lens and investigated the minimum lens size in which the slit dipole antenna works as a leaky-wave antenna. The slit dipole antenna consists of a planar feeding structure, which is a center-fed and open-ended slot line. A slit dipole antenna backed by an extended hemisphe...

Background: Cellulose is a ubiquitous, renewable and environmentally friendly biopolymer, which has high promise to fulfil the rising demand for sustainable and biocompatible materials. Particularly, the recent progress in the synthesis of highly crystalline cellulose-based nanoscale biomaterials, namely cellulose nanocrystals (CNCs), draws signifi...

The present invention provides a color mixing lens which can improve color reproducibility, be made slim and adjust an emission pattern; and a liquid crystal display device having the same. The color mixing lens includes a light receiving portion having at least two light emission diodes positioned at a side for emitting color lights different from...

Combined with THz time-domain spectroscopy, THz near-field microscopy based on an atomic force microscope is a technique that, while challenging to implement, is invaluable for probing low-energy light-matter interactions of solid-state and biomolecular nanostructures, which are usually embedded in background media. Here, we experimentally demonstr...

We measured terahertz (THz) emission from the vertically aligned indium arsenide (InAs) nanowires using THz time-domain spectroscopy. The photoexcited InAs nanowires were grown by metalorganic chemical vapor deposition on type silicon substrate. Experimental results shows that THz emission mechanism of InAs nanowires are very different from that of...

We experimentally investigated the terahertz (THz) photoresponse of vertically-aligned single-walled carbon nanotube (SWCNT) films by using optical pumping method. The SWCNT film was grown on the fused quartz substrate by using the alcohol catalytic CVD process. We observed that the strong THz radiation is emitted from the photoexcited SWCNT film....

An antenna design to increase the directivity of a terahertz (THz) photomixer used as a THz source is presented. The antenna is consisted of a full-wavelength dipole on a GaAs membrane substrate and covered by a frequency-selective surface (FSS). The perfect mirror of ground plane and the partially reflective mirror of FSS form a resonator which pr...

The present invention provides a color mixing lens which can improve color reproducibility, be made slim and adjust an emission pattern; and a liquid crystal display device having the same. The color mixing lens includes a light receiving portion having light receiving recesses which respectively house light emission diodes formed at a side surface...

An antenna consisting of a full-wavelength dipole on a GaAs membrane covered by a frequency selective surface is proposed for improved output power from photomixers used as terahertz (THz) sources. The antenna structure reduces the vertical dimension of a typical photomixer antenna using a substrate lens while still exhibiting high total efficiency...

An antenna consisting of a full-wavelength dipole on a GaAs membrane covered by a frequency selective surface is proposed for improved output power from photomixers used as terahertz (THz) sources. The antenna structure reduces the vertical dimension of a typical photomixer antenna using a substrate lens while still exhibiting high total efficiency...

A variety of buffers are commonly used in biochemistry and molecular biology to maintain functions of various biomolecules such as nucleic acids and proteins. We present the high-precision terahertz (THz) time domain spectroscopy (TDS) of buffer and water to clarify differences between both liquids in complex dielectric functions and determine a di...

We present a THz scattering-type scanning near-field optical microscope (THz s-SNOM) system. To estimate pure material resolution of our THz s-SNOM system, near-field images were obtained for embedded metallic grating. The pure material resolution of our THz s-SNOM system was estimated to be less than 100 nm.

We present quantitative coherent measurements of scattering pulses and spectra in terahertz apertureless near-field microscopes. Broadband near-field image contrasts for both amplitude and phase spectra are measured directly from time-domain scattering signals with an unprecedentedly high single-scan signal-to-noise ratio (∼48 dB), with approach cu...

This paper presents a numerical study of self-complementary antennas on sub-strate lenses made of high-permittivity dielectric material. Bowtie, logarithmically periodic, and logarithmic spiral antennas with the same outer and inner dimensions were selected for study, and their overall performances were compared in the terahertz band at frequencies...

Articular cartilage forms a thin layer and has a specialized molecular composition and structural organization along its depth, which consists of superficial, transitional, deep, and calcified zones. Terahertz (THz) pulse imaging is an emerging technique that was developed as a result of recent advances in THz technology. THz pulse imaging is a coh...

The effective complex conductivity tensor of a highly anisotropic, vertically aligned silicon nanowire film was measured by terahertz time-domain spectroscopy. The silicon nanowires were fabricated on a p-type silicon substrate by metal-assisted chemical etching, which resulted in a film with uniaxially anisotropic optical properties. The measured...

Despite the wide application of liquid crystals (LCs) in the visible frequency range, their properties in the terahertz range have not yet been extensively investigated. In this paper we have investigated the terahertz properties of LCs E7, BL037, RDP-94990 and RDP-97304 using terahertz time-domain-spectroscopy. We find that RDP-94990 has the large...

This paper presents a full-wavelength dipole antenna on a membrane covered with a lens to improve the efficiency and directivity of a terahertz photomixer. A full-wavelength dipole antenna supported by a membrane structure has low directivity. An extended hemispherical lens introduced in front of the antenna can achieve high directivity. The optimi...

The water distribution in human osteoarthritic articular cartilage has been quantitatively characterized using terahertz time-domain spectroscopy (THz TDS). We measured the refractive index and absorption coefficient of cartilage tissue in the THz frequency range. Based on our measurements, the estimated water content was observed to decrease with...

A full-wavelength dipole antenna on a GaAs membrane, covered with a silicon lens to improve the output power of a terahertz (THz) photomixer, is proposed. A full-wavelength dipole antenna supported by a GaAs membrane structure has been proven to achieve both high input resistance and high radiation efficiency for improved overall efficiency. Howeve...

This paper presents antenna structures suitable for terahertz photomixer designs. Two antenna structures are selected: a four-leaf-clover-shaped antenna on an extended hemispherical substrate lens and a full-wavelength dipole antenna on a thin substrate with backside excitation. The input impedance and radiation characteristics of each antenna are...

New terahertz (THz) optical devices which can control the optical properties of THz waves are needed to broaden the application area of THz technology. Liquid crystals (LCs) are very attractive materials for developing such devices because they have outstanding properties such as sensitivity to applied electric fields, chemical stability, relativel...

Recent technological breakthrough in the field of Terahertz radiation has triggered new applications in biology and biomedicine. Particularly, biological applications are based on the specific spectroscopic fingerprints of biological matter in this spectral region. Historically with the discovery of new electromagnetic wave spectrum, we have always...

Liquid crystals (LCs) represent a very interesting material in the visible range but their properties in the terahertz (THz) range have not been extensively investigated yet. In this paper, we have investigated the THz properties of a set of LCs E7, BL037, and RDP97304 using terahertz time-domain spectroscopy. Refractive indices and absorption coef...

We present the analysis and measurements of scattering signals of a terahertz pulse scattering-type near-field microscope. We used a self-consistent line dipole image method for the quantitative analysis of the THz near-field interaction. The line scan across a gold film demonstrated that the terahertz microscope has a nanoscale resolution of ~80 n...

We demonstrated quantitative analysis and measurements of near-fields interactions in a terahertz pulse near-field microscope. We developed a self-consistent line dipole image method for the quantitative analysis of the near-field interaction in THz scattering-type scanning optical microscopes. The measurements of approach curves and relative contr...

In this paper, a full-wavelength dipole antenna, supported by a GaAs membrane structure, is examined. The antenna was designed with both high input resistance and high radiation efficiency to improve the overall efficiency of a terahertz photomixer design. The geometrical effects of the GaAs membrane structure on the overall antenna performance wer...

Lymph node metastasis is an important prognostic factor in cervical cancer patients. We report THz imaging for detecting micro-metastatic foci in the lymph nodes of early-stage uterine cervical cancer patients. Five paraffin-embedded metastatic lymph nodes from two cervical cancer patients were imaged using a THz time-domain spectroscopy system in...

There have been many issues which remain mysterious on the physics of high temperature superconductivity, magnetism and quantum criticality. The recent discovery of iron-based superconductors could enable physicists to break through the mystery of those issues. We review the recent works about the magnetic properties as well as the superconductivit...

A four-leaf-clover-shaped antenna composed of a highly resonant radiation element and a stable DC feed element and mounted on an extended hemispherical lens is described. The proposed antenna was optimized to have maximum total efficiency and maximum directivity on an extended hemispherical lens. Compared to a full-wavelength dipole antenna on a si...

To improve the output power of a photomixer used as a THz source, we propose a four-leaf-clover-shaped antenna structure composed of a highly resonant radiation element and a stable DC feed element. The resonance characteristics of the proposed structure were first investigated on a half-infinite substrate as a simplified radiation environment to r...

In this paper, we present THz Yagi-Uda antennas with high input impedance, achieving greatly improved impedance matching compared to existing THz antennas. We show designs employing both full-wavelength straight-line dipole and full-wavelength U-shaped dipole drivers. Current leakage into the bias line was minimized by applying photonic band gap st...

This paper presents the design of terahertz Yagi-Uda antennas with high input resistance. The proposed Yagi-Uda antennas can achieve much higher input resistance than existing THz antennas by using a full-wavelength straight-line dipole and a full-wavelength U-shaped dipole as driver elements. Current leakage into the bias line was minimized by app...

This paper presents a Yagi-Uda antenna with a U-shaped dipole as the driver element for a terahertz photomixer. By constructing the antenna on a thin substrate, end-fire radiation patterns with high antenna impedance can be obtained even when the substrate has a high dielectric constant. The proposed antenna achieves a high input resistance of 4372...

Air guided single mode propagation of THz radiation in a photonic crystal waveguide has been experimentally demonstrated. The photonic crystal waveguide has been fabricated by introducing an air defect at the center of the air/Si 1D photonic crystal.

Two-dimensional (2D) photonic quasicrystals (PQCs) were fabricated by a holographic nanolithography technique. Using two laser beams with different angles incident on the sample, microcavities with 2D internal nanostructures are patterned with a few micrometer periods.

Guided-wave propagation of sub-ps terahertz (THz) pulses in a highly birefringent plastic photonic crystal fiber has been experimentally demonstrated. The fabricated fibers have exhibited an extremely high birefringence of ~ 0.021 at 0.3 THz.

Optically pumped terahertz emission has been observed in a wide range of semiconductors. We show that InAs quantum dots on GaAs can be used to significantly enhance terahertz emission compared with a bare GaAs surface.

Photoconductive folded half-wavelength dipole antennas have been introduced and demonstrated successfully for high power THz applications. The pulsed terahertz signals are measured in the time-domain terahertz setup. Using the simple model, the measured THz signals were analyzed. The calculated signals are in good agreements with the measured signa...

Terahertz (THz) radiation, which occupies a large portion of the electromagnetic spectrum between the mid-infrared and microwave bands, offers innovative imaging and sensing technologies that can provide information, which may not be available through conventional methods (i.e. microwave and X-ray techniques.) As THz wave (T-ray) technology improve...

We investigated the device performances for a post-growth thermally treated In0.5Ga0.5As/GaAs quantum-dot infrared detector (QDIP). Device characteristics, such as dark current, photoluminescence (PL), and photocurrent spectra, have been studied and compared for the as-grown and thermally treated QDIPs. After the thermal treatment with a SiO2 cappi...

THz wave near-field emission microscopes provide THz emission from nanoscale semiconductor through femtosecond laser excitation. Metal/semiconductor surface measurements show that the resolutions are less than 1 and 100 nm in vertical and lateral directions, respectively.

A scanning near-field microscope provides nano-scale imaging capability of field induced THz wave emission spectra from semiconductor surfaces and interfaces. Combined with a scanning probe tip and femtosecond optical pulse excitation, THz wave emission with sub-100 nm spatial resolution has been demonstrated. The scanning probe tip modulates semic...

Intermixing effects of MOCVD (metal organic chemical vapor deposition) grown InGaAs SAQDs (self-assembled quantum dots) covered with SiO2 and SiNx-SiO2 dielectric capping layers were investigated. The intermixing of SAQDs was isothermally performed at 700°C by varying annealing time under the N2-gas ambient. It was confirmed from the PL measurement...

We describe an optical sensor using photonic quantum ring lasers and resonance- enhanced photodetectors for the purpose of counting object discrimination. Due to the angle-dependent peak wavelength shift characteristics of the photonic quantum ring laser, an emission at an angle of 0 approximately 30 deg from a perpendicular direction was reflected...

Low-energy As-ion-implanted InAs was examined using double-crystal X-ray diffraction and transmission electron microscopy. For uniform defect distribution, multiple implantations were made at 0.05-0.4 MeV with 4 × 1014-5× 1015 ions/cm2. After annealing at 600°C for 20 min, As precipitates were observed, and the implantation-induced strain was signi...

We experimentally investigated presence of multi-frequency soliton complex that exist in high power Er/Yb-doped Fiber Amplifier. The complex with the spectral bandwidth in excess of 100 nm is bound by the Kerr nonlinearity and exhibit stable propagation.

We show numerically that a large phase-divergence and phase-locking can occur in mutually trapped orthogonally polarized spatial solitons in AlGaAs planar waveguides due to the presence of strong polarization coupling.

In conclusion we have described the impurity-free vacancy disordering of AlGaAs-AlGaAs GRIN quantum well structures by a single-step rapid thermal annealing (RTA). We have used the samples with different mesa widths to investigate the spectral shifts in the photoluminescence (PL) and lasing characteristics

Small signal direct modulation characteristics of InGaAs-GaAsP-InGaP multiple quantum well ridge waveguide lasers (4.5/spl times/220 /spl mu/m/sup 2/) are described. The compressive strain of four InGaAs quantum wells is compensated by the tensile strain of GaAsP barriers. The lasers have a threshold current of 8 mA and an internal differential qua...

The activation efficiencies of implanted Si, Be, and C in GaAs 0.93 P 0.07 have been measured in the annealing range 650–950 °C. Be provides much higher sheet hole densities than C, even when the latter is coimplanted with Ar to enhance the electrical activity. The maximum activation efficiency of Be is ∼60% at a dose of 5×10¹⁴ cm<sup>2<...

The performance characteristics of InGaAs-GaAsP-InGaP strain compensated laser emitting near 1 /spl mu/m are reported. The ridge waveguide lasers have room temperature threshold current of 18 mA and differential quantum efficiency of 0.45 W/A/facet. The linewidth enhancement factor is smaller and gain coefficient is larger for these strain compensa...

Single transverse and lateral mode InGaAs/GaAsP/InGaP 980 nm SPIN (spread index) ridge waveguide lasers (3 μm×750 μm) with symmetric far fields of Θ_⊥×Θ_||=17°×15°, slope efficiency of η_ext=0.9 W/A and threshold current of I_th=24 mA have been demonstrated. 90% coupling to singlemode lensed fibre and 52% di...

We describe a new InGaP/lnGaAs/GaAs quantum well laser structure, the SPIN (Spread INdex) laser design. The figure of merit for the 980 nm Er-doped fiber application is how much power is coupled into the single-mode fiber. This is determined by the beam divergence and external differential quantum efficiency. To this end, the SPIN laser design allo...

The processing and optical characteristics of an InGaAs-AlGaAs-GaAs strained-layer quantum-well heterostructure square ring laser (4 μm width and 50-μm/side length) with two output waveguides and two narrow physical gap couplers are described. The ridges and the total internal reflection (TIR) mirrors of the square ring lasers are fabricated by two...

Two-dimensional rectangular lattice distributed feedback lasers in a waveguide geometry are described by a coupled-mode analysis of a scalar wave equation derived from a vector field for TE guided modes. A detailed analysis is provided for a two-dimensional grating which allows the method of separation of variables for the scalar wave equation so t...

High-quality total internal reflection (TIR) mirrors are fabricated by a new self-aligned etch process using SiCl/sub 4/ reactive ion etching (RTE). The self-aligned etch process requires only a single SiO/sub 2/ mask layer without additional dielectric depositions and lift-off steps. The reflectivity of the TIR mirrors are measured by characterizi...

Lateral-mode discrimination by misaligned total internal reflection (TIR) mirrors in multimode ridge waveguide structures is proposed. The effects of angular misalignment in TIR mirrors and index steps in ridge waveguides on the modal reflectivity is analyzed by a plane wave expansion method. The modal reflectivity of a high order mode decreases mo...

Two types of integrable semiconductor lasers are investigated in this thesis. Semiconductor ring lasers with circular and square ring resonators are analyzed, and their fabrications and laser results are described. Two -dimensional rectangular lattice distributed feedback lasers are proposed and analyzed by a coupled mode theory. Strained-layer InG...

Top-contact monolithic serially-biased InGaAs-GaAs-AlGaAs (/spl lambda//spl sim/0.93 /spl mu/m) broad area strained-layer quantum well laser arrays have been fabricated on a semi-insulating GaAs substrate. The laser array consists of four individual laser diodes and operates up to 2.8 W at 3.6 A (supply limited) per uncoated facet under pulsed cond...

Selective-area growth and regrowth using conventional atmospheric pressure metalorganic chemical vapor deposition is investigated for wavelength tuning in strained layer InxGa1-xAsGaAs-Aly Ga1-yAs quantum well lasers. Growth inhibition from a silicon dioxide mask is the mechanism used for the selective-area growth rate enhancement. By varying the w...

The monolithic integration of a strained layer InGaAs-GaAs-AlGaAs quantum-well laser with a passive waveguide by conventional atmospheric pressure metalorganic chemical vapor growth is discussed. Growth inhibition from a silicon dioxide mask is the mechanism used for selective-area growth rate enhancement. Variation in the width of the oxide stripe...