[Editor's Pick] Current Optics and Photonics Vol. 5 no. 1 (2021 February) Shannon Entropy as an Indicator of the Spatial Resolutions of the Morphologies of the Mode Patterns in an Optical Resonator Kyu-Won Park1 *, Jinuk Kim1, and Songky Moon2 1 Department of Physics and Astronomy and Institute of Applied Physics, Seoul National University, Seoul 08826, Korea 2 Faculty of Liberal Education, Seoul National University, Seoul 08826, Korea Current Optics and Photonics Vol. 5 No.1 (2021 February) pp. 16-22 DOI: https://doi.org/10.3807/COPP.2021.5.1.016 Fig. 1 The eigenvalue trajectories and their corresponding Shannon entropies. (a) Real part of eigenvalues kR as the eccentricity ε is varied. (b) Imaginary part of eigenvalues kR as ε is varied. The Shannon entropies for N = 98, 212, 398, 596, 810, and 1040 are shown in (c), (d), (e), (f), (g), and (h), respectively. Abstract We present the Shannon entropy as an indicator of the spatial resolutions of the morphologies of the resonance mode patterns in an optical resonator. We obtain each optimized number of mesh points, one of minimum size and the other of maximum one. The optimized mesh-point number of minimum size is determined by the identifiable quantum number through a chi-squared test, whereas the saturation of the difference between Shannon entropies corresponds to the other mesh-point number of maximum size. We also show that the optimized minimum mesh-point increases as the (real) wave number increases and approximates the proportionality constant between them. [Editor's Pick] Current Optics and Photonics Vol. 4 no. 6 (2020 December) Comparative Measurement of Transverse Nuclear Magnetization of Polarized 129Xe and 131Xe by Spin-exch ange Optical Pumping Ye Jin Yu, Seong Ho Min, and Han Seb Moon* Department of Physics, Pusan National University, Busan 46241, Korea Current Optics and Photonics Vol. 4 No. 6 (2020 December) pp. 466-471 DOI: https://doi.org/10.3807/COPP.2020.4.6.466 Fig. 1 The eigenvalue trajectories and their corresponding Shannon entropies. (a) Real part of eigenvalues kR as the eccentricity ε is varied. (b) Imaginary part of eigenvalues kR as ε is varied. The Shannon entropies for N = 98, 212, 398, 596, 810, and 1040 are shown in (c), (d), (e), (f), (g), and (h), respectively. Abstract We analyze the transverse nuclear magnetizations of 129Xeand 131Xe in a vapor cell containing natural Xe, 87Rb, and buffer gases. Th e Xe atoms are polarized th rough spin-exch ange optical pumping (SEOP) with Rb atoms under low-magnetic-field conditions. From the free-induction-decay (FID) signal, we measure the nuclear magnetization of the Xe atoms in the Xe-Rb vapor cell. Furthermore, we measure the dependence of the gyromagnetic ratio on the magnetization of 129Xe and 131Xe by examining the amplitude of the FID signal of each isotope, and we evaluate the relationship between the magnetic field gradient and transverse relaxation rate for both of the 129Xe and 131Xe isotopes. [Editors Pick] Current Optics and Photonics Vol. 4 no. 5 (2020 October) A Privacy-protection Device Using a Directional Backlight and Facial Recognition Hyeontaek Lee, Hyunsoo Kim, and Hee-Jin Choi* Department of Physics and Astronomy, Sejong University, Seoul 05006, Korea Current Optics and Photonics Vol. 4 No. 5 (2020 October) pp. 421-427 DOI: https://doi.org/10.3807/COPP.2020.4.5.421 Fig. 1 Calculation of the true position of the face’s center. Abstract A novel privacy-protection device to prevent visual hacking is realized by using a directional backlight and facial recognition. The proposed method is able to overcome the limitations of previous privacyprotection methods that simply restrict the viewing angle to a narrow range. The accuracy of user tracking is accomplished by the combination of a time-of-flight sensor and facial recognition with no restriction of detection range. In addition, an experimental demonstration is provided to verify the proposed scheme. [Editors Pick] Current Optics and Photonics Vol. 4 no. 4 (2020 August) Terahertz Generation by a Resonant Photoconductive Antenna Kanghee Lee1,2, Seong Cheol Lee1, Won Tae Kim1, Jagang Park2, Bumki Min2, and Fabian Rotermund1 * 1 Department of Physics, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Korea 2 Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Korea Current Optics and Photonics Vol. 4 No. 4 (2020 August) pp. 373-379 DOI: https://doi.org/10.3807/COPP.2020.4.4.373 Fig. 1 (a) Microscopic images of the NRPCA (top panel) and schematic of its cross-section (bottom). The inset in the top panel presents an enlarged view. The bidirectional arrow indicates the THz field direction for the transmission measurement in (c). (b) Microscopic images of the RPCA (top panel) and schematic of its cross section (bottom). (c) Measured amplitude transmission from the NRPCA. (d) Measured amplitude transmission from the RPCA. Abstract In this study, we investigate terahertz (THz) generation by a photoconductive antenna with electrodes in the shape of split-ring resonators. According to our theoretical investigation based on a lumped-circuit model, the inductance of this electrode structure leads to resonant behavior of the photo-induced current. Hence, near the resonance frequency the spectral components generated by a resonant photoconductive antenna can be greater than those produced by a non-resonant one. For experimental verification, a resonant photoconductive antenna, which possesses a resonance mode at 0.6 THz, and a non-resonant photoconductive antenna with stripe-shaped electrodes were fabricated on a semi-insulating GaAs substrate. The THz generation by both of the photoconductive antennas demonstrated a good agreement with the theoretically expected results. The observed relationship between the resonant electrodes of the photoconductive antenna and the generated THz spectrum can be further employed to design a narrow-band THz source with an on-demand frequency. [Editors Pick] Current Optics and Photonics Vol. 4 no. 3 (2020 June) Recent Progress in High-Luminance Quantum Dot Light-Emitting Diodes Seunghyun Rhee1, Kyunghwan Kim1, Jeongkyun Roh2 *, and Jeonghun Kwak1 ** 1 Department of Electrical and Computer Engineering, Inter-university Semiconductor Research Center (ISRC), Seoul National University, Seoul 08826, Korea 2 Department of Electrical Engineering, Pusan National University, Busan 46241, Korea Current Optics and Photonics Vol. 4 No. 3 (2020 June) pp. 161-173 DOI: https://doi.org/10.3807/COPP.2020.4.3.161 Fig. 1 (a) Display applications of QD-based light sources, with respect to required brightness. (b) Schematic of a standard QLED structure, and its energy-band diagram. Abstract Colloidal quantum dots (QDs) have gained tremendous attention as a key material for highly advanced display technologies. The performance of QD light-emitting diodes (QLEDs) has improved significantly over the past two decades, owing to notable progress in both material development and device engineering. The brightness of QLEDs has improved by more than three orders of magnitude from that of early-stage devices, and has attained a value in the range of traditional inorganic LEDs. The emergence of high-luminance (HL) QLEDs has induced fresh demands to incorporate the unique features of QDs into a wide range of display applications, beyond indoor and mobile displays. Therefore it is necessary to assess the present status and prospects of HL-QLEDs, to expand the application domain of QD-based light sources. As part of this study, we review recent advances in HL-QLEDs. In particular, based on reports of brightness exceeding 10 5 cd/m 2 , we have summarized the major approaches toward achieving high brightness in QLEDs, in terms of material development and device engineering. Furthermore, we briefly introduce the recent progress achieved toward QD laser diodes, being the next step in the development of HL-QLEDs. This review provides general guidelines for achieving HL-QLEDs, and reveals the high potential of QDs as a universal material solution that can enable realization of a wide range of display applications. [Editors Pick] Current Optics and Photonics Vol. 4 no. 2 (2020 April) Analysis of the Design Parameters for a Lightfield Near-eye Display Based on a Pinhole Array Hyeontaek Lee1, Ungyeon Yang2, and Hee-Jin Choi1 * 1 Department of Physics and Astronomy, Sejong University, Seoul 05006, Korea 2 Electronics and Telecommunications Research Institute, Daejeon 34129, Korea Current Optics and Photonics Vol. 4 No. 2 (2020 April) pp. 121-126 DOI: https://doi.org/10.3807/COPP.2020.4.2.121 Fig. 1 The basic principles and parameters of a pinhole-array-based NED. Abstract With the increasing demand for head-mounted display applications, the image quality provided by a near-eye display device is a key factor in satisfying the consumer. Among various techniques to realize a near-eye display that has a thinner volume than the working distance of a human eye, a lightfield image-generation method based on a pinhole array is attracting much attention, with its simple and thin structure. In this paper, we propose a numerical analysis of the visual parameters and verifications with computational reconstruction. [Editors Pick] Current Optics and Photonics Vol. 4 no. 1 (2020 February) Simultaneous Generation of Orthogonally Polarized Signals in an Optical Parametric Oscillator Based on Periodically Poled Lithium Niobate CH. S. S. Pavan Kumar, Byoung Joo Kim, Deok Woo Kim, and Myoungsik Cha* Department of Physics, Pusan National University, Busan 46241, Korea Current Optics and Photonics Vol. 4 No. 1 (2020 February) pp. 63-68 DOI: https://doi.org/10.3807/COPP.2020.4.1.063 Fig. 1 Schematic diagram of experimental setup for OPO. L: Focusing lens, M1 and M2: Cavity mirrors, F: Filters, P: Polarizer. Abstract We built an optical parametric oscillator (OPO) generating orthogonally polarized signals at different wavelengths simultaneously, based on a periodically poled lithium niobate (PPLN) crystal. The OPO was pumped by ns-pulses at 1.064 µm from a diode-pumped solid-state laser, where we found the type-0 and the type-1 quasi-phase matching conditions were satisfied simultaneously in the PPLN crystal. This enabled us to create a coherent light source which can emit dual signals which could be accessed easily by rotating a polarizer. [Editors Pick] Current Optics and Photonics Vol. 3 no. 6 (2019 December) Frequency Response Estimation of 1.3 µm Waveguide Integrated Vertical PIN Type Ge-on-Si Photodetector Based on the Analysis of Fringing Field in Intrinsic Region Dongjun Seo1, Won-Bae Kwon2, Sung Chang Kim2, and Chang-Soo Park1 * 1 School of Electrical Engineering and Computer Science, Gwangju Institute of Science and Technology (GIST), Gwangju 61005, Korea 2 Honam Research Center, Electronics and Telecommunications Research Institute (ETRI), Gwangju 61012, Korea Current Optics and Photonics Vol. 3 No. 6 (2019 December) pp. 510-515 DOI: https://doi.org/10.3807/COPP.2019.3.6.510 Fig. 1 Photodetector structure diagrams-(a) perspective view; (b) front view; (c) top view. Abstract In this paper, we introduce a 1.3-µm 25-GHz waveguide-integrated vertical PIN type Ge-on-Si photodetector fabricated using a multi-project wafers service based on fringing field analysis in the depletion region. In general, 1.3-µm photodetectors fabricated using a commercial foundry service can achieve limited bandwidths because a significant amount of photo-generated carriers are located within a few microns from the input along the device length, and they are influenced by the fringing field, leading to a longer transit time. To estimate the response time, we calculate the fringing field in that region and the transit time using the drift velocity caused by the field. Finally, we compare the estimated value with the measured one. The photodetector fabricated has a bandwidth of 20.75 GHz at -1 V with an estimation error of <3 GHz and dark current and responsivity of 110 nA and 0.704 A/W, respectively. [Editors Pick] Current Optics and Photonics Vol. 3 no. 5 (2019 October) Two-dimensional Laser Drilling Using the Superposition of Orthogonally Polarized Images from Two Computer-generated Holograms Hwihyeong Lee1 *, Seongwoo Cha2, Hee Kyung Ahn1, and Hong Jin Kong2 1 Space Optics Team, Advanced Instrumentation Institute, Korea Research Institute of Standards and Science, Daejeon 34113, Korea 2 Department of Physics, Korea Advanced Institute of Science and Technology, Daejeon 34141, Korea Current Optics and Photonics Vol. 3 No. 5 (2019 October) pp. 451-457 DOI: https://doi.org/10.3807/COPP.2019.3.5.451 Fig. 1 (a) A line target. (b) A CGH phase made with the target. (c) A reconstructed image. The insets show each of the magnified parts. Abstract Laser processing using holograms can greatly improve processing speed, by spatially distributing the laser energy on the target material. However, it is difficult to reconstruct an image with arrays of closely spaced spots for laser processing, because the specklelike interference pattern prevents the spots from getting close to each other. To resolve this problem, a line target was divided in two, reconstructed with orthogonally polarized beams, and then superposed. Their optical reconstruction was performed by computer-generated holograms and a pulsed laser. With this method, we performed two-dimensional (2D) laser drilling of polyimide film, with a kerf width of 20 µm and a total processing length of 20 mm. [Editors Pick] Current Optics and Photonics Vol. 3 no. 4 (2019 August) Chirality in Non-Hermitian Photonics Sunkyu Yu, Xianji Piao, and Namkyoo Park* Photonic Systems Laboratory, Department of Electrical and Computer Engineering, Seoul National University, Seoul 08826, Korea Current Optics and Photonics Vol. 3 No. 4 (2019 August) pp. 275-284 DOI: https://doi.org/10.3807/COPP.2019.3.4.275 Fig. 1 Chirality in different physical domains of nonHermitian photonics. For the optical field, E = eA(r,R)exp (iωt - ik(R) · rr), where R is the system parameter vector and r is the position vector, the extended definition of optical chirality in non-Hermitian photonics can be classified according to each physical quantity: polarization e for SAM, wavefrontA(r,R) for OAM, canonical momentum k(R) for wave propagation, and the geometry of state evolution in the system parameter space R. The system parameter R represents the complex optical potential that determines the condition of PT symmetry, including on-site and hopping constants defined by structural and material parameters. Abstract Chirality is ubiquitous in physics and biology from microscopic to macroscopic phenomena, such as fermionic interactions and DNA duplication. In photonics, chirality has traditionally represented differentiated optical responses for right and left circular polarizations. This definition of optical chirality in the polarization domain includes handedness-dependent phase velocities or optical absorption inside chiral media, which enable polarimetry for measuring the material concentration and circular dichroism spectroscopy for sensing biological or chemical enantiomers. Recently, the emerging field of non-Hermitian photonics, which explores exotic phenomena in gain or loss media, has provided a new viewpoint on chirality in photonics that is not restricted to the traditional polarization domain but is extended to other physical quantities such as the orbital angular momentum, propagation direction, and system parameter space. Here, we introduce recent milestones in chiral light-matter interactions in non-Hermitian photonics and show an enhanced degree of design freedom in photonic devices for spin and orbital angular momenta, directionality, and asymmetric modal conversion. 12345
[Editor's Pick] Current Optics and Photonics Vol. 5 no. 1 (2021 February) Shannon Entropy as an Indicator of the Spatial Resolutions of the Morphologies of the Mode Patterns in an Optical Resonator Kyu-Won Park1 *, Jinuk Kim1, and Songky Moon2 1 Department of Physics and Astronomy and Institute of Applied Physics, Seoul National University, Seoul 08826, Korea 2 Faculty of Liberal Education, Seoul National University, Seoul 08826, Korea Current Optics and Photonics Vol. 5 No.1 (2021 February) pp. 16-22 DOI: https://doi.org/10.3807/COPP.2021.5.1.016 Fig. 1 The eigenvalue trajectories and their corresponding Shannon entropies. (a) Real part of eigenvalues kR as the eccentricity ε is varied. (b) Imaginary part of eigenvalues kR as ε is varied. The Shannon entropies for N = 98, 212, 398, 596, 810, and 1040 are shown in (c), (d), (e), (f), (g), and (h), respectively. Abstract We present the Shannon entropy as an indicator of the spatial resolutions of the morphologies of the resonance mode patterns in an optical resonator. We obtain each optimized number of mesh points, one of minimum size and the other of maximum one. The optimized mesh-point number of minimum size is determined by the identifiable quantum number through a chi-squared test, whereas the saturation of the difference between Shannon entropies corresponds to the other mesh-point number of maximum size. We also show that the optimized minimum mesh-point increases as the (real) wave number increases and approximates the proportionality constant between them. [Editor's Pick] Current Optics and Photonics Vol. 4 no. 6 (2020 December) Comparative Measurement of Transverse Nuclear Magnetization of Polarized 129Xe and 131Xe by Spin-exch ange Optical Pumping Ye Jin Yu, Seong Ho Min, and Han Seb Moon* Department of Physics, Pusan National University, Busan 46241, Korea Current Optics and Photonics Vol. 4 No. 6 (2020 December) pp. 466-471 DOI: https://doi.org/10.3807/COPP.2020.4.6.466 Fig. 1 The eigenvalue trajectories and their corresponding Shannon entropies. (a) Real part of eigenvalues kR as the eccentricity ε is varied. (b) Imaginary part of eigenvalues kR as ε is varied. The Shannon entropies for N = 98, 212, 398, 596, 810, and 1040 are shown in (c), (d), (e), (f), (g), and (h), respectively. Abstract We analyze the transverse nuclear magnetizations of 129Xeand 131Xe in a vapor cell containing natural Xe, 87Rb, and buffer gases. Th e Xe atoms are polarized th rough spin-exch ange optical pumping (SEOP) with Rb atoms under low-magnetic-field conditions. From the free-induction-decay (FID) signal, we measure the nuclear magnetization of the Xe atoms in the Xe-Rb vapor cell. Furthermore, we measure the dependence of the gyromagnetic ratio on the magnetization of 129Xe and 131Xe by examining the amplitude of the FID signal of each isotope, and we evaluate the relationship between the magnetic field gradient and transverse relaxation rate for both of the 129Xe and 131Xe isotopes. [Editors Pick] Current Optics and Photonics Vol. 4 no. 5 (2020 October) A Privacy-protection Device Using a Directional Backlight and Facial Recognition Hyeontaek Lee, Hyunsoo Kim, and Hee-Jin Choi* Department of Physics and Astronomy, Sejong University, Seoul 05006, Korea Current Optics and Photonics Vol. 4 No. 5 (2020 October) pp. 421-427 DOI: https://doi.org/10.3807/COPP.2020.4.5.421 Fig. 1 Calculation of the true position of the face’s center. Abstract A novel privacy-protection device to prevent visual hacking is realized by using a directional backlight and facial recognition. The proposed method is able to overcome the limitations of previous privacyprotection methods that simply restrict the viewing angle to a narrow range. The accuracy of user tracking is accomplished by the combination of a time-of-flight sensor and facial recognition with no restriction of detection range. In addition, an experimental demonstration is provided to verify the proposed scheme. [Editors Pick] Current Optics and Photonics Vol. 4 no. 4 (2020 August) Terahertz Generation by a Resonant Photoconductive Antenna Kanghee Lee1,2, Seong Cheol Lee1, Won Tae Kim1, Jagang Park2, Bumki Min2, and Fabian Rotermund1 * 1 Department of Physics, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Korea 2 Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Korea Current Optics and Photonics Vol. 4 No. 4 (2020 August) pp. 373-379 DOI: https://doi.org/10.3807/COPP.2020.4.4.373 Fig. 1 (a) Microscopic images of the NRPCA (top panel) and schematic of its cross-section (bottom). The inset in the top panel presents an enlarged view. The bidirectional arrow indicates the THz field direction for the transmission measurement in (c). (b) Microscopic images of the RPCA (top panel) and schematic of its cross section (bottom). (c) Measured amplitude transmission from the NRPCA. (d) Measured amplitude transmission from the RPCA. Abstract In this study, we investigate terahertz (THz) generation by a photoconductive antenna with electrodes in the shape of split-ring resonators. According to our theoretical investigation based on a lumped-circuit model, the inductance of this electrode structure leads to resonant behavior of the photo-induced current. Hence, near the resonance frequency the spectral components generated by a resonant photoconductive antenna can be greater than those produced by a non-resonant one. For experimental verification, a resonant photoconductive antenna, which possesses a resonance mode at 0.6 THz, and a non-resonant photoconductive antenna with stripe-shaped electrodes were fabricated on a semi-insulating GaAs substrate. The THz generation by both of the photoconductive antennas demonstrated a good agreement with the theoretically expected results. The observed relationship between the resonant electrodes of the photoconductive antenna and the generated THz spectrum can be further employed to design a narrow-band THz source with an on-demand frequency. [Editors Pick] Current Optics and Photonics Vol. 4 no. 3 (2020 June) Recent Progress in High-Luminance Quantum Dot Light-Emitting Diodes Seunghyun Rhee1, Kyunghwan Kim1, Jeongkyun Roh2 *, and Jeonghun Kwak1 ** 1 Department of Electrical and Computer Engineering, Inter-university Semiconductor Research Center (ISRC), Seoul National University, Seoul 08826, Korea 2 Department of Electrical Engineering, Pusan National University, Busan 46241, Korea Current Optics and Photonics Vol. 4 No. 3 (2020 June) pp. 161-173 DOI: https://doi.org/10.3807/COPP.2020.4.3.161 Fig. 1 (a) Display applications of QD-based light sources, with respect to required brightness. (b) Schematic of a standard QLED structure, and its energy-band diagram. Abstract Colloidal quantum dots (QDs) have gained tremendous attention as a key material for highly advanced display technologies. The performance of QD light-emitting diodes (QLEDs) has improved significantly over the past two decades, owing to notable progress in both material development and device engineering. The brightness of QLEDs has improved by more than three orders of magnitude from that of early-stage devices, and has attained a value in the range of traditional inorganic LEDs. The emergence of high-luminance (HL) QLEDs has induced fresh demands to incorporate the unique features of QDs into a wide range of display applications, beyond indoor and mobile displays. Therefore it is necessary to assess the present status and prospects of HL-QLEDs, to expand the application domain of QD-based light sources. As part of this study, we review recent advances in HL-QLEDs. In particular, based on reports of brightness exceeding 10 5 cd/m 2 , we have summarized the major approaches toward achieving high brightness in QLEDs, in terms of material development and device engineering. Furthermore, we briefly introduce the recent progress achieved toward QD laser diodes, being the next step in the development of HL-QLEDs. This review provides general guidelines for achieving HL-QLEDs, and reveals the high potential of QDs as a universal material solution that can enable realization of a wide range of display applications. [Editors Pick] Current Optics and Photonics Vol. 4 no. 2 (2020 April) Analysis of the Design Parameters for a Lightfield Near-eye Display Based on a Pinhole Array Hyeontaek Lee1, Ungyeon Yang2, and Hee-Jin Choi1 * 1 Department of Physics and Astronomy, Sejong University, Seoul 05006, Korea 2 Electronics and Telecommunications Research Institute, Daejeon 34129, Korea Current Optics and Photonics Vol. 4 No. 2 (2020 April) pp. 121-126 DOI: https://doi.org/10.3807/COPP.2020.4.2.121 Fig. 1 The basic principles and parameters of a pinhole-array-based NED. Abstract With the increasing demand for head-mounted display applications, the image quality provided by a near-eye display device is a key factor in satisfying the consumer. Among various techniques to realize a near-eye display that has a thinner volume than the working distance of a human eye, a lightfield image-generation method based on a pinhole array is attracting much attention, with its simple and thin structure. In this paper, we propose a numerical analysis of the visual parameters and verifications with computational reconstruction. [Editors Pick] Current Optics and Photonics Vol. 4 no. 1 (2020 February) Simultaneous Generation of Orthogonally Polarized Signals in an Optical Parametric Oscillator Based on Periodically Poled Lithium Niobate CH. S. S. Pavan Kumar, Byoung Joo Kim, Deok Woo Kim, and Myoungsik Cha* Department of Physics, Pusan National University, Busan 46241, Korea Current Optics and Photonics Vol. 4 No. 1 (2020 February) pp. 63-68 DOI: https://doi.org/10.3807/COPP.2020.4.1.063 Fig. 1 Schematic diagram of experimental setup for OPO. L: Focusing lens, M1 and M2: Cavity mirrors, F: Filters, P: Polarizer. Abstract We built an optical parametric oscillator (OPO) generating orthogonally polarized signals at different wavelengths simultaneously, based on a periodically poled lithium niobate (PPLN) crystal. The OPO was pumped by ns-pulses at 1.064 µm from a diode-pumped solid-state laser, where we found the type-0 and the type-1 quasi-phase matching conditions were satisfied simultaneously in the PPLN crystal. This enabled us to create a coherent light source which can emit dual signals which could be accessed easily by rotating a polarizer. [Editors Pick] Current Optics and Photonics Vol. 3 no. 6 (2019 December) Frequency Response Estimation of 1.3 µm Waveguide Integrated Vertical PIN Type Ge-on-Si Photodetector Based on the Analysis of Fringing Field in Intrinsic Region Dongjun Seo1, Won-Bae Kwon2, Sung Chang Kim2, and Chang-Soo Park1 * 1 School of Electrical Engineering and Computer Science, Gwangju Institute of Science and Technology (GIST), Gwangju 61005, Korea 2 Honam Research Center, Electronics and Telecommunications Research Institute (ETRI), Gwangju 61012, Korea Current Optics and Photonics Vol. 3 No. 6 (2019 December) pp. 510-515 DOI: https://doi.org/10.3807/COPP.2019.3.6.510 Fig. 1 Photodetector structure diagrams-(a) perspective view; (b) front view; (c) top view. Abstract In this paper, we introduce a 1.3-µm 25-GHz waveguide-integrated vertical PIN type Ge-on-Si photodetector fabricated using a multi-project wafers service based on fringing field analysis in the depletion region. In general, 1.3-µm photodetectors fabricated using a commercial foundry service can achieve limited bandwidths because a significant amount of photo-generated carriers are located within a few microns from the input along the device length, and they are influenced by the fringing field, leading to a longer transit time. To estimate the response time, we calculate the fringing field in that region and the transit time using the drift velocity caused by the field. Finally, we compare the estimated value with the measured one. The photodetector fabricated has a bandwidth of 20.75 GHz at -1 V with an estimation error of <3 GHz and dark current and responsivity of 110 nA and 0.704 A/W, respectively. [Editors Pick] Current Optics and Photonics Vol. 3 no. 5 (2019 October) Two-dimensional Laser Drilling Using the Superposition of Orthogonally Polarized Images from Two Computer-generated Holograms Hwihyeong Lee1 *, Seongwoo Cha2, Hee Kyung Ahn1, and Hong Jin Kong2 1 Space Optics Team, Advanced Instrumentation Institute, Korea Research Institute of Standards and Science, Daejeon 34113, Korea 2 Department of Physics, Korea Advanced Institute of Science and Technology, Daejeon 34141, Korea Current Optics and Photonics Vol. 3 No. 5 (2019 October) pp. 451-457 DOI: https://doi.org/10.3807/COPP.2019.3.5.451 Fig. 1 (a) A line target. (b) A CGH phase made with the target. (c) A reconstructed image. The insets show each of the magnified parts. Abstract Laser processing using holograms can greatly improve processing speed, by spatially distributing the laser energy on the target material. However, it is difficult to reconstruct an image with arrays of closely spaced spots for laser processing, because the specklelike interference pattern prevents the spots from getting close to each other. To resolve this problem, a line target was divided in two, reconstructed with orthogonally polarized beams, and then superposed. Their optical reconstruction was performed by computer-generated holograms and a pulsed laser. With this method, we performed two-dimensional (2D) laser drilling of polyimide film, with a kerf width of 20 µm and a total processing length of 20 mm. [Editors Pick] Current Optics and Photonics Vol. 3 no. 4 (2019 August) Chirality in Non-Hermitian Photonics Sunkyu Yu, Xianji Piao, and Namkyoo Park* Photonic Systems Laboratory, Department of Electrical and Computer Engineering, Seoul National University, Seoul 08826, Korea Current Optics and Photonics Vol. 3 No. 4 (2019 August) pp. 275-284 DOI: https://doi.org/10.3807/COPP.2019.3.4.275 Fig. 1 Chirality in different physical domains of nonHermitian photonics. For the optical field, E = eA(r,R)exp (iωt - ik(R) · rr), where R is the system parameter vector and r is the position vector, the extended definition of optical chirality in non-Hermitian photonics can be classified according to each physical quantity: polarization e for SAM, wavefrontA(r,R) for OAM, canonical momentum k(R) for wave propagation, and the geometry of state evolution in the system parameter space R. The system parameter R represents the complex optical potential that determines the condition of PT symmetry, including on-site and hopping constants defined by structural and material parameters. Abstract Chirality is ubiquitous in physics and biology from microscopic to macroscopic phenomena, such as fermionic interactions and DNA duplication. In photonics, chirality has traditionally represented differentiated optical responses for right and left circular polarizations. This definition of optical chirality in the polarization domain includes handedness-dependent phase velocities or optical absorption inside chiral media, which enable polarimetry for measuring the material concentration and circular dichroism spectroscopy for sensing biological or chemical enantiomers. Recently, the emerging field of non-Hermitian photonics, which explores exotic phenomena in gain or loss media, has provided a new viewpoint on chirality in photonics that is not restricted to the traditional polarization domain but is extended to other physical quantities such as the orbital angular momentum, propagation direction, and system parameter space. Here, we introduce recent milestones in chiral light-matter interactions in non-Hermitian photonics and show an enhanced degree of design freedom in photonic devices for spin and orbital angular momenta, directionality, and asymmetric modal conversion.
