• Current Optics and Photonics
• /
• v.5 no.4
• /
• pp.375-383
• /
• 2021

We present a simulation method to design antireflection coating (ARCs) for fiber-coupled superconducting nanowire single-photon detectors. Using a finite-element method, the absorptance of the nanowire is calculated for a defined unit-cell structure consisting of a fiber, ARC layer, nanowire absorber, distributed Bragg reflector (DBR) mirror, and air gap. We develop a method to evaluate the uncertainty in absorptance due to the uncontrollable parameter of air-gap distance. The validity of the simulation method is tested by comparison to an experimental realization for a case of single-layer ARC, which results in good agreement. We show finally a double-layer ARC design optimized for a system detection efficiency of higher than 95%, with a reduced uncertainty due to the air-gap distance.

• Korean Journal of Optics and Photonics
• /
• v.23 no.2
• /
• pp.71-76
• /
• 2012

A Sagnac-type Mach-Zehnder interferometer(SMZI) is a very stable instrument with respect to phase drift because all optical paths can share common optical components. We have observed very stable interference fringes in single photon interference experiments with an extremely attenuated laser beam, and with heralded single photons prepared from polarization correlated photon pairs in type-II spontaneous parametric down-conversion. The phase stability is measured to be about ${\pm}0.18$ rad for the SMZI, in contrast, conventional MZI showed the phase fluctuation of around ${\pm}1.02$ rad.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
• /
• 2009.05a
• /
• pp.678-681
• /
• 2009

A single-pixel photon counter is designed using a folded cascode CMOS operational amplifier which is self-biased. Since there is no need for a voltage bias circuit, the layout area and power consumption of the designed counter are reduced. The signal voltage of the designed charge sensitive amplifier (CSA) with the MagnaChip $0.18{\mu}m$ CMOS process is simulated to be 138mv, near the theoretical voltage of 151mV. And the layout area of the designed counter is $100{\mu}m{\times}100{\mu}m$.

• New Physics: Sae Mulli
• /
• v.68 no.12
• /
• pp.1374-1377
• /
• 2018

We achieved 95% visibility in the Hong-Ou-Mandel interference experiment while we achieved only 56% visibility in a previous report. We used a 120 mW 405 nm single-mode continuous wave laser, a 5-mm-thick type-1 ${\beta}$-barium borate single crystal, standard Hong-Ou-Mandel interferometer optics, two avalanche photodiode single-photon counters, and a homemade coincidence counting unit. The photon collection unit was the key difference between the present study and the previous study. In the present experiment, we used single-mode optical fibers for photon collection, which suppressed accidental coincidence between-different mode photons by acting as a spatial filter because of its core size being much smaller than a multi-mode fiber.

• Proceedings of the Optical Society of Korea Conference
• /
• 2009.10a
• /
• pp.343-344
• /
• 2009

Pump wavelength dependence of the gain spectra was investigated by simulating the model of single-pumped fiber optical parametric amplifiers (FOPAs). Conditions for the single-pumped FOPAs having broad bandwidths with relatively large gains were optimized and the gain bandwidth was found to be as large as 146.4 nm when signal power and pump power were 316 nW and 7 W, respectively.

• JSTS:Journal of Semiconductor Technology and Science
• /
• v.14 no.6
• /
• pp.789-796
• /
• 2014

A CMOS photo detection bias quenching circuit is developed to be used with single photon avalanche photodiodes (SPADs) operating in Geiger mode for the detection of weak optical signals. The proposed bias quenching circuits for the performance improvement reduce the circuit size as well as improve the performance of the quenching operation. They are fabricated in a $0.18-{\mu}m$ standard CMOS technology to verify the effectiveness of this technique with the chip area of only $300{\mu}m^2$, which is about 60 % of the previous reported circuit. Two types of proposed circuits with resistive and capacitive load demonstrated improved performance of reduced quenching time. With a commercial APD by HAMAMATSU, the dead time can be adjusted as small as 50 ns.

• Journal of the Optical Society of Korea
• /
• v.11 no.3
• /
• pp.97-100
• /
• 2007

We have investigated properties of a plastic photonic crystal fiber guiding terahertz radiations, THz photonic crystal fiber. The single-mode condition and dispersion of a plastic triangular THz photonic crystal fiber are investigated by using the plane wave expansion method and the beam propagation method. The THz photonic crystal fiber can perform as a single-mode fiber below 2.5 THz when the ratio of diameter (d) and period (${\Lambda}$) of air holes is less than 0.475. The THz photonic crystal fiber with ${\Lambda}=500{\mu}m$ and $d/{\Lambda}=0.4$ shows almost zero flattened dispersion behavior, $-0.03{\pm}0.02 ps/THz{\cdot}cm$, in the THz frequency range from 0.8 to 2.0 THz.

• Journal of the Korean Physical Society
• /
• v.73 no.10
• /
• pp.1502-1505
• /
• 2018

We present a design for hybrid circular Bragg gratings (hCBGs) for efficiently extracting single-photons emitted by InAs quantum dots (QDs) embedded in GaAs. Finite-difference time-domain simulations show that a very high photon collection efficiency (PCE) up to 96% over a 50 nm bandwidth and pronounced Purcell factors up to 19 at cavity resonance are obtained. We also systematically investigate the geometry parameters, including the $SiO_2$ thickness, grating period, gap width and the central disk radius, to improve the device performances. Finally, the PCEs and the Purcell factors of QDs located at different positions of the hCBG are studied, and the results show great robustness against uncertainties in the location of the QD.

• Korean Journal of Optics and Photonics
• /
• v.32 no.1
• /
• pp.29-33
• /
• 2021

We have experimentally simulated the violation of the Leggett-Garg inequality (LGI) using a heralded-single-photon source via spontaneous parametric down-conversion (SPDC) in a periodically poled potassium titanyl phosphate (PPKTP) crystal. We measured the polarization fringe as a function of the angle of the linear polarization of the photons, and analyzed the LGI according to n measurement. Simulation results for LGI based on the polarization of the heralded single photon were in good agreement with theoretical calculations.

• Bulletin of the Korean Chemical Society
• /
• v.10 no.3
• /
• pp.298-302
• /
• 1989

The strengths of two-photon transitions from the ground state to excited vibronic states in benzene are calculated by using the CNDO/2-U wave functions. The role of vibronic coupling in two-photon absorption process is discussed. The $A_{1{\bar{g}}}-A_{2g}^+$ two-photon transitions, which are forbidden by the identity-forbidden selection rule in single frequency two-photon absorption, are too weak to be experimentally observed even when two photons of different energies are used. It is because the transitions are forbidden also by the pseudo-parity selection rule which are applicable for alternant hydrocarbons such as benzene. It is also shown that the vibronic coupling is not very effective in altering the pseudo-parity property of the electronic state. The strength of the vibronically induced two-photon absorption is strongly affected by the presence of an electronic state from which two-photon absorption can borrow the intensity. It is pointed out that the pseudo-parity selection rule may be violated in such cases.