Ultra-high vacuum compatible full metal atom beam shutter for optical lattice clocks.

We developed a shutter driven by a solenoid to switch on/off the atomic beam of optical lattice clocks developed at KRISS [C. Y. Park et al.

Graphene Bridge Heterostructure Devices for Negative Differential Transconductance Circuit Applications.

Two-dimensional van der Waals (2D vdW) material-based heterostructure devices have been widely studied for high-end electronic applications owing to their heterojunction properties. In this study, we demonstrate graphene (Gr)-bridge heterostructure devices consisting of laterally series-connected ambipolar semiconductor/Gr-bridge/n-type molybdenum disulfide as a channel material for field-effect transistors (FET). Unlike conventional FET operation, our Gr-bridge devices exhibit non-classical transfer characteristics (humped transfer curve), thus possessing a negative differential transconductance.

Robust frequency stabilization and linewidth narrowing of a laser with large intermittent frequency jumps using an optical cavity and an atomic beam.

An experimental method is developed for robust frequency stabilization using a high-finesse cavity when the laser exhibits large intermittent frequency jumps. This is accomplished by applying an additional slow feedback signal from Doppler-free fluorescence spectroscopy in an atomic beam with increased frequency locking range. As a result, a stable and narrow-linewidth 556 nm laser maintains the frequency lock status for more than a week and contributes to more accurate evaluation of the Yb optical lattice clock.

Advanced Satellite-Based Frequency Transfer at the 10 Level.

Advanced satellite-based frequency transfers by two-way carrier-phase (TWCP) and integer precise point positioning have been performed between the National Institute of Information and Communications Technology and Korea Research Institute of Standards and Science. We confirm that the disagreement between them is less than at an averaging time of several days. In addition, an overseas frequency ratio measurement of Sr and Yb optical lattice clocks was directly performed by TWCP.

Atom shutter using bender piezoactuator.

A flag-type atom shutter based on a rotating lever that is driven by a bender piezoelectric actuator was developed to manipulate atomic beams. The shutter flag was displaced by ∼10 mm to open and close a 5-mm-diameter aperture with a shutter time of 13 ms that produced small mechanical vibrations. The short-term shutter time stability for each cycle was 0.

Neurological Change after Gamma Knife Radiosurgery for Brain Metastases Involving the Motor Cortex.

Background: Although Gamma Knife radiosurgery (GKRS) can provide beneficial therapeutic effects for patients with brain metastases, lesions involving the eloquent areas carry a higher risk of neurologic deterioration after treatment, compared to those located in the non-eloquent areas. We aimed to investigate neurological change of the patients with brain metastases involving the motor cortex (MC) and the relevant factors related to neurological deterioration after GKRS.

Methods: We retrospectively reviewed clinical, radiological and dosimetry data of 51 patients who underwent GKRS for 60 brain metastases involving the MC.

Continuous Renal Replacement Therapy for Acute Renal Failure in Patients with Traumatic Brain Injury.

Objective: The purpose of this study was to investigate the impact of continuous renal replacement therapy (CRRT) on survival and relevant factors in patients who underwent CRRT after traumatic brain injury (TBI).

Methods: We retrospectively reviewed the laboratory, clinical, and radiological data of 29 patients who underwent CRRT among 1,190 TBI patients treated at our institution between April 2011 and June 2015. There were 20 men and 9 women, and the mean age was 60.

Ultrafast Ramsey interferometry to implement cold atomic qubit gates.

Quantum computing is based on unitary operations in a two-level quantum system, a qubit, as the fundamental building block, and the ability to perform qubit operations in an amount of time that is considerably shorter than the coherence time is an essential requirement for quantum computation. Here, we present an experimental demonstration of arbitrary single-qubit SU(2) quantum gate operations achieved at a terahertz clock speed. Implemented by coherent control methods of tailored ultrafast laser interaction with cold rubidium atomic qubits, Bloch vector manipulation about all three rotational axes was successfully demonstrated.

Polarization characteristics of semipolar (112̄2) InGaN/GaN quantum well structures grown on relaxed InGaN buffer layers and comparison with experiment.

Partial strain relaxation effects on polarization ratio of semipolar (112̄2) InxGa1−xN/GaN quantum well (QW) structures grown on relaxed InGaN buffers were investigated using the multiband effective-mass theory. The absolute value of the polarization ratio gradually decreases with increasing In composition in InGaN buffer layer when the strain relaxation ratio (ε0y′y′−εy′y′)/ε0y′y′ along y′-axis is assumed to be linearly proportional to the difference of lattice constants between the well and the buffer layer. Also, it changes its sign for the QW structure grown on InGaN buffer layer with a relatively larger In composition (x > 0.

Generation of 578-nm yellow light over 10 mW by second harmonic generation of an 1156-nm external-cavity diode laser.

578-nm yellow light with an output power of more than 10 mW was obtained using a waveguide periodically-poled-lithium-niobate crystal as a nonlinear medium for second harmonic generation, which is the highest output power at this wavelength using second harmonic generation of a solid state laser source without an enhancement ring cavity, to our knowledge. To achieve this result we made a high power 1156-nm external-cavity diode laser with the maximum output power of more than 250 mW. This system is expected to be an excellent alternative to the system using the sum-frequency generation with the advantage of simplicity and cost-effectiveness, and will be used as a clock laser of the ytterbium optical lattice clock with robust and reliable operation.

Linewidth reduction of a distributed-feedback diode laser using an all-fiber interferometer with short path imbalance.

The linewidth of a distributed-feedback (DFB) diode laser at 1156 nm, of which free-running linewidth was 3 MHz, was reduced to 15 kHz using an all-fiber interferometer with 5-m-long path imbalance. Optical power loss and bandwidth limitation were negligible with this short optical fiber patch cord. This result was achieved without acoustic and vibration isolations, and the frequency lock could be maintained over weeks.

Strong-field quantum control of 2 + 1 photon absorption of atomic sodium.

We demonstrate ultrafast coherent control of multiphoton absorption in a dynamically shifted energy level structure. In a three-level system that models optical interactions with sodium atoms, we control the quantum interference of sequential 2 + 1 photons and direct three-photon transitions. Dynamic change in energy levels predicts an enormous enhancement of |7p>-state excitation in the strong-field regime by a negatively chirped pulse.

Narrow linewidth 578 nm light generation using frequency-doubling with a waveguide PPLN pumped by an optical injection-locked diode laser.

This study demonstrates 578 nm yellow light generation with a narrow linewidth using a waveguide periodically poled lithium niboate (PPLN) and an optical injection-locked diode laser. The frequency of an external cavity diode laser used as a master laser operating at 1156 nm in optical injection-locking mode was locked into a high-finesse cavity with the Pound-Drever-Hall technique, which results in a linewidth reduction of the master laser. The linewidth of the master laser was estimated to be approximately 1.

Demonstration of an optical frequency synthesizer with zero carrier-envelope-offset frequency stabilized by the direct locking method.

We developed an optical frequency synthesizer (OFS) with the carrier-envelope-offset frequency locked to 0 Hz achieved using the "direct locking method." This method differs from a conventional phaselock method in that the interference signal from a self-referencing f-2f interferometer is directly fed back to the carrier-envelope-phase control of a femtosecond laser in the time domain. A comparison of the optical frequency of the new OFS to that of a conventional OFS stabilized by a phase-lock method showed that the frequency comb of the new OFS was not different to that of the conventional OFS within an uncertainty of 5.

Frequency-stabilized high-power violet laser diode with an ytterbium hollow-cathode lamp.

We have demonstrated in an ytterbium laser cooling and trapping experiment a high-power violet extendedcavity diode laser (ECDL) stabilized to the Yb resonant transition at 398.9 nm in an Yb hollow-cathode lamp. A frequency-dispersion signal, which we obtained by applying a modulation-free dichroic-atomic-vapor laser lock technique, allowed us to stabilize the violet ECDL at a frequency stability below 1 MHz at 1-s average time and a useful output power of 15 mW.