Exciton-Dominated Ultrafast Optical Response in Atomically Thin PtSe2.

Strongly bound excitons are a characteristic hallmark of 2D semiconductors, enabling unique light-matter interactions and novel optical applications. Platinum diselenide (PtSe2 ) is an emerging 2D material with outstanding optical and electrical properties and excellent air stability. Bulk PtSe2 is a semimetal, but its atomically thin form shows a semiconducting phase with the appearance of a band-gap, making one expect strongly bound 2D excitons. However, the excitons in PtSe2 have been barely studied, either experimentally or theoretically. Here, the authors directly observe and theoretically confirm excitons and their ultrafast dynamics in mono-, bi-, and tri-layer PtSe2 single crystals. Steady-state optical microscopy reveals exciton absorption resonances and their thickness dependence, confirmed by first-principles calculations. Ultrafast transient absorption microscopy finds that the exciton dominates the transient broadband response, resulting from strong exciton bleaching and renormalized band-gap-induced exciton shifting. The overall transient spectrum redshifts with increasing thickness as the shrinking band-gap redshifts the exciton resonance. This study provides novel insights into exciton photophysics in platinum dichalcogenides.

Medical X-band linear accelerator for high-precision radiotherapy.

PURPOSE: Recently, high-precision radiotherapy systems have been developed by integrating computerized tomography or magnetic resonance imaging to enhance the precision of radiotherapy. For integration with additional imaging systems in a limited space, miniaturization and weight reduction of the linear accelerator (linac) system have become important. The aim of this work is to develop a compact medical linac based on 9.3 GHz X-band RF technology instead of the S-band RF technology typically used in the radiotherapy field. METHODS: The accelerating tube was designed by 3D finite-difference time-domain and particle-in-cell simulations because the frequency variation resulting from the structural parameters and processing errors is relatively sensitive to the operating performance of the X-band linac. Through the 3D simulation of the electric field distribution and beam dynamics process, we designed an accelerating tube to efficiently accelerate the electron beam and used a magnetron as the RF source to miniaturize the entire linac. In addition, a side-coupled structure was adopted to design a compact linac to reduce the RF power loss. To verify the performance of the linac, we developed a beam diagnostic system to analyze the electron beam characteristics and a quality assurance (QA) experimental environment including 3D lateral water phantoms to analyze the primary performance parameters (energy, dose rate, flatness, symmetry, and penumbra) The QA process was based on the standard protocols AAPM TG-51, 106, 142 and IAEA TRS-398. RESULTS: The X-band linac has high shunt impedance and electric field strength. Therefore, even though the length of the accelerating tube is 37 cm, the linac could accelerate an electron beam to more than 6 MeV and produce a beam current of more than 90 mA. The transmission ratio is measured to be approximately 30% ~ 40% when the electron gun operates in the constant emission region. The percent depth dose ratio at the measured depths of 10 and 20 cm was approximately 0.572, so we verified that the photon beam energy was matched to approximately 6 MV. The maximum dose rate was measured as 820 cGy/min when the source-to-skin distance was 80 cm. The symmetry was smaller than the QA standard and the flatness had a higher than standard value due to the flattening filter-free beam characteristics. In the case of the penumbra, it was not sufficiently steep compared to commercial equipment, but it could be compensated by improving additional devices such as multileaf collimator and jaw. CONCLUSIONS: A 9.3 GHz X-band medical linac was developed for high-precision radiotherapy. Since a more precise design and machining process are required for X-band RF technology, this linac was developed by performing a 3D simulation and ultraprecision machining. The X-band linac has a short length and a compact volume, but it can generate a validated therapeutic beam. Therefore, it has more flexibility to be coupled with imaging systems such as CT or MRI and can reduce the bore size of the gantry. In addition, the weight reduction can improve the mechanical stiffness of the unit and reduce the mechanical load.

Development of a compact X-band linear accelerator system mounted on an O-arm rotating gantry for radiation therapy.

A compact X-band linear accelerator (LINAC) system equipped with a small and lightweight magnetron was constructed to develop a high-precision image-guided radiotherapy system. The developed LINAC system was installed in an O-ring gantry where cone-beam computed tomography (CBCT) was embedded. When the O-arm gantry is rotated, an x-ray beam is stably generated, which resulted from the stable transmission of radio frequency power into the X-band LINAC system. Quality assurance (QA) tests, including mechanical and dosimetry checks, were carried out to ensure safety and operation performance according to the American Association of Physicists in Medicine's TG-51, 142, an international standard protocol established by accredited institutions. In addition, delivery QA of the radiotherapy planning system was conducted to verify intensity-modulated radiotherapy techniques. Therefore, it was demonstrated that the developed X-band LINAC system mounted on the O-arm gantry proved to be valid and reliable for potential use in CBCT image-guided radiation therapy.

Transmission characteristics of all-dielectric guided-mode resonance filter in the THz region.

In this study we report the first on the terahertz (THz) transmission characteristics of a guided-mode resonance (GMR) filter made of all-dielectric material. Two strong transverse electric (TE) resonance modes, TE0,1 and TE1,1, and one strong transverse magnetic (TM) resonance mode, TM0,1, were detected. The measured resonances can be explained by diffraction from the grating surface of the GMR filter, and by guiding along the inside of the filter (slab waveguide). Because two identical GMR filters were employed to overcome limited grating numbers, the measured Q-factors of the TM0,1, TE1,1, and TM0,1 modes were as high as 62.9, 71.0, and 74.4 respectively. Also, we obtained polarization efficiencies of up to 96.9, 96.3, and 92.9% for the TM0,1, TM1,1, and TM0,1 modes, respectively, when the GMR filter was rotated to 90°. By increasing the incident THz beam angle, one TE resonance can be divided into two TE resonances, and the resonant frequency can be adjusted like a THz tunable resonance filter. Furthermore, when the GMR filters were inserted between Teflon plates, only the TM1,1 mode was perfectly removed. The designed GMR filter has a high Q-factor, tunable filter, good polarizer, and good modulator characteristics. These experimental results were in good agreement with simulation results.

Enhanced THz guiding properties of curved two-wire lines.

We present experimental and simulation studies of enhanced terahertz (THz) guiding properties of curved two-wire lines for several surface conditions. When a THz-wave propagates through curved two-wire lines, a rough wire surface with dielectric coating contributes to a lower bending loss compared to a smooth or rough wire surface without coating. Dielectric coating and rough surface confine the THz field to the wire surface making the bending loss low. The guiding property at a curve depth of 30 mm of a rough wire surface with 25-µm-thick coating is improved by 34% compared to that of a smooth wire without coating. Furthermore, computer simulation technology (CST) software visually shows the bending loss as same as the experimental studies.

Accelerated esterification of free fatty acid using pulsed microwaves.

It was demonstrated that pulsed microwave irradiation is a more effective method to accelerate the esterification of free fatty acid with a heterogeneous catalyst than continuous microwave irradiation. A square-pulsed microwave with a 400 Hz repetition rate and a 10-20% duty cycle with the same energy as the continuous microwave were used in this study. The pulsed microwaves improved the esterification conversion from 39.9% to 66.1% after 15 min in comparison with the continuous microwave under the same reaction conditions. These results indicated that pulsed microwaves with repetitive strong power could enhance the efficiency of biodiesel production relative to the use of continuous microwave with mild power.

Microwave-accelerated energy-efficient esterification of free fatty acid with a heterogeneous catalyst.

This paper shows energy-efficiency of microwave-accelerated esterification of free fatty acid with a heterogeneous catalyst by net microwave power measurement. In the reaction condition of 5 wt% sulfated zirconia and 1:20 M ratio of oil to methanol at 60°C and atmospheric pressure, more than 90% conversion of the esterification was achieved in 20 min by microwave heating, while it took about 130 min by conventional heating. Electric energy consumption for the microwave heating in this accelerated esterification was only 67% of estimated minimum heat energy demand because of significantly reduced reaction time.

High speed scanning of terahertz pulse by a rotary optical delay line.

We demonstrate high speed scanning of a time-domain terahertz pulse by a rotary optical delay line (RODL) incorporated into a photoconductive antenna based terahertz system. The delay line of RODL consists of six rotating convex reflective blades with a rotating speed controlled for a wide range of scan repetition rates. It can perform path length scans of 2.1 cm at a scanning speed of 8.4 m/s while maintaining linearity. The comparison of the terahertz temporal waveform and the frequency spectrum measured by using the RODL and a linear stage optical delay line confirms the linearity of the RODL up to 140 ps at high scan repetition rates of 400 scan/s. A high signal-to-noise ratio, up to a few thousand, can be obtained by averaging multiple terahertz pulses at a high scan speed.

Fast scanning of a pulsed terahertz signal using an oscillating optical delay line.

We describe a fast measurement of a pulsed terahertz signal generated by a femtosecond laser and a photoconductive antenna using an oscillating optical delay line. The method to measure the amplitude of the retroreflector in the oscillating optical delay line is proposed and the displacement of the retroreflector is exactly calculated to acquire the optical delay time in the fast scan mode. With the different oscillation frequency and amplitude of the retroreflector, the pulsed terahertz signals are measured and analyzed. The comparison of the temporal waveform and frequency spectrum between the fast scan mode and the slow scan mode shows a good agreement with the decrease in the scanning time from 60 to 1 s at a signal to noise ratio of 430.