Bo-Yi Chen

Probing the exciton-phonon coupling strengths of O-polar and Zn-polar ZnO wafer using hard X-ray excited optical luminescence

The temperature-dependent hard X-ray excited optical luminescence (XEOL) spectroscopy was used to study the optical properties of O and Zn polarity of a c-plane single crystal ZnO wafer. By analyzing the XEOL and XRD, we found an unprecedented blue shift of the free exciton transition with increasing the excited carrier density as tuning the X-ray energy across the Zn K-edge, and the O-polar face possesses better crystal structure than the Zn-polar one. This spectral blue shift is attributed to the Coulomb screening of the spontaneous polarization by the excited free carriers that result in decreasing the exciton-phonon Frohlich interaction to reduce exciton binding energy.

Multimodal hard x-ray nanoprobe facility by nested Montel mirrors aimed for 40nm resolution at Taiwan Photon Source

The hard X-ray nanoprobe facility at Taiwan Photon Source (TPS) provides multimodal X-ray detections, including XRF, XAS, XEOL, projection microscope, CDI, etc. Resulting from the large numerical aperture obtained by utilizing nested Montel mirrors, the beamline with a moderate length 75 meters can conduct similar performance with those beamlines longer than 100 meters. The mirrors are symmetrically placed with a 45 degrees cut. The beamline optics is thus designed to take the advantage of the symmetry of mirrors such that a round focal spot is accomplished. The size and the divergence of the focus spot are simulated around 40 nm and 6.29 mrad, respectively. The whole facility including the beamline and the stations will be operated under vacuum to preserve the photon coherence as well as to prevent the system from unnecessary environmental interference. A SEM in close cooperation with laser interferometers is equipped to precisely locate the position of the sample. This endstation is scheduled to be commissioned in the fall of 2016.

X-ray nanoprobe project at Taiwan Photon Source

The hard X-ray nanoprobe facility at Taiwan Photon Source (TPS) provides versatile X-ray analysis techniques, with tens of nanometer resolution, including XRF, XAS, XEOL, projection microscope, CDI, etc. Resulting from the large numerical aperture obtained by utilizing Montel KB mirrors, the beamline with a moderate length 75 meters can conduct similar performance with those beamlines longer than 100 meters. The two silica-made Montel mirrors are 45 degree cut and placed in a V-shape to eliminate the gap loss and the deformation caused by gravity. The slope error of the KB mirror pair is less than 0.04 µrad accomplished by elastic emission machining (EEM) method. For the beamline, a horizontal DCM and two-stage focusing in horizontal direction is applied. For the endstation, a combination of SEM for quickly positioning the sample, a fly scanning system with laser interferometers, a precise temperature control system, and a load lock transfer system will be implemented. In this presentation, the design and...

The Precise Adjustment of X-ray Montel Mirrors for Diffraction-Limited Focal Spots

Synchrotron radiation newS, Vol. 31, No. 5, 2018 27 Introduction To achieve diffraction-limited focal spot size is a goal for nextgeneration synchrotrons. Mirrors are increasingly important for X-ray focusing optics following recent advances in mirror fabrications [1, 2]. The use of mirrors for focus spots with diffraction-limited size has been realized [3]. With these advanced techniques, the Montel mirror [4, 5] has demonstrated promise as a technique for focusing smaller spot size. Montel optics have three advantages over the KB mirror. First, Montel optics increase the numerical aperture (NA) more than the conventional KB mirror. Second, the setup of the Montel mirror minimizes the distance for two focusing mirrors. Therefore, the focal spot size, which is enlarged by slope error of the upstream mirror, can be minimized. Third, and the subject of this article, the distance of two mirrors is minimized and the mechanical design of the Montel mirror holder enables it to be far more stable than the two separated mirror holders for a conventional KB mirror. The construction of the X-ray nanoprobe beamline (TPS 23A) at the National Synchrotron Radiation Research Center was completed in February 2017, and the commissioning of the endstation began to test the performance of each component and system. The 45-degree cut Montel mirrors were designed to focus the spot size down to 40 nm with a 70-m-long beamline [6–8]. Both the focus ability of the Montel mirrors and the stability of the mirror holder have been measured. The focus ability of the Montel mirrors relies upon not only the quality of slope error of the mirror, but also the accuracy and stability of its holder. The performance of the mirror holder depends on the mechanical design and the ground vibration control. In this article, we will discuss and report on our experiences with the precise adjustment of the Montel mirrors. Precise nanopositioning is always an issue for synchrotron radiation facilities because it significantly affects the beam quality and performance of each endstation [9–15]. The overconstrained weak-link mechanism has a lot of excellent capacities, such as ultra-high stiffness, frictionlessness, high repeatability, and high precision. Most of the nanopositioning stages involve the assembly of weak-link structures and piezo-based actuators. This mechanism is also adopted in the design of the Montel mirror holder of TPS 23A. In this article, we discuss the design of a precise holder used for adjusting the Montel mirrors. The ground vibration evaluation, vibration isolation, foundation of the mirror holder, and design concept of mirror holder upgradation are discussed. The testing result of TPS 23A, before the 2017 summer shutdown, shows that the focal spot size is about 160 nm. At the same time, the peak-to-peak stability for pitch, roll, and yaw are around 1.5 μrad, 1.4 μrad, and 2.1 μrad, respectively. This demonstrates that the original design of Montel mirrors and holder is successful, but there is still room for improvement. The mechanical strength, vibration, and stability were re-evaluated during the 2017 summer shutdown, and the upgrade program for the mirror holder was completed. The results of the upgrade program show that the ambient vibration of the mirror holder is around 8 nm (peak to peak), and the focal spot size is about 50 nm.

Projection x-ray microscopy beamline mirror cooling and optomechanical analysis

The thermal transfer issue is an important problem associated with the synchrotron radiation optical system. During projection x-ray microscopy (PXM), x-ray light comes from the wiggler insert parts. The vertical collimating mirror (VCM) absorbs 40 W of energy on the mirror surface. The mirror length is 1000 mm, and its width is 87 mm. Here we apply an x-ray optical simulation software, named SHADOW, as well as the finite element method (FEM) software, ANSYS®, to calculate the surface thermal deformation at various thicknesses of VCM. The FEM software calculates the mirror surface deformation from heat absorption, and the surface deformation can be fit by the B-spline curve. The thermal deformation fitting results can be fed back to the SHADOW software and be used to evaluate how mirror thermal deformation affects the optical system performance.

New type of on-the-fly scanning data acquisition system for x-ray nanoprobe at Taiwan Photon Source (Conference Presentation)

This on-the-fly scanning control system is for the x-ray nanoprobe endstation at Taiwan Photon Source(TPS) and built base-on the high speed Hardware (H/W), high throughput data stream and multi-channel control interfaces. The main idea is to tag each data with information of time and position, which generates by circuit and laser interferometer. The data is then processed by a computer to be analyzed and visualized. By using high speed FPGA with embedded processer to process the input and output data which includes the DAC, ADC, Gigabit Ethernet (GbE), X-ray fluorescence (XRF) and laser interferometer control interfaces. Three DAC control the X,Y and Z axes of the flexure stage, four ADCs and sensor interfaces gather the data and packet it into data packet. GbE send data back to computer to do image processing then reconstruct the scanning image. The numerous data not only for rebuild the image but also good for information analysis. Including the vibration, time slide analysis. Our demo system is built by an e-beam source, flexure stage and laser interferometer. The current maximum scanning speed is up to 5 lines/sec which is limited by the mechanical, the sample rate can be as high as 20M samples/sec which limited by laser interferometer, and the maximum data rate is close to 100M bytes/sec which is limited by the GbE. Interferometer information combine with position data in data packet, makes easy for data analysis and also for image stitching. The system is going to commission on beamline at March, 2017. The commission result for this system will be presented.

Commissioning of the Montel nano-optics for the x-ray nanoprobe at Taiwan Photon Source (Conference Presentation)

The diffraction-limited Montel mirrors, equipped at the X-ray Nanoprobe (XNP) at Taiwan Photon Source (TPS), provide a 40 nm focal spot and working distance 55 mm under the total beamline length of 69 m. The underneath holder supporting for the Montel mirrors is a 12 axes flexure based manipulators in which 10 out of the 12 axes are motorized. To monitor the position and stability of individual holder motion, a monitoring system consisted of three optical encoders and three- axes laser interferometers for angle movement is implemented. The gap width between the two mirrors and their orthogonality can be adjusted by a tilting sensor and a high magnification optical microscope. The focusing properties, phase and amplitude, after the Montel mirrors will be investigated by means of coherent Ptychography, as well as by zone plate imaging. An SEM in close cooperation with laser interferometers is equipped to precisely position the samples and conduct the on-the-fly scan. A high speed FPGA based circuit is developed to address signal from XRF, XAS, XEOL and XRD. Data is in tag with position and time information and been processed by computers to allow 5nm precision stage scanning free from mechanical feedback. The XNP at TPS is under commissioning since February 2017. The commissioning result, particularly the performance of the Montel mirrors will be reported in this presentation.

Hard x-ray nanoprobe by Montel KB mirrors at Taiwan Photon Source

The hard X-ray nanoprobe at Taiwan Photon Source (TPS) makes use of the large numerical aperture obtained by nested Montel mirrors. To fully uptake the focusing power and flux, these mirrors requires the surface slope error no less than 0.05 μrad and are symmetrically placed with a 45 degrees cut for perfect surface matching. The beamline optics is designed to take the advantage of the symmetry of mirrors such that a round focal spot is accomplished. The final size of the focus spot are simulated below 40 nm at 9-15 keV. The whole facility including the beamline and the stations will be operated under vacuum to preserve photon coherence as well as to prevent the system from unnecessary environmental interference. The station equips with multimodal x-ray probes, including XRF, XAS, XEOL, projection microscope, CDI, etc. A SEM in close cooperation with laser interferometers is equipped to precisely locate the position of the sample. The beamline and the station are scheduled to be in commissioning phase in 2016.