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MELISSA: Laser ion source setup at CERN-MEDICIS facility. Blueprint
摘要： The Resonance Ionization Laser Ion Source (RILIS) has become an essential feature of many radioactive ion beam facilities worldwide since it offers an unmatched combination of efficiency and selectivity in the production of ion beams of many different chemical elements. In 2019, the laser ion source setup MELISSA is going to be established at the CERN-MEDICIS facility, based on the experience of the workgroup LARISSA of the University Mainz and CERN ISOLDE RILIS team. The purpose is to enhance the capability of the radioactive ion beam supply for end users by optimizing the yield and the purity of the final product. In this article, the blueprint of the laser ion source, as well as the key aspects of its development and operation are presented.
关键词： Laser resonance ionization,Lanthanides,Ti:sapphire,MELISSA,Isotope separation,Nuclear medicine,CERN-MEDICIS
更新于2025-09-16 10:30:52
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Enhanced terahertz radiation of photoconductive antenna fabricated on GaAs-on-sapphire
摘要： The terahertz (THz) radiation properties of a photoconductive antenna (PCA) fabricated on a GaAs-on-sapphire (GoS) substrate are reported at sub-THz band. The GaAs layer with a thickness of approximately 1 μm was directly deposited on a sapphire wafer by means of molecular beam epitaxy. A butterfly-shaped antenna structure was then fabricated on the GoS substrate by photolithography, and the device was tested as the emitter of an in-house built THz time-domain spectrometer. The performance of this antenna was compared with a commercial one, which had an identical antenna structure but was fabricated on low-temperature-grown GaAs (LT-GaAs). The results showed that the GoS-based PCA radiated an enhanced THz field, which could be as much as 1.9 times that of the LT-GaAs-based PCA, indicating that GoS could be a promising photoconductive material. In addition, the optical transparency of the sapphire substrate allows the device to be illuminated from the backside, which is crucial for THz near-field imaging applications where the sample is usually in close proximity to the front surface of the PCA device.
关键词： photoconductive antenna,molecular beam epitaxy,THz time-domain spectrometer,terahertz,GaAs-on-sapphire
更新于2025-09-12 10:27:22
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Comparative Study of the Pump Beam Quality Effect on the Slope Efficiency in Kerr-Lens Mode-Locked Ti:Sapphire Laser
摘要： A comparative study of the pump beam quality effect on the slope efficiency in Kerr-lens mode-locked Ti:sapphire laser is presented. The Ti:sapphire laser is a very attractive candidate for ultrafast laser applications because of its broad emission bandwidth and high gain. The main factor that limits the efficiency of Ti:sapphire lasers is the poor absorption of the pump beam in the Ti:sapphire crystal. The pump beam quality is a key parameter for the efficiency of the laser. In this work, we compared a 1.0 μm green and a 1.0 μm from a green laser in order to investigate the effects of spatial beam properties on the pumping efficiency in a Ti:sapphire laser. The beam quality factors of green and green lasers were measured to be 1.2 and 1.1, respectively. After the special beam combination, the combined beam was focused into a 2-mm-long Ti:sapphire crystal. The focused beam diameters of the green and green lasers were 24 μm and 22 μm in the slow axis and 22 μm and 23 μm in the fast axis, respectively. The slope efficiency of 21.1% more for the green laser than the green laser was achieved. After the beam combination, the slope efficiency of 24.8% was achieved. The positive effect of higher beam quality on the slope efficiency was demonstrated.
关键词： pump beam quality,Kerr-lens mode-locking,Ti:sapphire laser,slope efficiency
更新于2025-09-12 10:27:22
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Femtosecond Laser Inscribed Sapphire Fiber Bragg Grating for High Temperature and Strain Sensing
摘要： In this paper, a sapphire fiber Bragg grating (SFBG) is fabricated by femtosecond (fs) laser line-by-line scanning method. A third-order fiber Bragg grating is obtained in a single crystal sapphire fiber with a diameter of 60 μm. Compared to the SFBG written by point-by-point method, the SFBG written by the line-by-line method has a higher reflectivity and grating with about 15% reflectivity is achieved when the track length is about 40 μm. The temperature sensing characteristics of this SFBG from room temperature to 1600°C are studied, and the temperature sensitivity of SFBG is 34.96 pm/°C from 1000°C to 1600°C. The strain characteristics are also tested at the temperature of 26°C, 500°C, 1000°C and 1600°C, and the strain sensitivities are 1.42 pm/με, 1.42pm/με, 1.44 pm/με and 1.45 pm/με, respectively. These characteristics show that this SFBG has potential applications in harsh environment structural health monitoring.
关键词： structural health monitoring,femtosecond laser,line-by-line scanning,Sapphire fiber Bragg grating
更新于2025-09-11 14:15:04
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Effects of interfaces and current spreading on the thermal transport of micro-LEDs for kA-per-square-cm current injection levels
摘要： The three-dimensional thermal characteristics of micro-light-emitting diodes (mLEDs) on GaN and sapphire substrates were studied with forward-voltage methods, thermal transient measurements, and infrared imaging. The mLEDs on the GaN substrate showed an approximately 10 °C lower junction temperature and smaller amplitude of the K factors than those on the sapphire substrate under the current injection level of 4 kA cm?2. The thermal transient measurement showed that the spreading thermal resistances of the mesa, the GaN epilayer, and the interface of the GaN/substrate were reduced significantly for mLEDs on the GaN substrate because of the high-quality GaN crystal and the interfaces. The infrared thermal images showed lower total average junction temperatures and more uniform temperature distributions for the mLEDs on the GaN substrate, which were also simulated with APSYS software. The thermal transport mechanisms are discussed for the lateral and vertical directions in the mLEDs.
关键词： sapphire substrate,micro-light-emitting diodes,GaN substrate,junction temperature,thermal transport,infrared imaging,APSYS simulation
更新于2025-09-11 14:15:04
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Determination of Sapphire Off‐Cut and its Influence on the Morphology and Local Defect Distribution in Epitaxially Laterally Overgrown AlN for Optically Pumped UVC Lasers
摘要： In this paper a systematic study of the morphology and local defect distribution in epitaxial laterally overgrown (ELO) AlN on c-plane sapphire substrates with different off-cut angles is presented. Precise measurements of the off-cut angle, using a combination of optical alignment and X-ray diffraction with an accuracy of ±5° for the off-cut direction and ±0.015° for the off-cut angle were performed. For ELO AlN growth a transition from step flow growth at a < 0.14° with height undulations on the surface to step bunching with step heights up to 20 nm for a > 0.14° was observed. Furthermore, the terraces of the step bunched surface exhibit curved steps. An analysis of the local defect distribution by scanning transmission electron microscopy and a comparison with atomic force microscopy reveals a bunching of defects in line with the ELO pattern and a roughening of step edges in highly defective regions. In addition, a reduction of the threshold excitation power density for optically pumped UVC lasers with smooth surface morphologies was observed.
关键词： AlN,off-cut,optically pumped laser,sapphire,UVC,ELO
更新于2025-09-11 14:15:04
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Reduction in crystalline quality anisotropy for non-polar a-plane GaN directly grown on titanium patterned sapphire substrate
摘要： Nonpolar a-plane GaN films were directly grown on titanium patterned sapphire substrates (Ti-PSS) by metalorganic chemical vapor deposition (MOCVD). It is demonstrated that the GaN film grown on Ti-PSS has superior surface quality and less surface pits than that on flat sapphire substrate. More importantly, the anisotropic behavior of the X-ray rocking curve-full width at half maximum values for the on-axis reflections were significantly improved. It is found that, the increased mosaic block size along m-direction could be the main reason for the reduction in the crystalline quality anisotropy. This work provides a simple and effective method to improve the crystal quality of non-polar GaN films.
关键词： X-ray rocking curve,crystalline quality anisotropy,MOCVD,titanium patterned sapphire substrates,Nonpolar a-plane GaN
更新于2025-09-10 09:29:36
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Repeatable Room Temperature Negative Differential Resistance in AlN/GaN Resonant Tunneling Diodes Grown on Sapphire
摘要： Resonant tunneling diodes (RTDs) are candidates for high power terahertz oscillators, and form the basis for understanding the quantum confinement and vertical transport in quantum structures such as quantum cascade lasers and quantum cascade detectors. In this work, repeatable negative differential resistance (NDR) is achieved in AlN/GaN RTDs grown on sapphire substrate by plasma-assisted molecular-beam epitaxy. Two reproducible NDR regions sequentially following two preresonance replicas are demonstrated at room temperature. A current region exhibiting negative correlation with temperature and oscillation-like features is first identified under reverse bias, which is interpreted as a combined contribution of weak resonant tunneling channels through different bound states in the well. The revealed peak-to-valley current ratio ranges from 1.1 to 1.8, and peak current density ranges from 5 to 164 kA cm?2. Using an analytic model, resonant tunneling transports in both bias directions are quantitatively characterized and show good agreements with experiment results, demonstrating the capability of accurate quantum transport control using III-nitride grown on sapphire substrate. The findings will promote the implementation of low cost III-nitride monolithic microwave circuits and resonant tunneling structures based on sapphire, SiC, and even silicon substrates.
关键词： sapphire substrate,AlN/GaN resonant tunneling diodes,repeatable negative differential resistance
更新于2025-09-10 09:29:36
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Thermal lensing measurements of Ti: Sapphire crystal pumped at 80?MHz picosecond pulses by Shack-Hartmann wavefront sensor
摘要： The thermal lensing effects in the lasing crystal appear to be dominant when the pump laser focused on the lasing crystal which causes the local heating effect. The spatial refractive index variations observed if the crystal has non-uniform temperature distribution. In the present study, the demonstration of thermal lensing effect of Brewster-cut Ti: Sapphire crystal carried out when pumped by 25 ps pulses at 80 MHz repetition rate with 532 nm central wavelength, while the average pump power controlled by the attenuator. The thermal lensing effects measured first at the ～293 K temperature and later at ～40 K by using the cryogenic cooling system. The thermal lensing variations at different average pump power were calculated and measured by using the HASO4 Shack-Hartmann wavefront sensor.
关键词： Thermal lens,Cryogenic cooling,Ti: Sapphire crystal,Shack-Hartmann wavefront sensor
更新于2025-09-10 09:29:36
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Growth of Si-doped AlN on sapphire (0001) via pulsed sputtering
摘要： The Si-doped AlN films were grown on sapphire (0001) by pulsed sputtering deposition (PSD), and their structural and electrical properties were investigated. A combination of PSD and high-temperature annealing process enabled the growth of high-quality AlN (0001) epitaxial films on sapphire (0001) substrates with atomically flat stepped and terraced surfaces. The transmission electron microscopy observations revealed that the majority of the threading dislocations in AlN belonged to the mixed- or edge-type, with densities of 2.8 × 108 and 4.4 × 109 cm?2, respectively. The Si-doping of AlN by PSD yielded a clear n-type conductivity with a maximum electron mobility of 44 cm2 V?1 s?1, which was the highest value reported for AlN that was grown on sapphire. These results clearly demonstrated the strong potential of the PSD technique for growing high-quality conductive n-type AlN on sapphire.
关键词： n-type conductivity,Si-doped AlN,pulsed sputtering deposition,electron mobility,sapphire
更新于2025-09-10 09:29:36