Doughnut Profile Research Articles

Development of high-power VCSEL emitter of size 30mm x 30mm, scalable in two dimensions and applicable to Si wafer annealing

Two-dimensionally scalable vertical cavity surface emitting laser (VCSEL) emitter of size 30mm×30mm was designed and fabricated for practical applications in the rapid heating processes of large-area targets. Nine emitters were arranged in a 3×3 array for an experimental VCSEL module, which was used for annealing of a 4” Si wafer. Each emitter was composed of 120 chips of size 1.26mm×1.26mm, and each chip was composed of 770 VCSEL diodes of 30μm in diameter, emitting a continuous beam with wavelength of 940nm and maximum optical power density of 47.6W/cm2. Since VCSELs generated a significant amount of Joule heat during operation, an effective cooling system was required for reliable thermal management of the emitter. A water cooling block of 126 microchannel flow passages with the heat transfer area-to-volume ratio approximately 1270m2/m3 was designed and fabricated for each emitter, resulting in the thermal resistance of 0.012K/W, equivalent to the overall heat transfer coefficient of 92,000W/m2K at water flow rate of 1L/min, which was the minimum value for the normal operation of the emitter. By comparing the experimental images on the 4” Si wafer irradiated by the single emitter and by the module with the numerical predictions based on the ray-tracing method, the laser beam emitted from a single VCSEL diode of 30μm in diameter was estimated to have the mode of TEM01* (doughnut profile, multimode) and a divergence angle of 20∘. These were two factors that affected the irradiation uniformity on the target. In addition, to validate the applicability of the high-power VCSEL system to anneal large-area targets, the heating process of 4” Si wafers using the module of nine emitters was experimentally carried out at atmospheric pressure, and was compared with numerical predictions.

Dark hollow beams originating terahertz radiation in corrugated plasma under magnetic field

In the present process, two laser beams having the same doughnut profiles but different frequencies are explored in space-periodic nonlinear plasma to produce nonlinear oscillatory current that resonantly excites the terahertz (THz) radiation. The interactions are assumed to be under the effect of external magnetic field and electron-neutral collisions. The intensity gradient of the considered dark hollow beams has a doughnut-shaped distribution, which is responsible for the multifocal field profile of the emitted THz radiation. The present scheme is capable of producing laser-to-THz energy conversion efficiency up to ∼10−3 with the optimization of various laser and plasma parameters even in the presence of electron-neutral collisions.

Investigations on the microstructure and crack formation of IN738LC samples processed by selective laser melting using Gaussian and doughnut profiles

This paper contributes to a better understanding of processing the nickel-based superalloy IN738LC using selective laser melting (SLM). Initially, the basic workability of IN738LC using SLM is demonstrated. Subsequently, a comparison between a Gaussian and a doughnut profile is carried out, and clear correlations between the choice of process parameters and the resulting imperfections depending on the chosen laser beam profile are shown. Electron backscatter diffraction (EBSD) measurements show a significant influence of scan strategy and build direction on the texture of the sample, independent of the used laser beam profile. Regardless of the laser beam profile, two contradicting trends complicate the defect-free processing of IN738LC, i.e. a reduction in the crack density can only be realized with increasing porosity. Through microstructural investigations, observed hot cracks are identified to be solidification cracks. Based on a broad experimental study in combination with a numerical solidification study, a theory about the crack initiation mechanism is presented. Finally, by using atom probe tomography (APT), the element zirconium is confirmed as a possible reason for the occurrence of solidification cracks.

Numerical Simulation of Anomalous Vortex Beams on Different Fractional Fourier Transform Planes

The properties of fractional Fourier transform of anomalous vortex beams are studied. A new type of analytical expression of fractional Fourier transform for anomolous vortex beams is obtained. The properties of anomolous vortex beams on different fractional Fourier transform planes with different parameters are illustrated. The results show that the anomolous vortex beams always has a doughnut profile, the distribution of intensity on different fractional Fourier transform planes highly depends on the fractional order and the beam parameters, such as the beam order and the topological charge.

Lasing of ambient air with microjoule pulse energy pumped by a multi-terawatt infrared femtosecond laser

We report on the lasing action of atmospheric air pumped by an 800 nm femtosecond laser pulse with peak power up to 4 TW. Lasing emission at 428 nm increases rapidly over a small range of pump laser power, followed by saturation above ∼1.5 TW. The maximum lasing pulse energy is measured at 2.6 μJ corresponding to an emission power in the MW range, while a maximum conversion efficiency of 3.5×10(-5) is measured at moderate pump pulse energy. The optical gain inside the filament plasma is estimated to be in excess of 0.7/cm. Lasing emission shows a doughnut profile, reflecting the spatial distribution of the pump-generated white-light continuum that acts as a seed for the lasing. We attribute the pronounced saturation to the defocusing of the seed in the plasma amplifying region and to the saturation of the seed intensity.

Subdiffraction, Luminescence-Depletion Imaging of Isolated, Giant, CdSe/CdS Nanocrystal Quantum Dots

Subdiffraction spatial resolution luminescence depletion imaging was performed with giant CdSe/14CdS nanocrystal quantum dots (g-NQDs) dispersed on a glass slide. Luminescence depletion imaging used a Gaussian shaped excitation laser pulse overlapped with a depletion pulse, shaped into a doughnut profile, with zero intensity in the center. Luminescence from a subdiffraction volume is collected from the central portion of the excitation spot, where no depletion takes place. Up to 92% depletion of the luminescence signal was achieved. An average full width at half-maximum of 40 ± 10 nm was measured in the lateral direction for isolated g-NQDs at an air interface using luminescence depletion imaging, whereas the average full width at half-maximum was 450 ± 90 nm using diffraction-limited, confocal luminescence imaging. Time-gating of the luminescence depletion data was required to achieve the stated spatial resolution. No observable photobleaching of the g-NQDs was present in the measurements, which allowed imaging with a dwell time of 250 ms per pixel to obtain images with a high signal-to-noise ratio. The mechanism for luminescence depletion is likely stimulated emission, stimulated absorption, or a combination of the two. The g-NQDs fulfill a need for versatile, photostable tags for subdiffraction imaging schemes where high laser powers or long exposure times are used.

Nonlinear beam shaper for femtosecond laser pulses, from Gaussian to flat-top profile

We present a straightforward method to transform a spatially Gaussian femtosecond laser beam into a flat-top shaped beam. The proposed technique takes advantage of a nonlinear phase induced in positive Kerr medium followed by a simple optical system. The variation of the refractive index with the laser intensity creates a phase plate which induces changes in the beam profile after propagation; flat-top and doughnut profiles are observed. The shaping conditions are computed numerically and confirmed experimentally. The method does not introduce energy losses. The device is very simple, self-regulated, flexible and does not need a manufactured phase plate or precise alignment. This method can be useful for light-matter interaction and laser machining.

Gaussian beam conversion using an axicon

The paper presents a theoretical investigation of laser beam tailoring using a transparent diffracting phase plate. It is shown that a linear axicon is able to transform a Gaussian beam either into a super-Gaussian profile, a ring-shaped profile or a doughnut profile. All these different intensity distributions are easily obtained by only changing the width of the incident Gaussian beam.

Gravity gradient or gravity jitter induced viscous stress and moment fluctuations in microgravity

The dynamical behavior of fluids affected by the asymmetric gravity gradient and gravity jitter accelerations, in particular the effect of surface tension on partially-filled rotating fluids with a special example applicable to a full-scale Gravity Probe-B Spacecraft dewar tank, has been investigated. Results of slosh wave excitation along the liquid–vapor interface induced by gravity gradient acceleration indicated one-up one-down and one-down one-up oscillations of two bubbles in the cross-section of doughnut profiles in the vertical r–z plane of a rotating dewar, and an eccentric contour of a bubble rotating around the axis of the dewar in the horizontal r–θ plane. Results of slosh wave excitation along the liquid–vapor interface induced by gravity jitter acceleration showed an equivalent to the time-dependent oscillatory force which pushes a bubble in the combined bubble motion of down-and-up and leftward-and-rightward as the bubble is rotating with respect to a rotating dewar axis. Fluctuations of fluid stress forces, fluid stress moments, and moment arm of fluid moment exerted on the dewar wall of the container due to slosh wave excitations driven by gravity gradient acceleration or gravity jitter acceleration are also investigated.