Ilya Mingareev

Diffractive optical elements utilized for efficiency enhancement of photovoltaic modules

Common solar cells used in photovoltaic modules feature metallic contacts which partially block the sunlight from reaching the semiconductor layer and reduce the overall efficiency of the modules. Diffractive optical elements were generated in the bulk glass of a photovoltaic module by ultrafast laser irradiation to direct light away from the contacts. Calculations of the planar electromagnetic wave diffraction and propagation were performed using the rigorous coupled wave analysis technique providing quantitative estimations for the potential efficiency enhancement of photovoltaic modules.

Femtosecond laser post-processing of metal parts produced by laser additive manufacturing

High-repetition rate femtosecond laser radiation was utilized to improve surface quality of metal parts manufactured by laser additive techniques. Different laser scanning approaches were utilized to increase the ablation efficiency and to improve the surface finish. Processing of 3D-shaped parts made of titanium- and nickel-base alloys resulted in the reduction of the average surface roughness to a few microns. This approach can be used to post-process parts made of thermally and mechanically sensitive materials and to attain complex designed shapes with micrometer precision. Advantages and limitations of this novel post-processing technique are discussed.

Thulium fiber laser and application development

Within the past 10 years, thulium (Tm)-doped fiber lasers have emerged as a flexible platform offering high average power as well as high peak power. Many of the benefits and limitations of Tm:fiber lasers are similar to those for ytterbium (Yb)-doped fiber lasers, however the ~2 µm emission wavelength posses unique challenges in terms of laser development as well as several benefits for applications. In this presentation, we will review the progress of laser development in CW, nanosecond, picosecond, and femtosecond regimes. As a review of our efforts in the development of power amplifiers, we will compare large mode area (LMA) stepindex and photonic crystal fiber (PCF) architectures. In our research, we have found Tm-doped step index LMA fibers to offer relatively high efficiency and average powers at the expense of fundamental mode quality. By comparison, Tm-doped PCFs provide the largest mode area and quasi diffraction-limited beam quality however they are approximately half as efficient as step-index fibers. In terms of defense related applications, the most prominent use of Tm:fiber lasers is to pump nonlinear conversion to the mid-IR such as supercontinuum generation and optical parametric oscillators/amplifiers (OPO/A). We have recently demonstrated Tm:fiber pumped OPOs which generate ~28 kW peak power in the mid-IR. In addition, we will show that Tm:fiber lasers also offer interesting capabilities in the processing of semiconductors.

Studying the effect of zeolite inclusion in aluminum alloy on measurement of its surface hardness using laser-induced breakdown spectroscopy technique

Abstract. Laser-induced breakdown spectroscopy (LIBS) has been used to study the surface hardness of special aluminum alloys containing zeolite. The aluminum alloy has acquired pronounced changes in its metallurgical properties due to the zeolite inclusion. The surface hardness of the samples under investigation is determined by measuring the spectral intensity ratios of the ionic to atomic spectral lines in the LIBS spectra of samples having different surface hardness values that have been conventionally measured before for comparison. The presence of aluminum silicate mineral in the studied alloys enabled material volume to expand under compression. This feature gave new results in the measurement of hardness via LIBS. It has been proven that the trend of the alloy density change complies with the increase of ionic to atomic spectral line intensity ratio.

Laser Additive Manufacturing: Going Mainstream

Despite rapid progress, LAM remains largely a niche fabrication technology. What advances in technology and manufacturing science will bring it to the front ranks?

Post-processing of 3D-printed parts using femtosecond and picosecond laser radiation

Additive manufacturing, also known as 3D-printing, is a near-net shape manufacturing approach, delivering part geometry that can be considerably affected by various process conditions, heat-induced distortions, solidified melt droplets, partially fused powders, and surface modifications induced by the manufacturing tool motion and processing strategy. High-repetition rate femtosecond and picosecond laser radiation was utilized to improve surface quality of metal parts manufactured by laser additive techniques. Different laser scanning approaches were utilized to increase the ablation efficiency and to reduce the surface roughness while preserving the initial part geometry. We studied post-processing of 3D-shaped parts made of Nickel- and Titanium-base alloys by utilizing Selective Laser Melting (SLM) and Laser Metal Deposition (LMD) as additive manufacturing techniques. Process parameters such as the pulse energy, the number of layers and their spatial separation were varied. Surface processing in several layers was necessary to remove the excessive material, such as individual powder particles, and to reduce the average surface roughness from asdeposited 22-45 μm to a few microns. Due to the ultrafast laser-processing regime and the small heat-affected zone induced in materials, this novel integrated manufacturing approach can be used to post-process parts made of thermally and mechanically sensitive materials, and to attain complex designed shapes with micrometer precision.

Material response of semiconductors irradiated with IR ultrashort laser pulses

We utilize near- and mid-IR ultrafast laser radiation to investigate the processing of crystalline silicon with different dopants. A numerical model is adopted to simulate the material response depending on the wavelength and the dopant concentration.

Trans-wafer removal of metallization using a nanosecond Tm:fiber laser

By utilizing photon energies considerably smaller than the semiconductors’ energy band gap, space-selective modifications can be induced in semiconductors beyond the laser-incident surface. Previously, we demonstrated that back surface modifications could be produced in 500-600 μm thin Si and GaAs wafers independently without affecting the front surface. In this paper, we present our latest studies on trans-wafer processing of semiconductors using a self-developed nanosecond-pulsed thulium fiber laser operating at the wavelength 2 μm. A qualitative study of underlying physical mechanisms responsible for material modification was performed. We explored experimental conditions that will enable many potential applications such as trans-wafer metallization removal for PV cell edge isolation, selective surface annealing and wafer scribing. These processes were investigated by studying the influence of process parameters on the resulting surface morphology, microstructure and electric properties.

Utilizing the transparency of semiconductors via "backside" machining with a nanosecond 2 μm Tm:fiber laser

Semiconductors such as Si and GaAs are transparent to infrared laser radiation with wavelengths >1.2 μm. Focusing laser light at the back surface of a semiconductor wafer enables a novel processing regime that utilizes this transparency. However, in previous experiments with ultrashort laser pulses we have found that nonlinear absorption makes it impossible to achieve sufficient optical intensity to induce material modification far below the front surface. Using a recently developed Tm:fiber laser system producing pulses as short as 7 ns with peak powers exceeding 100 kW, we have demonstrated it is possible to ablate the “backside” surface of 500-600 μm thick Si and GaAs wafers. We studied laser-induced morphology changes at front and back surfaces of wafers and obtained modification thresholds for multipulse irradiation and surface processing in trenches. A significantly higher back surface modification threshold in Si compared to front surface is possibly attributed to nonlinear absorption and light propagation effects. This unique processing regime has the potential to enable novel applications such as semiconductor welding for microelectronics, photovoltaic, and consumer electronics.

Comparison of different ultra-short pulsed laser sources used for semiconductor substrate processing

Singulation of semiconductor substrates was carried out with ultra-short pulsed laser radiation. Experimental studies were aimed to improve the resulting edge quality by maintaining minimal cut width and sufficient cut depth, in addition to the minimizing of the heat-affected zone and debris generation. Single-pulse ablation thresholds were determined for both laser systems adopted. The influence of various process parameters on wafer processing was studied, and the resulting kerf geometries were investigated by determining the width and the depth obtained in different process conditions. Silicon wafers were processed using two different ultra-short pulsed laser sources with pulse durations 100 fs and 350 fs. Laser radiation was focused onto the wafer surface using three different microscope objectives with numerical apertures in the range NA=0.1–0.4. Irradiation was performed with varying process parameters such as the laser power and the translation speed, as well as by adopting different laser beam scanning strategies. A set of process parameters resulting in a combination of optimal edge quality, kerf geometry and overall ablation rate was identified for each laser system.Singulation of semiconductor substrates was carried out with ultra-short pulsed laser radiation. Experimental studies were aimed to improve the resulting edge quality by maintaining minimal cut width and sufficient cut depth, in addition to the minimizing of the heat-affected zone and debris generation. Single-pulse ablation thresholds were determined for both laser systems adopted. The influence of various process parameters on wafer processing was studied, and the resulting kerf geometries were investigated by determining the width and the depth obtained in different process conditions. Silicon wafers were processed using two different ultra-short pulsed laser sources with pulse durations 100 fs and 350 fs. Laser radiation was focused onto the wafer surface using three different microscope objectives with numerical apertures in the range NA=0.1–0.4. Irradiation was performed with varying process parameters such as the laser power and the translation speed, as well as by adopting different laser beam sca...