We present a scalable approach capable of manufacturing high-precision three-dimensional (3-D) GRIN nanocomposites based on multi-component bulk glass-ceramics where we spatially modulate the concentration of high refractive index nanocrystals within a glass matrix. Previously demonstrated in homogeneous thin films and bulk glasses containing large scale liquid-liquid phase separation, this work expands on our efforts to optimize processing protocols employing a near single-phase bulk glass starting material enabling true 3-D modification. Sub-bandgap laser exposure generates Pb-rich amorphous phases within a Ge-As-Pb-Se glass matrix, which undergo crystallization resulting in the formation of highindex nanocrystals upon controlled heat treatment. Nanocrystal density is modulated in both radial and axial geometries by the laser dose, providing spatially tailorable changes in index and dispersion.
Optical materials capable of advanced functionality in the infrared will enable optical designs that can offer lightweight or small footprint solutions in both planar and bulk optical systems. The University of Central Florida’s Glass Processing and Characterization Laboratory, together with our collaborators, have been evaluating compositional design and processing protocols for both bulk and film strategies employing multicomponent chalcogenide glasses (ChGs). These materials can be processed with broad compositional flexibility that allows tailoring of their transmission window, physical and optical properties, which allows them to be engineered for compatibility with other homogeneous amorphous or crystalline optical components. We review progress in forming ChG-based gradient refractive index (GRIN) materials from diverse processing methodologies, including solution-derived ChG layers, poled ChGs with gradient compositional and surface reactivity behavior, nanocomposite bulk ChGs and glass ceramics, and metalens structures realized through multiphoton lithography. We discussed current design and metrology tools that lend critical information to material design efforts to realize next-generation IR GRIN media for bulk or film applications.
We demonstrate a scalable photo-thermal process which enables manufacturing of infrared (IR) transmissive glass-ceramic films with gradient refractive index (GRIN) profiles. Spatiallycontrolled laser irradiation creates Pb-rich amorphous phases within Ge-As-Pb-Se glass films, which are subsequently crystallized and become high index phases upon heat treatment. The density of the high index nanocrystals is shown to be controlled by the laser irradiation power, and the extent of fraction crystallized is controlled by post heat treatment time and temperature. Both of these variables can be optimized to realize a localized effective refractive index change, enabling a spatially-modulated refractive index change up to ~ +0.1. We demonstrate IR GRIN functionality within 1 inch diameter GAP-Se films with thicknesses ranging from 1 to 40 μm, confirming the scalability of our photo-thermal process to component-relevant geometries.
The ability to employ spatially-selective control of refractive index and dispersion variation with a high magnitude of change is essential for the realization of functional infrared graded-index (GRIN) components. Thin films fabricated from multi-component GAP-Se glass-ceramic materials were processed using nanosecond laser radiation at the wavelength λ = 2 μm. Various irradiation and post-processing protocols were implemented to maximize the magnitude of the local refractive index change, and to quantify the evolution of the glass to glass ceramic ‘conversion’ on optical material physical properties. Irradiation of films possessing various thicknesses from 1 to 25 μm was performed using area-scan patterns, while the average laser power and the number of scans were varied. Irradiated materials were subsequently heat-treated, and the local refractive index was determined for different durations of the heat treatment. Depth-dependent composition and film morphology characterization of as-deposited films was evaluated, and surface morphology of the post laserprocessed and heat-treated areas was studied to evaluate effects on the photo-thermal refractive index change associated with nanocrystal formation. Initial studies demonstrated a maximum positive refractive index change of ▵n ≈ 0.07 in a broad spectral range in the infrared which scales with film thickness and exposure dose while maintaining required optical quality.
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.
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.
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.
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.
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.
Utilizing the transparency of silicon at 2 μm, we are able to ablate the backside of 500-μm thick
silicon wafers without causing any damage to the front surface using a novel nanosecond
Tm:fiber laser system. We report on our high energy/high peak power nanosecond Tm:fiber
laser and provide an initial description of the effects of laser parameters such as pulse duration
and energy density on the ablation, and compare thresholds for front and backside machining.
The ability to selectively machine the backside of silicon wafers without disturbing the front
surface may lead to new processing techniques for advanced manufacturing in solar cell and
microelectronics industries.
In recent years, a major interest in surface as well as bulk property modification of semiconductors using laser irradiation
has developed. A.Kar et al. [1][2] and E.Mazur et al. [3] have shown introduction and control of dopants by long-pulse
laser irradiation and increased absorption due to femtosecond irradiation respectively. With the development of mid-IR
sources, a new avenue of irradiation can be established in a spectral region where the semiconductor material is highly
transparent to the laser radiation. The characterization of the light-matter-interaction in this regime is of major interest.
We will present a study on GaAs and its property changes due to pulsed laser irradiation ranging from the visible to the
mid-IR region of the spectrum. Long-pulse as well as ultra-short pulse radiation is used to modify the material.
Parameters such as ablation threshold, radiation penetration depth and thermal diffusion will be discussed.
Modifications of bulk aluminum irradiated well above ablation threshold (F < 300 J.cm-2) have been investigated
in situ by means of shadowgraphy and transient quantitative phase microscopy (TQPm) using ultrafast
laser radiation (tp=80 fs, λ=800 nm). This novel pump-probe technique enables quantitative time-resolved measurements
of object's properties, e.g. dimensions of melt droplets and layer thickness or transient refractive index
changes. A series of time-resolved phase images of vaporized material and/or melt, which are induced by n=1..8
pulses on an aluminum target, are obtained using TQPm. Dynamics and characteristics of melting, dependence
of the ablated material volume on process parameters and thereby induced structural modifications have been
studied. An increase of material ejection rate is observed at delay time of approximately τ=300 ns and τ>800
ns after the incident pulse.
Transient refractive index modifications have been investigated in technical glass (Schott D263) by means of
TQPm. By using high-repetition rate ultra-short pulsed laser radiation (tp=400 fs, λ=1045 nm, frep=1 MHz)
focused by a microscope objective (w0 ≈ 4 μm) heat accumulation and thereby glass melting as well as welding
is enabled. Transient optical phase variation has been measured up to τ=2.1 μs after the incident pulse and can
be attributed to the generation of free charge carriers and compression forces inside glass.
Lighting applications like OLED or on silicon for electro-optical applications need a reproducible sealing process. The
joining has to be strong, the permeability for gasses and humidity very low and the process itself has to be very localized
not affecting any organic or electronic parts inside the sealed region. The actual sealing process using glue does not fulfil
these industrial needs.
A new joining process using ultra-fast laser radiation offers a very precise joining with geometry dimensions smaller
than 50 μm. Ultra-fast laser radiation is absorbed by multi-photon absorption in the glass. Due to the very definite
threshold for melting and ablation the process of localized heating can be controlled without cracking. Repeating the
irradiation at times smaller than the heat diffusion time the temperature in the focus is increased by heat accumulation
reaching melting of the glass. Mowing the substrate relatively to the laser beam generates a seal of re-solidified glass.
Joining of glass is achieved by positioning the laser focus at the interface. A similar approach is used for glass-silicon
joining.
The investigations presented will demonstrate the joining geometry by microscopy of cross-sections achieved by
welding two glass plates (Schott D263 and AF45) with focused IR femtosecond laser radiation (wavelength λ =
1045nm, repetition rate f = 1 MHz, pulse duration tp = 500 fs, focus diameter w0 = 4 μm, feeding velocity v= 1-10
mm/s). The strength of the welding seam is measured by tensile stress measurements and the gas and humidity is
detected. A new diagnostic method for the on-line detection of the welding seam properties will be presented. Using a
non-interferometric technique by quantitative phase microscopy the refractive index is measured during welding of glass
in the time regime 0-2 μs. By calibration of the measured refractive index with a relation between refractive index and
temperature a online-temperature detection can be achieved.
Irradiation of metals with ultrashort laser pulses reveals a variety of versatile microscopic processes compared to longer pulses. In particular, the impact of some material-specific characteristics, such as the electron-phonon coupling, seems to get more significance in order to meet the machining requirements. Finding the optimal process parameter area has been therefore a dominating problem in materials processing with sub-picosecond laser radiation.
Ablation of bulk metals (Al, Cu) has been investigated in-situ by means of high-resolution pump-probe photography using pump laser radiation of pulse duration tp=80 fs, at wavelength of 820 nm. This technique enables direct visualization of laser-induced processes up to 1 μs after the interaction of a single laser pulse with material. Variation of the fluence of the laser radiation, behavior and time characteristics of melting and post-melting processes have been matter of research.
Depending on metal-specific parameters, qualitatively different ablation phenomena have been observed. Structural analysis by electron and optical microscopies reveals rosette-like surface structures showing the morphology of the ablated regions. The temporal development of the ablation dynamics can be conditionally categorized into different characteristic time regions. Particularly, laser induced melt injection has been observed in the time range of 700 ns to 1.0 μs after the initial laser-metal interaction.
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