Collection of Ultrafast Laser Applications in Material Processing
This weeks blogpost we are creating a list of ultrafast laser material processing applications. Ultrashort pulse lasers are unbelievably versatile. The usual material processing need about 1uJ - 2000uJ pulse energy, 200-3000fs pulse length, 5-30W average power. The different applications will of course work in different parameter ranges within this region.
Before we create the list of applications, let's review the different parameters and what parameters and ideal laser would need to be plastically suitable for most applications.
Processing Optics and Average Power
The repetition rate of the laser in itself is not a major variable parameter. As a rule of thumb the lower pulse energy is used the more pulses per second are utilized. This gives us rather an average laser power which is in most cases between 5W - 100W. Due to this relation to the pulse repetition rate, the more power, the more pulses and the more demanding scanning speed is needed. Galvanometer based laser scanners often can reach 20m/s scanning speed but in reality up to 9m/s is really practical. The reason behind this is that the motor which is moving the mirror needs a finite time to pick up the target scanning speed (otherwise we get infinite acceleration). In the acceleration (and deceleration phase) the pulses will overlap differently and this is disturbing the material processing.
If you want to scan pulses which are in focus about 30micron diameter, you will end up with 6m/s scanning speed at about 9000mm/0.03mm=300 000 pulses or pulse repetition rate. A smaller spot size will need less pulse energy and more pulses, so the same average power.
The immediate conclusion of this quick example is that by using a higher average power ultrafast laser, you will need a much more expensive scanning system (for instance polygon scanner) or you need to split the beam into beamlets or sub-beams. So, about 30W laser power is the highest an average user will ever require from a laser.
Some glass cleaving systems often use XY stages to move around the beam. This particular application uses Bessel beams to produce an high depth of focus. The Bessel beam has a lateral diameter at the Airy disk of about 2um .. 6umm which in exchange gives the cutting speed of 600mm/s - 1800mm/s. This kind of speed is a better fit for motorized stages. The actual speed these systems can achieve will be very dependent on the wight of the laser and processing optics.
There are of course other specialty applications, but in most cases the above two solutions are used: galvo or XY stage based.
Pulse Energy
Let's analyse the pulse energy too. To damage the material or make any modifications, one needs a fluence of about 1-10J/cm^2. Materials which don't absorb very well the photons of the laser will be at the higher end of this range. If you only want to make smaller changes (like writing waveguides), you will end up in the lower range. The fluence is the pulse energy of a single pulse distributed over the area of the focal spot. So, for about 5J/cm^2 one needs 50uJ pulse energy if we stay with the previously stated 30um spot diameter in focus. Accounting for losses on the processing optics, being a bit maybe off-focus, considering beam quality being not perfect .. with 100uJ we would be very comfortable able to process any material.
Pulse length
The pulse length will affect the fluence needed to damage the material. Shorter pulses will need less energy. This rule holds when going from nanoseconds down to a single picosecond. Below a picosecond there is no more improvement. Going much below a picosecond like 100fs or longer pulses will have additional issues. See, the shorter the pulse is, the more it will elongate when passing through a certain thickness of glass. Lenses are always used not just inside the lasers but for focusing the beam on the workpiece. If the pulse is too short, it will quickly get longer while passing through just a couple millimetres of glass. Other than the elongation, shorter pulses have a much broader spectrum. While a picosecond laser pulse requires around 2 nanometer of spectral bandwidth, a 6fs pulse will have about 200nm spectrum. Making the optics thin (reduce pulse elongation) and work with 200nm broad light source is a serious challenge. So, the ideal pulse duration is about 1ps. I like to remember that light itself travels only 0.3mm within this time frame. So the 1ps pulse itself takes in space only 0.3mm. It's almost like a sheet of paper flying through space.
An ideal ultrafast laser for material processing:
So, the ideal ultrashort pulse laser fitting most applications: up to 100uJ pulse energy, 300kHz comfortable repetition rate, 30W average power, about 1ps pulse energy.
Now, that we know that laser and process parameters we should use, it's time to list the applications of these lasers. I'll expand this list as we go, let's start with the first 35 applications which come to mind:
Laser eye surgeries
As the laser pulses are focused on the eye, every one of them creates a little bubble. The bubbles end up forming a lens. You get your contact lenses directly cut into your eyes. It's a common thing so we will just link here the Wiki page of LASIK:
https://en.wikipedia.org/wiki/LASIK
Laser cleaving of glass
Glass is often cleaved using Bessel beams. The advantage of the Bessel beam is the very long depth of focus with an extremely small (2-6um) spot in the middle. Axicons, diffractive axicons or reflective axicons can generate Bessel beams for this. There are, however, aspheric axicons which are even better for this:
https://www.powerphotonic.com/laser-glass-cleaving/
Autoclavable black marking
Black markings produced by nanosecond lasers are typically not suitable for high temperature or wet/humid environment. This is because nanosecond lasers heat the material a lot and this will cause later on corrosion issues. The solution is making the black mark under the oxide layer using ultrafast lasers.
Trumpf has a very nice overview with a lot of images on their website:
https://www.trumpf.com/en_INT/solutions/applications/laser-marking/black-marking/
Black silicon
I think it was the company Sionyx which patented this process a long time ago. They are basically creating a very high surface area on the surface of the silicon based image sensor. The light gets absorbed much better after many reflections. As silicon can actually work up to 1300nm as a photon detector, this process enables some night vision capability. The appearance of the sensor is very black:
https://www.sionyx.com/pages/sensors?srsltid=AfmBOoopByBlM5xYQMkFeR7xOxZnS-GVldkbtvt5fkUlFPt9wDhX3Dn7
Water and dust repellent surfaces
The process called direct laser interference patterning (DLIP) can create about light-wavelength size microstructures on the surface. This results in interesting properties. Water get's repelled but for instance dust too if the dust particles are large enough. No more cleaning solar power stations. Look at for instance Fusion Bionic's website for more details:
https://fusionbionic.com/en/
Superhydrophilic surfaces
This is the other way around - water tends to rather stick to the surface. Often you want for instance cells to stick to a medical implant so recovery can be accelerated after surgery.
https://en.wikipedia.org/wiki/Superhydrophilicity
Generation of X-rays without high voltage
Did you know that femtosecond lasers generate x-rays when they are engraving? Well, how the process works is that the laser pulse has a stronger electric field than the one binding the electrons. The electrons get detached, accelerated and smashed back into the surface. This generates x-rays. The spectrum will depend on the material composition. I wonder why this is not used with a fluid to make x-ray images.
Here is a paper on the topic: https://doi.org/10.1364/OE.468135
Generation of THz radiation
Ultrashort pulses when sent through a semiconductor like Zinc-Telluride will generate exotic THz light. THz radiation is not typically present in nature because there are no natural sources. It's somewhat between radio waves and light. THz can be used to see through cloths and plastic bottles, identify guns which don't let it through. For instance here you can buy your crystals for the conversion from Del Mar Photonics.
PCB prototyping
Selectively etching away the copper layer on the PCB is not easy with nanosecond lasers. There is a lot of heat and carbonization going on so the result will be very bad. With ultrafast lasers the processing is rather cold - close to no thermal effects. LPFK sells such machines but they cost like around 100k, likely due to the expensive ultrashort pulse laser. These machines will get also cheaper in the future with our affordable femtosecond lasers. Check out their laser PCB prototyping machine:
https://www.lpkf.com/en/industries-technologies/research-in-house-pcb-prototyping/products/lpkf-protolaser
PIC (photonic integrated circuit) direct writing
Photonics integrated circuits are currently expensive as they are made in fabs similar to the good old semiconductor lithography process. But it's possible to write some of these using ultrafast lasers. Modular Photonics has a patent from many years ago but there was not much chance to make it mainstream due to high laser prices. Patent will expire soon and lasers will get affordable. If you are looking for a startup idea, I embrace you to state developing! Link to Modular Photonics:
https://www.modularphotonics.com/custompics/
Manufacturing of fibre sensors
Similar to PIC, the refractive index of fused silica can be altered by close to damage laser fluence. What actually changes in the material is the distribution of the number of atoms forming a ring. Yes, fused silica is amorphous but there is some order in there. Lumoscribe can write custom sensors for you and Northlab has recently released a machine which can manufacture fibre Bragg gratings.
https://lumoscribe.com/
https://www.northlab.se/fiber-products/noria-fiber-bragg-grating-manufacturing-solution/
Micromachining of stents
Stents and other microstructured medical devices are often made of nitinol and using a femtosecond laser. The process of designing these reliable must be hard enough. Lasers should enable the design to come to life. NKT has some nice images of such stents - see link below. Yes, NKT offers lasers too, buy they are far too expensive unless to have a very deep pocket.
https://www.nktphotonics.com/applications/medical-life-science/laser-micromachining-of-nitinol-stents/
Two-Photon Polymerization 3D printing
How this process works is that the laser is focused into a material which will only polymerize at half of the laser's wavelength which is used in the process. The laser is focused on the material but the intensity is only in focus large enough to create the second harmonic (IR --> Green) of the laser. Where there is green, there is polymerization.
Nanoscibre is one of the main players in this space. They can print lenses on the end of a fibre!
https://www.nanoscribe.com/en/microfabrication-technologies/2pp-two-photon-polymerization/
Etching of microfluidic cells
Is something these lasers are very suitable for. The laser will create an ablation spot in focus which is depending on the fluence but can be less lens 0.1um deep. Etching the glass surface or in volume creates capillaries.
https://en.wikipedia.org/wiki/Microfluidics
Dicing of silicon wafers
There is nothing new here, for the laser it could be glass or ceramic and work the same good. Semiconductor wafers full of transistor stacks are expensive to make due to the special needs of the lithography process. If you are dicing these with a laser, the amount of lost area can be minimalized so more chips fit on the same wafer. Precise cuts with no heat affected zone. A typical example of an industry where the price of the laser doesn't matter.
https://www.disco.co.jp/eg/solution/library/laser/laser.html
Electrodeless electrolysis
Yes, the electric field of the laser pulses can also split water! The laser itself is not very efficient so I guess that's not a very competitive approach. Anyway, if you only have a laser you can also split water. Without electrodes wearing out..
https://pubs.rsc.org/en/content/articlelanding/2022/ra/d2ra01337a
Supercontinuum light sources
Supercontinuums are actually lasers. Very white lasers. Often they produce spectrum in a very wide range. Nonlinearities are at work here, the ultrafast laser pulse being confined in a small space. Often the difficulty is keeping the spectrum stable. But it looks amazing! Hyperspectral LIDAR is an interesting application here. Imaging in true 4D. Supercontinuum lasers are also often used for OCT (optical coherence tomography) where depth information is measured by interfering a reference pulse with it's reflected self form different depths.
https://opg.optica.org/oe/viewmedia.cfm?uri=oe-31-20-33486&html=true
Free-Space Optical Communications
I heard from someone that the bandwidth of free space optical communications is limited by fluctuations in the refractive index of air (wind blowing, temperature..). Well it turns out that a very short pulse is so localized (100fs = 30um long in space) that these fluctuations are negligible. An interesting idea! It just needs a low cost laser with high repetition rate.
Dual-Comb laser range finder
Did you know that you can reconstruct 3D images from kilometres away with millimetre precision? Lidar is just one application of frequency combs. High precision spectroscopy is the other. These guys can deliver the laser for you: (but it will be expensive)
https://k2photonics.com/
Laser assisted lift-off (removal of coatings from substrates)
Using the laser from the back of a substrate, you can remove the coating locally. If there is some dirt on the surface, you can remove the dirt without affecting the surface. An interesting idea, used often for manufacturing displays.
https://www.researchgate.net/publication/333055781_Laser_Transfer_Printing_and_Assembly_Techniques_for_Flexible_Electronics
Welding of metal to glass or plastic to glass
This is an application I like a lot. You can weld materials together which have very different melting points. For instance Plastics: couple hundred degrees. Metals: couple thousand degrees. The trick is that the surfaces need to touch each other so the created plasma bubble mixes up the materials nicely. There is a lot of opportunity here once the lasers get low enough cost. Especially for instance plastic welding is a very cost sensitive market.
https://www.youtube.com/watch?v=pgdIynJc2Js
Trimming of resistors
This is once again just removal of coatings. Due to the heat-free process you can etch away materials from the edges of a coating so far that the remaining coating "wire" becomes extremely thin. By controlling the thickness you can simply change the resistance.
https://en.wikipedia.org/wiki/Laser_trimming
Fluorescence lifetime imaging microscopy
FLIM seems to be an emerging microscopy technique. If you measure the fluorescence lifetime over a large area, you can draw conclusions regarding interaction between the fluorescing molecules and their environment.
https://en.wikipedia.org/wiki/Fluorescence-lifetime_imaging_microscopy
Nonlinear microscopy (CARS, 2PEF, SHG..)
In this microscopy technique nonlinear effects are used to make a very localized or molecular vibration targeted excitation. It's possible to map the distribution of the certain molecular bond in your sample. For instance, fat will have more of a certain bonds than proteins. The main advantage is that there is no dyes needed which are usually toxic to the cells.
https://en.wikipedia.org/wiki/Coherent_Raman_scattering_microscopy
Femtosecond laser induced breakdown spectroscopy
The laser destroys the sample by ionizing it. The emitted light is very specific to the materials making up the sample. Atomic composition can be determined by linear unmixing of the spectra using a known database.
https://en.wikipedia.org/wiki/Laser-induced_breakdown_spectroscopy
Laser drilling of materials
This application is often done with burst-mode lasers. The idea of drilling is simple, laser etching the same place over an over again. But this will often accumulate some heat, even though the pulses are very short. With the burst mode one basically removes the heat which is generated simultaneously.
https://ceramics.org/ceramic-tech-today/femtosecond-laser-bursts-drill-crack-free-holes-in-glass/
High density data storage
Archiving large amount of data by creating microdefects in glass can be done using an ultrafast lasers. Often fused silica is used and the process is the same what we have mentioned at the fibre sensors (optical fibres are made of fused silica too).
https://en.wikipedia.org/wiki/5D_optical_data_storage
Writing 3D wires in glass
If the glass has silver nanoparticles in it, the laser can get these connected. By scanning the laser in 3D one can create circuits inside the glass. I can't find a link for this but I'm confident this is a thing.
Cutting of high hardness materials like diamonds
Diamond and similar high hardness materials are often used in machining tools. But cutting / dicing an artificially grown diamond substrate is hard. Ultrafast lasers come handy for this task.
https://pubs.aip.org/lia/jla/article/35/4/042042/2916234/Ultra-short-pulsed-laser-processing-of-single
Laser peening
I don't know if this is an application with practical relevance but AI keeps throwing this at me so I decided to add it here. Apparently nanosecond lasers were used for this earlier but I can imagine that femtosecond would even work better given the cold processing advantage.
https://en.wikipedia.org/wiki/Laser_peening
Laser induced forward transfer (LIFT)
This application is using a laser to ablate material from a substrate. Similar to the laser assisted lift-off process, but this time we have a received substrate on the other side where the ablated material deposits.
https://www.psi.ch/en/lmx-interfaces/lift
Fabrication of diffraction gratings (LIPSS)
By adjusting the fluence in focus, one can create surface structures which have the period of the laser half wavelength. Yes, it's interference. The electric field of the laser produces these waves and given their very precise period, these can be easily used as diffraction gratings. The direction of the structures is governed by beam polarization. So by rotating the polarization you can make even circular gratings (rather uncommon application).
https://link.springer.com/rwe/10.1007/978-3-030-63647-0_17
Pump-probe measurements
In pump probe measurement there are two laser pulses. One excites the molecules and the other probes them. By delaying the two pulses at different amount, one can do very high temporal resolution videos. It's a cool application. We once made a video of a high intensity laser creating plasma.
https://en.wikipedia.org/wiki/Pump%E2%80%93probe_microscopy
High harmonic generation (HHG)
If the laser pulse has a good temporal contrast, meaning the intensity increases in time very rapidly, then the plasma it creates in focus will convert the wavelength to higher harmonics. While second and third harmonics are common to have in the laser world, using this technique one can generate coherent light with wavelengths close to a couple nanometres. Such a very short wavelength laser could be used for lithography or very accurate x-rays if the water-window is achieved.
https://en.wikipedia.org/wiki/High_harmonic_generation
Compact Laser Particle Accelerators
Particle accelerators are huge but they are very useful. Creating isotopes for medical applications or making very precise radiation therapy. Often a Laguerre-Gaussian beam is used (ring beam) and it needs control of the phase of the pulse peak. It's good to mention but likely out of range for most material processing lasers.
https://www.science.org/content/article/compact-cheaper-laser-powered-particle-accelerators-get-real
Laser lightning rod (laser filamentation)
Another cool application where a very high pulse energy laser is focused very tightly in air. The repeated defocusing and refocusing created a plasma filament in air. If it's long enough, it will act as a lightning rod because the plasma is a very good conductor. Before the lightning gets to the expensive laser it hits a grounded metal rod close to the filamentation. Rockets can be protected from lightning strike like this. The teramobile project is very old but look at those pictures! Straight from Star Wars.
https://www.teramobile.org/teramobile.html
Making watch gears
Watches have small components, the details are often only visible under the microscope. Well, those tiny gears are laser cut. A laser machine can produce very quickly all the parts a watch needs, so the material cost of the watch stays low. This is an area where an affordable laser will certainly not reduce the watch prices. Lasea produces laser machines and has a very good showroom of these parts. Check out their website:
https://www.lasea.eu/en/industries/watch-industry-jewellery/
Laser contact opening
This is a simple engraving operation in the solar industry, as far as I understand.
https://www.sciencedirect.com/science/article/abs/pii/S0927024818303076
Producing nanoparticles in liquids (Pulsed Laser Ablation in Liquids)
Have you ever wondered where all the ablated material goes? Hopefully your fume extractor picks it up. The laser produced plasma cools, electrons recombine, molecular bonds form. The result is a soup of particles. Doing it in a liquid has apparently some advantage. Likely easier recovery of the particles afterwards.
https://www.intechopen.com/chapters/87041
Frequency Combs
Even though this is not material processing application, it is a very important one: Frequency combs are laser pulses which have a spectrum with fixed and known spacing spectral components. They can be used for many things but extremely precise spectroscopy is one of the main application.