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FAQ
What is the biggest advantage of dual photon absorption over single photon absorption based processing?
The answer to this question is at the heart of the 3D femtosecond laser nanofabrication. Compared to single photon absorption based processing, dual photon absorption based processing has the following merits:
1.
Single photon absorption based processing can only be used for photoresist materials that have linear absorption for the laser wavelength used. Based on the nature of the photoresist chosen, it is able to achieve single photon polymerization (e.g. negative photoresist. Exposed positions are retained while unexposed positions are washed out by the developer solutions) or single photon bond-breaking (e.g. positive photoresist. Unexposed positions are retained while exposed positions are washed out by the developer solutions).
However, dual photon absorption based processing can be applicable to various kinds of materials and is not limited by the linear absorption feature for laser wavelength. Through different reaction mechanisms on different materials, the possible processing mechanisms of dual photon absorption based processing include the following:
- It can also be used to achieve dual photon polymerization and bond breaking for photoresist materials.
- It enables material modification via dual-photon-driven chemical reactions e.g. photo-reduction and photo-oxidation.
- It enables phase change of materials, e.g. from crystalline phase to amorphous phase and it is evident by looking at the change in the material’s refractive index.
- It enables the stripping and etching processing of materials, e.g. layer-by-layer stripping of 2D materials, etching of metallic materials or semiconductors.
- It enables microbursts inside the materials, to realize hollow points or lines around which a high density shell layer will be formed. This allows forming lattice or waveguide structures in the materials, with high refractive index difference.
As a result, there are a wealth of mechanisms associated with dual photon absorption based processing that accommodate different materials and application requirements.
2.
Since single photon absorption based processing requires that the material to be processed has linear absorption for the selected laser wavelength, it is not possible to focus the laser inside the material for three-dimensional processing. In contrast, two-photon absorption occurs only at the focal point of the laser (subject to the requirement of material threshold), which allows three-dimensional processing with three-dimensional spatial resolution to be performed by focusing the laser inside the material even if it has no linear absorption for that laser wavelength. It enables fabrication of arbitrary three-dimensional structures by scanning the processing path inside the material. Even if the sample has a layer structure, the dual-photon approach can enable modifying or processing of any layer in the material. Such approach offers a significant degree of freedom and convenience. For example, conventional single-photon based processing of layer structures requires pre-processing to expose the desired layer to the surface for single-photon exposure processing, and then cladding the material layer by layer after processing according to the protection requirements. However, the dual-photon approach does not require any pre-processing and post-processing, which effectively reduces steps, shortens time, and lowers costs of processing.
3.
For the same laser wavelength, the dual-photon absorption based processing has a higher spatial resolution compared to the single-photon processing. For example, the resolution of the conventional diffraction limit based on the single-photon mechanism can be expressed as: ds=λ/2NA, where λ is the laser wavelength used and NA is the numerical aperture of the focusing microscope lens used. However, based on dual-photon mechanism, this resolution can be improved by a factor of √‾2, to dt= ds/√‾2 =λ/2√‾2 NA.
In sum, compared to single-photon absorption based processing using continuous-wave lasers, the dual-photon absorption-based femtosecond laser processing has the following advantages: abundant processing mechanisms, applicable to multiple materials, three-dimensional processing capability and high spatial resolution etc.
How can we inspect and subsequently trace the material surface using laser?
Automatically tracing the surface of the material to be processed is a great challenge, which requires an AI vision recognition function simultaneously with targeted algorithms. We need to consider the response of material itself to laser (e.g. whether it is highly reflective or it is fluorescent etc.) and the material modification after being irradiated by laser (e.g. modulation of refractive index and amplitude). Therefore, it is necessary to implement tracing functions accordingly based on different mechanisms and different materials. Here we provide an example of photoreduction processing on graphene oxide films: First, graphene oxide has a relatively high refractive index (>1.8), thus we can determine whether the laser focus is on the surface of the graphene oxide film by detecting the reflected light pattern through machine vision. Meanwhile, we can determine the performance of laser focus on the film surface by looking at the amplitude modulation caused by laser fabrication, and we can also determine whether the laser is in the best position by judging its processing resolution. Additionally, here we demand excellence not just in software but also in hardware. Design and calibration of the laser device is necessary to achieve the best imaging quality and highest imaging resolution. Surface tracing is a systems engineering task and this is just a case of processing graphene oxide films. It is a complicated topic that is constantly changing according to requirements of different materials. Innofocus is an expert in this field and holds patents for relevant inventions. Do not hesitate to enquire with more specific demands.
What is the mechanism behind refractive index imaging and are there any limitations on materials?
Refractive index imaging is a method of quantifying the phase difference caused by difference in refractive index using computational holography, through spatial light interference, which further reproduces the three-dimensional refractive index distribution of the measured material. To measure the 3D refractive index, the material to be measured first needs to have a high transmittance in the wavelength region of the laser to be used. (Innofocus provides laser of multiple wavelength options including 450nm, 488nm, 532nm, 633nm etc., and can customize laser wavelength to be used according to customer requirement.) Therefore, in theory, the refractive index imaging can be performed as long as the material is transparent to the laser wavelength used. However, in practice, due to the need for high numerical aperture lenses (NA>0.5) to achieve high spatial resolution, there are certain requirements for the thickness of the material to be imaged. The thickness should be less than 2 mm and the distance from the fabricated structure to the material surface should be less than the working distance of the imaging microscope lens (typically <400 µm). Since the refractive index difference and spatial distribution are the same when processed with the same laser parameters on the same material, we can perform refractive index imaging on the materials in thin sheet first to quantify refractive index, optimize laser processing parameters, and use the optimized parameters on the final thick blocks of the materials. This allows for quick optimization of processing parameters and can be used on block materials of different thicknesses depending on the application requirements.
What kind of material is the refractive index imaging capability mainly for? What is the main difference compared to other processing systems with built-in camera showing the process and imaging?
This question hits on the difference between refractive index imaging and normal CCD in-situ observation. First of all, the major strength of refractive index imaging is to observe the distribution of the three-dimensional refractive index difference in the material, with quantitative accuracy up to 0.0001, and currently there is no other alternative that can achieve the same function. It is mainly for observing of the refractive index difference produced by using femtosecond laser processing in different optically transparent materials. This refractive index difference has irreplaceable importance in the application of optical waveguide and fiber grating. The refractive index imaging can be used in a wide range of crystalline and non-crystalline materials, including traditional polymers (e.g. PET, acrylic material and etc.), traditional optical crystal materials (e.g. quartz, optical glass), and new optical crystal materials (lithium niobate crystals, sapphire, diamond, BGO crystals). It can also be used to arbitrary rectangular, cubic and cylindrical shaped material, and is also suitable for fiber optic materials.
The built-in camera in other laser processing system can display a two-dimensional picture of the process, or a modification of the material (such as an optical material showing a darker colour due to elevated absorption, or scattering created by glass or etching of the material, or areas of high transparency), and is a useful way to observe the process and the results in real time. However, in the need to observe laser-introduced refractive index differences, this traditional method faces two challenges:
1.
If it is purely refractive index change, it does not introduce a change in transmittance or absorption, and it cannot be clearly distinguished when the refractive index change is entirely in the focal plane (e.g., the core-cladding boundary of an optical fiber cannot be distinguished when the fiber is accurately focused). Therefore, it is often necessary to move the introduced refractive index difference slightly away from the focal plane of observation to obtain some contrast by observing the colour streaks caused by this refractive index difference. However, since the diffracted image is observed, its width of lines is difficult to determine directly, and although deliberately moving the sample away from the focal plane is beneficial to the observation, it also causes some errors in the processing position, which is only suitable when the requirements for the processing position are not very precise (allowing a few hundred nanometers of error).
2.
The observation of the three-dimensional spatial distribution of the refractive index difference is not possible with the conventional method. As mentioned in the first point, it is possible to see the effect of the introduced refractive index difference by observing diffraction pattern using the conventional observation method, but the diffraction pattern is controlled by the difference between the processing position and the position of the focal plane, and the three-dimensional spatial distribution cannot be fully determined.
3.
Conventional methods cannot quantify the refractive index difference measured. Meanwhile, a quantitative refractive index difference has a decisive impact on the structural design and final device performance in fields of optical communication devices and optical fiber gratings. Therefore, it is extremely important to accurately quantify the three-dimensional refractive index distribution of the structure being processed. In this regard, Innofocus Photonics has developed the world's first and only intelligent laser nanofabrication system with in-situ refractive index imaging characterization capability, which is a ground-breaking invention. Based on this patented invention, they have achieved the first in-situ characterization of three-dimensional refractive index difference distribution, which is a fundamental breakthrough in the field of laser processing.
Can your laser nanofabrication technique be applied to the manufacturing of micro lens arrays?
Micro lens arrays are one of the main applications where our technology is highly advantageous. A mature process for fabricating quartz micro lenses is the lithography method using photoresist to fabricate patterns with etching. This method currently faces the following problems.
a.
Low flexibility: The design of the lens is determined by the mask patterns and cannot be freely adjusted.
b.
Long cycle of testing: Since each design update involves processing new masks, the fabrication process is complicated and time-consuming.
c.
Long cycle of testing: Since each design update involves processing new masks, the fabrication process is complicated and time-consuming.
d.
Surface morphology cannot be precisely controlled: Since the lithography process is a technique to produce two-dimensional graphics, the three-dimensional surface morphology of the fabricated lens cannot be directly controlled. However, the focal length of the lens, the imaging quality mainly depends on the surface shape of the lens. Therefore, the focal length of the lens and the imaging quality are seriously affected when the surface morphology cannot be accurately controlled.
e.
Inability to fabricate aspheric lens, combined with the elimination of phase aberration function: due to the method of heating to form an arch, the most likely surface shape formed by its thermal stabilization is spherical according to the principle of uniform distribution of heat deformation, and it cannot be precisely controlled.
f.
Inability to fabricate high numerical aperture micro lenses: Due to the heating method to form an arch-shaped surface, it is not possible to fabricate lenses with a small radius of curvature to meanwhile achieve high numerical apertures. Therefore, the resolution of imaging is greatly limited.
g.
Low uniformity: Since it is controlled by heating, the curvature of each lens depends on the temperature applied to it. And the difference of temperature will cause the difference of curvature and thus affect the focal length. Also, even if the temperatures are all the same, there is no guarantee that each lens will form the same change in curvature under heating conditions. Therefore the uniformity of the lens array cannot be guaranteed.
h.
Specific requirements of photoresist choice: the photoresist to be used has three requirements: 1) photosensitive to UV light; 2) able to heat deform to an arch; 3) able to resist argon ion etching for pattern transfer. Therefore, the choice of photoresist is limited.
i.
Low yield: Even if the process can achieve stable control, during the heating, the quality of each of its micro lens arrays is difficult to be completely consistent when its surface morphology is not accurately controlled. This leads to a compromised yield and repeatability.
However, fabricating micro lens arrays using femtosecond laser nanofabrication technology has several advantages:
a.
High flexibility: Based on laser 3D nano-fabrication technology, no mask is required, and lens arrays with arbitrary spatial distribution can be fabricated.
b.
Low cost: The cost of laser processing is extremely low because there is no need for fabricating any mask.
c.
Short optimization time: Since no mask is required, for each new lens array design, only new processing files is needed to generate, the design can be quickly optimized for comparison.
d.
Precise control of surface morphology: The surface morphology of each lens can be controlled independently. The high resolution of 3D laser nanofabrication technology can be combined with the spatial position of each micro lens to produce the surface shape of the micro lens accordingly, and then fabricate the micro lens array to meet the specific light field distribution.
e.
Ability to fabricate aspheric achromatic lenses: Due to the precise control of the structure's three-dimensional surface morphology by 3D laser nanofabrication technology, the precise processing of micro lenses of arbitrary design can be realized. The error of the surface morphology can be controlled within a few tens of nanometers, fully meeting the application requirements.
f.
Ability to process lens with a small radius of curvature: High numerical aperture micro lenses can be produced to improve the imaging resolution.
g.
High uniformity: 3D Laser nanofabrication technology enables precise processing of each lens to achieve high uniformity.
h.
High yield: Based on high accuracy and high uniformity, precise control of the quality of micro lens arrays can be achieved and high yield can be realized.
On this basis, 3D laser nanofabrication technology can not only fabricate traditional micro lens arrays, but also micro lens arrays composed of new pure flat superlenses (including graphene superlenses and super-surface lenses) to realize ultra-light and ultra-thin micro lens arrays.
Is it possible to correct aberrations at different depths in real time by using a spatial light modulator (SLM) to compensate for phase and aberration?
The biggest advantage of using a spatial light modulator (SLM) for aberration correction is that it can dynamically update the phase diagram (basic frequency is around 60Hz, while customization can go up to 500Hz). Innofocus’s nanofabrication system have unique technical advantages in terms of light field modulation. For example, our software system can generate phase maps for aberration correction in real time, based on the processing depth calculated after locating the material surface, and quickly update them on the spatial light modulator to achieve optimal compensation for each processing depth. Furthermore, since the laser focal spot runs along a specific designed trajectory in 3D space during the processing, it may move continuously at different depths. We designed AI-assisted software system to realize real-time compensation according to the depth where the focal spot is located during the continuous processing, to ensure quality and consistency.
Are there any feasible alternatives for characterizing 3D refractive index modification of materials?
To the best of our knowledge, Innofocus's commercialized nanofabrication equipment based on multiple patented technologies is the world first system to ever achieve in-situ laser nanofabrication characterization of three-dimensional refractive index distribution, which is a fundamental breakthrough in the field of laser processing. The traditional method of refractive index measurement by refractive index liquid is very time-consuming and laborious, and can only perform two-dimensional refractive index characterization, but not three-dimensional refractive index characterization.
Are there any difficulties in fabrication of large-area, large-size (e.g., millimeter-scale) devices using two-photon lithography?
First of all, the 3D laser nanofabrication technology based on two-photon absorption mechanism can be understood as two-photon lithography in specific application areas (only in the case of using photoresist and in the etching process using a patterned photoresist as a mask). Although it is one of its functions, in general, 3D laser nanofabrication is not used as a photolithography technique. This is because one of the important advantages of 3D laser nanofabrication is the processing of 3D structures, which are generally used as stand-alone functional devices and not as etching masks, because etching techniques cannot transfer 3D structures. Two-photon absorption based processing mechanism has various advantages, one of which is that the material to be processed can be etched directly, so that patterning and etching can be done in one step, no additional etching steps are needed, saving processing steps, time and cost.
Using 3D laser nanofabrication technology to process large-area devices first requires a large-area displacement stage and its travel would determine the size of the final device that can be processed. The current air-bearing-based large-area displacement stage can achieve a processing range of 1m x 1m, which can fully meet the needs of large-area processing. However, this is only a prerequisite, and having a large-area displacement station does not guarantee the processing of large-area, high-quality optical components, especially those with nanometer-level precision. There are two major problems that need to be solved:
1.
Precise guarantee of the relative position of the laser focal spot to the sample. In large-area processing, the laser focus will deviate from the designed processing position during the large-area movement even if there is only a very small tilt angle (<0.1°) of the placement of the material to be processed, and precise processing cannot be achieved. However, a 0.1° tilt is basically unavoidable. It can be caused by the uneven thickness of the material being processed. The error caused by this tilt will significantly affect the quality of processing. Therefore, even if the displacement station is able to fulfil large-area movement, it is likely that only a small area resulted can meet the requirements of use. Therefore, real-time tracking of the sample surface to determine the processing position is especially important in large-area processing. The surface tracing function of Innofocus’s nanofabrication system is a valuable invention that ensures accurate processing position throughout the whole processing area, enabling large-area processing of micro and nano optical components.
2.
During high-resolution nanoscale processing achieved by 3D laser nanofabrication, nanoscale lines, points or grids will fill the required processing area. As a result, during the actual fabrication process, the displacement station would need to travel a really long distance (despite meter-level structure, the processing path would be up to 1012 meters, which is an enormous number). Even if the displacement station can travel at high speed (0.5 m / s), the entire fabrication process would take significant amounts of time, which has become a bottleneck in the current 3D laser nanofabrication technology that limits the yield. Therefore, the availability of multi-focus and making full use of laser energy, is a key factor of expanding the application area of 3D laser nanofabrication technology. Innofocus’s unique multi-focus parallel fabrication function, which divides the laser energy into hundreds or thousands of focal points for parallel processing, in accordance with the desired processing target, increases the productivity of 3D laser nanofabrication technology by hundreds or thousands of times and achieves a breakthrough in large-area processing from the principle.
How to improve the inscribed area of FBG for a large core diameter fiber like sapphire fiber?
The area that inscribed affects FBG’s performance. Therefore, it is especially important to increase the FBG inscribed area for large core diameter fibers. There are two methods to increase the FBG inscribed area:
1.
Conventional laser fabricating systems without beam shaping function, are able to achieve inscribing in large area through line-by-line method, where the area is determined by the length of the laser focus (along the z-direction, which is the direction of the optical axis) and the length of the swept line segment (x or y-direction, which is the direction of the writing line). If a large area is required, lens with a lower numerical aperture (NA) can be used (to increase the length of the focal point) as well as writing longer lines (to increase the length of the written line segment). This method inscribes in a square interface, and the writing time is relatively long. Thus, it requires longer processing time.
2.
With the support of Innofocus’s unique beam shaping function, we can adjust the beam focus and extend one of the focus dimensions (perpendicular to the fiber core direction) to inscribe in a pancake-shape focus. This method inscribes in a circular interface. Furthermore, this method can directly adjust the focus to match the specific parameters of the fiber core. The advantage of using this method is that it enables high-speed processing of FBGs with point-by-point writing.
Innofocus's NanoPrint 3D Intelligent Laser Nano-fabrication System.
Is reconstruction of refractive index fast?
Reconstruction of refractive index consists of two steps:
1.
Acquisition of interference fringe images with different illumination angles
2.
Based on the images, conduct calculation to reconstruct the 3D refractive index distribution of the structures.
Innofocus’s system has excellent performance in this area and completely automated the process for users, allowing automatic continuous image acquisition and computational reconstruction without multiple background acquisitions. The speed of the completed process depends on the configuration of the computer. A real-time reconstruction speed of 0.5 Hz can be achieved with the standard configuration of a small workstation. Speed can be increased with more advance-configured workstations.
What is the maximum energy threshold when you adjust the SLM? How to process the SLM center zero level?
This depends on the system configuration. For example, Innofocus’ standard SLM achieves a maximum power of 800 mW/cm2. Thus, depending on the SLM irradiated area, the maximum power is 1.06 W. If set higher customised SLMs, 2 W/cm2 can be achieved with a maximum power reaching 2.654 W.
The central zero stage of SLM can be resulted in two ways, depending on the scenarios:
1.
Direct blocking method: This is achieved by directly blocking the zero level and only allowing the +1 level to pass through. This method is convenient and suitable for most scenarios.
2.
Direct blocking method: This is achieved by directly blocking the zero level and only allowing the +1 level to pass through. This method is convenient and suitable for most scenarios.
Can in-situ refractive index measure the refractive index values at the femtosecond laser, or is it just possible to complete a 3D reconstruction of refractive index?
The most important purpose of using in-situ 3D refractive index reconstruction is to understand the refractive index difference resulting from the interaction between the femtosecond laser and the material. In addition, 3D refractive index reconstruction after laser processing not only measures the refractive index difference at the point of femtosecond laser interaction, but also demonstrates the entire specific morphology and spatial distribution of the laser-processed 3D structure, providing important information in line widths, positions, cross sections, etc.
How fast is the rate speed of surface tracing? Will there be any impact on surface tracing in spectroscopic processing?
Surface tracing performs in real time. The speed of the process is determined by the sampling speed and frequency. The goal surface tracing is to keep the focal point on the surface of the substrate during processing by compensating for the focal point offset in real time. Surface tracing can further output the real-time position of the focal point, which can reconstruct the entire surface profile. This function can be used to design the 3D position of the entire multi-focus array for multi-focus machining.
In the multi-focus scenario, in order to track each focus, the 3D spatial distribution of the entire multifocal array needs to be updated in real time, so that the multifocal array can be updated by refining the phase modulation. Ideally, this method is feasible. However, this process requires intensive computer computation, and the current computer capability is not prepared to enable the real-time surface tracing of multifocal arrays.
What is the uniformity of the multifocal points in the actual experiment in multifocal parallel processing?
Uniformity of focus in multi-focus machining is the key to achieving high quality parallel machining. Therefore, the requirement for uniformity is extremely high, and the optimisation algorithm can achieve more than 99% uniformity.
How is focus point tracked in real-time?
Uniformity of focus in multi-focus machining is the key to achieving high quality parallel machining. Therefore, the requirement for uniformity is extremely high, and the optimisation algorithm can achieve more than 99% uniformity.
