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For Scientific Research & Industry Modernisation.
Laser nanofabrication requires high numerical aperture microscope to achieve spatial resolution at nanoscale, and this leads to two major problems:
1. Due to high-resolution condition, it requires laser to move a long distance to process large-area structures and therefore it takes time. However, traditional laser nanofabrication technique uses only a single laser focus to perform linear processing, which is time-consuming and costly, and during laser processing, the instability of the laser and optical components will significantly affect the fabrication results. Meanwhile, a significant portion of laser energy is lost since the focal spot merely requires 1/100 of the laser output power.
2. High numerical aperture lenses are particularly sensitive to aberration caused by mismatch of refractive index. This is especially true in materials of relatively high refractive index such as sapphire, lithium niobate crystal, diamond, and sulfur-based glass. The aberration will significantly affect the focal spot shape, reducing the peak power and resolution of the focus. As the processing depth increases, the aberration accumulates and eventually the unshaped spot completely overlaps, severely limiting the processing depth.
During waveguide fabrication, the fabricating position is relatively deep in the dielectric material due to its large dimensions. The aberrations caused by the accumulation of refractive index mismatches therefore have a significant negative impact on the fabrication. And this effect progressively worsens with increasing processing depth. Laser spot shaping therefore plays a pivotal role in the fabrication of micro-nano and waveguides.
Without aberration correction, the normal elliptical spot will be stretched with the peak spot energy being extremely dragged down, causing the fabrication to not proceed properly. After correction, the spot returns to normal and the process can be carried out normally as designed. As the machining depth increases, the unshaped spot completely overlaps and cannot form the structure that needs to be fabricated, while after shaping a high quality dot structure will be produced.
To address the aforementioned challenges, Innofocus uses spatial light modulation technique to perform phase modulation hence achieving focal spot shaping. TRB is able to achieve the following functions:
Conventional laser fabrication with a single focus is time-consuming and inefficient, but Innofocus®’s unique multi-focal fabrication technology increases fabricating speed and efficiency. The laser is processed in a periodic array structure in the crystal, and the focal intensity at each point can be controlled at will. By adjusting the spatial light modulator, the focal array and the intensity of exposure at each focus can be arbitrarily selected in a single exposure.
1. Multifocal parallel fabrication: Enables to obtain focal spot with hundreds of resolutions (diffraction-limited resolution) in the focusing area and achieve parallel fabrication. It is possible to accelerate processing by up to two orders of magnitude. Also, parallel fabrication can further prevent defects caused by unstable laser or optical components during prolonged processing times, hence improve the quality and uniformity. Furthermore, it enables the laser’s output power to be utilized to its full potential, maximizing its efficiency and lowering its cost of operation.
Figure source from: Han Lin, Baohua Jia & Min Gu. Dynamic generation of Debye diffraction-limited multifocal arrays for direct laser printing nanofabrication. Opt. Lett., 36(3): 406-408 (2011).
2. Aberration correction by phase modulation: the required phase modulation is calculated based on the distribution of the material’s refractive index (including birefringent crystals) and the expected processing depth, to achieve aberration correction. By fully utilizing the material’s 3D volume, it increases the processing depth and improves the uniformity of processing at various depths, to fabricate stronger and finer structures.
3. Conventional laser fabrication with a single focus is time-consuming and inefficient, but Innofocus®’s unique multi-focal fabrication technology increases fabricating speed and efficiency. The laser is processed in a periodic array structure in the crystal, and the focal intensity at each point can be controlled at will. By adjusting the spatial light modulator, the focal array and the intensity of exposure at each focus can be arbitrarily selected in a single exposure.
Focal spot shaping based on the desired focus shape: for instance, circular cross-sections are often necessary for processing optical waveguides. However, conventional focal spot of laser generates elliptic cross-sections which does not help to reduce the transmission loss of the waveguide. Innofocus uses phase (or amplitude) modulation to obtain circular symmetric focus. It improves the symmetry of the processed structures to fabricate optical waveguides with high transmittance and other different structures which require high uniformity.
Figure source from: Benjamin P. Cumming, Sukanta Debbarma, Barry Luther-Davis and Min Gu. Simultaneous compensation for aberration and axial elongation in three-dimensional laser nanofabrication by a high numerical-aperture objective, Opt. Express 21 (16), 19135-19141 (2013); doi: 10.1364/OE.21.019135
Furthermore, by beam shaping Innofocus can also realize switching flexibly between different polarization states (such as a series of continuous modulation from circular polarization, to elliptical polarization, to linear polarization), and can even produce spatially individuals of independent modulation of polarization states, such as radial polarization state and tangential polarization state. Polarization modulation is used to achieve precise control of the light-matter interaction by modifying the polarization distribution of the light field. With the TransBeam module, users can now enjoy unprecedented flexibility to further improve processing efficiency, spatial resolution, and advanced light field modulation.
