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Principles Of Optics Electromagnetic Theory Of Propagation Interference And Diffraction Of Light

Two-photon excitation provides a means of activating chemical or physical processes with high spatial resolution in three dimensions and has made possible the development of three-dimensional fluorescence imaging, optical data storage, and lithographic microfabrication. These applications take advantage of the fact that the two-photon absorption probability depends quadratically on intensity, so under tight-focusing conditions, the absorption is confined at the focus to a volume of order λ3 (where λ is the laser wavelength). Any subsequent process, such as fluorescence or a photoinduced chemical reaction, is also localized in this small volume. Although three-dimensional data storage and microfabrication have been illustrated using two-photon-initiated polymerization of resins incorporating conventional ultraviolet-absorbing initiators, such photopolymer systems exhibit low photosensitivity as the initiators have small two-photon absorption cross-sections (δ). Consequently, this approach requires high laser power, and its widespread use remains impractical. Here we report on a class of π;-conjugated compounds that exhibit large δ (as high as 1, 250 × 10−50 cm4 s per photon) and enhanced two-photon sensitivity relative to ultraviolet initiators. Two-photon excitable resins based on these new initiators have been developed and used to demonstrate a scheme for three-dimensional data storage which permits fluorescent and refractive read-out, and the fabrication of three-dimensional micro-optical and micromechanical structures, including photonic-bandgap-type structures.

Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication

In this progress report the development of three-dimensional μ-printing and its impact as an enabling technology onto different scientific fields is reviewed. Driven by direct laser writing via two-photon absorption, the technology has reached a level of maturity and ease of application such that 3D printing on the micrometer scale can now be considered. While the underlying technology is still developing towards higher resolution and increasing speed of fabrication, the last five years have seen new fields rising that were obviously enabled by 3D μ-printing. Among the recent technological developments discussed in this progress report are the fields of super-resolution lithography and spatial light modulator-based lithography. Novel fields relying on these technological advances like aperiodic photonic structures, mechanical metamaterials, and structures for biological studies will be reviewed. A glimpse into developing research directions will also be provided.

Three-Dimensional μ-Printing: An Enabling Technology

We introduce optically clear and resilient free-form micro-optical components of pure (non-photosensitized) organic-inorganic SZ2080 material made by femtosecond 3D laser lithography (3DLL). This is advantageous for rapid printing of 3D micro-/nano-optics, including their integration directly onto optical fibers. A systematic study of the fabrication peculiarities and quality of resultant structures is performed. Comparison of microlens resiliency to continuous wave (CW) and femtosecond pulsed exposure is determined. Experimental results prove that pure SZ2080 is ∼20 fold more resistant to high irradiance as compared with standard lithographic material (SU8) and can sustain up to 1.91 GW/cm2 intensity. 3DLL is a promising manufacturing approach for high-intensity micro-optics for emerging ﬁelds in astro-photonics and atto-second pulse generation. Additionally, pyrolysis is employed to homogeneously shrink structures up to 40% by removing organic SZ2080 constituents. This opens a promising route towards downscaling photonic lattices and the creation of mechanically robust glass-ceramic microstructures.

Optically Clear and Resilient Free-Form µ-Optics 3D-Printed via Ultrafast Laser Lithography

3D meso scale structures that can reach up to centimeters in overall size but retain micro- or nano-features, proved to be promising in various science fields ranging from micro-mechanical metamaterials to photonics and bio-medical scaffolds. In this work, we present synchronization of the linear and galvanometric scanners for efficient femtosecond 3D optical printing of objects at the meso-scale (from sub-μm to sub-cm spanning five orders of magnitude). In such configuration, the linear stages provide stitch-free structuring at nearly limitless (up to tens-of-cm) working area, while galvo-scanners allow to achieve translation velocities in the range of mm/s-cm/s without sacrificing nano-scale positioning accuracy and preserving the undistorted shape of the final print. The principle behind this approach is demonstrated, proving its inherent advantages in comparison to separate use of only linear stages or scanners. The printing rate is calculated in terms of voxels/s, showcasing the capability to maintain an optimal feature size while increasing throughput. Full capabilities of this approach are demonstrated by fabricating structures that reach millimeters in size but still retain sub-μm features: scaffolds for cell growth, microlenses, and photonic crystals. All this is combined into a benchmark structure: a meso-butterfly. Provided results show that synchronization of two scan modes is crucial for the end goal of industrial-scale implementation of this technology and makes the laser printing well aligned with similar approaches in nanofabrication by electron and ion beams.

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Mesoscale laser 3D printing

We demonstrate a new 3D fabrication method to achieve the same results as those obtained by the two-photon excitation technique, by using a simple one-photon elaboration method in a very low absorption regime. Desirable 2D and 3D submicrometric structures, such as spiral, chiral, and woodpile architectures, with feature size as small as 190 nm have been fabricated, by using just a few milliwatts of a continuous-wave laser at 532 nm and a commercial SU8 photoresist. Different aspects of the direct laser writing based on ultralow one-photon absorption (LOPA) technique are investigated and compared with the TPA technique, showing several advantages, such as simplicity and low cost.

Submicrometer 3D structures fabrication enabled by one-photon absorption direct laser writing

The small correction volume for conventional wavefront shaping methods limits their application in biological imaging through scattering media. We demonstrate large volume wavefront shaping through a scattering layer with a single correction by conjugate adaptive optics and remote focusing (CAORF). The remote focusing module can maintain the conjugation between the adaptive optical (AO) element and the scattering layer during three-dimensional scanning. This new configuration provides a wider correction volume by better utilization of the memory effect in a fast three-dimensional laser scanning microscope. Our results show that the proposed system can provide 10 times wider axial field of view compared with a conventional conjugate AO system when 16,384 segments are used on a spatial light modulator. We also demonstrate three-dimensional fluorescence imaging, multi-spot patterning through a scattering layer and two-photon imaging through mouse skull tissue.

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Three-dimensional focusing through scattering media using conjugate adaptive optics with remote focusing (CAORF)

With respect to experimental condition, we have investigated the point spread function of a high numerical aperture objective lens, taking into account the absorption effect of the studied material. By using a material possessing an ultralow one-photon absorption (LOPA) coefficient at the excitation wavelength, the light beam can penetrate deeply inside the material and be tightly focused into a subwavelength spot, almost the same as in the absence of material. Combining tight focusing and ultralow absorption conditions, we show that LOPA-based microscopy is thus capable of three-dimensional imaging and fabrication with long penetration depth up to 300 μm. As compared to the commonly used two-photon absorption microscope, the LOPA method allows simplification of the experimental setup and also minimization of the photodamaging or bleaching effect of materials.

Concept for three-dimensional optical addressing by ultralow one-photon absorption method

A woofer–tweeter adaptive optical structured illumination microscope (AOSIM) is presented. By combining a low-spatial-frequency large-stroke deformable mirror (woofer) with a high-spatial-frequency low-stroke deformable mirror (tweeter), we are able to remove both large-amplitude and high-order aberrations. In addition, using the structured illumination method, as compared to widefield microscopy, the AOSIM can accomplish high-resolution imaging and possesses better sectioning capability. The AOSIM was tested by correcting a large aberration from a trial lens in the conjugate plane of the microscope objective aperture. The experimental results show that the AOSIM has a point spread function with an FWHM that is 140 nm wide (using a water immersion objective lens with NA=1.1) after correcting a large aberration (5.9 μm peak-to-valley wavefront error with 2.05 μm RMS aberration). After structured light illumination is applied, the results show that we are able to resolve two beads that are separated by 145 nm, 1.62× below the diffraction limit of 235 nm. Furthermore, we demonstrate the application of the AOSIM in the field of bioimaging. The sample under investigation was a green-fluorescent-protein-labeled Drosophila embryo. The aberrations from the refractive index mismatch between the microscope objective, the immersion fluid, the cover slip, and the sample itself are well corrected. Using AOSIM we were able to increase the SNR for our Drosophila embryo sample by 5×.

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Woofer-tweeter adaptive optical structured illumination microscopy

We demonstrated theoretically and experimentally the influence of polarization of an input light beam on the intensity and polarization distributions of a tight focusing spot, obtained by a high numerical aperture (NA) objective lens (OL). The numerical calculations, based on vectorial Debye theory, show that, under tight focusing condition (NA = 1.4), the shape of the focusing spot varies as a function of the input beam polarization, resulting in either asymmetric or symmetric spots. A home made confocal microscope constituted by a high NA OL is employed to scan the fluorescence image of single gold nanoparticles showing the actual polarization dependence of focusing spot. The input linear polarization allows generating a small but asymmetric focusing spot, while the circular polarization produces a larger but symmetric one.

Qinggele Li

Papers

Submicrometer 3D structures fabrication enabled by one-photon absorption direct laser writing

Three-dimensional focusing through scattering media using conjugate adaptive optics with remote focusing (CAORF)

Concept for three-dimensional optical addressing by ultralow one-photon absorption method

Woofer-tweeter adaptive optical structured illumination microscopy

Influence of incident beam polarization on intensity and polarization distributions of tight focusing spot