Low Numerical Aperture Objective Lens Research Articles

Multi‐Dimensional Shingled Optical Recording by Nanostructuring in Glass

AbstractIn the digital era, the need for high‐density data storage techniques has become increasingly imperative. To address this, the study has demonstrated a multi‐dimensional shingled optical recording technique, utilizing femtosecond laser‐induced nanostructures in silica glass. The evolution of the bulk nanostructures is investigated on a pulse‐by‐pulse basis using multiple microscopic analysis techniques. The formation of the nanostructures is attributed to a self‐adaptive near‐field anisotropic nanostructuring process. Furthermore, it is shown that writing in a 3D shingled configuration significantly reduces the volume per data voxel to a size of 500 nm × 500 nm × 1.0 µm, surpassing the diffraction limit. This reduction is achieved even when employing an infrared laser and a relatively low numerical aperture objective lens. As a result, the approach increases the data storage capacity by at least two orders of magnitude compared to conventional techniques. The work paves the way for advanced shingled optical recording techniques offering ultra‐dense capacity, ultra‐long lifetime, and low energy consumption.

Fabrication of microneedles using two photon-polymerization with low numerical aperture

Three-dimensional (3D) micro-nano fabrication through two-photon polymerization (TPP) is a prominent approach for fabricating complex 3D structures. However, the existing techniques face limitations, such as small fabrication area, dependence on high numerical aperture (e.g., oil immersion) objective lenses, which are more expensive. We developed a 780 nm fs laser system integrated with a 2D galvanometer system and Z axis to create a 3D micro-nano printing system. A method to print arbitrarily complex 3D structures while maintaining an extremely high lateral spatial resolution of sub-100nm is proposed by using a cheap low numerical aperture objective lens with the laser power and exposure time precisely controlled. The impact of the laser power and exposure time on the resolution of 3D micro-nano processing is investigated, and we identify the optimal processing environment for achieving the best resolution. The printed linewidth of 62 nm was achieved without the use of an oil immersed objective lens, and only one laser beam was used instead of stimulated emission depletion. Microneedle arrays for animal injection were fabricated using this strategy, which can improve the performance of animal organoid repair. Our results suggest that TPP micro-nano fabrication is an effective method for processing microneedle arrays for biomedical engineering, with the ability to improve the processing resolution with a cheap low numerical aperture objectives lens (e.g., NA = 0.4) through meticulous control of laser power and exposure time. This approach opens up possibilities for affordable and scalable fabrication in sub-100nm structures.

Fabrication of Plano-convex Microlenses using Two-Photon Polymerization for Bioimaging with Non-Linear Excitation Microscopy

A recent challenge in bioimaging is the observation and imaging of vital, thick, and complex tissues in real time and in non-invasive mode. In the last decade, non-linear excitation microscopy showed several advantages for in-vivo imaging compared to conventional confocal techniques. Nevertheless, deep tissue imaging remains challenging, especially for thick media, due to spherical aberrations induced on focused beams by the tissue. A low numerical aperture objective lens coupled to high dioptric power microlenses, implanted in the tissue, can be beneficial for the reduction of optical aberrations. In this context, we fabricated a system of plano-convex microlenses and microscaffolds on a single chip by means of two-photon polymerization), to be used for non-linear imaging of biological specimens.

Precise Brightfield Localization Alignment for Fourier Ptychographic Microscopy

Fourier ptychographic microscopy (FPM) is a recently developed microscope technology that overcomes the resolution limit of a low numerical aperture objective lens by employing angular varying illuminations. Combining the concepts of ptychography, synthetic aperture, and phase retrieval, FPM achieves high-resolution, wide-field, and quantitative phase imaging at the same time. In typical FPM systems, the angular varying illuminations are achieved with LED arrays whose positional misalignments bring significant errors in the reconstruction procedure. In previous studies, several LED array alignment methods are developed, which iteratively recover the positional misalignment parameters during the reconstruction. These methods consume additional calculations in FPM reconstruction and may not be practical in other microscopy system. In this work, we represent a preprocessing LED array alignment method by accurately localizing the brightfield area on the sample plane. By applying particle swarm optimization method and random sample consensus method, the global misalignment parameters can be estimated with high accuracy and speed. Both numerical simulations and actual system experiments are carried out to evaluate the effectiveness of our method and the results show that the reconstruction quality of high-resolution images is significantly improved by using our method.

13-fold resolution gain through turbid layer via translated unknown speckle illumination.

Fluorescence imaging through a turbid layer holds great promise for various biophotonics applications. Conventional wavefront shaping techniques aim to create and scan a focus spot through the turbid layer. Finding the correct input wavefront without direct access to the target plane remains a critical challenge. In this paper, we explore a new strategy for imaging through turbid layer with a large field of view. In our setup, a fluorescence sample is sandwiched between two turbid layers. Instead of generating one focus spot via wavefront shaping, we use an unshaped beam to illuminate the turbid layer and generate an unknown speckle pattern at the target plane over a wide field of view. By tilting the input wavefront, we raster scan the unknown speckle pattern via the memory effect and capture the corresponding low-resolution fluorescence images through the turbid layer. Different from the wavefront-shaping-based single-spot scanning, the proposed approach employs many spots (i.e., speckles) in parallel for extending the field of view. Based on all captured images, we jointly recover the fluorescence object, the unknown optical transfer function of the turbid layer, the translated step size, and the unknown speckle pattern. Without direct access to the object plane or knowledge of the turbid layer, we demonstrate a 13-fold resolution gain through the turbid layer using the reported strategy. We also demonstrate the use of this technique to improve the resolution of a low numerical aperture objective lens allowing to obtain both large field of view and high resolution at the same time. The reported method provides insight for developing new fluorescence imaging platforms and may find applications in deep-tissue imaging.

Large refractive index changes of a chemically amplified photoresist in femtosecond laser nonlinear lithography

We found that marked increases in refractive index of chemically amplified photoresists induced by highly repetitive femtosecond laser irradiation without post-exposure baking treatment. For laser writing speed less than 30 μm/s, the refractive index change of the nonlinear absorption region was as large as 8 × 10(-3). Moreover, cross-linking reactions of the resists were induced. The refractive index changes can generate optical confinement and subsequent channel propagations of femtosecond laser pulses. The coupling efficiency was estimated as high as 87% using a low numerical aperture objective lens. The peak intensities of the guiding modes exceeded the polymerization threshold of the resist.

Optical trapping and manipulation of live T cells with a low numerical aperture lens

An optical manipulation system that employs both optical and temperature gradients to simultaneously enable trapping, manipulating and imaging of live cells with a low magnification, low numerical aperture objective lens (10x/0.4 N.A.) is reported. This approach negates the requirement for a high N.A. lens used in traditional optical trapping. Our system comprised a dual scanning system and two independent lasers which provided the ability to move the trapping spot independently of the confocal imaging process in close to real-time and without pre-programming. To demonstrate the efficacy of this innovative method, trapping and manipulation of live T cells was simultaneously performed over a field of view exceeding 1 mm(2) for extended periods without compromising cell viability.

Structured illumination for the extension of imaging interferometric microscopy

Structured illumination applied to imaging interferometric microscopy (IIM) allows extension of the resolution limit of low numerical aperture objective lenses to ultimate linear systems limits (<approximately lambda/4 in air) without requiring a reference beam around the objective lens. Instead, the reference beam is provided by an illumination beam just at the edge of the optical system numerical aperture resulting in a shift of the recorded spatial frequencies (equivalent to an intermediate frequency). The restoration procedure is discussed. This technique is adaptable readily to existing microscopes, since extensive access to the imaging system pupil plane is not required.

Fluorescence coherence tomography

In this paper, we introduce a new form of cross-sectional, coherence-gated fluorescence imaging, which we term 'spectral-domain fluorescence coherence tomography' (SD-FCT). SD-FCT is accomplished by spectrally detecting self-interference of the spontaneous emission of fluorophores located along the axial (depth) dimension of the sample. We have built a first generation SD-FCT system that utilizes two opposing low numerical-aperture objective lenses in an interferometer and an imaging spectrometer for detecting self-interference of fluorescence emitted from a sample. Here, in proof-of-principle experiments we demonstrate cross-sectional profiling of layered fluorescence phantoms. Narrow (a few micrometers FWHM) axial point-spread functions, large ranging depths (a few hundreds of micrometers) and wide fields of view (>1 mm) were measured. Initial results suggest that SD-FCT may be a viable tool for the investigation of semi-transparent and selectively labeled fluorescent samples.