Method for testing small light spot produced by optical systems of high NA

We report the method of producing a spatio-temporal (ST) wave packet carrying pure transverse orbital angular momentum (OAM) with subwavelength spatial sizes. Due to the lack of temporal focusing, an ST wave packet focused by a high numerical aperture (NA) objective lens experiences a "spatio-temporal astigmatism" effect similar to the focusing action of a cylindrical lens on the transverse profile of optical field. Thus an ST vortex with a spiral phase in the ST domain focused through a high NA objective will be distorted and lose the ST characteristic spiral phase pattern. With the understanding of such an ST astigmatism, the ST wave packet can be pre-conditioned such that an ST vortex carrying OAM with subwavelength transverse sizes can be obtained after strong focusing. This is the first revelation that a tightly focused ST vortex beam with transverse OAM can be realized, paving the way for potential applications including microscopy, optical trapping, laser machining, nonlinear light-matter interactions, and so on. The ST astigmatism effect also offers insights for the focusing and propagation studies of other types of ST wave packets.

Since a large number of optical systems and devices are based on differently shaped focal intensity distributions (point-spread-functions, PSF), the PSF’s quality is crucial for the application’s performance. E.g., optical tweezers, optical potentials for trapping of ultracold atoms as well as stimulated-emission-depletion (STED) based microscopy and lithography rely on precisely controlled intensity distributions. However, especially in high numerical aperture (NA) systems, such complex laser modes are easily distorted by aberrations leading to performance losses. Although different approaches addressing phase retrieval algorithms have been recently presented[1–3], fast and automated aberration compensation for a broad variety of complex shaped PSFs in high NA systems is still missing. Here, we report on a Gerchberg-Saxton[4] based algorithm (GSA) for automated aberration correction of arbitrary PSFs, especially for high NA systems. Deviations between the desired target intensity distribution and the three-dimensionally (3D) scanned experimental focal intensity distribution are used to calculate a correction phase pattern. The target phase distribution plus the correction pattern are displayed on a phase-only spatial-light-modulator (SLM). Focused by a high NA objective, experimental 3D scans of several intensity distributions allow for characterization of the algorithms performance: aberrations are reliably identified and compensated within less than 10 iterations.

Hybrid-type 3-axis actuator for active tilt compensation in optical pickup

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

The paper is devoted to characterization of topography of micro optical elements with very high numerical aperture using digital holographic microscope. For very high numerical aperture we mean the one larger than numerical aperture of optical system conjugating the object plane with the detector plane. In this case the optical system is not capable of capturing any information about micro element areas with high numerical aperture (high shape gradients). In the paper we are presenting method that can be used for recovering high numerical aperture shape from few measurements with digital holographic microscope working in transmission and reflective configuration. We are focusing on metrology of microlenses of high numerical aperture. Within the presented method measurement of tilted object is necessary. When the element is tilted then some of optical field is coming from "new high gradient element area", when the element is tangential the area of high gradient is producing field with numerical aperture larger that numerical aperture of the measurement optical system. In our paper we therefore use data captured for tilted sample in order to reconstruct micro element topography within a region of high numerical aperture. Such data are then defocused to tilted plane and only then can be used for topography reconstruction.

Based on Richards-Wolf vectorial diffraction integral, the expression of electric cross-spectral density matrix of linearly polarized circular partially coherent (LPCPC) beams, near the focus of a high numerical aperture (NA) objective, are derived. Numerical calculations are performed to show the influence of initial coherent length and NA of the objective, on the intensity distribution and degree of coherence of the LPCPC beams in the focal region. The results indicated that after focusing the LPCPC beams through a high NA objective a super-length optical needle (<12λ) with wavelength beam size can be obtained. Moreover, the numerical calculations illustrate that the length of the optical needle can be controlled by adjusting the initial coherent length of the LPCPC beams. The influence of NA of the objective on the length of the optical needle is much less than the initial coherent length. Setting the reference vector position on the z-axis, we can find that the coherence of the optical needle on the same wavefront remains unchanged.

By the use of the vector diffraction theory, the focusing properties of radially polarized Bessel-like beam with radial cosine wave front phase through a high numerical-aperture (NA) lens are investigated theoretically and numerically in this work. The wave front phase distribution is a cosine function of radial coordinate. Calculation results show that focus shift is considerably influenced by changing frequency parameter C and topological charge. An increase on the focus shift C, the total intensity pattern changes remarkably and it focuses along the optical propagation axis. Thus, the focal shift direction can be adjusted by the change of the frequency parameter in cosine function. In this paper, the generation of the focal spot allows an increase in focal length in the axial direction of the incident beam propagation. Under higher numerical aperture (NA = 0.95), the effect of the frequency parameter and topological charge on the focal pattern gets stronger.

So far, all our studies have been based on the paraxial approximation described in Section 2.4. This approximation holds for an imaging lens (objective) whose numerical aperture is not large, in which case many effects such as apodization, depolarization and aberration can be ignored. When the numerical aperture of a lens becomes approximately larger than 0.7, these effects become pronounced and must be included in the imaging theory. This chapter is devoted to the study of imaging with a high numerical-aperture objective. After the effects of a high numerical-aperture objective are described in Section 6.1, the Debye imaging theory for a high numerical-aperture objective, which has been mentioned in Section 2.3, is presented in Section 6.2. From the Debye theory, apodization, one of the effects related to a high numerical-aperture objective, is studied in detail in Section 6.3. Transfer functions under the Debye theory are derived in Section 6.4. The vectorial diffraction theory for a high numerical-aperture objective in a uniform medium is presented in Section 6.5. The focusing of a high numerical-aperture objective through dielectric interfaces is discussed in Section 6.6.

As minimum feature size shrinks to a metal pitch of 21 nm, the current extreme ultra violet (EUV) lithographic tool with a numeric aperture (NA) of 0.33 will face resolution limit for some critical layers. High NA (0.55) EUV with anamorphic optics or EUV double patterning (DP) at 0.33 NA are being considered for the next generation of lithographic technology. Both the high NA EUV system and EUV DP will enhance resolution relative to current EUV single patterning (SP). Nevertheless, in order to be able to compare EUV DP and High NA EUV processes, important lithographic factors including image contrast, mask three dimension (M3D) effects, process variation band, stochastic effects and local critical dimension uniformity need to be investigated to understand their contributions to process variations. This study was carried out using rigorous lithographic model simulations in Sentaurus Lithography, where strong M3D effects in EUVL are computed physically. We have simulated patterns with both isomorphic and anamorphic optical proximity corrections (OPC) using the rigorous model. The study focuses on 3nm node Via layer designs. These vias need to connect to metal features which have pitches of 21 nm. Simulation results using 0.33 NA SP, 0.33 NA DP, and 0.55 NA anamorphic SP are presented. The benefit of using an alternative mask absorber and a thinner resist as well as the impact of stochastic effects have also been explored. Although a 0.55 NA EUV is expected to produce a superior image to 0.33 NA EUV and to have less impact from overlay errors and stochastic effects, an analysis of process margins of 0.33 NA EUV SD and DP versus 0.55 NA anamorphic systems helps to better understand the benefits, challenges and optimal insertion point for introducing High-NA EUV.

Optical profilometers based on light reflection may fail at surfaces presenting steep slopes and highly curved features. Missed light, interference and diffraction at steps, peaks and valleys are some of the reasons. Consequently, blind areas or profile artifacts may be observed when using common reflection micro-optical profilometers (confocal, scanning interferometers, etc…). The Topographic Optical Profilometry by Absorption in Fluids (TOPAF) essentially avoids these limitations. In this technique an absorbing fluid fills the gap between a reference surface and the surface to profile. By comparing transmission images at two different spectral bands we obtain a reliable topographic map of the surface. In this contribution we develop a model to obtain the profile under micro-optical observation, where high numerical aperture (NA) objectives are mandatory. We present several analytical and experimental results, validating the technique's capabilities for profiling steep slopes and highly curved micro-optical surfaces with nanometric height resolution.

High numerical aperture focusing is becoming increasingly important for nanotechnology related applications. Rigorous, vector evaluation of the focused field, in such cases, is usually performed using the Richards-Wolf method which is based on the Debye approach. The resulting field is known to have a piecewise quasi planar phase. A corresponding result, produced by a Fresnel-Kirchhoff integral for aplanatic optical systems of medium and low numerical apertures, leads to the well known physical fact that a quadratic phase exists when the entrance pupil is not located at the front focal plane. Yet, the amplitudes produced in both ways are in a good agreement. In this work we investigated the difference, presented above, in a 2D system with the help of the Stratton-Chu diffraction integral. The amplitude obtained by the Stratton-Chu integral was quite similar to the classic results while the phase exhibited a quadratic behavior, with the quadratic coefficient depending on the numerical aperture of the optical system. For lower numerical apertures it approached the result obtained by the Fresnel-Kirchhoff integral while for higher numerical apertures it was approaching the Richards-Wolf result. A mathematical expression for the quadratic coefficient was derived and verified for various values of numerical aperture.

We are using a gold nanoparticle coated film to achieve high spatially resolved biosensing that is based on localized surface plasmon resonance (LSPR). This special film possesses the unique optical properties of being not sensitive to the changes of incident angle and relies exclusively on the spectral shift of absorption peak for biosensing. This uniqueness enables it to be compatible with high numerical aperture (NA) optics and to achieve high spatial resolution. We demonstrate a spatial resolution of 25μm assuming a maximum of spectral fluctuation of 0.1nm is acceptable.

Volumetric imaging of dynamic processes with microscopic resolution holds a huge potential in biomedical research and clinical diagnosis. Using supercontinuum light sources and high numerical aperture (NA) objectives, optical coherence tomography (OCT) achieves microscopic resolution and is well suited for imaging cellular and subcellular structures of biological tissues. Currently, the imaging speed of microscopic OCT (mOCT) is limited by the line-scan rate of the spectrometer camera and ranges from 30 to 250 kHz. This is not fast enough for volumetric imaging of dynamic processes in vivo and limits endoscopic application. Using a novel CMOS camera, we demonstrate fast 3-dimensional OCT imaging with 600,000 A-scans/s at 1.8µm axial and 1.1 µm lateral resolution. The improved speed is used for imaging of ciliary motion and particle transport in ex vivo mouse trachea. Furthermore, we demonstrate dynamic contrast OCT by evaluating the recorded volumes rather than en face planes or B-scans. High-speed volumetric mOCT will enable the correction of global tissue motion and is a prerequisite for applying dynamic contrast mOCT in vivo. With further increase in imaging speed and integration in flexible endoscopes, volumetric mOCT may be used to complement or partly replace biopsies.

We demonstrate an all-optical strategy for realizing spherical three-dimensional (3D) super-resolution (∼λ3/22) spot arrays of pure longitudinal magnetization by exploiting a 4π optical microscopic setup with two high numerical aperture (NA) objective lenses, which focus and interfere two modulated vectorial beams. Multiple phase filters (MPFs) are designed via an analytical approach derived from the vectorial Debye diffraction theory to modulate the two circularly polarized beams. The system is tailored to constructively interfere the longitudinal magnetization components, while simultaneously destructively interfering the azimuthal ones. As a result, the magnetization field is not only purely longitudinal but also super-resolved in all three dimensions. Furthermore, the MPFs can be designed analytically to control the number and locations of the super-resolved magnetization spots to produce both uniform and nonuniform arrays in a 3D volume. Thus, an all-optical control of all the properties of light-induced magnetization spot arrays has been demonstrated for the first time. These results open up broad applications in magnetic-optical devices such as confocal and multifocal magnetic resonance microscopy, 3D ultrahigh-density magneto-optic memory, and light-induced magneto-lithography.

In this paper, we report on the low energy-density recording with a high-repetition-rate femtosecond pulsed beam in homogenous gold-nanorod-dispersed discs by using low numerical aperture (NA) micro-optics. By focusing a femtosecond pulsed beam at a repetition rate of 82 MHz using a low NA DVD optical head, the spatially-stretched energy density introduces a temperature rising of the polymer matrix. This temperature rising facilitates the surface melting of gold nanorods, which leads to over one-order-of-magnitude reduction in the energy-density threshold for recording, compared with that by focusing single pulses through a high NA objective. Applying this finding, we demonstrate the dual-layer recording in gold-nanorod-dispersed discs with an equivalent capacity of 69 GB. Our results demonstrate the potential of ultra-high density three-dimensional optical memory with a low-cost and DVD-compatible apparatus.