Aplanatic Solid Immersion Lens Research Articles

Integrated plasmon-enhanced Raman scattering (iPERS) spectroscopy

A new strategy named integrated plasmon-enhanced Raman scattering (iPERS) spectroscopy that features a configuration of evanescent field excitation and inverted collection is presented, which well unites the local field enhancement and far field emission, couples localized and propagating surface plasmons, integrates the SERS substrates and Raman spectrometers via a self-designed aplanatic solid immersion lens. A metallic nanoparticle-on-a film (NOF) system was adopted in this configuration because it improves the amplification of the incidence light field in near field by 10 orders of magnitude due to the simultaneous excitation of quadrupolar and dipolar resonance modes. This iPERS allows for higher excitation efficiency to probed molecules and full collection of the directional-radiation Raman scattering signal in an inverted way, which exhibits a practical possibility to monitor plasmonic photocatalytic reactions in nanoscale and a bright future on interfacial reaction studies.

Feature-based filter design for resolution enhancement of known features in microscopy.

We present a new feature-based design concept for filter design in which a filter is designed specifically to image a known feature with dimensions lower than the optical resolution of the system. Unlike the conventional filter design based on focal spot engineering, we use the complete response of the microscope to form a resolution factor and consider minimizing the resolution factor as the design goal. We consider three design goals (i.e., resolution factors) based on the system response and show that a feature matching approach is more suitable in practice. It is shown that a three-bar pattern and grid of square dots of critical dimension, 0.08λ, can be resolved using a simple two-layer feature-based filter designed for radially polarized illumination in aplanatic solid immersion lens microscopy. An example that requires a more complex filter is also shown. Applicability of the concept of feature-based filter design in diverse microscopy scenarios is discussed as well.

Effect of vector asymmetry of radially polarized beams in solid immersion microscopy

We theoretically and experimentally investigate the effect of imperfect vector symmetry on radially polarized beams focused by an aplanatic solid immersion lens at a numerical aperture of 3.3. We experimentally achieve circularly symmetric focused spot with a full-width-half-maximum of ~λ0/5.7 at λ0 = 1,310 nm, free-space wavelength. The tight spatial confinement and overall circular symmetry of the focused radially polarized beam are found to be sensitive to perturbations of its cylindrical polarization symmetry. The addition of a liquid crystal based variable retarder to the optical path can effectively ensure the vector symmetry and achieve circularly symmetric focused spots at such high numerical aperture conditions.

Aberration compensation in aplanatic solid immersion lens microscopy

The imaging quality of an aplanatic SIL microscope is shown to be significantly degraded by aberrations, especially when the samples have thicknesses that are more than a few micrometers thicker or thinner than the design thickness. Aberration due to the sample thickness error is modeled and compared with measurements obtained in a high numerical aperture (NA ~3.5) microscope. A technique to recover near-ideal imaging quality by compensating aberrations using a MEMS deformable mirror is described and demonstrated.

Imaging using cylindrical vector beams in a high-numerical-aperture microscopy system

Imaging of object structures using cylindrical vector beams in an aplanatic solid immersion lens (SIL) microscope is investigated. Based on a complete optical model of an aplanatic SIL microscope, images of some object structures using radial polarization, azimuthal polarization, and azimuthal vortex beams are simulated. Some interesting imaging effects of these object structures are observed. For example, counterintuitively, it is found that, compared to linear and circular polarizations, radial polarization requires a larger pinhole to acquire a good image and resolution. Similarly, it is shown that an azimuthal vortex beam provides good images for a variety of object structures and pinhole sizes. Theories and explanations are provided to justify the observed effects. The presented results play an important role in high-numerical-aperture optical imaging.

A complete and computationally efficient numerical model of aplanatic solid immersion lens scanning microscope

This paper presents a computational model for modeling an aplanatic solid immersion lens scanning microscope. The scanning microscope model consists of three subsystems, each of which can be computed as a separate system, connected to the preceding or succeeding subsystem through the input/output only. Numerical techniques are used to enhance the computational efficiency of each subsystem. A distinct merit of the proposed model is that it can be used to simulate imaging results for diverse setups of the scanning microscope, like various polarizations, numerical aperture, and different detector pinhole sizes. It allows the study and analysis of both theoretical aspects like achievable resolution, and practical aspects like expected images for different object patterns and experimental setups. Further, due to its computational efficiency, diverse large scale structures can be easily simulated in scanning microscope and good experimental approaches determined before indulging into the time consuming and costly process of experimentation.

Complete modeling of subsurface microscopy system based on aplanatic solid immersion lens

A general model of a subsurface microscopy system based on an aplanatic solid immersion lens (ASIL) is presented. This model is composed of three components: generation of incident light into the ASIL, interaction of the incident light with the sample, and imaging of the scattered light. Interaction of incident light with sample can be calculated numerically using electromagnetic scattering theory, while vector diffraction theory is used to treat the other two components. Examples of imaging small and extended scatterers are shown. For small scatterers, we show the differences between the actual resolution of the whole system and the resolution predicted by considering only one subsystem of the whole system. For extended scatterers, two types of illuminations-focusing light illumination and plane wave direct illumination-are used to image the scatterers, and observations are explained using interaction of the incident light with the sample.

Spherical aberration correction in aplanatic solid immersion lens imaging using a MEMS deformable mirror

Aplanatic solid immersion lens (SIL) microscopy is required to achieve the highest possible resolution for next generation silicon IC backside inspection and failure analysis. However, aplanatic SILs are very susceptible to spherical aberrations introduced by substrate thickness mismatch. We correct this aberration using a MEMS deformable mirror. Good agreement between theory and experiment is achieved and spot intensity increases by a factor of two to three are demonstrated.

Resolution of aplanatic solid immersion lens based microscopy

We study the resolution of a subsurface microscopy system based on the use of an aplanatic solid immersion lens. Resolution limits under various criteria are calculated theoretically as well as numerically. Images of combinations of dipoles of various orientations are considered. Both lateral and longitudinal resolutions are studied. The theoretical criteria are compared against the visually resolvable simulated images of the dipoles. The observations are explained explicitly through a detailed analysis of the dyadic Green's function. A new resolution criterion is also proposed, which provides a very accurate estimate of the resolution limits.

High numerical aperture hybrid optics for two-photon polymerization

We report on an immersion hybrid optics specially designed for focusing ultrashort laser pulses into a polymer for direct laser writing via two-photon polymerization. The hybrid optics allows for well-corrected focusing over a large working distance range of 577 μm with a numerical aperture (NA) of 1.33 and low internal dispersion. We combine the concepts of an aplanatic solid immersion lens (ASIL) for achieving a high NA with a diffractive optical element (DOE) for correction of aberrations. To demonstrate the improvements for volume structuring of the polymer, we compare the achievable structure sizes of our optics with a commercially available oil-immersion objective (100x, NA=1.4).

Highly confined surface imaging by solid immersion total internal reflection fluorescence microscopy

We report the use of a high-refractive-index aplanatic solid immersion lens (ASIL) in total internal reflection fluorescence (TIRF) microscopy. This new solid immersion total internal reflection fluorescence (SITIRF) microscopy allows highly confined surface imaging with a significantly reduced imaging depth compared with conventional TIRF microscopy. We explore the application of a high refractive index, low optical dispersion material zirconium dioxide in the SITIRF microscope and also introduce a novel system design which enables the SITIRF microscope to work either in the epi-fluorescence or TIRF modes with variable illumination angles. We use both synthetic and biological samples to demonstrate that the imaging depth in the SITIRF microscope can be confined to a few tens of nanometers. SITIRF microscopy has the advantages of performing highly selective imaging and high-resolution high-contrast imaging. Potential applications in biological imaging and future developments of SITIRF microscopy are proposed.

Dyadic Green’s function for aplanatic solid immersion lens based sub-surface microscopy

We present the derivation of the dyadic Green's function for the aplanatic solid immersion lens based microscopy system. The presented dyadic Green's function is general and is applicable at non-aplanatic points as well in the object plane. Thus, the electromagnetic wave formulation is used to describe the optical system without paraxial assumptions. Various important and useful properties of SIL based microscopy system are also presented. The effect of the numerical aperture of the objective on the peak intensities, resolutions and the depth of field are also reported. Some interesting longitudinal effects are also reported.

Chromatic and spherical aberration correction for silicon aplanatic solid immersion lens for fault isolation and photon emission microscopy of integrated circuits

Current state-of-the-art in backside fault isolation and logic analysis utilizes solid immersion lens (SIL) imaging in the central configuration. An attractive advancement is the development and integration of an aplanatic SIL, which allows significant improvement in resolution, signal acquisition and isolation capabilities, especially for the 22nm node and beyond. However, aplanatic SIL configurations introduce both chromatic and spherical aberrations. We have developed backing objective designs capable of correcting for chromatic aberrations allowing application in photon emission microscopy, as well as deformable mirror designs and experiments that eliminate spherical aberrations of aplanatic SILs to account for variations in substrate thickness and off-axis imaging.

Enhancing near-field photolithography by surface plasmon polaritons excited from negative-refractive-index nanofilms

We report a novel method for enhancing near-field nanophotolithography using negative-refractive-index slabs. In our method, a negative-refractive-index-metal film (NIF) is attached to the plane surface of an aplanatic solid immersion lens (SIL), and the polymer layer is coated on the negative-refractive-index film. The intensity of the transmitted light is enhanced by the surface plasmon polaritons excited in the NIF-polymer composite layer. The numerical results based on the vector diffraction theory show that under the illumination of the SIL-NIF-polymer composite by a sharp focused radially polarized beam, the composite configuration under investigation can substantially increase the spot intensity, suppress the spot expansion along the distance from the interface, and improve the depth of focus, in comparison with conventional focusing systems using bare SILs. In the case where a photoresist film is placed near the composite configuration, a subtle deep-etching pattern with high resolution can be achieved.

Wide-field high-resolution solid immersion fluorescence microscopy applying an aplanatic solid immersion lens

The use of aplanatic solid immersion lenses (ASILs) made of high refractive index optical materials provides a route to wide-field high-resolution optical microscopy. We analyze the aberrations that need to be circumvented for wide-field, high-resolution imaging by hybrid optical simulations based on both ray and wave optics. The conclusions on the relationships among ASIL size, fluorescence imaging spectral width, and field of view not only guide us to design the microscopic system demonstrated in this article, but also clearly demonstrate the general design considerations necessary when applying an ASIL in fluorescence microscopy. Based on the simulation results, we develop wide-field high-resolution solid immersion fluorescence (SIF) microscopy employing an ASIL with an effective numerical aperture (NA) of 1.85. We demonstrate wide-field, high-resolution imaging of synthetic and biological samples with our SIF system. In the presence of a gap between the ASIL and the sample, we experimentally demonstrate that the effective NA of the SIF system is determined by the refractive index of the gap medium in addition to that of the ASIL material; thus, in general, degrading the resolution. Future developments of the SIF system to suit routine use and make it achieve still higher resolution by combining SIF and other microscopic techniques are proposed.

Imaging performance of widefield solid immersion lens microscopy

We investigate the performance of a widefield imaging system employing an aplanatic solid immersion lens. Off-axis imaging quality is examined theoretically at different radii and thicknesses of the aplanatic solid immersion lens. It is found that field curvature is the major aberration affecting the imaging quality. Aberrations are measured experimentally, and the results are in very good agreement with those obtained from simulations and demonstrate the situations where high quality images can be obtained with the aplanatic solid immersion lens.

High-density near-field optical recording with a solid immersion lens, conventional actuator, and a robust air gap servo

We present a near-field optical recording system with a solid immersion lens (SIL) in a conventional biaxial actuator. We manufactured a super-hemispherical aplanatic SIL with a numerical aperture (NA) of NA = 1.9. Our system utilizes a gap error signal (GES) and a digital servo system to drive a conventional actuator in order to maintain a constant air gap of 25 nm between the SIL and a spinning optical disc.

Wide-field surface plasmon microscopy with solid immersion excitation

This letter shows how an aplanatic solid immersion lens system may be used for high-resolution wide-field surface plasmon microscopy. The lens configuration means that no fluid couplant is required; it has the additional advantage that very high refractive indices may be used to reduce the incident angle at which surface plasmons are excited. A transmission spatial light modulator is incorporated conjugate with the back focal plane; this allows the image contrast to be controlled in a precise and flexible manner. Its use also provides definitive demonstration that the image contrast arises from surface plasmon excitation.