Evaluation of bone growth around bioactive glass S53P4 by scanning acoustic microscopy co-registered with optical interferometry and elemental analysis.

Bioactive glass (BAG) is a bone substitute that can be used in orthopaedic surgery. Following implantation, the BAG is expected to be replaced by bone via bone growth and gradual degradation of the BAG. However, the hydroxyapatite mineral forming on BAG resembles bone mineral, not providing sufficient contrast to distinguish the two in X-ray images. In this study, we co-registered coded-excitation scanning acoustic microscopy (CESAM), scanning white light interferometry (SWLI), and scanning electron microscopy with elemental analysis (Energy Dispersive X-ray Spectroscopy) (SEM-EDX) to investigate the bone growth and BAG reactions on a micron scale in a rabbit bone ex vivo. The acoustic impedance map recorded by the CESAM provides high elasticity-associated contrast to study materials and their combinations, while simultaneously producing a topography map of the sample. The acoustic impedance map correlated with the elemental analysis from SEM-EDX. SWLI also produces a topography map, but with higher resolution than CESAM. The two topography maps (CESAM and SWLI) were in good agreement. Furthermore, using information from both maps simultaneously produced by the CESAM (acoustic impedance and topography) allowed determining regions-of-interest related to bone formation around the BAG with greater ease than from either map alone. CESAM is therefore a promising tool for evaluating the degradation of bone substitutes and the bone healing process ex vivo.

Subsurface metrology using scanning white light interferometry: absolute z coordinates deep inside displays.

Mobile devices with interactive displays are ubiquitous commodities. Efficient quality control (QC) drives competitiveness. Scanning white light interferometry imaging offers a fast and nondestructive tool for QC purposes. Relying on optical compensation and image stitching, one can rapidly and cost-effectively produce sharp 3D images of a display's inner structures with a few nanometers' accuracy along the z direction. As a practical example, 3D images of a mobile device display revealed 0.92±0.02 µm height variation in the top glass assembly. The proposed method improves quality assurance methods of display manufacturing.

Microsphere-assisted phase-shifting profilometry.

In the present work, we have investigated the combination of a superresolution microsphere-assisted 2D imaging technique with low-coherence phase-shifting interference microscopy. The imaging performance of this technique is studied by numerical simulation in terms of the magnification and the lateral resolution as a function of the geometrical and optical parameters. The results of simulations are compared with the experimental measurements of reference gratings using a Linnik interference configuration. Additional measurements are also shown on nanostructures. An improvement by a factor of 4.7 in the lateral resolution is demonstrated in air, thus giving a more isotropic nanometric resolution for full-field surface profilometry in the far field.

System spectrum conversion from white light interferogram.

Capability to simulate the coherence function is important when tuning an interference microscope in an effort to reduce sidelobes in interference signals. The coherence function cannot directly be derived from the light source spectrum since the microscope's effective spectrum is affected by e.g. spatial coherence effects. We show this by comparing the true system spectrum measured using a spectrometer against the effective system spectrum obtained by Fourier analysis of the interference data. The results show that a modulation function that describes the scattering-induced spatial coherence dampening in the system is needed to correct the observed difference between these two spectra. The validity of this modulation function is further verified by quantifying the arithmetic mean roughness of two specified roughness standards. By providing a spectral transfer function for scattering, our method can simulate a sample specific coherence function, and thus shows promise to increase the quality of interference microscope images.

3D Super-Resolution Optical Profiling Using Microsphere Enhanced Mirau Interferometry.

We present quantitative three dimensional images of grooves on a writable Blu-ray Disc based on a single objective Mirau type interferometric microscope, enhanced with a microsphere which is considered as a photonic nanojet source. Along the optical axis the resolution of this microsphere assisted interferometry system is a few nanometers while the lateral resolution is around 112 nm. To understand the physical phenomena involved in this kind of imaging we have modelled the interaction between the photonic jet and the complex disc surface. Agreement between simulation and experimental results is demonstrated. We underline that although the ability of the microsphere to generate a photonic nanojet does not alone explain the resolution of the interferometer, the nanojet can be used to try to understand the imaging process. To partly explain the lateral super-resolution, the potential role of coherence is illustrated. The presented modality may have a large impact on many fields from bio-medicine to nanotechnology.

Nanometer depth resolution in 3D topographic analysis of drug-loaded nanofibrous mats without sample preparation.

We showed that scanning white light interferometry (SWLI) can provide nanometer depth resolution in 3D topographic analysis of electrospun drug-loaded nanofibrous mats without sample preparation. The method permits rapidly investigating geometric properties (e.g. fiber diameter, orientation and morphology) and surface topography of drug-loaded nanofibers and nanomats. Electrospun nanofibers of a model drug, piroxicam (PRX), and hydroxypropyl methylcellulose (HPMC) were imaged. Scanning electron microscopy (SEM) served as a reference method. SWLI 3D images featuring 29 nm by 29 nm active pixel size were obtained of a 55 µm × 40 µm area. The thickness of the drug-loaded non-woven nanomats was uniform, ranging from 2.0 µm to 3.0 µm (SWLI), and independent of the ratio between HPMC and PRX. The average diameters (n=100, SEM) for drug-loaded nanofibers were 387 ± 125 nm (HPMC and PRX 1:1), 407 ± 144 nm (HPMC and PRX 1:2), and 290 ± 100 nm (HPMC and PRX 1:4). We found advantages and limitations in both techniques. SWLI permits rapid non-contacting and non-destructive characterization of layer orientation, layer thickness, porosity, and surface morphology of electrospun drug-loaded nanofibers and nanomats. Such analysis is important because the surface topography affects the performance of nanomats in pharmaceutical and biomedical applications.

High-speed stroboscopic imaging with frequency-doubled supercontinuum.

We present a supercontinuum (SC) light source designed for stroboscopic white light interferometry. The compact, cost-effective SC source is built from off-the-shelf optical components and operates both in the visible and near-IR at arbitrary repetition rates in the 10 kHz-1 MHz frequency range. We estimate that our source allows performing dynamic white-light interferometric characterization of rapidly oscillating objects up to several tens of megahertz. Its current potential is demonstrated by capturing the movement of a microelectromechanical system oscillating at 2.16 MHz with sub-100 nm accuracy.

Stroboscopic scanning white light interferometry at 2.7 MHz with 1.6 µm coherence length using a non-phosphor LED source.

Stroboscopic scanning white light interferometry (SSWLI) allows precise three dimensional (3D) measurements of oscillating samples. Commercial SSWLI devices feature limited pulsing frequency. To address this issue we built a 400-620 nm wideband 150 mW light source whose 1.6 µm wide interferogram is without side peaks. The source combines a non-phosphor white LED with a cyan LED. We measured a calibration artifact with 10 nm precision and obtained 40 nm precision when measuring the 3D profile of a capacitive micromachined ultrasonic transducer membrane operating at 2.72 MHz. This source is compatible with solid state technology.