Single-shot 4-step phase-shifting multispectral fringe projection profilometry.

Phase-shifting profilometry (PSP) is considered to be the most accurate technique for phase retrieval with fringe projection profilometry (FPP) systems. However, PSP requires that multiple phase-shifted fringe patterns be acquired, usually sequentially, which has limited PSP to static or quasi-static imaging. In this paper, we introduce multispectral 4-step phase-shifting FPP that provides 3D imaging using a single acquisition. The method enables real-time profilometry applications. A single frame provides all four phase-shifted fringe patterns needed for the PSP phase retrieval algorithm. The multispectral nature of the system ensures that light does not leak between the spectral bands, which is a common problem in simultaneous phase-shifting with color cameras. With the use of this new concept, custom composite patterns containing multiple patterns can be acquired with a single acquisition.

Surface plasmon resonance sensing properties of a 3D nanostructure consisting of aligned nanohole and nanocone arrays.

Molecular surface plasmon resonance (SPR) sensing is one of the most common applications of an array of periodic nanoholes in a metal film. However, metallic nanohole arrays (NHAs) with low-hole count have lower resolution and SPR sensing performance compared to NHAs with high-hole count. In this paper, we present a compact three-dimensional (3D) plasmonic nanostructure with extraordinary optical transmission properties benefiting from surface plasmon matching and enhanced localized surface plasmon coupling. The 3D nanostructure consisted of a gold film containing a NHA with an underlying cavity and a gold nanocone array (NCA) at the bottom of the cavity. Each nanocone was aligned with the nanohole above and the truncated apex of each nanocone was in close proximity (100 nm) to the gold film. The NHA-NCA structures outperformed conventional NHA structures in terms of bulk sensitivity and Figure of Merit (FOM). Furthermore, the NHA-NCA structure with 525 nm periodicity was capable of sensing streptavidin down to 2 nM exhibiting a 10-fold increase in streptavidin sensitivity compared to conventional NHA structures. The sensitivity and performance of the 3D nanostructure can be further improved by exploiting multiplexing methods in combination with stable light sources and detection systems.

Radial angular filter arrays for angle-resolved scattering spectroscopy.

The radial angular filter array (RAFA) consists of a series of radially-distributed micro-machined channels, where the long axes of the channels converge at a focal point. The high aspect ratio of each channel provides a means to reject photons with trajectories outside the acceptance angle of the channel. The output of the RAFA represents the angular distribution of photons emitted from the focal point. A series of RAFAs were designed, fabricated, and tested to evaluate the impact of device geometry, inter-channel cross talk, achromaticity, and channel leakage on device performance. As an application example, an RAFA was used together with an imaging spectrometer to capture angle-resolved spectra of turbid samples.

Effect of surface plasmon cross-talk on optical properties of closely packed nano-hole arrays.

The integration and miniaturization of nanostructure-based optical devices based on interaction with surface plasmons requires the fabrication of patterns of multiple nanostructures with tight spacing. The effect of surface plasmon energy interchange (cross-talk) across large grids of nanostructures and its effect on the optical characteristics of individual nanostructures have not been investigated. In this paper, we experimentally fabricated a large grid of individual nano-hole arrays of various hole diameter, hole spacing, and inter-array spacing. The spectral optical transmission of each nano-hole array was measured and the effect of inter-array spacing on the transmission spectra and resonance wavelength was determined.

Optical resonance transmission properties of nano-hole arrays in a gold film: effect of adhesion layer.

In this paper, we present a systematic study on the influence of composition of the adhesion layer between gold and a Pyrex substrate on the optical resonance transmission properties of nano-hole arrays in an optically thick gold film. Large nano-hole arrays with different hole periodicities in a square lattice arrangement were fabricated using Electron Beam Lithography using different adhesion layers (chromium, titanium, or etched adhesion layer). The fabricated nano-hole arrays were optically characterized using transmission spectroscopy. The optical performance of each nano-hole array was numerically simulated using a Finite Difference Time Domain (FDTD) method. The experiments and simulations revealed that the optical resonance transmission properties (i.e. the resonance wavelength, the spectral transmission modulation ratio, and the resonance bandwidth) of the nano-hole arrays depended highly on the composition and the thickness of the adhesion layer. The optical resonance bandwidths were larger for the nano-hole arrays with chromium or titanium adhesion layers. Also, a red-shift of the optical resonance peak was observed for nano-hole arrays with a metal adhesion layer compared to the corresponding nano-hole arrays with an etched adhesion layer, but the red-shift was greatest for the nano-hole array with the titanium adhesion layer. For adhesion layers of greater thickness, the optical resonance peaks were reduced in magnitude. Finally, nano-hole arrays with an etched adhesion layer had a significant blue-shift in the optical resonance peak and a narrower optical resonance bandwidth compared to nano-hole arrays with a titanium or a chromium adhesion layer. Consequently, a narrow optical resonance bandwidth characteristic of a nano-hole array with an etched adhesion layer can potentially enhance the spectral selectivity and offer improved optical performance.

Single-shot detection of 8 unique monochrome fringe patterns representing 4 distinct directions via multispectral fringe projection profilometry.

Spatial resolution in three-dimensional fringe projection profilometry is determined in large part by the number and spacing of fringes projected onto an object. Due to the intensity-based nature of fringe projection profilometry, fringe patterns must be generated in succession, which is time-consuming. As a result, the surface features of highly dynamic objects are difficult to measure. Here, we introduce multispectral fringe projection profilometry, a novel method that utilizes multispectral illumination to project a multispectral fringe pattern onto an object combined with a multispectral camera to detect the deformation of the fringe patterns due to the object. The multispectral camera enables the detection of 8 unique monochrome fringe patterns representing 4 distinct directions in a single snapshot. Furthermore, for each direction, the camera detects two π-phase shifted fringe patterns. Each pair of fringe patterns can be differenced to generate a differential fringe pattern that corrects for illumination offsets and mitigates the effects of glare from highly reflective surfaces. The new multispectral method solves many practical problems related to conventional fringe projection profilometry and doubles the effective spatial resolution. The method is suitable for high-quality fast 3D profilometry at video frame rates.

Multispectral intravital microscopy for simultaneous bright-field and fluorescence imaging of the microvasculature.

Intravital video microscopy permits the observation of microcirculatory blood flow. This often requires fluorescent probes to visualize structures and dynamic processes that cannot be observed with conventional bright-field microscopy. Conventional light microscopes do not allow for simultaneous bright-field and fluorescent imaging. Moreover, in conventional microscopes, only one type of fluorescent label can be observed. This study introduces multispectral intravital video microscopy, which combines bright-field and fluorescence microscopy in a standard light microscope. The technique enables simultaneous real-time observation of fluorescently-labeled structures in relation to their direct physical surroundings. The advancement provides context for the orientation, movement, and function of labeled structures in the microcirculation.

Experimental and numerical analysis on the optical resonance transmission properties of nano-hole arrays.

In this paper, we present experimental and numerical analysis on Extraordinary Optical Transmission (EOT) or optical resonance transmission through various nano-hole arrays constructed from an optically thick metal film within the visible and near infra-red spectrum. Nano-hole arrays with different geometrical parameters (hole size, hole shape, and hole periodicity) having their EOT properties in the visible and near-infrared regime were simulated based on Finite Difference Time Domain (FDTD). Large nano-hole arrays with geometric properties similar to the simulated arrays were fabricated using Electron Beam Lithography (EBL). The optical resonance transmission properties (resonance position, transmission efficiency, and spectral bandwidth of resonance peak) of the fabricated nano-hole arrays were characterized. Finally, the experimental and numerical results were analyzed to determine the dependencies and discrepancies between optical resonance transmission properties for various nano-hole arrays.

Angular domain fluorescence lifetime imaging: a tissue-like phantom study.

We describe a fluorescence lifetime imaging technique employing the collimation detection capabilities of an angular filter array (AFA). The AFA accepts minimally scattered photons emitted from fluorophores up to 2 mm deep within turbid media. The technique, referred to as Angular Domain Fluorescence Lifetime Imaging (ADFLI), is described and its performance evaluated in comparison to a conventional (lens and pinhole) system. Results from a tissue-mimicking phantom demonstrated that ADFLI provides better spatial resolution and image contrast for fluorescent probes at greater depths compared to a lens and pinhole system.

Bolus tracking with nanofilter-based multispectral videography for capturing microvasculature hemodynamics.

Multispectral imaging is a highly desirable modality for material-based analysis in diverse areas such as food production and processing, satellite-based reconnaissance, and biomedical imaging. Here, we present nanofilter-based multispectral videography (nMSV) in the 700 to 950 nm range made possible by the tunable extraordinary-optical-transmission properties of 3D metallic nanostructures. Measurements made with nMSV during a bolus injection of an intravascular tracer in the ear of a piglet resulted in spectral videos of the microvasculature. Analysis of the multispectral videos generated contrast measurements representative of arterial pulsation, the distribution of microvascular transit times, as well as a separation of the venous and arterial signals arising from within the tissue. Therefore, nMSV is capable of acquiring serial multispectral images relevant to tissue hemodynamics, which may have application to the detection and identification of skin cancer.

Nanohole-array-based device for 2D snapshot multispectral imaging.

We present a two-dimensional (2D) snapshot multispectral imager that utilizes the optical transmission characteristics of nanohole arrays (NHAs) in a gold film to resolve a mixture of input colors into multiple spectral bands. The multispectral device consists of blocks of NHAs, wherein each NHA has a unique periodicity that results in transmission resonances and minima in the visible and near-infrared regions. The multispectral device was illuminated over a wide spectral range, and the transmission was spectrally unmixed using a least-squares estimation algorithm. A NHA-based multispectral imaging system was built and tested in both reflection and transmission modes. The NHA-based multispectral imager was capable of extracting 2D multispectral images representative of four independent bands within the spectral range of 662 nm to 832 nm for a variety of targets. The multispectral device can potentially be integrated into a variety of imaging sensor systems.

Large area periodic, systematically changing, multishape nanostructures by laser interference lithography and cell response to these topographies.

The fabrication details to form large area systematically changing multishape nanoscale structures on a chip by laser interference lithography (LIL) are described. The feasibility of fabricating different geometries including dots, ellipses, holes, and elliptical holes in both x- and y- directions on a single substrate is shown by implementing a Lloyd's interferometer. The fabricated structures at different substrate positions with respect to exposure time, exposure angle and associated light intensity profile are analyzed. Experimental details related to the fabrication of symmetric and biaxial periodic nanostructures on photoresist, silicon surfaces, and ion milled glass substrates are presented. Primary rat calvarial osteoblasts were grown on ion-milled glass substrates with nanotopography with a periodicity of 1200 nm. Fluorescent microscopy revealed that cells formed adhesions sites coincident with the nanotopography after 24 h of growth on the substrates. The results suggest that laser LIL is an easy and inexpensive method to fabricate systematically changing nanostructures for cell adhesion studies. The effect of the different periodicities and transition structures can be studied on a single substrate to reduce the number of samples significantly.

Transillumination hyperspectral imaging for histopathological examination of excised tissue.

Angular domain spectroscopic imaging (ADSI) is a novel technique for the detection and characterization of optical contrast in turbid media based on spectral characteristics. The imaging system employs a silicon micromachined angular filter array to reject scattered light traversing a specimen and an imaging spectrometer to capture and discriminate the largely remaining quasiballistic light based on spatial position and wavelength. The imaging modality results in hyperspectral shadowgrams containing two-dimensional (2D) spatial maps of spectral information. An ADSI system was constructed and its performance was evaluated in the near-infrared region on tissue-mimicking phantoms. Image-based spectral correlation analysis using transmission spectra and first order derivatives revealed that embedded optical targets could be resolved. The hyperspectral images obtained with ADSI were observed to depend on target concentration, target depth, and scattering level of the background medium. A similar analysis on a muscle and tumor sample dissected from a mouse resulted in spatially dependent optical transmission spectra that were distinct, which suggested that ADSI may find utility in classifying tissues in biomedical applications.

Angular domain transillumination imaging optimization with an ultrafast gated camera.

By employing high-aspect-ratio parallel microchannels as an angular filter, quasiballistic photons sensitive to internal structures in a turbid medium can be captured. Scattered photons exiting the turbid medium typically exhibit trajectories with random angles compared to the initial trajectory and are mostly rejected by the filter. However, angular filter arrays cannot differentiate between quasiballistic photons (early arriving) and photons that happen to attain a scattered trajectory that is within the acceptance angle (late arriving). Therefore, we have two objectives: (1) to experimentally characterize the angular distribution and proportion of minimally deviated quasiballistic photons and multiply scattered photons in a turbid medium and (2) to combine time and angular gating principles so that early and late arriving photons can be distinguished. From the angular distribution data, the angular filter with angular acceptance about 0.4 deg yields the highest image contrast for transillumination images. The use of angular domain imaging(ADI) with time-gating enables visualization of submillimeter absorbing objects with approximately seven times higher image contrast compared to ADI in a turbid medium with a scattering level of six times the reduced mean free path.