Photon counting for enhanced resolution spatial frequency modulation imaging.

Spatial frequency modulation imaging (SPIFI) has previously been demonstrated in multiphoton modalities with resolution enhancement. When signal light levels are low enough, signals consist of discrete pulses: this is a photon-counting regime. By binning photon counts into discrete time bins, SPIFI signals can be built up by incrementing the photon counts over many modulation periods.

Wavelength domain spatial frequency modulation imaging: enabling fiber optic delivery and detection.

Spatial frequency modulation imaging (SPIFI) provides a simple architecture for modulating an extended illumination source that is compatible with single pixel imaging. We demonstrate wavelength domain SPIFI (WD-SPIFI) by encoding time-varying spatial frequencies in the spectral domain that can produce enhanced resolution images, like its spatial domain counterpart, spatial domain (SD) SPIFI. However, contrary to SD-SPIFI, WD-SPIFI enables remote delivery by single mode fiber, which can be attractive for applications where free-space imaging is not practical.

Cascaded domain multiphoton spatial frequency modulation imaging.

Significance: Multiphoton microscopy is a powerful imaging tool for biomedical applications. A variety of techniques and respective benefits exist for multiphoton microscopy, but an enhanced resolution is especially desired. Additionally multiphoton microscopy requires ultrafast pulses for excitation, so optimization of the pulse duration at the sample is critical for strong signals.

Super-resolution computational saturated absorption microscopy.

Imaging beyond the diffraction limit barrier has attracted wide attention due to the ability to resolve previously hidden image features. Of the various super-resolution microscopy techniques available, a particularly simple method called saturated excitation microscopy (SAX) requires only simple modification of a laser scanning microscope: The illumination beam power is sinusoidally modulated and driven into saturation. SAX images are extracted from the harmonics of the modulation frequency and exhibit improved spatial resolution.

Design and analysis of polygonal mirror-based scan engines for improved spatial frequency modulation imaging.

Spatial frequency modulation imaging (SPIFI) is a structured illumination single pixel imaging technique that is most often achieved via a rotating modulation disk. This implementation produces line images with exposure times on the order of tens of milliseconds. Here, we present a new architecture for SPIFI using a polygonal scan mirror with the following advances: (1) reducing SPIFI line image exposure times by 2 orders of magnitude, (2) facet-to-facet measurement and correction for polygonal scan design, and (3) a new anamorphic magnification scheme that improves resolution for long working distance optics.

Single-shot spatial frequency modulation for imaging.

Spatial frequency modulation for imaging (SPIFI) has traditionally employed a time-varying spatial modulation of the excitation beam. Here, for the first time to our knowledge, we introduce single-shot SPIFI, where the spatial frequency modulation is imposed across the entire spatial bandwidth of the optical system simultaneously enabling single-shot operation.

Wavelength-multiplexed single-shot ptychography.

We present the first experimental demonstration of wavelength-multiplexing in single-shot ptychography. Specifically, we experimentally reconstruct the complex transmission profile of a wavelength-independent and wavelength-dependent object simultaneously for 532 nm and 633 nm probing wavelengths. In addition, we discuss the advantages of a more general approach to detector segmentation in single-shot ptychography.

Investigating low salinity waterflooding via glass micromodels with triangular pore-throat architectures.

Glass micromodels have been extensively used to simulate and investigate crude oil, brine, and surface interactions due to their homogeneous wettability, rigidity, and ability to precisely capture a reservoir's areal heterogeneity. Most micromodels are fabricated via two-dimensional patterning, implying that feature depths are constant despite varying width, which sub-optimally describes a three-dimensional porous architecture. We have successfully fabricated micromodels with arbitrary triangular cross sections via femtosecond pulsed laser direct writing resulting in depth-dependent channel width.

Fabrication of sealed sapphire microfluidic devices using femtosecond laser micromachining.

Due to its hardness, strength, and transparency, sapphire is an attractive material for the construction of microfluidic devices intended for high-pressure applications, but its physiochemical properties resist traditional microfabrication and bonding techniques. Here a femtosecond pulsed laser was used to directly machine fluidic channels within sapphire substrates and to form bonds between machined and flat sapphire windows, resulting in the creation of sealed microfluidic devices. Sapphire-sapphire bond strength was determined by destructive mechanical testing, and the integrity of the bond was verified by the capillary filling of the channel with air and ethanol.

Two-dimensional random access multiphoton spatial frequency modulated imaging.

Spatial frequency modulated imaging (SPIFI) enables the use of an extended excitation source for linear and nonlinear imaging with single element detection. To date, SPIFI has only been used with fixed excitation source geometries. Here, we explore the potential for the SPIFI method when a spatial light modulator (SLM) is used to program the excitation source, opening the door to a more versatile, random access imaging environment.

Fourier computed tomographic imaging of two dimensional fluorescent objects.

We introduce a new form of tomographic imaging that is particularly advantageous for a new class of super-resolution optical imaging methods. Our tomographic method, Fourier Computed Tomography (FCT), operates in a conjugate domain relative to conventional computed tomography techniques. FCT is the first optical tomography method that records complex projections of the object spatial frequency distribution.

Fluorescent coherent diffractive imaging with accelerating light sheets.

Fluorescence microscopy is a powerful method for producing high fidelity images with high spatial resolution, particularly in the biological sciences. We recently introduced coherent holographic image reconstruction by phase transfer (CHIRPT), a single-pixel imaging method that significantly improves the depth of field in fluorescence microscopy and enables holographic refocusing of fluorescent light. Here we demonstrate that by installing a confocal slit conjugate to the illuminating light sheets used in CHIRPT, out-of-focus light is rejected, thus improving lateral spatial resolution and rejecting noise from out-of-focus fluorescent light.

Spectral phase and amplitude retrieval and compensation technique for measurement of pulses.

In this Letter, an in-line, compact, and efficient quantitative pulse compensation and measurement scheme is demonstrated. This simple system can be readily deployed in multiphoton imaging systems and advanced manufacturing where multiphoton processes are exploited.

Interferometric spatial frequency modulation imaging.

Interferometric spatial frequency modulation for imaging (I-SPIFI) is demonstrated for the first time, to our knowledge. Significantly, this imaging modality can be seamlessly combined with nonlinear SPIFI imaging and operates through single-element detection, making it compatible for use in scattering specimens. Imaging dynamic processes with submicrometer axial resolution through long working distance optics is shown, and high contrast images compared to traditional wide-field microscopy images.

Three-dimensional single-pixel imaging of incoherent light with spatiotemporally modulated illumination.

We derive analytic expressions for the three-dimensional coherent transfer function (CTF) and coherent spread function (CSF) for coherent holographic image reconstruction by phase transfer (CHIRPT) microscopy with monochromatic and broadband illumination sources. The 3D CSF and CTF were used to simulate CHIRPT images, and the results show excellent agreement with experimental data. Finally, we show that the formalism presented here for computing the CSF/CTF pair in CHIRPT microscopy can be readily extended to other forms of single-pixel imaging, such as spatial-frequency-modulated imaging.

Fabrication and characterization of modulation masks for multimodal spatial frequency modulated microscopy.

Spatial frequency modulated imaging (SPIFI) is a powerful imaging method that when used in conjunction with multiphoton contrast mechanisms has the potential to improve the spatial and temporal scales that can be explored within a single nonlinear optical microscope platform. Here we demonstrate, for the first time to our knowledge, that it is possible to fabricate inexpensive masks using femtosecond laser micromachining that can be readily deployed within the multiphoton microscope architecture to transform the system from a traditional point-scanning system to SPIFI and gain the inherent advantages that follow.

Two-phase displacements in microchannels of triangular cross-section.

Varying microfluidic channel cross-sectional geometry can dramatically alter fluid flow behavior, particularly for capillary-driven flow. Most fabrication techniques, however, are planar and therefore incapable of providing depth-dependent variations in width. We introduce an ultrafast laser ablation technique that enables the fabrication of microchannels with arbitrary triangular cross sectional geometry.

Plasmon-less surface enhanced Raman spectra induced by self-organized networks of silica nanoparticles produced by femtosecond lasers.

Raman spectroscopy is the workhorse for label-free analysis of molecules. It relies on the inelastic scattering of incoming monochromatic light impinging molecules of interest. This effect leads to a very weak emission of light spectrum that provides a signature of the molecules being observed.

Superresolved multiphoton microscopy with spatial frequency-modulated imaging.

Superresolved far-field microscopy has emerged as a powerful tool for investigating the structure of objects with resolution well below the diffraction limit of light. Nearly all superresolution imaging techniques reported to date rely on real energy states of fluorescent molecules to circumvent the diffraction limit, preventing superresolved imaging with contrast mechanisms that occur via virtual energy states, including harmonic generation (HG). We report a superresolution technique based on spatial frequency-modulated imaging (SPIFI) that permits superresolved nonlinear microscopy with any contrast mechanism and with single-pixel detection.

Spatial-spectral characterization of focused spatially chirped broadband laser beams.

Proper alignment is critical to obtain the desired performance from focused spatially chirped beams, for example in simultaneous spatial and temporal focusing (SSTF). We present a simple technique for inspecting the beam paths and focusing conditions for the spectral components of a broadband beam. We spectrally resolve the light transmitted past a knife edge as it was scanned across the beam at several axial positions.

High-throughput linear optical stretcher for mechanical characterization of blood cells.

This study describes a linear optical stretcher as a high-throughput mechanical property cytometer. Custom, inexpensive, and scalable optics image a linear diode bar source into a microfluidic channel, where cells are hydrodynamically focused into the optical stretcher. Upon entering the stretching region, antipodal optical forces generated by the refraction of tightly focused laser light at the cell membrane deform each cell in flow.

Broadband interferometric characterization of divergence and spatial chirp.

We demonstrate a spectral interferometric method to characterize lateral and angular spatial chirp to optimize intensity localization in spatio-temporally focused ultrafast beams. Interference between two spatially sheared beams in an interferometer will lead to straight fringes if the wavefronts are curved. To produce reference fringes, we delay one arm relative to another in order to measure fringe rotation in the spatially resolved spectral interferogram.