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.

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.

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.

Measurement of energy contrast of amplified ultrashort pulses using cross-polarized wave generation and spectral interferometry.

We present a method using spectral interferometry (SI) to characterize a pulse in the presence of an incoherent background such as amplified spontaneous emission (ASE). The output of a regenerative amplifier is interfered with a copy of the pulse that has been converted using third-order cross-polarized wave generation (XPW). The ASE shows as a pedestal background in the interference pattern.

Advances in multiphoton microscopy technology.

Multiphoton microscopy has enabled unprecedented dynamic exploration in living organisms. A significant challenge in biological research is the dynamic imaging of features deep within living organisms, which permits the real-time analysis of cellular structure and function. To make progress in our understanding of biological machinery, optical microscopes must be capable of rapid, targeted access deep within samples at high resolution.

Intuitive analysis of space-time focusing with double-ABCD calculation.

We analyze the structure of space-time focusing of spatially-chirped pulses using a technique where each frequency component of the beam follows its own Gaussian beamlet that in turn travels as a ray through the system. The approach leads to analytic expressions for the axially-varying pulse duration, pulse-front tilt, and the longitudinal intensity profile. We find that an important contribution to the intensity localization obtained with spatial-chirp focusing arises from the evolution of the geometric phase of the beamlets.

Direct diode-pumped Kerr-lens mode-locked Ti:sapphire laser.

We describe a Ti:sapphire laser pumped directly with a pair of 1.2 W 445 nm laser diodes. With over 30 mW average power at 800 nm and a measured pulsewidth of 15 fs, Kerr-lens-modelocked pulses are available with dramatically decreased pump cost.

Eliminating the scattering ambiguity in multifocal, multimodal, multiphoton imaging systems.

In this work we present how to entirely remove the scattering ambiguity present in existing multiphoton multifocal systems. This is achieved through the development and implementation of single-element detection systems that incorporate high-speed photon-counting electronics. These systems can be used to image entire volumes in the time it takes to perform a single transverse scan (four depths simultaneously at a rate of 30 Hz).

Differential Multiphoton Laser Scanning Microscopy.

Multifocal multiphoton microscopy (MMM) in the biological and medical sciences has become an important tool for obtaining high resolution images at video rates. While current implementations of MMM achieve very high frame rates, they are limited in their applicability to essentially those biological samples that exhibit little or no scattering. In this paper, we report on a method for MMM in which imaging detection is not necessary (single element point detection is implemented), and is therefore fully compatible for use in imaging through scattering media.

Remote focusing for programmable multi-layer differential multiphoton microscopy.

We present the application of remote focusing to multiphoton laser scanning microscopy and utilize this technology to demonstrate simultaneous, programmable multi-layer imaging. Remote focusing is used to independently control the axial location of multiple focal planes that can be simultaneously imaged with single element detection. This facilitates volumetric multiphoton imaging in scattering specimens and can be practically scaled to a large number of focal planes.

Blind frequency-resolved optical-gating pulse characterization for quantitative differential multiphoton microscopy.

We use a unique multifocal multiphoton microscope to directly characterize the pulse in the focal plane of a high-NA objective using second-harmonic generation frequency-resolved optical gating (FROG). Because of the nature of the optical setup, femtosecond laser pulses of orthogonal polarization states are generated in the focal plane, each acquiring a different spectral dispersion. By applying an additional constraint on the phase extraction algorithm, we simultaneously extract both the gate and probe pulses from a single spectrogram with a FROG error of 0.

Optimizing the fluorescent yield in two-photon laser scanning microscopy with dispersion compensation.

A challenge for nonlinear imaging in living tissue is to maximize the total fluorescent yield from each fluorophore. We investigated the emission rates of three fluorophores-rhodamine B, a red fluorescent protein, and CdSe quantum dots-while manipulating the phase of the laser excitation pulse at the focus. In all cases a transform-limited pulse maximized the total yield to insure the highest signal-to-noise ratio.

Spatiotemporal dynamics of cross-polarized wave generation.

We use time-domain spatially and spectrally resolved interferometry to investigate cross-polarized wave (XPW) generation in barium fluoride. We find that the XPW pulse is square root of 3 smaller than the input in the spatiotemporal domain, regardless of input chirp. Additionally, we calculate a temporally dependent focal length resulting from the nonlinear interaction and discuss its implications.

Measurement of pump-induced transient lensing in a cryogenically-cooled high average power Ti:sapphire amplifier.

The transient thermal lensing in a liquid-nitrogren cooled kilohertz multipass amplifier is quantitatively measured with spatially-resolved Fourier transform spectral interferometry. A pump-probe arrangement allows the observation of a polarization-dependent non-thermal component following the fluorescence timescale: additional cooling would not suppress this residual lensing. We also observe a time-dependent thermal component that has a timescale sufficiently fast to indicate that there is cooling between shots even at a repetition rate of 1 kHz.

Characterization of coupled nonlinear spatiospectral phase following an ultrafast self-focusing interaction.

We use collinear spatially resolved spectral interferometery to characterize the nonlinear phase changes experienced by an intense ultrashort pulse propagating in glass. The measurement yields the spectrally dependent wavefront, allowing us to measure the spatial and chromatic aberrations of the nonlinearly induced lens. For these conditions, we find that while the shape of the spatial wavefront follows the beam profile as expected, the spectral dependence of the lensing power is determined by the self-phase modulation.

A modular approach to the analytic calculation of spectral phase for grisms and other refractive/diffractive structures.

Analytic expressions for spectral phase for optical systems are very important for the design of wide-bandwidth optical systems. We describe a general formalism for analytically calculating the spectral phase for arbitrary optical structure made up of nested pairs of plane-parallel interfaces that can be diffractive or refractive. Our primary application is the calculation of the spectral phase of a grism pair, which is then used to analyze the behavior of higher-order phase terms.

Advancing multifocal nonlinear microscopy: development and application of a novel multibeam Yb:KGd(WO4)2 oscillator.

We present a novel Yb:KGd(WO(4))(2) oscillator design that generates six beams of temporally delayed, 253 fs, 11 nJ pulses. This allows multifocal nonlinear microscopy to be performed without the need for complicated optical multiplexers. We demonstrate our design with twelve simultaneously acquired two-photon, second-harmonic and/or third-harmonic images generated from six laterally separated foci.

In situ quantitative measurement of concentration profiles in a microreactor with submicron resolution using multiplex CARS microscopy.

In situ quantitative imaging of concentration profiles of reactants and products inside a microfluidic reactor is achieved, with submicron spatial resolution with mM sensitivity and on ms time scales, for a given position. The label-free approach relies on quantitative vibrational spectroscopy, using Coherent Anti-Stokes Raman scattering microscopy in a spectrally resolved fashion, and is demonstrated on an elementary acid-base reaction.

Simultaneous multifocal, multiphoton, photon counting microscopy.

We demonstrate a novel multifocal, multiphoton microscope that is capable of simultaneous dynamic imaging of multiple focal planes. We show for the first time that multimodal, multiphoton images excited with orthogonal polarizations can be acquired simultaneously in both the transmission and epi directions.

Characterization of a High Efficiency, Ultrashort Pulse Shaper Incorporating a Reflective 4096-Element Spatial Light Modulator.

We demonstrate pulse shaping via arbitrary phase modulation with a reflective, 1×4096 element, liquid crystal spatial light modulator (SLM). The unique construction of this device provides a very high efficiency when the device is used for phase modulation only in a prism based pulse shaper, namely 85%. We also present a single shot characterization of the SLM in the spatial domain and a single shot characterization of the pulse shaper in the spectral domain.