Michael E. Durst

Deep tissue multiphoton microscopy using longer wavelength excitation

We compare the maximal two-photon fluorescence microscopy (TPM) imaging depth achieved with 775-nm excitation to that achieved with 1280-nm excitation through in vivo and ex vivo TPM of fluorescently-labeled blood vessels in mouse brain. We achieved high contrast imaging of blood vessels at approximately twice the depth with 1280-nm excitation as with 775-nm excitation. An imaging depth of 1 mm can be achieved in in vivo imaging of adult mouse brains at 1280 nm with approximately 1-nJ pulse energy at the sample surface. Blood flow speed measurements at a depth of 900 mum are performed.

Simultaneous spatial and temporal focusing of femtosecond pulses

We experimentally demonstrate the concept of simultaneous spatial and temporal focusing of femtosecond pulses. Our technique has the potential to significantly reduce background excitation which fundamentally limits the imaging depth in scattering biological specimens.

Simultaneous spatial and temporal focusing for axial scanning

We show theoretically and experimentally that simultaneous spatial and temporal focusing can scan the temporal focal plane axially by adjusting the group velocity dispersion in the excitation beam path. When the group velocity dispersion is small, the pulse width at the temporal focal plane is transform-limited, and the amount of shift depends linearly upon the dispersion. By adding a meter of large mode area fiber into the system, we demonstrate this axial scanning capability in a fiber delivery configuration. Because a transform-limited pulse width is automatically recovered at the temporal focal plane, simultaneous spatial and temporal focusing negates the need for any dispersion pre-compensation, further facilitating its integration into a fiber delivery system. A highly promising application for simultaneous spatial and temporal focusing is an axial scanning multiphoton fluorescence fiber probe without any moving parts at the distal end and without dispersion pre-compensation.

High speed multiphoton axial scanning through an optical fiber in a remotely scanned temporal focusing setup

Simultaneous spatial and temporal focusing is used to acquire high speed (200Hz), chemically specific axial scans of mouse skin through a single-mode fiber. The temporal focus is remotely scanned by modulating the group delay dispersion (GDD) at the proximal end of the fiber. No moving parts or electronics are required at the distal end. A novel GDD modulation technique is implemented using a piezo bimorph mirror in a folded grating pair to achieve a large GDD tuning range at high speed.

Tunable dispersion compensation by a rotating cylindrical lens

We present a technique for tunable dispersion compensation that is low cost, high speed, and has a large tuning range. By rotating a cylindrical lens at the Fourier plane of a folded 4f grating pair system, the group-velocity dispersion can be tuned over a range greater than 10(5) fs(2), sufficient for compensating the dispersion of several meters of optical fiber.

Kinetics of the Growth of Anthracene Nanoparticles

Abstract Nanocrystals of anthracene were formed by “solvent shifting”—reducing the solute solubility by changing the ratio of two components of a binary solvent. Dispersions of slowly‐growing nanoparticles with sizes ranging from 100 to 1000 nm were prepared in an acetone/water binary solvent. A model of the growth was developed that described the rapid initial increase in particle size over several minutes, followed by much slower growth over several hours to days. Dynamic light scattering (DLS) measurements of particle size could be described well using this model. UV–VIS and fluorescence spectra suggest that the anthracene particles are crystalline, and exist in equilibrium with molecular anthracene in solution.

Enhanced axial confinement of sum-frequency generation in a temporal focusing setup.

We demonstrate enhanced axial confinement in a temporal focusing setup with a shaped spectrum and a narrow emission filter, achieving a reduction of 1 order of magnitude of the out-of-focus background when compared with conventional point-scanning two-photon microscopy. By rejecting the background in the optical domain, our technique circumvents the noise problems common in other background subtraction techniques.

In vivo deep tissue imaging with long wavelength multiphoton excitation

As a result of the large difference between scattering mean free paths and absorption lengths in brain tissue, scattering dominates over absorption by water and intrinsic molecules in determining the attenuation factor for wavelengths between 350 nm and 1300 nm. We propose using longer wavelengths for two-photon excitation, specifically the 1300-nm region, in order to reduce the effect of scattering and thereby increase imaging depth. We present two photon fluorescence microscopy images of cortical vasculature in in vivo mouse brain beyond 1 mm. We also explore the capabilities of the 1300-nm excitation for third harmonic generation microscopy of red blood cells in in vivo mouse brain.

All-Fiber, Wavelength and Repetition-Rate Tunable, Ultrafast Pulse Generation in the 2.0 μm Region Without Mode-Locking

We show 3.0 ps pulses from 1877 nm to 2008 nm at variable repetition rates up to 18 GHz using time-lens compression of a tunable CW laser. The center wavelength is changed by tuning the CW seed laser, and the repetition rate is changed by electronically tuning the drive of the master RF clock. The repetition rate of 18 GHz represents a record speed for pulse generation in this spectral region. This simple all-fiber platform uses standard 1550 nm telecom components, offering a turn-key, flexible, robust alternative to pulse generation in the 2.0 μm region with both wavelength and repetition rate tunability.

Kilohertz tunable dispersion compensation with a bimorph piezo deformable mirror

We present a new technique for dispersion compensation with ≫10⁵ fs² range and kilohertz tuning speed, enabling high-speed focal plane scanning of two-photon excited fluorescence in a temporal focusing setup.