Harmonic mode locking in a sliding-frequency fiber laser.

We demonstrate a sliding-frequency mode-locked (SFM) erbium fiber laser generating 20 ps pulses with center wavelengths rapidly sweeping across a spectral range of 50 nm. Excess optical nonlinearity in the laser cavity leads to multipulsing, with a tendency to tight pulse bunching (<3 ns) at the fundamental cavity frequency of 25 MHz. The addition of a parallel optical delay line, with a path difference equal to a rational fraction of the cavity length, distributes the pulses uniformly across the entire cavity and achieves a harmonic SFM up to 1 GHz. The result establishes cavity nonlinearity as a critical design parameter for picosecond wavelength-swept lasers.

Two-photon microscopy by wavelength-swept pulses delivered through single-mode fiber.

Nonlinear microscopy through flexible fiber-optic catheters has potential in clinical diagnostic applications. Here, we demonstrate a new approach based on wavelength-swept narrowband pulses that permits simple fiber-optic delivery without need of the dispersion management and allows nonmechanical beam scanning. Using 0.86 ps pulses rapidly tuned from 789 nm to 822 nm at a sweep rate of 200 Hz, we demonstrate two-photon fluorescence and second-harmonic generation imaging through a 5-m-long standard single-mode fiber.

Optical microassembly platform for constructing reconfigurable microenvironments for biomedical studies.

Cellular development is highly influenced by the surrounding microenvironment. We propose user-reconfigurable microenvironments and bio-compatible scaffolds as an approach for understanding cellular development processes. We demonstrate a model platform for constructing versatile microenvironments by fabricating morphologically complex microstructures by two-photon polymerization (2PP) and then assembling these archetypal building blocks into various configurations using multiple, real-time configurable counterpropagating-beam (CB) traps. The demonstrated capacity for handling feature-rich microcomponents may be further developed into a generalized microassembly platform.

Computerized "drag-and-drop" alignment of GPC-based optical micromanipulation system.

In the past, aligning the counterpropagating beams in our 3D real-time generalized phase contrast (GPC) trapping system has been a task requiring moderate skills and prior experience with optical instrumentation. A ray transfer matrix analysis and computer-controlled actuation of mirrors, objective, and sample stage has made this process user friendly. The alignment procedure can now be done in a very short time with just a few drag-and-drop tasks in the user-interface. The future inclusion of an image recognition algorithm will allow the alignment process to be executed completely without any user interaction. An automated sample loading tray with a loading precision of a few microns has also been added to simplify the switching of samples under study. These enhancements have significantly reduced the level of skill and experience required to operate the system, thus making the GPC-based micromanipulation system more accessible to people with little or no technical expertise in optics.

2D optical manipulation and assembly of shape-complementary planar microstructures.

Optical trapping and manipulation offer great flexibility as a non-contact microassembly tool. Its application to the assembly of microscale building blocks may open new doors for micromachine technology. In this work, we demonstrate all-optical assembly of microscopic puzzle pieces in a fluidic environment using programmable arrays of trapping beams. Identical shape-complimentary pieces are optically fabricated with submicron resolution using two-photon polymerization (2PP) technique. These are efficiently assembled into space-filling tessellations by a multiple-beam optical micromanipulation system. The flexibility of the system allows us to demonstrate both user-interactive and computer-automated modes of serial and parallel assembly of microscale objects with high spatial and angular positioning precision.

Autonomous and 3D real-time multi-beam manipulation in a microfluidic environment.

The Generalized Phase Contrast (GPC) method of optical 3D manipulation has previously been used for controlled spatial manipulation of live biological specimen in real-time. These biological experiments were carried out over a time-span of several hours while an operator intermittently optimized the optical system. Here we present GPC-based optical micromanipulation in a microfluidic system where trapping experiments are computer-automated and thereby capable of running with only limited supervision. The system is able to dynamically detect living yeast cells using a computer-interfaced CCD camera, and respond to this by instantly creating traps at positions of the spotted cells streaming at flow velocities that would be difficult for a human operator to handle. With the added ability to control flow rates, experiments were also carried out to confirm the theoretically predicted axially dependent lateral stiffness of GPC-based optical traps.

GPC-based optical micromanipulation in 3D real-time using a single spatial light modulator.

Using a novel dual-beam readout with the generalized phase contrast (GPC) method, a multiple-beam 3D real-time micromanipulation system requiring only one spatial light modulator (SLM) has been realized. A theoretical framework for the new GPC scheme with two parallel illumination beams is presented and corroborated with an experimental demonstration. Three-dimensional arrays of polystyrene microbeads were assembled in the newly described system. The use of air immersion objective lenses with GPC-based optical trapping allowed the simultaneous viewing of the assemblies in two orthogonal bright-field imaging perspectives.

Two-photon excited fluorescence of intrinsic fluorophores enables label-free assessment of adipose tissue function.

Current methods for evaluating adipose tissue function are destructive or have low spatial resolution. These limit our ability to assess dynamic changes and heterogeneous responses that occur in healthy or diseased subjects, or during treatment. Here, we demonstrate that intrinsic two-photon excited fluorescence enables functional imaging of adipocyte metabolism with subcellular resolution. Steady-state and time-resolved fluorescence from intracellular metabolic co-factors and lipid droplets can distinguish the functional states of excised white, brown, and cold-induced beige fat. Similar optical changes are identified when white and brown fat are assessed in vivo. Therefore, these studies establish the potential of non-invasive, high resolution, endogenous contrast, two-photon imaging to identify distinct adipose tissue types, monitor their functional state, and characterize heterogeneity of induced responses.

Laser projection using generalized phase contrast.

We demonstrate experimental laser projection of a gray-level photographic image with 74% light efficiency using the generalized phase contrast (GPC) method. In contrast with a previously proposed technique [Alonzo, New J. Phys. 9, 132 (2007)], a new approach to image construction via GPC is introduced. An arbitrary phase shift filter eliminates the need for high-frequency modulation and conjugate phase encoding. This lowers device performance requirements and allows practical implementation with currently available dynamic spatial light modulators.