Carlo Amadeo Alonzo

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.

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.

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.

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.

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.

Picosecond sliding frequency mode-locked fiber laser

We demonstrate an Er3+-doped fiber laser with a broad bandwidth (2.6 nm) intracavity filter that produces 10-ps pulses at 38-MHz repetition. Pulse center-wavelengths shift at 1.6-pm intervals over a 60-nm range about 1542 nm.

Track and trap in 3D

Three-dimensional light structures can be created by modulating the spatial phase and polarization properties of an an expanded laser beam. A particularly promising technique is the Generalized Phase Contrast (GPC) method invented and patented at Risø National Laboratory. Based on the combination of programmable spatial light modulator devices and an advanced graphical user-interface the GPC method enables real-time, interactive and arbitrary control over the dynamics and geometry of synthesized light patterns. Recent experiments have shown that GPC-driven micro-manipulation provides a unique technology platform for fully user-guided assembly of a plurality of particles in a plane, control of particle stacking along the beam axis, manipulation of multiple hollow beads, and the organization of living cells into three-dimensional colloidal structures. 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.

Three-dimensional intensity distribution of helico-conical optical beams

Helico-conical optical beams are a recently introduced class of beams that multiplicatively combine helical and conical phase fronts. Focusing these beams leads to a spiral intensity distribution at the focal plane of the lens. Further theoretical and experimental examination reveals interesting three-dimensional intensity patterns near the focal region, including a cork-screw structure around the optical axis. Variations on these light distributions based on the superposition of multiple helico-conical beams are also presented here. These beams are expected to yield interesting dynamics when applied to the optical trapping of microscopic particles, such as dielectric microspheres or even biological cells.

Single-SLM 3D interactive micromanipulation based on the generalized phase contrast (GPC) approach

Using a single phase-only spatial light modulator (SLM), we present a compact GPC-based optical trapping system for interactively manipulating microscopic particles in three dimensions (3D) and in real-time. We employ only one GPC 4f setup, which transforms 2D phase into intensity patterns, and utilize the SLM to form two phase-encoding regions defined by two equally sized apertures - one centered at x = x0 and the other at x = -x0 (with the optical axis centered at x = 0). Reconfigurable intensity patterns associated with the two independently addressable SLM-apertures are relayed to the sample volume to form a dynamic array of counterpropagating-beam traps. We discuss the experimental demonstrations showing 3D trapping of microparticles using the presented optical setup.