Woei Ming Lee

Optical vortex beam shaping by use of highly efficient irregular spiral phase plates for optical micromanipulation

Optical dark traps such as Laguerre-Gaussian beams, modulated optical vortices, and high-order Bessel beams have been used in the micromanipulation of microparticles. Such optical traps are highly versatile, as they are able to trap both high- and low-index microparticles as well as to set them into rotation by use of the orbital angular momentum of light. Holography has been widely used to modulate the shape of an optical vortex for new optical traps. We show that, by designing the shape of a spiral phase plate and using electron-beam lithography for fabrication, one can modulate the amplitude and the phase of an optical vortex with respect to the specific shape of the spiral phase plate as required. Furthermore, to the best of our knowledge this is the first report of transferring orbital angular momentum from a spiral phase plate to an absorptive microparticle in an experiment. Hence, with this technique, optical dark traps can easily be designed and fabricated.

In-fiber common-path optical coherence tomography using a conical-tip fiber

Common-path optical coherence tomography (CPOCT) is known to reduce group velocity dispersion and polarization mismatch between the reference and the sample arm as both arms share the same physical path. Existing implementations of CPOCT typically require one to incorporate an additional cover glass within the beam path of the sample arm to provide a reference signal. In this paper, we aim to further reduce this step by directly making use of the back-reflected signal, arising from a conical lens-tip fiber, as a reference signal. The conical lens, which is directly manufactured onto the optical fiber tip via a simple selective-chemical etching process, fulfils two functions acting as both the imaging lens and the self-aligning reference plane. We use a Fourier-domain OCT system to demonstrate the feasibility of this technique upon biological tissue. An in-fiber CPOCT technique may prove potentially useful in endoscopic OCT imaging.

Construction and calibration of an optical trap on a fluorescence optical microscope

The application of optical traps has come to the fore in the last three decades. They provide a powerful, sterile and noninvasive tool for the manipulation of cells, single biological macromolecules, colloidal microparticles and nanoparticles. An optically trapped microsphere may act as a force transducer that is used to measure forces in the piconewton regime. By setting up a well-calibrated single-beam optical trap within a fluorescence microscope system, one can measure forces and collect fluorescence signals upon biological systems simultaneously. In this protocol, we aim to provide a clear exposition of the methodology of assembling and operating a single-beam gradient force trap (optical tweezers) on an inverted fluorescence microscope. A step-by-step guide is given for alignment and operation, with discussion of common pitfalls.

Experimental observation of optical vortex evolution in a Gaussian beam with an embedded fractional phase step

Abstract Laguerre–Gaussian beams may possess an azimuthal phase variation of 2π l , where l is an integer, around the beam axis, resulting in an annular intensity distribution. This azimuthal phase variation and associated vortices can be visualized through the appearance of forked fringes, when interfering the LG beam with its mirror image (a beam of opposite helicity) at an angle. In this paper, we examine the evolution of optical beams with a fractional phase step hosted within a Gaussian beam by experimental analysis of both the phase and intensity distribution. To generate these beams, we introduce differing fractional (non-integer) topological charge variations within a Gaussian beam generated using a spatial light modulator (SLM). We detect the evolution of the vortex from the increase of the fractional phase step by interfering two beams of opposite but equal fractional phase step increment. The interference pattern generated shows evidence of the birth of an additional single extra charge as the fractional phase step increase extends above a half-integer value.

Dynamic optical manipulation with a higher-order fractional bessel beam generated from a spatial light modulator.

Higher-order Bessel beams have been demonstrated to have the ability to trap and rotate low- and high-index particles simultaneously [Phys. Rev. A 66, 063402 (2002)]. The rotation and trapping is caused by the presence of orbital angular momentum arising from its azimuthal phase variation (that changes at integer multiples of 2pi) and the concentric rings of the Bessel mode. We demonstrate for the first time to our knowledge a branch from the family of higher-order Bessel beams that has fractional azimuthal variation at its beam axis. This new family of laser beams has the ability to perform dynamic optical manipulation with dynamic control of a spatial light modulator. Furthermore, we take the opportunity to explore the propagation characteristics of higher-order Bessel beams for which the azimuthal phase changes at noninteger multiples of 2pi.

Direct electron-beam writing of continuous spiral phase plates in negative resist with high power efficiency for optical manipulation

Laser beams propagating in Laguerre-Gaussian (LG) modes are of considerable interest due to their widespread applications in the areas of optical manipulation of microparticles, quantum entanglement of photons, nonlinear optics, optical vortex interactions, and atomic studies. However, the proliferation of LG beams has been hampered due to the absence of reliable and reproducible fabrication technologies in producing the required optical elements for their generation. In this letter, we describe a simple, reliable, and reproducible fabrication technique for a micron-sized spiral phase plate with high power efficiency (80%–90%) and good beam uniformity. This facilitates the widespread use of LG beams in various applications: as an example the fabricated elements can easily and readily be incorporated into an existing optical trapping system with minimum modification.

Fabrication and operation of GRIN probes for in vivo fluorescence cellular imaging of internal organs in small animals

Intravital fluorescence microscopy has emerged as a powerful technique to visualize cellular processes in vivo. However, owing to their size, the objective lenses required have limited physical accessibility to various tissue sites in the internal organs of small animals. The use of small-diameter probes using graded-index (GRIN) lenses expands the capabilities of conventional intravital microscopes to minimally invasive imaging of internal organs. In this protocol, we describe the detailed steps for the fabrication of front- and side-view GRIN probes and the integration and operation of the probes in a confocal microscope to enable visualization of fluorescent cells and microvasculature in various mouse organs. Some experience in building an optical setup is required to complete the protocol. We also present longitudinal imaging of immune cells in renal allografts and tumor development in the colon. Fabrication and integration can be completed in 5–7 h, and a typical in vivo imaging session takes 1–2 h.

Nonlinear optical response of colloidal suspensions.

We experimentally probed the nonlinear optical response of aqueous nano-colloidal suspensions to provide a test of the theoretical approaches that have been proposed for the nonlinearity, namely an exponential model, an artificial Kerr medium, and a non-ideal gas model. The best agreement with experiment is found using the non-ideal gas model for the colloidal suspension which in turn can be used to infer values for the second virial coefficient of the medium and the nonlinear coefficients.

Fabricating low cost and high performance elastomer lenses using hanging droplets

Existing methods for low cost lenses using parallel mold stamping and high temperature reflow requires complex engineering controls to produce high quality lenses. These manufacturing techniques rely on expensive equipment. In this paper, we propose a low cost (<

Experimental study of holographic generation of fractional Bessel beams

0.01 per pc) flexible moldless lens fabrication method based on curing a hanging transparent polydimethylsiloxane (PDMS) elastomer droplet on a curved substrate. Additional deposition of hanging droplets in the same manner led to a substantial increase in the lens curvature and concomitant decrease in the focal length of the PDMS lenses down to ~2 mm. The shortest focal length lenses were shown to collimate light from a bare light emitting diode (LED) and image microscopic structures down to around 4 µm with 160x magnification. Our hanging droplet lens fabrication technique heralds a new paradigm in the manufacture of low cost, high performance optical lenses for the masses. Using these lenses, we were able to transform an ordinary commercial smartphone camera into a low-cost digital dermascope (60x magnification) that can readily visualize microscopic structures on skin such as sweat pores.