Masud Mansuripur

Evanescent field-based optical fiber sensing device for measuring the refractive index of liquids in microfluidic channels

We report a simple optical sensing device capable of measuring the refractive index of liquids propagating in microfluidic channels. The sensor is based on a single-mode optical fiber that is tapered to submicrometer dimensions and immersed in a transparent curable soft polymer. A channel for liquid analyte is created in the immediate vicinity of the taper waist. Light propagating through the tapered section of the fiber extends into the channel, making the optical loss in the system sensitive to the refractive-index difference between the polymer and the liquid. The fabrication process and testing of the prototype sensing devices are described. The sensor can operate both as a highly responsive on-off device and in the continuous measurement mode, with an estimated accuracy of refractive-index measurement of approximately 5 x 10(-4).

Femtosecond laser pulse generation with a fiber taper embedded in carbon nanotube/polymer composite

We propose and demonstrate a new saturable absorber based on a fiber taper embedded in a carbon nanotube/polymer composite. Greater than a 10% reduction in absorption (due to saturation) is directly measured for our saturable absorber. Using an embedded fiber-taper saturable absorber, we built an all-fiber mode-locked ring laser, which produces 594 fs/1.7 nJ pulses with a repetition rate of 13.3 MHz.

Experimental and theoretical investigations of laser-induced crystallization and amorphization in phase-change optical recording media

We describe the numerical procedure for calculating three-dimensional profiles of temperature in a multilayer stack illuminated by a laser beam, and model the crystallization and amorphization kinetics for phase-change rewritable media. Experimental methods have been used to determine indirectly the probabilities of nucleation and growth for Ge2Sb2Te5 alloy. Some of the fundamental behaviors of phase-change erasable media, such as the crystallization of as-deposited amorphous phase, amorphization of supercooled liquid, and recrystallization of quenched amorphous phase, have been illustrated based on our three-dimensional temperature calculations and the model kinetics. The calculated transient reflectance behavior of as-deposited Ge2Sb2Te5 amorphous films in a single layer and in a quadrilayer stack, as well as the erasure behavior of Ge2Sb2Te5 alloy in a quadrilayer disk are in good agreement with experimental observations.

Laser-induced local heating of multilayers

For a multilayer structure illuminated by a laser beam, absorption of optical energy in the absorptive layers and the diffusion of the resultant heat throughout the structure are studied. Analytical and numerical procedures for this study are described, and, as a specific example, the profiles of temperature distribution during recording on a magnetooptical disk are presented. The technique is also expected to be of value for studies of thermal marking and laser annealing.

Certain computational aspects of vector diffraction problems

Fourier decomposition of a given amplitude distribution into plane waves and the subsequent superposition of these waves after propagation is a powerful yet simple approach to diffraction problems. Many vector diffraction problems can be formulated in this way, and the classical results are usually the consequence of a stationary-phase approximation to the resulting integrals. For situations in which the approximation does not apply, a factorization technique is developed that substantially reduces the required computational resources. Numerical computations are based on the fast-Fourier-transform algorithm, and the practicality of this method is shown with several examples.

Biconical Fiber Taper Sensors

We present several simple sensitive fiber-optics-based sensors that utilize a biconical fiber taper. A displacement sensor with 100-nm accuracy, a temperature monitor with sensitivity DeltaT~1degC, and a refractive-index sensor capable of measuring Deltan~1.42times10-5 are demonstrated using tapers made from a standard single-mode fiber

Transmission of light through slit apertures in metallic films

Transmission of polarized light through subwavelength slit apertures is studied based on the electromagnetic field distributions obtained in finite difference time domain computer simulations. The results show the existence of a cutoff for E/sub /spl par// and a strong transmission (with no cutoff) for E/sub /spl perp//, where /spl par/ and /spl perp/ refer to the direction of the incident E-field relative to the long axis of the slit. Interference between the charges and currents induced in the vicinity of two adjacent slits is shown to result in enhanced transmission through both slits when the slits are separated by about one- half of one wavelength.

Analysis of multilayer thin‐film structures containing magneto‐optic and anisotropic media at oblique incidence using 2×2 matrices

A complete analysis of multilayer structures containing an arbitrary number of dielectric, metal, magnetic, and birefringent/dichroic layers is presented. An algorithm, based on simple 2×2 matrices, is derived which allows reflection, transmission, absorption, magneto‐optic conversion, birefringence, and dichroism of the structure to be computed on a personal computer. The incident beam is assumed to be plane monochromatic with arbitrary angle of incidence. There are no approximations involved, and the results are direct consequences of Maxwell’s equations.

Distribution of light at and near the focus of high-numerical-aperture objectives

Classical diffraction theory is used to investigate the effects of high numerical aperture on the focusing of coherent light. By expanding the diffracted beam in plane waves, we show that the lens action can be expressed as a succession of three Fourier transforms. Furthermore, polarization effects are included in the model in a natural way. Some numerical results of the theory are also presented.

Magnetization reversal, coercivity, and the process of thermomagnetic recording in thin films of amorphous rare earth-transition metal alloys

A model is proposed for the mechanism of magnetization reversal in thin films of amorphous alloys with perpendicular magnetic anisotropy. Examples of these alloys are TbFe, GdCo, DyFe, and GdTbFeCo, which are currently under investigation as storage media for erasable optical recording applications. The model exhibits the observed behavior of the media such as nucleation and growth of reverse‐magnetized domains under external magnetic fields; square hysteresis loops; temperature dependence of coercivity; formation and stability of domains under conditions of thermomagnetic recording; and incomplete erasure with insufficient applied fields.