Daniel Day

Formation of voids in a doped polymethylmethacrylate polymer

We report on the formation of submicrometer voids within a doped polymethylmethacrylate (PMMA) polymer under multiphoton absorption excited by an infrared laser beam. An ultrashort pulsed laser beam of pulse width 80 fs at a repetition rate of 82 MHz and a wavelength of 800 nm is focused into a PMMA-based photorefractive polymer consisting of 2,5-dimethyl-4(p-nitrophenylazo)anisole, 2,4,7-trinitro-9-fluorenone, and N-ethylcarbazole. The large change in refractive index associated with a void allows confocal reflection microscopy to be used as a detection method. Voids can be arranged in a multilayered structure for read-only high-density optical data storage.

Microchannel fabrication in PMMA based on localized heating by nanojoule high repetition rate femtosecond pulses

Microchannels are fabricated in a poly(methyl methacrylate) substrate by high repetition rate, nanojoule femtosecond laser pulses. The mechanism for channel fabrication is based on the localized heating of the substrate due to the high repetition rate of the laser, resulting in smooth walled cylindrical channels. Microchannels with diameters of 8 - 20 microm can be fabricated at 800 microm/s using 80 fs pulses at a repetition rate of 80 MHz and energy of 0.9 nJ/pulse.

Rewritable 3D Bit Optical Data Storage in a PMMA‐Based Photorefractive Polymer

3D bit optical data storage has the ability to reach Tbytes on a recording medium no larger than a compact disc using two-photon excitation and multi-layer recording. The introduction of a poly-vinyl carbazole (PVK) based photorefractive polymer as the recording material allows the system to rewrite the recorded information via the photorefractive effect.[1,2] In this paper, we demonstrate the use of continuous wave illumination for three-dimensional (3D) bit optical data storage under two-photon excitation in a new poly(methylmethacrylate)-based (PMMA) photorefractive polymer. This achievement makes it possible to develop a cheap, compact, sub Tbyte rewritable optical data storage system to further extend the capabilities of compact disc and digital versatile disc technology.

A method for prolonged imaging of motile lymphocytes

With new imaging technologies and fluorescent probes, live imaging of cells in vitro has revolutionized many aspects of cell biology. A key goal now is to develop systems to optimize in vitro imaging, which do not compromise the physiological relevance of the study. We have developed a methodology that contains non‐adherent cells within the field of view. ‘Cell paddocks’ are created by generating an array of microgrids using polydimethylsiloxane. Each microgrid is up to 250 × 250 μm2 with a height of 60 μm. Overlayed cells settle into the grids and the walls restrict their lateral movement, but a contiguous supply of medium between neighboring microgrids facilitates the exchange of cytokines and growth factors. This allows culture over at least 6 days with no impact upon viability and proliferation. Adaptations of the microgrids have enabled imaging and tracking of lymphocyte division through multiple generations of long‐term interactions between T lymphocytes and dendritic cells, and of thymocyte–stromal cell interactions.

A microfluidic refractive index sensor based on an integrated three-dimensional photonic crystal

We present the concept of using three-dimensional photonic crystals for refractive index sensing in a microfluidic channel. The sensors are based on a three-dimensional void channel photonic crystal fabricated by femtosecond laser writing in a polymer substrate. It is demonstrated that a change in the refractive index of the fluid in the microchannel results in a shift in the band gap or band gap defect position of the photonic crystal. According to Fourier transform infared spectroscopy of the photonic crystal sensor, a change of 6×10−3 in the refractive index of the fluid can be detected.

Multi-level optical data storage in a photobleaching polymer using two-photon excitation under continuous wave illumination

A multi-level recording method is implemented in a photobleaching polymer using two-photon excitation under continuous wave illumination. It is experimentally shown that information encoded with ten levels of darkness can be successfully stored. This multi-level bit optical data storage method is of importance in increasing the storage density of a recording material and the speed of data access.

Microchannel fabrication in PMMA based on localized heating using high-repetition rate femtosecond pulses

Femtosecond laser pulses with energy of 0.9 nJ per pulse and a 80 MHz repetition rate at a wavelength of 750 nm were used to fabricate straight microchannels in a PMMA substrate. The size and shape of the microchannels can be controlled by changing the fabrication parameters of speed, the number of fabrication repeats and delay in-between fabrication repeats. It has been proposed that the absorption of energy in the focal region modifies the density of the polymer matrix, which after annealing the sample above the glass transition temperature results in the formation of the microchannels. Diffusion of heat through the substrate is a uniform process which has the effect of creating symmetrically shaped channels. This fabrication method is expected to have applications in the fabrication of microstructures or microfluidic devices in polymer substrates.

Femtosecond fabricated photomasks for fabrication of microfluidic devices.

This paper describes the direct write laser fabrication of a photolithography mask for prototyping of microfluidic devices in polydimethylsiloxane. An amplified femtosecond pulse laser is used to selectively remove the aluminium metal layer from the poly(methyl methacrylate) photomask substrate. The use of a femtosecond pulse laser to selectively etch a metal layer has several advantages over other conventional methods for binary photomask fabrication, namely rapid prototyping of microfluidic devices using soft lightography. Control of the energy density and defocus position of the focusing objective lens results in the etching of features with widths ranging from 2 microm to 35 microm when using an objective lens with numerical aperture of 0.25.

Shear stress mapping in microfluidic devices by optical tweezers

We present an optical tweezer sensor for shear stress mapping in microfluidic systems of different internal geometries. The sensor is able to measure the shear stress acting on microspheres of different sizes that model cell based biological operations. Without the need for a spatial modulator or a holographic disk, the sensor allows for direct shear stress detection at arbitrary positions in straight and curved microfluidic devices. Analytical calculations are carried out and compared with the experimental results. It is observed that a decrease in the microsphere size results in an increase in the shear stress the particle experiences.

Three-dimensional coherent transfer function for reflection confocal microscopy in the presence of refractive-index mismatch.

The three-dimensional (3-D) coherent transfer function for reflection confocal microscopy of high-numerical-aperture objectives is derived and calculated in the presence of refractive-index mismatch when a laser beam is focused into a medium of refractive index different from its immersion medium. This aberrated coherent transfer function is then used to estimate the readout efficiency of 3-D data bits recorded in a thick medium. It is shown that the readout efficiency of confocal microscopy for 3-D bit data storage is decreased with the focal depth of an objective in a recording medium. However, a high readout efficiency can be maintained if the tube length of a reading objective is linearly altered to compensate for the spherical aberration caused by the refractive-index mismatch.