Several biotechnologies are currently available to quantify how cells allocate resources between growth and carbon storage, such as mass spectrometry. However, such biotechnologies require considerable amounts of cellular biomass to achieve adequate signal-to-noise ratio. In this way, existing biotechnologies inevitably operate in a ‘population averaging’ mode and, as such, they cannot unmask how cells allocate resources between growth and storage in a high-throughput fashion with single-cell, or subcellular resolution. This methodological limitation inhibits our fundamental understanding of the mechanisms underlying resource allocations between different cellular metabolic objectives. In turn, this knowledge gap also pertains to systems biology effects, such as cellular noise and the resulting cell-to-cell phenotypic heterogeneity, which could potentially lead to the emergence of distinct cellular subpopulations even in clonal cultures exposed to identical growth conditions. To address this knowledge gap, we applied a high-throughput quantitative phase imaging strategy. Using this strategy, we quantified the optical-phase of light transmitted through the cell cytosol and a specific cytosolic organelle, namely the lipid droplet (LD). With the aid of correlative secondary ion mass spectrometry (NanoSIMS) and transmission electron microscopy (TEM), we determined the protein content of different cytosolic organelles, thus enabling the conversion of the optical phase signal to the corresponding dry density and dry mass. The high-throughput imaging approach required only 2 μL of culture, yielding more than 1,000 single, live cell observations per tested experimental condition, with no further processing requirements, such as staining or chemical fixation.
In this report, we review our recent results in the optical micromanipulation of vesicles.
Traditionally, vesicle manipulation has been possible by employing photon momentum and
optical trapping, giving rise to unique observations of vesicle shape changes and soft matter
mechanics. Contrary to these attempts, we employ photon energy rather than momentum, by
sensitizing vesicles with an oxidizing moiety. The later converts incident photons to reactive
oxygen species, which in turn attack and compromise the stability of the vesicle membrane.
Both coherent and incoherent radiation was employed. Polymersome re-organization into
smaller diameter vesicles was possible by focusing the excitation beam in the vicinity of the
polymersomes. Extended vesicle illumination with a collimated beam lead to their complete
destabilization and micelle formation. Single particle analysis revealed that payload release
takes place within seconds of illumination in an explosive burst. We will discuss the
destabilization and payload release kinetics, as revealed by high resolution microscopy at the
single particle level, as well as potential applications in single cell biomodulation.
By employing anisotropic fluids and namely liquid crystals, fluid flow becomes an additional degree of freedom in
designing optofluidic devices. In this paper, we demonstrate optofluidic liquid crystal devices based on the direct flow
of nematic liquid crystals in microfluidic channels. Contrary to previous reports, in the present embodiment we employ
the effective phase delay acquired by light travelling through flowing liquid crystal, without analysing the polarisation
state of the transmitted light. With this method, we demonstrate the variation in the diffraction pattern of an array of
microfluidic channels acting as a grating. We also discuss our recent activities in integrating mechanical oscillators for
on-chip peristaltic pumping.
Microfluidic structures in poly(dimethylsiloxane) (PDMS) are usually fabricated by optically patterning a resist and
subsequently transferring this pattern to PDMS. Such microsystems suffer from reduced resolution, which in turn inhibit
the manipulation of sub-micron scale entities such as bacteria, as well as their optical properties (e.g. long period
gratings and radiation losses). In this paper we discuss how electron beam lithography (EBL) can be employed in
prototyping SU8 moulds and nanostructures for poly(dimethylsiloxane) (PDMS) based microfluidics and optofluidics.
In comparison to conventional optical methods, direct patterning of SU8 with an electron beam enabled both sub- and
few micron scale structures with reduced complexity and duration. We will also discuss how to synthesize optofluidic
circuits with this method and discuss our preliminary results on how such sub-micron scale structures can be employed
in the optofluidic analysis of bacteria.
By replacing common buffers with anisotropic liquids in microfluidics, an enhanced range of optofluidic functionalities
is enabled. Such an anisotropic liquid is nematic liquid crystals (NLC), which exhibits optical properties that can be
tuned by optical, electrical or mechanical fields, such as flow. We demonstrate an optofluidic modulator based on direct
flow of nematic liquid crystals in microfluidic channels. We discuss this optofluidic paradigm both under steady state
conditions, and under flow. Rapid pulsatile flows are detrimental towards more compact and ultra-fast devices. These
were enabled via peristaltic pumps, demonstrating liquid crystal modulators operating above the limit of 3 kHz. We
discuss the latter results, but also assess the feasibility of performing ultra-fast optics and additional functionalities for
on- and off-chip imaging.
Surfaces -defined as the interfaces between solids and liquids- have attracted much attention in optics and biology, such
as total internal reflection imaging (TIRF) and DNA microarrays. Within the context of optofluidics however, surfaces
have received little attention. In this paper, we describe how surfaces can define or enhance optofluidic function. More
specifically we discuss chemical interfaces that control the orientation of liquid crystals and the stretching of individual
nucleic acids, diffractive and plasmonic nanostructures for lasing and opto-thermal control, as well as microstructures
that read pressure and form chemical patterns.
We demonstrate optofluidic evanescent dye lasers based on two types of solid distributed feedback (DFB) grating
cavities- the first order linear DFB gratings which gives in-plane laser emitting and second order circular DFB gratings
which gives surface laser emitting. For both of them, the laser mode is confined within the waveguide and experience
optical gain via evanescent wave coupling with the dye solution. Benefitting from the solid waveguide cores, stable and
narrow linewidth laser output were observed with a large tolerance of fluid refractive indices, which prove the feasibility
of integrating fluid evanescent gain dye laser into passive waveguide circuit.
We present a pressured mediated tunable optofluidic dye laser with novel cavity. The dye laser chip was fabricated with Polydimethylsiloxane (PDMS) via replica molding and has none nano-features. It comprises a liquid waveguide and micro-scale air-gap chambers which function as mirrors to provide feedback. The lasing wavelength was determined by the interference of the reflected beams from the two PDMS-air interfaces of the air-gap chamber acting as Fabry-Perot etalon, while the tuning was realized by varying the width of the air-gap by applying air pressure. The lasing with linewidth of 3 nm and tuning range of 14 nm was demonstrated.
Silicon photonics is a rapidly progressing field, where silicon structures are developed
for optical information generation, transmission and processing. Although substantial
progress has been achieved in the fields of transmission and processing,
significant challenges remain to be addressed in generating light on silicon. In this
paper we show that by integrating a silicon resonator with organic semiconductors,
light generation on silicon chips can be achieved in the visible spectral range. Unlike
similar attempts in the telecommunication spectral region, the signal from our device
can be directly measured by silicon photodetectors.
In this paper we describe the design and performance of diode-pumped organic lasers based on the poly(paraphenylene-vinylene)
derivative MEH-PPV. To achieve the very low oscillation thresholds required for direct diode pumping, we
use a novel surface-emitting distributed Bragg reflector cavity. We describe the operating characteristics of such devices
when operating below and above threshold, and show that they can combine low threshold operation with the favourable
spectral and emission characteristics of DFB lasers. We also describe and characterize an energy transfer gain medium
using coumarin 102 laser dye as the host, which has been optimized for efficient harvesting of the diode laser excitation.
Semiconducting polymers are a rapidly advancing class of optoelectronic materials. They give efficient light emission
under optical or electrical stimulation, and offer promise as compact, lightweight and simple to fabricate lasers. The
development of such active polymer components complements developments in polymer fibre and planar lightwave
circuits opening new directions in polymer integrated optics. In this article progress towards making compact practical
polymer lasers is described. The potential for polymer lasers to operate in the space radiation environment is also discussed.
Semiconducting conjugated polymers have recently attracted significant interest as amplifying media for solid-state lasers due to their functional photo-physical properties and simple fabrication. Distributed feedback (DFB) cavities are proving to be the most attractive for polymer lasers, since they can combine the properties of transverse optical pumping, low threshold and practical output beams. To date, in most polymer DFB lasers the feedback is provided by second order diffraction. This has the advantage of surface emission, though it also imposes extensive scattering losses. In this work, we present the use of alternative structures that attempt to reduce the threshold of polymer DFB lasers, and also achieve dual wavelength operation. The former was addressed with cavities formed by alternative symmetries of the Brillouin zone of a square lattice. Using the diagonal ΓM symmetry first order feedback was attained. The threshold energy was thus reduced by almost an order of magnitude as compared with the more commonly used ΓX symmetry of second order square gratings. Finally, we show that two lasing wavelengths may be set independently in a semiconducting polymer laser by using a doubly periodic (i.e. Moiré) DFB grating.
SU8 is a commercial negative photoresist, which is highly transparent in the visible and near-infrared and extremely resistant to many organic solvents. Here we show that sub-micron period diffraction gratings, and 2D photonic crystal structures, can be readily formed holographically over extended areas. By coating the SU8 layer with a suitable gain medium, such structures may be used as feedback and output-coupling gratings for organic waveguide lasers. Thin films of SU8, were initially deposited by spin casting onto glass substrates. These films were then mounted in one arm of a Lloyd's mirror interferometer and exposed with the expanded beam of a HeCd laser, operating at 325 nm. Subsequent baking and developing steps lead to both volume gratings with index contrast of 0.014, and surface gratings with corrugation depths of up to 140 nm. By varying the incidence angle of the HeCd laser beam to the SU8 film we have tuned the microstructure period from 500 nm down to 200 nm. Using multiple exposures, both doubly-periodic diffraction gratings and square-lattice crystal structures have been produced.