Optofluidic-tunable color filters and spectroscopy based on liquid-crystal microflows

The integration of color filters with microfluidics has attracted substantial attention in recent years, for on-chip absorption, fluorescence, or Raman analysis. We describe such tunable filters based on the micro-flow of liquid crystals. The filter operation is based on the wavelength-dependent liquid crystal birefringence that can be tuned by modifying the flow velocity field in the microchannel. The latter is possible both temporally and spatially by varying the inlet pressure and the channel geometry, respectively. We explored the use of these optofluidic filters for on-chip absorption spectroscopy in poly(dimethylsiloxane) microfluidic systems; by integrating the distance-dependent color filter with a dye-filled micro-channel, the absorption spectrum of a dye could be measured. Liquid crystal microflows substantially simplify the optofluidic integration, actuation and tuning of color filters for lab-on-a-chip spectroscopic applications.

Étude expérimentale des propriétés optiques des cristaux photoniques bidimensionnels et de leur accordabillité

Barbara Wild

Photonic crystals are periodic dielectric structures, where the periodicity varies in one, two or three dimensions. Analogous to the periodic potential of electrons in semiconductors, the periodic variation of the dielectric constant influences the electromagnetic properties. The energy of the light is separated in bandgaps, energy ranges in which the propagation of the light is forbidden for certain directions and energies. These properties suggest that photonic crystals may be suitable for fabrication of the components needed for integrated optics. Since the fabrication of three dimensional photonic crystals is still limited by complex fabrication problems we have studied two dimensional photonic crystals. These photonic crystals can be fabricated by standard microelectronic technology. The photonic crystals studied in this thesis consist of GaAs and InP based low index vertical waveguides with a matrix of holes etched into it. The fabrication of photonic crystals with good optical properties is an important factor for photonic crystal devices in integrated optics. In the first part we studied the optical properties of photonic crystal slabs and of Fabry-Pérot cavities. These structures were measured by the internal light source technique, which allows quantitative normalized transmission measurements. The photonic crystals were etched by three different dry etching technologies. Out-of plane scattering is described by a 2D-FDTD fit of the transmission spectra and by a semi-analytical model. The finite hole depth and the conical shape of the holes are important structural parameters. The detailed study of their effects yields an important feedback for the critical parameters of the fabrication process. Photonic crystal waveguides are another group of components in integrated optics. We measured two types of straight photonic crystal waveguides by the endfire technique. Here the light is coupled by optical fibers and ridge waveguides in the photonic crystal waveguides. The transmission spectra reveal fringes which are due to multiple interference of the light in the sample. This interference contains information about propagation losses of the waveguides and an analysis based on the Fourier transform of the transmission spectra enabled us to deduce the propagation losses of the photonic crystal waveguides. The necessity of tuning or trimming the optical properties of photonic crystals is an important issue either compensating fabrication imperfections (trimming) or controlling the optical properties on demand (tuning) for devices like filters. We have shown that it is possible to tune the optical properties of planar photonic crystal cavities by temperature. The experimental results were validated by theoretical calculations. Also we have shown that is possible to tune the optical properties of photonic crystals by infiltrating liquid crystals in the holes. Liquid crystals are a birefringent material whose optical axis can be tuned by external fields (electric, temperature, photonic source, etc.). Phase transitions exhibiting abrupt changes in the refractive index are important characteristics of liquid crystals. The holes of the photonic crystal are infiltrated by a reversible and reliable infiltration process. The infiltration efficiency and the orientation of the molecules in the holes were determined by polarization resolved internal light source measurements. We have shown that the frequency of the Fabry-Pérot resonance changes at the phase transitions of the liquid crystal.

EPFL2006

Optofluidic modulator based on peristaltic nematogen microflows

Julien Cuennet, Luciano De Sio, Demetri Psaltis, Andreas Vasdekis

Nematogens rotate by the application of external fields, thereby enabling optical modulation. This principle has had a profound impact on our daily lives through the plethora of liquid-crystal displays in use around us(1,2). However, the wider use of nematic liquid crystals, particularly in microdisplays(3) and information processing, has been hampered by their slow response times. In nematogens, rotational and translational molecular motions are coupled(4), so flow is inevitably linked with optical modulation(5,6). This linkage motivated us to fuse microfluidics with anisotropic liquids and introduce an optofluidic(7,8) modulator that exhibits a submillisecond (250 mu s) symmetric response and can operate at frequencies up to 1 kHz. The modulator is based on peristaltic nematogen microflows(9) realized in polydimethylsiloxane microfluidics. The latter simultaneously permits peristalsis by means of elastomeric deformation, nematogen alignment and rapid prototyping through cast-moulding. Together with large-scale, vertical integration and piezoelectric nanotechnologies, this optofluidic paradigm can enable high-density and three-dimensional architectures of fast modulators.

2011

Optofluidics based on liquid crystal microflows

Julien Cuennet, Luciano De Sio, Demetri Psaltis, Andreas Vasdekis

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.

Spie-Int Soc Optical Engineering, Po Box 10, Bellingham, Wa 98227-0010 Usa2011

Étude expérimentale des propriétés optiques des cristaux photoniques bidimensionnels et de leur accordabillité

Barbara Wild

EPFL2006