We report a highly sensitive ultrasound sensor based on an integrated photonics silicon Mach-Zehnder interferometer (MZI). One arm of the MZI is located on a thin membrane, acting as the sensing part of the device. Ultrasound waves excite the membrane’s vibrational mode, thus inducing modulation of the MZI transmission. The measured sensor transfer function is centered at 0.47 MHz and has a −6 dB bandwidth of 21.2%. For 1.0 mW optical input power, we obtain a high sensitivity of 0.62 mV/Pa, a low detection limit of 0.38 mPa/Hz1/2 at the resonance frequency and a large dynamic range of 59 dB. In preliminary ultrasound imaging experiments using this sensor, an image of a wire phantom is obtained. The properties of this sensor and the generated image show that this sensor is very promising for ultrasound imaging applications.
We experimentally demonstrate optical trapping of single B. subtilis spores using the enhanced field of a cavity at resonance in a planar silicon photonic crystal. By tracking the suppressed Brownian motion of a spore in three types of optical traps, generated with three types of cavities (H0, H1 and L3) we derive trap stiffnesses of around 7.6 pN/nm/W and find good agreement with calculated values obtained with 3D FDTD simulations. We envision that planar photonic crystals provide a suitable platform for the manipulation and sensing of bio-particles.
We re-evaluate the Macdonald-Vaughan model for Raman depth profiling [J. Raman Spectrosc. 38, 584 (2007)]. The model is an geometrical description of the sample regions from which Raman signal is collected in a confocal geometry and indicates that Raman signal also originates from far outside the focus. Although correct shapes of Raman depth profiles were obtained, quantitatively the results were not satisfactory, in view of the highly deviating values of the fitted extinction coefficients of the sample material. Our re-evaluation, based on a new numerical implementation of the model, indicates that the model is not only capable of predicting the proper profiles but also yields the right extinction coefficients. As a result, the model now is highly useful for interpretation of depth profiles, also for biomedical samples such as the human skin.
In monitoring the quality of drinking water with respect to the presence of hazardous bacteria there is a strong need for
on-line sensors that allow quick identification of bacterium species at low cost. In this respect, the combination of
photonics and microfluidics is promising for lab-on-a-chip sensing of these contaminants. Photonic crystal slabs have
proven to form a versatile platform for controlling the flow of light and creating resonant cavities on a wavelength scale.
The goal of our research is to use photonic crystal cavities for optical trapping of microorganisms in water, exploiting the
enhanced evanescent field of the cavity mode. We optimize the H0, H1 and L3 cavities for optical trapping of bacteria in
water, by reducing out-of-plane losses and taking into account the trapping-induced resonance shift and the in-plane
coupling with photonic crystal waveguides. The cavities are fabricated on silicon-on-insulator material, using e-beam
lithography and dry etching. A fluidic channel is created on top of the photonic crystal using dry film resist techniques.
Transmission measurements show clear resonances for the cavities in water. In the present state of our research, we
demonstrate optical trapping of 1 μm diameter polystyrene beads for the three cavities, with estimated trapping forces on
the order of 0.7 pN.
Thermal tuning of the transmission of an elastomer infilled photonic crystal cavity is studied. An elastomer has a thermal
expansion-induced negative thermo-optic coefficient that leads to a strong decrease of the refractive index upon heating.
This property makes elastomer highly suitable for thermal tuning of the transmission of a cavity, which is demonstrated
by global infilling of a hole-type silicon photonic crystal slab and global thermal tuning. In the temperature range 20-60
0C the cavity peak shows a pronounced elastomer-induced blue shift of 2.7 nm, which amply overcompensates the red
shift arising from the thermo-optic property of the silicon. These results qualify the elastomer for tuning by local optical heating.
Based on the self-collimation effect of light propagating inside a photonic crystal, we demonstrate a novel concept for a
compact Mach-Zehnder interferometer. The properties of these self-collimated beams are such that we can manipulate
them to form the beam splitters and mirrors of the Mach-Zehnder interferometer in a very compact area of 20x20 μm2.
We obtain the unidirectional output behaviour characterized by the high contrast in the telecommunication-wavelength
signal at the two outputs of the photonic crystal Mach-Zehnder interferometer. The experiments are done using optical
transmission spectroscopy and far-field optical microscopy. This photonic crystal Mach-Zehnder interferometer holds a
promise for a compact Mach-Zehnder modulator, inspired by recent reports of NEMS-based photonic crystal membrane.
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