Julien Vaillant

DNA biosensor combining single-wavelength colorimetry and a digital lock-in amplifier within a smartphone

Smartphone camera based gold nanoparticle colorimetry (SCB-AuNP colorimetry) has shown good potential for point-of-care applications. However, due to the use of a camera as a photo-detector, there are major limitations to this technique such as a low bit resolution (∼8 bits mainstream) and a low data acquisition rate. These issues have limited the ultimate sensitivity of smartphone based colorimetry as well as the possibility to integrate efficiently a more sensitive approach such as detection based on a lock-in amplifier (LIA). In this paper, we improve the metrological performance of the smartphone to overcome existing issues by adding the LIA capability to AuNP sensing. In this work, instead of using the camera as a photo-detector, the audio jack is used as a photo-detector reader and function generator for driving a laser diode in order to achieve a smartphone based digital lock-in amplifier AuNP colorimetric (SBLIA-AuNP colorimetry) system. A full investigation on the SBLIA design, parameters and performance is comprehensively provided. It is found that the SBLIA can reduce most of the noise and provides a detection noise-to-signal ratio down to -63 dB, which is much better than the -49 dB of the state-of-the-art SCB based method. A DNA detection experiment is demonstrated to reveal the efficacy of the proposed metrological method. The results are compared to UV-visible spectrometry, which is the gold standard for colorimetric measurement. Based on our results, the SBLIA-AuNP colorimetric system has a detection limit of 0.77 nM on short strand DNA detection, which is 5.7 times better than the 4.36 nM limit of a commercial UV-visible spectrometer. Judging from the results, we believe that the sensitive SBLIA would be further extended to other optical diagnostic tools in the near future.

Generalized lock-in detection for interferometry: application to phase sensitive spectroscopy and near-field nanoscopy.

A generalized lock-in detection method is proposed to extract amplitude and phase from optical interferometers when an arbitrary periodic phase or frequency modulation is used. The actual modulation function is used to create the reference signals providing an optimal extraction of the useful information, notably for sinusoidal phase modulation. This simple and efficient approach has been tested and applied to phase sensitive spectroscopy and near-field optical measurements. We analyze the case where the signal amplitude is modulated and we show how to suppress the contribution of unmodulated background field.

Development of a polarization resolved mid-IR near-field microscope

We have developed a scattering-type microscope operating in the mid-IR range with a polarization analysis. The experimental development and the operation of the microscope are described. The optical system can provide for each pixel of the image a matrix similar to a Jones matrix. Examples of polarization resolved images obtained on a SiO2/Si surface grating with a tungsten probe are shown and a high optical resolution is clearly demonstrated through the imaging of submicron metallic lines.

Interferometry Using Generalized Lock-in Amplifier (G-LIA): A Versatile Approach for Phase-Sensitive Sensing and Imaging

A large number of interferometric setups make use of non-linear phase modulators. In the past, specific extraction methods have been proposed mostly to cover the important case of sinusoidal phase modulation with certain limits in term of signal-to-noise ratio. Recently, a detection method based on “Generalized Lock-in Amplifier” (G-LIA) was proposed to extract optimally amplitude and phase information in two-arm interferometers when nearly arbitrary phase modulations are used such as triangular or sinusoidal phase modulations. This method offers the opportunity to develop highly sensitive interferometers with simple-phase modulators such as piezo-actuated mirrors, piezo stretchers, or power-modulated laser diodes in unbalanced interferometers. Here we present the basics of the approach and we give application examples for monitoring displacement, sensing, and digital holography. The case where an amplitude modulation is also present is also detailed and discussed in the context of unbalanced interferometry and near-field nanoscopy.

Measurement of phonon damping by nanostructures

The understanding of phonon lifetime and scattering rates is attracting an increasing interest due to the major role of phonon in thermal and electrical conductivity which are key properties for technological applications. The infrared complex dielectric function of a crystal is determined by the harmonic characteristics of the phonon together with the intrinsic and extrinsic phonon scattering rates. In order to investigate the interplay between the phonon intrinsic scattering and the scattering of the phonon by a nanostructured surface, infrared reflectivity measurements from SiC nano-pyramids on SiC substrate have been analysed using a Kramers-Kronig conversion technique to deduce the infrared complex dielectric function. Then, the real and imaginary parts of the dielectric function were fitted simultaneously by using a theoretical model for the dielectric constant that considers frequency-dependent phonon damping at the center of the Brillouin zone. It has been found that surface nanostructuring strongly enhances the overall scattering rate of the phonon at the Brillouin zone center.

Mid-IR imaging of doped silicon gratings at a decananometer scale

Mid-IR nanoscopy has proved its ability to recognize material at nanometer scale through spectroscopic or even single wavelength imaging [1,2]. A high optical contrast can indeed be obtained between different materials as the permittivity of solid materials exhibit strong variations near phononic resonances, usually below 1000cm−1. One limitation of the technique is that a weak contrast is obtained if the permittivities of two materials are too close, which can be the case for example far from phonon resonances. As another example, doping imaging in silicon is difficult still difficult at 10.6µm, although the free electrons or holes can modify notably the permittivities as shown in figure 1. It has been shown previously that even longer wavelengths seem to be necessary to image unambiguously different doped zones [3,4] at least with common near-field probes. However, in a recent paper we have calculated that the phonon confinement occurring in decananometric scale objects such as near-field probes can lead to a drastic modification of the probes permittivity [5]. This effect can be very useful as for a range of probe permittivity, much stronger contrast can be expected. To experimentally demonstrate these theoretical expectations, we have built a mid-IR near field microscope and imaged doped Si gratings with a period of 2 µm (fabricated in CEA-LETI) with carefully prepared tungsten tip stuck on a tuning fork. The detection has been done using tips with different radius (30 nm to 100 nm) and a very strong contrast between P doped and P+ doped can be observed for a variety of small-radius probes. An example of highly contrasted profile obtained on the doped silicon grating is shown in figure2.