Chi Thanh Nguyen

Vertically Coupled Polymer Microracetrack Resonators for Label-Free Biochemical Sensors

We report on the efficient design and fabrication of polymeric microracetrack optical resonators for label-free biosensing purposes. Vertically-coupled microresonators immersed in deionised water display high Q-factors (>;35000)and finesses up to 25. A surface sensing experiment performed with these microresonators using 5-TAMRA cadaverine as a test molecule demonstrated both the high sensitivity and low detection limit of our device.

High-performance electro-optic modulators realized with a commercial side-chain DR1-PMMA electro-optic copolymer

Several high-performance polymeric electro-optic modulators have been demonstrated in the last decade. Most of them have been elaborated using specific high-performance electro-optic polymers designed for their exceptional electro-optic response and their thermal stability. In this paper we report the high performance of electro-optic modulators made of a commercial side-chain electro-optic copolymer DR1-PMMA as the active core material and of a passive epoxy polymer NOA73 as cladding material. The electro-optic polymer used in these modulators is a Disperse Red 1- poly-methylmethacrylate (DR1-MMA) side-chain copolymer with relative molar concentrations of DR1-substituted (resp. MMA unsubstituted) groups equal to 30% (resp. 70%). We have designed, elaborated and tested phase modulator and pushpull Mach-Zehnder modulators in order to optimize their figure of merit VπL. A push-pull Mach-Zehnder modulator with 2 cm-long electrodes and an inter-electrode distance of 8.8 μm displays a half-wave voltage of 2.6 V at 1550 nm, corresponding to a figure of merit of 5.2 V.cm. This result was obtained with a moderate poling electric field of 75 V/μm applied to the core of the modulator waveguide. We report here the best figure of merit which has never been observed in a modulator realized with a commercial side-chain electro-optic polymer.

General approach for the sensitivity analysis and optimization of integrated optical evanescent-wave sensors

The optimization of integrated optical evanescent-wave sensors includes two parts. For optimal performance, we require waveguides with both maximal sensitivity to the measurand—the quantity intended to be measured—and minimal sensitivity to perturbations. In this context, fully numerical approaches are extremely powerful but demand huge computer resources. We address this issue by introducing a general and efficient approach, based on the formal derivation of analytical dispersion equations, to express and evaluate all waveguide sensitivities. In particular, we apply this approach to rectangular waveguides and discuss its accuracy and its use within sensitivity optimization procedures.

Vertically coupled polymer microresonators for optofluidic label-free biosensors

In this paper we report on the design and fabrication of polymeric microracetracks optical resonators for optofluidic label-free biosensing. In the domain of optical integrated devices, polymer materials offer the advantages of low cost, easy fabrication, low scattering loss on waveguide sidewalls, and high coupling efficiency to optical fibres and waveguides. Moreover, for biochemical sensing, polymer surfaces can be easily modified to immobilize a wide choice of target molecules. Polymers are also well compatible with microfluidic circuits, favoring the insertion of photonic circuits into optofluidic cells. The vertical coupling configuration, in which resonators are vertically coupled to the buried bus waveguide, presents several advantages in comparison with the lateral coupling configuration, particularly in the context of optofluidic biosensors. Polymeric microracetracks were fabricated using the SU-8 negative photoresist and the CYTOP fluorinated polymer, using a combination of a simple near UV lithography and reactive ion etching technology. Vertically coupled microracetracks immersed in deionized water display high Q-factors (> 35000) and finesse up to 25. Surface sensing experiments performed with these microresonators using TAMRA-cadaverine as a test molecule, which can be quantified through fluorescence analysis, demonstrated a very low detection limit of 0.22 attogram.

Instrumentation system for determination and compensation of electro-optic modulator transfer function drift

This paper presents an instrumentation system developed to improve the operation of an electro-optic modulator (EOM). During their operating time, EOM are subject to a drift of the optical transfer function; therefore the initial tuning of the bias point no longer corresponds to the best characteristics of the device. Because of this drift the EOM no longer behaves linearly and there is degradation during time of the performances of the system in which the EOM is included. To determine the drift, a low frequency modulation signal (at 500 Hz) is applied to the EOM and the second harmonic component at 1 kHz is detected. A new criterion is introduced for estimating the nonlinearity and for compensating the drift of the transfer function, keeping the optical bias point at the quadrature position. Temperature changes significantly influence the EOM characteristics. Thus, the instrumentation system has to be simultaneously developed with temperature control and drift compensation of the optical transfer function. The design is based on PSOC microcontrollers for tuning the different parameters, for data acquisition and regulation process. By setting the temperature to some specific values, it is possible to test the behaviour of the modulator. Finally, by using both temperature and bias point control, a significant reduction of the nonlinearity can be obtained during 2 h of experiment: the biasing point at the quadrature point of the transfer function which corresponds to the most linear behaviour can be stabilized within ±0.22% of the half-wave voltage. All the works presented here were carried out with a Mach–Zehnder intensity modulator made of lithium niobate, but it is also possible to apply this method to other kinds of material, for example polymer material.

Electrooptic probe based on an organic microcavity

We present the concept of an electrooptic (EO) probe based on a thin organic layer of DR1-PMMA embedded in a high finesse Fabry-Pe/spl acute/rot cavity with optimal orientation of the DR1 molecules, parallel to the faces of the microcavity. This optimal orientation is obtained through a lateral poling method and an r/sub 33/ value of 2.5 pm/V is reached for a 16-/spl mu/m-thick DR1-PMMA layer. This EO probe shows a high sensitivity of 2 V/spl middot/cm/sup -1//spl middot/Hz/sup -1/2/.

Study of all-polymer-based waveguide resonant gratings and their applications for optimization of second-harmonic generation

We investigated theoretically and experimentally the optical properties of all-polymer-based one-dimensional waveguide resonant gratings (WRGs) and their important applications for the optimization of second-harmonic generation (SHG). We first studied the basic theory of the resonant modes of a simple grating-coupled waveguide realized on a material possessing a low refractive index contrast. The optical properties of any WRG were numerically simulated by using the finite-difference time domain method, performed by commercial Lumerical software. The polymer-based surface relief gratings were fabricated on azopolymer Disperse Red 1-Poly-Methyl-Methacrylate (DR1–PMMA) thin films by using the two-beam interference method and mass transport effect. Their experimental reflection spectra measured as a function of incident light wavelength are in good agreement with the theoretical predictions. We then demonstrated a first application of such a polymer-based WRG for nonlinear optics. Thanks to the strong local electrical field in the WRG, due to a guided-mode resonance condition, the SHG signal of an infrared light beam was strongly enhanced by a factor of 25 as compared to the result obtained in a sample without a grating.

Enabling patterning of polymer optical devices working at visible wavelength using thermal nano-imprint lithography

Thermal Ultraviolet NanoImprint Lithography is a fast and reliable process to manufacture large scale integrated polymer-based optical components from a soft stamp. This technology already provides polymer integrated components for optical communications operating in the infrared region. However, several fabrication issues must be addressed to enable reliable mass production of optical components working in the visible region, especially when patterning large devices with nanometric features. In this work, we report our fabrication results on grating coupler and optical microring resonators with SU-8 resist. The device is conceived for monomode visible wavelength operation dedicated to future optical sensing applications. For this purpose, sub-micron waveguides are needed. We reported two main defects on soft stamp fabrication: partial sidewall detachment and shifted or double embossing of the features. Nanoimprinting SU-8 waveguides was achieved with the operational devices of the soft stamp.

Optimizing detection limits of optical resonator based sensors by optimization of real-time measurements of resonators response

Sensors based on functionalized optical resonators provide high specificity, high sensitivity, very high detection limit and fast response. The sensing principle of such sensors is based on the detection of a change of environment at the vicinity of the optical resonator surface. This change induces an effective index variation of the guided mode circulating in the resonator, resulting in a resonance spectral shift in the optical resonator response. The detection limit (i.e. the smallest amount of analyte that can be detected by sensors) depends on the resolution of the sensor and of its background noise. Improving the detection limits of sensors requires then to minimize their background noise by exploring various real-time configurations. In this report, we present direct measurements methods at different points of the resonance transmittance response (at minimum point and at inflexion points where the slope of transmittance is maximum) and indirect methods (resonance transmittance fit with Lorentzian and microresonator transmission models) to determine the resonance wavelength. Using an optofluidic label-free sensor based on a polymeric vertically coupled optical microracetrack, we demonstrated that measuring a spectral shift at the minimum of the sensor spectral transmittance at resonance is not the best solution to reduce the measurement background noise of sensors. For different analyte concentrations, the background noise obtained with the inflexion point method is reduced 3 times as compared to the minimum point method. On another hand, the sensor response time is 1000 times less than that obtained with the method using transmission model fitting or Lorentzian fitting. This method of measurement can be extended for any sensors based on optical resonators.

Dual-polarization optofluidic biodetection based on polymer microring resonators

We previously demonstrated that vertically-coupled polymer microring resonators are good candidates as transducers in label-free biosensing instruments, for several reasons: good optical performances, low production costs, excellent biocompatibility and state-of-the-art surface sensitivity. We later presented the association of our original opto-electronic setup to microfluidics to perform homogeneous optofluidic detection, using glucose solutions to characterize the behavior of the microring transducer. In parallel, the same transducer devices have been associated to another detection scheme, phase-sensitive OLCI, with promising results.