F. Hamouda

Soft UV nanoimprint lithography-designed highly sensitive substrates for SERS detection

AbstractWe report on the use of soft UV nanoimprint lithography (UV-NIL) for the development of reproducible, millimeter-sized, and sensitive substrates for SERS detection. The used geometry for plasmonic nanostructures is the cylinder. Gold nanocylinders (GNCs) showed to be very sensitive and specific sensing surfaces. Indeed, we demonstrated that less than 4 ×106 avidin molecules were detected and contributed to the surface-enhanced Raman scattering (SERS) signal. Thus, the soft UV-NIL technique allows to obtain quickly very sensitive substrates for SERS biosensing on surfaces of 1 mm 2.

Frequency performances of a miniature optically pumped cesium beam frequency standard

The optically pumped cesium beam clock named Cs IV is operated with a new short Ramsey cavity satisfying strict requirements on the microwave leakage level. The most relevant characteristics of the device are presented. Cs IV is presently driven by standard electronics coming from a HP 5061 B clock that provides a sinusoidal modulation of the interrogation microwave signal and a microwave power stability of about 1% at a temperature of 20/spl plusmn/1/spl deg/C. The short- and medium-term frequency stability measurement gives /spl sigma//sub y/(1 day)=2/spl times/10/sup -14/: this value holds up to 3 days. The accuracy evaluation results in an uncertainty of 10/sup -12/, and the repeatability is evaluated to 3/spl times/10/sup -13/. It appears that the flicker floor is beginning at 2/spl times/10/sup -14/ and is mainly due to both the power fluctuations of the free running microwave interrogating signal and the fluctuations of the external static magnetic field. The accuracy is limited by the lack of knowledge of the end-to-end cavity phase shift.

Controlling the microwave amplitude in optically pumped cesium beam frequency standards

Assuming square wave frequency modulation, the response, versus the amplitude of the microwave field, of an optically pumped cesium beam tube is considered. The properties of the first maximum of this response are analyzed. The effect of the neighboring lines is taken into consideration, and a model of the profile of the microwave field in each interaction region is validated. A symmetry property of the response considered is pointed out. It enables us to implement a feedback control of the microwave amplitude with a large depth of the amplitude modulation. Residual frequency offsets that may occur as a consequence of a spurious amplitude modulation correlated with the frequency modulation are assessed. And, with a cavity designed such that /spl sigma/=/spl pi/ between the two oscillatory fields, it also is possible to measure the microwave amplitude at the first maximum of the sole contribution of the reference atomic line.

Sub-200nm gap electrodes by soft UV nanoimprint lithography using polydimethylsiloxane mold without external pressure

In this article, the authors present an alternative approach to electron beam lithography allowing the fabrication of gap electrodes with dimensions less than 200nm. This approach consists of the soft ultraviolet nanoimprint lithography on nonstandard sample sizes, using polydimethylsiloxane (PDMS) as a flexible mold material. By simple deposit of the PDMS mold on the substrate, they succeeded in imprinting gaps in Amonil and the transfer of patterns of polymethylmethacrylate/Amonil bilayer was achieved by etching the substrate with a suitable reactive ion etching process. The imprint pressure was ensured by only the weight of the PDMS mold.

Highly sensitive detection of paclitaxel by surface-enhanced Raman scattering

The surface-enhanced Raman scattering (SERS) technique was shown to be an effective molecular analytical tool due to its high sensitivity. Here, we propose to exploit soft UV assisted nanoimprint lithography (UV-NIL) for the development of a reproducible and highly-sensitive SERS biosensor. Soft lithography is known to be advantageous for biological applications since it is compatible with insulating supports and large-area samples. In the present investigations, soft UV-NIL is used for the fabrication of large-sized arrays of gold nanocylinders on glass which were shown to be highly sensitive and highly specific sensing surfaces, with a limit of detection measured down to 1 nM. Employing the UV-NIL SERS substrate enable working ranges of nanomolar to micromolar concentrations in regards to our model paclitaxel analyte.

Processing strategies for accurate frequency comparison using GPS carrier phase

We use GPS phase and code measurements from geodetic or geodetic-like receivers to compare distant hydrogen maser clocks. We show that the frequency comparison may be accurately carried out only when using specialised processing strategies, where main attention is paid to the continuity of the comparison result. Standard strategies used in the real time massive processing of global networks, which have proven very efficient in producing accurate, e.g. geodetic and orbit results, are less adapted to the needs of the accurate comparison of primary frequency standards.

Limitation of the frequency stability by local oscillator phase noise: new investigations

In passive frequency standards where atoms have a permanent interaction with the probe signal, the local oscillator phase noise limits the short and medium term frequency stability. It has been shown that this spurious effect cannot be suppressed whenever there is any truncation in the spectrum of the resonator response. However, a simultaneous processing of the probe signal, similar to that of the NIST, and of the resonator response-by means of an appropriate demodulation waveform-makes it possible to significantly reduce the limiting effect. First achieved with a square wave frequency modulation, this result is now extended to various modulation waveforms. It is shown that, for various modulation waveforms, the use of an optimized demodulation provides a moderate additional improvement to a moderate 2f/sub M/ rejection: this result confirms that, for a cesium clock using a slow frequency modulation, a high 2f/sub M/ rejection and a proper demodulation are necessary to strongly reduce the limiting effect. It is also shown that a non controlled distortion in the demodulation waveform may degrade the performance. At last, for a square wave frequency modulation and in the case of a flicker phase noise, a convenient demodulation reduces the spurious effect by a few dBs without any rejection in the microwave spectrum.

Conventional and Un-Conventional Lithography for Fabricating Thin Film Functional Devices

Thin film devices are conquering many aspects of today’s life, and continuous shrinking of building block dimensions of these structures enhances their performances and makes them economically attractive. This chapter is an overview of some conventional and unconventional lithography techniques used to fabricate thin film functional structures. Several aspects of pattern transfer were addressed with emphasis on the limits of these lithography techniques. We have thus highlighted the issue of pitch resolution for optical lithography and discussed some aspect of proximity effects for electron beam lithography. Pattern transfer from resist image to the wafer was also discussed. Considered as unconventional, we discussed several aspects linked to thin film fabrication using nanoimprint and nanosphere lithography techniques.

Transient response following frequency or amplitude switching in a cesium beam tube

The transient responses of an optically pumped cesium beam tube to square wave frequency and amplitude modulation is considered. The frequency transient is computed assuming a phase difference /spl phi/ of either 0 or /spl pi/ between the two oscillatory fields. We present theoretical and experimental data showing that, contrary to the frequency transient, the amplitude transient depends on the direction of switching. The knowledge of this property is useful for the design of the servo-loop controlling the amplitude of the microwave signal applied to the atomic resonator. A justification of this asymmetrical behavior is given. Experimental results confirm the theoretical predictions in the case /spl phi/=/spl pi/.

Frequency shift analysis tools for high performance optically pumped cesium beam frequency standard

We have implemented a digital servo system on an optically pumped cesium beam frequency standard. It controls three parameters: the frequency of the Ultra Stable Oscillator, the microwave power of the signal which interrogates the cesium atoms and the static magnetic field in the cavity. The short tube developed at LHA has been evaluated. It shows a very satisfactory level of short and medium term frequency stability. The frequency offset (-4.10/sup -12/) results mainly from the residual phase difference of the oscillatory field between the two interaction regions which is due to imperfections in the cavity symmetry. We describe means for evaluating the spurious frequency offset, through theoretical and experimental considerations. Assuming a square wave frequency modulation, a numerical simulation of the beam tube response is performed as a function of the microwave field amplitude, for different values of the residual phase difference /spl Delta//spl Phi/ and including the cavity pulling effect. Compared with the measurement frequency offset, the numerical simulation leads to a second order Doppler shift equal to -3.5 mHz and a residual phase difference, /spl Delta//spl Phi/ of +150 /spl mu/rad. An experimental method of measurement of /spl Delta//spl Phi/ without beam reversal is used. We obtain /spl Delta//spl Phi/=+155 /spl mu/rad /spl plusmn/17 /spl mu/rad. Finally, the clock accuracy is determined. It is equal to 4.10/sup -13/.