Julien Gratier - Academia.edu (original) (raw)

Papers by Julien Gratier

Chemical and Biological Sensing VIII, 2007

Differential reflectometry (DR) is an effective tool to supplement existing explosives detection ... more Differential reflectometry (DR) is an effective tool to supplement existing explosives detection systems thus making the combined unit more effective than one tool alone. It is an optical technique in which the investigative light beam emanates from an extended distance onto the substance under investigation, thus rendering it to be a standoff method. The applicable distance still needs to be determined but could be well within the 50 to 100 m range. Specifically, differential reflectometry (also known as Differential reflection spectroscopy) is a surface analytical technique that reveals details about the electron structure. In other words, the instrument allows the measurement of the energies that electrons absorb from photons as they are raised into higher, allowed energy states. Since each material has a specific electron structure the measurement of the characteristic energies for "electron transitions" serves as a means (i.e. a fingerprint) for identifying these substances. The DR device can be made portable, it is fast, safe for the public, does not require human involvement, is cost effective, and most of all, it is a standoff technique which does not require ingestion of a suspicious substance into an instrument.

2011 IEEE International Ultrasonics Symposium, 2011

ABSTRACT Buried electrodes SAW resonators on lithium niobate suffer from transverse mode spurii e... more ABSTRACT Buried electrodes SAW resonators on lithium niobate suffer from transverse mode spurii even if the acoustic velocity inside the resonator is faster than the acoustic velocity outside the resonator (i.e. when a 'waveguiding' condition is not established). In practical devices, the electrode end gaps play an important role since they constitute a very fast region and reflect the acoustic waves creating multiple transverse modes. Guiding conditions need to be created in resonators to obtain good quality factors. This can be done by using sufficiently large gaps sizes. Unfortunately, this leads to even stronger transverse modes. The proposed solution consists in tailoring the shape of the main mode. A “piston mode” shape is obtained by adding a slow region at the edge of the active region. The mode shape is matched to the rectangular excitation and therefore, the coupling to the higher order modes becomes negligible. Results exhibit low spurious, lower losses and better effective coupling coefficients than apodized devices. Quality factors in the 1500 range are measured on resonators while CRF losses are significantly reduced.

2010 IEEE International Ultrasonics Symposium, 2010

High coupling substrates using overcoat layers of silicon oxide for temperature compensation exhi... more High coupling substrates using overcoat layers of silicon oxide for temperature compensation exhibit a strong affinity toward multimode behavior. Apodization, which is the standard technique to suppress the transverse modes, suffers from drawbacks such as reduction of the transduction efficiency and additional losses. By changing the transducer layout, the guiding can be improved and a “piston mode” can be obtained. This approach is demonstrated on resonators and DMS test devices manufactured on lithium niobate with an orientation close to YX128 deg.

2008 IEEE Ultrasonics Symposium, 2008

ABSTRACT In order to improve the temperature stability of RF SAW devices, it is worth adding a la... more ABSTRACT In order to improve the temperature stability of RF SAW devices, it is worth adding a layer of SiO2 on the surface of the piezoelectric substrate. This paper presents an original method, which allows the computation of the Harmonic Admittance of an infinite periodic grating covered with an SiO2 overlay. The origins of this method can be found in a previous paper. By comparison with S. Ballandras and K. Hashimoto works, this methods allows to take into account the very efficient Chebyshev-like charge distribution, together with a classical FEM basis interpolation for the stresses. FEM is used to model mechanical behavior of complex shape electrodes and BEM using a periodic Green's function Kernel is used to take into account the semi-infinite piezoelectric substrate. Validations are presented by comparison with K. Hashimoto's software.

IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, 2000

It is known that a part of the loss of leaky SAW resonators is due to radiation of acoustic energ... more It is known that a part of the loss of leaky SAW resonators is due to radiation of acoustic energy in the bus-bars. Many researchers are working on so-called phononic crystals. A 2-D grating of very strong reflectors allows these devices to fully reflect, for a given frequency band, any incoming wave. A new device based on the superposition of a regular SAW resonator and a 2-D periodic grating of reflectors is proposed. Several arrangements and geometries of the reflectors were studied and compared experimentally on 48 degrees rotated Y-cut lithium tantalate. In particular, a very narrow aperture (7.5 lambda) resonator was manufactured in the 900 MHz range. Because of its small size, this resonator has a resonance Q of only 575 when using the standard technology, whereas a resonance Q of 1100 was obtained for the new device without degradation of the other characteristics. Because of the narrow aperture, the admittance of the standard resonator showed a very strong parasitic above the resonance frequency, whereas this effect is drastically reduced for the new device. These results demonstrate the feasibility of the new approach.

IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, 2000

It is well known that transverse effects contribute significantly to the loss of SAW resonators o... more It is well known that transverse effects contribute significantly to the loss of SAW resonators on lithium tantalate. In particular, for frequencies above resonance, the surface wave is not guided inside the transducer and radiates into the busbars. In addition, because bulk modes can also be excited, scalar models are not sufficient to accurately predict transverse effects. It is also known that the layout of a SAW resonator (electrode gaps and dummy electrodes) has a strong impact on the transverse effects. In this paper, a periodic FEM/BEM model is presented and is used to simulate the transverse effects for various SAW resonator layouts. Test devices matching those simulated are fabricated and measured; the measured results are compared with the simulated results and show good agreement. By analyzing the dispersion curves produced from the FEM/BEM model in the different regions of the device, several frequency bands corresponding to different transverse behaviors are identified. These results are consistent with the elastic displacements, also computed by the FEM/BEM model. It is further shown that guided conditions in the gap between the transducer and the busbar occur for a frequency range above resonance. This result is in agreement with measurements showing that resonators with smaller gaps exhibit less [AU1: fewer?]spurious responses in their admittance.

Applied Optics, 2007

Differential reflectometry (DR) is an effective tool to supplement existing explosives detection ... more Differential reflectometry (DR) is an effective tool to supplement existing explosives detection systems thus making the combined unit more effective than one tool alone. It is an optical technique in which the light beam (UV) emanates from an extended distance onto the substance under investigation, thus rendering it to be a standoff method. DR allows the measurement of the energies that electrons absorb from photons as they are raised into higher, allowed energy states. These electron transitions serve as a "fingerprint" for identifying substances. The device can be made portable; it is fast, safe for the public, does not require human involvement, is cost effective, and most of all, does not require ingestion of a suspicious substance into an instrument. Various embodiments are presented.

2009 IEEE International Ultrasonics Symposium, 2009

Abstract—Rigorous SAW device analysis methods, such as FEM/SDA, rely upon the accurate characteri... more Abstract—Rigorous SAW device analysis methods, such as FEM/SDA, rely upon the accurate characterization of the elastic moduli and mass density for both the piezoelectric substrate as well as the materials comprising the devices' thin film electrodes. As the values for the ...

Chemical and Biological Sensing VIII, 2007

Differential reflectometry (DR) is an effective tool to supplement existing explosives detection ... more Differential reflectometry (DR) is an effective tool to supplement existing explosives detection systems thus making the combined unit more effective than one tool alone. It is an optical technique in which the investigative light beam emanates from an extended distance onto the substance under investigation, thus rendering it to be a standoff method. The applicable distance still needs to be determined but could be well within the 50 to 100 m range. Specifically, differential reflectometry (also known as Differential reflection spectroscopy) is a surface analytical technique that reveals details about the electron structure. In other words, the instrument allows the measurement of the energies that electrons absorb from photons as they are raised into higher, allowed energy states. Since each material has a specific electron structure the measurement of the characteristic energies for "electron transitions" serves as a means (i.e. a fingerprint) for identifying these substances. The DR device can be made portable, it is fast, safe for the public, does not require human involvement, is cost effective, and most of all, it is a standoff technique which does not require ingestion of a suspicious substance into an instrument.

2011 IEEE International Ultrasonics Symposium, 2011

ABSTRACT Buried electrodes SAW resonators on lithium niobate suffer from transverse mode spurii e... more ABSTRACT Buried electrodes SAW resonators on lithium niobate suffer from transverse mode spurii even if the acoustic velocity inside the resonator is faster than the acoustic velocity outside the resonator (i.e. when a 'waveguiding' condition is not established). In practical devices, the electrode end gaps play an important role since they constitute a very fast region and reflect the acoustic waves creating multiple transverse modes. Guiding conditions need to be created in resonators to obtain good quality factors. This can be done by using sufficiently large gaps sizes. Unfortunately, this leads to even stronger transverse modes. The proposed solution consists in tailoring the shape of the main mode. A “piston mode” shape is obtained by adding a slow region at the edge of the active region. The mode shape is matched to the rectangular excitation and therefore, the coupling to the higher order modes becomes negligible. Results exhibit low spurious, lower losses and better effective coupling coefficients than apodized devices. Quality factors in the 1500 range are measured on resonators while CRF losses are significantly reduced.

2010 IEEE International Ultrasonics Symposium, 2010

High coupling substrates using overcoat layers of silicon oxide for temperature compensation exhi... more High coupling substrates using overcoat layers of silicon oxide for temperature compensation exhibit a strong affinity toward multimode behavior. Apodization, which is the standard technique to suppress the transverse modes, suffers from drawbacks such as reduction of the transduction efficiency and additional losses. By changing the transducer layout, the guiding can be improved and a “piston mode” can be obtained. This approach is demonstrated on resonators and DMS test devices manufactured on lithium niobate with an orientation close to YX128 deg.

2008 IEEE Ultrasonics Symposium, 2008

ABSTRACT In order to improve the temperature stability of RF SAW devices, it is worth adding a la... more ABSTRACT In order to improve the temperature stability of RF SAW devices, it is worth adding a layer of SiO2 on the surface of the piezoelectric substrate. This paper presents an original method, which allows the computation of the Harmonic Admittance of an infinite periodic grating covered with an SiO2 overlay. The origins of this method can be found in a previous paper. By comparison with S. Ballandras and K. Hashimoto works, this methods allows to take into account the very efficient Chebyshev-like charge distribution, together with a classical FEM basis interpolation for the stresses. FEM is used to model mechanical behavior of complex shape electrodes and BEM using a periodic Green's function Kernel is used to take into account the semi-infinite piezoelectric substrate. Validations are presented by comparison with K. Hashimoto's software.

IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, 2000

It is known that a part of the loss of leaky SAW resonators is due to radiation of acoustic energ... more It is known that a part of the loss of leaky SAW resonators is due to radiation of acoustic energy in the bus-bars. Many researchers are working on so-called phononic crystals. A 2-D grating of very strong reflectors allows these devices to fully reflect, for a given frequency band, any incoming wave. A new device based on the superposition of a regular SAW resonator and a 2-D periodic grating of reflectors is proposed. Several arrangements and geometries of the reflectors were studied and compared experimentally on 48 degrees rotated Y-cut lithium tantalate. In particular, a very narrow aperture (7.5 lambda) resonator was manufactured in the 900 MHz range. Because of its small size, this resonator has a resonance Q of only 575 when using the standard technology, whereas a resonance Q of 1100 was obtained for the new device without degradation of the other characteristics. Because of the narrow aperture, the admittance of the standard resonator showed a very strong parasitic above the resonance frequency, whereas this effect is drastically reduced for the new device. These results demonstrate the feasibility of the new approach.

IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, 2000

It is well known that transverse effects contribute significantly to the loss of SAW resonators o... more It is well known that transverse effects contribute significantly to the loss of SAW resonators on lithium tantalate. In particular, for frequencies above resonance, the surface wave is not guided inside the transducer and radiates into the busbars. In addition, because bulk modes can also be excited, scalar models are not sufficient to accurately predict transverse effects. It is also known that the layout of a SAW resonator (electrode gaps and dummy electrodes) has a strong impact on the transverse effects. In this paper, a periodic FEM/BEM model is presented and is used to simulate the transverse effects for various SAW resonator layouts. Test devices matching those simulated are fabricated and measured; the measured results are compared with the simulated results and show good agreement. By analyzing the dispersion curves produced from the FEM/BEM model in the different regions of the device, several frequency bands corresponding to different transverse behaviors are identified. These results are consistent with the elastic displacements, also computed by the FEM/BEM model. It is further shown that guided conditions in the gap between the transducer and the busbar occur for a frequency range above resonance. This result is in agreement with measurements showing that resonators with smaller gaps exhibit less [AU1: fewer?]spurious responses in their admittance.

Applied Optics, 2007

Differential reflectometry (DR) is an effective tool to supplement existing explosives detection ... more Differential reflectometry (DR) is an effective tool to supplement existing explosives detection systems thus making the combined unit more effective than one tool alone. It is an optical technique in which the light beam (UV) emanates from an extended distance onto the substance under investigation, thus rendering it to be a standoff method. DR allows the measurement of the energies that electrons absorb from photons as they are raised into higher, allowed energy states. These electron transitions serve as a "fingerprint" for identifying substances. The device can be made portable; it is fast, safe for the public, does not require human involvement, is cost effective, and most of all, does not require ingestion of a suspicious substance into an instrument. Various embodiments are presented.

2009 IEEE International Ultrasonics Symposium, 2009

Abstract—Rigorous SAW device analysis methods, such as FEM/SDA, rely upon the accurate characteri... more Abstract—Rigorous SAW device analysis methods, such as FEM/SDA, rely upon the accurate characterization of the elastic moduli and mass density for both the piezoelectric substrate as well as the materials comprising the devices' thin film electrodes. As the values for the ...