Patent Grant
8389954
This invention relates to a system for generating fast ions and a method thereof.
Fast ion beams are of interest for various applications including production of radioactive isotopes, neutron production, radiography, fusion, and various forms of radiation therapy.
Beams of fast ions are typically produced in accelerators of various configurations such as cyclotrons or synchrotrons. Accelerators are relatively large and expensive machines that are costly to run and maintain. The development of lasers that are capable of providing extremely high intensities and electric fields has stimulated research in exposing matter to laser light electric fields to generate fast ions and interest in using lasers to provide relatively inexpensive fast ion sources.
U.S. Pat. No. 6,906,338 describes using laser pulses “having a pulse length between approximately 1 to 500 femtoseconds (fs)” focused to energy densities of between about 1018 to about 1023 Watts/cm2 (W/cm2) to produce a high flux of energetic ions such as protons that may be used for medical purposes. The pulses are directed to interact with targets of various designs and provide radiation components that “include different species of ions (e.g., protons), x-rays, electrons, remnants of the pulse 102, and different energy components (e.g., MeV, 10's MeV, and 100's MeV within a certain energy band or window)”. The targets may comprise a thin foil layer for absorbing pre-pulse energy of the pulses. A beam transport system allows ions, such as protons, produced in the target and having a predetermined beam emittance and energy to propagate to a “treatment field” for therapeutic applications. The patent describes targets that are concave on a side of the target downstream relative to a propagation direction of the laser pulses and may be formed having grooves, or comprising fibers, clusters, or foams. “The size of grooves, 402, fibers 404, clusters 406 or foams 408 may be designed to be shorter than the size of electron excursion in the pulse field (less than approximately 1 micron)”.
An article, “Efficient Coupling of High Intensity Short Laser Pulses into Snow Clusters”; by T. Palchan et al; Applied Physics Letters 90, 041501 (2007); published online 24 Jan. 2007 by some of the same inventors of the present invention, the disclosure of which is incorporated herein by reference, describes coupling intense laser light to a target comprising “elongated snowflakes smaller in diameter than the laser wavelength”. The snowflakes are formed on a sapphire (Al2O3) substrate located in a vacuum chamber and cooled to less than −70° C. The inventors found that about 94% of the energy in pulses of laser light of wavelength at 800 nm focused on the snowflakes to intensities between about 1×1015 W/cm2 to about 2×1016 W/cm2 was absorbed by the snowflakes. The pulses had a pulse width of about 150 fs and a contrast ratio of about 10−3.
Another article “Generation of Fast Ions by an Efficient Coupling of High Power Laser Into Snow Nanotubes”; by T. Palchan et al; Applied Physics Letters 91, 251501 (2007); published online Dec. 18, 2007 by some of the same inventors as inventors of the present invention, describes “generation of fast ions during interaction of a short laser pulse at moderate intensity, I˜1016-1017 W/cm2, with snow nanotubes”. The article, the disclosure of which is incorporated herein by reference, notes that H-like and He-like oxygen having kinetic energy up to 100 keV were generated in the interaction. The target of snow nanotubes “were snow clusters . . . grown by depositing H2O vapor into vacuum onto 1 mm thick sapphire (Al2O3) plate at a temperature of 100 K. The snow clusters were randomly deposited to form a layer on the sapphire substrate about 100 microns thick and comprised “elongated cluster with characteristic size in the range of 0.01-0.1 μm”.
The inventors have found that for a given intensity of high power coherent electromagnetic radiation, a non-oriented target (T) such as described in the articles referenced above, interacting with the radiation beam tends to produce relatively large fluxes of relatively high energy ions.
The inventors have now created oriented patterned targets (OPT) and investigated the interaction of such oriented patterned target (OPT) with incident electromagnetic radiation. The pattern on a surface of the target substrate has pattern features having certain longitudinal axes (so-called “elongated features”) which are uniformly oriented along a certain common axis. Such pattern features of the OPT may be constituted by wire-like elements, e.g. nano-wires, filaments, etc. These oriented pattern features present roughness on the OPT surface, which roughness may or may not be implemented as a continuous-surface relief.
The use of such OPT allows for optimizing parameter(s) of the incident electromagnetic radiation to enhance the efficiency of the radiation coupling into the OPT contributing to creation of fast ions with high kinetic energy. Such optimizable parameters include an angle of incidence of a beam of electromagnetic radiation onto the OPT surface and/or polarization of the incident beam. As will be described further below, the angle of incidence is a so-called “grazing angle”, i.e. angle less than 45° between the beam propagation axis and the OPT surface (or higher than 45° in the meaning of “incident angle” being an angle between the beam propagation axis and the normal to the OPT surface). It should be understood that the optimal value of the grazing angle (magnitude as well as azimuth and elevation) should be appropriately selected and/or gradually varied, in accordance with the critical dimensions of the pattern (including the depth of pits), as well as the direction of orientation, to achieve the generation of an optimal fast ion beam.
As for the polarized electromagnetic radiation e.g. linear polarized light, it should be understood that this means light having a predetermined preferred polarization direction. The polarization direction has been selected relatively to the orientation axis of the OPT, and the fluxes and the energy of the ions, seem to be enhanced in comparison with non-oriented targets (T). Therefore, using an OPT target is more efficient than using T targets for producing relatively fast ions at relatively large fluxes.
It should be understood that, a target comprising randomly oriented filaments is referred to as a “target (T)”, and that a target having a surface pattern exhibiting a preferred direction of orientation is referred to as an “oriented patterned target (OPT)”.
In particular, a laser pulse having intensity between about 5×1019 W/cm2 and about 5×102° W/cm2 interacting with an OPT target, would produce a burst of protons having energy between about 20 and 200 MeV. The burst may comprise more than 106 protons, more than 107 protons, more than 108 protons, more than 109 protons or even 1010 protons.
Therefore, the present invention provides a new system and method for generating fast ions (a beam of fast ions). The system comprises a target substrate having a surface relief with nanoscale feature (i.e. roughness) (i.e. a patterned surface, the pattern comprising nanoscale pattern features) oriented substantially homogeneously/uniformly along a certain axis/a common axis (i.e. having a predetermined direction of orientation) and a beam unit to be used with a high power coherent electromagnetic radiation source (e.g. laser); the beam unit being adapted to receive a high power coherent electromagnetic radiation beam and to focus the radiation beam onto the patterned surface of the target substrate to cause interaction between the radiation beam and the substrate enabling creation of fast ions.
In some embodiments, the beam unit is adapted to direct the electromagnetic radiation beam onto the patterned surface of the target substrate with a predetermined grazing angle. The grazing angle is selected in accordance with the pattern such that the interaction provides an efficient coupling between the radiation beam and the substrate enabling creation of fast ions of desirably high kinetic energy.
It should be noted that generally, the grazing angle refers to the angle between the beam and the surface, i.e. 90° minus the angle of incidence. In some embodiments, the grazing angle is lesser than 45°. In some embodiments, the grazing angle is in the range of about 20°-40° (i.e. angle of incidence 50°-70°.
In some embodiments, the electromagnetic beam has a pre-defined polarization direction defining a certain angle between the polarization direction and the orientation axis of the pattern features of the target substrate is selected such that the interaction provides an efficient coupling between the radiation beam and the substrate enabling creation of fast ions having a desirably high kinetic energy.
Thus, an angle between a polarization direction of the beam of electromagnetic radiation and the orientation axis of the pattern features of the target substrate, and the grazing angle are selected such that interaction between the radiation beam and the substrate provides an efficient coupling between the radiation beam and the substrate enabling creation of fast ions. By this, the invention enables providing ion sources producing ions in relatively large quantities. In some embodiments, the angle between the polarization direction and the orientation axis is in a range of 0°-30°.
The system of the present invention provides fast ions having kinetic energy about equal to or greater than at least one of 5 MeV; 50 MeV; 100 MeV; 150 MeV; 200 MeV.
In some embodiments of the invention, the ions comprise protons. In some embodiments of the invention, the ions comprise Oxygen ions.
In some embodiments of the invention, the system comprises a beam unit configured and operable to selectively adjust the direction of polarization to different angles relative to the direction of orientation of the OPT.
According to some embodiments of the invention, the radiation beam comprises polarized beam having a desired direction of polarization relative to the direction of orientation of the OPT. In some embodiments, the polarization direction is substantially parallel to the orientation axis.
In some embodiments, the beam unit is configured to orient the polarization direction such that the polarization direction is substantially parallel to the direction of orientation.
In other embodiments, the beam unit is configured to orient the polarization direction such that the polarization direction has a relatively small angle (0°-30° to the direction of orientation.
In some embodiments of the invention, the beam unit is configured and operable to focus the radiation beam to a spot size in the target for which the beam has a maximum intensity about equal to or greater than at least one of 1016 W/cm2; 1017 W/cm2; 1018 W/cm2; 1019 W/cm2; 1020 W/cm2.
In this connection, it should be understood that, an electric field produced by a laser beam with intensity
For a short powerful laser beam of 1012 Watt focused to a spot diameter of 5 microns, an electric field of about
is generated at the focal region. This field is larger than the electric field binding the electrons in the Hydrogen atom. Therefore, while interacting, the electrons are photo-ionized through one of three mechanisms. The dominate process would depend on the laser intensity and ionization potential. The first mechanism is a multi-photon ionization mechanism in which a number of photons hit the atom simultaneously to overcome the energetic gap need for ionization (one photon of 800 nm beam has about 1.5 eV). The second mechanism is a tunnel ionization mechanism in which the atom's electric field is distorted by the laser beam and the probability of an electron to tunnel is non negligible due to the reduced potential barrier. The third mechanism is an ionization mechanism over the barrier in which the electric field of the laser beam is large compared to the ionization potential in which the electrons are essentially free and gain kinetic energy from the laser electric field. The Keldysh parameter which is defined by
where Ip is the ionization potential and
is the ponderomotive potential. When γ>>1 multi-photon ionization is the dominate mechanism for ionization. In the present invention, the radiation beam at the focal point on the target has a maximum intensity about equal to or greater than at least one of 1016 W/cm2, 1017 W/cm2, 1018 W/cm2, 1019 W/cm2, 1020 W/cm2, therefore γ<1 and the mechanisms involved are the second and in some eases the third mechanism. Therefore, when the leading edge of the radiation beam reaches the target it ionizes the atoms, such that the interaction between the radiation beam and the OPT is essentially with plasma.
In some embodiments, the patterned surface of the target substrate is a continuous surface and the pattern comprises grooves.
In some embodiments, the nanoscale features comprises discrete nanostructures which may be elongated.
For example, the nanoscale features have a characteristic width less than or about equal to at least one of 0.5λ; 0.25λ; 0.1λ; 0.05λ; 0.02λ and a characteristic length greater than or about equal to at least one of λ; 2λ; 5λ; 10λ.
The inventors believe that the surface pattern of the targets acts as a field concentrator for the electric field of the electromagnetic radiation (e.g. light pulses) interacting with the target.
In particular, according to some embodiments of the invention, the surface pattern comprises a layer of filaments/nanowires characterized by a substantially uniform direction of orientation. In this case, the filaments may act as conductive needles concentrating and amplifying the laser electric field at their ends, like a macroscopic metal needle in an electric field generates an intense electric field at its point, or the local field enhancement measured at plasmon resonances.
In some embodiment of the invention, the surface pattern comprises nano-crescent shaped structures scattered on the substrate all aligned in the same direction. In this case, the nano-cresents can act as bent conducting needles concentrating and amplifying the laser electric field at their ends.
In some embodiments of the invention, the filaments are ice filaments. It should be noted that the terms “ice”, “snow”, and “H2O vapor” in the context of this patent application are used interchangeably all to refer to pattern features made from water vapor.
In some embodiments of the invention, the patterned surface has a thickness greater than or about equal to at least one of 1 μm; 10 μm; 20 μm; 50 μm; 100 μm.
In some embodiments, the target substrate is made of at least one of sapphire, silicon, carbon or plastics material.
In some embodiments, the target substrate is made by interacting the substrate with water vapor in a vacuum chamber while under biasing electric field across the substrate, thereby creating nanoscale features oriented along the electric field.
In some embodiments of the invention, the radiation beam comprises at least one pulse of laser light. Optionally, the pulse has duration less than or about equal to at least one of 1 ps; 0.5 ps; 0.2 ps; 0.1 ps; 0.03 ps.
In some embodiments of the invention, the invention enables a new way of employing “pre-pulses” for plasma production. A pre-pulse is an energy pulse that precedes the main plasma-producing pulse. It should be noted that generally pre-pulses are an artifact of laser amplification and typically have intensities between 10−3 and 10−6 that of a laser light pulse that they precede. Pre-pulses generally interfere with interaction of laser light pulses with matter in a target. A pre-pulse typically creates plasma on a surface of a target that reflects energy in the laser light pulse incident on the target surface following the pre-pulse and reduces thereby efficiency with which energy in the following light pulse couples to the target. However, it appears that pre-pulses accompanying laser pulses that interact with an OPT target, in accordance with an embodiment of the invention, are dissipated by ablation and ionization of a portion of the targets. The plasma created by a pre-pulse ablating and ionizing a portion of an OPT target, in accordance with an embodiment of the invention, is generally sub-critical density plasma, which does not interact strongly with energy in a subsequent pulse associated with, and following, the pre-pulse. As a result, the subsequent pulse is able to interact relatively efficiently with remaining, non-ablated, portions of the targets substantially without interference from plasma generated by the pre-pulse.
Although energy pulses in the form of laser pulses are preferred, other types of energy pulses are also conceivable, such as ultra short electron beam pulses. However, in the following description, energy pulses in the form of laser pulses will be taken as the preferred example. The electromagnetic radiation may be a laser light pulse which typically comprises a pre-pulse preceding the main pulse. However, the system of the present invention may also be used with laser systems reaching very low contrast ratios (i.e. the pre-pulse have intensities between of about 10−14 of the main pulse). The beam source may be controlled such that the pre-pulse may precede the pulse by a period equal to or greater than about 10 ns. Additionally or alternatively, the surface pattern has a characteristic dimension greater than or about equal to a path length of the beam in the surface pattern sufficient to absorb substantially all the energy in the pre-pulse.
According to another broad aspect of the present invention, there is also provided a method for generating fast ions. The method comprises irradiating a target substrate with a high power polarized coherent electromagnetic radiation beam, wherein the target substrate has a patterned surface with a pattern comprising nanoscale pattern features oriented substantially uniformly along a common orientation axis. A relation between the pattern and at least one parameter of the electromagnetic radiation is optimized by selecting at least one of an angle between a polarization direction of the beam of electromagnetic radiation and the orientation axis, and an incident angle for the beam of electromagnetic radiation, such that interaction between the radiation beam and the patterned surface of the substrate provides an efficient coupling between the radiation beam and the substrate resulting in generation of a fast ions' beam.
In some embodiments, the method comprises receiving the high power coherent polarized electromagnetic radiation beam and directing the radiation beam onto the surface of the target substrate at a desired grazing angle.
In some embodiments, the method comprises fabricating the target substrate by interacting a substrate with water vapor in a vacuum chamber while under biasing electric field across the substrate, thereby creating a target in the form of patterned substrate, the pattern having nanoscale features oriented in a predetermined substantially homogeneous direction along the electric field.
In order to understand the invention and to see how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:
Region D of the figure represents the common knowledge in the field using various laser configurations. The proton energy is approximately scaled as the square root of the laser intensity (i.e. Eprotons˜I0.6). As clearly seen in the figure, OPT target (filled circles) provides about an order of magnitude above the results obtained by the other targets (square and triangle marks).
In a specific and non-limiting example, the OPT target is formed by H2O nano-wires layed on a substrate of sapphire. The diameter of the wires is about 100 nm while their length is a few micrometers. The wires are therefore sub-resonant, e.g. the diameter of the wires is smaller than the irradiated laser electric field wavelength which is about 0.8 μm. The inventors have found that, when exposed, the target absorbs over 95% of incident light. Moreover, as detailed below, the target is less susceptible to the pre-pulse, which typically reduce the coupling of radiation beam to target. The target also enhances the electric field associated with the interaction and acceleration of charged particles.
In some embodiments, the surface pattern of the targets acts as a field concentrator for the electric field of the electromagnetic radiation (e.g. light pulses) interacting with the target. In particular, according to some embodiments of the invention, the surface pattern comprises a layer of filaments/wires characterized by a direction of orientation. In this case, the filaments may act as conductive needles concentrating and amplifying the laser electric field at their ends, like a macroscopic metal needle in an electric field generates an intense electric field at its point. The geometrical dimensions of the narrow tips at the end of the wires generate a large charge-separation when irradiated by the electric field. As mentioned above, the high intensity laser pulse ionizes the wires. The charge separation induced by the wire geometry is locally added to the electric field of the laser interacting with the individual particles (electron and protons).
The main parameter for calculating the field enhancement is the geometrical ratio, g, which is the ratio between the diameter and length of a nanoscale feature.
The field enhancement factor (FEE) scales with g linearly,
Here Elaser is the corresponding electric field to irradiated laser pulse and Eenhanced is the effective electric field that is involved in the acceleration process of the ions.
Reference is made to
The radiation beam 32 is directed towards to target 40 at a desired grazing angle θ. The radiation beam 32 has a predetermined polarization direction indicated by an arrow 34. For example, the beam unit 30 is controllable to provide polarized laser beam pulses that are focused to a focal region in OPT 40 schematically indicated by a circle 60.
In this specific and non-limiting example, the surface pattern of the OPT 40 comprises oriented filaments formed on and supported by a target pedestal 50. An arrow 44 indicates a direction of orientation that characterizes orientation of nanoscale features 42 and OPT 40. In an embodiment of the invention, polarization direction 34 is substantially parallel to direction 44 of orientation of OPT 40.
Pedestal 50 may comprise a sapphire substrate 51 coupled to a cooling unit 52 configured in accordance with any of various techniques known in the art. Optionally, cooling unit 52 comprises a Cu heat exchanger block 54 coupled to a liquid nitrogen circulation system (not shown) that pumps liquid nitrogen through the heat exchanger to remove heat from sapphire substrate 51. The substrate is sandwiched between bias electrodes 56 that are connected to a power supply 55. OPT 40 and pedestal 50 are located in a vacuum chamber (not shown).
To produce OPT 40, in accordance with an embodiment of the invention, pressure in the vacuum chamber is reduced to between about 5×10−4 mBar to about 10−5 mBar and the cooling unit is operated to cool substrate 51 to about 80° K. Power supply 55 is controlled to apply a potential voltage between electrodes 56 that generates a biasing electric field in substrate 51, which is parallel to direction of orientation 44. Water vapor is then introduced into the vacuum chamber and condenses on substrate 51 in the form of elongated ice filaments 42. Because water is a polar molecule, as the molecules condense onto the substrate and grow ice filaments 42, the molecules, and the ice filaments tend to orient parallel to the electric biasing field and thereby direction of orientation 44. Other materials having the ability to be patterned, the pattern having nanoscale pattern features oriented substantially uniformly along a common axis, such as silicon, carbon or plastics (i.e. C—H composites) can also be used to form the target substrate having a substantially uniform direction of orientation according to the teachings of the present invention.
In some embodiments, the radiation beam 32 includes a beam pulse.
In an embodiment of the invention, water vapor is introduced into the vacuum chamber for a period long enough to grow layer 41 of surface pattern to thickness sufficient to absorb substantially all the energy in pre-pulse 33 and pulse 32. Pre-pulse 33 energy would therefore be dissipated by ablating and ionizing a portion of layer 41 and leave in place of the ablated material a relatively thin, sub-critical density, plasma overlaying a remaining portion of layer 41 prior to pulse 32 reaching the layer. The sub-critical density plasma does not interact strongly with energy in pulse 32, and as a result, energy in pulse 32 couples efficiently to the nanoscale features 42 in the remaining non-ablated portion of layer 41.
The presence of the electric field generated in substrate 51 would of course not result in all nanoscale features 42 that condense on the substrate being substantially aligned along direction 44. However, the electric field results in a density of aligned surface pattern (e.g. ice filaments) that characterizes layer 41 and OPT 40 with orientation direction 44. And it is expected that interaction of OPT 40 with pulse 33 of beam polarized in a direction, e.g. direction 34, parallel to direction of target orientation 44, in accordance with an embodiment of the invention, would be enhanced relative to interaction of the pulse with a non-oriented target T. Ion fluxes and energies provided by interaction of radiation beam (e.g. laser light pulse) with OPT 40 are therefore expected to be enhanced relative fluxes and energies provided by interaction of the light pulse with a T target.
The inventors have conducted experiments with a T target comprising a layer of non-oriented ice filaments interacting with intense, 800 nm wavelength laser light pulses to produce fast ions. An experiment conducted by the inventors was reported in the article entitled “Generation of Fast Ions by an Efficient Coupling of High Power Laser into Ice Nanotubes”, referenced above. The experiments indicate that fluxes of 150 KeV protons are produced per laser light pulse having pulse width less than about 0.1 ps and “moderate” intensity of about 1016 W/cm2 incident on a 1 mm thick T ice filament target formed on a target pedestal similar to pedestal 50. To produce same energy protons from conventional interaction of a laser light pulse and a solid, non-filamentary target, the laser pulse typically requires intensity of about 1017 W/cm2, which is about an order of magnitude greater than that required using a T target.
In some embodiments of the invention, beam unit 30 focuses beam radiation 32 (e.g. laser light pulse) to a maximum intensity about equal to or greater than at least one of the followings: 1016 W/cm2; 1017 W/cm2; 1018 W/cm2; 1019 w/cm2; 1020 W/cm2.
Because the surface pattern has sub-resonant nanoscale features 42 e.g. the width of the surface pattern is much smaller than the wavelength of light in pulse 32, the electric field of the pulse at any given moment is substantially constant within and in the neighborhood of the surface pattern. Without being bound by any particular theory, as mentioned before, the inventors believe that the surface pattern therefore acts similarly to a conducting needle in, and parallel to, an electric field, and concentrates the field at its tips, and that the concentrated field of a plurality of oriented nanoscale features 42 is particularly advantageous for generating a relatively large flux of fast protons. An inset 70 schematically shows nanoscale features 42 in the electric field of a localized region of pulse 32 smaller than a wavelength λ of light in the pulse. A block arrow 72 represents the electric field of light pulse 32 near feature 42 and dashed field lines 76 converging towards a tip 74 of the feature schematically represent the concentrated field at the tip.
Concentrated field 76 generates a plume of hot electrons, schematically represented by circles 80, that leave feature 42 near its tip 74 by ionizing hydrogen and oxygen atoms (not shown) in the feature. The plume of electrons and ionized atoms in feature 42 produce an intense double layer field (not shown) that accelerates hydrogen ions in the filament to relatively high energies producing the flux of protons represented by cluster of arrows 68.
It is noted that efficacy with which light pulse 32 produces fast ions 68 by interacting with OPT 40 (
This application was filed under 35 USC 371 as a National Phase of International Application PCT/IL2009/001201, filed Dec. 20, 2009, having a claim of priority to U.S. Provisional Application 61/138,533, filed Dec. 18, 2008, the entirety of which is incorporated herein by reference.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IL2009/001201 | 12/20/2009 | WO | 00 | 6/16/2011 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2010/070648 | 6/24/2010 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 6906338 | Tajima | Jun 2005 | B2 |
| 7420687 | Pfaff | Sep 2008 | B2 |
| 7704684 | Rogers et al. | Apr 2010 | B2 |
| 7728958 | Pfaff | Jun 2010 | B2 |
| 20020090194 | Tajima | Jul 2002 | A1 |
| Entry |
|---|
| T. Palchan et al., “Generation of fast ions by an efficient coupling of high power laser into snow natubes” Appl. Phys. Lett., vol. 91, No. 25, Dec. 18, 2007, pp. 251501-1-251501-3, XP002579654 New York. |
| Kahaly et al., “Near-Complete Absorption of Intense, Ultrashort Laser Light by Sub-Lambda Gratings” Phys. Rev. Letters, vol. 101, 145001, Oct. 20, 2008, pp. 1-4, XP002579655. |
| Hutley, Maystre: “The total absorption of light by a diffraction grating” Optics Communications, vol. 19, No. 3, Dec. 1976, pp. 431-436, XP002579656 Amsterdam ISSN: 0030-4018. |
| Murnane et al., “Efficient coupling of high-intensity subpicosecond laser pulses into solids” Appl. Phys. Lett, vol. 62, No. 10, Mar. 8,1993, pp. 1068-1070, XP002579657 New York. |
| T. Palchan et al., “Efficient coupling of high intensity short laser pulses into snow clusters” Appl. Phys. Lett., vol. 90, No. 4, Jan. 24, 2007, pp. 41501-1-41501-3, XP002579658 New York. |
| Number | Date | Country | |
|---|---|---|---|
| 20110248181 A1 | Oct 2011 | US |
| Number | Date | Country | |
|---|---|---|---|
| 61138533 | Dec 2008 | US |
