Patent Description:
More particularly, the present disclosure relates to techniques for beam forming in both the near-field zone and the far-field zone.

The disclosure can be of interest in any field where beam forming is needed, as for instance in dense and photonic systems using focusing devices, like integrated optical sensors used in photo/video cameras that are essential components of the today and tomorrow mobile technology (e.g. smartphones, tablets, augmented reality (AR) and virtual reality (VR) glasses), or in integrated lens antenna systems, light communication systems, including quantum computers, microscopy, spectroscopy and metrology systems, etc..

In this document, we more particularly detail an existing problem in the field of optical devices to which the inventors of the present patent application were faced. The disclosure is of course not limited to this particular field of application, and is of interest in any field were EM wave patterns need to be formed.

Optical focusing, beam and field forming are of a great interest for the emerging technology known as AR and VR glasses, like those produced by Google, Magic Leap, Microsoft, etc. Here, various types of refractive and diffractive lenses and beam-forming devices are used to guide the light from a micro-display or a projector towards the human eye forming a virtual image. In case of AR glasses this virtual image is superimposed with an image of the physical world seen with a naked eye.

Among other, devices allowing to form EM wave patterns in a form of a mastered beam shape and EM field structure, in a preferred configuration with a single lobe of known deviation angle and spread angle are of interest. The availability of such mastered beam indeed removes spurious contributions due to multiple secondary lobes or to a non-optimal (e.g. larger) spread angle of the beam. It cleans the composition of different edge portions into field forming microstructure shapes or ridges. A device with structural elements having nano-scale dimensions for imaging applications is disclosed in <CIT>.

The today level of technologies enables fabrication of highly-integrated components (e.g. chips and optical sensors) with structural elements having nano-scale dimensions, which are close to or even smaller than the wavelength of visible light. The possibility of manipulating light with the same level of accuracy would become a great breakthrough compared to the state of the art.

There is thus a need for new focusing devices allowing to form EM wave patterns, for instance in a mastered beam shape comprising a single lobe of known deviation angle and spread angle.

There is a need for having such new focusing device that can be adapted to visible light applications, preferably using structures having nano-scale dimensions.

Aspects of the present invention are defined by the appended claims.

Thus, the present disclosure proposes solutions for forming electromagnetic wave patterns.

More particularly, when the claimed device is illuminated by an incoming electromagnetic wave, an outgoing electromagnetic wave (or jet wave) is generated at the level of the contact area and propagates from the material of lower refractive index (i.e. the first material) toward the material of higher refractive index (i.e. the second material). Furthermore, having the height of the projection of the contact area that is less than <NUM>% greater than the critical height, the outgoing electromagnetic wave comprises a single lobe so that the device behaves as a point source generating the jet wave in question.

According to one embodiment not according to the claimed invention, an angle of incidence between said direction of propagation of said incoming electromagnetic wave and a direction of extension of said single contact area lays between -<NUM> degrees and +<NUM> degrees, preferably between -<NUM> degrees and +<NUM> degrees, more preferably between -<NUM> degrees and +<NUM> degrees.

According to one embodiment, the wavelength in vacuum of the incoming electromagnetic wave belongs to the visible light spectrum.

Thus, devices with nano-scale dimensions can be obtained for forming visible light wave patterns.

According to one embodiment, the first material is a liquid and the second material belongs to the group comprising:.

According to the claimed invention, the second part, forms at least a container, the first material being a fluid filling at least part of the container.

Thus, when the claimed device is illuminated by an incoming electromagnetic wave, the characteristics of the outgoing electromagnetic wave (spread angle, jet direction, etc.) generated from the first material toward the second material at the level of the contact area depend on the refractive index of the fluid. This allows for an estimation of the refractive index in question based on a measurement of the characteristics of the outgoing electromagnetic wave.

According to one embodiment, the height of the single contact area is sensibly equal to the critical height.

Thus, the outgoing electromagnetic wave is of highest intensity, the single lobe having a minimum spread angle.

According to the claimed invention, the device comprises an electromagnetic source configured for generating the incoming electromagnetic wave and a photodiode configured for capturing at least part of the outgoing electromagnetic wave radiated by the single contact area when the device is illuminated by the incoming electromagnetic wave.

Thus, the refractive index of the fluid can be estimated based on a measurement of the characteristics of the outgoing electromagnetic wave generated when the claimed device is illuminated by an incoming electromagnetic wave.

According to one embodiment, the incoming electromagnetic wave interferes with the outgoing electromagnetic wave radiated by the single contact area for generating a corresponding nanojet hotspot.

Another aspect of the present disclosure not according to the claimed invention relates to a use of a device according to the disclosure for trapping or moving nano-particles in the outgoing electromagnetic wave radiated by the single contact area when the device is illuminated by the incoming electromagnetic wave.

Another aspect of the present disclosure relates to a use:.

Other features and advantages of embodiments shall appear from the following description, given by way of indicative and non-exhaustive examples and from the appended drawings, of which:.

In all of the figures of the present document, the same numerical reference signs designate similar elements and steps.

We now describe in relationship with <FIG>, the characteristics of an outgoing EM wave 100o (or jet wave) radiated by one contact area <NUM> between a first part <NUM> of a first material (e.g. glass, plastic, a liquid, a polymer material, etc.) having a first refractive index n1 and a second part <NUM> of a second material (e.g. glass, plastic, a liquid, a polymer material, etc.) having a second refractive index n2 higher than n1 when the contact area <NUM> is illuminated by an incoming EM wave 100i radiated by an electromagnetic source <NUM>. The outgoing EM wave 100o is radiated from the first part <NUM> of low refractive index n1 toward the second part <NUM> of high refractive index n2.

More particularly, the contact area <NUM> extends along the direction of propagation of the incoming EM wave 100i (i.e. along the Oz axis as illustrated in <FIG>) and has a non-vanishing height h along that direction. Due to complex EM phenomenon at the interface between the first <NUM> and second <NUM> parts (involving e.g. both diffraction and refraction), the outgoing EM wave 100o radiated by the contact area <NUM> has particular characteristics.

For instance, depending on the height h of the contact area <NUM>, the outgoing EM wave 100o presents a different number N of lobes.

An explication for this dependency of N on the height h comes from the observation of the different periodic variations of:.

Indeed, the outgoing EM wave 100o radiated by the contact area <NUM> can be seen as correlated to a difference between the first and second EM wave that each propagate along the Oz axis (i.e. along the direction of propagation of the incoming EM wave 100i). As a result, the amplitude of the outgoing EM wave 100o along the contact area extension is weighted by the function: <MAT> which can be rewritten as a product of sine functions according to: <MAT> with λ the equivalent wavelength in vacuum of the incoming EM wave 100i.

The various terms involved in this equation are plotted in <FIG>; i.e.:.

The low frequency beat of D<NUM>(z) shapes the lobes of the outgoing EM wave 100o. More particularly, the fraction of the contact area <NUM> extending:.

Thus, in order to achieve an outgoing EM wave 100o comprising a single lobe of higher intensity, one has to select h=hc.

In practice, with h up to <NUM>% more than hc, only the first lobe is entirely present in the outgoing EM wave 100o. Only a fraction of the second lobe starts to be present in the outgoing EM wave 100o so that a simple decrease in the intensity of the first lobe is experienced in practice. When h reached <NUM>% to <NUM>% of hc the second lobe orientates toward the denser medium and comes into interference with the first lobe. There is a π phase shift between the first and second lobe such that the spread angle of the first lobe decrease when the height h increases above hc. For a height h higher than hc, secondary lobes appear progressively, with a dark fringe limit between lobes being identified for secondary critical heights of the form <MAT> with k = <NUM>,<NUM>,.

Consequently, a device according to the disclosure comprises at least one contact area according to the geometry of the contact area <NUM>, but with a height h lower than <NUM> times the critical height hc.

In some embodiments, a device according to the disclosure comprises a single contact area according to the geometry of the contact area <NUM>. For instance, a device <NUM> according to one embodiment of the invention is now described in relationship with <FIG>.

More particularly, the geometry of the device <NUM> is the same as discussed above in relation with <FIG> except that the height h of the contact area <NUM> comprised in the device <NUM> is targeted to be equal to the critical height hc. Accordingly, when illuminated by the incoming EM wave 100i that propagates along the Oz axis, the contact area <NUM> radiates the outgoing EM wave 100o that now comprises a single lobe whose characteristics are the following:.

Furthermore, when the incoming EM wave 100i radiated by the electromagnetic source <NUM> interferes with the outgoing EM wave 100o radiated by the contact area <NUM>, a nanojet hotspot <NUM> (i.e. a localized area where the EM field reaches a high intensity) is generated in the vicinity of the contact area <NUM>.

In some embodiments, the wavelength in vacuum λ of the incoming EM wave 100i belongs to the visible light spectrum (e.g. the wavelength in vacuum λ lays between <NUM> and <NUM>, or equivalently the frequency of the incoming EM wave lays between <NUM> THz to <NUM> THz). Thus, devices with nano-scale dimensions can be obtained for forming visible light wave patterns. In other embodiments, the frequency of the incoming EM wave belongs to the group comprising:.

In other embodiments (not illustrated), the device according to the disclosure exhibits a symmetry of the structure along the x axis. In that case, for a TM polarization of the incoming EM wave 100i, the outgoing EM wave 100o presents a π phase shift whereas there is no phase shift for TE polarization.

In other embodiments (not illustrated), the height h of the contact area <NUM> comprised in the device <NUM> is lower than the critical height hc. In that case, the spread angle γC increases for decreasing values of the height h.

In other embodiments not according to the claimed invention not illustrated), the height h of the contact area <NUM> comprised in the device <NUM> is comprised between the critical height hc and <NUM>% more than the critical height hc. In that case, the spread angle γC is stable.

In other embodiments not according to the claimed invention (not illustrated), the contact area <NUM> does not extend along the direction of propagation of the incoming EM wave 100i. In that case, the technical effect is achieved (i.e. a single lobe is radiated by the contact area <NUM>) as long as a projection of the contact area <NUM> along the direction of propagation of the incoming EM wave has a non-vanishing height lower than <NUM> times the critical height hc.

Thus, a device according to the disclosure comprises a first part <NUM> of a first material having a first refractive index n1 and a second part <NUM> of a second material having a second refractive index n2 higher than n1. Such device further comprises at least one contact area <NUM> in between the first <NUM> and second <NUM> parts, radiating an outgoing EM wave 100o when the device is illuminated by an incoming EM wave 100i. A projection of the at least one contact area <NUM> along a direction of propagation of the incoming EM wave 100i has a non-vanishing height h lower than <NUM> times the critical height hc.

In some embodiments not according to the claimed invention, an angle of incidence between the direction of propagation of the incoming EM wave 100i (i.e. the direction given by the wave vector) and a direction of extension of the contact area <NUM> lays between -<NUM> degrees and +<NUM> degrees. In other words, there is a direction of extension of the contact area <NUM>, i.e. the axis oz in the present case, that forms an angle with the wave vector of the incoming EM wave 100i that lays in the specified range of -<NUM> degrees and +<NUM> degrees. Preferably, such angle lays between -<NUM> degrees and +<NUM> degrees, more preferably between -<NUM> degrees and +<NUM> degrees. In some embodiments, the direction of propagation of the incoming electromagnetic wave is tangential to the contact area <NUM>.

We now describe in relationship with <FIG>, not according to the claimed invention, the use of a device according to the disclosure for trapping and moving nano-particles.

Indeed, the force created by the interaction of the EM field on a polarizable nano-particle can trap or move the nano-particle in the EM field. The trajectory for the move depends on the EM field distribution.

To calculate the optically induced force on a nano-particle, one can use the Rayleigh criteria which holds for particles much smaller than the incident wavelength. In this regime, the optically induced force can be calculated from the EM field using the following equation (see e.g. <NPL>, or <NPL>): <MAT> where xi is a spatial coordinate (x, y and z), α is the particle's complex polarizability and E is the electric field acting on the particle. The polarizability of the nano-particle can be estimated based on the following equation: <MAT> where r is the radius of the nano-particle and ε is its relative permittivity of the nano-particle, which is a complex number.

From the two above equations we see that the optically induced force is different for particles with similar material but different sizes (dependency on r). We can also observe the dependency of the optically induced force on ε of the nano-particle. These dependencies provide the means for applications such as nano-particle sorting based on e.g. their size or refractive index.

For instance, considering the outgoing EM wave 100o radiated by the geometry illustrated in <FIG> and discussed above, the simulations depicted in <FIG> show that:.

Thus, a device according to the disclosure (i.e. a device having at least one contact area <NUM> with a height h lower than <NUM> times hc as discussed above, in any of its embodiments) can be used to radiate a structured EM field which can create separate (non-crossing) trajectories. This provides a good control over the exact position of the nano-particles without ambiguity about the future path. Such device can thus be used for trapping or moving nano-particles in the outgoing EM wave 100o radiated by the contact area <NUM> when the device is illuminated by the incoming EM wave 100i radiated by the electromagnetic source <NUM>. For instance, the applications can be:.

We now describe in relationship with <FIG>, not according to the claimed invention, a device <NUM>' for focusing toward a focal point <NUM> an incoming EM wave 100i when the device <NUM>' is illuminated by the incoming EM wave 100i radiated by the electromagnetic source <NUM>.

More particularly, as for the device <NUM> discussed above in relation with <FIG>, the device <NUM>' comprises:.

However, the device <NUM>' further comprises at least two contact areas <NUM>'<NUM>, <NUM>'<NUM> in between the first <NUM>' and second <NUM>' parts, radiating each an outgoing EM wave 100o1, 100o2 propagating toward the focal point <NUM> when the device <NUM>' is illuminated by the incoming EM wave 100i. A projection of each of the at least two contact areas <NUM>'<NUM>, <NUM>'<NUM> along the direction of propagation of the incoming EM wave 100i has a non-vanishing height h lower than <NUM> times the critical height hc as defined above.

The device thus behaves as a lens and can be used for focusing the incoming EM wave 100i toward the focal point <NUM>.

Furthermore, as shown in <FIG>, the width of the focal point <NUM> is minimum when the height h of the projection of the at least two contact areas <NUM>'<NUM>, <NUM>'<NUM> along the direction of propagation of the incoming EM wave 100i is equal to the critical height hc. In one embodiment, the height h of the projection of the at least two contact areas <NUM>'<NUM>, <NUM>'<NUM> is thus equal to the critical height hc.

In the embodiment illustrated in <FIG>, the at least two contact areas <NUM>'<NUM>, <NUM>'<NUM> are located on different sides (or faces) of a cuboid filled with the second material.

In other embodiments, the at least two contact areas are located on a different sub-area of a cylinder extending along the direction of propagation of the incoming EM wave 100i and filled with the second material.

We now describe in relationship with <FIG>, a system comprising a photodiode <NUM> and a device <NUM>" for measuring the index of refraction of a fluid according to one embodiment of the invention.

More particularly, as for the devices <NUM> and <NUM>' discussed above, the device <NUM>" comprises:.

However, in the present embodiment, the second part <NUM>" forms at least a container. The first material that compose the first part <NUM>" is a fluid filling at least part of the container.

Thus, when the device <NUM>" is illuminated by the incoming EM wave 100i, the characteristics (spread angle, direction, etc.) of the outgoing EM wave 100o generated from the first material toward the second material at the level of the contact area <NUM>" depend on the refractive index n1 of the fluid following the derivation discussed above in relation with <FIG>.

This allows for an estimation of the refractive index n1 based on a measurement of the characteristics of the outgoing EM wave 100o.

The photodiode <NUM> can thus be used for capturing at least part of the outgoing EM wave 100o.

The device <NUM>", in combination or not with the photodiode <NUM>, can thus be used for measuring the index of refraction n1 of the fluid.

In some embodiments not according to the claimed invention, this is the first part of the first material of low refractive index n1 that forms at least a container. The second part of the second material of high refractive index n2 is composed of a fluid filling at least part of the container. In this configuration, the index of refraction n2 of the fluid can be measured based on the characteristics of the outgoing EM wave 100o.

In some embodiments, the height h of the contact area <NUM>" is equal to the critical height hc so that the measurement is of higher accuracy through the thinner width achieved for the lobe of the outgoing EM wave 100o.

Claim 1:
A method comprising:
directing an incoming electromagnetic wave on a device (<NUM>");
wherein the device comprises:
a first part (<NUM>") comprising a first material having a first refractive index n1, wherein the first material is a liquid;
a second part (<NUM>) comprising a second material having a second refractive index n2 higher than n1, wherein the first part is disposed inside a container formed by the second part;
at least one contact area (<NUM>") between the first part and the second part, wherein the at least one contact area radiates the outgoing electromagnetic wave (100o) when the device is illuminated by an incoming electromagnetic wave (100i), wherein the at least one contact area extends along a direction of propagation of the incoming electromagnetic wave and has a non-vanishing height (h) along said direction lower than <NUM> times a critical height (hc) equal to a wavelength in vacuum of the incoming electromagnetic wave divided by the difference between the second refractive index n2 and the first refractive index n1;
measuring at least one characteristic of the outgoing electromagnetic wave (100o); and
estimating the first refractive index n1 based on the at least one characteristic.