Patent Application
20180204974
This application claims priority from French Patent Application No. 17 50299 filed on Jan. 13, 2017. The content of this application is incorporated herein by reference in its entirety.
The field of the invention is that of sources of light, and more particularly that of sources of single photons.
Sources of single photons are capable of emitting a single photon at a time. They generally consist of an emitter such as a quantum dot, and the emission of a photon is carried out therein using an injected electron-hole pair.
The interest of these sources is the fundamental study of optical and quantum processes, but also quantum cryptography. Indeed, if it is possible to transmit information in bits each consisting of a single photon, the interception of messages can be protected against or at least detected.
In order for a source of single photons to be usable, it must be possible to efficiently collect the emitted photon. For this, the source must be able to emit the photon in a directional manner.
A well-known source of single photons is described in the article “Quantum dots as single-photon sources for quantum information processing” (D C Unitt et al 2005 J. Opt. B: Quantum Semiclass. Opt. 7 S129). It consists of a pillar etched via reactive-ion etching, with a quantum dot inside it, surrounded by Braggs mirrors. Bragg mirrors allow the formation of a Fabry-Perot cavity that exacerbates the probability of emission of the quantum dot in this resonance mode. However, it is observed that this type of source diverges. Moreover, because of the strong resonance of this source (long lifetime of the photon emitted in the cavity), there is a risk that the photon will be diffracted by the etching-edge roughness of the pillar.
Another type of source of single photons is presented in the article “A highly efficient single-photon source based on a quantum dot in a photonic nanowire” (J Claudon et al., Nature Photonics 4, 174-177 (2010)). This source is in the form of a pillar, the upper tip of which is refined by suitable etching conditions. A mirror is positioned under the pillar in order to reflect the light upwards. This source is not very resonant, which prevents the light from being diffracted by roughness. Moreover, the refining of the top of the pillar into a point allows the mode of the pillar to be enlarged spatially and thus be made less angularly divergent. This source thus has good emission directivity.
However, the positioning of the quantum dot inside a pillar requires precise alignment, which is not easy. Moreover, it is difficult to precisely control the shape given to the tip of the pillar, and thus the collimation of the source cannot be controlled well.
One goal of the invention is to propose a source of collimated light that does not have these disadvantages. For this purpose, the invention proposes a source of collimated light comprising a pyramidal cavity formed in a substrate having a front face. The pyramidal cavity has an axis of symmetry, a base at the front face of the substrate, a centre of the base, sides and an apex below the centre of the base along the axis of symmetry. At least one quantum dot suitable for emitting light with a wavefront is arranged at the apex of the pyramidal cavity. A structure having an index gradient fills the pyramidal cavity. Its effective index decreases from the centre of the base towards the sides in such a way as to flatten the wavefront of the light emitted by the at least one quantum dot.
Certain preferred but not limiting aspects of this source are the following:
The invention extends to the use of this source for the emission of a sequence of single photons, for example in a quantum-cryptography process. The invention also relates to a method for manufacturing such a source of collimated light.
Other aspects, goals, advantages and features of the invention will be better understood upon reading the following detailed description of preferred embodiments of said invention, given as a non-limiting example, and made in reference to the appended drawings in which:
The invention relates to a source of collimated light, for example a source intended to emit a sequence of single photons.
In reference to
In an embodiment shown in
The source also comprises at least one quantum dot Bq suitable for emitting light with a wavefront. The at least one quantum dot is arranged at the apex 2, 2′ of the pyramidal cavity C, C′. In order to emit single photons, a single quantum dot is provided.
A structure 4 having an index gradient fills the pyramidal cavity. The effective index of this structure decreases from the centre C towards the sides F1-F4. Thus, the wavefront of the light emitted by the at least one quantum dot Bq is flattened, thus collimating the light emitted by the at least one quantum dot. Effective index designates the average index seen by the light. This effective index can be different from the local refractive index, this local index corresponding for example to the index of the material(s) forming the levels of a stack formed in the pyramidal cavity as described below.
In
It is noted here that a single photon cannot simultaneously go upwards and downwards. Thus, when a simulation shows a portion of the light going upwards, and a portion downwards, this means that a single photon has a certain probability of going upwards, and a complementary probability of going downwards, these probabilities being prorated according to the quantities of light given per simulation. With the reflective structure, it is imposed that all the photons go upwards with a probability of 1.
The formation of the pyramidal cavity typically involves etching of the substrate, for example anisotropic wet etching. The solution for anisotropic wet etching is generally KOH (potassium hydroxide) or TMAH (tetramethylammonium hydroxide). The etching kinetics are dependent on the crystalline planes, which leads to the inverted-pyramid shape. A mask of resin is first made on the front face of the substrate, the mask having small openings made via photolithographies that allow the etching to be localised and initiated. More particularly, a substrate of Si provided with an SiO2 or Si3N4 surface layer can be used. The surface layer is covered with a mask of resin defined by lithography and a hard mask is defined via wet or dry etching of the surface layer through the mask of resin. Then, the resin is removed and the etching of the pyramid is carried out. At the end of this etching, the hard mask can be removed via dry or wet etching.
The substrate is for example a substrate of silicon. Its anisotropic chemical etching leads to the formation of an inverted pyramid, with a characteristic angle of 54.7° between a side of the cavity and the horizontal plane corresponding to the front face of the substrate. This angle is designated by a in
The substrate can also be made from a III-V material, for example from InP or InGaAs that can also be etched in a wet anisotropic way.
The base of the pyramidal cavity can have various shapes, in particular according to the nature of the crystal. It can in particular be square or hexagonal. It must have a dimension greater than the wavelength emitted in order to prevent phenomena of diffraction.
The arrangement of the at least one quantum dot at the apex of the pyramidal cavity can involve the deposition of a colloidal solution of quantum dots on the substrate. Via capillarity, these quantum dots place themselves at the bottom of the pyramidal cavities. Self-alignment is thus achieved.
The control of the concentration of quantum dots in the colloidal solution allows the number of cavities provided with a single quantum dot to be controlled. A concentration of one quantum dot per volume of a pyramidal cavity is thus preferably chosen. The fact that there is only one quantum dot in a cavity can be verified by carrying out the photoluminescence of the bottom of the pyramid and by verifying that the optical signal emitted corresponds to the spectral signature of a single emitter (observation of rays of excitonic or even multiexcitonic origin, as well as the observation of the antibunching of photons on the emission ray implying the emission of a single photon at a given time).
In an alternative embodiment, a resin for electron-beam lithography is manufactured, said resin being enriched with colloidal quantum dots. The sample is coated with this resin, also including at the bottom of the pyramids. Electron-beam lithography allows a block of resin to be left at the bottom of the pyramid containing a colloidal quantum dot. An oxygen plasma allows the resin to be removed in order to only leave the quantum dot.
When the substrate is made of a III-V material, because of the compatibility of the materials, the growth of quantum dots can be carried out directly from the apex of the cavities.
The structure having an index gradient that fills the cavity can have a continuous variation in refractive index. For this, the composition of an alloy (for example SiGe deposited via epitaxy) or the composition of a mixture of materials (for example Nb2O5/SiO2, SiN/SiO2 or TiO2/SiO2) can be continuously modified during the formation of the structure having an index gradient in the cavity via deposition of such an alloy or of such a mixture in the cavity conformally to the sides.
In an alternative embodiment, the filling of a pyramidal cavity by a structure having an index gradient, the effective index of which decreases from the centre of the base towards the sides of the cavity, can involve the formation of a stack of levels in the cavity, the levels being successively deposited in the pyramidal cavity conformally to the sides.
The effective index of each level increases gradually from one level to another in the succession of the deposited levels. Thus, at the base, the effective index is lower at the contour (1st level deposited) than at the centre (last level deposited). A pseudo index gradient is created in this way (the variation in index is of a discrete nature that approximates a continuous variation).
The levels preferably have the same thickness, noted as P1 in
In one embodiment, the levels are deposits of an alloy or of a mixture of materials, the composition of which differs from one level to another.
In another embodiment shown in particular in
In each bilayer, the material of the first layer has a refractive index lower than the refractive index of the material of the second layer. A factor of filling of a level by the first layer decreases from one level to another in the succession of the deposited levels.
The material of the first layer 7 can be silica (index nSiO2=1.5), the material of the second layer 8 being amorphous silicon (index nSi=3.5). Considering this example of an embodiment, f is the local concentration of silica that is a function of the distance x from the centre of the base of the pyramidal cavity along a diagonal of the base of the pyramidal cavity. The effective index is expressed as:
{circumflex over (n)}(x)=√{square root over (f(x)·nSiO22+(1−f(x))·nSi2)} (1)
In order to carry out the collimation, the variation in local index must approximately verify the following relationship in order to compensate for the difference in distance travelled by the light from the apex of the pyramid between the centre of the base and a point on the base located at a distance x from the centre.
where X designates the length of a half-diagonal of the base of the pyramid (x=X designating the intersection of the diagonal and the contour of the base), and where a corresponds to the etching angle.
In particular,
{circumflex over (n)}(X)={circumflex over (n)}(0)·sin α (3)
With an etching angle of α=54.7° for the silicon, the following indices, for example, can be adopted: {circumflex over (n)}(0)=3 and {circumflex over (n)}(X)=2.5.
The filling factor of the silica changes according to the previous equations according to
In
It is noted above that the sides of the cavity can be coated with a reflective structure 3. Such a structure is formed after the etching of the pyramidal cavity and before the arrangement of the at least one quantum dot in the latter. In the above formulas, if such a reflective structure is present, X corresponds to the border between the structured 4 having an index gradient and the reflective structure 3.
In reference for example to
One advantage of the pyramid configuration is that the Bragg mirror is not planar, but surrounds the quantum dot. It thus always sees a wavefront more or less at normal incidence, which allows it to be efficient (a Bragg mirror functions less efficiently with a high angle of incidence).
In an alternative embodiment, the reflective structure comprises a metal layer (for example made of aluminium, copper or gold) deposited on the sides of the cavity after the etching of the substrate, and a spacer layer, for example a dielectric such as silica, covering the metal layer and allowing contact between the quantum dot and the metal layer to be prevented. This spacer layer is for example a quarter-wave layer.
Simulations via calculation of finite differences in the time domain were carried out at the telecom wavelength of λ=1.55 μm. They relate to a source (i) not corresponding to the invention in that it does not have a structure having an index gradient (cf.
For these three sources, the cavity resulting from the etching has a maximum depth (height of the pyramid) of 10 μm. In the sources (ii) and (iii), the structure having an index gradient has a period of P1=0.15 μm and the filling factor f of silica in the amorphous silicon-silica bilayers varies from 20% to 80%. In the source (iii), the Bragg mirror comprises three bilayers with w1=0.26 μm and w2=0.11 μm.
The simulation of the source (i) is shown in
The invention is also of interest due to the fact that a scale factor can be applied to the pyramid, with the light still remaining collimated. Indeed, even if the size of the pyramid, that is to say X, is reduced, as long as the index relationship (2) indicated above is verified, the invention functions homothetically, the relationship (3) not being dependent on X. The invention can thus be applied to any pyramid size, and thus provide any desired directivity, since the wider the emission beam, the better the angular directivity.
The first embodiment involves the etching of the substrate S in order to form a cavity therein resulting from the etching C (
The three first steps of the second embodiment (
The third embodiment involves the etching of the substrate S in order to form a cavity resulting from etching C (
The invention also relates to the use of the source as described above for the emission of a sequence of single photons.
The device can thus consist of a pulse pump laser and a pair of two APD (avalanche photodiode) fast detectors coupled with a pulse counter that measures the correlation function. The two detectors are each located on either side of a beam splitter receiving the flow of photons coming from the sample excited by the laser.
In an embodiment forming an alternative to the optical pumping, electric injection via the tunnel effect in the quantum dot can be carried out. For this purpose, the quantum dot is arranged in a dielectric layer sandwiched between two doped semiconductor layers, one n-type, the other p-type. The dielectric layer is for example an oxide such as silica, and the doped layers are for example layers of silicon. The thickness of the dielectric layer is several nanometres, adapted to the size of the quantum dot.
Since the doped layers are deposited on the whole wafer, they are found not only at the bottom of the pyramidal cavity, but also on the surface of the substrate, where metal contacts with these layers can be easily made using tracks or the metal tips. These electric contacts allow electric injection to be carried out, and the current cannot pass from one doped layer to another because of the dielectric layer, except via the quantum dot via the tunnel effect. By thus forcing the current to pass through the dot, good injection efficiency is provided, the injection of an electron-hole pair allowing the emission of a photon.
This electric injection is compatible with the presence of a reflective structure on the sides of the cavity. When this structure takes the form of a metal layer, the doped layer in contact with the metal layer acts as a spacer layer. This is preferably a quarter-wave layer. The metal layer can be used on the surface of the substrate to create electric contact, as an alternative to the doped layer in contact of the metal layer.
When the reflective structure is in the form of a Bragg layer, the last layer of the mirror (i.e. the upper layer) can be a semiconductor layer doped in such a way as to form one of the doped layers of the electric injection. This layer is a quarter-wave layer.
| Number | Date | Country | Kind |
|---|---|---|---|
| 17 50299 | Jan 2017 | FR | national |
