Patent Application: US-29710103-A

Abstract:
apparatus for single photon source based on emitters with frequencies distributed in a chosen manner . in one embodiment , the apparatus comprises an opto - electronic component capable or emitting light pulses containing a single photon comprising a resonant optical cavity and a group of photon emitters placed in this optical cavity , a single one of these emitters having an emission frequency equal to approximately the resonant frequency of the cavity characterised in that all emitters have a spectral distribution with a concentration of emitter frequencies at a given frequency , and in that the cavity is made so that its resonant frequency is different from this concentration frequency so that the number of emitters with an emission frequency corresponding to the resonant frequency of the cavity is close to one .

Description:
characteristically , the invention comprises a three - dimensional optical microcavity 100 in which one or several planes 200 of high density (& gt ; 100 boxes per μm 2 ) quantum boxes 250 are inserted such that there is a strong spectral shift a between the useful mode ( fundamental mode ) of the cavity 100 and the maximum emission peak of all quantum boxes 250 namely the concentration point of emission peaks of quantum boxes 250 . this peak or concentration point is marked as reference p in fig4 . the fundamental mode of the cavity is marked m . this spectral shift δ is adjusted such that the spectral density of quantum boxes 250 with mode m energy is less than 0 . 5 / δe , where δe is the spectral width of mode m of the cavity 100 . this shift is easily and very reproducibly obtained by auto - organized epitaxial growth of quantum box planes 250 with a high box density . the density and the mean emission wavelength of these planes do not critically depend on the quantity of inas deposited , and are very uniform over the surface of the sample . the introduction of a strong spectral shift a between the mean emission peak p of the boxes 200 and the useful mode of the cavity 100 on the different devices made can reduce the number of boxes 250 coupled to mode m to an average of 0 . 5 ( or less ) quantum boxes on all devices made . a single quantum box can then be put into resonance with this mode by adjusting the temperature . similarly , this type of resonance can be set up by applying a magnetic field with a selected value to the structure . this mean peak or the global emission ray of the plane of boxes 250 may be placed on the “ high energies ” side or the “ low energies ” side of the cavity mode m . however , it is preferable to place it on the “ high energies ” side of mode m in order to avoid collecting the emission originating from transitions between excited states of quantum boxes 250 . for some families of materials such as inas / gaas , the spectral distribution of quantum boxes 250 can be described by a gaussian description as a very good approximation ; it is then easy to calculate the difference δ between mean peak p / mode m . δ can always be adjusted experimentally if this distribution is not as well known or if it is not as regular . in the example shown in fig3 , the microcavity 100 is in the form of a cylindrical column with an elliptical section . fig3 shows a structure conform with the invention , operating at a wavelength of 1 μm , using a gaas / alas micro - column and ingaas quantum boxes . more precisely , the cavity is sandwiched between two bragg mirrors each of which is composed of an alternating series of gaas and alas quarter wavelength layers . the eccentricity of the section is sufficient such that the fundamental mode of this cavity is not degenerated ( 6 ) ( for example small axis 1 μm , large axis 2 μm ). during growth , a plane of inas or ingaas quantum boxes 200 with a density of 400 boxes per μm 2 and emitting at close to e 0 = 1 . 29 ev at 77k , with a typical spectrum width σ = 80 mev , is inserted at the heart of the structure . the number of boxes in this column is therefore close to n = 310 , and their distribution as a function of the energy is written as follows : n ⁡ ( e ) ⁢ de = 2 ⁢ n / σ ⁢ π ⁢ ⅇ ⁢ - 4 ⁢ ( e - e 0 ) 2 σ 2 ⁢ de for a mode width δe of 1 mev ( typical of the state of the art , see ref . ( 6 )), it is found that the shift δ must be equal to at least 60 mev so that an average of 0 . 5 boxes are coupled at mode m . therefore , for example , we will calculate the structure of microcavity 200 so as to adjust the energy of its fundamental mode to 1 . 23 ev . as described previously , in this case this microcavity 200 is made in a known manner in the form of a micro - column with mirrors made from gaas / alas . the structure is placed on a base 300 ( stirling cooler ) that will keep its temperature at a set point close to 77k , for which a single quantum box is coupled to the fundamental mode m . a gaas substrate 350 is placed between the base 300 and the micro - column . the structure is optically pumped by an impulse laser . fig6 and 7 show the results obtained with a structure similar to that in fig3 but adapted to an emission at a wavelength of 1 . 3 μm and at ambient temperature . the fundamental mode is at 0 . 954 ev ( 1 . 3 μm ) at 300k ; the plane of quantum boxes contains 400 boxes / μm 2 , emits at close to 0 . 99 ev at 300k , and has a spectral width of 40 mev . it is easy to check that the average number of boxes 250 coupled to mode m is less than 0 . 5 . fig7 more particularly shows a random distribution of boxes around mode m . these fig6 and 7 show a mode density and an average spectral distribution of quantum boxes ( fig6 ) such that there is a large difference between m and p , and an example of a particular embodiment of the random distribution of boxes ( fig7 ). the structure in fig8 corresponds to a structure similar to that in fig3 , except that the mirrors of the micro - column are doped and in electrical contact with the ends of the column by an electrical pulse generator , so as to achieve electrical excitation of the system . in this case , the upper bragg mirror is composed of alternating layers of p doped gaas / alas and the lower mirror is composed of alternating layers of n doped gaas / alas . the layer 100 forming the cavity is made of gaas that is not intentionally doped . the micro - column is located on an n doped gaas substrate . fig9 shows a structure identical to the structure in fig3 , except that it is monolithically integrated into a semiconducting surface emission microlaser . the micro - column forms a single photon source with wavelength λ and the microlaser forms a source at wavelength λ ′, where λ ′ is less than λ . the microlaser is composed of three layers , in other words one active layer 400 sandwiched between two bragg mirrors 500 and 600 , the first at the micro - column end being p doped , and the second at the end opposite to the micro - column being n doped . there are electrodes in contact with the two ends of the microlaser , one of the two electrodes surrounding the bottom of the micro - column . fig1 and 11 show a structure identical to the structure in fig3 , coupled to an optical fibre 700 for the collection of emitted photons and electrically pumped ( fig1 ) or optically pumped by an optical fibre ( fig1 ). in these two examples , the structure in fig3 with its cooler 300 and its substrate 350 is placed in a box 800 and the collection optical fibre 700 passes through the box 800 from an upper end of the micro - column . in the case shown in fig1 , the excitation electrodes are placed in the same way as in fig8 . in the case shown in fig1 , an excitation optical fibre 900 passes through the box laterally to excite the cavity 100 of the micro - column . fig1 shows a micro - disk 1 000 based on gaas and gaalas containing a plane of quantum boxes 200 made of ingaas . this micro - disk 1000 is placed on an elongated base made of gaalas , itself placed on a gaas substrate . the thickness of the micro - disk is of the order of 200 nm , and the disk diameter ( 1 to 5 μm ) is adjusted such that one of its confined modes , mode m in fig1 , is sufficiently offset from the mean emission of all the quantum boxes , in other words the concentration point of the frequencies of quantum boxes 250 , and on average is coupled to a small number of boxes ( 0 . 1 to 0 . 5 ). an optical fibre 700 ( or a semiconductor wave guide ) collects emitted photons . a pass band filter 750 is used to select photons emitted in mode m from the parasite emission emitted in the other modes . in one particular embodiment , this filter 750 may be integrated into the fibre as shown in fig1 . fig1 shows a cylindrical micro - column with an elliptical section composed of a cavity layer 100 made of gan or gaaln containing a plane of ingan quantum boxes 200 , and dielectric bragg mirrors ( for example based on sio2 and ta2o5 ) on each side of the layer 100 . the lower bragg mirror may also be made based on a stack of gan and gaaln layers . in one variant , the quantum boxes are made of gan and the cavity layer is made of gaaln . an emission wavelength in the visible range ( particularly blue and green ) or the near ultraviolet can be chosen for this single photon source , by varying the composition and size of the ingan or gan quantum boxes . 2 ) add a system to collect single photons ( fibre , guide , etc . ); 3 ) add a system for spectral filtering of the parasite emission of quantum boxes not coupled to the mode ; 4 ) integrate means of optical or electrical impulse pumping of quantum boxes in the microcavity . there are preferred embodiments , for example corresponding to one of the different variants in the appended figures , in which we preferably adopt the following , for a particularly easy and reliable embodiment : quantum boxes made of a material with formula in x ga 1 - x as , where x is between 0 . 5 and 1 ; the cavity layer made of ga y al 1 - y as where y is between 0 . 5 and 1 , particularly in the case of quantum boxes according to the previous paragraph ; quantum boxes made of inas and the cavity layer made of gaas ( a particularly good choice ); in one variant , the quantum boxes are made of in x ga 1 - x n , where x is between 0 and 1 . 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