Semi-transparent photocathode with improved absorption rate

Abstract

The invention relates to a semi-transparent photocathode (1) for a photon detector having an increased absorption rate for a preserved transport rate. According to the invention, the photocathode (1) includes a transmission diffraction grating (30) able to diffract said photons and provided in the support layer (10) on which the photoemissive layer (20) is deposited.

Claims

1. A semi-transparent photocathode (1) for a photon detector, including: a transparent support layer (10) having a front face (11) to receive said photons and an opposite back face (12), and a photoemissive layer (20) deposited directly on said back face (12) and having an opposite emitting face (22), intended to receive said photons from said support layer (10) and to responsively emit photoelectrons from said emitting face (22), characterized in that it includes a transmission diffraction grating (30) able to diffract said photons, provided in the support layer (10) and located at said back face (12); said diffraction grating (30) being formed of a periodical arrangement of patterns (31) filled with a pattern material having an optical index different from the material of the support layer (10); said diffraction grating (30) being further provided so as to bound at least partly the back face (12) of the support layer (10) by being flush with the same.

2. The photocathode (1) according to claim 1, characterized in that a layer of said pattern material is directly provided on the back face, in continuity with said patterns.

3. The photocathode (1) according to claim 1 or 2, characterized in that it includes at least a further diffraction grating (40) able to diffract said photons, which is located in the support layer (10) and provided in the vicinity of said first diffraction grating (30), formed of a periodical arrangement of patterns (41) along a direction distinct from that of the patterns of the first grating.

4. The photocathode (1) according to claim 3, characterized in that the diffraction grating (30) and the further diffraction grating (40) are located in a same plane and made by means of two-dimensional patterns.

5. The photocathode (1) according to claim 4, characterized in that the photoemissive layer (20) comprises antimony and at least one alkaline metal.

6. The photocathode (1) according to claim 5, characterized in that the photoemissive layer (20) is made of a material selected from SbNaKCs, SbNa.sub.2KCs, SbNaK, SbKCs, SbRbKCs, or SbRbCs.

7. The photocathode (1) according to claim 4, characterized in that the photoemissive layer (20) is formed of AgOCs.

8. The photocathode (1) according to claim 1, characterized in that the photoemissive layer (20) has a substantially constant thickness.

9. The photocathode (1) according to claim 8, characterized in that the photoemissive layer (20) has a thickness lower than or equal to 300 nm.

10. A photon detection optical system including a photocathode (1) according to claim 1, and an output device for emitting an output signal in response to the photoelectrons emitted by said photocathode (1).

11. The photon detection optical system according to claim 10, being an image intensifier tube or a photomultiplier tube, of the EB-CCD or EBCMOS type.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the invention will now be described, by way of non limiting examples, referring to the appended drawings, wherein:

(2) FIG. 1, already described, is a schematic transverse cross-section view of a photocathode according to an example of prior art;

(3) FIG. 2, already described, illustrates an example of the time course of the absorption, transmission and reflection rates as a function of the wavelength of a 140 nm-thickness photoemissive layer of a S25-type photocathode according to an example of prior art;

(4) FIG. 3 is a schematic transverse cross-section view of the photocathode according to a first preferred embodiment of the invention;

(5) FIG. 4 is a schematic enlarged cross-section view of a part of the photocathode illustrated in FIG. 3;

(6) FIG. 5 illustrates an example of the time course of the quantum yield as a function of the wavelength for a photocathode according to the prior art and for a photocathode according to the first preferred embodiment of the invention;

(7) FIG. 6 is a schematic transverse cross-section view of the photocathode according to another preferred embodiment of the invention, wherein the diffracted photons are fully reflected at the emitting layer of the photocathode; and

(8) FIG. 7 is a schematic transverse cross-section view of the photocathode according to another preferred embodiment of the invention, wherein the photocathode comprises two diffraction gratings.

DETAILED DISCLOSURE OF A PREFERRED EMBODIMENT

(9) FIGS. 3 and 4 illustrate a semi-transparent photocathode 1 according to a first preferred embodiment of the invention.

(10) It should be noted that the scales are not respected, for the sake of the drawing's clarity.

(11) The photocathode 1 according to the invention can equip any type of photon detector, such as for example an image intensifier tube or an electron multiplier tube.

(12) The photocathode has a function to receive a flow of incident photons and to responsively emit electrons, called photoelectrons.

(13) It comprises a transparent support layer 10, a layer 20 of a photoemissive material and, according to the invention, at least one diffraction grating 30 able to diffract the incident photons.

(14) The support layer 10 is a layer of a transparent material on which the photoemissive layer 20 is deposited.

(15) It is indicated as transparent given that the incident photons pass through it without being absorbed. The transmittance of the support layer 10 is thus substantially equal to one.

(16) It includes a front face 11, called a photon receiving face, and an opposite back face 12.

(17) At least one transmission diffraction grating 30 is provided in the support layer 10 at said back face 12.

(18) In the preferred embodiment of the invention illustrated in FIGS. 3 and 4, a single diffraction grating 30 is provided.

(19) The diffraction grating 30 is formed of a periodical arrangement of patterns 31 filled with a material having an optical index different from the material of the support layer 10.

(20) By patterns, it is intended indentations, nicks, recesses, notches, or scratches, having a sinusoidal, with steps, trapezoidal, or other shape, provided in the support layer.

(21) The difference between the optical indices of the material of the diffraction grating 30 present in said patterns 31 on the one hand and of the material of the support layer 10 on the other hand is higher than or equal to 0.2.

(22) The diffraction grating 30 is in particular characterized by the distance, called the grating spacing, between two neighboring patterns 31. The grating spacing is defined as a function of the wavelength of the incident photons, so as to be able to diffract them.

(23) As shown in detail in FIG. 4, the diffraction grating 30 can be provided in the support layer 10 at the back face 12, thus bounding at least partly the back face 12.

(24) Alternatively, the diffraction grating can be provided inside the support layer and located in close vicinity to the back face, at a distance thereof being negligible with respect to the thickness of the support layer.

(25) It is to be noted that the back face 12 of the support layer 10 is substantially planar. It can however be curved in the case of a photocathode itself having a defined curvature.

(26) In FIG. 4, the diffraction grating 30 is located in the support layer 10, such that the material filling the patterns 31 of the grating does not project from said patterns. However, as will be seen during the manufacture of the photocathode, the material filling the patterns 31 can, according to one alternative, form a layer between the back face 12 of the support layer and the photoemissive layer 20.

(27) The photoemissive layer 20 is provided against the back face 12 of the support layer 10.

(28) It has an upstream face 21, in contact with the back face 12 of the support layer 10, and an opposite downstream face 22, called the photoelectron emitting face.

(29) The photoemissive layer 20 has a substantially constant mean thickness, noted e. The thickness is preferably lower than or equal to 300 nm.

(30) The photoemissive layer 20 is made of a suitable semi-conductor material, preferably an antimony-based alkaline compound. Such an alkaline material can be selected from SbNaKCs, SbNa.sub.2KCs, SbNaK, SbKCs, SbRbKCs, or SbRbCs. The photoemissive layer 20 can also be formed of silver oxide AgOCs.

(31) The emitting face 22 can be treated with hydrogen, cesium, or cesium oxide to decrease its electronic affinity. Thus, the photoelectrons which reach the downstream emitting face 22 of the photoemissive layer 20 can be naturally extracted therefrom and thus be emitted in the vacuum.

(32) An electrode (not represented), forming an electron reservoir, is in contact with the photoemissive layer 20 and is brought to an electric potential.

(33) It can be provided against a side face of the photoemissive layer 20, not to decrease or disturb the electron emission from the downstream emitting face 22.

(34) The electron reservoir enables holes generated by the incident photons to be recombined. Thus, the overall electric charge of the photoemissive layer 20 remains substantially constant.

(35) It should be noted that the photoemissive layer 20 is thin enough for the generated electrons to be naturally moved to the emitting face 22.

(36) It is therefore not required to generate an electric field in the photoemissive layer 20 to ensure the electrons transport to the emitting face. The generation of such an electric field would indeed require to deposit two bias electrodes, one against the upstream face 21 of the photoemissive layer 20 and the other against the downstream emitting face 22.

(37) The operation of the photocathode according to the invention is described hereinafter.

(38) Photons enter the photocathode 1 through the front receiving face 11 of the support layer 10.

(39) They pass through the support layer 10 up to the back face 12 thereof.

(40) They are then diffracted by the diffraction grating 30 and transmitted in the photoemissive layer 20. They have statistically a diffraction angle substantially higher, in absolute value, to the incidence angle, the incidence and diffraction angles being defined with respect to the normal of the back face 12.

(41) More precisely, if =.sub.i is the incidence angle on the grating, () the angular distribution of the incident beam, .sub.d the diffraction angle, the angular distribution of the diffracted beam can be written as:
F()=f()f(+)+f()
where is the diffraction figure of the grating and the approximation is made by restricting to the first order of diffraction with =/p where p is the grating spacing.

(42) The angular distribution of the diffracted beam is consequently more spread than that of the incident beam. The electrons face a photoemissive layer 20 having a mean apparent thickness:

(43) $e_d=e\underset{-{}_{\max }+}{\overset{{}_{\max }+}{}}\frac{F\left({}_d\right)}{.Math.\cos {}_d.Math.}{}_d$
where e is the actual thickness of the layer and .sub.max is the maximum incidence angle on the grating.

(44) The mean apparent thickness e.sub.d of the photoemissive layer is substantially higher than its actual thickness e, in other words the mean distance traveled by the photons in the layer is substantially higher than in prior art. As a result, a higher percentage of the diffracted photons is absorbed.

(45) The absorption of the diffracted photons causes the generation of electron-hole pairs. The electrons generated are propagated in the photoemissive layer 20 up to the downstream emitting face 22 where they are emitted in vacuum.

(46) Since the transport of electrons in the photoemissive layer 20 is independent of the prior propagation direction of the photons, the transport rate of the photoemissive layer 20 is substantially equal to that of a photocathode according to prior art, that is without diffraction grating. The transport rate is thus preserved.

(47) The photocathode 1 according to the invention thus has a high absorption rate and a preserved transport rate, which results in an optimized quantum yield, in particular for energies close to the photoemission threshold.

(48) The photocathode 1 according to the invention can be made as follows.

(49) The support layer 10 is made of a suitable transparent material, for example of quartz or borosilicate glass.

(50) The patterns 31 of the diffraction grating 30 are etched in the support layer 10 at the back face 12 by known etching techniques, such as, for example, the holography and/or ionic etching, or even diamond engraving techniques.

(51) The patterns 31 are then filled with a diffraction material the optical index of which is different from that of the support layer, as, for example, Al.sub.2O.sub.3 (n1.7), TiO.sub.2 (n2.3-2.6) or Ta.sub.2O.sub.5 (n2.2), or even HfO.sub.2.

(52) This material can be deposited by known physical vapor deposition techniques, such as, for example, sputtering, evaporation, or Electron Beam Physical Vapor Deposition (EBPVD). Known chemical vapor deposition techniques such as, for example, Atomic Layer Deposition (ALD) can also be used, as well as known so-called hybrid techniques such as, for example, reactive spraying and Ion Beam Assisted Deposition (IBAD).

(53) According to a first advantageous alternative, illustrated in FIG. 4, the back face 12 is polished so as to remove any extra diffraction material projecting from the patterns 31 of the diffraction grating 30.

(54) According to a second alternative, not represented, the back face is polished without being flush with the back face. As a result, a uniform layer of diffraction material remains present on the back face 22, in continuity with the patterns.

(55) Regardless of the alternative, a thin diffusion barrier can then be deposited to prevent any chemical migration/interaction between the material of the photoemissive layer and the material of the diffraction grating. The thickness of the diffusion barrier is selected thin enough (less than /4 and preferably in the order of /10).

(56) In any case, the photoemissive layer 20 is then deposited by one of the previously mentioned deposition techniques.

(57) By way of illustration, a S25-type photocathode 1 according to the first preferred embodiment of the invention can be made in the following way.

(58) The support layer 10 is made of quartz.

(59) The diffraction grating 30 is etched in the support layer 10 at the back face 12, in the form of a periodic arrangement of grooves 31 parallel to each other.

(60) The grooves 31 are 341 nm wide and 362 nm deep. The grating spacing, that is the distance separating two neighboring and parallel grooves 31, is 795 nm.

(61) The grooves 31 are filled for example with TiO.sub.2, the optical index of which is between 2.3 and 2.6.

(62) The TiO.sub.2 can be deposited by the known atomic layer deposition (ALD) technique.

(63) A step of polishing the back face 12 is carried out to remove any extra diffraction material projecting from the grooves 31.

(64) Thus, the back face 12 is substantially planar, and partly bounded by the material (quartz) of the support layer 10 and partly by the diffraction material (TiO.sub.2) of the grooves 31 of the diffraction grating 30.

(65) The photoemissive layer 20 is finally made of SbNaK or SbNa.sub.2KCs and is deposited on the back face 12 of the support layer 10 so as to be substantially constantly 50 to 240 nm thick.

(66) FIG. 5 illustrates the time course of the quantum yield as a function of the wavelength of the incident photons, for such a photocathode on the one hand and for a photocathode according to the example of prior art previously described on the other hand.

(67) It is noticed that the quantum yield is improved throughout the wavelength range, and more particularly at great wavelengths.

(68) Thus, for 825 nm, the quantum yield of the photocathode according to the invention is in the order of 18%, whereas it is in the order of 10% in the case of a photocathode without a diffraction grating, which yields an improvement close to 80% of the quantum yield.

(69) FIG. 6 illustrates a photocathode according to a second embodiment of the invention.

(70) Reference numerals identical to those of FIG. 3 previously described designate identical or similar elements.

(71) The photocathode 1 only differs from the first preferred embodiment in that the diffraction grating 30 is dimensioned such that any photon arriving under normal incidence (.sub.i=0), diffracted and not absorbed in the photoemissive layer 20, is reflected at the downstream emitting face 22.

(72) Alternatively, the diffraction grating 30 is advantageously dimensioned such that the mean diffraction angle .sub.d (in view of the angular distribution F(.sub.d)) is strictly higher than arcsin(1/n.sub.p) where n.sub.p is the optical index of the photoemissive layer. More precisely, the spacing p of the grating and/or the optical index of the diffraction material filling the patterns 31 are selected such that the mean diffraction angle .sub.d is strictly higher than arcsin(1/n.sub.p).

(73) Thus, these reflected photons remain located in the photoemissive layer 20 until the absorption thereof and the generation of electron-hole pair.

(74) This enables the transmission rate of the photons of the photoemissive layer 20 to be significantly decreased in benefit of the absorption rate.

(75) Since the transport rate of the electrons remains unchanged, the quantum yield of the photocathode is consequently further improved, in particular for photons having an energy close to the photoemission threshold.

(76) FIG. 7 illustrates a photocathode, viewed from above, according to a third embodiment of the invention, wherein two diffraction gratings 30, 40 are present in the support layer 10 at the back face 12.

(77) The reference numerals identical to those of FIG. 3 previously described designate identical or similar elements.

(78) The photocathode only differs from the first preferred embodiment in the presence of a further diffraction grating 40 in the support layer 10.

(79) This further grating 40 is provided in the vicinity of the first diffraction grating 30, upstream the same along the propagation direction of the photons.

(80) Both these gratings 30, 40 are oriented along distinct, preferably orthogonal directions, and are distant from each other by a distance negligible with respect to the thickness of the support layer, for example by a distance in the order of /10 to 10.

(81) The further grating 40 is for example of the same spacing as the previously described first diffraction grating 30.

(82) According to an alternative, the first diffraction grating and the further grating are made in a same plane according to a two-dimensional pattern the transmission function of which is the product of the respective transmission functions of the first grating and the further grating. The two-dimensional pattern can be obtained by holographic techniques.

(83) In the hypothesis of two orthogonal gratings, the angular distribution of the diffracted photons can thus be written as:
F(,)=f(a,)f(+,+)+f(+,)+f(,)+f(,)
by keeping the same notations, where and are respectively the incidence angles of the photon in the plane perpendicular to the direction of the first grating and in the plane perpendicular to the direction of the further grating, =/p; =/p where p and p are the spacings of the first grating and the further grating.

(84) Thus, the angular distribution is more spread than in the first embodiment and the apparent thickness of the photoemissive layer 20 for the photons is higher, which improves the absorption rate.

(85) Those skilled in the art will understand that this embodiment is not restricted to two diffraction gratings. A greater number of diffraction gratings having distinct directions can be present in the support layer at the back face.

(86) On the other hand, various modifications can be made by those skilled in the art to the invention just described only by way of non limiting examples.

(87) Finally, the abovedescribed photocathode can be integrated in a photon detection optical system. Such an optical system comprises an output device suitable for converting photoelectrons into an electrical signal. This output device can include a CCD array, the optical system being known as an Electron Bombarded CCD (EB-CCD). Alternatively, the output device can include a CMOS array on a thinned passivated substrate, the optical system being then known as an Electron Bombarded CMOS (EBCMOS).