Patent Application: US-51097709-A

Abstract:
infrared imaging at wavelengths longer than the silicon bandgap energy typically require expensive focal plane arrays fabricated from compound semiconductors or use of slower silicon microbolometer technology . furthermore , these technologies are available in relatively small array sizes , whereas silicon focal plane arrays are easily available with 10 megapixels or more array size . a new technique is disclosed to up convert infrared light to wavelengths detectable by silicon focal plane arrays , or other detector technologies , thereby enabling a low - cost , high pixel count infrared imaging system .

Description:
as used herein [ re 1 , re 2 , . . . re 10 ] are chosen from the lanthanide series of rare earths from the periodic table of elements consisting of 57 la , 58 ce , 59 pr , 60 nd , 61 pm , 62 sm , 63 eu , 64 gd , 65 tb , 66 dy , 67 ho , 68 er , 69 tm , 70 yb and 71 lu , plus yttrium , 39 y , and scandium , 21 sc , are included as well for the invention disclosed . optionally , rare earth oxides , nitrides , and phosphides and / or combinations thereof may be employed . as used herein the terms , “ oxides ” and “ rare - earth oxide [ s ]” are inclusive of rare earth oxides , nitrides , and phosphides and / or combinations thereof . a further advantage of the disclosed system is that by utilizing different pump wavelengths , the uc material , or one layer of the uc material may be excited such that the imaging system becomes sensitive to an alternate wavelength of infrared light . for example , by using er and dy co - doping with ˜ 1600 nm pump radiation , when a er ion is excited form ground state to its 4 i 13 / 2 state , it can be quickly excited further to its 4 i 11 / 2 state by etu since the energy spacing between the 4 i 13 / 2 and 4 i 11 / 2 states is close to the energy of excited dy ions in their 6 h 13 / 2 states . then consider that a portion of the same uc device is also doped with ho and tm . when the pump wavelength is changed to ˜ 1850 nm ho ions will absorb the pump radiation and become excited from ground state to the 5 i 7 state . tm ions are excited from ground state to 3 f 4 state by absorption of the target radiation at 1650 nm . etu can then take the energy from an excited tm ion to further excite a ho ion into the 5 i 5 state . a ho ion in the 5 i 5 state may then radiatively decay to produce a 870 nm photon which is sensed by a semiconductor detector . in effect this enables a multi - wavelength , pixel - registered ir imaging system , by modulating the pump wavelength . fig7 and 8 provide exemplary schematics of a dual wavelength imaging detector system employing multiple pump wavelength sources enabling up conversion over an extended wavelength spectrum and exemplary energy levels in reo and semiconductor detector . a semiconductor imaging device with integrated re containing up conversion device . the up conversion device consists of one or more layers of re containing material . when one layer is used the re material contains two or more re species . when a layered uc device is used layers containing two or more re are interspersed between layers containing at least one re species so as to limit the resonant energy transfer between active layers containing two or more re species . in this embodiment , the uc device is illuminated solely by light from the target . same device structures as detailed in 1 , but in this embodiment , the uc device is illuminated by a secondary ‘ pump ’ source of radiation . the pump radiation , λ p , is absorbed in the uc device so that certain re species in the uc device are excited to higher energy states ; optionally , more than one pump wavelength is provided , λ p1 and λ p2 , etc ., for example . by exciting the re atoms uc of the target radiation is enabled , either through excited state absorption ( esa ) or ground state absorption ( gsa )/ energy transfer up conversion ( etu ) gsa / etu . the pump radiation may be either continuous wave or modulated radiation , where modulated pump radiation may be used to enable synchronous signal recovery techniques . the pump radiation may be provided by either a led , superluminescent led , or laser radiation impinging upon the uc device . the wavelength of the pump radiation may be the same as the target wavelength , in which case either the pump or the target radiation must be modulated in order to recover a signal from the detector . in the case of gsa / etu up conversion , it is well know that the uc process becomes more efficient as the power density increases . radiation from the pump can be used to increase the average power density in the uc device , thus allowing additional radiation from the target to be up converted more efficiently . the radiation from the pump source may also be chosen to be at a wavelength different to the target wavelength . this scheme may be used to either populate the same excited state as the target radiation , in which case a modulation scheme can be used to extract the signal . alternatively , the pump radiation can be use to excite the re ions into intermediate states which then enable absorption and up conversion of the target radiation , in a esa up conversion scheme . the up conversion device consists of one or more layers of re containing material . when one layer is used the re material contains two or more re species . when a layered uc device is used layers containing two or more re are interspersed between layers containing at least one re species so as to limit the resonant energy transfer between active layers containing two or more re species . the uc device may also incorporate an optical element so that optical radiation within a particular range of wavelengths is selectively reflected back into the uc device , while other wavelengths are selectively transmitted into the semiconductor imaging device . as is well known , alternating layers of different refractive indices can be used to create a distributed bragg reflector ( dbr ). such a layer structure may be disposed between the uc layers and the semiconductor detector structure such that wavelengths from the pump and / or the target are reflected back into the uc device , while radiation resulting from up conversion is transmitted through the dbr for detection by the semiconductor detector . in one embodiment a uc device is grown epitaxially on a silicon detector device , optionally by molecular beam epitaxy , using solid sources or rare - earth elements and molecular oxygen . in this embodiment , a detector device , optionally silicon , and uc device are a monolithically integrated uc imaging device . the uc device comprises active re ions within a host matrix of re oxide . the doping profile of the active re ions need not be uniform and is chosen to ( a ) suppress resonant energy transfer to quenching sites and ( b ) coincide with regions of high optical intensity that arise from a standing wave pattern created within the uc layer due to optical interference . the preferred doping profile is also such that the upper and lower surfaces of the uc device do not contain active re species ; fig6 provides one exemplary example of a doping profile , showing active re ion doping coinciding with peaks in the optical standing wave produced in the film due to optical interference . in one embodiment , uc takes place via ground state absorption / energy transfer up conversion gsa / etu , such that the pump radiation is absorbed by a ground state absorption , gsa , in one of the active re ions , and the target radiation is also absorbed by a ground state absorption in the same or a different active re ion . up conversion may then proceed via energy transfer up , etu , conversion , where the energy of one excited re ion is transferred to another excited re ion , such that the second re ion becomes excited to an energy level more than one level above its ground state . a specific example is an uc device consisting of a gd 2 o 3 host matrix doped with er and dy for imaging at 1550 nm , with a pump at around 1600 nm . the dy absorbs the pump radiation at 1600 nm , while er absorbs the 1550 nm radiation from the target . through etu , an er ion in its 4 i 13 / 2 state can be excited to its 4 i 11 / 2 state through energy transfer from the dy . the 4 i 11 / 2 state can then decay to ground level through emission of a photon at 980 nm which is detected by a silicon imaging device . in all examples listed herein it is disclosed that a rare earth layer ( s ) for up converting occurs with one or more layers comprising one or more rare earths in combination with one or more elements chosen from the group comprising oxygen , nitrogen , phosphorus , silicon , germanium , and carbon . a rare earth layer may be grown on a single crystal substrate or not ; the substrate may be silicon or not ; a rare earth layer for converting may be transferred to a different substrate for the converting . a rare earth layer may be deposited as single crystal , or multi - crystalline , amorphous or quantum dots ; subsequent processing may be required to change a physical state of a rare earth layer to make it suitable for up converting , such as converting an amorphous layer to a large grained layer . a rare earth layer for up converting may be used in combination with one or more reflectors , bragg layers , textured layers , or other optical components known to one knowledgeable in the field . in some embodiments a rare earth layer for up converting is also a reflector layer , bragg layer , and / or textured layer . in one embodiment a device for detecting incoming radiation comprises a first region comprising one or more layers each comprising a rare earth ; and a radiation detector operable between wavelengths λ 1 and λ 2 wherein λ 2 is greater than λ 1 and the first region is between the incoming radiation and the radiation detector such that the first region is operable to convert at least a portion of the incoming radiation from wavelength λ 3 to λ 4 wherein λ 3 is greater than λ 2 and λ 1 ≦ λ 4 ≦ λ 2 ; optionally , a device has a range of λ 1 from about 300 nm to about 1 , 000 nm ; λ 2 has a range from about 800 nm to about 1 , 400 nm ; and λ 3 has a range from about 1 , 000 nm to about 12 , 000 nm ; optionally , a device has a first region comprising at least one layer comprising two or more rare earths ; optionally , a device has a first region comprising a first and second layer such that the first layer comprises at least two rare earths and the second layer comprises at least one rare earth ; optionally , a device comprises a second region between said first region and said radiation detector operable as a distributed bragg reflector . in one embodiment a device for detecting incoming radiation comprises a radiation detector operable between wavelengths λ 1 and λ 2 wherein λ 2 is greater than λ 1 ; and a first region of one or more layers each comprising a rare earth , such that the radiation detector is between the incoming radiation and the first region and the first region is operable to convert at least a portion of the incoming radiation from wavelength λ 3 to λ 4 wherein λ 3 is greater than λ 2 and λ 1 ≦ λ 4 ≦ λ 2 ; optionally , a device has a range of λ 1 from about 300 nm to about 1 , 000 nm ; λ 2 has a range from about 800 nm to about 1 , 400 nm ; and λ 3 has a range from about 1 , 000 nm to about 12 , 000 nm ; optionally , a device comprises a first region comprising one layer comprising two or more rare earths ; optionally , a device comprises a first region comprising a first and second layer such that the first layer comprises two or more rare earths and the second layer comprises one or more rare earths . in one embodiment an apparatus for detecting incoming radiation comprises a device for emitting pump radiation of wavelength , λ p , into the radiation detector ; a first region of one or more layers each comprising a rare earth ; and a radiation detector operable between wavelengths λ 1 and λ 2 wherein λ 2 is greater than λ 1 and the first region is between the incoming radiation and the radiation detector such that the first region is operable to convert at least a portion of the pump radiation from wavelength λ p to λ 4 and the first region is operable to convert at least a portion of the incoming radiation from wavelength λ 3 to λ 5 wherein λ 3 is greater than λ 2 and less than λ 4 and x 1 ≦ λ 5 ≦ λ 2 ; optionally , an apparatus has pump radiation of wavelength , λ p ≧ 1 , 100 nm ; optionally , an apparatus as has pump radiation of wavelength , λ p ≦ 1 , 100 nm ; optionally , an apparatus has a first region comprising one layer comprising two or more rare earths ; optionally , an apparatus has a first region comprising a first and second layer such that the first layer comprises two or more rare earths and the second layer comprises one or more rare earths . in one embodiment an apparatus for detecting incoming radiation comprises a device for emitting pump radiation of wavelengths , λ p1 and λ p2 into the radiation detector ; a first region comprising a plurality of layers each comprising a rare earth ; and a radiation detector operable between wavelengths λ 1 and λ 2 wherein λ 2 is greater than λ 1 and the first region is between the incoming radiation and the radiation detector such that the first region is operable to convert at least a portion of the pump radiation from wavelength λ p1 to λ 4 in the presence of incoming radiation λ 5 and from λ p2 to λ 6 in the presence of incoming radiation λ 7 , wherein λ 1 ≦ λ 4 , λ 6 ≦ λ 2 and λ 2 ≦ λ p1 ≦ λ p2 ; optionally , an apparatus has pump radiation of wavelength , λ p2 ≦ 3 , 000 nm ; optionally , an apparatus has a plurality of layers comprising at least one layer comprising two or more rare earths ; optionally , an apparatus has a second region between said first region and said radiation detector operable as a distributed bragg reflector . the foregoing described embodiments of the invention are provided as illustrations and descriptions . they are not intended to limit the invention to a precise form as described . in particular , it is contemplated that functional implementation of invention described herein may be implemented equivalently in hardware and / or various combinations of hardware and software and / or other available functional components or building blocks . other variations and embodiments are possible in light of above teachings to one knowledgeable in the art , and it is thus intended that the scope of invention not be limited by this detailed description , but rather by claims following . all patents , patent applications , and other documents referenced herein are incorporated by reference in their entirety for all purposes , unless otherwise indicated . 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