Patent Application: US-201414208000-A

Abstract:
an imaging / detection device includes a hemispherical lens having a surface opposite a curvature of the hemispherical lens , where the hemispherical lens defines an optical axis . the imaging / detection device also includes a plurality of detectors arranged on a focal plane array that is positioned near the surface of the hemispherical lens . each of the detectors respectively includes a diode and an antenna monolithically integrated with the diode . additionally , at least one of the detectors is offset by a distance from the optical axis of the hemispherical lens and is configured such that a radiating pattern of the respective antenna is tilted by an angle and directed toward the optical axis of the hemispherical lens . a maximum direction of the radiating pattern of the respective antenna is related to the distance by which the detector is offset from the optical axis of the hemispherical lens .

Description:
fig1 demonstrates an overview of the disclosed thz imaging system . the object ( e . g ., tissue sample ), 10 , that is to be imaged is illuminated with uniform thz radiation , 12 . the transmitted waves that have amplitudes modified according to the absorption properties of the object are guided to the imaging array through a lens system , 14 . the array elements are attached to the back of an extended hemispherical lens , 16 , for detection from different angles of incidence . the lens is made from high resistivity silicon and its shape provides high gain and gaussian beam coupling efficiency . each focal plane array detector consists of a slot antenna structure ( e . g ., double slot ) and sb - heterostructure diode printed on it . the radiation received through the antenna is rectified on the diode , which acts as a half wave rectifier . the output of the rectification is dc voltage and higher frequency harmonics . the power level of the dc component corresponds to the magnitude of the received radiation . the detector subsequently is connected to an nd converter , which quantizes the dc voltage and determines the pixel contrast of the generated digital image , 18 . because each antenna - diode detector collects radiation from a specific direction , the nd converters measured voltage values form the pixels of a digital image ( a . k . a ., the image of the tissue under test . the critical design aspects for each detector element are the matching between the antenna and the diode , the absorption loss in the high resistivity silicon lens and the antenna aperture efficiency . all these losses and mismatches directly affect rf power transfer to the diode , thus decreasing the overall responsivity of the thz detector . fig1 a is an image of the actual tissue being imaged , while fig1 b is an image of the actual display of the imaged tissue . fig2 ( a ) depicts a magnified view of the fabricated detector , 18 , with the dual slot antenna with the sb - heterostructure [ 5 ] diode printed on it . the dual slot dipoles are designed to be 0 . 94 mm long , 0 . 08 mm wide , and 0 . 5 mm apart to resonate at 100 ghz . the slots are fed with a 0 . 1 mm wide coplanar waveguide ( 0 . 06 mm inner conductor width ) leading to 10 ghz bandwidth on silicon half - space ( 50ω impedance ). the detector is attached to a 25 mm diameter extended silicon lens ( ε r = 11 . 7 ) having a high resistivity of & gt ; 10 kω . the diode area is adjusted to match 50ω and a 5 mm extension length is chosen for operation close to the diffraction - limited directivity with large gaussian coupling efficiency [ 6 ]. during the design process , radiation into the silicon half - space ( fig3 ) is evaluated via a method of moment ( mom ) solution of the magnetic field integral equation . subsequently , aperture currents on the lens are determined from a ray tracing technique , and then integrated to calculate radiation pattern of the overall system [ 6 ] ( fig4 ). the received radiation is rectified by the zero - bias diode and converted to a dc voltage measured through a wire - bonded pigtail . a 0 . 1 mm × 0 . 2 mm pad , 20 , with 5 mm gap is formed adjacent to the antenna layout as depicted in fig2 ( a ) for wire bonding . although the gap is very narrow as compared to the slot widths , its presence significantly affects the x - z plane radiation pattern . as shown in fig3 , the x - z plane half - power beam width ( hpbw ) is 8 ° wider than the y - z plane . also , the x - z plane pattern is 8 ° tilted along − x and a − 10 db side lobe is observed towards + x . this pattern broadening is expected to decrease the gaussian coupling efficiency and associated detector responsivity in an optically coupled imaging system . on the other hand , far field patterns in fig4 are still very close to the diffraction limited size with 21 mm × 21 mm effective aperture area ( with only a 1 ° shift in the x - z plane ). fig5 depicts the experimental setup for testing the direct detection and sb - heterojunction diode responsivity . a chopper , 22 , rotating with a frequency of 341 hz was mounted in front of the source horn of the backward wave oscillator , 24 , radiating at 103 . 8 ghz . a mirror , 26 , turns the beam towards a lens detector , 28 , with readout provided by an oscilloscope , 30 . initial measurements resulted in a diode responsitivity of 1000 v / w . new improved antenna designs presented below allow for realized responsitivities exceeding 150 , 000 v / w . the number of detectors , 34 , used to form a focal plane - imaging array under the extended hemispherical silicon lens , 32 , is limited by the internal reflections at the lens surface [ 4 ]. to illustrate this , let us consider the off - axis radiation properties of the dual slot antenna elements designed in the previous section . since the slots are positioned symmetrically around the feed in the x and y directions , the antenna radiates a pencil beam into the silicon half - space along the positive z ( θ = 0 °) axis , 36 , with 10 . 9 db directivity . from the ray optics illustration shown in fig6 a , it can be observed that the antennas radiate / detect along off - axis directions as they are positioned further away from the center element at the lens axis . therefore , obtaining high - resolution thz images ( i . e ., large number of detectors ) with a fixed size extended hemispherical lens implies detection from larger incidence angles . although ray optics demonstrates that any off - axis detection is simply possible with an appropriate element location , antennas positioned further way from the lens axis experience significant power loss due to the reflections , 42 , at the lens / air boundary . as shown in fig6 b , most of the radiation from the dual slot antenna element , 38 , gets internally ( 42 ) reflected when the angle between lens surface normal and main beam direction increases . to alleviate this fundamental limitation and increase the number of allowable array elements for the same frequency and lens size , the disclosure proposes to minimize internal reflection using antenna structures , 40 , with radiating patterns tilted towards the optical axis , 44 , as shown in fig6 b . in the dual slot antenna configuration , 46 , ( see fig7 ), such pattern tilting can be easily achieved via asymmetrical feeding or additional parasitic slots . for example , the half - space pattern of the antenna layout depicted in fig8 is tilted 26 ° in the y - z plane towards + y direction . this tilting is simply achieved by moving the coplanar waveguide feed line 150 um along − y direction . the modified antenna element has a pencil beam with − 7 db side lobe level at 100 ghz and exhibits a real input impedance of 55ω due to inductive loading applied around the feed position ( see fig1 ). solid lines , 50 , in fig1 depict the computed y - z plane radiation patterns when the original detector layout in fig7 is displaced off - axis along − y direction with 1 mm intervals . these patterns are normalized with respect to the center element located at the optical axis . it is observed that for scan angles larger than 20 ° ( off - axis distance & gt ; 2 mm ) a significant drop at the radiated power (& lt ;− 6 db ) is observed . therefore , 25 mm the silicon lens can only support 5 elements for a linear imaging array ( 2 along − y , center element , 2 along + y ) and 13 elements ( or equivalently pixels ) for a rectangular 2d imaging array at 100 ghz ( d & lt ;= 2 mm , where d is the distance from the lens axis . the dashed lines , 52 , in fig1 demonstrate radiation patterns when detector layout , 48 , in fig9 is positioned at − 2 , − 3 , and − 4 mm off the optical axis . due to much reduced internal reflection , the elements receive 1 db , 3 . 1 db , and 4 . 8 db more power , respectively . hence , the 25 mm silicon lens can now support 4 more elements for a linear array configuration and 36 more elements ( d ≦ 4 mm ) for a 2d imaging array as compared to the original configuration . fig1 depicts an illustration of a possible 2d array utilizing modified dual slot antenna elements . due to different element orientations , a polarization scan is necessary for the best imaging performance . on the other hand , different types of modifications , such as feed location shift along x / y axes or parasitic slots , can be combined to tilt element patterns in an arbitrary cut without changing the antenna element . however , beam tilting supported by asymmetrically fed double slot antennas turn out to be limited by 25 °. in order to enable beam tilting beyond 25 °, the detector must consist of multiple antennas as illustrated in fig1 . in addition the overall size of the detector layout must be kept smaller than the maximum pixel size dictated by the wavelength and lens , f - number . hence , miniature antenna layouts and detector topologies as demonstrated in fig1 alleviate these challenges and enable high - resolution thz focal plane imaging arrays that will work without expensive and large silicon lens optics . broadband focal plane array performance enhancement via antenna beam - correction for thz imaging double slot antenna configuration is quite flexible for designs that can achieve excellent performance within a focal plane array ( fpa ) as seen in fig1 - 16 . however , they exhibit narrow operation bandwidths . thus , it is imperative to show that the pattern correction techniques ( outlined above for double slot antennas ) also are applicable for broadband antenna to correct for the off - axis element performance degradation due to refractions and wavefront aberrations . the slot spiral antenna features broadband operation while maintaining a very small footprint . it is compact in size ; thus , can easily be incorporated into a densely packed fpa . it exhibits a rather uniform radiation pattern throughout the operating frequency band . in fig1 and 18 , the spiral antenna layouts for boresight ( fig1 ) and tilted beam ( fig1 ) reception are depicted . in order to tilt the receiving beam of the antenna , the feed ( or the location of the detector element ) was shifted off of the geometrical center and the horizontal slots were elongated . also shown in fig1 is the broadband impedance performance of the beam - tilted spiral element . in addition to a spiral design , broadband performance also can be achieved by different antenna topologies , such as the one shown in fig2 and 21 . this antenna is a modification of the double - slot antenna and is termed the “ butterfly antenna ” due to its appearance . the beam - correction technique applied to the butterfly antenna is outlined below . the narrow bandwidth of the versatile double slot antenna can be improved by modifying the antenna slots as depicted in fig2 and 21 . this design accommodates wideband around the center frequency . the resulting impedance performance of the butterfly antenna is shown in fig2 , displaying a much broader bandwidth covering the 500 ghz to 1 thz range . the antenna has a relatively uniform behavior in terms of both impedance and radiation pattern . as opposed to the broadband spiral antenna , the butterfly antenna layout features improved flexibility in controlling the beam - tilt angle throughout the operation frequency band . as seen in fig2 a - 22c , the feed ( or detector ), 54 , is placed off the center , 56 , closer to one of the two slots resulting in a phase difference between the currents in the two slots ( as in the case of double slot antennas ). however , for the lower frequencies phase difference is small meaning that the angle of the tilted beam also is small . thus , adding meander section into the feed line ( see fig2 ) increases the electric length of the feeding line ; thus , the phase difference . the achievable beam tilts for the 500 ghz - 1 thz bands are shown in fig2 a - 23c . as demonstrated in the previous section , focal plane thz imaging arrays ( positioned at the back surface of an extended hemispherical lens ) can employ small and directive slot antenna configurations in order to deliver the best detection performance in terms of sensitivity , loss , and resolution . that is , the antenna size must not be larger than that set by the diffraction limit of the lens aperture ( 1 . 22λf / d ), while the pattern is directive and symmetric with respect to the lens axis . the size of the antenna element becomes even more critical at multiband applications to obtain the best possible resolution at the high frequency regime . for our frequencies of interest ( breast cancer detection ) at 500 ghz and 800 ghz , this disclosure proposes the double folded slot antenna configuration shown in fig2 as an initial design . since folded slot length is about half of a regular slot , the 111 um × 94 um resolution of the double folded slot antenna is very close to the optimal 85 um × 85 um at 800 ghz . both of the 500 / 800 ghz radiation patterns are symmetric and directive implying that individual radiators ( i . e ., inner and outer folded slots ) can be excited without coupling and distorting each other . fig2 demonstrates the | s 11 |& lt ;− 10 db bandwidths of the antenna element at 500 / 800 ghz frequencies . specifically , 15 % bandwidth can be obtained when the antenna is matched to 140ω . clearly , smaller slot antenna / detector topologies are highly beneficial for high - resolution focal plane thz imaging arrays that will employ multiband detection . in addition , enabling pattern tilting for these multiband antennas will result in higher resolution without resorting to large and bulky lenses , as was discussed in the above . a device structure ( see fig2 ) has demonstrated noise equivalent powers ( nep ) as low as 240 fw / hz 1 / 2 . 1 this sensitivity is sufficient to enable passive imaging arrays based on direct detection , without requiring either cryocooling or low - noise amplifier ( lna ) front - ends . this reduces not only the cost , but also the front - end engineering needed for arrays based on these materials . improved noise performance translates directly into improved system signal - to - noise ratio and reduced component part count and complexity . this detection array also involves a dc choke at each pixel . this choke directly converts from intensity - to - dc output on - chip , removing the requirement of transporting thz signals that results in large losses and has historically precluded a number of terahertz applications . hbds , 58 , includes layers : 62 , s . i . gaas substrate ; 64 , buffer layer ; 66 , gasb ; 68 , al 0 . 1 ga 0 . 9 sb ; 70 , aisb ; 72 , inas ; and 74 ; inas . prior work by others using custom - grown structures has demonstrated extremely low 1 / f noise and an intrinsic sensitivity that exceeds the theoretical limits of thermionic devices ( e . g ., schottky diodes , planar - doped barrier diodes ). to date , these demonstrations have been limited to w - band and below (& lt ; 110 ghz ). this effort sees the aggressive scaling of deep - submicron devices for extending their frequency range into the thz regime . these nanoscale devices will be integrated with antennas to form broadband fpa arrays that operate in the 100 ghz through thz regime . work on the present disclosure has already demonstrated a scalable 6 × 11 fpa monolithically - integrated with matched antenna - diode structures . 1 several alternative thz antenna architectures shown in fig2 - 34 . in particular , fig2 is terahertz antenna elements for integration with hbds in a bow - tie configuration . fig2 is terahertz antenna elements for integration with hbds in a planar log - periodic configuration . fig2 is terahertz antenna elements for integration with hbds in a double - slot with microstrip feed configuration . fig3 is terahertz antenna elements for integration with hbds in a spiral antenna configuration . fig3 is terahertz antenna elements for integration with hbds in a helical antenna configuration . fig3 is terahertz antenna elements for integration with hbds in a ring antenna configuration . fig3 is terahertz antenna elements for integration with hbds in a dielectric rod antenna configuration . finally , fig3 is terahertz antenna elements for integration with hbds in a double slot antenna with co - planar waveguide feed configuration . 1 n . su , r . rajavel , p . deelman , j . n . schulman , and p . fay , “ sb - heterostructure millimeter - wave detectors with reduced capacitance and noise equivalent power ,” ieee electron device lett . 29 , no . 6 , pp . 536 - 539 , 2008 . after considering the several alternative thz antenna architectures shown in fig2 - 34 , the antenna design of choice consisted of the double - slot antenna element ( fig3 ) printed on a high - resistivity silicon substrate and tuned to match the hbd &# 39 ; s impedance at 0 . 1 thz . a 0 . 5 thz prototype has been demonstrated by scaling the design and re - tuning the impedance match to this particular frequency . this single element prototype , 80 , shown in fig3 and 36 , is situated behind an extended hemispherical imaging lens , 82 . the integrated antenna - diode is shown in fig2 and 2a ( above ), along with the computed receiving pattern of the single element detector . this first prototype was successfully tested using the standard setup shown in fig5 . at the design frequency of 0 . 1 thz , this matched prototype achieved an unprecedented responsivity of r = 100 , 000 v / w , and a noise equivalent power of nep = 0 . 2 × 10 − 12 ( fig3 and 38 ). when scaled to 0 . 5 thz , the design sustains a responsivity & gt ; 20 , 000 v / w with nep & lt ; 1 × 10 − 12 . there exists no other detector option offering this performance without a considerable form factor and liquid helium cryocooling requirements . this monolithic integration of sensor element and antenna allows the designer to flexibly modify the antenna topology . this modification can be done according to well - developed antenna design and microwave matching and filter theory techniques , in order to achieve a perfect match to the complex diode impedance . this highly promising integration of antenna and radiofrequency ( rf ) engineering is already opening up new avenues to develop a high - efficiency coupling of incident radiation into high - speed non - linear detectors . for example , similar approaches are being pursued to improve sensor responsivity and speed in the infrared ( ir ) and optical bands ( using nano - antennas ). disclosed herein is a dual slot antenna with tunable main - beam direction integrated with zero bias sb - heterostructure backward diode for direct detection of thz radiation . the compact layout and high responsivity of such detector elements make them suitable to design 2d focal plane thz imaging arrays . further , the disclosure proposed and demonstrated that the number of detectors supported by a fixed size silicon lens can be significantly increased by tuning the main radiation beam of each pixel to illuminate the optical axis . this can simply be achieved by shifting feed locations and / or introducing small parasitic slots into the antenna geometry . in addition , the disclosure demonstrated that double folded slot antennas are promising candidates for dual band thz detection of breast cancer . improving resolution and enabling multiband detection are current challenges in the thz imaging arrays and can be addressed with the development of smaller antenna / detector topologies , as outlined in this disclosure . besides healthcare use of the disclosed detector system , other likely uses include , for example , thz imaging and diagnostics cancer detection : breast , oral , skin , prostate , and cervical dental imaging skin assessment : burn diagnostics and dermatology , cosmetic diagnostics and treatment orthopedic imaging ( of exposed bones ) ablation : cancer treatment , cosmetic surgery , cosmetic therapeutics volatile organic compound detection and spectroscopy identifying polymorphs and hydrides in drug compounds counter detection of drugs and pharmaceutical microchip inspection corrosion identification gaseous compound inspection and detection pharmaceutical quality control package inspection wind blade inspection jet - propulsion inspection material integrity and structural analysis fuel cell quality control and diagnostics process monitoring pollution monitoring diagnostics of natural gas pipelines detection of separation of laminated materials systems for 3d inspection of fragile art products molecular recognition and protein folding checkpoint screening concealed contraband detection controlled substances detection harmful compound identification biometrics ( gates , hand , walking behavior ) imaging through fog evaluation of agricultural products ( imaging of fresh frozen samples , moisture differences , etc .) line of sight high - speed digital and analog communications , & gt ; 10 gbits military applications for high speed extensions of broadband fiber optics high magnetic field application sub - mmw and mmw astronomy atmospheric observations / monitoring ( such as ozone depletion ) guidance and id systems counter riot protection additional fields of use include , for example , rf / eo / optics imaging systems , rf / eo / optics phased arrays , mmw imaging , mmw communications mmw adaptive arrays , mmw smart antennas , and uv communications / radios . while the device and its use have been described with reference to various embodiments , those skilled in the art will understand that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope and essence of the disclosure . in addition , many modifications may be made to adapt a particular situation or material to the teachings of the disclosure without departing from the essential scope thereof . therefore , it is intended that the disclosure not be limited to the particular embodiments disclosed , but that the disclosure will include all embodiments falling within the scope of the appended claims . in this application all units are in the metric system and all amounts and percentages are by weight , unless otherwise expressly indicated . also , all citations referred herein are expressly incorporated herein by reference . 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[ 6 ] d . f . filipovic , s . s . gearheart , and g . m . rebeiz , “ double - slot antennas on extended - hemispherical and elliptical silicon dielectric lenses ,” ieee transactions on microwave theory and techniques , vol . 41 , pp . 1738 - 1749 , october 1993 .