Patent Application: US-53030408-A

Abstract:
a method and apparatus for enhanced thz radiation coupling to molecules , includes the steps of depositing a test material near the discontinuity edges of a slotted member , and enhancing the thz radiation by transmitting thz radiation through the slots . the molecules of the test material are illuminated by the enhanced thz radiation that has been transmitted through the slots , thereby producing an increased coupling of em radiation in the thz spectral range to said material . the molecules can be bio - molecules , explosive materials , or species of organisms . the slotted member can be a semiconductor film , a metallic film , in particular insb , or layers thereof . thz detectors sense near field thz radiation that has been transmitted through said slots and the test material .

Description:
as employed herein , the term “ slots ” is inclusive of a structure having a linear array of thin opaque strips , a structure in which slots are formed in a solid material , and slits or slots having a periodic spacing and suspended on a solid matrix . the term slots is inclusive of hole and gratings . the geometry of slots includes : a closed curve , the intersection of a right circular cone ( see cone ) and a plane that is not parallel to the base , the axis , or an element of the cone . it may be defined as the path of a point moving in a plane so that the ratio of its distances from a fixed point ( the focus ) and a fixed straight line ( the directrix ) is a constant less than one . any such path has this same property with . . . elongated slot , such as , a flattened circle : a two - dimensional shape like a stretched circle with slightly longer flatter sides ii — egg shape : something shaped like an egg or a flattened circle iii — oval — a closed plane curve resulting from the intersection of a circular cone and a plane that is non - parallel to the plane of the base of cone the cutting completely through it ; “ the sums of the distances from the foci to any point on an ellipse is constant ”. a conic section whose plane is not parallel to the axis , base , or generatrix of the intersected cone . the locus of points for which the sum of the distances from each point to two fixed points is equal . a four sided polygon having opposing sides equal to each other but not equal to their adjacent sides . viii — an elongated parallelogram — a quadrilateral whose opposite sides are both parallel and equal in length to each other but not equal in length to adjacent sides an aspect of various embodiments of the present invention comprises , but is not limited thereto , a method and related system for detection of the thz spectroscopic signatures of bio - molecules or other materials of interest , such as explosives , in 0 . 1 - 3 thz range that is based on the local em field enhancement with respect to the incident field in structures with slot or slot arrays fabricated using semiconductor or metallic films or multilayer structures . this enhancement leads to an increased coupling of em radiation in the thz spectral range to materials of interest and , therefore , results in dramatic improvements to the sensitivity , selectivity , reliability and spatial resolution of thz detection systems . a prototypical embodiment of this application to deliver the enhanced coupling of thz radiation with bio - or chemical materials is through periodic structures of sub wavelength slots in semiconductor or metallic films . in the thz region , interaction between radiation and metals is quite different from higher frequency regions due to the change in material dielectric properties . in the visible and near - ir regions , where frequencies are only slightly less than plasma frequency , the permittivity is predominantly real and negative ( for example , at wavelength 1 μm , ∈ au =− 51 . 4 + j1 . 6 ), and metals are reflective . on the contrary , as the frequency is lowered to the thz range , the real part continues to be negative and large , but the dissipative imaginary part becomes larger , and hence metals are very conducting and absorbing ( at wavelength 500 μm , ∈ au =− 5 . 5 × 10 4 + j8 . 5 × 10 5 ). therefore , to reduce radiation losses , it is preferable to substitute metals with doped semiconductors with plasma frequencies in the low thz range . insb with high electron mobility and low effective mass is most suited for this purpose , but still has a substantial absorbing imaginary part compared to the real component . in the semiconductor structure with periodic gratings , the material properties are periodic functions of coordinates as well . the absorbing component in semiconductors ( insb and si ) requires the assumption of a small film thickness , which makes the semiconductor skin depth at both semiconductor - air interfaces larger than half the film thickness throughout the frequency range of interest . this renders the surface impedance boundary conditions for perfect conductors [ 32 , 33 ] to be unsuitable for semiconductor structures . on the other hand , in contrast with the behavior of metals in short wavelength ranges , the fourier expansion method for field diffracted from gratings [ 9 ] can be applied in the thz region for insb and si films , since the imaginary permittivity component damps the gibbs oscillations [ 34 ]. the fourier expansion of the electro - magnetic fields and the permittivity were used to solve the terahertz transmission / absorption / reflection problem and to calculate the total distribution of the electro - magnetic field in the system . at the same time , the fourier expansion method is unsuitable for au owing to its dielectric properties . however since the skin depth for au is small compared to thickness , surface impedance boundary conditions can be used . even in this case , the perfect conducting walls approximation [ 35 ] for fields inside slots is employed since the thickness assumed is very small compared to the wavelength . using a rigorous theoretical model of the enhancement effect derived from the numerical solution of maxwell &# 39 ; s equations for semiconductor based periodic structures with one dimensional slot arrays in 0 . 3 - 0 . 75 thz range [ described originally in refs . 5 , 6 ], the “ edge effect ”, a localization of em field that can be used to implement novel bio - and chemical sensors , was discovered . maxwell &# 39 ; s equations with appropriate boundary conditions on interfaces were solved with the frequency - dependent permittivity of the doped semiconductor . for polar materials like insb , the frequency dependence of the relative permittivity , ∈( ω ), includes terms describing the interaction of light with free carriers ( drude model ) and with the optical phonons . an aspect of various embodiments of the present invention can comprise a structure suitable for sensing applications , as illustrated in fig1 . the structure includes a structure 102 having a subwavelength array of slots 104 with the periodicity in the x - direction and extending in the y - direction . the z - direction is perpendicular to the plane of incidence . since the structural geometry is not altered in the y - direction , it would suffice to analyze a one - dimensional periodic slot structure as shown in fig1 with spacing ( or periodicity ) denoted by ( d ), the slot width by ( s ) and the thickness of the film by ( h ). the structure is considered to be illuminated at normal tm incidence 106 . fig2 shows the electric field amplitude ( with incident field normalized to unity ) at the interface of incidence , as a function of position with a slot width ( s ) of 55 μm , periodicity ( d ) of 381 μm , height ( h ) of 4 μm . the simulation frequency is chosen to be 420 ghz ( wavenumber of 14 cm − 1 ) because absorption peaks of interest for many biological molecules have been shown to occur in this region . the enhancement of the field intensity at this frequency was obtained at all points in the slots . the half - power peak field near the slot edges occurs over a sub - micron region (˜ 500 nm ). in practice , most of the field is confined to the edges ( i . e . sharp regions ) of the conducting medium . the maximum power enhancement is approximately 1100 and also occurs for a slot height of 4 μm . the enhancement persists across the slots , decreasing slightly from the incident interface to the outgoing ( transmission ) interface . it cannot be attributed to a surface plasmon mode because the plasmon matching condition is not applicable for permittivities with substantial imaginary parts . using insb as an example , it has been shown that the 30 - fold em field enhancement within the sub - micron region of the slot edges , translates into a 1000 fold increase in power ( fig2 and 3 ). this “ edge effect ” at sub - thz frequencies caused by discontinuity effects is an important new result that can be applied to guide designs for enhanced thz coupling , as described below . the em field enhancement at other points inside the slot , away from the edges is smaller , on the order of 3 - 5 fold . the enhancement of the amplitude of the electric field with respect to the incident field is demonstrated in fig2 where the relative x - component of the electric field amplitude is plotted as a function of a coordinate across the slot , x , with s = 55 μm and h = 4 μm , for radiation with the frequency of 14 cm − 1 . the electric field enhancement occurred within the sub - micron region around the slot edges i . e . at discontinuities as illustrated in fig2 . practically most of the fields were confined to the edges i . e . sharp regions of the conducting medium . the enhancement at the edges is an order of magnitude higher than at the other points within the slot . the maximum field enhancement is 33 . 3 at the incident interface and 31 . 8 at the outgoing interface for h = 4 μm . for h = 6 μm , these values are 27 . 7 and 25 respectively and for h = 12 μm , 20 . 5 and 14 . 7 respectively . the half power width around the slot edges was ˜ 500 nm with maximum power enhancement ˜ 1100 for the h = 4 μm case . this region did not change much for the other h values . the enhancement exists across the slots , slightly decreasing from the incident interface to the outgoing interface . the decay into the metallic region is more abrupt than into free space as expected , as seen in fig3 , and around the edges is approximately proportional to χ − 1 / 3 , consistent with edge effects . fig3 illustrates the basic concept of an instrument of the present invention . fig3 shows thz power , ( e x / e o ) 2 , enhancement as a function of a coordinate x ( μm ) across a slot for the structure with the same parameters as in fig2 at two frequencies 14 cm − 1 ( the wavelength λ = 714 μm ) and 24 cm − 1 . it is seen that an imaging sensor is capable of measuring the thz response as well as resolving spatial features of samples under the test with a micron - submicron resolution . the instrument employs a terahertz source radiation that is collimated using optical components . the thz radiation is directed at a thin film slot grating integrated with a microfluidic channel with the sample material to be measured where the sample is illuminated with the terahertz energy . an integrated thz micro - detector assembly is composed of three essential parts , i . e . a sub - micron probe ( antenna ) that is connected to a miniature bolometer detector ( for example , schottky - diode ), and control circuit with the corresponding impedance matching network to achieve the precise detection of the electric field in the near - field configuration . the detector assembly with a micro probe is mounted on the stage , which provides precise scanning , with a resolution of less than 1 μm , over the sample under test along xyz direction with nanometer accuracy controlled by the control circuit . the technology for fabricating the miniature detector with micron size antenna to affectively couple with thz radiation transmitted through the slit is disclosed in publications noted herein as 26 and 27 . fig4 compares the enhancement of two electric field components , e x and e z , that are perpendicular and along the direction of the incident radiation . the enhancement at the slot edge as a function of a slot width is plotted in fig5 for three different thickness . the calculated far field transmission through the structure is plotted in fig6 as a function of a periodicity , d / λ , for different slot widths . the “ edge effect ” at sub - thz frequencies for two other materials ( silicon and gold ) is demonstrated in fig7 a and 7b . the effect is significantly less than for insb structure , however these materials still can be used due to technological advances . in all these cases , a sub micron narrow thz beam along the edge is a local , highly intense radiation source for probing biological and other material properties using near field configuration for specific microscopic sensing and imaging instruments in the thz range . the invention is illustrated by the example structure consisting of a one - dimensional array of rectangular slots with the period less than the wave length λ of applied em radiation , which contains small quantities of biological material embedded in the nano - size regions of the edges where enhancements of radiation in the thz gap are observed . this array is made of a thin - doped insb film with a free electron concentration of 1 . 1 × 10 16 cm − 3 fabricated on a substrate transparent for thz radiation . this is not to be construed in any way as imposing limitations upon the scope of the invention . structures with slot arrays or hole arrays of different periodicity and different geometry can be used as well . different materials such as semiconductor films or metallic films can be used separately or in combinations as in multilayer structures . it should be understood that resort can be had to various other embodiments , modifications , and equivalents to the embodiments of the invention described herein which , after reading the description of the invention herein , can suggest themselves to those skilled in the art without departing from the scope and spirit of the present invention . fig8 shows an embodiment of the present invention for the application of the periodic array of semiconductor slots to enhance thz coupling to materials of interest for thz sensing and imaging . the basic concept of the instrument is an imaging instrument capable of measuring the thz spectral response as well as resolving spatial features of samples under test with submicron resolution . the design consists of three parts : a supporting plate from plastic or quartz onto which a periodic slots structure is bonded or electroplated that also comprise materials sample chamber ; a miniaturized thz detector assembly which can be adjusted with a movable stage so that sub micron probe ( s ) of detector ( s ) are within ˜ 1 μm of the plane where the thz radiation exiting the slots . the preferable miniaturized thz detector is a schottky micro - diode from virginia diode inc ., charlottesville , va ., integrated with a coupling circuit and a nano - probe ( antenna ); and a motorized movable stage with controller that provides sub micron steps . a terahertz source is collimated using standard optical components onto the sample material that is induced into channels of a microfluidic periodic structure integrated with a thin film periodic slot grating . the detector assembly with a micro probe is mounted to an xyz nanopositioner and is scanned over the sample under test . high - resolution piezoelectric positioners with nanometer accuracy and travel ranges up to 1 cm are commercially available and can be used for probe placement and positional control . in this configuration , the rectangular slots of a periodic structure are concurrently used as channels for the sample material and the materials or molecules of interest can be immobilized on the surface of the film structure or trapped at the slot edges . in another embodiment , these two functional elements can be separated . small quantities of biological material are embedded in the nano - size regions of the edges where enhancements of radiation in the thz gap are observed . very small amount of material would be enough for detection using this approach . the detector probes ( antennas ) can be scanned in two perpendicular directions across the sample chamber and sample material to improve sensitivity and selectivity of thz sensing or to generate a 2d thz imaging . such application modes provide a new class of devices using bio - or chemical fluidic chips combined with near field thz detectors . the effect of local near field enhancement of electromagnetic field is used to maximize the coupling of terahertz radiation to both biological and chemical molecules . the new process for coupling provides dramatic improvements in spatial resolution , sensitivity , reliability , and selectivity of terahertz detection systems . the imaging mechanism of the present invention is appertureless , all optical , and utilizes low thz frequency range radiation to achieve a spatial resolution well below the diffraction limit . this new detection platform can produce a new class of resonant , highly sensitive and selective portable bio and chemical devices for uses in many different applications . by interacting the thz vibration absorption modes from organic or biological molecules with a locally enhanced em field of terahertz radiation in a sub - micron region , the developed imaging mechanism : is capable of sub wavelength spatial resolution , ideally 10 3 orders less than the radiation wavelength . is an all - optical instrument , with no required mechanical tips or probes to contact testing material . requires no apertures . can allow for spectral selectivity . can test biological monolayers , and molecules in dilute solutions . the applications of terahertz frequencies for identification and detection uses is virtually almost endless , ranging from military and transportation detection devices to real time drug development monitoring of anti - bacterial or anti - viral drugs . new imaging mechanism integrated with a “ lab - on - a chip ” device for sub - wavelength thz spectroscopic microscope . real time monitoring of drug - bacteria cell wall interaction in drug development . the research work included sensor modeling and design , fabrication of a beam lead antenna and a diode integrated with a circuit , and demonstrated the successful implementation of the imaging mechanism of the present invention . all elements of a thz detector assembly to measure the electrical field distribution around the periodic slots were modeled and fabricated and the detector assembly was completed and tested . the periodic slots structure of fig9 , indicated generally as 900 , was made of gold thin film on silicon wafers and quartz using the photolithographic process and electroplating . the key challenge associated with the fabrication of the slot arrays 902 is to obtain a high degree of sharpness at the edges of slots due to the fact that the enhancement of electrical field is in a micron region . the microfabrication processes have been optimized to obtain high sharpness at the edge 1002 of the slots 1006 , as shown in fig1 . the edge extrude of 0 . 2 - 0 . 5 μm has been attained . the key function of the thz micro - sensor is to detect the electrical field in the vicinities of the slot edges where the enhanced coupling occurs . in other word , the thz micro - sensor system is responsible to detect the near field distribution of radiation transmitted through the biological or other materials of interest that are located around the edges of the slots . since the electrical field enhancement is only available in a region of several microns , the sub - micron sharp probe ( antenna ) 1102 of fig1 , is required for the sensing of the transmitted electrical field through biological sample in order to obtain high sensitivity and spatial resolution . thus , it is crucial to provide an integrated thz sensor detector with high - sensitivity and sub - micron spatial resolution for subwavelength thz spectroscopy . one example of such a sensor is a miniature sensing device which incorporates a room - temperature detector , schottky micro - diode 1112 , integrated with a coupling circuit and a nano - probe 1108 , ( also referred to as an antenna ) mounted on a silicon substrate 1102 , as shown in fig1 a . other types of miniature detectors can be used as well . the zero biased schottky diode 1112 , which in this example incorporates gaas islands , transforms the input thz radiation coupled from the sharp beam lead probe 1108 tip to the output dc voltage . the magnitude of the output dc voltage is proportional to the input power of the thz radiation . in fig1 b , a low pass filter 1120 and the rf choke 1126 , are the components for blocking the high frequency radiation for the measurement of the dc voltage across the diode 1112 . thin ( 50 μm ) fused quartz material is chosen as the substrate 1102 for the detector circuit to minimize the possible surface mode excitation . the detector assembly chip in this example is 1 mm wide and 1 . 5 mm long . as illustrated in fig1 the beam lead micro - tip 1200 has a length of about is ˜ 60 μm long , as indicated by arrow 1204 , has a tip length of about 15 μm as indicated by arrows 1206 , a tip width of about 15 μm as indicated by arrow 1202 , and a tip 1208 of about 0 . 64 μm . other types of miniature detectors that produce the same results as the detector set forth above can be used as well . a sharp coupling device can modify the original electrical field distribution produced by the slots structure . thus , the size of the coupling device and the distance between the coupling device and the slots has to be designed and optimized in order to obtain the balance between measurement and disturbance of the local electrical field around the periodic slots , while being a physically realizable tip geometry . the local electrical field enhancement at the edge of slot is confirmed by our electrical field simulation work using the commercial full - wave solver . from fig1 it is seen that although the beam lead antenna disturbs the electric field distribution , the enhancement effect near slot edges is preserved . another aspect of the research was fabrication and characterization of sample or microfluidic chambers . to apply the local enhancement of thz coupling , the bio - or chemical material can be immobilized on the surface , trapped at slot edges , or scanned across a microfluidic chamber . the materials of interest can be in solid or fluidic form . microfluidic channels were fabricated using polydimethylsiloxane ( pdms ) as the polymeric material onto which channels were micromolded . inexpensive disposable periodic lab - on - chip structures can be used for enhanced thz coupling and detection . the slots can be scanned across the material sample to enhance local coupling and thereby improve the chemical resolution and sensitivity of the detector to thz imaging . the linear array of several integrated thz sensor detectors can be designed and fabricated to provide the capability for a two - dimensional imaging . one of possible solutions for realization a proposed imaging technology is to use a linear detector array of micron / sub - micron size detector elements with a coupling structure , antenna , at each element to probe several slots . only short distance movement of the detector assembly over the slot width will be required in this case . fig1 and 15 demonstrate the existing capabilities to fabricate a schottky diode or bolometer detector array with the spacing between elements ˜ 40 μm [ 27 , 28 ]. fig1 illustrates an array section 1500 including low band pass filters 1504 and slot ring antennas 1502 , and an heb superconducting bridge 1506 . fig1 shows ( not in scale ) an example of a detector assembly 1606 combined with a sample or microfluidic channel 1612 ( 5 - 50 μm wide , 1 μm deep , 1 - 2 cm long ), with a 10 - 50 μm transparent substrate , that is , a backing support 1610 to enable handing , is filled with bio - material 1604 . in this embodiment a 2 - 5 μm au edge layer 1602 is patterned on the top of channel structure , although other semiconductors as taught herein can be used . a movable stage with an xyz controller 1608 is placed at one end of the channel 1612 . as can be seen , linear polarized thz radiation 1614 is presented at right angles to the substrate 1610 . the precise control of the thz sensor position , especially of the sensing probe , has to be implemented in order to enable the sensor to approach near the surface of a sample and to scan along the plane of the periodic structure . long focused optical components can be utilized for precise location of sensing antennas at the distance of about 1 - 3 μm from the sampling material . electric ( for example , capacitive ) sensors can be used as well . the disclosed detection system can include variety of miniaturized thz near - field sensors as listed above . another application of the invention is monitoring changes of dielectric properties of bio - materials in biophysical processes , for example , denaturation of dna , folding - unfolding of proteins , structural conformational changes of biomolecules in interactions with drags , and monitoring other processes for a broad bio - medical and pharmaceutical research . fig1 illustrates a detection device 1700 wherein the thz illumination 1701 is applied from the top down . a plate of quartz 1710 has insb 1712 bonded to the plate 1710 effectively forming slots 1714 . the mid - plate 1720 contains the fluidic cells 1722 with an inlet and outlet . the mid - plate 1720 is adjacent to the quartz bottom plate 1730 that contains a translatable piezo stage with thz detectors . the near field detectors can be less than 0 . 1 μm from the fluidic cells . fig1 a and 18b are illustrative arrangements for the microfluidic cells . in fig1 a the cell 1800 has an inlet 1802 that is connected to an outlet 1804 by channel 1808 . in fig1 b the biomolecules enter the cell at inlet 1 ( 1842 ) and the reagent at inlet 2 ( 1840 ) and are mixed at the joining point 1830 . the biomolecules are moved into the trapping region 1836 where they are exposed to thz radiation 1832 . a thz detector receives the resulting radiation 1834 . the biomolecules then move to the outlet 1838 . the cell can be used for real time monitoring of processes . fig1 demonstrates the dramatic difference in transmission spectra of a single and double - stranded dna that can be used in the proposed monitors . fig1 a shows similar possibilities for monitoring conformational change of proteins . thz source ( ghz signal generator 2010 , frequency multiplier 2008 and power supply 2006 for the source ); collimating devices 2012 ( an off - axis parabolic mirror 2012 and an hemispheric silicon lens ); horn 2012 ; periodic slot chip 2014 combined with microfluidic cell and mounted on the planar surface of silicon lens 2024 ; detector assembly chip 2016 ( beam lead probe , transmission line , schottky diode and detector circuit ); motorized xyz stage with controller 2022 ; the dc voltage measurement device ( i . e . a lock - in amplifier ) 2002 , and controlling computer 2004 . the thz radiation required to illuminate the periodic structure is generated by multiplying the low frequency radiation using a frequency multiplier ( 36 times ) as can be obtained from a source such as virginia diodes inc ., of charlottesville va . system path loss is minimized by using reflectors ( rather than lens ) as well as an anti - refection coating on the surface of the hemispherical lens . the lens assembly is mounted to a platen . the integrated thz antenna is scanning transmitted beam over the sample material put into a microfluidic channel using precision xyz positioners . some exemplary products and services that various embodiments of the present invention method and system may be utilized for may comprise , but not limited thereto , the following : portable scanners to detect explosive residues or bio hazards on clothing , bags , in vehicles , in trains , metro stations , airports , on board of ships , on bridges , in tunnels . portable scanners to detect explosive residues or bio hazards in public areas , buildings . compact remote sensors to detect explosives or bio hazards that can be installed as stand alone devices , as well as on buildings , structures , put on unmanned airplanes , unmanned land vehicles . detectors for real - time monitoring of drug - bacteria cell wall interaction , for testing the effectiveness of bacteria or virus destruction by drugs under development . the following publications as listed below and throughout this document are hereby incorporated herein by reference in their entirety . t . globus , d . woolard , m . bykhovskaia , b . gelmont , l . werbos , and a . samuels , int . j . of high speed electron . syst . 13 , 903 ( 2003 ). 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