Patent Application: US-201113281064-A

Abstract:
disclosed are systems and methods for improving the performance of systems for generating and detecting electromagnetic radiation at terahertz frequencies . embodiments of the systems and methods include the fabrication and use of coupling tapers to provide efficient transfer of thz radiation between a photomixer and a waveguide that supports a propagating thz mode . a representative system comprises of a photomixer to convert high - frequency components of an optical pump signal into corresponding electrical thz frequencies , a waveguide that supports a propagating thz mode , and a matching taper that effectively converts the highly localized currents generated by the photomixer to the mode supported by the waveguide .

Description:
as used in this application and in the claims , the singular form &# 39 ; s “ a ,” “ an ,” and “ the ” include the plural forms unless the context clearly dictates otherwise . additionally , the term “ includes ” means “ comprises .” further , the term “ coupled ” does not exclude the presence of intermediate elements between the coupled items . the systems , apparatus , and methods described herein should not be construed as limiting in any way . instead , the present disclosure is directed toward all novel and non - obvious features and aspects of the various disclosed embodiments , alone and in various combinations and sub - combinations with one another . the disclosed systems , methods , and apparatus are not limited to any specific aspect or feature or combinations thereof , nor do the disclosed systems , methods , and apparatus require that any one or more specific advantages be present or problems be solved . any theories of operation are to facilitate explanation , but the disclosed systems , methods , and apparatus are not limited to such theories of operation . in what follows , the term photomixer refers to any photoconductive or other photoresponsive material configured to receive either a pulsed or continuous wave optical input ( typically from a pulsed or continuous wave laser source ) to generate currents with terahertz frequency components . the term terahertz frequency band refers to the region of the electromagnetic spectrum ranging between 100 ghz and 10 thz . the term active terahertz device refers to any photomixer , photodetector , electronic diode or transistor or collection of such devices into integrated circuits operable to generate or detect electromagnetic radiation at terahertz frequencies . such active terahertz devices are also referred to herein as terahertz devices , terahertz generators , or terahertz detectors . the term terahertz waveguide refers to any guided wave structure , including but not limited to two wire waveguides , rectangular metal waveguides , and dielectric waveguides operable to transmit terahertz electromagnetic radiation . the term mode - matching taper is used to describe the gradual transition from one mode size and shape to another . mode - matching tapers can be based on slotlines , coplanar striplines , microstrips , striplines , coplanar waveguides , or other waveguides . the term radiation loss suppression mechanism refers to mechanisms by which shock - wave radiation loss is prevented , including but not limited to the addition of a symmetric cap layer , thinning of the substrate under the waveguide , and the incorporation of a periodic layered structure . a conventional tgad module 100 as shown in fig1 can be replaced with an illustrative example of an embodiment of the disclosed technology . the representative improved module uses a two - wire waveguide and novel pm - waveguide coupling tapers , shown in fig2 , to produce a small high - performance thz spectrometer . in one preferred embodiment , a tgad module 200 consists of a compact 2 - wire ( gold ) waveguide 250 supported within a small glass sample tube 260 and driven by fiber - coupled pms 210 . the typically 10 cm long and 300 μm diameter gold wires are separated by roughly 1 mm and supported within the glass tube by stretching with modest tension between two plastic end caps . gas ports allow purge gas and the admission of gas samples . novel aspects include the design of the waveguide 250 and the coupling from pms into and out of the waveguide via novel tapers 270 . fig3 shows an alternative view of tgad module 200 , showing tapers and pms 210 and tapers 270 integrated onto the same chip , as discussed in more detail below with regard to fig5 . module 200 can be substituted for module 100 in fig1 . a variety of methods can be devised to place liquid or solid samples within the gap between the gold wires . this configuration also presumes that a pm is used as a detector , as is well known in the prior art . alternatively , the pm - taper device can be used only at one end . for example , the waveguide may alternatively be terminated on another type of detector like a golay cell . also , as might be encountered in a circuit operating at thz frequencies , active ( e . g . mixer , transistor ) or passive ( e . g ., antenna , matching stub ) components might be connected between the waveguide conductors . it should also be noted that the discussion so far has concentrated on photomixers , or equivalently photoconductors , that convert modulated optical intensity into a change in conductance . embodiments of the inventions disclosed herein would also be applicable if the source of the high - frequency electrical signals were high - speed photodetectors , which convert to modulated optical intensity directly into photocurrent with high - frequency components , or any form of an active electronic circuit using , for example , very high speed diodes or transistors to form an oscillator , frequency multiplier , mixer , or other terahertz device . as will be obvious to one skilled in the art , significant advantage may be achieved in coupling any of these components to low loss terahertz waveguides . fig4 shows the electric field distribution of the tem mode of the two - wire waveguide . it must be understood that a variety of waveguide structures could be contemplated within the scope of the present invention and that this two - wire waveguide is merely representative . the field distribution is highly dependent on the radii of the wires and the separation distance . designs can be based on a detailed understanding [ 16 ] of the field distribution and loss as a function of physical dimensions . waveguide loss over a 10 cm length may be a small fraction of a db . direct coupling from pms or photodetectors to the waveguide is based on novel tapers . ( in conventional systems , pms are coupled to antennas .) the first stage of this taper ( fig5 a ) expands the field to two parallel gold strips , or a slot waveguide , all fabricated on the planar pm substrate . the terahertz current , generated at the small gap in the active area 510 , excites the tem mode of a slot - line ; the separation of the plates becomes gradually wider in the form of a taper 570 to expand the field to the size comparable to the waveguide &# 39 ; s 580 dimensions . it should also be understood that this particular implementation of the taper is just one example , and that other means of expanding the field size may be contemplated , depending on the form of the waveguides used . the second stage converts from the planar strips 580 to lower - loss round gold wires that have been tapered to flat for attachment to the chip , or to whatever form of waveguide is used . one skilled in the art will recognize that a variety of similar tapers can be contemplated to connect to a variety of waveguide structures . fabricating waveguides on the surface of a high dielectric substrate such as gaas requires careful consideration . the high velocity mismatch between fields propagating above the substrate and below does not allow the tem mode to propagate , and simulations presented in fig5 b show that the electric field is drawn into the substrate and lost . this loss mechanism has been referred to as shock wave radiation loss . to prevent this , a variety of techniques may be employed . since the photomixers are typically fabricated on gaas or similar alloys , a matching upper gaas , silicon , or other cap layer can be used to compensate for this velocity mismatch . the efficacy of this approach can be seen in fig6 , which shows a simulated mode propagating with low loss through such a structure . another possible solution for the mismatch problem is to make the thickness of the substrate right below the slot , small compared to the wavelength ( less than typically 10 μm for gaas ) so that the shock waves cannot be excited as shown in fig7 ( a ). yet another solution , as shown in fig7 b , is to use a layered substrate to avoid propagation of the terahertz wave into the substrate . the layered substrate is a 1 - dimensional photonic crystal that creates a band - gap to avoid propagation of waves into the substrate for a certain range of frequency . it can be shown that surface waves can be supported by the layered substrate with exponentially decaying field amplitude in air and the substrate for the appropriate thicknesses of the layers . therefore , we confine the waves in one dimension by the metal plates and in the other by the layered substrate . fig8 shows how the layered structure confines most of the electromagnetic energy at the surface . the second stage of the taper is used since the field distribution of the tem modes supported by the slot - line 980 and the two - wire waveguide 950 are rather different , as shown in fig9 . in the slot - line the field is mostly concentrated on the edges of the plates while it is more distributed on the surfaces of the wires for the two - wire waveguide . a two - wire waveguide with the wires squeezed to rectangular shapes at the input port of the waveguide can be used . this way , the field distribution changes gradually from the slot - line field distribution to the tem mode supported by the two - wire waveguide . the disclosed tgad modules based on waveguides and tapers offer advantages in mechanical design , performance , and cost . mechanically , the device is compact , requires no alignment , and is easier to isolate from environmental variability , including humidity and vibration . advantages in performance include reduced thz loss , which translates to higher dynamic range , and potentially very long interaction lengths , an important factor for trace gas - phase measurements . a major advantage is that the frequency response is no longer constrained by the resonant response of the antennas . finally , since the thz signal is coupled from the edge of the pm chip , the receiver is more easily isolated from laser - induced thermal interference from the transmitter . as for cost , the parts for the thz waveguide assembly cost a small fraction of the cost of typical bulk optic components . optical pump coupling is normal to the chip surface , facilitating easier attachment of optical fibers . operational cost is reduced as alignment is not needed and need for mechanical isolation is reduced . in view of the many possible embodiments to which the disclosed principles of invention may be applied , it should be recognized that the illustrated embodiments are only preferred examples and should not be taken as limiting in scope . we claim all that is encompassed by the pending claims . the disclosed technology is described above with reference to the following documents , all of which are incorporated herein by reference . 1 . h . h . mantsch , d . naumann , “ terahertz spectroscopy : the renaissance of far infrared spectroscopy ”, journal of molecular structure 964 , 1 - 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