CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Patent application Serial No. 61/592,295 entitled "THz PHOTOMIXING PROBE FOR CONTACT-FREE

DEVICE AND INTEGRATED CIRCUIT MEASUREMENT OR CHARACTERIZATION" and filed January 30, 2012. The entirety of the above-noted application is incorporated by reference herein.

NOTICE ON GOVERNMENT FUNDING

[0002] This innovation was made with government support under a grant awarded by the Office of Naval Research (ONR) Award N00014-11-1-0721. The government has certain rights in the innovation.

ORIGIN

[0003] The innovation disclosed herein relates to contact-free measurement or characterization of solid-state devices or integrated circuits at RF and THz frequencies, and more specifically, to a contact- free, ac-coupled photomixing probe for use in such measurement or characterization.

BACKGROUND

[0004] One challenge to the development of THz semiconductor devices, particularly transistors, is THz measurement and characterization. As it stands today, vector network analysis (VNA) with standard (metal-to -metal) dc-coupled contact probing is

commercially available up to 500 GHz (Oleson Microwave,

http://www.omlinc.com/products/vna-extension-modules/wr-0 22-325-500-ghz.html ) using coaxial probes (GGB Industries, Inc.; PicoProbe Model 325;

http://www.ggb.com/325.html), with promise of extending to 750 GHz using silicon- micromachined probes (Dominion MicroProbes, Inc; Model DMPI 1.5V01MPR;

http://dmprobes.com/Products%20DMPI.html). However, this technology is very expensive and fragile, with little hope of working beyond 1.0 THz in the foreseeable future, in large part because it requires application of vector network analyzers and custom front-end frequency-extension modules and down-conversion units. The fragility stems from the small size of the ground-signal-ground (GSG) probes and their need to make intimate metal contact. In technical terms, the existing technology couples the conduction current term, J, in Maxwell's equations (specifically, Maxwell' s

generalization of Amperes Law (VxH = J + dO/dt)). In circuit terminology, this is "dc coupling."

[0005] Photomixing entails the use of two fiber-coupled, frequency-offset diode lasers (usually distributed feedback lasers) to generate or receive a single difference-frequency tone in a photomixer - an ultrafast photoconductive gap easily embedded in a terahertz (THz) antenna or planar transmission line. The tone is "pure" in the sense that there are no harmonics and no intermodulation products of any sort. An important development over the past 10 years is fully-coherent photomixing. There is a transmit (Tx) photomixer and a receive (Rx) photomixer, both driven by the same pair of diode lasers so that the difference frequency tone at each photomixer is mutually coherent. Difference-frequency sweeping occurs by temperature tuning of one or both lasers. Mixing of the Tx -radiated tone with the Rx-generated tone produces a dc component that is easily read out to a transimpedance amplifier. This is the same "homodyne" conversion common in RF and photonic transceivers. To avoid dc-drift and 1/f-noise issues, the Tx photomixer is easily amplitude modulation (AM) or frequency modulation (FM) modulated, and the phase- preserving Rx baseband is raised to the modulation frequency (quasi-homodyne).

SUMMARY

[0006] The following is a simplified summary to provide a basic understanding of some aspects of the innovation. This summary is not an extensive overview of the innovation. It is not intended to identify key/critical elements or to delineate the scope of the innovation. Its sole purpose is to present some concepts of the innovation in a simplified form as a prelude to the more detailed description that is presented later.

[0007] In accordance with aspects of the innovation, a method for measuring solid- state devices at frequencies up to 1.0 THz and beyond is provided that includes a transmitting photomixing probe structure and a receiving photomixing probe structure. The transmitting photomixing probe structure and the receiving photomixing probe structure are ac-coupled to the solid-state-device circuit in a contact-free manner.

[0008] In accordance with another aspect of the innovation, the method utilizes contact-free, ac-coupled probes that are fabricated from the same high-resistivity (e.g., silicon (Si), gallium arsenide (GaAs), indium phosphide (InP), gallium nitride (GaN) etc.) substrates as the photomixers and have either a fork-like or orthorhombic shape for probing balanced THz transmission lines (e.g., coplanar waveguide (CPW)). The ac coupling occurs through polarization current, which is naturally strong in Si, GaAs, InP, and GaN because of their high dielectric constant (ε > 12) and low THz dielectric loss. And unlike metal-to-metal coupling that becomes increasingly lossy with frequency, dielectric coupling improves approximately linearly with frequency in accordance with Maxwell's equation (Ampere's generalized equation).

[0009] The dielectric-coupling probe concept was inspired in part by "electro-optic sampling" first demonstrated in the 1980s and applied to a variety of high-frequency devices and structures in the 1990s. Electro-optic sampling is a time-domain technique that takes advantage of the short pulses (-100 fs) and very stable repetition frequency (typically 10 to 100 MHz) of modern solid-state mode-locked lasers (e.g., titanium sapphire). The optical pulses create short THz pulses (typically < 1 ps), which then interact with the device or circuit under test, and produce an output that is essentially the impulse response. So like all time-domain techniques, the THz power is spread over 1 THz or more, which degrades the instantaneous signal-to-noise ratio considerably compared to a coherent transceiver. This can be overcome by averaging but then the data acquisition time becomes quite long, on the order of many minutes, which is usually unacceptable for device characterization.

[0010] To create the foregoing and related capabilities, certain illustrative aspects of the innovation are described herein in connection with the following description and the annexed drawings. These aspects are indicative, however, of but a few of the various ways in which the principles of the innovation can be employed and the subject innovation is intended to include all such aspects and their equivalents. Other advantages and novel features of the innovation will become apparent from the following detailed description of the innovation when considered in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1 is a perspective view of THz ac-coupled probe structures coupled to a device in accordance with aspects of the innovation.

[0012] FIG. 2 is an illustrative graph of a typical output signal, noise floor, and signal- to-noise ratio from a coherent photomixing spectrometer.

[0013] FIG. 3 is a plan view of a proposed balanced "microfork" THz probe structure in accordance with aspects of the innovation.

[0014] FIG. 4 is a plan view of an unbalanced "microfork" probe structure.

[0015] FIG. 5 is an illustration of micrograph views of laser micromachining fabricating techniques in accordance with aspects of the innovation.

[0016] FIGS. 6-9 are illustrations of fabrication techniques in accordance with aspects of the innovation.

[0017] FIG. 10 displays the physical methodology behind the proposed technique of measuring or characterizing a solid-state device or integrated circuit with contact-free probes at frequencies up to 1.0 THz and beyond in accordance with aspects of the innovation.

[0018] FIGS. 11A-B show a plan view of a proposed "orthorhombic" THz probe structure in accordance with aspects of the innovation, and designed with coplanar- waveguide coupling for measuring the signals in similar balanced circuits such as that shown in FIG. 11B.

[0019] FIGS. 12A-B show a plan view of a proposed "orthorhombic" THz probe structure in accordance with aspects of the innovation, and designed with slot-line coupling for measuring signals in similar unbalanced circuits such as that shown in FIG. 12B.

DETAILED DESCRIPTION

[0020] The innovation is now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the subject innovation. It may be evident, however, that the innovation can be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate describing the innovation.

[0021] While specific characteristics are described herein (e.g., specific dimensions of example components, etc.), it is to be understood that the features, functions and benefits of the innovation can employ characteristics that vary from those described herein. These alternatives are to be included within the scope of the innovation and claims appended hereto.

[0022] While, for purposes of simplicity of explanation, the one or more

methodologies shown herein, e.g. , in the form of a flow chart, are shown and described as a series of functions, it is to be understood and appreciated that the subject innovation is not limited by the order of functions, as some may, in accordance with the innovation, occur in a different order and/or concurrently with other functions from that shown and described herein. For example, those skilled in the art will understand and appreciate that a methodology could alternatively be represented as a series of interrelated states or events, such as in a state-flow diagram. Moreover, not all illustrated functions may be required to implement a methodology in accordance with the innovation.

[0023] With reference now to the figures, the innovation disclosed herein provides a contact-free system and method of creating an input signal, and probing the output signal of a solid-state device well beyond the frequency limits of existing contact probing. The subject innovation enables operation at higher frequencies and with greater instantaneous bandwidth than currently possible. The approach is to probe the device output by way of the displacement current term (dO/dt), referred to as "ac coupling" in circuit terminology. Although ac coupling is lower in efficiency than dc coupling at microwave frequencies, it becomes increasingly more efficient as the frequency increases. It also improves with the dielectric constant of the coupling material. Accordingly, the innovation disclosed herein can provide a new device measurement and characterization capability at frequencies up to 1.0 THz and beyond the capabilities of existing probing and vector network analysis (VNA) techniques. In addition, the subject innovation allows one to measure and characterize the device without the probes coming into electrical contact with the planar transmission line in which the sample is embedded. As such, the subject innovation avoids the mechanical stress on the probe and scratch-induced damage (of the sample transmission line) that existing contact probe technology entails.

[0024] Referring to FIG. l, illustrated is a perspective view of a solid-state device 102, such as but not limited to a high-speed THz Si-, GaAs-, InP-, or GaN-based

semiconductor device, to be tested while embedded in a planar transmission line 104 with the innovative contact-free, ac-coupled probes. In some examples discussed herein, specific examples of a planar transmission line 104 are provided (e.g., a coplanar waveguide (CPW), etc.), but it is to be appreciated that other coplanar transmission lines may be used, such as slot line, microstrip, and twin line. The ac-coupled probes include a transmitting photomixing probe structure, Tx 106, and a receiving photomixing probe structure, Rx 108. Each of the probes 106 and 108 include a photomixer 106A, 108 A at the photoconductive gap of the probes. Optical fibers 110, 112 couple the photomixers 106A, 108 A, respectively, to two external lasers (not shown). The two lasers need to be mutually coherent, but their frequency difference needs to be tunable. In an aspect of the innovation, both the lasers are single frequency diode lasers. In addition, each of the probes 106 and 108 includes a planar transmission line structure 106C, 108C at the photomixer 106A, 108C end of the probes, where the planar transmission line structure 106C, 108C includes a metalized surface 106E, 108E. Each of the probes 106, 108 is made of a substrate 106D, 108D, and a metalized surface 106E, 108E. In aspects, the substrate 106D, 108D can be made of high resistivity, semiconducting material, including but not limited to, silicon (Si), gallium arsenide (GaAs), indium phosphide (InP), or gallium nitride (GaN). The probes 106 and 108 can have many different designs and some but not all of them will be discussed in connection with FIGS. 3, 4, 11A-B, and 12A-B. In some embodiments of the innovation, such as the examples shown in FIGS. 3 and 4, each of the probes 106 and 108 can also include a dielectric waveguide structure 106B, 108B (without any metalized surfaces) at the circuit end of the probes, where the circuit end of the probes is the end closer to the device or circuit under test. The thickness of the probes 106, 108 can be in the range of 10 to 1000 micron, for example, around 100 micron. The height is in the range of 0.1 to 10 mm, for example, around 1.0 mm. In various aspects, probes 106, 108 can be made using standard semiconductor fabrication and packaging methods, especially if they are made with a silicon substrate.

[0025] Coupling to the solid-state device 102 occurs through the dielectric

transmitting and receiving photomixing probes 106, 108 acting in part like near-field, dielectric-rod or dielectric- waveguide antennas. The output field of the solid-state device 102 can be sampled in the coplanar-waveguide or other planar transmission line 104 in which the solid-state device 102 is embedded. The gap distances Di and D ₂ from the probes 106 and 108 to the planar transmission line 104 is a trade-off between weak coupling (at large D) and strong perturbation of the circuit 102 (at small D). While Di and D ₂ do not have to be the same, they can fall in the range between 1 and 10 micron, for example, around 10 micron. The coupling in this range is "near- field," meaning that the electromagnetic mode excited in the dielectric waveguide structure 106B and 108B of the probes propagates through polarization currents dP/dT = χθΕ/θΤ, where χ is the electric susceptibility given by χ = ε - 1, where ε is the THz dielectric constant. The polarization current is, in turn, transformed to conduction current by a metallic transforming transmission line (106C and 108C) located well above the dielectric prongs. An example transforming structure (106C and 108C) can be a terminated section of coplanar waveguide (CPW) or other planar transmission-line structure. Tapering of the transmission line may be necessary to achieve acceptable efficiency, and may be achieved by a variety of methods including "adiabatic" transformation as is done in the well-known Klopfenstein coupler at microwave frequencies. As such, because Di and D ₂ are not equal to zero, the subject innovation can enable the probes 106, 108 to measure or characterize the device/circuit under test 102 without the probes 106, 108 coming into electrical contact with the planar transmission line 104 in which the device or circuit under test 102 is embedded. This contact- free capability gives the subject innovation the ability to avoid the mechanical stress on the probe 106, 108 and scratch-induced damage (of the sample transmission line 104) that the prior art experiences.

[0026] Once the polarization current is transformed to conduction current, the field strength can be measured by an optical fiber-coupled photomixer 108D monolithically embedded into the CPW or other transmission line 106C, 108C at the top of the receiving photomixing probe 108. THz photomixing is well-suited to this task because of its huge operational bandwidth, its spectral purity, and lack of harmonics, spurs, and other nonlinear effects that occur when scaling the conventional vector-network analyzer into the THz regime. Photomixing is so pure because the THz tone it generates is exactly the beat frequency lv ₁ - v ₂ l between two frequency-offset, single-frequency diode lasers, readily available in either DFB (distributed feedback) or DBR (distributed Bragg reflector) designs. There are no harmonics in this process, nor spurs, because the nonlinearity that the mixing action is based upon is pure quadratic (the fundamental internal photoelectric effect associated with cross-gap absorption in any semiconductor). Assuming it is sinusoidal at frequency f, the incident signal across the transmission line in the receive probe is then down-converted to an intermediate frequency (IF) given by If - Ivi - V ₂ II.

[0027] A notable aspect of the subject innovation is that the THz coupling of the contact-free probe is reciprocal. That is, the coupling to a transmission-line under test 104, and the subsequent transformation to a transmission-line-embedded photomixer, can also operate in reverse. Hence, a THz signal generated by the photomixer 106A will propagate in the probe planar transmission line 106C, transform to a dielectric-waveguide mode in dielectric waveguide structure 106B, and couple to the device or circuit under test 102 through near- field, polarization-current coupling without electrical contact with planar transmission line 104. The signal from the device or circuit under test 102 will couple to the dielectric waveguide structure 108B without electrical contact with planar transmission line 104, transform and propagate to the probe planar transmission line 108C before reaching photomixer 108 A.

[0028] The operation of the transmit and receive photomixers is based on the same laser- generated THz difference-frequency tone. However, the transmit photomixer 106A must generate a THz signal while the receive photomixer 108 A only needs to accept it. This difference can be readily controlled by DC bias. The transmit photomixer 106 A can be DC-biased so that the laser-difference-frequency tone transforms DC bias power to AC THz power through photoconductance modulation. The receive photomixer 108 A need not be DC biased, in which case an incident THz signal will mix with the laser- difference-frequency tone. Lacking any modulation of the transmit laser signals, this is pure homodyne conversion. To address 1/f and related problems, one of the transmit laser powers can be AM modulated to produce an IF well within the bandwidth of lock-in amplifiers and ultra-low-noise transimpedance amplifiers. This "quasi-homodyne" approach carries the fully advantages of traditional homodyne and heterodyne conversion typically used in vector network analyzers and coherent RF transceivers of all sorts. One significant advantage over incoherent detection (which spectrum analyzers usually do) is signal-to-noise ratio. Homodyne and heterodyne transceivers can routinely operate near the theoretical limits dictated by electronic noise. This provides a very high dynamic range, usually in the range of 50-to-100 dB depending on the frequency band and the electronics involved.

[0029] The transmitting photomixer probe 106 and receiving photomixer probe 108 can be driven by the same pair of frequency-offset lasers, so the typical frequency jitter in the diode lasers is cancelled perfectly, meaning that there is an inherent capability to do frequency or phase locking. This assumes, of course, that the jitter is tolerably small compared to the THz frequencies of interest. Modern DBR and DFB lasers both tend to jitter by about 100 MHz or less, or 1 part in 10 ⁴ of a THz tone, and there is no

anticipation of resolution requirements any finer than this in the course of the typical THz device characterization.

[0030] FIG. 2 illustrates the spectral response, quadrature-component amplitude, and dynamic range of a commercial quasi-homodyne photomixing transceiver (model PB7100 manufactured by Emcore, Corp.). The dynamic range is seen to be 80 dB at 100 GHz, 60 dB at 1.1 THz, and still 40 dB at 2.0 THz. Although the THz radiation propagates as quasi-Gaussian beams rather than the evanescent/surface or dielectric- waveguide modes disclosed herein, the operating principle will be the same, and comparable values of dynamic range are expected once the dielectric probe designs are optimized.

[0031] FIG. 3 illustrates a plan view of an example contact-free, ac-coupled probe structure 106 or 108 (hereinafter "probe") in a balanced microfork design configuration that is compatible with a planar transmission line 104 that is a CPW in accordance with aspects of the innovation (FIG. 4 is an illustration of a probe structure 400 in an unbalanced microfork design that is compatible with a planar transmission line 104 that is a microstrip). The probe 300 includes dielectric prongs/dielectric waveguide structure 302, a tapered region/planar transmission line structure 304 and an interdigital electrode region 306. The probe can be made from a high-resistivity Si, GaAs, InP, or GaN substrate material between, 10 and 1000 microns thick, for example, around 100 microns thick. The overall height of the probe 300 can be between 0.1 and 10 mm, for example, around 1.0 mm high. The dielectric prongs 302 can be between 0.05 and 5 mm high. Further, the bottom portion of each dielectric prong 302 can be between 10 and 100 microns wide.

[0032] The approximate dimensions illustrated in FIGS. 3 and 4 are based on preliminary design considerations, including the width of the CPW-transmission-line 104 circuit in which the device-under-test 102 is likely to be embedded, the necessary offset of the dielectric- wave guide portion of the probe to the transmission-line circuit, and the area needed for fiber-optic coupling to the photomixer element. It is to be understood, of course, that the specific dimensions provided in connection with example probe 300 are meant to illustrate one possible embodiment, and greater or lesser values can be used in other embodiments, such as where some of these design considerations vary. The non- rectilinear geometry of the "microfork" does not comply with standard dicing or scribe- and-break tools. And because the thickness wants to be approximately 100 micron or more for mechanical strength, wet- or reactive-ion etching of the substrate, be it Si, GaAs, InP, or GaN, would be cumbersome and expensive. Thus, an alternative approach to dicing or scribing of the chips is laser micromachining - a technique that works very well on all common semiconductors and produces small kerf (10s of microns) and nearly vertical smooth sidewalls, see FIG. 5. This can be accomplished via high peak-power Q- switched solid-state (e.g., Nd:YAG) lasers that have become very popular in

microelectronic fabrication the past decade. The dicing action occurs by laser ablation, which can penetrate up to several hundred microns over a short period of time (minutes). And complex chip shapes and sizes can be obtained by routing the laser beam with mirrors mounted on precision translation stages.

[0033] The Si-, GaAs-, InP-, or GaN-based photomixers and associated circuitry can be fabricated by standard methods developed over the past decade. This includes interdigital electrodes defined in the photoconductive gap of the probe transmission line. The ultrafast Si-, GaAs-, InP-, or GaN-based active layers will either be implanted Si, epitaxial low-temperature- grown (LTG) GaAs, epitaxial ErAs:GaAs, or some form of InGaAs epitaxial film (lattice-matched to InP) developed by materials scientists over the past few decades.

[0034] A second fabrication challenge exists in fiber-coupling of the diode lasers to the Tx and Rx photomixing probes 106, 108. The GaAs-based photomixers work best with 780-nm laser diodes, which are available as commercial off-the-shelf (COTS) components. But 780-nm fiber is not as easy to couple into and out-of single-mode fiber as the standard 9-micron-core fiber so common in 1550-nm optical telecommunications. The ~6 micron core of 780-nm fiber is much more lossy and fragile than 1550-nm fiber, so must be butt-coupled with special machines common to the fiber-optic industry.

[0035] Referring to FIGS. 6-9, other manufacturers have been identified as having a vast experience in fiber-coupling at both 1550 and 780 nm; techniques of these manufacturers can be employed in fabricating probes of the subject innovation. Their bonding technique involves precision fiber alignment using laser-radiation-induced photocurrent as the alignment metric. After the fiber is in place, a photo-active epoxy is applied and then cured with UV light. They routinely perform fiber packaging for a variety of fiber-coupled instruments that necessarily have to be field-deployable, so can survive manipulation, vibration, elevated temperature, and moisture.

[0036] One challenge created by the THz probe technology is the electromagnetics. Dielectric rod antennas have never been scaled to be used at THz frequencies. And although they are fairly common at microwave frequencies, it is not clear whether the design rules that have been developed apply to THz frequencies because these antennas are generally used for standard far-field transmit and receive functions, not the near-field coupling as required by our probe technology. Hence, research on the coupling included full- wave electromagnetic simulation. Electromagnetic simulation at THz frequencies using High Frequency Structure Simulator (HFSS) is still probably the best option in the numerical simulation industry. Research conducted in connection with the subject innovation used HFSS- 12 to model and design the photomixer probes over a huge range of frequencies from roughly 100 GHz to 2.0 THz, and as expected, the coupling to the probe increases with frequency.

[0037] Once an optimal design is obtained in simulation, there is the required step of verifying the performance experimentally. This is especially important at THz frequencies where various practical effects, such as surface roughness, can introduce extraneous losses that are nearly impossible to simulate realistically. A related challenge of any high-frequency probe technology, dc- or ac-coupled, is calibration. What nearly every engineer desires from high-frequency probing of any device are the associated scattering (S) parameters. For two-port devices these can be obtained accurately only through a careful calibration procedure, the standard one utilizing "SOTL" standards (short, open, through, load). And such calibration is critical to the success of standard vector network analysis at all frequencies. Unfortunately, such SOTL standards do not exist for the subject innovation, and it is not even clear how to fabricate them. Therefore, an important aspect of the innovation is the ability to carry out cross-calibration. The photomixer probe technology can easily work down to 100 GHz or lower, thereby overlapping the frequency range of existing VNAs, which operate up to -500 GHz. Thus, the subject innovation can utilize a common, shared test structure compatible with both conventional techniques and the innovative probing techniques of the subject innovation. This can allow the use of standard- VNA S parameters to quantify the photomixer probe performance.

[0038] Given the anticipated levels of signal-to-noise ratio of the photomixer

"microfork" probes (at least 50 dB) and comparable levels of dynamic range, another possibility is characterization of prospective THz devices by measurement of the noise spectrum. It is often true that the noise spectrum from solid-state devices is governed by the same device physics and forward transfer function as the signal spectrum. In the case of transistors, the output noise will be present even if the forward gain is very low, as required by thermodynamics. And at the high bias and current density levels that THz solid-state devices tend to require, the shot-noise and thermal-noise effects are bound to be very strong. Of course, this may require long integration times and tedious data acquisition, but the noise spectrum could provide important diagnostic information about devices in which amplification is not even possible.

[0039] FIG. 10 is an illustration of the physical methodology of measuring or characterizing a solid-state device or circuit under test having a frequency up to 1.0 THz and beyond in accordance with aspects of the innovation. At 1002, a Tx photomixing probe structure 106 and a Rx photomixing probe structure 108 can be provided. At 1004, the Tx photomixing probe structure 106 creates an electromagnetic signal. At 1006, the Tx photomixing probe structure 106 and the Rx photomixing probe structure 108 can be ac-coupled to the solid-state device or circuit under test 102 via an electromagnetic field, i.e. without electrical contact between the probes 106, 108 and the planar transmission line 104. At 1008, the ac-coupling can occur by an electromagnetic mode excited in the Tx photomixing probe structure and the Rx photomixing probe structure, and can propagate through polarization currents. In each probe 1010, the polarization current can be transformed to conduction current, or vice versa. At 1012, once the polarization current is transformed to conduction current, the electromagnetic field strength created by the Tx photomixing probe structure 106 can be measured by the Rx photomixing probe structure 108.

[0040] FIG. 11 A illustrates a plan view of an alternative contact-free, ac-coupled probe structure 106, 108 in a balanced "orthorhombic" design configuration compatible with the CPW planar transmission line 104 in accordance with aspects of the innovation (FIG. 12A is an illustration of a corresponding probe structure 1200 that is an unbalanced orthorhombic design configuration, compatible with microstrip, etc). The "orthorhombic" probes 1100 and 1200 differ from the "microfork" structure of FIGS. 3 and 4 in that dielectric-waveguide structure of the microfork is eliminated. Instead, the tapered transmission-line section common to all probes of the subject innovation can start at the bottom of the probe - the end closest to the transmission-line 104 under test. In other words, the "orthorhombic" design includes the tapered, coplanar-waveguide region 1104 and an interdigital-electrode photomixer 1106, but does not have the prongs of the "microfork" design shown in FIGS. 3 or 4. The probe can be made from a high-resistivity Si, GaAs, InP, or GaN substrate material between 50 and 500 microns thick. The overall width of the probe 1100 can be between 10 and 100 micron, consistent with the size of planar transmission lines used in integrated circuits at frequencies up to 1 THz and beyond.

[0041] The approximate dimensions illustrated in FIGS. 1 IB and 12B are based on preliminary design considerations, including the width of the balanced or unbalanced transmission-line circuit in which the device-under-test is likely to be embedded and the area needed for fiber-optic coupling to the photomixer element. The size of the gap distance shown in FIGS 11B and 12B is again a tradeoff between strong coupling to the transmission line under test and strong perturbation of the transmission line signal, both of which depend inversely on gap size. The rectilinear geometry of the "orthorhombic" probe complies with standard dicing and scribing tools, so it does not necessarily require the intricate laser micromachining of the "microfork" described earlier. However, such laser micromachining may be necessary for accuracy and surface-morphology (i.e., smoothness) reasons.

[0042] What has been described above includes examples of the innovation. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the subject innovation, but one of ordinary skill in the art may recognize that many further combinations and permutations of the innovation are possible. Accordingly, the innovation is intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the appended claims. Furthermore, to the extent that the term "includes" is used in either the detailed description or the claims, such term is intended to be inclusive in a manner similar to the term "comprising" as "comprising" is interpreted when employed as a transitional word in a claim. In addition, while the subject innovation uses examples with THz signals, it is applicable to signals with other frequencies, especially within the range of 0.5THz to 1.6THz.