Patent Description:
The invention lends itself to several applications such as medical and security imaging, submillimeter-astronomy, the detection of gases and - more particularly - non-destructive material analysis.

The expression "terahertz radiation" (THz) designates electromagnetic waves having frequencies in an intermediate range between infrared and microwave. More precisely, in the following, "terahertz radiation" will designate radiation having a (central) frequency comprised between <NUM> and <NUM> THz (<NUM> THz=<NUM><NUM> Hz), corresponding to wavelengths approximately comprised between <NUM> and <NUM>.

Over the last decade, technological solutions have emerged that are extremely promising to take advantage of this part of the electromagnetic spectrum. However, polarimetric measurements in THz range, where the polarization of the THz light is controlled, need to be developed further to provide interesting information in the context of material science. For instance, photoelastometry may provide information on mechanical constraints in opaque materials. In the case of one of the most widespread THz techniques, THz time domain spectroscopy (TDS), used for probing transitory or non-equilibrium regimes with nanosecond or picoseconds temporal resolution, the generation of THz pulses is typically performed through ultrafast optical excitation of photoconductive generators (or "switches") comprising at least two non-contacting electrodes on a photoconductive surface.

Here, the emission consists of one given polarization, fixed by the direction of the electrodes geometry itself. Therefore, most polarization measurements are performed through the use of mechanically controlled elements, such as rotational mounts for the switches or wired grid polarizers.

This inherently limits the rapidity and the precision of the measurement.

The paper by <NPL> describes a photoconductive terahertz generator comprising <NUM> triangular electrodes on a surface of a photoconductive LT-GaAs substrate. This generator allows rotating the polarization direction of the emitted THz radiation, but only in a stepwise manner, with <NUM>° steps. Such a rough polarization control is insufficient for many applications.

Photoconductive switches may also be used for detecting terahertz radiation. In this case they typically show a similar limitation, namely they are only sensitive to a single polarization component. Therefore at least two separate measurements using a rotatable switch are required to characterize an arbitrary polarization direction.

The paper by <NPL>) describes a three-electrode photoconductive switch allowing performing polarization-sensitive detection of terahertz radiation. This switch has a small sensitive area and is not scalable.

<CIT> discloses a photoconductive switch comprising interdigitated electrodes extending along two perpendicular directions, suitbable to generate terahertz radiation with radial, azimuthal or quadirpolar polarization. This device, however, is not suitable for generating or detecting linearly-polarized terahertz radiation with an arbitrary and electrically-controlled polarization direction.

The paper by <NPL> describes a four-contact photoconductive antenna and a total-reflection Si prism to generate a circularly polarized terahertz electromagnetic radiation. The four-contact photoconductive antenna comprises a four metal contacts on a low-temperature-grown GaAs (LT-GaAs) substrate by a standard photolithographic and chemical etching method. By applying a bias voltage to two adjacent contacts with the other two are grounded, the bias electric field in the photoconductive gap is directed to +<NUM>° or -<NUM>° from the horizontal axis. When a short pulse light (pump beam) is irradiated to the biased photoconductive gap, the transient photocurrent generates THz radiation which is linearly polarized in the direction of the bias field.

The invention aims at overcoming the drawbacks of the prior art. More precisely it aims at providing full and continuous (or, at least, fine-grained) control of the polarization direction of the emitted THz radiation by purely electrical means, and/or at allowing the determination of the polarization direction of a received THz radiation in a single measurement.

The invention achieves these aims by the use of two intermixed orthogonal - or, more generally, nonparallel - photoconductive switches on a same substrate, with independent bias control (for generation) or current measurement (for detection). In emission mode, by adjusting relative field amplitude between the two intermixed switches the direction of the polarization can be adjusted with a high degree of precision. In detection mode, the ratio of the current signals issued from the two intermixed switches is indicative of the polarization direction of the impinging THz radiation.

An object of the present invention is then a photoconductive switch for generating or detecting linearly-polarized terahertz radiation comprising: a photoconductive substrate; and a plurality of electrodes on a surface of the photoconductive substrate; characterized in that said plurality of electrodes comprises: a first pair of structured electrodes separated by a first gap comprising at least a plurality of first rectilinear sections extending along a first direction ; and a second pair of structured electrodes separated by a second gap comprising at least a plurality of second rectilinear sections extending along a second direction, different from the first direction; and in that it further comprises a patterned opaque layer, opaque to at least one of terahertz radiation and visible or infrared radiation suitable to induce an increase of the substrate conductivity, selectively masking portions of the first and second gaps between the electrodes, in such a way that only remain unmasked: first rectilinear sections of the first gap such that, upon application of a first voltage between the electrodes of the first pair and illumination by said visible or infrared radiation, a first electric current flows with a same direction and orientation across all said first rectilinear sections; and second rectilinear sections of the second gap such that, upon application of a second voltage between the electrodes of the second pair and illumination by said visible or infrared radiation, a second electric current flows with a same direction and orientation across all said second rectilinear sections (two opposite orientations being associated to a direction).

Another object of the invention is a device for generating terahertz radiation with a controlled polarization direction comprising such a photoconductive switch; a first controllable voltage generator connected to the electrodes of the first pair, for imposing a first voltage across the first gap; and a second, independently controllable, voltage generator connected to the electrodes of the second pair, for imposing a second voltage across the second gap.

Another object of the invention is a method for generating terahertz radiation with a controlled polarization direction using such a device, comprising the step of: using the first controllable voltage generator for imposing the first voltage across the first gap, and the second controllable voltage generator for imposing the second voltage across the second gap, a ratio of the first and the second voltage being determined as a function of a target polarization direction of the terahertz radiation to be generated; and directing pulsed light towards said region of the surface of the photoconductive substrate.

Another object of the invention is a device for detecting terahertz radiation comprising: a photoconductive switch as above; a first readout circuit connected to the electrodes of the first pair, for detecting a first current flowing through said electrodes; and a second readout circuit connected to the electrodes of the second pair, for detecting a second current flowing through said electrodes.

A further object of the invention is a method for detecting terahertz radiation using such a device, comprising the step of: directing pulsed light towards said region of the surface of the photoconductive substrate; using the first readout circuit for detecting the first current, and the second readout circuit for detecting the second current; and determining a polarization direction of an impinging terahertz radiation from a ratio of the first and the second current.

Additional features and advantages of the present invention will become apparent from the subsequent description, taken in conjunction with the accompanying drawings, which show:.

As shown on <FIG>, a conventional photoconductive switch (or "photoconductive antenna") TPS comprises a photoconductive substrate SUB made of a semiconductor material such as GaAs, having a surface SS carrying two metal electrodes E<NUM> and E<NUM> facing each other and separated by a gap G. A voltage V is applied through the gap by connecting electrode E<NUM> to a voltage generator VG and electrode E<NUM> to the ground. The electric resistance of the semiconductor material being quite large (greater than <NUM><NUM> Ω·cm for GaAs), the current density flowing through the gap is small.

In order to generate THz radiation, an ultrashort (i.e. picosecond or femtosecond) laser pulse LP, having a photon energy larger than the bandgap of the semiconductor material of the substrate SUB, is directed towards the surface SS, and more precisely toward the gap G. The light absorption from the substrate generates pairs of electrons and holes, which migrate towards respective electrodes (electrons towards E<NUM> and holes towards E<NUM>, assuming that E<NUM> is kept at a higher potential than E<NUM>) resulting in a sudden current surge. The current density then decreases at a rate depending of the pair recombination time, or carrier lifetime, of the semiconductor material, typically in a time of a few picoseconds. According to the laws of electrodynamics, the current surge and decrease generate an electromagnetic radiation pulse TR, whose main spectral components are in the THz range. Pulse TR is linearly polarized along the direction of a line connecting electrodes E<NUM> and E<NUM>, i.e. the direction along which the gap G extends (direction y on the figure, z being the propagation direction of both the light pulse LP and the THz pulse TR).

The device of <FIG>, <FIG> differs from that of <FIG> in that it comprises two pairs of facing electrodes: EV1, EV2, separated by a first gap Gv extending in the y direction, and EH1, EH2, separated by a first gap Gv extending in the x direction. A first controllable voltage generator VVG applies a first voltage VV between electrodes EV1 and EV2, i.e. across the first gap, and a second controllable voltage generator HVG applies a second voltage VH between electrodes EH1 and EH2, i.e. across the first gap. The voltage generators are driven by a controller CTR (e.g. a computer or a microprocessor system), which determines the values taken by VV and VH. <FIG> represents a situation wherein VH=<NUM> while VV≠<NUM>. In this case, the photogenerated current flows in the y direction, and the THz pulse TR is also polarized along this direction. <FIG> represents a situation wherein VH≠<NUM> while VV=<NUM>. In this case, the photogenerated current flows in the x direction, and the THz pulse TR is also polarized along this direction. In the case of <FIG>, both VV and VH are different from zero, and the polarization direction of the THz pulse TR forms and angle θ=tan-<NUM>(VH/VV) with the y-axis. It can then be seen that by varying the ratio between the two voltage values VV and VH it is possible to finely control the polarization direction of the THz radiation.

As illustrated on <FIG>, a similar approach allows measuring the polarization direction of an incoming THz pulse. The device of <FIG> is similar to that of <FIG>, <FIG> except in that the voltage generators VVG and HVG and the controller CTR are replaced by two readout circuits RCV, RCH measuring the current flowing through respective electrode pairs and a processor PR. Moreover, as it is well known in the art, the photoconductive materials used for THz detection preferably exhibit shorter carrier lifetimes than those used for THz generation. A suitable choice for the substrate of the switch of <FIG> is LT-GaAs (i.e. GaAs grown at low temperature).

Processor PR receives the measured current values and outputs an indication of the polarization direction. The THz pulse TR whose polarization is to be measured impinges onto the surface of the photoconductive switch TPS, temporally and spatially overlapping with a light pulse LP. The light pulses photogenerate charge carriers, which are accelerated by the electric field of the THz pulse, resulting in an electric current flowing along the polarization direction of the latter (assumed to form an angle θ with the y-axis). The two readout circuits measure the x- and y-components of the electric current density, from which the value of θ can be deduced.

The photoconductive switch of <FIG> and <FIG> has a very small photosensitive area, which severely limits the power of the THz pulses it can generate, as well as its sensitivity when it is used as a detector. Moreover, it is not easily scalable.

The present invention allows overcoming these drawbacks by replacing the simple linear electrodes of <FIG>, <FIG> and <FIG> by at least two pairs of structured electrodes. The electrodes of the first pair face each other defining a first gap having a complex shape, comprising a plurality of rectilinear section extending along a first direction (say, the x direction). Similarly, the electrodes of the second pair face each other defining a second gap also having a complex shape, comprising a plurality of rectilinear section extending along a second direction different from - and preferably perpendicular to - the first one (say, the y direction). The complex geometry of the electrodes allows obtaining a comparatively large emitting area, and therefore to increase the power of the generated THz radiation with respect to the previously-considered cases. As in the case of <FIG>, polarization control of the generated THz radiation is obtained by applying controlled voltages between the electrodes of the two pairs. The photoconductive switch may also be used for detecting THz radiation, as discussed above with reference to <FIG>.

For the generated THz radiation to exhibit a spatially uniform polarization state, the two pairs of electrodes should be intermixed, forming a substantially homogeneous pattern at the scale of the wavelength of the THz radiation. More precisely, the electrode patterns should be homogeneous at a scale L satisfying: <MAT> where λTHz_min is the shortest wavelength of the THz band of interest and N - typically of the order magnitude of, but smaller than one - is the numerical aperture of the THz radiation collecting optics.

For instance, the rectilinear sections of the first and second electrode pairs should occupy surfaces of a same order of magnitude over a region of the substrate having a radius of at least <NUM>, and preferably more. Ideally, the surface occupied by the rectilinear sections of the first and second electrode pairs should be identical, but differences of up to <NUM>% or <NUM>% are acceptable and can be compensated by suitably modifying the voltages applied to the electrodes. This condition should also be fulfilled when the photoconductive switch is used in reception in order to obtain uniform sensibility to the polarization direction.

Moreover, as it will be explained below, a patterned opaque layer must be provided to mask some parts of the gaps, to avoid destructive interference between their contributions to the radiated THz field. The same is true when the photoconductive switch is used in reception.

<FIG> shows the electrode patterns of a photoconductive switch according to a first embodiment of the inventions. The switch comprises two pairs of interdigitated electrodes EV1, EV2 and EH1, EH2 forming a square region R divided into four quadrants, each having a side of <NUM> length. Electrodes EV1, EV2 are separated by a first gap GV and electrodes EH1, EH2 are separated by a second gap GH. Both gaps have a complex shape, similar to that of the electrodes.

Electrodes EV1 and EV2 occupy the first and third quadrant; they both comprise fingers which extend in the x-direction from a "stem" oriented along the y-direction. The stems are disposed at opposite ends of each quadrant, and the fingers of an electrode protrude towards the stem of the other electrode of the pair. Each finger of an electrode (except for those at the border of the pattern) is surrounded by two fingers of the other electrode of the pair, separated by them by a rectilinear section of gap GV extending in the "vertical" y-direction. Similarly, electrodes EH1 and EH2 occupy the second and fourth quadrants; they both comprise fingers which extend in the y-direction from a stem oriented along the x-direction. The stems are disposed at opposite ends of each quadrant, and the fingers of an electrode protrude towards the stem of the other electrode of the pair. Each finger of an electrode (except for those at the border of the pattern) is surrounded by two fingers of the other electrode of the pair, separated by them by a rectilinear section of gap GH extending in the "horizontal" x-direction.

For each point of a "vertical" section of the first gap GV, suitable to emit y-polarized THz radiation, there is a corresponding point of a "horizontal" section of the second gap GH, suitable to emit x-polarized THz radiation, which is at most <NUM> away. These two points can only be resolved by radiation having a wavelength shorter than λTHzmin = <NUM>D·N e.g. <NUM>, corresponding to a frequency of <NUM> THz, for a numerical aperture of <NUM>. For longer-wavelength (i.e. lower frequency) radiation, they can be considered as a single point-like source. Therefore it can be expected that the photoconductive switch of <FIG> can be used to generate electromagnetic radiation with a frequency up to <NUM> THz. Numerical simulations show that a very well defined, spatially uniform linear polarization state is obtained for frequencies up to <NUM> - <NUM> THz. Higher frequency radiation can also be generated, but with a less uniform polarization state, which can be acceptable or not depending on the specific application considered.

However, by itself, the electrode pattern of <FIG> does not allow generating any THz radiation, or a vanishingly small amount of it, because of destructive interferences between different points of the emitting gaps. Let us assume that electrode EV1 is kept at a higher voltage than EV2; a finger of electrode EV1 is surrounded by two fingers of electrode EV2, one situated "above" it (i.e. at a position corresponding to a larger value of y) and the other one "below" it (i.e. at a position corresponding to a smaller value of y). Electric field lines going from the EV1 finger to the first E2V neighboring finger are directed along the positive direction of the y-axis, and the electric field lines going from the EV1 finger to the second EV2 neighboring finger are directed along the negative direction of the y-axis. It will then be easily understood that, upon illumination of the surface of the photoconductive substrate, two opposite current densities will flow from the EV1 finger towards the two neighboring EV2 fingers, whose contribution to the generation of THz radiation will cancel each other. To avoid this destructive interference, it is necessary to mask one of every two linear sections of the gaps in such a way that the electric current flows with a same first direction (either positive-y or negative-y) across all the unmasked gap sections of the first and third quadrants, and with a same second direction (either positive-x or negative-x) across all the unmasked gap sections of the second and fourth quadrants.

<FIG> shows a sectional view of a portion of the first or third quadrant of the photoconductive switch of <FIG>. Reference Δ designates the width of the gap GV between neighboring fingers of electrodes EV1 and EV2. A transparent, electrically insulating layer TL, e.g. made of SiO<NUM> covers the surface SS of the substrate SUB. A patterned opaque layer PML, e.g. made of metal, is deposited over the transparent layer to mask one gap GV out of two, as explained above. It can be easily seen that, due to the presence of the patterned opaque layer, only current density flowing to the right of the figure is generated.

It is important to note that layer TL must be transparent to both the light used for photogenerating carriers in the substrate and to THz radiation, while it is sufficient that the PML layer is opaque to either light or THz radiation (in the latter case, interfering radiation is generated, but cannot propagate away from the surface of the photoconductive switch).

<FIG> illustrates the structure of a photoconductive switch according to a second exemplary embodiment of the invention, which can be considered an improvement of the first embodiment. This switch also comprises two pairs of interdigitated electrodes; however, the "ground" electrodes of each pair are connected together, effectively constituting a single electrode EG (the two other electrodes - intended to be connected to voltage generators or readout circuits - are labeled EV and EH). These electrodes also form a square pattern, which is larger than that of <FIG>, having a side of <NUM>. This square is subdivided into four "quadrants" which are shaped like pieces of a jigsaw puzzle, in such a way that the characteristic scale of the pattern is <NUM> - and not <NUM> as it would have been the case if the pattern of <FIG> had simply been magnified by a factor of two. Numerical simulations show that this pattern, too, allows generating electromagnetic radiation with a frequency up to <NUM> THz and showing a well defined, spatially uniform linear polarization state. Satisfactory operation at higher frequency (up to approximately <NUM> - <NUM> THz, the precise value depending, of course, on the acceptability criterion) may be achieved by only illuminating the central part of the pattern but, of course, this reduces the power level of the generated THz radiation.

An opaque masking pattern must also be provided to suppress destructive interference.

A third embodiment of a photoconductive switch according to the invention is illustrated on <FIG>. This switch comprises three electrodes - a ground electrode EGR and two other electrodes intended to be connected to voltage generators or readout circuits, E<NUM> and E<NUM> - having complex shapes. More precisely, each of these electrodes comprises a plurality of stair-shaped or "zigzag" electrodes including alternating rectilinear segments extending in the x and y-direction. Three appendages, one for each electrode, form a band, an appendage of the ground electrode EG being disposed between an appendage of E<NUM> and an appendage of E<NUM>. Within each band, the appendage of E<NUM> is separated from that of EG by a first stair-shaped gap, and the appendage of E<NUM> is separated from that of EG by a second stair-shaped gap. This electrode pattern, illustrated on <FIG>, is partially masked by a patterned absorbing layer, shown on <FIG>. It can be seen that only:.

This embodiment allows achieving a more uniform polarization state of the THz radiation that the photoconductive switches of <FIG> and <FIG>, and is easily scalable to larger surfaces. However, given that most of the photoconductive surface is masked, the THz power generated per unit surface is lower.

The electrode patterns of <FIG>, <FIG> and <FIG> are by no mean limitative of the scope of the invention. For instance, it is not strictly necessary that the "active" linear sections of the gaps extend in mutually perpendicular direction, even if this is a preferred feature: it is only required that they are not parallel to each other.

<FIG> illustrates a TDS setup used to test the operation of a prototype photoconductive switch according to the invention. The prototype used the electrode pattern of <FIG> with a <NUM> x <NUM> area. The photoconductive substrate was a <NUM>-thick semi-insulating wafer of GaAs. Electrodes were fabricated by lithography, and were made of a <NUM> gold layer deposited on a <NUM> chrome layer. Electrode separation was <NUM>. The transparent layer TL was <NUM> thick and made of SiO<NUM> deposited by ionic sputtering. The patterned opaque layer had the same structure and composition as the electrodes.

A Ti:Sa laser source LAS generates <NUM>-fs laser pulses LP at a wavelength of <NUM>. A beam splitter BS separates each pulse LP into two pulses LP1, LP2 propagating along a first and a second path, respectively. The second path, along which LP1 propagates, comprises a variable delay line DL. The second pulse LP2 propagates through a focusing mirror FM1 which is highly reflective in the terahertz region of the spectrum but transparent at <NUM> - or which is traversed by a hole through which the laser pulse can pass - and impinges onto a photoconductive switch TPS according to the invention. The THz pulse TR generated by the photoconductive switch propagates towards mirror FM1 which collimates it; then it is focused by a second mirror FM2, collimated again by a third mirror FM3 and focused, by a fourth mirror FM4, on a <NUM>-thick ZnTe crystal EOS. The second laser pulse LP2 also impinges on crystal EOS through mirror FM4. The laser pulse LP2 and the THz pulse TR spatially overlap; delay line DL can be adjusted to make them temporally overlap, too.

Both the laser pulse LP2 and the THz pulse TR have a linear polarization forming an angle of <NUM>° with the ordinary and extraordinary axis of the ZnTe crystal EOS and of a quarter-wave plate QWP following it. In the absence of THz radiation, the quarter-wave plate converts the polarization of the LP2 pulses from linear to circular. A Wollastone prism WP decomposes this circular polarization into two spatially separated linear components, which impinges onto respective photodiodes of a balanced photodetector BPD. The two components having a same intensity, the output signal of the balanced photodetector is zero. Due to electro-optic effects in the EOS crystal, the electric field of the THz pulse TR induces a rotation of the polarization plane of the laser pulses, proportional to its amplitude. Due to this rotation, the laser polarization state downstream the quarter-wave plate is no longer circular, but elliptical. This induces an imbalance between the two components separated by the Wollastone prism, and therefore a non-zero output signal of the balanced photodetector. By varying the delay between the laser pulse LP2 and the THz pulse (which can be done by using the variable delay line DL), one obtains a signal representative of the THz electric field in the time domain. This is illustrated on <FIG> showing the corresponding spectrum. These results are typical of a conventional photoconductive THz generator.

In order to test the polarization control properties of the inventive device, the generated THz power was measured using a conventional pyroelectric detector associated to a mechanical THz polarizer.

Claim 1:
A photoconductive switch for generating or detecting linearly-polarized terahertz radiation (TR) comprising:
- a photoconductive substrate (SUB); and
- a plurality of electrodes on a surface (SS) of the photoconductive substrate;
characterized in that said plurality of electrodes comprises
- a first pair of structured electrodes (E<NUM>, EGR) separated by a first gap comprising at least a plurality of first rectilinear sections (GV) extending along a first direction (x); and
- a second pair of structured electrodes (E<NUM>, EGR) separated by a second gap comprising at least a plurality of second rectilinear sections (GH) extending along a second direction (y), different from the first direction; and
in that it further comprises a patterned opaque layer (PML), opaque to at least one of terahertz radiation and visible or infrared radiation suitable to induce an increase of the substrate conductivity, selectively masking portions of the first and second gaps between the electrodes, in such a way that only remain unmasked:
- first rectilinear sections of the first gap such that, upon application of a first voltage between the electrodes of the first pair and illumination by said visible or infrared radiation, a first electric current flows with a same direction and orientation across all said first rectilinear sections; and
- second rectilinear sections of the second gap such that, upon application of a second voltage between the electrodes of the second pair and illumination by said visible or infrared radiation, a second electric current flows with a same direction and orientation across all said second rectilinear sections.