FR2950700A1 - TERAHERTZ OPTOELECTRONIC DEVICE AND METHOD FOR GENERATING OR DETECTING TERAHERTZ ELECTROMAGNETIC WAVES - Google Patents

Abstract

The invention relates to a terahertz optoelectronic device (1) adapted to cooperate with an electronic device (2) for generating or detecting terahertz electromagnetic waves (3), and a method for generating or detecting terahertz electromagnetic waves. The electronic apparatus (2) comprises at least two distant metal conductors (4a, 4b), and means (6, 6 ') for generating an electric field or measuring an electric current between the metal conductors (4a, 4b). The terahertz optoelectronic device (1) comprises a light excitation source (7) capable of delivering light signals, and an optical fiber (8) comprising a first and a second end (9a, 9b). According to the invention, the terahertz optoelectronic device (1) comprises a semiconductor component (10) integral with the second end of the optical fiber (8). The light excitation source (7) is able to excite the semiconductor component (10) to produce charge carriers therein, for generating or measuring terahertz electromagnetic waves (3), when the semiconductor component conductor (10) is near or in contact with the metal conductors (4a, 4b).

Description

The present invention relates to a terahertz optoelectronic device capable of cooperating with an electronic apparatus for generating or detecting terahertz electromagnetic waves, and a method for generating or detecting terahertz electromagnetic waves.

Terahertz electromagnetic wave generators and detectors are used in various fields such as microelectronics for circuit characterization, telecommunications, biology, or defense, for example. In the field of microelectronics, electronic and optoelectronic components are becoming faster and more complex. It becomes difficult to characterize and locally measure, for example, rapid electric field variations in these components, with conventional measuring tools. Indeed, the operating frequencies of the electronic and optoelectronic components to be characterized are greater than the bandwidths of conventional measuring tools, limited today to 210 GHz. Characterization tools are known associating a terahertz electromagnetic wave generator with an electrical-optical conversion probe. Terahertz electromagnetic wave generators can be divided into two types of generators: those based on the generation of terahertz electromagnetic waves propagating in free space and those based on the generation of terahertz electromagnetic waves propagating in a waveguide . The terahertz electromagnetic wave generators make it possible to emit terahertz electromagnetic waves propagating in a waveguide of the circuit to be characterized, for example. These generated terahertz electromagnetic waves are then detected by the electrical-optical conversion probe to locally measure fast electric field variations in the circuit. It is possible to determine the direction of propagation of electrical signals, the presence of clutter and to characterize these echoes.

These characterization tools make it possible to reach bandwidths greater than 400 GHz, ie subpicosecond temporal resolutions. In the field of wireless telecommunication, these Terahertz electromagnetic wave generators can be used as an emitter / detector of electromagnetic radiation to detect or transmit high speed data (> 10Gbit / s) on carrier at terahertz frequencies. Document FR 2 870 386 discloses an emitter / detector for terahertz electromagnetic radiation, as illustrated in FIG. 1, making it possible to generate or detect terahertz electromagnetic waves. The terahertz electromagnetic radiation emitter / detector comprises on the one hand an electronic apparatus comprising a substrate, a photoconductor comprising an epitaxial layer of unintentionally doped semiconductor material formed on the substrate and metal electrodes or contacts deposited on the substrate. substrate, and secured to the semiconductor material. The design of the electronic device depends on the coupling envisaged either in a waveguide or towards a free space.

The emitter of electromagnetic radiation further comprises a light excitation source of the photoconductor for generating charge carriers in the epitaxial layer, means for creating a continuous electric field between the metallic contacts, means for emitting electromagnetic waves from the generated charge carriers, and a coupling optics for directing the light flux onto the semiconductor component and which is placed between the excitation source and the photoconductor. In the field of the characterization of electronic circuits, the electronic apparatus of the Terahertz electromagnetic wave generator can be connected to the electronic circuit to be characterized by bonds of "bonding" type or directly integrated with the latter during the manufacturing step. as described above. However, bonding bonds are micron gold wires, which have high radiation losses at terahertz frequencies. In addition, the impedance mismatches generated by this type of connection induce numerous parasitic echoes of the terahertz pulse. In addition, the integration of the photoconductor directly on the electronic circuit to be characterized is difficult to achieve technologically and limits the characterization to that of the microelectronic circuit. The microelectronic circuit to be characterized is modified specifically for the measurement, and the generation of the terahertz pulse is located at a precise and fixed location of the studied apparatus. Similarly, in the field of the emission / detection of Terahertz electromagnetic radiation, the emission or detection of the Terahertz electromagnetic radiation is only possible at a precise and fixed location of the electronic device of the emitter / detector of electromagnetic radiation. terahertz. In addition, for the generation of electromagnetic waves in free space, the terahertz electromagnetic wave propagates through the substrate of the electronic device. However, this substrate may have significant losses in the terahertz domain and high permittivity. The objective of the invention is therefore to propose a Terahertz optoelectronic device independent of the electronic device and enabling the generation or detection of terahertz electromagnetic waves at different locations of the electronic device. This terahertz optoelectronic device is able to cooperate with an electronic device for generating or detecting terahertz electromagnetic waves. This Terahertz optoelectronic device works well with a wide range of electronic devices. To this end, the invention relates to a Terahertz optoelectronic device adapted to cooperate with an electronic apparatus for generating or detecting Terahertz electromagnetic waves, said electronic apparatus comprising at least two distant metallic conductors, and means for generating an electric field or measuring a electrical current between said metal conductors, said terahertz optoelectronic device comprising: a light excitation source capable of delivering light signals; an optical fiber comprising a first and a second end, said first end being able to receive the light signals delivered. by said excitation source, said optical fiber directing these light signals towards said second end.

According to the invention, the terahertz optoelectronic device comprises: a semiconductor component integral with said second end of the optical fiber, said light excitation source being able to excite the semiconductor component to produce charge carriers in this region; last, to generate or measure terahertz electromagnetic waves, when the semiconductor component is near or in contact with the metal conductors. In various possible embodiments, the present invention also relates to the following characteristics which may be considered individually or in all their technically possible combinations and each bring specific advantages: said semiconductor component is bonded to said second end of the optical fiber; with an optical adhesive, the terahertz optoelectronic device comprises a lens positioned between the semiconductor component and said second end of the optical fiber for focusing said luminous flux on the semiconductor component; said semiconductor component comprises an epitaxial layer; of a semiconductor material, the epitaxial layer being a layer of InGaAs previously irradiated by ions, - said semiconductor component is formed of a stack of several semiconductor materials, - the lifetime of the carriers of charge of said epitaxial layer, generated by said luminous flux is less than one picosecond, the mobility of these carriers being greater than 200 cm 2 V -s-1 at room temperature and the resistivity of said layer being between 104 0 cm and 1 0 cm, the semiconductor component has a section greater than that of the optical fiber, - the excitation source is a high-speed ultra-short laser source capable of generating light pulses centered on a wavelength between 0.7 lm and 1.6 1m and having a duration of the order of one hundred femtoseconds, the excitation source comprises two coherent continuous laser sources each emitting a light wave, the frequency difference of said light waves being in the terahertz frequency range. The invention also relates to a method for generating or detecting Terahertz electromagnetic waves with a terahertz optoelectronic device as defined above, and an electronic apparatus comprising at least two distant metallic conductors, and means for generating an electric field or measuring a electric current between said metal conductors.

According to the invention, to generate or detect the terahertz electromagnetic waves, the semiconductor component integral with the second end of the optical fiber is placed at a distance from the two metal conductors. The invention provides a Terahertz optoelectronic device independent of the electronic device (or external to the latter). The terahertz optoelectronic device is compatible with a large number of electronic devices using in particular substrates of low permittivity. This Terahertz optoelectronic device thus enables the generation of higher power levels and the detection with greater sensitivity of terahertz electromagnetic waves in free space. This terahertz optoelectronic device is also suitable for the characterization of a large number of complex electronic circuits. The generation of an terahertz electromagnetic wave can be performed at any accessible point of the electronic circuits studied. The Terahertz optoelectronic device can be freely positioned without the need for tedious adjustments or complex manipulations. When the terahertz optoelectronic device is associated with an electrical-optical conversion probe, such as that of document FR 2 906 039, based on electro-optical sampling, which is also independent or external, the characterization of the electronic circuits may be made through the protective housings enclosing them. The terahertz optoelectronic device can be mounted on a production line to characterize on-line circuits. The Terahertz optoelectronic device may be associated with an electronic device optimized for generating or detecting terahertz electromagnetic waves propagating in a waveguide or for generating and detecting the terahertz electromagnetic wave in free space. The invention can be applied to terahertz spectroscopy. The invention will be described in more detail with reference to the accompanying drawings in which: - Figure 1 is a schematic representation of an emitter of electromagnetic radiation of the prior art; FIG. 2 is a schematic representation of an terahertz optoelectronic device, according to a first embodiment of the invention; FIG. 3 is a detailed view of this terahertz optoelectronic device; Figure 1 shows an emitter of electromagnetic radiation of the prior art. This emitter of electromagnetic radiation, which is disclosed in the document FR 2,870,386, comprises a semiconductor or photoconductive component 10 comprising an epitaxial layer of a non-intentionally doped semiconductor material 11 formed on a substrate 5. Advantageously, this epitaxial layer 11 is encapsulated between, on the one hand, an electrical insulation layer 13 placed on the surface of the substrate 5, and on the other hand, a contact layer 14 for depositing metal electrodes 18a, 18b.

The substrate 5 consists of an InP semiconductor material, or GaAs, or the like. Preferably, the semiconductor material of the epitaxial layer 11 is made of InGaAs. The semiconductor component 10 further comprises metal electrodes 18a, 18b formed on the epitaxial layer 11 by evaporation of a gold-based alloy. Means make it possible to create a continuous electric field 6 between these metal electrodes 18a, 18b, the epitaxial layer being placed in this electric field. These means 6 comprise for example a voltage generator applying a potential difference between the metal electrodes 18a, 18b.

The transmitter also includes a broadband ultra-short laser source 7 for emitting ultra-short pulses centered on a wavelength λ between 1.3 μm and 1.6 μm. In this embodiment, this ultra-short laser source 7 is a fiber laser. The ultra-short pulses emitted by this laser source 7 generate charge carriers in the epitaxial layer of the unintentionally doped semiconductor material 11 whose lifetime is less than the picosecond. The transmitter comprises an optical fiber 8 which sends the ultra-short pulses generated by the laser source onto the semiconductor component 10. This optical fiber 8 is placed between the excitation source 7 and the semiconductor component 10. this example of the prior art, the transmitter comprises means for emitting electromagnetic radiation from the generated charge carriers. These means comprise a transmission line 15a, 15b which is coplanar with the metal electrodes 18a, 18b. The metal electrodes 18a, 18b have a potential difference that can be modified by the photogenerated carriers. This transmission line 15a, 15b is connected to a Hertz dipole 16. This Hertz dipole 16 comprises two arms 17a, 17b, each arm 17a, 17b being respectively connected to one end of the metal strips of the transmission line 15a, 15b . The space between the metal electrodes 18a, 18b of the semiconductor component 10 is illuminated by the light source 7. The charge carriers generated by the interaction between the ultra-short laser pulses and the semiconductor component 10 are sent via the transmission line 15a, 15b to the arms 17a, 17b of the Hertz dipole 16 which emits electromagnetic radiation. With this emitter of electromagnetic radiation of the prior art, the semiconductor material is secured to the substrate on which the metal electrodes are deposited. The generation of electromagnetic waves in free space is then of limited power because of the losses in the semiconductor substrate. In addition, the generation of electromagnetic waves in a waveguide on a dielectric substrate of low permittivity is tricky. FIG. 2 is a schematic representation of a terahertz optoelectronic device, according to a first embodiment of the invention. FIG. 3 is a detailed view of this terahertz optoelectronic device. The terahertz optoelectronic device 1 comprises a light excitation source 7 capable of delivering light signals, and an optical fiber 8 comprising two ends 9a, 9b including a first end 9a adapted to receive the light signals delivered by the excitation source 7 The optical fiber 8 directs these light signals towards its second end 9b. A semiconductor component 10 is integral with the second end of the optical fiber 8. The excitation source 7, the optical fiber 8 and the semiconductor component 10 form a unitary unit. The semiconductor component 10 is capable of performing an optical-electrical conversion to generate charge carriers. The semiconductor component 10 may be glued directly to the second end 9b of the optical fiber 8 by means of an optical adhesive. The terahertz optoelectronic device 1 is able to cooperate with an electronic device 2 either to generate terahertz electromagnetic waves 3, which can then be measured by an electrical-optical conversion probe, or to detect terahertz 3 external electromagnetic waves at the device. 2. The electronic apparatus 2 comprises at least two distant metal conductors or electrodes 4a, 4b. These may be formed on a semiconductor or dielectric substrate, for example. These can also be suspended in the air.

When the terahertz 1 optoelectronic device is used in cooperation with the electronic apparatus 2 to generate terahertz electromagnetic waves, the electronic apparatus 2 comprises means 6 for generating an electric field between the metal conductors 4 a, 4 b.

The means for generating an electric field 6 may comprise a voltage generator, for example. The substrate 5 may be of a semiconductor material or formed of another material. It may have a low permittivity.

The two metallic conductors 4a, 4b can form a coplanar waveguide. The light excitation source 7 is able to excite the semiconductor component 10 to generate charge carriers. The generated charge carriers interact with the electric field generated between the metallic conductors 4a, 4b of the electronic apparatus 2 to produce terahertz electromagnetic waves 3. The semiconductor component 10 of the invention may comprise a single epitaxial layer of a semiconductor material, for example. Alternatively, the semiconductor component 10 may be formed of a stack of several semiconductor materials. Preferably, the epitaxial layer of the semiconductor component is InGaAs. It can be of another material as well. One approach for reducing charge carrier lifetime is to introduce defects in the crystalline photoconductive material to create capture centers for trapping charge carriers. The method of document FR 2 870 386 consists of using an irradiated semiconductor component, that is to say subjected to ion bombardment, to generate defects in a controlled and uniform manner and to reduce the lifetime of the carriers charge.

The epitaxial layer is a layer of InGaAs previously irradiated with ions. The lifetime of the charge carriers of the epitaxial layer, generated by the luminous flux is less than the picosecond. It can be also greater than the picosecond. The mobility of these carriers is greater than 200 cm.sup.2 at room temperature and the resistivity of the layer is between 104 0 cm and 10 cm. The technological realization of the terahertz 1 optoelectronic device is particularly delicate, the diameter of the optical fiber 8 being 125 microns. Other diameters of optical fiber 8 are also possible. The optical fiber 8 may be a monomode optical fiber.

In an exemplary possible embodiment, a thin layer of Ino, 53Gao, 47As previously irradiated is bonded at the end of optical fiber 8 with the aid of an optical glue polymerizing with UV radiation. The charge carrier lifetime in Ino material, 53Gao, 47As is reduced to values of the order of one picosecond. In the example of FIGS. 2 and 3, the InGaAs layer has a thickness of 1 μm. The semiconductor component 10 preferably has a dimension greater than that of the optical fiber 8 so that all the light flux coming from the fiber is absorbed by the semiconductor component 10. The diameter of the optical fiber 8 can also be less than that of the semiconductor component 10. Other forms are also possible. It is also possible to use a diaphragm suspended from InGaAs in the center, obtained by chemical etching. The optical fiber 8, the end of which has been stuck with glue, then punctures the InGaAs membrane. The optical fiber 8 can be mounted on precision motorized translation stages. The choice of the material of the epitaxial layer advantageously makes it possible to generate ultra-short electric field pulses from an ultrashort optical excitation whose wavelength is between 0.7 μm and 1.6 μm. The ultra-short laser source 7 of the terahertz optoelectronic device 1 is a wide-spectrum laser source 7 for emitting ultrashort light pulses centered on a wavelength λ of between 0.7 μm and 1.6 μm. In the example of Figures 2 and 3, the wavelength λ is 1.55 μm. The light pulses have a duration of the order of one hundred femtoseconds. In another possible embodiment, the terahertz optoelectronic device 1 comprises a lens (not shown) positioned between the semiconductor component 10 and the second end of the optical fiber 9b to focus the light flux on the semiconductor component 10 to increase the power density incident on the semiconductor component 10. This lens is, for example, silicon or glass.

In another possible embodiment, the excitation source 7 comprises two coherent continuous laser sources each emitting a light wave. The charge carriers are then generated by photomixing. Two light waves emitted for example by two coherent laser sources of slightly different frequency are spatially superimposed. The reference frequency used is between 0.7 μm and 1.6 μm. The photo-mixing of the two coherent light waves then has a pulsation beat term equal to the difference in frequency between the two waves lying in the terahertz frequency range. The two-wave mixture is produced in the semiconductor component 10. As mentioned above, the terahertz 1 optoelectronic device can be used in cooperation with the electronic device 2 to detect outside and incident terahertz electromagnetic waves on the electronic device 2 For this application, the electronic apparatus 2 comprises means for measuring a current 6 'between the metal conductors 4a, 4b. These means 6 'may include an ammeter for example. The light excitation source 7 is able to excite the semiconductor component 10 to generate charge carriers in the latter, and the generated charge carriers interact with the electric field produced by the outer terahertz electromagnetic waves and incident on the electronic device 2, to induce an electric current between the metal conductors 4a, 4b. This electric current is measured by the means for measuring an electric current 6 '. The terahertz 1 optoelectronic device and the electronic device 2 can be combined to form a terahertz electromagnetic wave transmission / reception device that can be used in the field of mobile telephony.

The metal contacts 4a, 4b can be connected to a transmission line, which is itself connected to an antenna or to a "Hertz dipole". Two modes of operation are possible. The generated current transient can result solely from the displacement current or the conduction current and the displacement current.

In the first case, the semiconductor component 10 is placed near the metal conductors 4a, 4b or waveguide but without contact. The ultra-short electric pulse generated is therefore only the result of the displacement current.

In the second case, the semiconductor component 10 is brought into contact with the metal conductors 4a, 4b. The ultra-short electric pulse 3 therefore results from the displacement current and the conduction current. Contact generation provides more electromagnetic signal. On the other hand, large leakage currents exist with a high electric field applied between the two metal electrodes, which leads to the destruction of the terahertz optoelectronic device 1. On the contrary, when the semiconductor component 10 is not in contact, electric fields much higher standards are allowed. In this case, the charge carriers in the semiconductor component may have a life time greater than one picosecond. To generate terahertz electromagnetic waves 3, the semiconductor component 10 can be brought closer to the metal contacts 4a, 4b so that its surface is substantially parallel to the surface of the metal contacts 4a, 4b.

Alternatively, the surface of the semiconductor component 10 may be inclined relative to the surface of the metal contacts 4a, 4b. The approximation of the lateral edge of the semiconductor component 10 is sufficient to generate terahertz 3 electromagnetic waves. The terahertz 1 optoelectronic device of the invention makes it possible to generate or detect terahertz 3 electromagnetic waves propagating either in free space or in a waveguide. Systems in free space have the advantage of a very large bandwidth. On the other hand, guided wave systems have a much lower spatial resolution than diffraction, limited only by the lithographic stresses encountered during the fabrication of the waveguides.

Claims (10)

REVENDICATIONS1. Terahertz optoelectronic device (1) adapted to cooperate with an electronic apparatus (2) for generating or detecting Terahertz electromagnetic waves (3), said electronic apparatus (2) comprising at least two distant metallic conductors (4a, 4b), and means (6, 6 ') for generating an electric field or measuring an electric current between said metal conductors (4a, 4b), said terahertz optoelectronic device (1) comprising: - a light excitation source (7) capable of delivering signals an optical fiber (8) comprising a first and a second end (9a, 9b), said first end (9a) being able to receive the light signals delivered by said excitation source (7), said optical fiber ( 8) directing these light signals towards said second end (9b), characterized in that it comprises: a semiconductor component (10) integral with said second end of the optic fiber; that (8), said light excitation source (7) being able to excite the semiconductor component (10) to produce charge carriers therein, for generating or measuring terahertz electromagnetic waves (3), when the semiconductor component (10) is near or in contact with the metal conductors (4a, 4b). 2. terahertz optoelectronic device according to claim 1, characterized in that said semiconductor component (10) is bonded to said second end of the optical fiber (9b) with an optical adhesive. 3. terahertz optoelectronic device according to claim 1, characterized in that it comprises a lens positioned between the semiconductor component (10) and said second end of the optical fiber (9b) for focusing said luminous flux on the semiconductor component (10). 4. terahertz optoelectronic device according to any one of claims 1 to 3, characterized in that said semiconductor component (10) comprises an epitaxial layer of a semiconductor material, the epitaxial layer being a layer of InGaAs previously irradiated by ions. 5. terahertz optoelectronic device according to claim 4, characterized in that the service life of the charge carriers of said epitaxial layer, generated by said luminous flux is less than the picosecond, the mobility of these carriers being greater than 200 cm²V- at room temperature and the resistivity of said layer being between 104 0cm and 1 0cm. 6. terahertz optoelectronic device according to any one of claims 1 to 5, characterized in that said semiconductor component (10) is formed of a stack of several semiconductor materials. 7. terahertz optoelectronic device according to any one of claims 1 to 6, characterized in that the semiconductor component (10) has a section greater than that of the optical fiber (8). 8. terahertz optoelectronic device according to any one of claims 1 to 7, characterized in that the excitation source (7) is a broad-spectrum ultra-short laser source capable of generating ultra-short light pulses centered on a wavelength between 0.7 lm and 1.6 lm and having a duration of the order of one hundred femtoseconds. 9. terahertz optoelectronic device according to any one of claims 1 to 7, characterized in that the excitation source (7) comprises two coherent continuous laser sources each emitting a light emonde, the frequency difference of said light waves lying in the terahertz frequency domain. 10. A method for generating or detecting terahertz electromagnetic waves (3) with a terahertz optoelectronic device (1) as defined in any one of claims 1 to 9, and an electronic device (2) comprising at least two conductors. remote metal elements (4a, 4b), and means (6, 6 ') for generating an electric field or measuring an electric current between said metal conductors (4a, 4b), characterized in that: - for generating or detecting electromagnetic waves Terahertz, the semiconductor component (10) integral with the second end of the optical fiber (8) is placed at a distance from the two metal conductors (4a, 4b).