Three-dimensional photoconductive transducer for terahertz signals or picosecond electrical pulses.

Abstract

The present invention relates to a photoconductive transducer for generating or detecting waves in the terahertz frequency range or the picosecond pulse range. The transducer comprises a three-dimensional structure including, in this order, a first planar electrode (El), an array of nanocolumns (C) embedded in a layer of resist (R), and a second planar electrode (E2) parallel to the first planar electrode. The transducer design utilizes photonic and plasmonic resonances and a high, homogeneous electric field to improve the efficiency of light-to-terahertz conversion. The height of the nanocolumns and the thickness of the resist range between 100 nm and 400 nm. The width of the nanocolumns is between 100 nm and 400 nm, and the distance between two adjacent nanocolumns is between 300 nm and 500 nm. The nanocolumns are made of III-V semiconductors.

Description

[Technical field]

[0001] The field of the invention is that of sources for generating or detecting very high frequency radiation and / or electrical pulses of very short duration, on the order of picoseconds or even shorter. One of the preferred applications of the invention is the development of highly efficient terahertz sources. [Background technology]

[0002] Terahertz radiation has a wide variety of applications, particularly related to its high penetrating power in most materials. For example, applications of the present invention may be in security, detection, quality control, communications and electronic applications using nanotechnology. These sources can be used at room temperature or at low temperatures.

[0003] The development of this efficient terahertz source is one of the difficulties encountered. One possible production technique involves converting optical radiation produced by a continuous wave or pulsed laser into terahertz radiation and / or electrical pulses. Various conversion means are possible.

[0004] One of the most common means is to use optoelectronic devices known as photoconductive switches, which can generate continuous wave terahertz radiation or picosecond electrical pulses via the following mechanisms:

[0005] A laser source emits radiation in the form of continuous wave radiation or ultrashort pulses. Ultrashort means pulses with a duration of less than 100 femtoseconds. The radiation provided by the laser source is located in the visible or near infrared region of the electromagnetic spectrum. The light provided by the laser source is absorbed by a semiconductor material such as gallium arsenide (GaAs) or indium gallium arsenide (InGaAs). Using light absorption, charge carriers are generated in the semiconductor and are then collected by metal contacts, referred to herein as electrodes, thus creating an electrical signal. The process of charge carrier generation using light absorption will be referred to herein as photogeneration.

[0006] The duration and frequency of this signal depend on the frequency and duration of the light provided by the laser source. The very high dynamic range of optoelectronic devices makes it possible to generate electrical signals in the gigahertz or terahertz frequency range.

[0007] The main drawback of existing technologies is their low conversion efficiency, which makes them unsuitable for certain terahertz power demanding applications or for systems and applications requiring operation in cryogenic environments.

[0008] The conversion efficiency is defined as the ratio of the optical intensity delivered by the laser source to the terahertz power delivered by the optoelectronic device. This efficiency is the result of two factors. The first factor is associated with the efficiency of absorption of the laser light by the semiconductor. The absorption efficiency is generally low.

[0009] The second factor is related to the efficiency of collection of the photogenerated charge carriers by the electrodes of the optoelectronic device, which is also low. A certain number of technical solutions have been proposed to improve these two factors.

[0010] The publication by Nezih T. Yardimci et al. entitled "High-Power Terahertz Generation Using Large-Area Plasmonic Photoconductive Emitters" published in IEEE Transactions on Terahertz Science and Technology, Vol. 5, N°2, March 2015, pp. 223-229, and the patent application WO 2013 / 112608 entitled "Photoconductive device with plasmonic electrodes" describe a photoconductive device including the following components: a photoconductive assembly made of a semiconductor substrate, an antenna assembly, and one or more plasmonic contact electrodes positioned on the semiconductor substrate. The photoconductive assembly is made to improve the overall efficiency of the photoconductor device by enhancing the laser absorption efficiency in the light absorbing regions of the semiconductor substrate through plasmonic resonance. The device only improves the laser absorption efficiency.

[0011] A publication by Shang-Hua Yang et al., entitled “7.5% Optical-to-Terahertz Conversion Efficiency Offered by Photoconductive Emitters With Three-Dimensional Plasmonic Contact Electrodes”, published in IEEE Transactions on Terahertz Science and Technology, Vol. 4, N°5, September 2014 pp. 575-581, proposes a similar photoconductive device that includes an array made up of parallel rows of plasmonic contact electrodes.

[0012] A publication by Nezih Tolga Yardimci et al., entitled “A High-Power Broadband Terahertz Source Enabled by Three-Dimensional Light Confinement in a Plasmonic Nanocavity”, published in Scientific Reports, 7(1), pp. 4166, describes a device similar to the one described above, but which includes an additional distributed Bragg reflector as part of the light-absorbing semiconductor to facilitate further increase in laser absorption.

[0013] A publication by Burford et al., entitled “Computational modeling of plasmonic thin-film terahertz photoconductive antennas,” published in the Journal of the Optical Society of America B, 33(4), pp. 748-759, describes a photoconductive device that includes a thin plasmonic array of nanodisks placed on contact electrodes.

[0014] The publication by Peytavit et al. entitled "Low-Temperature-Grown GaAs Photoconductor with High Dynamic Responsivity in the Millimeter Wave Range", published in Applied Physics Express 4 (2011) 104101, and the French patent application FR 2 949 905, by the same inventors, entitled "Photodetecteur, photomelangeur et leur application a la generation de rayonnement terahertz" ("Photodetector, photomixer and application thereof to generate terahertz radiation"), describe a photodetector comprising a layer of a photoconductive material capable of absorbing optical laser radiation. The so-called thin layer has a thickness that is smaller than the absorption length of the laser radiation inside the light-absorbing material. The light-absorbing material is positioned between two electrodes: an at least partially transparent electrode, called the top electrode, and a reflective electrode, called the bottom electrode. The so-called bottom electrode and the top electrode form a resonant cavity for the laser radiation. The photodetector is coupled to an antenna to generate free-space terahertz radiation. This device, unlike the devices described above, allows for increased conversion efficiency by increasing the efficiency of charge carrier collection by the metal electrodes.

[0015] US 2014 / 175283 discloses a terahertz detector that includes a one- or two-dimensional array of slabs made of polar semiconductors in a dielectric matrix. On each side of the slabs, an interface with the dielectric matrix supports interface phonon polaritons. Each slab has a top electrical contact and a bottom electrical contact, forming an optical antenna, allowing resonant coupling of the interface phonon polaritons with incident radiation. [Prior art documents] [Patent documents]

[0016] [Patent Document 1] International Publication No. 2013 / 112608 Brochure [Patent Document 2] French Patent Application Publication No. 2 949 905 [Patent Document 3] US Patent Application Publication No. 2014 / 175283 [Non-patent literature]

[0017] [Non-Patent Document 1] Nezih T. Yardimci et al., “High-Power Terahertz Generation Using Large-Area Plasmonic Photoconductive Emitters”, IEEE Transactions on Terahertz Science and Technology, Vol. 5, N°2, March 2015, pp. 223-229 [Non-Patent Document 2] Shang-Hua Yang et al., “7.5% Optical-to-Terahertz Conversion Efficiency Offered by Photoconductive Emitters With Three-Dimensional Plasmonic Contact Electrodes”, IEEE Transactions on Terahertz Science and Technology, Vol. 4, N° 5, September 2014, pp. 575-581 [Non-Patent Document 3] Nezih Tolga Yardimci et al., “A High-Power Broadband Terahertz Source Enabled by Three-Dimensional Light Confinement in a Plasmonic Nanocavity”, Scientific Reports, 7(1), pp. 4166 [Non-Patent Document 4] Burford et al., “Computational modeling of plasmonic thin-film terahertz photoconductive antennas”, Journal of the Optical Society of America B, 33(4), pp.748-759 [Non-Patent Document 5] Peytavit et al., “Low-Temperature-Grown GaAs Photoconductor with High Dynamic Responsivity in the Millimeter Wave Range”, Applied Physics Express 4(2011)104101 Summary of the Invention [Problem to be solved by the invention]

[0018] None of these publications makes it possible to achieve both high laser absorption efficiency and satisfactory collection efficiency of the generated carriers. One objective of the present invention is to significantly improve these two efficiencies using a single and simple structure that combines the advantages of parallel electrode devices and plasmonic / photonic structures. [Means for solving the problem]

[0019] More precisely, one subject of the invention is a photoconductive transducer intended for generating or detecting waves and signals in the terahertz frequency range or in the picosecond pulse range, as claimed in claim 1.

[0020] Dependent claims 2 to 7 correspond to specific embodiments of such a photoconductive transducer.

[0021] The invention also relates to a terahertz emitter according to claim 8, comprising such a photoconductive transducer.

[0022] The invention also relates to the use of such a photoconductive transducer for emitting and / or detecting waves in the terahertz frequency range.The invention also relates to a method according to claim 11 for manufacturing such a photoconductive transducer.

[0023] Other characteristics, details and advantages of the invention will become apparent on reading the description given with reference to the attached drawings, given by way of example, in which: [Brief description of the drawings]

[0024] [Figure 1] 1 shows an example of a plan view of a photoconductive transducer structure according to the present invention. [Diagram 2] 1 shows an embodiment of a partial perspective view of a nano-column including a photoconductive transducer according to the present invention. [Diagram 3] 1 shows a specific example of a cross-sectional view of a nanocolumn according to the present invention. [Figure 4] 1 shows an example of a first embodiment of a photoconductive transducer used as a source of terahertz radiation. [Diagram 5] 1 illustrates an example of a second embodiment of a photoconductive transducer used as a source of terahertz radiation. [Figure 6] 1 illustrates an example of a third embodiment of a photoconductive transducer used as a receiver of terahertz radiation. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] The photoconductive transducer according to the invention can operate either as an emitter or receiver of waves in the terahertz frequency range or in the picosecond pulse range. Except for a few details, the structures used for these two applications are the same.

[0026] The photoconductive transducer according to the invention can also operate as a source of THz frequency waves or picosecond duration electrical signals for micro- or nanoelectronic circuits.

[0027] The transducer core comprises a structure that includes the following three elements in this order: - a first planar electrode, - a layer of resist containing an embedded array of identical nano-columns oriented perpendicular to the plane defined by the first electrode, the distance between two adjacent columns being constant; - a second planar electrode parallel to the first planar electrode.

[0028] The area of ​​the structure is 1 μm depending on the transducer used. 2 ~1000μm 2 It is between.

[0029] Figure 1 shows a plan view of a nanocolumn array C placed on a first electrode E1. Figure 2 shows a partial perspective view of a nanocolumn C of a photoconductive transducer according to the invention. In the case of Figure 2, the column has a rectangular cross section. The cross section of the column may also be circular or polygonal.

[0030] When a transducer is used as an emitter, it is used in combination with a laser light source emitting at a defined wavelength, which is generally located in the visible or near infrared region of the electromagnetic spectrum, where certain properties of the structure depend on this wavelength.

[0031] The first electrode is formed by a conductive deposit which can be made of gold or titanium or silver or aluminium.

[0032] The second electrode E2 must be transparent at the aforementioned wavelengths so as to transmit the incident laser radiation into the structure. To this end, the second electrode can be formed by depositing a layer of indium tin oxide, which is transparent to wavelengths greater than 300 nanometers.

[0033] The thickness of the second electrode E2 is specific to the aforementioned wavelength and must be such that it forms an anti-reflective coating for the particular wavelength.

[0034] Likewise, the resist R must also be transparent at the wavelength of the laser light source. It is possible to use, for example, a negative photoresist such as the resist known as SU-8, which is commonly used in the fabrication of this type of microsystem and which comes in different variants. Of course, other types of transparent resists are possible. The layer of resist generally has a thickness comprised between 100 nanometers and 400 nanometers.

[0035] The height of the nanocolumns is equal to the height of the layer of resist. The width of the columns and the distance separating two adjacent columns are adapted to the wavelength of the laser and the refractive index of the resist.

[0036] By optimizing the spatial dimensions of the nanocolumns with respect to the spatial dimensions of the laser wavelength, the absorption of the laser light is maximized, which is the desired objective. The length and width of each nanocolumn are such that the laser is absorbed by excitation of plasmon resonances at the top and bottom surfaces of the structure. In addition, guided light photonic modes propagating through the heterogeneous layers of polymer resist and nanocolumn arrays will also be excited. Furthermore, resonant cavity modes will also be excited inside the nanocolumns in the vertical direction between the two electrodes. Typically, the width of the columns ranges between 100 nanometers and 400 nanometers.

[0037] The pitch of the array of nanocolumns corresponds to the distance between two consecutive columns and is responsible for the excitation of the collective photonic resonance mentioned above. This is due to the periodic arrangement of the nanocolumns which efficiently diffracts the laser light into the interior of the heterogeneous layer. The distance between two columns is between 300 nm and 500 nm. By combining the plasmonic and photonic effects, it is possible to obtain an absorption higher than 95%.

[0038] The material of the columns must be optimized to obtain the best possible picosecond pulses. Therefore, materials with very fast carrier response times, i.e. with very fast carrier dynamics, must be used. Given the scale, this time is preferably shorter than 10 picoseconds. For example, the nanocolumns can be made of specially processed III-V semiconductors such as gallium arsenide or indium gallium arsenide or indium phosphide.

[0039] Each column C carries a contact at its upper end that ensures electrical continuity with a transparent second electrode, made of the same conductive material as that of the first electrode.

[0040] 3 shows a cross-sectional view of a nanocolumn C according to the invention. It is surrounded by a resist R. It contains an electrical contact CE. The resist R is located between two electrodes E1 and E2.

[0041] Figure 4 shows a photoconductive transducer according to the invention, operating as a source of terahertz radiation. It comprises a laser L emitting at a defined wavelength, symbolically represented by a solid arrow, a second electrode E2 being transparent at said wavelength. The laser is arranged to illuminate through the second electrode E2 an array of nanocolumns C embedded in a resist R, said array being referenced R+C in Figure 4.

[0042] The arrangement of the structure, with a conversion medium consisting of a resist layer and an array of nano-columns placed between two planar parallel electrodes, allows the creation of a uniform electric field by subjecting the two electrodes E1 and E2 to a potential difference V of several volts.

[0043] Because the distance separating the two electrodes is very small, this potential difference can create very strong electric fields. The magnitude of these fields is on the order of 100 kV / cm. Depending on the application, the applied voltage can be DC or AC.

[0044] The created electric field uniformly accelerates the charge carriers generated by the absorption of the laser radiation. A uniform electric field makes it possible to maximize the number of carriers collected by the electrodes and therefore to increase the signal generated. It is therefore essential to reduce the height of the pillars so that the distance these carriers have to travel is minimized. The minimum travel distance ensures that the carriers are not lost through recombination mechanisms.

[0045] The transducer generates terahertz radiation or a picosecond electrical signal R T Generate the above R T can be converted into free space terahertz radiation with the aid of antenna elements or guided through waveguide elements to micro- and nanoelectronic circuits, as shown in Figure 5. The references in Figure 5 are the same as those in Figure 4. The duration, intensity and frequency bandwidth generated depend on the duration of the laser source, its power, its wavelength and the potential difference V applied between the electrodes E1 and E2.

[0046] 6 shows a photoconductive transducer according to the invention operating as a detector of terahertz radiation, which accelerates electrons flowing between two electrodes E1 and E2.

[0047] For example, a method for manufacturing a transducer structure according to the present invention includes the steps described below.

[0048] Step 1: Etch an array of columns into a substrate made of a III-V semiconductor such as gallium arsenide or indium gallium arsenide or indium phosphide.

[0049] Step 2: Deposit a metal layer on the top surface of the columns and on the bottom surface of the substrate supporting the columns. The metal layer will form the metal contacts. This process is carried out at low pressure, i.e., 10 -6It can be carried out at pressures below millibar and with slow deposition rates, of the order of magnitude of 0.5 nm / s. The deposited metals can be made of gold or silver or aluminum or titanium. In this process, it is essential that there is no contact between the bottom metal layer, which acts as the first electrode, and the metal layer on the top surface of the nanocolumns, which connects to the second electrode.

[0050] Step 3: Spin-coat a layer of negative epoxy photoresist, such as SU-8, completely covering the array of nanocolumns. The deposited thickness can be three times larger than the height of the nanocolumns for uniformity.

[0051] Step 4: Expose the layer of resist with e-beam or UV lithography.

[0052] Step 5: Polish or etch away the layer of resist until the metal contacts on the top of the nanocolumns are revealed.

[0053] Step 6: A transparent metal layer is deposited on the layer of resist so as to connect the various metal contacts. This layer can be made of indium tin oxide. The thickness of the latter layer ranges between 100 nanometers and 300 nanometers, depending on the wavelength of the laser source.

[0054] The following are the main advantages of the photoconductive transducer according to the present invention:

[0055] Its three-dimensional structure makes it possible to obtain a high efficiency, much higher than that achievable with prior art devices, thus providing a ten-fold efficiency improvement compared to current transducers.

[0056] Moreover, its "vertical" structure, consisting of an array of nanocolumns sandwiched between two electrodes, makes it possible to further optimize this efficiency.

[0057] Another advantage is that the same structure can be used as a source of terahertz radiation and as a receiver of the same radiation.

[0058] The transducers can be used on-chip in micro- and nano-electronic circuits.

[0059] The transducer can be used in a wide range of applications covering the fields of security, biological or chemical detection, quality control, telecommunications, electronics and quantum electronics.

[0060] Its architecture allows this transducer to operate at room temperature and cryogenic temperatures for ultrafast electronic switching applications, thus enabling its use in quantum technology: it can be used as an ultrafast source to test, trigger or drive qubits in quantum computers.

Claims

1. 1. A photoconductive transducer intended to generate or detect waves in the terahertz frequency range or in the picosecond pulse range, characterized in that the photoconductive transducer comprises a three-dimensional structure comprising a first planar electrode (E1), a second planar electrode (E2) parallel to the first planar electrode, and an array of identical nanocolumns (C) embedded in a layer of resist (R) located between the first and second planar electrodes, the resist and the second planar electrode being transparent at a given wavelength in the visible or near infrared range of the electromagnetic spectrum, the height of the nanocolumns and the thickness of the resist range between 100 nanometers and 400 nanometers, the width of the nanocolumns is between 100 nanometers and 400 nanometers, the distance between two successive nanocolumns is between 300 nanometers and 500 nanometers, the nanocolumns are made of III-V semiconductors, and the top of each nanocolumn comprises a metal contact (CE) electrically connected to the second planar electrode.

2. 2. The photoconductive transducer of claim 1, wherein the width of the nano-columns, the distance separating two adjacent nano-columns, and the refractive index of the resist are selected such that illuminating the photoconductive transducer at the given wavelength through the second planar electrode excites guided light photonic modes propagating through heterogeneous layers of polymer resist and the array of nano-columns, plasmon resonances at the top and bottom surfaces of the three-dimensional structure, and vertical resonant cavity modes inside the nano-columns between the two planar electrodes.

3. The area of ​​the three-dimensional structure is 1 μm ２ ~1000μm ２ The photoconductive transducer according to claim 1 or 2, wherein

4. The photoconductive transducer of any one of claims 1 to 3, wherein the second planar electrode is made of indium tin oxide.

5. The photoconductive transducer according to any one of claims 1 to 4, wherein the cross section of the nano-columns is rectangular, circular or polygonal.

6. The photoconductive transducer according to any one of claims 1 to 5, wherein the material of the nano-columns is a III-V semiconductor selected among gallium arsenide or indium gallium arsenide or indium phosphide.

7. The photoconductive transducer according to any one of claims 1 to 6, wherein the resist is a negative epoxy photoresist.

8. 8. A terahertz emitter comprising a photoconductive transducer according to any one of claims 1 to 7, a laser (L) emitting light at the defined wavelength, the laser being arranged to illuminate the array of columns through the second planar electrode, and means for establishing a potential difference between the first and second planar electrodes.

9. A photoconductive transducer according to any one of claims 1 to 7, a laser (L) emitting at the defined wavelength, the laser being arranged to irradiate the array of columns through the second planar electrode, and a detector for detecting the radiation from the laser and the incoming terahertz radiation (R Ｔ and a voltmeter (V) that measures an output signal resulting from a photo-generated current resulting from the photodiode.

10. Use of a photoconductive transducer according to any one of claims 1 to 7 for emitting waves in the terahertz frequency range.

11. Use of a photoconductive transducer according to any one of claims 1 to 7 for detecting waves in the terahertz frequency range.

12. A method for manufacturing a photoconductive transducer according to any one of claims 1 to 7, comprising the steps of: - etching said array of nanocolumns (C) in a substrate made of a III-V semiconductor; - depositing a metal layer (E1, CE) on the upper surface of said nanocolumns and on the lower surface of said substrate supporting said nanocolumns, the tops of said nanocolumns thus comprising metal contacts (CE); - spin-coating a layer of negative epoxy photoresist (R) to cover said array of columns; - exposing said layer of resist by electron beam or UV lithography; - polishing or etching the layer of resist until the metal contacts (CE) at the tops of the nanocolumns are revealed; - depositing a transparent metal layer (E2) on said layer of resist so as to connect said various metal contacts (CE); A method comprising:

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