Patent ID: 12206039

The photoconductive transducer, according to the invention, may operate either as an emitter or as a receiver of waves in the terahertz frequency domain or in the picosecond pulse domain. Excluding a few details, the structure used for these two applications is the same.

The photoconductive transducer, according to the invention, may also operate as a source for THz frequency waves or electrical signals with picosecond time duration for micro- or nano-electronic circuits.

The core of the transducer comprises a structure including, in this order, the following three elements:a first planar electrode,a layer of resist comprising—an embedded array of identical nano-columns placed perpendicular to the plane defined by the first electrode. The distances between two adjacent columns is constant,a second planar electrode parallel to the first planar electrode.

The area of the structure is between 1 μm2and 1000 μm2, depending on the use of the transducer.

FIG.1shows a top view of the nano-column array C placed on top of the first electrode E1.FIG.2shows a partial perspective view of the nano-columns C of the photoconductive transducer according to the invention. In the case ofFIG.2, the columns are of rectangular cross section. The cross section of the columns may also be circular or polygonal.

When the transducer is used as an emitter, it is used in combination with a laser source that emits at a defined wavelength. This wavelength is generally located in the visible or in the near infrared region of the electromagnetic spectrum. In this case, certain characteristics of the structure depend on this wavelength.

The first electrode is formed by a conductive deposit that may be made of gold or titanium or silver or aluminum.

The second electrode E2must be transparent at the aforementioned wavelength so as to let the incident laser radiation penetrate into the structure. To this end, the second electrode may be formed by depositing a layer of indium-tin oxide, which is transparent for wavelengths larger than 300 nanometres.

The thickness of the second electrode E2is specific to the aforementioned wavelength and must be such that it forms an anti-reflection coating for the particular wavelength.

In the same way, the resist R must also be transparent at the wavelength of the laser source. For example, it is possible to use a negative photoresist, such as the resist known as SU-8, which is commonly used in the fabrication of micro-systems of this type and of which there are various variants. Other types of transparent resists are, of course, possible. The layer of resist generally has a thickness comprised between 100 nanometres and 400 nanometres.

The height of the nano-columns 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 to the refractive index of the resist.

By optimizing the spatial dimensions of the nano-columns with respect to the spatial dimension of the laser wavelength, the absorption of the laser light is maximized, with this being the desired aim. The length and width of each nano-column are such that the laser is absorbed by excitation of plasmonic resonances at the upper and lower surfaces of the structure. In addition, guided optical photonic modes, which propagate through the heterogeneous layer consisting of the polymer resist and of the array of nano-columns, will also be excited. Furthermore, a resonant cavity mode will also be excited inside the nano-column and in the vertical direction between the two electrodes. Generally, the width of the columns is in the range between 100 nanometres and 400 nanometres.

The pitch of the array of the nano-columns, which corresponds to the distance between two successive columns, is responsible for the excitation of collective photonic resonances described above. This is due to the periodic arrangement of the nano-columns which diffract efficiently the laser light into the interior of the heterogeneous layer. The distance between two columns is between 300 nanometres and 500 nanometres. By combining plasmonic and photonic effects, it is possible to obtain an absorption higher than 95%.

The material of the columns must be optimized to obtain the best possible picosecond pulse. Materials with very fast carrier response times, i.e. with very fast carrier dynamics, must therefore be used. To give an order of magnitude, it is preferable for this time to be shorter than 10 picoseconds. For example, the nano-columns can be made of specially treated III-V semiconductors such as gallium arsenide or gallium indium arsenide or indium phosphide.

Each column C bears a contact on the top end that is made of a conductive material that is identical to that of the first electrode and that ensures electrical continuity with the transparent second electrode.

FIG.3shows a cross-sectional view of a nano-column C according to the invention. It is encircled by a resist R. It comprises—an electrical contact CE. The resist R is situated between the two electrodes E1and E2.

FIG.4shows the photoconductive transducer according to the invention operating as a source of terahertz radiation. It comprises—a laser L, which has been represented symbolically by a solid arrow, emitting at a defined wavelength, the second electrode E2being transparent at the aforementioned wavelength. The laser is arranged so as to irradiate, through the second electrode E2, the array of nano-columns C, which are embedded in the resist R, with the aforementioned array being referenced by R+C inFIG.4.

The arrangement of the structure with the conversion medium being composed of the resist layer and the array of nano-columns placed between the two planar and parallel electrodes, allows for the creation of a uniform electric field by subjecting the two electrodes, E1and E2, to a potential difference V of a few volts.

Because of the very small distance separating the two electrodes, this potential difference is capable of creating very strong electric fields. The order of magnitude of these fields is of 100 kV/cm. Depending on the application, the applied voltages are DC or AC.

The created electric field accelerates uniformly the charge carriers generated by the absorption of the laser radiation. The uniform electric field allows the number of carriers collected by the electrodes to be maximized and thus the generated signal to be increased. It is therefore essential for the height of the pillars to be small such that the distance travelled by these carriers is minimized. Minimum travelled distance ensures that the carriers are not lost through recombination mechanisms.

The transducer generates terahertz radiation or picosecond electrical signals RT. The aforementioned RTcan be converted to a free space terahertz radiation through the aid of an antenna element or guided through waveguide elements towards micro- and nano-electronic circuits as shown inFIG.5. The references ofFIG.5are identical to those ofFIG.4. The duration, the intensity and the frequency bandwidth generated, depend on the duration of the laser source, on its power, on its wavelength and on the potential difference V applied between the electrodes E1and E2.

FIG.6shows the photoconductive transducer according to the invention operating as a detector of terahertz radiation. The terahertz radiation accelerates the electrons, which flow between the two electrodes E1and E2.

For example, the process for producing the transducer structure, according to the invention, comprises of the steps described below.

Step1: etching the array of columns in a substrate made of III-V semiconductors such as gallium arsenide or indium-gallium arsenide or indium phosphide;

Step2: depositing a metal layer on the upper surface of the columns and on the lower surface of the substrate that bears the columns. The metal layers will constitute the metal contacts. This operation may be carried out at low pressure, i.e. at a pressure below 10−6millibars and with a slow evaporation rate, the order of magnitude of the evaporation rate being 0.5 nm/s. The deposited metal may be made of gold or silver or aluminum or titanium. It is essential, in this operation, that there is no contact between the bottom metal layer, which serves as first electrode, and the metal layer on the upper surface of the nano-columns, which connects to the second electrode;

Step3: spin coating a layer of negative epoxy photoresist such as SU-8 and cover completely the array of nano-columns. The deposited thickness can be for homogeneity three times larger than the height of the nano-columns;

Step4: exposing the layer of resist with an electron beam or UV lithography;

Step5: polishing or etching the layer of resist until the metal contacts at the top of the nano-columns appear;

Step6: depositing a transparent metal layer on the layer of resist so as to connect the various metal contacts. This layer may be made of indium-tin oxide. The thickness of the latter layer is in the range between 100 nanometers and 300 nanometers, depending on the wavelength of the laser source.

The following are the main advantages of the photoconductive transducer according to the invention:

Its three-dimensional structure allows high efficiencies to be obtained, much higher than those obtained with prior-art devices. An improvement of a factor of 10 in efficiency is thus obtained over current transducers.

Moreover, its “vertical” structure composed of the array of nano-columns sandwiched between two electrodes allows this efficiency to be further optimized.

Another advantage is that the same structure may be used as a source of terahertz radiation and as a receiver of the same radiation.

The transducer can be used on-chip in micro and nano-electronic circuits.

This transducer may be employed in a wide range of applications covering the fields of security, of biological or chemical detection, of quality control, of telecommunications, of electronics and quantum electronics.

Its architecture allows it to operate at room temperature and at cryogenic temperatures for ultra-fast electronic switching applications. Thus, it is possible to use it in quantum technologies. It is possible to use it as an ultra-fast source for inspecting, triggering or driving qubits for quantum computers.