rus
Issue #5/2023
V. M. Petrov, G. A. Ludnikov
Design Aspects Sillenite-Based Terahertz Radiation Receivers: Design Aspects
Design Aspects Sillenite-Based Terahertz Radiation Receivers: Design Aspects
DOI: 10.22184/1993-7296.FRos.2023.17.5.372.377
The possible application of crystals from the sillenite group (Bi12SiO20, Bi12GeO20, Bi12TiO20) to detect radiation in the terahertz (near infrared) range is considered. Photosensitivity for the spectral range of 3–30 μm is provided by application of the shallow traps located near the bottom of the conduction band. The crystal sections are determined, at which the electro-optical and piezoelectric effects can be used to develop a voltage on the surface electrodes. The electrodes made in the form of an interdigital transducer or a spiral complement the device with new functional capabilities.
The possible application of crystals from the sillenite group (Bi12SiO20, Bi12GeO20, Bi12TiO20) to detect radiation in the terahertz (near infrared) range is considered. Photosensitivity for the spectral range of 3–30 μm is provided by application of the shallow traps located near the bottom of the conduction band. The crystal sections are determined, at which the electro-optical and piezoelectric effects can be used to develop a voltage on the surface electrodes. The electrodes made in the form of an interdigital transducer or a spiral complement the device with new functional capabilities.
Теги: electro-optical effect nanoantennas piezoelectric effect terahertz sensors наноантенны пьезоэлектрический эффект терагерцевые сенсоры электрооптический эффект
Sillenite-Based Terahertz Radiation Receivers:
Design Aspects
V. M. Petrov 1 , G. A. Ludnikov 2
Saint-Petersburg State University, Saint-Petersburg, Russia
National Research University ITMO, Saint-Petersburg, Russia
The possible application of crystals from the sillenite group (Bi12SiO20, Bi12GeO20, Bi12TiO20) to detect radiation in the terahertz (near infrared) range is considered. Photosensitivity for the spectral range of 3–30 μm is provided by application of the shallow traps located near the bottom of the conduction band. The crystal sections are determined, at which the electro-optical and piezoelectric effects can be used to develop a voltage on the surface electrodes. The electrodes made in the form of an interdigital transducer or a spiral complement the device with new functional capabilities.
Keywords: terahertz sensors, nanoantennas, electro-optical effect, piezoelectric effect
Article received: 02.04.2023
Article accepted: 02.06.2023
Introduction
During the development of highly efficient terahertz nanoantennas one of the objectives is the possible fine tuning of their optical response using the electric fields. For this purpose, it is necessary to select a material that would have both light sensitivity, for example, photoconductivity, and availability of a physical mechanism that can be used to control the antenna response. The electrical tuning capabilities of nanoantennas have been demonstrated for the semiconductor structures due to the density control of charge carriers [1–3], nanoantennas based on a graphene sheet located on any substrate by controlling the dipole moment [4–8], and also due to the nano-mechanical deformations caused by the forces of various nature [9, 10], including the Casimir interaction [11].
The selection of a suitable material for a controlled nanoantenna is not limited to the materials listed above. Thus, back in 1995, the optical data recording in the bismuth silicate crystals Bi12SiO20 was demonstrated in the so-called shallow traps [12], located in the range almost from 0 to 1.5–1.9 eV from the bottom of the conduction band (see Fig. 1). The concentration of such traps is estimated at the level of 1014–1015 cm−1 [13]. Later, the efficient optical recording of dynamic holograms in the bismuth titanate crystals Bi12TiO20 was demonstrated [14].
The sillenite crystals belong to the point group 23, with a body-centered cell without an inversion center that allows for the availability of optical activity, electro-optical, piezoelectric, and inverse flexoelectric effects [15]. It is especially important to note the availability of an inverse flexoelectric effect in the crystals of this group that is shown to a large extent at the distances between the charged regions of the order of 100 nm [16].
The availability of shallow traps with a formation depth ΔWTR ensures the photosensitivity of crystals of the sillenite group in the red and infrared ranges that allows them to be used as the photodetectors in the range from a few THz to 100 THz (0.4 eV).
In the initial state, the optical indicatrix of such crystals is a sphere. This means that the refractive index n is the same in all directions. However, when an external electric field is applied, the sphere is deformed. Depending on the mutual orientation of the crystallographic axes and direction of the applied field E0, such deformations can have various shapes. The purpose of this work is to determine the most suitable sections of a Bi12SiO20 bismuth silicate crystal and to select the appropriate electrode geometry that makes it possible to record an electric signal resulting from the interaction of terahertz radiation and a photodetector.
Selection of crystal orientation
The main optical properties of crystals of the sillenite group are given in Table 1.
The tensors of the electrooptical coefficients r41 and d41 have the same form:
. (1)
We have identified two crystal section orientations suitable for detection of the terahertz radiation (Fig. 2a, b).
As can be seen from the figures, in the first case, the cross section of the optical indicatrix by the sample plane with the circle shape, takes the shape of an ellipse, the x' and y' axes of which are rotated by an angle of 45° relative to the initial position (Fig. 2a). As a result, there are changes in the refractive index. In one case, this will be the value −Δn (along the x' axis); in the other case, it will be the value +Δn (along the y' axis). To use radiation with the ordinary polarization:
(2)
For the section shown in Fig. 2b, the initial cross section of the optical indicatrix changes only its radius when a longitudinal electric field is applied. There is no change in the optical indicatrix shape. In this case, we have only one change in the refractive index along the radial direction Δnrad:
. (3)
The electrooptical and piezoelectric properties of these crystals have the same symmetry and are interconnected.
Illumination of the crystal surface by radiation leads to the photogeneration of electrons from the shallow traps to the conduction band resulting into a redistribution of the space charge inside the crystal. The occurrence of a space charge inside the crystal leads to its local mechanical deformations that develops an electrical voltage on the electrodes deposited on the surface (Fig. 2 c, d). We propose to use two types of electrodes. For the crystal section “a”, we propose to use electrodes in the form of interdigital transducers. The system of such electrodes must be oriented either along the x' axis or along the y' axis. For the crystal section “b”, the system of electrodes must be made in the form of a spiral.
The absolute voltage value that can be tapped off from the electrodes largely depends on the electrode design and the crystal area on which they are deposited. Our estimates show that for the crystal surface area, on which, for example, the electrodes in the form of interdigital converters are applied, the voltage value can be represented in microvolts that is sufficient for further output signal amplification and processing.
Discussion
In this work, we propose to use the shallow trap in the forbidden band of crystals of the sillenite group to detect the terahertz radiation up to 100 THz. Two crystal sections were found, in which the electrooptical effect – piezoelectric effect chain can be used to convert the terahertz radiation into voltage tapped off from the electrodes. For the efficient operation of such a photodetector, it is necessary to apply an additional electric field. It is interesting to note that, depending on the applied field polarity, we will obtain a various sign of the output voltage for the same radiation incident on the photodetector. When the voltage polarity is reversed, the ellipse of optical indicatrix, marked in red in Fig. 2a rotates by 90° in a clockwise direction and becomes elongated along the x’ axis. This feature makes it possible to use not just a constantly applied electric field, but a periodic alternating field. In this case, the photodetector sensitivity can be significantly improved due to the narrow-band filtering of the detected voltage, or application of the lock-in amplifier method.
Another interesting feature is demonstrated by the electrodes made in the form of a spiral. Such electrodes can be successfully applied to detect radiation in the form of optical vortices with axial symmetry. In this case, depending on the sign of the applied field E0, the cross-sectional radius of the optical indicatrix will be changed (either +Δn or −Δn relative to the initial refractive index value) that will also ensure generation of an alternating signal.
In addition, such photodetectors have the significant beam-forming properties in relation to the sensitivity to incident radiation. It is obvious that only the radiation incident on the plate along the [001] or [111] axis will provide the desired interaction with the tensor of electrooptical and piezoelectric coefficients. The radiation directed at an angle to these axes will not contribute to the recorded electrical signal.
AUTHORS
Petrov V. M., Dr. of Physical and Mathematical Sciences (radiophysics), Dr. of Physical and Mathematical Sciences (optics), professor, Department of General Physics - 1, Saint-Petersburg State University, Saint-Petersburg, Russia
ORCID: 0000-0002-8523-0336
Ludnikov G. A., stud , Department of Photonics, National Research University ITMO, Saint-Petersburg, Russia
ORCID: 0009-0004–9436–0118
CONFLICT OF INTEREST
The authors declare that they have no conflict of interest: the scientific mission of the research was determined by Petrov V. M., a professor; in accordance with the mission set, student Ludnikov G. A. performed calculations and simulations to detect radiation in the form of optical vortices with axial symmetry. The study results were discussed and indicated in the manuscript being a joint paper.
Design Aspects
V. M. Petrov 1 , G. A. Ludnikov 2
Saint-Petersburg State University, Saint-Petersburg, Russia
National Research University ITMO, Saint-Petersburg, Russia
The possible application of crystals from the sillenite group (Bi12SiO20, Bi12GeO20, Bi12TiO20) to detect radiation in the terahertz (near infrared) range is considered. Photosensitivity for the spectral range of 3–30 μm is provided by application of the shallow traps located near the bottom of the conduction band. The crystal sections are determined, at which the electro-optical and piezoelectric effects can be used to develop a voltage on the surface electrodes. The electrodes made in the form of an interdigital transducer or a spiral complement the device with new functional capabilities.
Keywords: terahertz sensors, nanoantennas, electro-optical effect, piezoelectric effect
Article received: 02.04.2023
Article accepted: 02.06.2023
Introduction
During the development of highly efficient terahertz nanoantennas one of the objectives is the possible fine tuning of their optical response using the electric fields. For this purpose, it is necessary to select a material that would have both light sensitivity, for example, photoconductivity, and availability of a physical mechanism that can be used to control the antenna response. The electrical tuning capabilities of nanoantennas have been demonstrated for the semiconductor structures due to the density control of charge carriers [1–3], nanoantennas based on a graphene sheet located on any substrate by controlling the dipole moment [4–8], and also due to the nano-mechanical deformations caused by the forces of various nature [9, 10], including the Casimir interaction [11].
The selection of a suitable material for a controlled nanoantenna is not limited to the materials listed above. Thus, back in 1995, the optical data recording in the bismuth silicate crystals Bi12SiO20 was demonstrated in the so-called shallow traps [12], located in the range almost from 0 to 1.5–1.9 eV from the bottom of the conduction band (see Fig. 1). The concentration of such traps is estimated at the level of 1014–1015 cm−1 [13]. Later, the efficient optical recording of dynamic holograms in the bismuth titanate crystals Bi12TiO20 was demonstrated [14].
The sillenite crystals belong to the point group 23, with a body-centered cell without an inversion center that allows for the availability of optical activity, electro-optical, piezoelectric, and inverse flexoelectric effects [15]. It is especially important to note the availability of an inverse flexoelectric effect in the crystals of this group that is shown to a large extent at the distances between the charged regions of the order of 100 nm [16].
The availability of shallow traps with a formation depth ΔWTR ensures the photosensitivity of crystals of the sillenite group in the red and infrared ranges that allows them to be used as the photodetectors in the range from a few THz to 100 THz (0.4 eV).
In the initial state, the optical indicatrix of such crystals is a sphere. This means that the refractive index n is the same in all directions. However, when an external electric field is applied, the sphere is deformed. Depending on the mutual orientation of the crystallographic axes and direction of the applied field E0, such deformations can have various shapes. The purpose of this work is to determine the most suitable sections of a Bi12SiO20 bismuth silicate crystal and to select the appropriate electrode geometry that makes it possible to record an electric signal resulting from the interaction of terahertz radiation and a photodetector.
Selection of crystal orientation
The main optical properties of crystals of the sillenite group are given in Table 1.
The tensors of the electrooptical coefficients r41 and d41 have the same form:
. (1)
We have identified two crystal section orientations suitable for detection of the terahertz radiation (Fig. 2a, b).
As can be seen from the figures, in the first case, the cross section of the optical indicatrix by the sample plane with the circle shape, takes the shape of an ellipse, the x' and y' axes of which are rotated by an angle of 45° relative to the initial position (Fig. 2a). As a result, there are changes in the refractive index. In one case, this will be the value −Δn (along the x' axis); in the other case, it will be the value +Δn (along the y' axis). To use radiation with the ordinary polarization:
(2)
For the section shown in Fig. 2b, the initial cross section of the optical indicatrix changes only its radius when a longitudinal electric field is applied. There is no change in the optical indicatrix shape. In this case, we have only one change in the refractive index along the radial direction Δnrad:
. (3)
The electrooptical and piezoelectric properties of these crystals have the same symmetry and are interconnected.
Illumination of the crystal surface by radiation leads to the photogeneration of electrons from the shallow traps to the conduction band resulting into a redistribution of the space charge inside the crystal. The occurrence of a space charge inside the crystal leads to its local mechanical deformations that develops an electrical voltage on the electrodes deposited on the surface (Fig. 2 c, d). We propose to use two types of electrodes. For the crystal section “a”, we propose to use electrodes in the form of interdigital transducers. The system of such electrodes must be oriented either along the x' axis or along the y' axis. For the crystal section “b”, the system of electrodes must be made in the form of a spiral.
The absolute voltage value that can be tapped off from the electrodes largely depends on the electrode design and the crystal area on which they are deposited. Our estimates show that for the crystal surface area, on which, for example, the electrodes in the form of interdigital converters are applied, the voltage value can be represented in microvolts that is sufficient for further output signal amplification and processing.
Discussion
In this work, we propose to use the shallow trap in the forbidden band of crystals of the sillenite group to detect the terahertz radiation up to 100 THz. Two crystal sections were found, in which the electrooptical effect – piezoelectric effect chain can be used to convert the terahertz radiation into voltage tapped off from the electrodes. For the efficient operation of such a photodetector, it is necessary to apply an additional electric field. It is interesting to note that, depending on the applied field polarity, we will obtain a various sign of the output voltage for the same radiation incident on the photodetector. When the voltage polarity is reversed, the ellipse of optical indicatrix, marked in red in Fig. 2a rotates by 90° in a clockwise direction and becomes elongated along the x’ axis. This feature makes it possible to use not just a constantly applied electric field, but a periodic alternating field. In this case, the photodetector sensitivity can be significantly improved due to the narrow-band filtering of the detected voltage, or application of the lock-in amplifier method.
Another interesting feature is demonstrated by the electrodes made in the form of a spiral. Such electrodes can be successfully applied to detect radiation in the form of optical vortices with axial symmetry. In this case, depending on the sign of the applied field E0, the cross-sectional radius of the optical indicatrix will be changed (either +Δn or −Δn relative to the initial refractive index value) that will also ensure generation of an alternating signal.
In addition, such photodetectors have the significant beam-forming properties in relation to the sensitivity to incident radiation. It is obvious that only the radiation incident on the plate along the [001] or [111] axis will provide the desired interaction with the tensor of electrooptical and piezoelectric coefficients. The radiation directed at an angle to these axes will not contribute to the recorded electrical signal.
AUTHORS
Petrov V. M., Dr. of Physical and Mathematical Sciences (radiophysics), Dr. of Physical and Mathematical Sciences (optics), professor, Department of General Physics - 1, Saint-Petersburg State University, Saint-Petersburg, Russia
ORCID: 0000-0002-8523-0336
Ludnikov G. A., stud , Department of Photonics, National Research University ITMO, Saint-Petersburg, Russia
ORCID: 0009-0004–9436–0118
CONFLICT OF INTEREST
The authors declare that they have no conflict of interest: the scientific mission of the research was determined by Petrov V. M., a professor; in accordance with the mission set, student Ludnikov G. A. performed calculations and simulations to detect radiation in the form of optical vortices with axial symmetry. The study results were discussed and indicated in the manuscript being a joint paper.
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