rus
Issue #7/2023
A. S. Kadochkin, V. V. Amelichev, S. S. Generalov, D. V. Gorelov
Study of Integrated Optical Switch for Development of Logical Element Controlled by the Thermal Influence on Ge2Sb2Te5 Layer (GST)
Study of Integrated Optical Switch for Development of Logical Element Controlled by the Thermal Influence on Ge2Sb2Te5 Layer (GST)
DOI: 10.22184/1993-7296.FRos.2023.17.7.556.564
The alternating phase layer Ge2Sb2Te5 (GST) demonstrates a significant refractive index difference between the amorphous (a-GST) and crystalline (c-GST) states. The fast and reversible phase transition between the two states allows for high speed and thermal stability of integrated optical devices. The paper presents a study of an integrated optical switch based on an annular microresonator with a superimposed GST layer controlled by the thermal influence. Such switches may be used in the optical circuits to implement logic functions.
The alternating phase layer Ge2Sb2Te5 (GST) demonstrates a significant refractive index difference between the amorphous (a-GST) and crystalline (c-GST) states. The fast and reversible phase transition between the two states allows for high speed and thermal stability of integrated optical devices. The paper presents a study of an integrated optical switch based on an annular microresonator with a superimposed GST layer controlled by the thermal influence. Such switches may be used in the optical circuits to implement logic functions.
Теги: gst layer integrated optical switch logical element microresonator optical resonance интегрально-оптический ключ логический элемент микрорезонатор оптический резонанс слой gst
Study of Integrated Optical Switch for Development of Logical Element Controlled by the Thermal Influence on Ge2Sb2Te5 Layer (GST)
A. S. Kadochkin, V. V. Amelichev, S. S. Generalov, D. V. Gorelov Research and Production Complex “Technological Center”, Zelenograd, Moscow, Russia
The alternating phase layer Ge2Sb2Te5 (GST) demonstrates a significant refractive index difference between the amorphous (a-GST) and crystalline (c-GST) states. The fast and reversible phase transition between the two states allows for high speed and thermal stability of integrated optical devices. The paper presents a study of an integrated optical switch based on an annular microresonator with a superimposed GST layer controlled by the thermal influence. Such switches may be used in the optical circuits to implement logic functions.
Key words: integrated optical switch, microresonator, GST layer, logical element, optical resonance
Article received: 04.10.2023
Article accepted: 31.10.2023
Introduction
One of the promising ways of information technology development is the use of optical methods for information signal processing. The optical methods have numerous advantages over the electronic ones, such as signal propagation velocity, possible multiplexing and parallel data processing. To implement the optoelectronic devices designed for optical data processing, it is required to develop both the passive elements of integrated optical circuits, such as the waveguides, splitters, polarizing filters, passive logical elements, and the active logical elements controlled by the external influences, including the optical switches being an optical analogue of transistors operating in the signal switching mode.
The paper [1] proposed the implementations of such devices using the carrier injection mechanism to change the states in an annular resonator using an external influence. In some way or another, such an influence is based on a change in the efficient refractive index of the mode propagating in the resonator, and can be performed using the charge carrier injection [2], electro-optical [3] or magneto-optical effects [4]. There are a number of papers [5] that propose an application of so-called phase-changing materials. In [6], to change the efficient mode refractive index of a silicon microresonator, it is proposed to use Ge2Sb2Te5 (GST) material applied to a section of an annular microresonator, used in the form of GST thin films in the optical data recording technology. This GST application is due to the possible ultrafast reversible phase transformations (<50 ns) in this material that occur between the amorphous (a-GST) and crystalline (c-GST) states and are accompanied by a significant change in the optical properties and electrical resistivity [1]. The refractive index (n) and extinction coefficient (k) represent the main optical specifications of any material. In thin GST films, these coefficient values for a-GST and c-GST are widely different (Figure 1).
Analysis of optical switch
with the GST coating
Figure 2 shows a general view of the studied element of a switch-type integrated optical circuit based on an annular microresonator with a generate GST layer. The figure insert demonstrates distribution of the TE0 mode in the waveguide cross section with a superimposed layer of c-GST and a-GST. The paper [6] reports on the experimental implementation of controlled annular microresonators based on silicon (Si). In contrast to the paper [6] in paper [7] silicon nitride (Si3N4) has been selected as the resonator material.
At present, there are various materials for development of the integrated photonics elements, each of which has its own benefits and considerations related to the presence or absence of the necessary specifications required to solve specific problems. The selection of Si3N4 is due to the wide operating spectral range of wavelengths, good mechanical properties, low intrinsic losses (less than 0.3 dB/cm), absence of two-photon absorption in the telecommunications wavelength range that makes it possible to achieve the high optical power density inside the waveguide. Figure 3 shows the schematic view of the studied annular microresonator with a superimposed 90° GST segment. The element is an annular microresonator made of Si3N4 generated on a silicon oxide (SiO2) substrate. A 90° GST layer segment (red) is placed on top of the annular microresonator and can be in the a-GST or c-GST state. In this case, the heating process can be performed depending on the design requirements, either using an external optical source (impulse laser) or an electric heater located on top.
The ability to control the microresonator mode properties is based on the fact that with the used resonator thickness (d1 = 300 nm) and wavelength (1.55 μm), the propagating mode cannot be completely localized in the resonator’s cross section; a significant part of its energy is localized outside its cross section in the form of evanescent (exponentially decaying) “tails”. Therefore, a GST layer placement in this region with the optical properties that are widely different from the optical properties of the waveguide and its surrounding environment leads to a change in the efficient refractive index of the waveguide modes. It should also be noted that c-GST has the significant absorption properties, whereas a-GST has close to zero absorption at the telecommunications wavelength around 1.55 μm (Figure 1). Thus, the microresonator with a superimposed a-GST segment (Figure 2) can maintain a high-Q resonance, and after switching to c-GST, the efficient mode refractive index obtains a significant imaginary part and the resonance is suppressed. In this case, the signal at the microresonator outputs is changed significantly that makes it possible to use such a device as a switch. The geometric parameters of the microresonator model are given in Table.
A finite difference time domain (FDTD) method is used to simulate the Si3N4 microresonator with a GST control element. The simulated structure parameters are given in Table 1. Figures 4 and 5 demonstrate the simulation results. Figure 4 shows distribution of the electric field strengths |E| for TE0 mode in the annular microresonator. Figure 5 demonstrates dependence of the signal reflection factor from the annular microresonator (transmittance factor T12) on the wavelength.
It can be seen on Figure 5 that the highest reflection coefficient value from the annular microresonator T12 in the case of a-GST is 35%, while the reflection from the annular resonator in the case of c-GST tends towards zero.
Impact assessment
of the GST overlay segment size
Similar calculations have been performed for the GST 180 and 360 degree segments (Figures 6–8 and 9–11).
Results
and discussion
The obtained dependencies shown in Figures 4, 7 and 10 allow to draw the following conclusion: in the case of c-GST (Figures 4a, 7a, 10a) the transmission of the output port No.2 is close to zero; in the case of a-GST (Figures 4b, 7b, 10b) the transmission of the output port No. 2 has a resonant nature (the same as in the absence of GST layer) and reaches a value of several tens of percent. This appears to be due to the strong absorption in the c-GST material (see Figure 4) and low absorption in a-GST.
In addition, a dependence on the GST segment size has been obtained (Figures 5, 8, 11) – a larger segment size (angular measure in degrees) provides better transmission (up to 100% in the case of a full 360‑degree segment) for a-GST. This appears to be due to the mode dissipation of annular microresonator on the GST segment placed on top of the resonator. This dissipation is decreased as the segment increases and is completely absent in the case of a full ring, allowing complete transmission (see Figure 11). Thus, a full ring (a 360‑degree segment) provides better contrast between logical “0” and “1” at the output port No.2.
Conclusion
The novelty of this paper is selection of the Si3N4 film in combination with a GST film as the resonator material. This selection is due to the wide operating spectral wavelength range, good mechanical properties of Si3N4, low intrinsic losses (less than 0.3 dB/cm) and absence of two-photon absorption in the telecommunications wavelength range. By changing the phase state of the GST film, it is possible to switch the output port No. 2 signal (i. e., there is a clearly distinguishable logical “0” and logical “1”). The enhanced throughput of the output port No.2 as the GST segment size is increased improves the discrimination between logical “0” and logical “1”.
A positive property of GST material for application in the switch-type device is significant absorption for c-GST and nearly total absence of absorption for a-GST. This fact ensures the resonance quenching in the annular microresonator in the first case and availability of a resonance condition in the second case (practically the same as in the microresonator without a GST coating). In the first case, this provides the lack of transmission, and in the second case, it provides high transmission. Switching between the GST status is performed by heating (laser, electrical heating), is reversible and stable over time.
Thus, as a result of simulation, it has been established that the application of a GST film over the microresonator region of a Si3N4 film makes it possible to implement the functions of logical elements.
Acknowledgment
This article has been prepared with the financial support of the Ministry of Education and Science of the Russian Federation as a part of the research work FNRM‑2022-0007.
Information about the authors
A. S. Kadochkin, Cand. of Sciences (Phyth.&Math.), senior researcher of the microsystems engineering department of the Federal State Budgetary Scientific Institution “Scientific and Production Complex “Technological Center”, Moscow, Zelenograd, Russia.
ORCID: 0000-0002-7960-1583
V. V. Amelichev, Cand. of Sciences (Eng.), head of the microsystems engineering department of the “Scientific and Production Complex “Technological Center”, Moscow, Zelenograd, Russia.
ORCID: 0000-0002-4204-2626
S. S. Generalov, head of the research laboratory for nano- and micromechanical systems, microsystems engineering department of the “Scientific and Production Complex “Technological Center”, Moscow, Zelenograd, Russia.
ORCID: 0000-0002-7455-7800
D. V. Gorelov, head of the research laboratory for integrated microcircuits, microsystems engineering department of the “Scientific and Production Complex “Technological Center” Moscow, Zelenograd, Russia.
ORCID: 0000-0002-0887-9406
Author contributions
The article has been prepared based on the work of all authoring team members: A. S. Kadochkin – provision of calculations, collected data analysis, processing and discussion; V. V. Amelichev – work arrangement, discussion; S. S. Generalov – work arrangement, discussion; D. V. Gorelov – search and translation of foreign references, discussion.
Conflict of interest
The authors declare no conflict of interest. All authors have participated in the manuscript preparation in terms of their contribution to the work and agree with the full text of the manuscript.
A. S. Kadochkin, V. V. Amelichev, S. S. Generalov, D. V. Gorelov Research and Production Complex “Technological Center”, Zelenograd, Moscow, Russia
The alternating phase layer Ge2Sb2Te5 (GST) demonstrates a significant refractive index difference between the amorphous (a-GST) and crystalline (c-GST) states. The fast and reversible phase transition between the two states allows for high speed and thermal stability of integrated optical devices. The paper presents a study of an integrated optical switch based on an annular microresonator with a superimposed GST layer controlled by the thermal influence. Such switches may be used in the optical circuits to implement logic functions.
Key words: integrated optical switch, microresonator, GST layer, logical element, optical resonance
Article received: 04.10.2023
Article accepted: 31.10.2023
Introduction
One of the promising ways of information technology development is the use of optical methods for information signal processing. The optical methods have numerous advantages over the electronic ones, such as signal propagation velocity, possible multiplexing and parallel data processing. To implement the optoelectronic devices designed for optical data processing, it is required to develop both the passive elements of integrated optical circuits, such as the waveguides, splitters, polarizing filters, passive logical elements, and the active logical elements controlled by the external influences, including the optical switches being an optical analogue of transistors operating in the signal switching mode.
The paper [1] proposed the implementations of such devices using the carrier injection mechanism to change the states in an annular resonator using an external influence. In some way or another, such an influence is based on a change in the efficient refractive index of the mode propagating in the resonator, and can be performed using the charge carrier injection [2], electro-optical [3] or magneto-optical effects [4]. There are a number of papers [5] that propose an application of so-called phase-changing materials. In [6], to change the efficient mode refractive index of a silicon microresonator, it is proposed to use Ge2Sb2Te5 (GST) material applied to a section of an annular microresonator, used in the form of GST thin films in the optical data recording technology. This GST application is due to the possible ultrafast reversible phase transformations (<50 ns) in this material that occur between the amorphous (a-GST) and crystalline (c-GST) states and are accompanied by a significant change in the optical properties and electrical resistivity [1]. The refractive index (n) and extinction coefficient (k) represent the main optical specifications of any material. In thin GST films, these coefficient values for a-GST and c-GST are widely different (Figure 1).
Analysis of optical switch
with the GST coating
Figure 2 shows a general view of the studied element of a switch-type integrated optical circuit based on an annular microresonator with a generate GST layer. The figure insert demonstrates distribution of the TE0 mode in the waveguide cross section with a superimposed layer of c-GST and a-GST. The paper [6] reports on the experimental implementation of controlled annular microresonators based on silicon (Si). In contrast to the paper [6] in paper [7] silicon nitride (Si3N4) has been selected as the resonator material.
At present, there are various materials for development of the integrated photonics elements, each of which has its own benefits and considerations related to the presence or absence of the necessary specifications required to solve specific problems. The selection of Si3N4 is due to the wide operating spectral range of wavelengths, good mechanical properties, low intrinsic losses (less than 0.3 dB/cm), absence of two-photon absorption in the telecommunications wavelength range that makes it possible to achieve the high optical power density inside the waveguide. Figure 3 shows the schematic view of the studied annular microresonator with a superimposed 90° GST segment. The element is an annular microresonator made of Si3N4 generated on a silicon oxide (SiO2) substrate. A 90° GST layer segment (red) is placed on top of the annular microresonator and can be in the a-GST or c-GST state. In this case, the heating process can be performed depending on the design requirements, either using an external optical source (impulse laser) or an electric heater located on top.
The ability to control the microresonator mode properties is based on the fact that with the used resonator thickness (d1 = 300 nm) and wavelength (1.55 μm), the propagating mode cannot be completely localized in the resonator’s cross section; a significant part of its energy is localized outside its cross section in the form of evanescent (exponentially decaying) “tails”. Therefore, a GST layer placement in this region with the optical properties that are widely different from the optical properties of the waveguide and its surrounding environment leads to a change in the efficient refractive index of the waveguide modes. It should also be noted that c-GST has the significant absorption properties, whereas a-GST has close to zero absorption at the telecommunications wavelength around 1.55 μm (Figure 1). Thus, the microresonator with a superimposed a-GST segment (Figure 2) can maintain a high-Q resonance, and after switching to c-GST, the efficient mode refractive index obtains a significant imaginary part and the resonance is suppressed. In this case, the signal at the microresonator outputs is changed significantly that makes it possible to use such a device as a switch. The geometric parameters of the microresonator model are given in Table.
A finite difference time domain (FDTD) method is used to simulate the Si3N4 microresonator with a GST control element. The simulated structure parameters are given in Table 1. Figures 4 and 5 demonstrate the simulation results. Figure 4 shows distribution of the electric field strengths |E| for TE0 mode in the annular microresonator. Figure 5 demonstrates dependence of the signal reflection factor from the annular microresonator (transmittance factor T12) on the wavelength.
It can be seen on Figure 5 that the highest reflection coefficient value from the annular microresonator T12 in the case of a-GST is 35%, while the reflection from the annular resonator in the case of c-GST tends towards zero.
Impact assessment
of the GST overlay segment size
Similar calculations have been performed for the GST 180 and 360 degree segments (Figures 6–8 and 9–11).
Results
and discussion
The obtained dependencies shown in Figures 4, 7 and 10 allow to draw the following conclusion: in the case of c-GST (Figures 4a, 7a, 10a) the transmission of the output port No.2 is close to zero; in the case of a-GST (Figures 4b, 7b, 10b) the transmission of the output port No. 2 has a resonant nature (the same as in the absence of GST layer) and reaches a value of several tens of percent. This appears to be due to the strong absorption in the c-GST material (see Figure 4) and low absorption in a-GST.
In addition, a dependence on the GST segment size has been obtained (Figures 5, 8, 11) – a larger segment size (angular measure in degrees) provides better transmission (up to 100% in the case of a full 360‑degree segment) for a-GST. This appears to be due to the mode dissipation of annular microresonator on the GST segment placed on top of the resonator. This dissipation is decreased as the segment increases and is completely absent in the case of a full ring, allowing complete transmission (see Figure 11). Thus, a full ring (a 360‑degree segment) provides better contrast between logical “0” and “1” at the output port No.2.
Conclusion
The novelty of this paper is selection of the Si3N4 film in combination with a GST film as the resonator material. This selection is due to the wide operating spectral wavelength range, good mechanical properties of Si3N4, low intrinsic losses (less than 0.3 dB/cm) and absence of two-photon absorption in the telecommunications wavelength range. By changing the phase state of the GST film, it is possible to switch the output port No. 2 signal (i. e., there is a clearly distinguishable logical “0” and logical “1”). The enhanced throughput of the output port No.2 as the GST segment size is increased improves the discrimination between logical “0” and logical “1”.
A positive property of GST material for application in the switch-type device is significant absorption for c-GST and nearly total absence of absorption for a-GST. This fact ensures the resonance quenching in the annular microresonator in the first case and availability of a resonance condition in the second case (practically the same as in the microresonator without a GST coating). In the first case, this provides the lack of transmission, and in the second case, it provides high transmission. Switching between the GST status is performed by heating (laser, electrical heating), is reversible and stable over time.
Thus, as a result of simulation, it has been established that the application of a GST film over the microresonator region of a Si3N4 film makes it possible to implement the functions of logical elements.
Acknowledgment
This article has been prepared with the financial support of the Ministry of Education and Science of the Russian Federation as a part of the research work FNRM‑2022-0007.
Information about the authors
A. S. Kadochkin, Cand. of Sciences (Phyth.&Math.), senior researcher of the microsystems engineering department of the Federal State Budgetary Scientific Institution “Scientific and Production Complex “Technological Center”, Moscow, Zelenograd, Russia.
ORCID: 0000-0002-7960-1583
V. V. Amelichev, Cand. of Sciences (Eng.), head of the microsystems engineering department of the “Scientific and Production Complex “Technological Center”, Moscow, Zelenograd, Russia.
ORCID: 0000-0002-4204-2626
S. S. Generalov, head of the research laboratory for nano- and micromechanical systems, microsystems engineering department of the “Scientific and Production Complex “Technological Center”, Moscow, Zelenograd, Russia.
ORCID: 0000-0002-7455-7800
D. V. Gorelov, head of the research laboratory for integrated microcircuits, microsystems engineering department of the “Scientific and Production Complex “Technological Center” Moscow, Zelenograd, Russia.
ORCID: 0000-0002-0887-9406
Author contributions
The article has been prepared based on the work of all authoring team members: A. S. Kadochkin – provision of calculations, collected data analysis, processing and discussion; V. V. Amelichev – work arrangement, discussion; S. S. Generalov – work arrangement, discussion; D. V. Gorelov – search and translation of foreign references, discussion.
Conflict of interest
The authors declare no conflict of interest. All authors have participated in the manuscript preparation in terms of their contribution to the work and agree with the full text of the manuscript.
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