Jury

 Summary:

The development of THz transmission technologies quickly came up against numerous limitations, particularly in terms of bandwidth and transmission power. The conventional technologies used until now are no longer suitable for applications requiring very wide bandwidths without losses. In addition, THz polarisation control is generally not easy to implement, despite the many potential applications in ellipsometry or telecommunications. This manuscript presents the emergence of THz spintronic transmitters, which use the spin's degree of freedom to produce a very broadband and uninterrupted THz emission over a range that can exceed 30 THz. Using the inverse spin Hall effect, these transmitters generate linear pulsed THz waves whose polarisation can be easily controlled by manipulating the magnetisation in the devices. These properties, hitherto little exploited, have led to the development of new device architectures to improve control. The standard structure of these emitters, a nanometric stack of W|FeCoB|Pt, is generally polarised and oriented using permanent magnetic fields, requiring mechanical rotation to cause the polarisation to rotate. By creating a device with magnetic layers coupled by exchange interactions, it is possible to induce uniaxial magnetic anisotropy in the structure. This new emitter configuration, whose active layers are then W|FeCo|TbCo2|FeCo|Pt, enables polarisation to be controlled over 360° using a scalar fixed magnetic field, according to the Stoner-Wohlfarth model. However, this principle of polarisation control, although innovative, still requires the use of strong magnetic fields, generally produced by bulky electromagnets. The magnetic structure developed previously also turns out to be a magneto-elastic configuration, sensitive to mechanical stress. So, by coupling this new structure with a piezoelectric element, it is possible to achieve magnetoelectric control of THz spintronic transmitters. The development of an emitter with uniaxial anisotropy deposited on a PMN-PT piezoelectric substrate has made it possible to establish THz polarisation control over a range of 90°, solely by applying an electric field to the device. Until now, polarisation control has been static. Dynamic polarisation control would therefore be a clear advantage in ellipsometry or communications systems. For example, it is possible to modulate the polarisation in transmitters using sinusoidal magnetic fields generated by coils. This modulation is also facilitated by involving the Spin Reorientation Transition in the anisotropic structures, to drastically increase magnetic sensitivity and enable modulation at a record frequency of 10 MHz. Finally, these spintronic emitters are integrated into optical and THz cavities in order to mitigate the main shortcoming of these emitters, their general lack of efficiency compared with standard THz generation methods. In addition, the emitters are also deposited on optical fibres, so that they can be easily integrated and used on conventional optical systems. This integration also brings its own set of challenges, both in terms of manufacture, but also when used in high-power regimes.

Abstract:

The development of THz transmission technologies quickly came up against numerous limitations, particularly in terms of bandwidth and transmission power. The conventional technologies used until now are no longer suitable for applications requiring very wide bandwidths without losses. In addition, THz polarisation control is generally not easy to implement, despite the many potential applications in ellipsometry or telecommunications. This manuscript presents the emergence of THz spintronic transmitters, which use the spin's degree of freedom to produce a very broadband and uninterrupted THz emission over a range that can exceed 30 THz. Using the inverse spin Hall effect, these transmitters generate linear pulsed THz waves whose polarisation can be easily controlled by manipulating the magnetisation in the devices. These properties, hitherto little exploited, have led to the development of new device architectures to improve control. The standard structure of these emitters, a nanometric stack of W|FeCoB|Pt, is generally polarised and oriented using permanent magnetic fields, requiring mechanical rotation to cause the polarisation to rotate. By creating a device with magnetic layers coupled by exchange interactions, it is possible to induce uniaxial magnetic anisotropy in the structure. This new emitter configuration, whose active layers are then W|FeCo|TbCo2|FeCo|Pt, enables polarisation to be controlled over 360° using a scalar fixed magnetic field, according to the Stoner-Wohlfarth model. However, this principle of polarisation control, although innovative, still requires the use of strong magnetic fields, generally produced by bulky electromagnets. The magnetic structure developed previously also turns out to be a magneto-elastic configuration, sensitive to mechanical stress. So, by coupling this new structure with a piezoelectric element, it is possible to achieve magnetoelectric control of THz spintronic transmitters. The development of an emitter with uniaxial anisotropy deposited on a PMN-PT piezoelectric substrate has made it possible to control THz polarisation over a range of 90°, solely by applying an electric field to the device. Until now, polarisation control has been static. Dynamic polarisation control would therefore be a clear advantage in ellipsometry or communications systems. For example, it is possible to modulate the polarisation in transmitters using sinusoidal magnetic fields generated by coils. This modulation is also facilitated by involving the Spin Reorientation Transition in the anisotropic structures, to drastically increase magnetic sensitivity and enable modulation at a record frequency of 10 MHz. Finally, these spintronic emitters are integrated into optical and THz cavities in order to mitigate the main shortcoming of these emitters, their general lack of efficiency compared with standard THz generation methods. In addition, the emitters are also deposited on optical fibres, so that they can be easily integrated and used on conventional optical systems. This integration also brings its own set of challenges, both in terms of manufacture, but also when used in high-power regimes.