Stimulated Microwave Emission Research Articles

Superradiance of Spin Defects in Silicon Carbide for Maser Applications

Masers as telecommunication amplifiers have been known for decades, yet their application is strongly limited due to extreme operating conditions requiring vacuum techniques and cryogenic temperatures. Recently, a new generation of masers has been invented based on optically pumped spin states in pentacene and diamond. In this study, we pave the way for masers based on spin S = 3/2 silicon vacancy (VSi) defects in silicon carbide (SiC) to overcome the microwave generation threshold and discuss the advantages of this highly developed spin hosting material. To achieve population inversion, we optically pump the VSi into their mS = ±1/2 spin sub-states and additionally tune the Zeeman energy splitting by applying an external magnetic field. In this way, the prerequisites for stimulated emission by means of resonant microwaves in the 10 GHz range are fulfilled. On the way to realising a maser, we were able to systematically solve a series of subtasks that improved the underlying relevant physical parameters of the SiC samples. Among others, we investigated the pump efficiency as a function of the optical excitation wavelength and the angle between the magnetic field and the defect symmetry axis in order to boost the population inversion factor, a key figure of merit for the targeted microwave oscillator. Furthermore, we developed a high-Q sapphire microwave resonator (Q ≈ 104–105) with which we find superradiant stimulated microwave emission. In summary, SiC with optimized spin defect density and thus spin relaxation rates is well on its way of becoming a suitable maser gain material with wide-ranging applications.

Highly Efficient Optical Pumping of Spin Defects in Silicon Carbide for Stimulated Microwave Emission

Microwaves (MWs) are at the heart of many technologies, $e.g.$ for communication, timekeeping, remote sensing, and quantum information processing. Masers can boost faint MW signals quite nicely, but not under real-world conditions. The authors demonstrate efficient population inversion of optically pumped vacancy-related spins in SiC, at room temperature, and find realistic operating conditions for a SiC maser to serve as a continuous-wave amplifier. These spins can also be coherently coupled to superconducting cavities efficiently at cryogenic temperatures. These findings suggest this SiC system as a promising platform for MW photonics and quantum electronics.

Assessment of Transcapillary Water Exchange in the Lungs by Active Radiometry

The possibility of noninvasive assessment of transcapillary water exchange in the lungs using stimulated microwave emission of the chest organs is discussed. The results of the study showed that the distribution of 1 GHz microwave emission of tissues probed using low-intensity electromagnetic waves at 65 GHz corresponded to the distribution of pulmonary blood flow. Calculations showed that the intensity of transcapillary water exchange in the lungs was distributed exponentially from the apices to the basal parts of the lungs. The distribution of the transcapillary water exchange was shown to vary according to the pulmonary perfusion gradient, which is indicative of a correlation between the parameters of pulmonary perfusion and the intensity of stimulated microwave emission of tissues at the frequency of 1 GHz. This correlation can be used for noninvasive assessment of pulmonary perfusion by active radiometry.

Spin Centres in SiC for Quantum Technologies

Atomic-scale colour centres in bulk and nanocrystalline SiC are promising for quantum information processing, photonics and sensing at ambient conditions. Their spin state can be initialized, manipulated and readout by means of optically detected magnetic resonance. It has been shown that there are at least two families of colour centres in SiC with S = 1 and S = 3/2, which have the property of optical alignment of the spin levels and allows a spin manipulation. The ground state and the excited state were demonstrated to have spin S = 3/2 and a population inversion in the ground state can be generated using optical pumping, leading to stimulated microwave emission even at room temperature. By controlling the neutron irradiation fluence, the colour centres concentration can be varied over several orders of magnitude down to a single defect level. Several, separately addressable spin centres have been identified in the same crystal for each polytype, which can be used either for magnetic field or temperature sensing. Some of these spin centres are characterised by nearly temperature independent zero-field splitting, making these centres very attractive for vector magnetometry. Contrarily, the zero-field splitting of the centres in the excited state exhibits a giant thermal shift, which can be used for thermometry applications. Finally coherent manipulation of spin states has been performed at room temperature and even at temperatures higher by hundreds of degrees. SiC is taking on a new role as a flexible and practical platform for harnessing the new quantum technologies.

Stimulated emission at transitions between Wannier–Stark ladders in semiconductor superlattices

New intraband semiconductor lasers—Wannier–Stark lasers—based on simple GaAs (150 A, quantum well)/GaAlAs (19 A with an aluminum fraction of 12%, barrier) superlattices have been demonstrated. The amplification mechanism in these lasers is based on population inversion between the ground Wannier–Stark level in the superlattice quantum wells and the weakly populated upper Wannier–Stark level in the wells two, three, or four periods down in the applied potential. Multiple regions of intense stimulated microwave emission near voltages of 8, 13, and 20 V (i.e., in the vicinity of resonances between these Wannier–Stark levels of the superlattice) have been discovered in the laser chips. The stimulated emission emerges in the circuit formed by the chip and its wiring. The emission from one of the chips at a temperature of up to 150 K (near 20 V applied to the chip) occurs at a frequency of about 7.3 GHz and has an estimated power of up to 1 W. It has been shown that the negative conductivity responsible for the emission still persists at 300 K but the emission is unseen owing to high losses in the circuit at this temperature. The superlattice wafer has been grown by metalorganic chemical vapor deposition. It consists of 1000 periods and a stop layer, to produce a metal–superlattice–metal terahertz resonator. Terahertz radiation has not been observed owing to a low amplification, as compared to losses in the resonator. According to the performed experiments, calculations, and discussions, such superlattices as radiation sources in gigahertz, terahertz, and higher frequency ranges could compete with quantum cascade lasers under appropriate optimization of their parameters.

Spin transfer stimulated microwave emission in MgO magnetic tunnel junctions

Measurements of noise properties of MgO-based magnetic tunnel junctions were performed in the frequency range from 100MHzto18GHz. A pronounced narrow peak at the frequency of around 7GHz with a full width at half-maximum as low as 21MHz has been observed. The peak amplitude and 1∕f noise have a threshold dependence on the dc bias current. The narrow peak is present only at one current polarity for an antiparallel orientation of the magnetic layers adjacent to the barrier, which is consistent with spin-momentum transfer theoretical predictions.

Stimulated microwave emission from E x B drifting electrons in slow-wave cavities: A quantum approach.

Stimulated microwave emission from E\ifmmode\times\else\texttimes\fi{}B drifting electrons in slow-wave cavities occurs when the Doppler-shifted radiation frequency is either near zero or the electron cyclotron frequency. The former case, characterized by the synchronous drift velocity u, \ensuremath{\omega}-ku\ensuremath{\simeq}0, corresponds to the ``pure drift'' instability, while the latter, satisfying \ensuremath{\omega}-ku\ensuremath{\simeq}\ifmmode\pm\else\textpm\fi{}\ensuremath{\Omega}, is termed the ``drift-cyclotron'' mode. In both cases the drift kinetic energy and momentum are invariant during radiative transitions. The momentum of the emitted or absorbed radiation quantum comes from the vector potential associated with the static magnetic field and induces a shift of the electron guiding center in a direction transverse to the drift velocity. In the pure drift case the radiation energy comes from the change in the electrostatic potential energy. In the drift-cyclotron case both electrostatic and cyclotron rotation energies are converted into radiation. In the nonrelativistic regime the gain is symmetric with respect to the frequency detuning from resonance. The difference between the stimulated absorption and emission probabilities, responsible for the gain, is caused by field gradients across the direction of the electron drift. These gradients come from the waveguide mode structure and the collective field of the electron beam. The drift mode is always unstable, while there exists one stable and one unstable drift-cyclotron branch. Relativistic mass effects influence only the drift-cyclotron instability, adding a gain contribution that is antisymmetric in frequency detuning.

Quantum theory of stimulated microwave emission from E×B drifting electrons

The quantum theory of stimulated microwave emission from electrons drifting in crossed electric and magnetic fields inside slow wave cavities is considered. The energy and momentum of the emitted radiation quanta come from the electron electrostatic and vector potentials, respectively. The kinematic momentum and energy are invariant during the transition. This recoilless emission explains the gain symmetry relative to the resonant frequency. The difference in absorption and emission rates responsible for the gain is caused by the transverse gradients in the waveguide mode structure and the collective beam field. The per-pass gain formula in the low-gain (nonexponential) regime is obtained and found to agree with recent classical results. >

Erratum: ‘‘Nonsingular linear theory for stimulated microwave emission in crossed-field devices’’ [Phys. Fluids B 5, 4162 (1993)

Erratum: ‘‘Nonsingular linear theory for stimulated microwave emission in crossed-field devices’’ [Phys. Fluids B <b>5</b>, 4162 (1993)

Nonsingular linear theory for stimulated microwave emission in crossed-field devices

The small signal stability of crossed-field devices fed by a thin electron beam is analyzed. The situation differs from diocotron modes in that the interaction cavity supports slow wave eigenmodes in vacuum. The rippling of the beam causes a modification of the vacuum dispersion relation and mode profiles. The growth rate is found by equating the rate of change of the power flux with the fast scale averaged wave–particle energy exchange rate. The radio frequency (rf) power flow including the energy circulating in the anode structure is related to the wave amplitude via the interaction impedance. The singularities at resonance, the trademark of any linear theory, are avoided by following the particle guiding center (GC) orbits in reference frame with the wave synchronous. The small signal gain is found by expansion in powers of the rf amplitude. A finite linear growth results, even for symmetric particle excursions, due to the self-field of the rippled beam. Near resonance the growth rate is independent of the detuning between the phase and drift velocities. Higher-order contributions to the instability are caused by the nonlinear bunching of the GC distribution in space and are symmetric relative to resonance. Symmetric frequency response is a unique feature of crossed-field devices (CFD’s) opposed to the antisymmetric growth characterizing other ‘‘unbound electron’’ devices [gyrotrons, free-electron lasers (FEL’s), traveling-wave tubes (TWT’s)]. The growth rate goes over to the diocotron growth when all the impedance comes from the rippled beam (i.e., smooth anode at infinite distance). The perturbed mode profiles are nonsingular; the profile singularities of standard linear theories reflect the singularities of the particle orbits at resonance.