- 1Experimental Physics 6 and Würzburg-Dresden Cluster of Excellence, Julius Maximilian University of Würzburg, Würzburg, Germany
- 2Helmholtz-Zentrum Dresden-Rossendorf, Institute of Ion Beam Physics and Materials Research, Dresden, Germany
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.
Introduction
Masers are established systems in deep-space communications and navigation due to their application as low-noise amplifiers and oscillators (Gordon et al., 1955). However, their field of application is limited to these niche utilizations because of the required operation conditions such as low temperatures and high vacuum (Benmessai et al., 2008). A new generation of solid-state room-temperature masers can offer a solution here (Arroo et al., 2021). A few years ago, a maser at room-temperature was demonstrated for optically pumped pentacene-doped p-terphenyl (Oxborrow et al., 2012; Breeze et al. 2015; Breeze et al., 2017; Salvadori et al., 2017). Its operating mode is restricted to a pulsed output, as the organic material, for instance, would be damaged at the required high pump power (maser threshold ≈230 W). This was followed by the demonstration of a diamond-based maser using NV-centres where continuous maser operation could be realized (Jin et al., 2015; Breeze et al., 2018). Silicon carbide (SiC) is being considered as another potential material for masers, as it hosts optically addressable high-spin defects and can be manufactured on a large scale in various polytypes due to its industrial importance in high-power electronics (Kraus et al., 2014). If a SiC-based maser succeeds, this technologically mature material system can quickly find its way into everyday applications. Besides the technological advantages of SiC, it also has disadvantages such as shorter spin coherence times or peculiarities related to its spin quartet energy level structure in the case of the VSi defect, which lead to a less efficient pumping scheme (Wimbauer et al., 1997; Carter et al., 2015; Janzén et al., 2009). To overcome these challenges, optimized spin pumping is essential which will be demonstrated in this work. We studied the influence of the excitation wavelength and the orientation of defects in the external magnetic field by means of the population inversion—the figure of merit determined via electron paramagnetic resonance (EPR). Finally, in combination with a high-Q resonator (Q ≈ 104–105), we demonstrate a superradiant amplification of the incident microwaves, analogous to the superluminescence of laser diodes in the optical spectrum, which is a precursor to a functional coherent microwave emitter. Finally, we discuss the missing pieces of the puzzle to realize a SiC maser.
Optical Spin Polarization of V2 in SiC
Silicon carbide possesses a large variety of different spin defects in its crystal lattice (Vainer and Il'in, 1981; Baranov et al., 2005; Son et al., 2006; Koehl et al., 2011). For the present work, the most relevant defect type is the negatively charged silicon vacancy
FIGURE 1. Optical pumping and stimulated emission of the V2 spin defect in 4H SiC: (A) Schematic of the optical excitation cycle leading to a preferential population of the ±1/2 GS sublevels. The application of an external magnetic field leads to a splitting of the energy levels due to the Zeeman effect and results in a population inversion between the −1/2 and the −3/2 states. (B) EPR setup for detecting microwave emission from a laser-pumped SiC sample placed in a resonator. Note that the resonant microwaves also serve to induce stimulated emission. (C) EPR spectrum of V2 in 4H SiC under optical excitation at room-temperature. The low (B−) and high (B+) field EPR transitions show different phase, indicating population inversion under optical excitation, while the signal at B0 is insensitive to light. The bottom inset displays the pump power dependency of the population differences at B− (blue) and B+ (red) in relation to the Boltzmann population (black dashed line).
The prerequisite for the maser is—as for the laser—a population inversion of the states where the stimulated emission can occur. This can be achieved, when the pump rate
In addition to critical parameters such as the decay rate
Electron Paramagnetic Resonance
Figure 1B illustrates the used EPR spectrometer, which is the basic tool to examine the spin population differences (Fischer et al., 2018; Breeze et al., 2018). The SiC sample is placed in a cylindrical dielectric sapphire resonator inside a copper cavity (Q ≈ 17,000 at ambient temperatures) and can be optically pumped using an 808 nm laser. An external magnetic field is applied to tune the Zeeman splitting between the spin sublevels into the range corresponding to the resonance frequency of the resonator. A home-made microwave bridge is used to determine and to optimize the spin polarization of the spin defects. An Anritsu source generates microwaves with a frequency of ≈9.3 GHz that are then attenuated to a sufficiently low power to avoid saturating the transitions (PMW ≈ −80 dBm = 10 pW). The microwaves are guided via a circulator to a loop antenna in the cavity, where they interact with the spin defects. The circulator directs the reflected microwaves to a low-noise amplifier before they are fed into a mixer with phase-adjusted local oscillator. A sinusoidal modulation of the external magnetic field results in a modulated mixer output that is detected by a lock-in amplifier. The resulting EPR signal is the first derivative of the sample’s microwaves absorption spectrum (Figure 1C). The V2 EPR spectrum consists of three features corresponding to the
The figure of merit for characterizing the spin polarization is the population difference
Here, the peak-to-peak amplitudes of the EPR spectra
Wavelength-Dependent Optical Pumping
One way of optimizing the spin polarization of VSi is to vary the wavelength of optical excitation. Obviously, it is more efficient to resonantly excite the spin centre directly at the zero-phonon line (ZPL) absorption instead of in the broad phonon side bands. However, this effect is strongly temperature-dependent. In Figure 2A, PL spectra of 4H SiC are shown for several temperatures in the range of 10–300 K. At cryogenic temperatures, a zoo of sharp ZPLs appears. They can be attributed to different optically active defects, such as Frenkel and Schottky defects, divacancies and silicon vacancies located at different lattice sites (Vainer and Il'in, 1981; Baranov et al., 2005; Son et al., 2006; Koehl et al., 2011). For our EPR studies we focus on the sharp line located at 917 nm which is attributed to the V2 defect (Wagner and Bechstedt, 2000). However, as can be clearly seen in Figure 2A, the ZPL is only detectable at low temperatures and disappears into the phonon background at temperatures between 50 and 100 K.
FIGURE 2. Resonant optical excitation of V2 in 4H SiC: (A). Photoluminescence spectra under 633 nm excitation at different temperatures (10–300 K). At low temperatures V2 emits a sharp ZPL at 917 nm. (B) EPR spectra taken at different excitation wavelengths, as indicated in the legend. (C) Population difference as a function of excitation wavelengths at T = 50 K. The values (circles) are calculated by Eq. 2 using single EPR measurements performed with the respective excitation wavelength (same colour code as in B). The grey trace is the normalized EPR intensity during a sweep of the excitation wavelength. Both measurements show efficient optical pumping for resonant excitation at the ZPL wavelength.
In Figure 2B, we show the dependence of the emissive B+ transition EPR spectrum on different excitation wavelengths in steps of 0.5 nm at T = 50 K and laser power of 10 mW. The highest spin polarization is achieved at 916.5 nm which coincides with the literature value of the ZPL of ca. 917 nm (Wagner and Bechstedt, 2000). Figure 2C shows the resulting population difference, which was calculated by Eq. 2 (Fischer et al., 2018). Each circle represents one EPR measurement with colours corresponding to the spectra in Figure 2B. At a wavelength matching the ZPL, the population inversion increases by a factor of 5. To rule out the possibility of additional features appearing between the individual measurements, we performed EPR measurements at a fixed magnetic field (B+) while the wavelength of the laser excitation was continuously adjusted (grey trace). The profile coincides perfectly with the single measurements and does not show any additional features. Thus, we conclude that at low temperatures, resonant optical pumping at the ZPL is particularly efficient.
At higher temperatures, the ZPL of V2 vanishes and the effect of resonant excitation can be neglected, suggesting using photons with higher energy than the ZPL (Stokes excitation). Anti-Stokes excitation, e.g., above 1,000 nm exploiting phonons is also conceivable, but its advantages, such as high ODMR contrast, are expected only well above room temperatures, making such excitation more suitable for realizing a maser at T > 300 K (Wang et al., 2021). To take advantage of a prolonged spin-lattice relaxation time and thus high spin polarization, we will continue to search for an optimal set of physical parameters at temperatures below room temperature and thus using Stokes excitation (808 nm).
Influence of SiC Crystal Orientation in the Magnetic Field
Another parameter that strongly affects the spin polarization is the polar angle
Here,
FIGURE 3. Anisotropy of V2 in 4H SiC: (A) Room temperature EPR spectra with fits (solid traces) by Eq. 3 for different angles between the external field
One possible candidate would be the magnetic field-dependent Boltzmann distribution given by
Previous works have shown that the spin interactions of NV− defects in diamond are also anisotropic and that the maximum polarization is achieved when the defect axis
With
Combination of Optimized Optical Pumping With a High Q-Factor Resonator
To enhance the interaction of microwave photons with the spin system in a resonator, a higher Q-factor is required. Thereby the Q-factor describes the dampening or energy losses and therefore can only be increased by reducing the losses of the standing waves in the resonator. This can be accomplished by reducing coupling losses using an undercoupled resonator and, additionally, by cooling the system to reduce the ohmic losses in the copper walls. At T = 110 K we thus reach a Q-factor of 40,000–110,000 by varying the depth of the coupling loop in the resonator. Another practical side effect of low temperatures is the prolonged spin lattice relaxation time T1 leading to a lower required optical pump power (Simin et al., 2017).
To investigate this in more detail, we examine an EPR spectrum for a Q-factor of 84,000 as shown in Figure 4A. We choose the optimal sample orientation from Figure 3
FIGURE 4. Stimulated emission and signs of superradiance for optimized settings: (A) EPR spectrum at T = 110 K for a high-Q resonator (Q ≈ 84,000). The emissive transition B+ is much more intense compared to the absorptive transition B−. (B) EPR intensity of the emissive transition B+ for different Q-factors. (C) Zoomed-in region of the EPR spectrum to illustrate the HF structure of the emissive transition (experimental data in red with black dotted fit). The expected trace based on the natural abundance of 29Si is shown in grey. The experimentally observed intensity of the main peak deviates strongly from the expectations and is 4 times more intense than expected according to the isotope ratio of silicon nuclei. (D) Population difference and linewidth as a function of laser pump power. Circles represent the main peak of the B+ (red) and B− (blue) EPR transitions, while crosses represent the values for the HF satellites. The B+ linewidth collapses by a factor of 0.5 upon optical pumping and also the population difference of B+ vastly exceeds that of B−. The increased stimulated emission together with the linewidth collapse are signs of superradiance.
A remarkable result is the tremendous population asymmetry between the absorbing B− and emitting B+ transition
In order to emphasize that we are just surpassing the onset of superradiance, we examine the hyperfine structure of the zoomed-in region around B+ as shown in Figure 4C. We will use the different natural abundances of silicon isotopes to show that the onset is clearly surpassed for V2 centres with just spin-less nuclei in their vicinity, but just barely for the lower number N of V2 interacting with spin-bearing Si isotopes. As pointed out earlier, the EPR transitions B+ or B− are not single lines but have two equally separated satellites that emerge due to HF interactions of the electron spin of V2 centres with one 29Si, I = 1/2 nuclear spin each. The central peak, however, is due to V2 centres that have only spin-less isotopes in their vicinity (28Si, 30Si with I = 0). To remind, the three major stable silicon isotopes, 28Si, 29Si and 30Si have abundances of 92.2%, 4.7%, and 3.1%, respectively (Meija et al., 2016). Consequently, the probability of having only spin-less isotopes as the 12 silicon neighbours of a specific V2 centre is (92.2% + 3.1%)12 = 56.1%. The probability of having just one out of 12 being 29Si is 12 4.7% (100%–4.7%)11 = 33.2%, while the remaining 10.7% of V2 centres have multiple 29Si neighbours. The additional satellite features of the latter are weaker and do not contribute significantly to the observed spectrum. The influence of spin-bearing 13C isotopes (I = 1/2) with abundance of just 1.1% is also not taken into account. The ratio of 56.1%/33.2% is therefore the basis of the simulated EPR spectra in Figure 1C that is also added as the dashed grey trace in Figure 4C. The fixed isotope abundances thus lead to a predictable intensity ratio of the central peak in comparison to the satellites. However, the central emission transition is much more dominant than expected (approximately factor of 4). This discrepancy between the ratio of signal amplitudes
The ratio of the spin numbers is given by the isotope ratio
Furthermore, varying the applied laser power P results in additional indications of superradiance as shown in Figure 4D. The effect on the population difference can be described by:
with
Furthermore, for superradiance a narrowing of the EPR linewidth due to monochromatization of the microwave photons is also expected. An analysis of the extracted linewidth is shown in Figure 4D (bottom). The observed EPR linewidth is typically in the range of 45 μT for the absorptive B− transition (blue symbols). In contrast, the linewidth of the emissive B+ transition changes by a factor of 0.5 with rising pump power (red circles), while this effect is less pronounced for the HF peaks (red crosses).
All three observations, the superlinear behaviour of microwave photon emission intensity with the number of involved spin centres N, the increased population difference and the spectral narrowing of the microwave emission upon optical pumping are strong indicators of superradiance and, at the same time, evidence that we have almost reached the maser threshold with our SiC-based quantum system.
Conclusion
In summary, on the way to the realization of a SiC-based maser, we have experimentally identified several physical parameters which have a strong impact on the spin polarization and, consequently, on the population inversion. They are related to the properties of the spin system itself as well as to the quality of the resonator hosting the crystal. Investigating the dependence of the spin polarisation on the excitation wavelength, we found that at low temperature (T = 50 K) the spin defect can be efficiently pumped via resonant excitation into the ZPL resulting in a higher population difference. At higher temperatures, i.e., when the ZPL has vanished, a Stokes excitation (λexc > λZPL) is sufficient. Whether anti-Stokes excitation, where the sample temperature exceeds 300 K due to pump heating, would be more suitable for a potential SiC maser requires further investigation. Furthermore, we have analysed the influence of the spin defect orientation in the external magnetic field
Materials and Methods
Silicon Carbide Samples
The investigated defects were created by 2 MeV electron irradiation (3–10 ⋅ 1017 cm−2) of a 4H SiC wafer (Kasper et al., 2020). In this work we used two different samples: First, a small wafer piece (5.9 mm3) with a defect density of 5.68 ⋅ 1014 cm−3. This sample is ideal to study the influence of excitation wavelength and defect orientation since it fits into standard EPR spectrometers (measurements shown in Figures 2, 3). Furthermore, we investigated a stack of SiC wafers (34.3 mm3) with a higher defect density of 2.27 ⋅ 1015 cm−3 for a larger ensemble of ≈7.8 ⋅ 1013 V2 spin defects (Figures 1,4).
Electron Paramagnetic Resonance
The EPR measurements were performed in three different setups with different advantages: The measurements for Figures 1, 4 were performed in a self-built EPR spectrometer as illustrated in Figure 1B. Here, we were able to perform measurements on a large ensemble in a customized high-Q resonator. Measurements for Figure 2B were performed in a modified Bruker spectrometer (E300) because the cavity (Bruker ER4104OR) possesses the best optical access for the tuneable laser (Sacher Lion 920). In order to perform angle-dependent measurements (see Figure 3) we used a commercial benchtop spectrometer (Bruker Magnettech ESR5000) with a motorized precision goniometer.
Photoluminescence Spectroscopy
The PL measurements (Figure 2A) were performed with a commercial LabRAM HR800 (Horiba) confocal micro Raman spectrometer with 633 nm laser excitation. The temperature was varied by a MicrostatHe (Oxford Instruments) cold finger cryostat.
Data Availability Statement
The raw data supporting the conclusion of this article will be made available by the authors, without undue reservation.
Author Contributions
AG and SS performed the wavelength and orientation dependent EPR studies. CK and AS determined optimum irradiation conditions. VB designed the microwave resonator. MK designed and realized the home-made spectrometer together with AG and AS. MW performed theoretical calculations of the relevant parameters affecting the maser threshold. MW and MK contributed equally to this work. AG, GA, VD, and AS wrote the manuscript. AS conceived the idea and supervised the project. All authors discussed the results and commented on the manuscript.
Funding
This publication was supported by the Open Access Publication Fund of the University of Wuerzburg and the German Science Foundation (DFG) under the project DY 18/13-1.
Conflict of Interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Publisher’s Note
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Keywords: stimulated emission, maser, population inversion, silicon vacancy, spin polarization, superradiance
Citation: Gottscholl A, Wagenhöfer M, Klimmer M, Scherbel S, Kasper C, Baianov V, Astakhov GV, Dyakonov V and Sperlich A (2022) Superradiance of Spin Defects in Silicon Carbide for Maser Applications. Front. Photonics 3:886354. doi: 10.3389/fphot.2022.886354
Received: 28 February 2022; Accepted: 18 April 2022;
Published: 16 May 2022.
Edited by:
Mehran Kianinia, University of Technology Sydney, AustraliaReviewed by:
Kasturi Saha, Indian Institute of Technology Bombay, IndiaJonas Nils Becker, Michigan State University, United States
Copyright © 2022 Gottscholl, Wagenhöfer, Klimmer, Scherbel, Kasper, Baianov, Astakhov, Dyakonov and Sperlich. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Vladimir Dyakonov, dyakonov@physik.uni-wuerzburg.de; Andreas Sperlich, sperlich@physik.uni-wuerzburg.de