Linewidth Measurements Research Articles

Photophysics of Intrinsic Single‐Photon Emitters in Silicon Nitride at Low Temperatures

AbstractA robust process for fabricating intrinsic single‐photon emitters in silicon nitride is recently established. These emitters show promise for quantum applications due to room‐temperature operation and monolithic integration with technologically mature silicon nitride photonics platforms. Here, the fundamental photophysical properties of these emitters are probed through measurements of optical transition wavelengths, linewidths, and photon antibunching as a function of temperature from 4.2 to 300 K. Important insight into the potential for lifetime‐limited linewidths is provided through measurements of inhomogeneous and temperature‐dependent broadening of the zero‐phonon lines. At 4.2 K, spectral diffusion is found to be the main broadening mechanism, while spectroscopy time series reveal zero‐phonon lines with instrument‐limited linewidths.

Effect of the optical power factors on the laser-linewidth measurements.

In this paper, the effects of optical power factors like laser power, the powers of the laser beams in the two arms of the optical system, and the power of the photodetector on laser-linewidth measurements are studied. From the experiments, it can be found that when the average optical input power for the photodetector is about 50% of its linear saturation power, the measured laser line width is a minimum. When the optical powers of the laser beams in the two arms are equal in short-delay self-homodyne system, the measured laser line width is narrowest. In the low output power range of the laser, its line width decreases with the increase in optical power. By comparing experiments, it can also be clear that the conventional measurement method is seriously affected by different noise types, which causes the measured line width to become wider and not change even if the laser linewidth changes. However, based on the short-delay coherent envelope method, the measured coherent envelope changes significantly when the laser line width changes slightly, and its corresponding laser-linewidth values are also clearly visible. It confirms the low noise and high resolution of the short-delay self-homodyne coherent-envelope laser-measurement method. The outcomes of this study can provide helpful information for precision ultra-narrow laser-linewidth measurements.

Are Nonthermal Velocities in Active Region Coronal Loops Anisotropic?

We have measured line widths in active region coronal loops in order to determine whether the nonthermal broadening is anisotropic with respect to the magnetic field direction. These nonthermal velocities are caused by unresolved fluid motions. Our analysis method combines spectroscopic data and a magnetic field extrapolation. We analyzed spectra from the Extreme Ultraviolet Imaging Spectrometer on Hinode. A differential emission measure analysis showed that many spectral lines that are commonly considered to be formed in the active region have a substantial contribution from the background quiet Sun. From these spectra we identified lines whose emission was dominated by the active region loops rather than background sources. Using these lines, we constructed maps of the nonthermal velocity. With data from the Helioseismic Magnetic Imager on the Solar Dynamics Observatory and the Coronal Modeling System nonlinear force-free magnetic field reconstruction code, we traced several of the magnetic field lines through the active region. Comparing the spectroscopic and magnetic data, we looked for correlations of the nonthermal velocity with the viewing angle between the line of sight and the magnetic field. We found that nonthermal velocities show a weak anticorrelation with the viewing angle. That is, the tendency is for the nonthermal velocity to be slightly larger in the parallel direction. This parallel broadening may be due to acoustic waves or unresolved parallel flows.

Using elliptical galaxy kinematics to compare the strength of gravity in cosmological regions of differing gravitational potential – a first look

ABSTRACT Various models of modified gravity invoke ‘screening’ mechanisms that are sensitive to the value of the local gravitational potential. This could have observable consequences for galaxies. These consequences might be seen by comparing two proxies for galaxy mass – their luminosity and their internal kinematics – as a function of local galaxy density. Motivated by this prospect, we have compared the observed properties of luminous red galaxies (LRGs) inside and outside of voids in the cosmic large scale structure. We used archival measurements of line widths, luminosities, redshifts, colours, and positions of galaxies in conjunction with recent void catalogues to construct comparison LRG samples inside and outside of voids. We fitted these two samples to the well-established fundamental plane of elliptical galaxies to constrain any differences between the inferred value of the Newtonian gravitational constant G for the two samples. We obtained a null result, with an upper limit on any fractional difference in G within and outside of cosmological voids to be α = δG/G ∼ 40 per cent. This upper bound is dominated by the small-number statistics of our N ∼ 100 within-void LRG sample. With the caveat that environmental effects could influence various parameters such as star formation, we estimate that a 1 per cent statistical limit on α could be attained with data from 105 elliptical galaxies within voids. This is within the reach of future photometric and spectroscopic surveys, both of which are required to pursue this method.

Kalman filtering-enhanced short-delay self-heterodyne interferometry for linewidth measurement.

We demonstrate an extended Kalman filtering-enhanced linewidth measurement in short-delay self-heterodyne interferometry (SDSHI). We found that a modified SDSHI trace closely resembles a biased cosine wave, which would enable convenient linewidth estimation by its uniform envelope contrast without any correction factor. Experimentally, we adopted this approach for kHz laser linewidth measurement, taking advantages of extended Kalman filtering (EKF) to adaptively track the cosine wave. Apart from the measurement noise suppression, this approach could use as many data points as possible in the noisy trace to make a linewidth estimation at each tracked data point, from which we can deduce valuable statistical parameters such as the mean and standard deviation. This approach involves no more equipment than conventional SDSHI and sophisticated EKF so that it can be easily implemented. Therefore, we believe it will find wide applications in ultra-narrow laser linewidth measurement.

Accurate measurements of the ferromagnetic resonance linewidth of single crystal BaM hexaferrite spheres employing magnetic plasmon resonance theory

Accurate measurements of the intrinsic ferromagnetic linewidth of a single-crystal Sc-doped hexagonal M-type barium ferrite (BaM) 308-µm-diameter sphere at millimeter-wave frequencies employing a subwavelength cavity are described. The magnetic hysteresis of the resonance frequency and Q-factor of the dominant resonance (which has a plasmonic character) in a hexaferrite sphere are reported. Measurements performed in a fixture open to free space also show that the ferromagnetic linewidth significantly increases as compared to measurements in a closed cavity. This effect can be attributed to radiation losses, as revealed by an electrodynamic magnetic plasmon resonance model, and is relevant for measurements in open structures, such as microstrip lines or coplanar waveguides. The impact of finite resistivity on the linewidth was also analyzed.

Achieving High Stability of Dual-Wavelength U-Shaped Laser in the C-Band Using Optical Feedback

Dual-wavelength lasers are attractive for various fields such as terahertz generation and high-precision spectroscopy, particularly when exhibiting narrow-linewidth performance. This study focuses on linewidth measurements of a monolithically-integrated InP dual-wavelength laser emitting in the telecom C-band. Linewidths below 3 MHz are demonstrated for both lines, which reduce to values below 40 kHz after introducing optical feedback from both sides of the laser cavity. The device has a small form-factor and is made with mature technologies, and as such has direct applications in the aforementioned fields.

Accurate evaluation of self-heterodyne laser linewidth measurements using Wiener filters.

Self-heterodyne beat note measurements are widely used for the experimental characterization of the frequency noise power spectral density (FN-PSD) and the spectral linewidth of lasers. The measured data, however, must be corrected for the transfer function of the experimental setup in a post-processing routine. The standard approach disregards the detector noise and thereby induces reconstruction artifacts in the reconstructed FN-PSD. We introduce an improved post-processing routine based on a parametric Wiener filter that is free from reconstruction artifacts, provided a good estimate of the signal-to-noise ratio is supplied. Building on this potentially exact reconstruction, we develop a new method for intrinsic laser linewidth estimation that is aimed at deliberate suppression of unphysical reconstruction artifacts. Our method yields excellent results even in the presence of strong detector noise, where the intrinsic linewidth plateau is not even visible using the standard method. The approach is demonstrated for simulated time series from a stochastic laser model including 1/f-type noise.

Revisiting Emission-line Measurement Methods for Narrow-line Active Galactic Nuclei

Measuring broad emission-line widths in active galactic nuclei (AGN) is not straightforward owing to the complex nature of flux variability in these systems. Line width measurements become especially challenging when the signal-to-noise ratio is low, profiles are narrower, or spectral resolution is low. We conducted an extensive correlation analysis between emission-line measurements from the optical spectra of Markarian 142 (Mrk 142; a narrow-line Seyfert galaxy) taken with the Gemini North Telescope (Gemini) at a spectral resolution of 185.6 ± 10.2 km s−1 and the Lijiang Telescope (LJT) at 695.2 ± 3.9 km s−1 to investigate the disparities in the measured broad-line widths from both telescopes’ data. Due to its narrow broad-line profiles, which were severely affected by instrumental broadening in the lower-resolution LJT spectra, Mrk 142 posed a challenge. We discovered that allowing the narrow-line flux of permitted lines having broad and narrow components to vary during spectral fitting caused a leak in the narrow-line flux to the broad component, resulting in broader broad-line widths in the LJT spectra. Fixing the narrow-line flux ratios constrained the flux leak and yielded the Hβ broad-line widths from LJT spectra ∼54% closer to the Gemini Hβ widths than with flexible narrow-line ratios. The availability of spectra at different resolutions presented this unique opportunity to inspect how spectral resolution affected emission-line profiles in our data and adopt a unique method to accurately measure broad-line widths. Reconsidering line measurement methods while studying diverse AGN populations is critical for the success of future reverberation-mapping studies. Based on the technique used in this work, we offer recommendations for measuring line widths in narrow-line AGN.

Evolution of the thermodynamic properties of a coronal mass ejection in the inner Corona

The thermodynamic evolution of Coronal Mass Ejections (CMEs) in the inner corona (≤1.5 Rsun) is not yet completely understood. In this work, we study the evolution of thermodynamic properties of a CME core observed in the inner corona on 20 July 2017, by combining the MLSO/K-Cor white-light and the MLSO/CoMP Fe XIII 10747 Å line spectroscopic data. We also estimate the emission measure weighted temperature (TEM) of the CME core by applying the Differential Emission Measure (DEM) inversion technique on the SDO/AIA six EUV channels data and compare it with the effective temperature (Teff) obtained using Fe XIII line width measurements. We find that the Teff and TEM of the CME core show similar variation and remain almost constant as the CME propagates from ∼1.05 to 1.35 Rsun. The temperature of the CME core is of the order of million-degree kelvin, indicating that it is not associated with a prominence. Further, we estimate the electron density of this CME core using K-Cor polarized brightness (pB) data and found it decreasing by a factor of ∼3.6 as the core evolves. An interesting finding is that the temperature of the CME core remains almost constant despite expected adiabatic cooling due to the expansion of the CME core, which suggests that the CME core plasma must be heated as it propagates. We conclude that the expansion of this CME core behaves more like an isothermal than an adiabatic process.

Measurement and numerical analysis of intrinsic spectral linewidths of photonic-crystal surface-emitting lasers

Photonic-crystal surface-emitting lasers (PCSELs) feature high-power coherent lasing over a large area, which are potentially suitable for various applications requiring narrow spectral linewidths. In this paper, we experimentally and theoretically investigate intrinsic spectral linewidths of PCSELs. We first measure the frequency noise spectra of a fabricated PCSEL with a 250-μm lasing diameter and realize single-mode lasing with an intrinsic spectral linewidth below 70 kHz. To investigate the feasibility of narrower spectral linewidths in PCSELs, we next perform the theoretical analysis of intrinsic spectral linewidths of PCSELs by a time-dependent three-dimensional coupled-wave analysis considering carrier–photon interactions as well as thermal effects. We reveal that intrinsic spectral linewidths below 1 kHz can be obtained with a 500-μm-diameter PCSEL by reducing the cavity loss and by compensating the temperature-induced band-edge-frequency distribution.

Microfabricated strontium atomic vapor cells.

We demonstrate strontium (Sr) atomic vapor cells having a total external volume of 0.63 cm3 that can operate above 300 °C for times exceeding 380 h. The cells are fabricated using micromachined silicon frames anodically bonded to glass windows that have a 20-nm thick protective layer of Al2O3 deposited on the interior surfaces. The presence of Sr vapor in the cell is confirmed through laser absorption spectroscopy for the 1S0 → 1P1 transition in Sr at 461 nm. Measurements of sub-Doppler linewidths indicated negligible (<3 MHz) broadening of this transition from residual background gas collisions. This compact and manufacturable, high-temperature atomic vapor cell can enable narrow-line optical frequency references based on strontium and other alkaline earth species.

A Width Measurement Method of Line Shape Based on Second Harmonic Peak and Modulation Amplitude.

The line width of different line shapes is a very important parameter in absorption spectroscopy sensing techniques. Based on the high sensitivity and low noise properties of wavelength modulation spectroscopy, we report a novel line width measurement method. After theoretically proving the relationship between line width, modulation amplitude and the amplitude of the second harmonic at the center frequency, the absorption lines of CH4 near 6046.96 cm-1 and CO2 4989.97 cm-1 were chosen for simulation, and the relative errors of the line width between our method and theoretical data were kept at about 1%. A distributed feedback laser diode operating near 1653 nm with three different concentrations of CH4 was used for experimental validation, and the results were consistent with the numerical simulation. Additionally, since only the peaks of second harmonic need to be measured, the advantages of wavelength modulation can be utilized while reducing the difficulty of data acquisition.

Linewidth Measurement of Visible Single-Frequency Laser Based on Short-Fiber Delay Self-Heterodyne

Linewidth Measurement of Visible Single-Frequency Laser Based on Short-Fiber Delay Self-Heterodyne

Convolution Error Reduction for a Fabry–Pérot-Based Linewidth Measurement: A Theoretical and Experimental Study

Linewidth measurement of a short pulse single-longitudinal mode laser with a low repetition rate has been a big challenge. Although the Fabry–Pérot (FP) etalon in combination with a beam profiler is an effective approach to measure the linewidth, the convolution error introduced by the inherent transmission spectrum width of an FP restricts the measurement accuracy. Here, the source of convolutional errors of the FP etalon-based linewidth measurement is analyzed, and the convolutional fitting method is proposed to reduce the errors. The results show that the linewidth measurement using the FP cavity with low reflectance (95%) can achieve the same resolution as that with high reflectance (99.5%) based on this convolution error reduction method. The study provides a simple approach to accurately measuring the linewidth of pulsed lasers, even with low energy.

Opto-mechanical material characterization via dual-geometry Brillouin confocal microscopy

Brillouin scattering is a light-matter interaction that induces a frequency shift in the scattered photons. Such shift directly depends on the optical and mechanical properties of the material itself. In the last decade, Brillouin frequency shift has become a reliable proxy for mechanical stiffness in biological samples such as cells and tissues, with the subsequent development of Brillouin confocal microscopy; however, such correlation does not apply to samples with high degree of inhomogeneity. Recently, we showed that a dual geometry Brillouin microscopy enables the direct measurement of refractive index and speed of sound. We now report an improved dual-geometry microscopy technique that allows measurement of optoacoustical properties within a confocal volume. This goal has been achieved through the measurement of frequency, linewidth, and intensity of Brillouin components of the scattered light spectrum. Finally, we proved our three-dimensional imaging capability by mapping, with micron-scale resolution, the refractive index, density, and viscoelastic properties of a liquid-liquid phase separation system. This technique can be applied to a multitude of biological heterogenous scenarios such as nucleolus characterization, high concentration lipid sites, dynamic protein aggregates and more.

A Lorentzian narrow-linewidth demodulation scheme based on a short fiber delayed self-heterodyne technique

We propose a narrow-linewidth demodulation method which utilizes a short-fiber-delayed self-heterodyne structure and coherent envelope spectral properties to restore the Lorentzian line shape of the laser output. The Lorentzian spectrum obtained using our scheme is consistent with that obtained using a traditional long-delay method, and here, the utilization of a short fiber avoids the broadening caused by 1/f noise. Both simulated and experimental results demonstrate that our scheme is effective and accurate. We demonstrate the recovery of a Lorentzian linewidth of 6.0 kHz, while maintaining higher accuracy than that achieved using traditional schemes (31.7 kHz). Our approach provides a feasible means of improving the accuracy and computational efficiency of narrow linewidth measurements.

Superradiance and Exciton Delocalization in Perovskite Quantum Dot Superlattices.

Achieving superradiance in solids is challenging due to fast dephasing processes from inherent disorder and thermal fluctuations. Perovskite quantum dots (QDs) are an exciting class of exciton emitters with large oscillator strength and high quantum efficiency, making them promising for solid-state superradiance. However, a thorough understanding of the competition between coherence and dephasing from phonon scattering and energetic disorder is currently unavailable. Here, we present an investigation of exciton coherence in perovskite QD solids using temperature-dependent photoluminescence line width and lifetime measurements. Our results demonstrate that excitons are coherently delocalized over 3 QDs at 11 K in superlattices leading to superradiant emission. Scattering from optical phonons leads to the loss of coherence and exciton localization to a single QD at temperatures above 100 K. At low temperatures, static disorder and defects limit exciton coherence. These results highlight the promise and challenge in achieving coherence in perovskite QD solids.

Dealing with Large Gaps in Asteroseismic Time Series

With long data sets available for asteroseismology from space missions, it is sometimes necessary to deal with time series that have large gaps. This is becoming particularly relevant for TESS, which is revisiting many fields on the sky every two years. Because solar-like oscillators have finite mode lifetimes, it has become tempting to close large gaps by shifting time stamps. Using actual data from the Kepler Mission, we show that this results in artificial structures in the power spectrum that compromise the measurements of mode frequencies and linewidths.

Analyzing and measuring the diode laser’s linewidth affected by the driving current’s white noise

It is important to study the effect of injection current’s noise on the diode laser’s linewidth. In this paper, the effect of the white noise applied to the injection current on the linewidth of diode laser is investigated theoretically and experimentally. Theoretically, by adding the non-Markov noise term into the Langevin equation, the output laser’s linewidth of the diode laser with the white noise current is obtained. In experiment, an optical Fabry–Perot (F–P) cavity with a linewidth of ∼ 1.83 M H z and a finesse of ∼ 136 is employed to measure a 1560.5 nm distributed-feedback (DFB)-type diode laser’s linewidth. With an increase of the white noise intensity, the laser linewidth is broadened from ∼ 2 to ∼ 60 M H z gradually, and the line shape is characterized evolving from Lorentzian function to Gaussian function. At the same level of white noise intensity, the laser linewidth decreases with increase of the injection current. The accuracy of our measurements is verified based on the fiber-delayed and acousto-optic modulator (AOM)-shifted self-heterodyne scheme and the optical F–P cavity linewidth measurement method. The 1560.5 nm laser is boosted by using an erbium-doped fiber amplifier (EDFA) and is frequency doubled to 780.25 nm by a fiber-pigtailed, single-pass PPMgO:LN waveguide module, and the laser linewidth is visually observed and verified by broadening of the saturated absorption spectra of rubidium atoms. Clearly this linewidth manipulation method of diode lasers based on broadband white noise current coupling can be extended to other wavelength diode lasers and can be applied to laser measurement, laser communication, and other fields for different requirements.