Cavity Bandwidth Research Articles

Deepest sensitivity to wavelike dark photon dark matter with superconducting radio frequency cavities

Wavelike, bosonic dark matter candidates like axions and dark photons can be detected using microwave cavities known as haloscopes. Traditionally, haloscopes consist of tunable copper cavities operating in the TM010 mode, but ohmic losses have limited their performance. In contrast, superconducting radio frequency (SRF) cavities can achieve quality factors of ∼1010, perhaps 5 orders of magnitude better than copper cavities, leading to more sensitive dark matter detectors. In this paper, we first derive that the scan rate of a haloscope experiment is proportional to the loaded quality factor QL, even if the cavity bandwidth is much narrower than the dark matter halo line shape. We then present a proof-of-concept search for dark photon dark matter using a nontunable ultrahigh-quality SRF cavity. We exclude dark photon dark matter with kinetic mixing strengths of χ>1.5×10−16 for a dark photon mass of mA′=5.35 μeV, achieving the deepest exclusion to wavelike dark photons by almost an order of magnitude. Published by the American Physical Society 2024

Retracted: Calibration of superconducting radio-frequency cavity forward and reflected channels based on stored energy dynamics

Modern superconducting radio-frequency linear accelerators require the cavity bandwidth and detuning to be within a specified range to maximize the efficiency of the machine. To correctly estimate these states during operation, the measured RF signals should be calibrated. Due to the finite isolation of the waveguide directional couplers, cross-coupling effects in the forward and reflected channels complicate the calibration of the RF signals in Low-Level RF control systems. Past work proposed a compensation method employing least-squares optimization. This method requires the directivity of the directional couplers to be much higher than one. Additionally, the algorithm requires a tuning parameter to optimize the calculated calibration. However, for some accelerating systems, finding an acceptable value for such a parameter is challenging and time-consuming. In this paper, we present a way to overcome these limitations by performing a nonlinear least square optimization constrained by energy conservation laws. The method is tested with L-band superconducting resonators at loaded quality factors of 4.6⋅106 and 2.8⋅107.

Feedforward resonance control for the European X-ray free electron laser high duty cycle upgrade

The High Duty Cycle (HDC) upgrade is a proposed improvement to the existing European X-ray Free Electron Laser (EuXFEL) to extend the pulsed RF duty factor from the actual value of around 1% to more than 5% up to Continuous Wave (CW). To implement this upgrade, the loaded quality factor (QL) of the superconducting cavities will increase by more than one order of magnitude. This will result in shrinking the cavity bandwidth to values as low as a few Hertz. Since the Lorentz Force Detuning (LFD) experienced during the accelerating field buildup is of hundreds of Hertz, the Low-Level RF (LLRF) system has to accurately track and control the cavity resonance frequency to obtain the desired accelerating gradient. Moreover, ponderomotive instabilities have to be suppressed to achieve stability during beam acceleration. Since LFD is a repetitive disturbance in cavity frequency, the correction to its effects can be implemented as a feedforward compensation on the piezoelectric tuners of the cavity. Initial results on the simulation of feedforward resonance control in the HDC regime are discussed in this proceeding.

Measurement of the refractive index at cryogenic temperature of absorptive silver thin films used as reflectors in a Fabry-Perot cavity.

Data on the refractive index of silver thin films are scarce in the literature, and largely dependent on both the deposition method and thickness. We measure the refractive index of silver films at cryogenic temperature with a technique that takes advantage of the absorption of the films and the corresponding peculiar properties of Fabry-Perot cavities: a frequency shift between the reflection and transmission peaks, together with a modified cavity bandwidth. We demonstrate a decrease in the real value of the refractive index, together with a decrease in its imaginary value at 4 K.

Online Detuning Computation and Quench Detection for Superconducting Resonators

Superconducting cavities are responsible for beam acceleration in superconducting linear accelerators. Challenging cavity control specifications are necessary to reduce radio frequency (RF) costs and to maximize the availability of the accelerator. Cavity detuning and bandwidth are two critical parameters to monitor when operating particle accelerators. Cavity detuning is strongly related to the power required to generate the desired accelerating gradient. Cavity bandwidth is related to the cavity RF losses. A sudden increase in bandwidth can indicate the presence of a quench or multipacting event. Therefore, calculating these parameters in real time in the low-level RF (LLRF) system is highly desirable. A real-time estimation of the bandwidth allows for a faster response of the machine protection system in the case of quench events, whereas the estimation of cavity detuning can be used to drive piezoelectric tuner-based resonance control algorithms. In this article, a new field programmable gate array (FPGA)-based estimation component is presented. Such a component is designed to be used either in continuous wave (CW) or pulsed operation mode with loaded quality factors between $10^{6}$ and $10^{8}$ . Results of this component with free-electron LASer in Hamburg (FLASH), European X-ray free electron laser (EuXFEL), cryo module test bench (CMTB), and electron linac for beams with high brilliance and low emittance (ELBE) are presented.

Atomic line versus lens cavity filters: a comparison of their merits

We present a comparison between lens cavity filters and atomic line filters, discussing their relative merits for applications in quantum optics. We describe the design, characterization, and stabilization procedure of a lens cavity filter, which consists of a high-reflection coated commercially available plano-convex lens, and compare it to an ultra-narrow atomic band-pass filter utilizing the D2 absorption line in atomic rubidium vapor. We find that the cavity filter peak transmission frequency and bandwidth can be chosen arbitrarily but the transmission frequency is subject to thermal drift and the cavity needs stabilization to better than a few mK, while the atomic filter is intrinsically stable and tied to an atomic resonance frequency such that it can be used in a non-laboratory environment.

Bismuth plasmonics for extraordinary light absorption in deep sub-wavelength geometries.

In this Letter, we demonstrate an ultra-broadband metamaterial absorber of unrivaled bandwidth (BW) using extraordinary optical response of bismuth (Bi), which is the material selected through our novel analysis. Based on our theoretical model, we investigate the maximum metal-insulator-metal (MIM) cavity BW, achievable by any metal with known n-k data. We show that an ideal metal in such structures should have a positive real permittivity part in the near-infrared (NIR) regime. Contrary to noble and lossy metals utilized by most research groups in the field, this requirement is satisfied only by Bi, whose data greatly adhere to the ideal material properties predicted by our analysis. A Bi nanodisc-based MIM resonator with an absorption above 0.9 in an ultra-broadband range of 800 nm-2390 nm is designed, fabricated, and characterized. To the best of our knowledge, this is the broadest absorption BW reported for a MIM cavity in the NIR with its upper-to-lower absorption edge ratio exceeding best contenders by more than 150%. According to the findings in this Letter, the use of proper materials and dimensions will lead to realization of deep sub-wavelength efficient optical devices.

Overcoming the efficiency-bandwidth tradeoff for optical harmonics generation using nonlinear time-variant resonators

Highly resonant photonic structures, such as cavities and metasurfaces, can dramatically enhance the efficiency of nonlinear processes by utilizing strong optical field enhancement at the resonance. The latter, however, comes at the expense of the bandwidth. Here, we overcome such tradeoff by utilizing time-varying resonant structures. Using harmonics generation as an example, we show that the amplitude and phase format of the excitation, as well as the time evolution of the resonator, can be optimized to yield the strongest nonlinear response. We find the conditions for an efficient synthesis of electromagnetic signals that surpass the cavity bandwidth, and discuss a potential experimental realization of this concept.

Investigating tunable bandwidth cavity via three-level atomic systems

We propose a scheme for intracavity electromagnetically induced transparency and white light cavity via three-level Ladder-type Rb atoms. The system is driven by coherent and incoherent fields. Due to the position dependent atom-field interaction, the tunable optical susceptibility of the probe field can be achieved. By using an incoherent pump field and choosing proper parameters, one can control dispersion behavior of the probe field. In weak probe field limit, cavity bandwidth narrowing and broadening could be controlled via atomic systems in different conditions. Assuming the intracavity electromagnetic-induced transparency and the white light cavity conditions, it’s possible to control the susceptibility to satisfy the resonance condition over a wide frequency range. Tuning and controlling bandwidth of the optical cavity may find interesting applications in investigating cavity-QED phenomena and designing novel all-optical devices such as optical switches.

Resonance frequency tuning of an RF cavity through sliding mode extremum seeking

Tuning of the natural resonance frequency of an RF cavity is crucial to achieve maximum power transfer from the cavity to the beam, to reduce power requirements, and to guarantee a predictable particle speed gain within each accelerator structure. So far, most operational cavities are frequency tuned through a phase comparison technique, which is sensitive to temperature variations if the facility or signal transportation cables are not temperature controlled. Temperature induced phase errors render this technique labor and time intensive. This paper presents an RF cavity tuning scheme, based on reflected power measurements and sliding mode extremum seeking control, to eliminate the phase measurement related difficulties. A stability analysis provides conditions on how to choose the controller parameters to guarantee stable system operation up to twice the cavity bandwidth, which is a controllable bandwidth improvement of a factor of two. The system has been implemented in two of TRIUMF’s (Canada’s particle accelerator centre) room temperature resonators and long term measurements show that the system effectively counteracts the RF heating effect as well as diurnal temperature variations.

Fundamental limitations of cavity-assisted atom interferometry

Atom interferometers employing optical cavities to enhance the beam splitter pulses promise significant advances in science and technology, notably for future gravitational wave detectors. Long cavities, on the scale of hundreds of meters, have been proposed in experiments aiming to observe gravitational waves with frequencies below 1 Hz, where laser interferometers, such as LIGO, have poor sensitivity. Alternatively, short cavities have also been proposed for enhancing the sensitivity of more portable atom interferometers. We explore the fundamental limitations of two-mirror cavities for atomic beam splitting, and establish upper bounds on the temperature of the atomic ensemble as a function of cavity length and three design parameters: the cavity g-factor, the bandwidth, and the optical suppression factor of the first and second order spatial modes. A lower bound to the cavity bandwidth is found which avoids elongation of the interaction time and maximizes power enhancement. An upper limit to cavity length is found for symmetric two-mirror cavities, restricting the practicality of long baseline detectors. For shorter cavities, an upper limit on the beam size was derived from the geometrical stability of the cavity. These findings aim to aid the design of current and future cavity-assisted atom interferometers.

The use of composite pulses for improving DEER signal at 94 GHz

The sensitivity of pulsed electron paramagnetic resonance (EPR) measurements on broad-line paramagnetic centers is often limited by the available excitation bandwidth. One way to increase excitation bandwidth is through the use of chirp or composite pulses. However, performance can be limited by cavity or detection bandwidth, which in commercial systems is typically 100–200MHz. Here we demonstrate in a 94GHz spectrometer, with >800MHz system bandwidth, an increase in signal and modulation depth in a 4-pulse DEER experiment through use of composite rather than rectangular π pulses. We show that this leads to an increase in sensitivity by a factor of 3, in line with theoretical predictions, although gains are more limited in nitroxide-nitroxide DEER measurements.

Preparation of single-photon states via cavity-assisted spontaneous parametric down-conversion for quantum memory based on Y7LiF4:Nd3+ crystal

We report on the realization of a tunable single-photon source compatible with quantum memories based on isotopically pure Y 7 LiF 4 crystals doped with Nd 3+ ions. The source is based on spontaneous parametric down-conversion in a PPLN crystal placed in a resonator. The latter is formed by two mirrors which are transparent for the pump (532 nm) and idler (1377 nm) fields but high reflective for the signal field (867 nm). The width of the second-order cross-correlation function between the signal and idler photons is determined to be 1.5 ns for the cavity length of 8 cm, which corresponds to a cavity bandwidth of 100 MHz.

Enhanced nonlinear interaction in a microcavity under coherent excitation.

The large field enhancement that can be achieved in high quality factor and small mode volume photonic crystal microcavities leads to strengthened nonlinear interactions. However, the frequency shift dynamics of the cavity resonance under a pulsed excitation, which is driven by nonlinear refractive index change, tends to limit the coupling efficiency between the pulse and the cavity. As a consequence, the cavity enhancement effect cannot last for the entire pulse duration, limiting the interaction between the pulse and the intra-cavity material. In order to preserve the benefit of light localization throughout the pulsed excitation, we report the first experimental demonstration of coherent excitation of a nonlinear microcavity, leading to an enhanced intra-cavity nonlinear interaction. We investigate the nonlinear behavior of a Silicon-based microcavity subject to tailored positively chirped pulses, enabling to increase the free carrier density generated by two-photon absorption by up to a factor of 2.5 compared with a Fourier-transform limited pulse excitation of equal energy. It is accompanied by an extended frequency blue-shift of the cavity resonance reaching 19 times the linear cavity bandwidth. This experimental result highlights the interest in using coherent excitation to control intra-cavity light-matter interactions and nonlinear dynamics of microcavity-based optical devices.

Simultaneous squeezing of coherent fields using coherent population trapping

We study the effects of equal and unequal Rabi frequencies as well as cavity bandwidth on the optimal noise of the output fields from a two-mode optical cavity, which is fed by two coherent fields and encloses a collection of three-level Λ atoms. Each of the output modes is shown to be highly squeezed in the case of smaller symmetrical atomic detunings compared to the spontaneous-emission rates, where the system is in the vicinity of perfect coherent population trapping (CPT). We find that the squeezing on each output mode is larger than that reported by Dantan et al (2006 Phys. Rev. Lett. 97 023605) when adjusting the equal Rabi frequencies. The cavity bandwidth affects, not the degree of squeezing, but the linewidth of the spectral components. In an alternative way, the squeezing of each outgoing mode can be understood as a consequence of the enhanced cross-phase modulation, different from the nonlinear Faraday effect in Dantan’s paper (2006 Phys. Rev. Lett. 97 023605). The application of squeezed light is to obtain high-precision measurement, optical communication, and interferometric detection of gravitational radiation.

Narrowing the filter-cavity bandwidth in gravitational-wave detectors via optomechanical interaction.

We propose using optomechanical interaction to narrow the bandwidth of filter cavities for achieving frequency-dependent squeezing in advanced gravitational-wave detectors, inspired by the idea of optomechanically induced transparency. This can allow us to achieve a cavity bandwidth on the order of 100 Hz using small-scale cavities. Additionally, in contrast to a passive Fabry-Pérot cavity, the resulting cavity bandwidth can be dynamically tuned, which is useful for adaptively optimizing the detector sensitivity when switching amongst different operational modes. The experimental challenge for its implementation is a stringent requirement for very low thermal noise of the mechanical oscillator, which would need a superb mechanical quality factor and a very low temperature. We consider one possible setup to relieve this requirement by using optical dilution to enhance the mechanical quality factor.

Dual-wavelength single-frequency fiber laser based on FP-LD injection locking for millimeter-wave generation

We propose and successfully demonstrate a dual-wavelength single-frequency fiber laser based on two cascaded Fabry Pérot laser diodes (FP-LDs) and a WaveShaper. Both the cascaded FP-LDs and the WaveShaper perform the dual-wavelength selection. Injection locking of the FP-LDs in laser cavity and narrow bandwidth filtering effect of the WaveShaper guarantee single-frequency operation of the laser. By adjustment of the drive currents of the two FP-LDs and the filtering characteristic of the WaveShaper, dual-wavelength single-frequency output with switchable wavelength spacing can be obtained, which can be used for switchable millimeter-wave generation.

Achieving ground state and enhancing optomechanical entanglement by recovering information

For cavity-assisted optomechanical cooling experiments, in order to achieve the quantum ground state of the mechanical oscillator, the cavity bandwidth needs to be smaller than the mechanical frequency. In the literature, this is the so-called resolved-sideband or good-cavity limit, and this is based on an understanding of optomechanical dynamics. We provide a different but physically equivalent explanation of such a limit: that is, information loss due to finite cavity bandwidth. With an optimal feedback control to recover the information in the cavity output, we can surpass the resolved-sideband limit and achieve the quantum ground state. In addition, recovering this information can also significantly enhance the entanglement between the cavity mode and the mechanical oscillator. Especially when the environmental temperature is high, such optomechanical entanglement will either exist or vanish critically depending on whether information is recovered or not. This provides a vivid example of a quantum eraser in the optomechanical system.

Adapting TESLA technology for future cw light sources using HoBiCaT

The HoBiCaT facility has been set up and operated at the Helmholtz-Zentrum-Berlin and BESSY since 2005. Its purpose is testing superconducting cavities in cw mode of operation and it was successfully demonstrated that TESLA pulsed technology can be used for cw mode of operation with only minor changes. Issues that were addressed comprise of elevated dynamic thermal losses in the cavity walls, necessary modifications in the cryogenics and the cavity processing, the optimum choice of operational parameters such as cavity temperature or bandwidth, the characterization of higher order modes in the cavity, and the usability of existing tuners and couplers for cw.

Impact of Nonlinear Memory Effects on Digital Communications in a Klystron

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Nonlinear memory effects in a klystron and their impact on digital communications are investigated using a time-domain physics-based model. The simulation results are compared to an idealized block model based on the frequency response and amplitude/phase drive curves typically used in system design with vacuum electronic amplifiers. Significant departures in transient behavior are noted in the physics-based model in comparison to the block model when the klystron is at or near saturation, provided that the signal bandwidth is simultaneously large ( <formula formulatype="inline"><tex Notation="TeX">$\geq\ \sim$</tex></formula>65% of the klystron output cavity bandwidth). Such nonlinear memory effects exist for pure amplitude, pure phase, and mixed transients of both the step and ramp variety. The effects of these nonlinear phenomena on 16-state phase-shift keying (16-PSK) digital communications waveforms (with preequalized symbols) are examined using symbol constellation diagrams and symbol error rate (SER) plots. Operation at saturation with signal bandwidths of 32%, 65%, and 93% of the klystron output cavity bandwidth, for a bit-energy-based signal-to-noise ratio of 18 dB, yields SER values of <formula formulatype="inline"><tex Notation="TeX">$ \hbox{2.0}\times \hbox{10}^{-5}$</tex></formula>, <formula formulatype="inline"><tex Notation="TeX">$\hbox{2.3} \times \hbox{10}^{-4}$</tex></formula>, and <formula formulatype="inline"><tex Notation="TeX">$\hbox{2.7} \times \hbox{10}^{-3}$</tex> </formula>, respectively, in comparison to the ideal 16-PSK value of <formula formulatype="inline"><tex Notation="TeX">$\hbox{1.1} \times \hbox{10}^{-5}$</tex></formula>. </para>