Cavity Tuning Research Articles

Radio-frequency cavity field measurements through free falling bead

This paper introduces a novel bead-falling measurement method for the precise and efficient mapping of electromagnetic fields within radio-frequency (rf) cavities, which are crucial components in the design of accelerators. The traditional bead-pull method, while effective, involves mechanical complexities and is prone to errors from wire perturbations. The innovative method reported here leverages the simplicity and accuracy of free-falling beads to mitigate these issues. This technique eliminates the need for a wire-pulley system, thereby simplifying the experimental setup and reducing potential mechanical errors. We detail the development and operational principles of this new method, including the design of a compact, portable measurement device that integrates a bead/droplet release system and a bead detection system linked to a vector network analyzer. The device has been tested with a three-gap buncher cavity and a scaled Alvarez-type cavity, demonstrating its ability to perform rapid, reliable field measurements under various challenging conditions, including low signal-to-noise ratios and environmental vibrations. The results confirm the method’s superiority in precision and operational efficiency, potentially setting a new standard for rf cavity diagnostics and tuning. Published by the American Physical Society 2025

Optical ring cavity for homogeneous quantum nondemolition measurement in atom interferometer

Quantum nondemolition (QND) measurement aided by high-finesse optical cavities is an important method for generating high-gain spin or momentum squeezed states, which can enhance the sensitivity of atom interferometers to surpass the standard quantum limit. Conventional two-mirror Fabry-Perot cavities have the drawback of a standing wave pattern, leading to inhomogeneous atom-light coupling and subsequent degradation of squeezing enhancement. In this study, we present a novel method for achieving homogeneous quantum nondemolition measurement using an optical ring cavity to generate momentum squeezed states in atom interferometers. We designed and demonstrated a high-finesse (<i>F</i> =2.4(1)×10<sup>4</sup>), high-vacuum compatible (1×10<sup>-10</sup> mbar) optical ring cavity that utilizes the properties of traveling wave fields to address the issue of inhomogeneous atom-light interaction. A strontium cold atomic ensemble was prepared and coupled into the cavity mode; the dispersive cavity phase shift caused by the atoms passing through was extracted through differential Pound-Drever-Hall measurement, enabling nondemolition measurement of the atom number. Experimental results indicate that, under a probe laser power of 20 µW, the dispersive phase shift of the ring cavity was measured to be 40 mrad. The effective number of atoms coupled into the cavity mode is around 1×10<sup>6</sup>. Verification of the consistency between the ring cavity dispersive phase shift and QND measurement theory was achieved by adjusting parameters such as matching the atomic position with the cavity mode and tuning the frequency of the probe laser. The optical ring cavity developed in this study provides a significant approach for generating spin or momentum squeezed states in atom interferometers, thus holding promise for enhancing their sensitivity and is expected to find wide applications in cavity-enhanced quantum precision measurements.

Strain-concentration for fast, compact photonic modulation and non-volatile memory

A critical figure of merit (FoM) for electro-optic (EO) modulators is the transmission change per voltage, dT/dV. Conventional approaches in wave-guided modulators maximize dT/dV via a high EO coefficient or longer light-material interaction lengths but are ultimately limited by material losses and nonlinearities. Optical and RF resonances improve dT/dV at the cost of spectral non-uniformity, especially for high-Q optical cavity resonances. Here, we introduce an EO modulator based on piezo-strain-concentration of a photonic crystal cavity to address both trade-offs: (i) it eliminates the trade-off between dT/dV and waveguide loss—i.e., enhancement of the resonance tuning efficiency dv c /dV for the fixed EO coefficient, waveguide length, and cavity Q—and (ii) at high DC strains it exhibits a non-volatile (NV) cavity tuning Δvc,NV for passive memory and programming of multiple devices into resonance despite fabrication variations. The device is fabricated on a scalable silicon nitride-on-aluminum nitride platform. We measure dv c /dV=177±1MHz/V, corresponding to Δv c =40±0.32GHz for a voltage spanning ±120V with an energy consumption of δU/Δv c =0.17nW/GHz. The modulation bandwidth is flat up to ωBW,3dB/2π=3.2±0.07MHz for broadband DC-AC and 142±17MHz for resonant operation near a 2.8 GHz mechanical resonance. Optical extinction up to 25 dB is obtained via Fano-type interference. Strain-induced beam-buckling modes are programmable under a “read-write” protocol with a continuous, repeatable tuning range of 5±0.25GHz, allowing for storage and retrieval, which we quantify with mutual information of 2.4 bits and a maximum non-volatile excursion of 8 GHz. Using a full piezo-optical finite-element-model (FEM) we identify key design principles for optimizing strain-based modulators and chart a path towards achieving performance comparable to lithium niobate-based modulators and the study of high strain physics on-chip.

High precision tuning of RF cavity for 6 MeV SKKU X-band medical LINAC

High precision tuning of RF cavity for 6 MeV SKKU X-band medical LINAC

Optimization design of the cavity tuning and RF input coupler for an 11 MeV superconducting cyclotron

This study addresses the compact design requirements for a 11 MeV superconducting cyclotron utilized in isotope production by proposing a design and optimization scheme for a radio frequency (RF) input coupling system. The system, comprising a coupler loop, transmission lines, and RF window, has been tailored to accommodate the spatial constraints imposed by the super conducting magnet radius, thereby facilitating efficient RF power transfer to the RF cavity and minimizing input power. Structural optimization of the coupler loop and RF window has achieved S11 parameter of -82.23 dB, significantly reducing input power while maintaining high coupling efficiency. A coaxial coupler suitable for lower frequency ranges has been designed, employing a tapered structure for the transmission line to ensure a smooth transition to reduce impedance discontinuity effects and enhance adjustability. The multi-stage design of the RF window has achieved fine impedance matching, significantly lowering the S11 parameter. In response to frequency drift caused by RF loss and temperature increase, the study utilizes perturbation theory to analyze the perturbation effect of coupling components on the resonant frequency, designing an adjustable structure with both coarse and precise tuning capabilities, realizing tuning precision at the kHz/mm level and a tuning range of several MHz. A conjectural formula has been proposed based on the impact of the tuner's structural parameters on the resonant frequency, accurately calculating the equivalent inductance of the tuner and the resonant frequency of the RF cavity post-tuner installation, with the maximum estimation error of the resonant frequency kept within 0.1%, offering significant guidance for engineering design.

Engineering the Impact of Phonon Dephasing on the Coherence of a WSe_{2} Single-Photon Source via Cavity Quantum Electrodynamics.

Emitter dephasing is one of the key issues in the performance of solid-state single-photon sources. Among the various sources of dephasing, acoustic phonons play a central role in adding decoherence to the single-photon emission. Here, we demonstrate that it is possible to tune and engineer the coherence of photons emitted from a single WSe_{2} monolayer quantum dot via selectively coupling it to a spectral cavity resonance. We utilize an open cavity to demonstrate spectral enhancement, leveling, and suppression of the highly asymmetric phonon sideband, finding excellent agreement with a microscopic description of the exciton-phonon dephasing in a truly two-dimensional system. Moreover, the impact of cavity tuning on the dephasing is directly assessed via optical interferometry, which points out the capability to utilize light-matter coupling to steer and design dephasing and coherence of quantum emitters in atomically thin crystals.

SPIRAL2 commissioning and operations

The SPIRAL2 linac is now successfully commissioned; H+, 4He2+, D+ and 18O6+ have been accelerated up to nominal parameters and 18O7+ and 40Ar14+ beams have been also accelerated up to 7 MeV/A. The main steps with 5 mA H+, D+ beams and with 0.6 mA 18O6+ are described. The general results of the commissioning of the RF, cryogenic and diagnostics systems, as well as the preliminary results of the first experiments on NFS are presented. In addition of an improvement of the matching to the linac, the tuning procedures of the 3 Medium Energy Beam Transport (MEBT) rebunchers and 26 linac SC cavities were progressively improved to reach the nominal parameters in operation, starting from the classical “signature matching method”. The different cavity tuning methods developed to take into account our particular situation (very low energy and large phase extension) are described. The tools developed for an efficient linac tuning in operation, e.g. beam energy and intensity changes are also discussed.

Cavity tuning and beam commissioning of the radio-frequency quadrupole linac for a transportable neutron source

A transportable, compact, accelerator-based neutron source is under development at Xi’an Jiaotong University. At the heart of the linear accelerator is a radio-frequency quadrupole (RFQ) unit that has been designed to accelerate a proton beam from 30 keV to 2.5 MeV with a peak current of 12 mA and a duty factor of 3%. Recently, the manufacturing of the RFQ cavity has been completed. Cavity tuning, including field unevenness, resonant frequency, and rf coupling coefficient, was carried out after finishing brazing and assembly. The quadrupole perturbative component was controlled within ±2.0%, and the two dipole components were about ±2.5%. The value of the intrinsic Q was 9609, 89% of that of the theoretical simulation. We have completed the installation of the RFQ experimental setup and performed beam commissioning experiments. The output beam current reached 10.6 mA with a transmission efficiency of 93.8%.

Understanding the Nature of Vibro-Polaritonic States in Water and Heavy Water.

Very recent experiments on vibrational strong coupling tend to modify chemical reactivity, energy transfer, and many other physical properties of the coupled system. This is achieved without external stimuli and is very sensitive to the vibrational envelope. Water is an excellent vibrational oscillator, which is being used for similar experiments. However, the inhomogeneously broad OH/OD stretching vibrational band make it complicated to characterize the vibro-polaritonic states spectroscopically. In this paper, we performed vibrational strong coupling and mapped the evolution of vibro-polaritonic branches from low to high concentration of H2 O and measured the on-set of strong coupling. The refractive index dispersion is correlated with the cavity tuning experiments. These results are further compared with transfer matrix simulations. Simulated data deviate as noted in the dispersion spectra as the system enters into ultra-strong coupling due to enhanced self-dipolar interactions. Hopfield coefficients calculation shows that even at ±400 cm-1 detuning, the vibro-polaritonic states still possess hybrid characteristics. We systematically varied the concentration of H2 O and mapped the weak, intermediate, and strong coupling regimes to understand the role of inhomogeneously broad OH/OD stretching vibrational band. Our finding may help to better understand the role of H2 O/D2 O strong coupling in the recent polaritonic chemistry experiments.

Piezoelectric and microfluidic tuning of an infrared cavity for vibrational polariton studies.

We developed a microfluidic system for vibrational polariton studies, which consists of two microfluidic chips: one for solution mixing and another for tuning an infrared cavity made of a pair of gold mirrors and a PDMS (polydimethylsiloxane) spacer. We show that the cavity of the system can be accurately tuned with either piezoelectric actuators or microflow-induced pressure to result in resonant coupling between a cavity mode and a variational mode of the solution molecules. Acrylonitrile solutions were chosen to prove the concept of vabriational strong coupling (VSC) of a CN stretching mode with light inside the cavity. We also show that the Rabi splitting energy is linearly proportional to the square root of molecular concentration, thereby proving the relevance and reliability of the system for VSC studies.

Construction and measurement of the prototype side-coupled standing-wave tube for electron Linac

Due to Iran's growing need for accelerators in various applications, NSTRI (Nuclear Science and Technology Research Institute) has defined a 6 MeV e-Linac (electron linear accelerator) project for medical and inspection applications. This electron accelerator has a side-coupled standing wave tube that operates in π/2 mode at the frequency of 2998.5 MHz. This paper presents a summary of the construction and cold test stage of the prototype tube for this accelerator. The prototype tube was constructed from aluminum and clamped with bolts.In the cold test stage, low-power RF measurements werecarried out using a side-coupled cavity tuning method and a bead-pull measurement technique. Using a network analyzer, magnetic and electric probes, and a mechanical structure constructed for this tube made the RF tuning possible. After tuning, the resonant frequency, unloaded quality factor, and effective shunt impedance of the aluminum tube were achieved at 2998.57 MHz, 7970, and 83.25 MΩ/m respectively.

Single-Mode-VCSEL With a Ring-Shaped Self-Aligned Recessed Metal Mode Filter

This article presents a single-mode vertical-cavity surface-emitting laser (VCSEL) using a novel ring-shaped self-aligned recessed metal (SARM) mode filter on the emitting window as a means of transverse mode control. The SARM structure acts as a spatial filter to suppress the higher-order transverse modes through selective recessed-etching of the top mirror and the self-aligned deposition of the p-type contact metals for cavity tuning and blocking the higher-order modes. The SARM mode filter could be a potentially high-yield alternative technique for the mass production of single-mode VCSELs, demonstrating a single-mode emission with 1.55-mW power and a 30.7-dB side mode suppression ratio.

Modification of ground-state chemical reactivity via light-matter coherence in infrared cavities.

Reaction-rate modifications for chemical processes due to strong coupling between reactant molecular vibrations and the cavity vacuum have been reported; however, no currently accepted mechanisms explain these observations. In this work, reaction-rate constants were extracted from evolving cavity transmission spectra, revealing resonant suppression of the intracavity reaction rate for alcoholysis of phenyl isocyanate with cyclohexanol. We observed up to an 80% suppression of the rate by tuning cavity modes to be resonant with the reactant isocyanate (NCO) stretch, the product carbonyl (CO) stretch, and cooperative reactant-solvent modes (CH). These results were interpreted using an open quantum system model that predicted resonant modifications of the vibrational distribution of reactants from canonical statistics as a result of light-matter quantum coherences, suggesting links to explore between chemistry and quantum science.

Versatile OSCAT time-domain THz spectrometer.

We report on a compact and versatile time-domain spectrometer operating in the THz spectral region from 0.2 to 2.5 THz based on ultrafast Yb:CALGO laser and photo-conductive antennas. The spectrometer operates with the optical sampling by cavity tuning (OSCAT) method based on laser repetition rate tuning, which allows at the same time the implementation of a delay-time modulation scheme. The whole characterization of the instrument is presented and compared to the classical THz time-domain spectroscopy implementation. THz spectroscopic measurements on a 520-μm thick GaAs wafer substrate together with water vapor absorption measurements are also reported to further validate the instrument capabilities.

Phonon-phonon interaction and parametric down-conversion generation in multimode optomechanical systems

We studied the process of polariton conversion in a 3-mode nonlinear optomechanical system. Compared with the standard 2-mode optomechanical system, we find a much larger conversion rate of polariton modes can be achieved under typical dissipation conditions. To obtain a transparent understanding of the relevant physical process, we show that in the large detuned case, the cavity can be eliminated adiabatically, resulting in a parametric down-conversion (PDC) interaction between two phononic polariton modes. By tuning cavity detuning, the nonlinear interaction can be enhanced with the frequency-matching condition. Results from analytical treatment based on the effective PDC model agree with the numerical simulation. Such a system provides potential applications in nonlinear phononics.

Characteristics of cavity tuning of half-external cavity Nd:YAG and Nd:YVO<sub>4</sub> microchip solid-state lasers

微片固体激光器具有体积小、寿命长等优点，是精密测量仪器的重要光源。构建了平-平、半外腔的Nd:YAG和Nd:YVO₄微片激光器，通过控制压电陶瓷伸缩改变激光器的谐振腔长，同时使用F-P扫描干涉仪和波长计观察纵模和波长。研究了这2种微片固体激光器的腔调谐特性，包括腔长与光功率的关系，激光纵模扫过出光带宽过程的光功率和单、双纵模变化的规律。实验结果表明：腔调谐过程中单、双纵模交替出现，腔长和泵浦电流共同影响激光器的输出模式和光功率。

Ultrafast true-green Ho:ZBLAN fiber laser inspired by the TD3 AI algorithm.

Ultrafast lasers in the true-green spectrum, which are scarce due to the "green gap" in semiconductor materials, are in high demand for the surging field of biomedical photonics. One ideal candidate for efficient green lasing is Ho:ZBLAN fiber, as ZBLAN-hosted fibers have already reached picosecond dissipative soliton resonance (DSR) in the yellow. When attempting to push the DSR mode locking further into the green, traditional manual cavity tuning is faced with extreme difficulty, as the emission regime for these fiber lasers is so deeply concealed. Breakthroughs in artificial intelligence (AI), however, provide the opportunity to fulfill the task in a fully automated manner. This work, inspired by the emerging twin delayed deep deterministic policy gradient (TD3) algorithm, represents the first application, to the best of our knowledge, of the TD3 AI algorithm to generate picosecond emissions at the unprecedented true-green wavelength of ∼545 nm. The study thus extends the ongoing AI technique further into the ultrafast photonics region.

Broad tuning range, high power quantum cascade laser at λ ∼ 7.4 µm.

In this article, we report a high power quantum cascade laser (QCL) at λ∼7.4 µm with a broad tuning range. By carefully designing and optimizing the active region and waveguide structure, a continuous-wave (CW) output power up to 1.36 W and 0.5 W is achieved at 293 K and 373 K which shows the excellent temperature stability. A high wall-plug efficiency (WPE) of 8% and 13.6% in CW and pulsed mode at 293 K are demonstrated. The laser shows a characteristic temperature T0 of 224 K and T1 of 381 K over a temperature range from 283 K to 373 K. In addition, a far field of pure zero order transverse mode and a fairly wide external cavity (EC) tuning range (280 cm-1) from 6.54 µm to 8 µm are achieved in pulsed operation. In addition, an EC single mode output power of 226 mW is obtained under CW operation at 293K.

Search for Galactic axions with a high- Q dielectric cavity

A haloscope of the QUAX--$a\gamma$ experiment, composed of an high-Q resonant cavity immersed in a 8 T magnet and cooled to $\sim 4.5$~K is operated to search for galactic axion with mass $m_a\simeq42.8~\mu\text{eV}$. The design of the cavity with hollow dielectric cylinders concentrically inserted in a OFHC Cu cavity, allowed us to maintain a loaded quality-factor Q $\sim 300000$ during the measurements in presence of magnetic field. Through the cavity tuning mechanism it was possible to modulate the resonance frequency of the haloscope in the region $10.35337-10.35345$~GHz and thus acquire different dataset at different resonance frequencies. Acquiring each dataset for about 50 minutes, combining them and correcting for the axion's signal estimation-efficiency we set a limit on the axion-photon coupling $g_{a\gamma\gamma}< 0.731\times10^{-13}$ GeV$^{-1}$ with the confidence level set at $90\%$.

Autotuning of Vibrational Strong Coupling for Site-Selective Reactions.

Site-selective chemistry opens new paths for the synthesis of technologically important molecules. When a reactant is placed inside a Fabry-Perot (FP) cavity, energy exchange between molecular vibrations and resonant cavity photons results in vibrational strong coupling (VSC). VSC has recently been implicated in modified chemical reactivity at specific reactive sites. However, as a reaction proceeds inside an FP cavity, the refractive index of the reaction solution changes, detuning the cavity mode away from the vibrational mode and weakening the VSC effect. Here we overcome this issue, developing actuatable FP cavities to allow automated tuning of cavity mode energy to maintain maximized VSC during a reaction. As an example, the site-selective reaction of the aldehyde over the ketone in 4-acetylbenzaldehyde is achieved by automated cavity tuning to maintain optimal VSC of the ketone carbonyl stretch during the reaction. A nearly 50 % improvement in site-selective reactivity is observed compared to an FP cavity with static mirrors, demonstrating the utility of actuatable FP cavities as microreactors for organic chemistry.