Fabry-Perot Cavity Research Articles

High Q and sub-wavelength THz electric field confinement in ultrastrongly coupled THz resonators

The control of light–matter coupling at the single electron level is currently a subject of growing interest for the development of novel quantum devices and for studies and applications of quantum electrodynamics. In the terahertz (THz) spectral range, this raises the particular and difficult challenge of building electromagnetic resonators that can conciliate low mode volume and high quality factor. Here, we report on hybrid THz cavities based on ultrastrong coupling between a Tamm cavity and an LC circuit metamaterial and show that they can combine high quality factors of up to Q=37 with a deep-subwavelength mode volume of V=3.2×10−4λ3. Our theoretical and experimental analysis of the coupled mode properties reveals that, in general, the ultrastrong coupling between a metamaterial and a Fabry–Perot cavity is an effective tool to almost completely suppress radiative losses and, thus, ultimately limit the total losses to the losses in the metallic layer. These Tamm cavity-LC metamaterial coupled resonators open a route toward the development of single photon THz emitters and detectors and to the exploration of ultrastrong THz light–matter coupling with a high degree of coherence in the few to single electron limit.

Load and Vibration Optical Fiber Sensor Based on Miniature Fabry–Perot Cavity Encapsulated by Micrometer Thickness of Silica Film

In this paper, an optical fiber sensor has been proposed to determine load and vibration signals based on the Fabry-Perot cavity. The load sensitivity has been experimentally demonstrated to be about 0.0148 nm/g. The vibration signal was measured by tracing the changes in frequency and amplitude intensity. Experimental results reveal that this sensor has the resonant frequency of 150 Hz, at which the signal-to-noise ratio is 35 dB. The operating frequency range for the proposed vibration sensor is 200-400 Hz, and the response sensitivity to vibration signals with different amplitudes is 0.0042 V/V.

A low‐profile dual‐band shared‐aperture antenna based on AMC with semitransparent elements

AbstractA dual‐band shared‐aperture antenna with a frequency ratio of 3 is presented in the letter. A low‐band antenna consists of two printed dipoles. An artificial magnetic conductor (AMC) is used to reduce the height of the low‐band antenna. A high‐band antenna consists of four printed dipoles. It is placed between the AMC elements and the antenna ground plane. The AMC elements and antenna ground plane form a Fabry–Perot cavity and allow increasing gain of the high‐band antenna. An additional structure is inserted between the antenna ground plane and the AMC elements to decrease heights of the Fabry–Perot cavity and the high‐band antenna simultaneously. The total size of the proposed antenna is 1.02λL × 1.02λL × 0.103λL (λL is the central wavelength of the low operating band). Simulated and measured results show that the designed antenna has 57% and 16.7% bandwidths in the low‐ and high‐frequency bands.

Molecular and solid-state topological polaritons induced by population imbalance

Abstract Strong coupling between electronic excitations in materials and photon modes results in the formation of polaritons, which display larger nonlinearities than their photonic counterparts due to their material component. We theoretically investigate how to optically control the topological properties of molecular and solid-state exciton–polariton systems by exploiting one such nonlinearity: saturation of electronic transitions. We demonstrate modification of the Berry curvature of three different materials when placed within a Fabry–Perot cavity and pumped with circularly polarized light, illustrating the broad applicability of our scheme. Importantly, while optical pumping leads to nonzero Chern invariants, unidirectional edge states do not emerge in our system as the bulk-boundary correspondence is not applicable. This work demonstrates a versatile approach to control topological properties of novel optoelectronic materials.

Enhanced emission from hBN in sputtered microcavities.

In this observational study, we embed few-layer hexagonal boron nitride (hBN) inside a planar Fabry-Perot cavity fabricated using a pulsed DC magnetron sputtering system and show that the hBN retains its inherent visible range, defect-based luminescent properties following relatively energetic deposition processing. The observed surface-normal emission enhancement factor of ∼40 is in good agreement with theoretical predictions. We also found that embedded hBN subjected to a rapid thermal annealing treatment exhibits a cracking effect where the edges of the material glow distinctly brighter than adjacent regions. Our results might inform future efforts involving monolithic integration of hBN active layers.

Cavity Jahn-Teller polaritons in molecules

We investigate Jahn-Teller (JT) polaritons, which emerge from the interaction of the two normal-incidence electromagnetic modes with perpendicular polarizations in a Fabry-Perot cavity resonator with JT active systems. These JT polaritons are characterized by a mixed $(+/-)$--circular electromagnetic polarization that originates from the molecular JT vibronic coupling of the material subsystem. Consequently, the exchange of photonic and vibronic angular momenta can be very efficient; exciting the cavity-JT system with short, polarized light pulses results in a dynamical and oscillatory response of the polarization in the cavity medium. Due to the photonic-vibronic coupling, we show how the cavity polarization direction becomes frequency dependent and does not necessarily coincide with the polarization direction of the external fields used to drive the system.

Fabrication of Selective Thermal Emitter with Multilayer Films for Mid-/Low-Temperature Infrared Stealth with Radiative Cooling

Infrared selective emitters are attracting more and more attention due to their modulation ability of infrared radiance, which provides an efficient ability to blend objects into the surrounding environment. In this paper, an Ag/ZnS/Si/Ag/Si multilayered emitter is proposed by virtue of impedance matching as well as Fabry-Perot cavity effect to achieve selective radiation in the infrared band. The emissivity of the fabricated selective emitter is measured to be ε3–5μm = 0.16 and ε8–14μm = 0.23 in the atmosphere windows, respectively, meeting the requirements of infrared stealth. Meanwhile, the emissivity at the non-atmospheric window (5–8 μm) is as high as 0.78, which allows efficient heat dissipation to achieve radiative cooling. Furthermore, the selective emitter maintains excellent stealth performance until 350 °C, indicating its good heat resistance and dissipation at medium temperature. The proposed emitter with spectral selectivity provides a new strategy for the facile fabrication of mid-/low-temperature infrared stealth devices.

A reference-free dual-comb spectroscopy calibrated by passive devices

Dual-comb spectroscopy has enabled new approaches for optical precision measurements. Although Doppler-limited resolution can be achieved over long-time scales across a large bandwidth, the development of dual-comb spectroscopy is hindered by strict demands for light source stability. Typically, expensive and complex self-reference systems are required to lock the carrier-envelope offset frequency (fceo) of the laser. Additionally, simply locking the repetition frequency (frep) to a radio frequency reference source still results in residual relative timing jitter between light sources. Here we extracted the relative fceo fluctuation between the frep-locked lasers from the high-precision passive notch filtering characteristics of the phase-shifted fiber Bragg grating and then eliminated it through online phase calibration. By introducing a passive broadband Fabry–Perot cavity with excellent thermal wavelength stability, we subsequently corrected residual relative timing jitter with online wavelength calibration, and the standard deviation of the relative wavelength drift was reduced to less than 0.4 pm within the full operating range. The spectral profile can also be extracted and removed by the Fabry–Perot cavity through intensity calibration. By calibrating these three dimensions, we built a reference-free post-calibration dual-comb spectroscopy and used this powerful tool to measure the Fabry–Perot cavity resonance peaks, the notch filtering narrow band of phase-shifted fiber Bragg gratings, and the absorption characteristics of hydrogen cyanide gas. The system achieves a spectral resolution of 0.8 pm over a bandwidth of more than 100 nm. This low-cost and convenient scheme provides new ideas for the application of dual-comb spectroscopy systems.

Accurate Determination of the Temperature Dependence of the Refractive Index of 4H-SiC at the Wavelength of 632 nm

The growing interest for the use of 4H-SiC in photonics is triggering the interest for more accurate characterizations of this semiconductor from the optical and opto-electronic point of view. In this work we report about new measurements run on an undoped 4H-SiC substrate, finalized at determining the precise dependence of its refractive index on temperature in the visible spectrum, and precisely at the wavelength of λ=632.8 nm, in a temperature range from room temperature (RT) to 400K. Measurements are performed by exploiting the properties of a Fabry-Perot cavity interrogated with a laser beam. It is known that the transmitted radiation intensity shows fringes that shift with temperature and the refractive index. By precisely monitoring the transmitted signal, the thermo-optic coefficient dn/dT can be determined with a resolution that approaches 10-6 K-1.

Femtosecond Laser Line-by-Line Inscribed Seven Core Fiber Cascaded Fabry–Perot Cavity and Its Vectorial Bending Sensing Application

Multi-core fibers have been widely used for vector-bending sensing due to their off-axis distributed cores. In contrast to vector-bending sensors based on Bragg gratings, fiber Fabry–Perot (F–P) interferometers are more advantageous due to their ease of fabrication and potential for introducing the Vernier effect to further improve sensitivity. We propose and experimentally demonstrate a cascaded Fabry–Perot (F–P) cavity vector bending sensor. From the experimental results, the sensor has a strong bending dependence with a maximum sensitivity of 123.12 pm/m−1, and the curvature magnitude and direction can be reconstructed from the tilted wavelength shift of the asymmetric fiber-core F–P cavities.

Design of embedded metamaterial solar absorber based on genetic algorithm

To effectively collect the solar energy, a metamaterial solar absorber operating in the UV to NIR wavelength range is designed, which consists of an embedded structure stacked with Ti and SiO2. The embedded structure can enhance the magnetic and electric fields to improve the absorption rate. However, the parameter space of the embedded structure is very huge to increase the optimization search time. Therefore, the absorption rate is calculated by the finite difference time domain method and the parameters of the absorber are optimized by genetic algorithm. The results show that the absorption rates of the optimized absorber are all over 95% in the 300–2500 nm and the average absorption rate is 98.7%, and the absorber is of polarization independence and the average absorption rate remains above 95% at an oblique incidence angle of 60°. By analyzing the electromagnetic field distribution of the absorber at the resonance wavelength, it can be concluded that the high absorption characteristics in the wideband are mainly achieved by the combined effects of local surface plasmon resonance, electric dipole resonance, propagating surface plasmon resonance and Fabry-Perot cavity resonance. Perfect absorber in the solar spectral range have significant value in fields such as solar energy harvesting and information detection.

Low-Threshold Optical Bistability Based on Photonic Crystal Fabry–Perot Cavity with Three-Dimensional Dirac Semimetal

In this paper, we investigate theoretically the tunable low threshold optical bistability (OB) in the terahertz range based on photonic crystals’ Fabry–Perot (FP) cavity with a three-dimensional Dirac semimetal (3D DSM). On the one hand, a 3D DSM with a high nonlinear refractive index coefficient creates conditions for the generation of OB. Additionally, the finite film thickness of 3D DSMs leads to significantly a enhanced interaction volume compared to graphene, which allows easier preparation and has stable properties. On the other hand, the resonance of the FP cavity plays a positive role in promoting the tunable low-threshold OB. It was found that the OB threshold and hysteresis curve can be continuously adjusted by manipulating the Fermi energy and relaxation times of a 3D DSM. Additionally, the bistable curve of the composite structure is also closely related to the angle of incident light. Through parameter optimization, OB with a threshold approaching 105 V/m can be obtained. The photonic crystal’s FP cavity with a 3D DSM structure provides a feasible way to achieve low-threshold OB and a building block for future integrated all-optical devices.

Combined Interrogation Algorithm of Phase Function and Minimum Mean Square Error for Fiber-Optic Fabry-Perot Micro-Pressure Sensors Based on White Light Interference

In this paper, a demodulation algorithm using the phase function of the Fabry-Perot (FP) cavity combined with the minimum mean squared error (MMSE) is proposed for the pressure measurement with low-pressure sensitivity fiber-optic FP micro-pressure sensors, which exhibits reasonable demodulation results. In this algorithm, the initial phase variation during the measurement process is focused, which is usually disregarded in traditional demodulation, and then, use the phase change features of phase function to achieve precise pressure demodulation. A functional relation between the reference pressure and the amount of phase function displacement is developed for demodulation based on interferometric spectra of reference pressures at 0 kPa and 35 kPa. The algorithm has been investigated theoretically and experimentally. The experiment results demonstrate that the measurement accuracy is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pm$</tex-math></inline-formula> 0.2 kPa in the pressure measurement range from 0 kPa to 35 kPa, the pressure resolution is up to 0.0218 kPa, and the reading error is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pm 3\%$</tex-math></inline-formula> in the pressure range from 6.7 kPa to 35 kPa, which indicate that the proposed algorithm can meet medical pressure analysis standards in the cardiovascular sector for low-pressure sensitivity sensors. Moreover, this work illustrates that inaccurate reference pressure calibration would cause substantial demodulation errors. These errors can be mitigated by appropriately increasing the number of reference pressures to keep the reading error at a reasonable level according to the specific application.

Tunable and ultrasensitive sensor covering terahertz to telecommunication range based on a Fabry–Perot interference of graphene plasmonic waves

We present an ultrasensitive tunable terahertz (THz) sensor that comprises monolayer graphene-based gratings integrated with a Fabry-Perot (FP) cavity through precisely numerical modeling. Due to the vertical polarization of the incident light on the graphene gratings, they act as an electric dipole, which causes interference in the propagation of plasmonic waves, and provides an excellent foundation for a wide range of ultrasensitive refractive index (RI) sensing with an ultra-high-quality factor (Q). The main advantage of the proposed structure is not only sensing a wide range of refractive indices but due to its unique physical mechanism, it covers electromagnetic waves from a few terahertz to 300 THz, including the telecommunication bands. The primary physical mechanism in the proposed sensor is the FP interference in its active area. Also, due to the extreme sensitivity of FP interference (FPi) to the (RI) of the medium and the active area, the proposed structure owns an ultra-high sensitivity and Q. Our full-wave electromagnetic simulations demonstrate a frequency sensitivity of up to 83 THz per RI unit (RIU) and a high value of Q near 275. In addition, the proposed structure for index sensing has the functionality to be tuned in any desired frequency of interest from THz to the Mid-IR range and telecommunication band, which entirely covers 1–300 THz. The chemical potential of graphene can be electrically controlled to tailor the sensing capability and incident frequency. According to the applied gate voltage, the proposed structure can detect ammonia concentration in water with a high sensitivity of 70–85 THz/RIU and a quality factor of more than 275. The error of the ammonia concentration sensing is smaller than 0.5%. In addition, the proposed structure can detect different types of matter, including various microfluidics, air, gasoline, liquid paraffin, ethanol, glycerin, and glucose. The graphene plasmonic grating method suggested here is a good contender for on-chip integrated THz to mid-IR, including telecommunication bio-sensors.

Temperature Self-Adaptive Ultra-Thin Solar Absorber Based on Optimization Algorithm

In solar applications, the solar absorber is paramount to converting solar radiation to heat energy. We systematically examined the relationship between the efficiency of the solar absorber and operating temperature and other factors. By combining inverse designs with surface plasmonic and Fabry-Perot cavity solar absorption theories, we have developed several solar absorber devices with excellent performance at different temperatures. One of these devices displays a solar spectral absorption of 95.6%, an ultra-low emission rate of 5.7%, and optical-to-thermal conversion efficiency exceeding 90%, all within an ultra-thin depth of 0.45 μm under working temperatures of 600 K. The device has the potential to surpass the Shockley-Queisser limit (S-Q limit) in solar power generation systems. Our method is adaptable, enabling the design of optimal-performance devices to the greatest extent possible. The design was optimized using modern optimization algorithms to meet complex conditions and offers new insights for further study of the conversion from solar to thermal energy and the advancement of solar energy applications.

Driving DNA Origami Coassembling by Vibrational Strong Coupling in the Dark

Vibrational strong coupling (VSC) has been an emerging tool to regulate the energy landscape of chemical bonds. VSC can be achieved by tuning the optical mode of Fabry-Pérot (FP) cavities to resonantly couple with molecular vibrations, which generates new light-molecule hybrid states and energy levels, thereby directly influencing chemical reactions. Herein, VSC with FP cavities can significantly reduce the melting temperature (Tm) of dsDNA, which can be precisely measured using DNA origami coassembly, benefiting from the precise hybridization-dependent origami conformation. An 8.5-μm-spaced FP cavity can strongly couple with the vibration of the pyridine bases, exhibiting the most optimized enhancing effect on directly driving the coassembly of DNA origami in FP cavities. This work sheds light on the quantum mechanism in life and in turn holds great promise to develop innovative techniques to regulate life.

WS2 Monolayer in Fabry–Perot Cavity Support for Plasmonic Fano Resonance

WS2 Monolayer in Fabry–Perot Cavity Support for Plasmonic Fano Resonance

Metal-coated high-temperature strain optical fiber sensor based on cascaded air-bubble FPI-FBG structure.

Metal coatings can protect the fragile optical fiber sensors and extend their life in harsh environments. However, simultaneous high-temperature strain sensing in a metal-coated optical fiber remains relatively unexplored. In this study, a nickel-coated fiber Bragg grating (FBG) cascaded with an air bubble cavity Fabry-Perot interferometer (FPI) fiber optic sensor was developed for simultaneous high temperature and strain sensing. The sensor was successfully tested at 545 °C for 0-1000 µɛ, and the characteristic matrix was used to decouple temperature and strain. The metal layer allows easy attachment to metal surfaces that operate at high temperatures, enabling sensor-object integration. As a result, the metal-coated cascaded optical fiber sensor has the potential to be used in real-world structural health monitoring.

Non-invasive detection of haemoglobin, platelets, and total bilirubin using hyperspectral cameras

Hyperspectral imaging has emerged as a promising high-resolution and real-time imaging technology with potential applications in medical diagnostics and surgical guidance. In this study, we developed a high-speed hyperspectral camera by integrating a Fabry-Perot cavity filter on each CMOS pixel. We used it to non-invasively detect three blood components (haemoglobin, platelet, and total bilirubin). Specifically, we acquired transmission images of the subject's fingers, extracted spectral signals at each wavelength, and used dynamic spectroscopy to obtain non-invasive blood absorption spectra. The prediction models were established using the PLSR method and were modelled and validated based on the standard clinical-biochemical test values. The experimental results demonstrated excellent performance. The best predictions were obtained for haemoglobin, with a high related coefficient (R) of 0.85 or more in both the calibration and prediction sets and a mean absolute percentage error (MAPE) of only 5.7%. The results for total bilirubin were also ideal, with R values exceeding 0.8 in both sets and a MAPE of 10.6%. Although the prediction results for platelets were slightly less satisfactory, the error was still less than 15%, indicating that the results were also acceptable. Overall, our study highlights the potential of hyperspectral imaging technology for the development of portable and affordable devices for blood analysis, which can be used in various settings.

Rabi splitting and unidirectional reflectionless propagation in one-dimensional topological plasmonic crystal

In this paper, we study the strong coupling between the topological edge state and Fabry–Perot cavity state in a one-dimensional plasmonic crystal heterostructure. Finite element method simulations show that a significant plasmonic Rabi splitting is achieved in the near-infrared region and Rabi energy can reach up to 45.5 meV. A dual-band near-perfect absorption phenomenon can be observed, and a coupled oscillator model is proposed to explain the origin of Rabi splitting. In addition, the dual-band unidirectional reflectionless plasmonic propagation in the Rabi splitting region is investigated, and the non-Hermitian scattering matrix is used to verify the existence of double exceptional points. The study may find applications in plasmonic switchers, on-chip sensors, diode-like devices, and filters.