This PDF file contains the front matter associated with SPIE Proceedings Volume 12895, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

In the dynamic field of quantum photonics, our research explores the promising convergence with interband cascade lasers (ICLs), focusing on their applications in free-space communications and quantum photonics. The pressing need for space-to-ground high-speed transmission in the global broadband network development aligns seamlessly with the unique advantages of mid-infrared wavelengths. From minimal atmospheric attenuation to eye-safe operation and resilience against bad weather conditions, mid-infrared wavelengths are expected to provide a robust foundation for these systems. Our work shows that the utilization of interband cascade technology is very much promising for high-speed transmission at a wavelength of 4.2 μm. The low power consumption of both the laser and the detector, combined with a substantial modulation bandwidth and good output power, positions this technology as an ideal solution for free-space optical communications hence enabling multigigabit data rate operations. Concurrently, our research also explores the potential of harnessing squeezed light using high quantum efficiency ICLs. Through a stochastic model approach, we demonstrate that these midinfrared semiconductor devices can exhibit significant amplitude squeezing across a broad bandwidth of several gigahertz when powered by low-noise constant current sources. These collective efforts pave the way for accelerated advancements in mid-infrared ICLs, encompassing both quantum photonics and future free-space laser communication systems include novel quantum key distribution protocols.

In this talk, challenges and solutions associated with the monolithic, epitaxial integration of mid- and longwave- infrared, InP-based quantum cascade lasers on GaAs and Si wafers will be discussed. Initial results, including room temperature, high power, and continuous wave operation, will be described.

The nonclassical light sources, such as frequency-time entangled photons, are anticipated to offer significant benefits for emerging quantum optical sensing or spectroscopic measurements and manifest on ultrafast time scales (sub-ps to fs). However, the constrained time resolution (ns to ps) of photon-counting detectors poses challenges in comprehensively characterizing their detailed properties on ultrafast time scales. Therefore, we present a novel asynchronous optical sampling (ASOPS) technique utilizing two-color optical frequency combs to demonstrate highly precise and sensitive ultrafast time-resolved cross-correlation measurements at the single-photon level. By employing photon counting statistics, this method successfully reconstructed the picosecond pulse width cross-correlation waveforms at extremely low power level (<1 photon per pulse), while effectively suppressing the residual temporal jitter between the two combs via optically triggered averaging using asynchronous optical sampling of combs. The use of repetition frequency stabilized distinct-wavelength pulses allowed for the effective suppression of strong background light from the pump through spectral filtering, achieving single-photon sensitivity. Subsequently, we parametrically down converted the frequency doubled light from the Er comb in the nonlinear ppKTP waveguide to generate quantum entangled photons at telecom band. A 9.04% Klyshko efficiency with a photon pair generation rate of 0.98 MHz/mW was obtained using heralding detection. Employing the established ASOPS technique to the generated photon pairs enabled the realization of ultrafast time-resolved and quantum mechanical correlation measurements. This paves the way for a versatile and comprehensive manipulation of quantum-entangled photon pairs in the time-domain, with potential applications in ultrafast optical quantum technology and ultrashort fluorescence measurements.

Single Photon Avalanche Diodes (SPADs) are semiconductor devices capable of accurately timing the arrival of single photons of light. Previously, we have demonstrated a pseudo-planar Ge-on-Si SPAD that operates in the short-wave infrared, which can be compatible with Si foundry processing. Here, we investigate the pseudo-planar design with simulation and experiment to establish the spatial contributions to the dark-count rate, which will ultimately facilitate optimisation towards operation at temperatures compatible with Peltier cooler technologies.

In learning quantum mechanics, an essential question has always been: How does one go about developing a “physical feel” for quantum phenomena? Naturally, one needs a basis or ground zero to start from, and that basis must be unlike anything with which we are already familiar in consequence of our experiences with the world of classical physics. We argue (channeling Richard Feynman) that the most elementary and the least cumbersome concept to build upon is the existence of complex probability amplitudes for physical events. An event that can take place in multiple alternative ways should be treated by adding the corresponding amplitudes when the paths are, in principle, indistinguishable, and by adding the probabilities themselves when the paths are distinguishable. Once we accept this principle and hone our intuition by examining quantum phenomena in its light, we will be on the path to "understanding" quantum mechanics. Elementary examples from the field of quantum optics demonstrate how adherence to Feynman’s principle could lead to a better, more “intuitive” appreciation for the magic of quantum mechanics.

The Piccolo gated sensor features a 32x32 SPAD array of single-photon avalanche diodes (SPADs) operating in time-correlated single-photon counting (TCSPC). The chip enables event-driven readout and a maximum count rate of 220 Mcps. The sensor is based on the original Piccolo architecture, whereas the pixel was redesigned to accommodate a sub-nanosecond time gating circuitry. As a result, the pitch was increased by 3 μm to 31 μm with a slightly lower fill factor of 23.7%. The time-gating circuitry comprises active recharge to activate the gate and a fast switch to de-activate the SPAD. The sensor is equipped with 128 dynamically allocated, 50 ps time-to-digital converters (TDCs) at the bottom of the array. Four TDCs are shared among 32 SPADs in each column, where a mechanism of reallocation is used to optimize the use of TDCs and to minimize photon loss. Time gating can reduce both uncorrelated and correlated noise by reducing overall active time and by increasing relaxation time after detection, respectively. Upon acquisition of TCSPC data, the FPGA reorganizes it in histograms, which may be dynamically allocated and reduced in the number of bins to optimize memory use and data transfer from the FPGA to an external Mac/PC. The TDCs may also be calibrated to suppress differential and integral nonlinearities on-FPGA. Timestamps are stored in DDR3 and streamed out of the FPGA through PCIe with a data rate of 5.12 Gbps. Thanks to these techniques, the maximum count rate of the sensor was increased by about 3×. The time gating feature was implemented to extend dynamic range, and therefore depth, of near-infrared optical tomography (NIROT) and g(2) multi-depth time-domain diffuse correlation spectroscopy (TD-mDCS). Time gating is especially useful in NIROT and mDCS, as it helps suppress large numbers of early photons reflected back from the sample’s surface, e.g. the skull or skin. Thus, the Piccolo-gated architecture could show its suitability in these imaging modality.

In the last few decades, quartz has been the chosen material to fabricate piezoelectric resonators such as MEMS, racetrack resonators and tuning forks (TFs), which have been widely employed for a variety of sensing applications in several different fields, spanning from environmental monitoring to oil and gas industry. However, throughout the last decade, lithium niobate (LiN) started being employed in the integrated photonics field to build acousto-optical, electro-optical and nonlinear optical devices. In particular, 128° y-cut LiN has a higher density (4.64 g/cm³), Young modulus (145 GPa), and its piezoelectric coefficients are overall one order of magnitude greater compared to z-cut quartz. In this work, a custom lithium niobate tuning fork (LiNTF) is employed for the first time as a piezoelectric transducer in a photoacoustic spectroscopy-based apparatus devoted to gas sensing. The LiNTF was obtained from a 128° y-cut LN wafer and exhibits a resonance frequency f₀ = 39196.6 Hz and a quality factor Q = 5900 at atmospheric pressure. For this proof of concept, a water vapor absorption line falling at 7181.14 cm^-1 (1.392 μm) was targeted, achieving a signal to noise ratio (SNR) of 400 for a standard air sample having a 1.2% concentration of water vapor at atmospheric pressure and 100 ms of lock-in integration time. An Allan – Werle analysis showed a one order of magnitude improvement in the SNR when increasing the integration time up to 20 s. These preliminary results mark a first step towards the realization of LiNTF-based devices integrated on LiN platforms for gas sensing applications.

Simultaneous detection of different gas species represents an indispensable asset for several applications, such as instantaneous quantification of isotope concentration ratios, self-calibrating sensors, and monitoring of the temporal evolution of a chemical reaction. In this research work, a dual-gas quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor for a real-time analysis and in a continuous flow monitoring of one reactant and one product of a gas-phase chemical reaction involving nitrogen dioxide (NO₂) and water vapor (i.e., H₂O) – as reaction reactants – and nitrogen monoxide (NO) – as one of the reaction products – was realized. The QEPAS sensor implemented a spectrophone composed of a pair of metallic acoustic resonator tubes applied at both antinode points of a custom quartz tuning fork (QTF). In this configuration, two different quantum cascade lasers (QCLs) were used, having an emission wavelength centered at 5.26 μm – resonant with a nitrogen monoxide absorption feature located at 1,900.075 cm^-1 – and at 6.25 μm – resonant with a nitrogen dioxide absorption feature located at 1,601.77 cm^-1 –, respectively. The chemical reaction was studied by injecting in the gas line a certified concentration of 5,000 parts-per-million (ppm) of NO₂:N₂ and monitoring the QEPAS signals at four different total gas flow values, i.e., 10, 20, 30 and 50 standard cubic centimeters per minute (SCCM), respectively.

In the race toward increasingly high-performance trace-molecule sensors, one of the most significant steps forward in the last decade for photoacoustic sensors was their combination with high-finesse optical cavities. Validated with different configurations, this technique demonstrated enhanced sensitivities below the part-per-trillion level (ppt) and record dynamic ranges. Here we present our advanced cantilever-based photoacoustic setup, based on a custom-made silicon cantilever embedded in a doubly-resonant configuration. The combination of a high-quality-factor acoustic resonator and a high-finesse optical cavity allows a final sensitivity enhancement by several orders of magnitude. The sensor was tested on strong N₂O transitions around 4.5 μm wavelength with a continuous-wave quantum cascade laser.

Ga₂O₃ has become the new focal point of high-power semiconductor device research due to its superior capability to handle high voltages in smaller dimensions and with higher efficiencies compared to other commercialized semiconductors. However, the low thermal conductivity of the material is expected to limit device performance. To compensate for the low thermal conductivity of Ga₂O₃ and to achieve a very high density 2-dimensional electron gas (2DEG), an innovative idea is to combine Ga₂O₃ with III-Nitrides (which have higher thermal conductivity), such as AlN. However, metal-polar AlN/β-Ga₂O₃ heterojunction provide type-II heterojunction which are beneficial for optoelectronic application. because of the negative value of specific charge density. On the other hand, N-polar AlN/β-Ga₂O₃ heterostructures provide higher 2DEG concentration and larger breakdown voltage compared to conventional AlGaN/GaN devices. This advancement would allow the demonstration of RF power transistors with a 10x increase in power density compared to today’s State of the Art (SoA) and provide a solution to size, weight, and power-constrained applications.

Resonant cavity infrared detectors (RCIDs) can reduce the noise in sensing a laser signal by strongly suppressing background photocurrent at wavelengths outside the narrow spectral band of interest. We recently reported an RCID with 100-nm-thick InAsSb/InAs absorber, GaAs/AlGaAs bottom mirror, and Ge/SiO₂ top mirror. At T = 300 K, the external quantum efficiency reached 58% atλ_res ≈ 4.6 μm, with linewidth δλ = 27 nm. The characteristics at 125 K implied a specific detectivity of 5.5 × 10¹² cm Hz^½/W, which is more than 3× higher than for a state-of-the-art broadband HgCdTe device operating at that temperature. However, a prominent variation with mesa diameter of the deposited Ge spacer thickness made it difficult to predictably control λ_res for devices processed with a given diameter. This has been addressed by measuring the reflectivity spectrum following deposition of the spacer, so that thicknesses of the top mirror’s SiO₂ and Ge layers could be adjusted appropriately to attain a targeted resonance. This was especially beneficial in matching the λ_res for a small mesa, needed to minimize the capacitance in high-frequency measurements, to the emission wavelength of a given ewquantum cascade laser.

Wide (and ultra-wide) bandgap III-N semiconductors are promising for electronic devices operating at high power levels and in harsh environments, as well as for sensors such as UV detectors. The promise of these materials derives from a combination of their excellent carrier transport properties and the ability to operate at high internal electric fields. However, many current-generation devices do not fully exploit the material limits, and thus performance is below the fundamental performance limits expected from the material properties. Recent work on polarization-graded structures for internal electric field mitigation to enhance the breakdown voltage in HEMTs, cost-effective edge termination strategies for vertical power devices, and devices exploiting impact ionization and avalanche in GaN will be discussed. For example, we find that the use of polarization-grading can decrease the peak electric field in the channel, increase the breakdown voltage, and improve the power scaling of III-N based HEMTs, without the use of field plates that limit high-frequency performance; experimentally-validated power-added efficiency of 50% at 94 GHz has been achieved. In vertical devices, device high-field operation is often limited by edge effects; we report a strategy for edge termination that provides a large process window that is tolerant of both fabrication processing and epitaxial layer thickness and doping variations, and enables robust avalanche operation to be achieved in practice. In addition to increased breakdown voltage, the ability to harness impact ionization and avalanche for device functionality is also critical for avalanche photodiodes and negative-resistance oscillators such at IMPATT diodes. We report the recent demonstration of experimentally-measured negative resistance at microwave frequencies from GaN-based IMPATT diodes, illustrating direct exploitation of the high-field operation of GaN pn junctions for advanced functionality. These approaches are amenable to extension to ultra-wide bandgap III-N materials, particularly for applications in quantum sensing and ultra-high power density electronics.

We present a photothermal spectroscopy setup based on a broadband mid-infrared quantum cascade laser frequency comb (QCL-FC). In this PTS detection scheme, local refractive index changes of the gas sample due to absorption-induced local temperature changes are detected optically by a near-infrared heterodyne interferometer. Until now, this method has been demonstrated only with single-frequency lasers in the mid-infrared region, which limits its capability of targeting broadband absorption features. The QCL-FC used in this work covers the spectral range from 7.7 to 8.2 μm with a repetition rate of 9.9 GHz. A Fourier transform spectrometer modulates the intensity of QCL-FC, which excites photothermal effect of the gas sample in a Herriott multipass cell with optical path of 76 m. Spectroscopic measurements on nitrous oxide is performed as proof of concept. This technique combines the sensitivity of PTS detection and the broadband mid-infrared coverage of QCL-FC, which has a great potential to further promote the applications of QCL-FC in trace gas sensing.

Electromagnetic radiation in the mid-infrared portion of the spectrum is critical for sensing and spectroscopy. However, detecting mid-infrared radiation is challenging. Typically, mid-infrared detectors rely on photon absorption in exotic semiconductor structures, or they use relatively slow (low-bandwidth) thermal effects. Here, we demonstrate the detection of long-wave infrared laser pulses in metal-semiconductor-metal photodiode structures. The pulses span the spectral range from 7-12 microns and have pulse energies <1 nJ. Our detectors consist of gold and titanium nanoantenna emitters resting on an intrinsic silicon substrate and separated by sub-micron gaps from collector electrodes. Operating at room temperature, our detectors yield >2 μA currents and exhibit bandwidths exceeding 1.3 GHz.

The direct-current (DC) linearity of a photodiode is a parameter that indicates the direct proportionality between the input optical power and the output response current. This linear relationship defines the optical power range using which the performance limits of the photodiode control circuitry such as A/D conversion, trans-impedance amplifier, and quenching circuits are designed. Due to the absorption saturation and self-heating, the response current of the photodiode starts to saturate at higher optical power. A narrow absorber region in state-of-the-art photodiodes (avalanche and PIN photodiodes) results in an early absorption saturation in addition to a reduction in absorption efficiency. We present a photon-trapping microstructure (PTMS) equipped-avalanche photodiodes (APD) to enhance the absorption efficiency and DC linearity. We have fabricated a mesa-based APD using complementary metal oxide semiconductor (CMOS)-compatible processes. We present a DC current-voltage comparison of APDs in the dark and under the illumination of a wide wavelength range varying from 640 nm to 1100 nm. The fabricated PTMS-equipped APDs exhibit 5× increase in the external quantum efficiency as opposed to that of a flat device and a 70 unit multiplication gain. Further, the PTMS-equipped APDs demonstrate an increased linearity of 106.04 dB in comparison to 104.83 dB linearity in the flat device. The introduction of PTMS, despite the reduction of net-absorber volume, enables a uniform spread of the input illumination power by bending the light laterally and results in increased absorption efficiency and DC linearity.

Addressing the persistent speed-efficiency trade-off, advanced photodetector designs are increasingly incorporating nanophotonic structures to enhance detection efficiency. However, contemporary detector technologies continue to grapple with issues of high-power consumption and limited scalability potential. Embedded nanostructures in photodetectors have already been demonstrated to improve efficiency, gain, and slight improvement in high-speed performance. This paper presents a unique, scalable detector design that leverages nanophotonic enhancement while delivering an ultra-high response time with sub-picosecond full-width-half-maximum for 450 nm illumination wavelength with low breakdown voltage (~8V). Our innovative design strategy involves etching nanoholes into conventional p-i-n photodetectors (1 μm absorbing layer) and doping alternate nanoholes with p+ and n+ doping. The nanoholes are etched all the way through the intrinsic layer to connect with the top and bottom highly doped p+ and n+ doped layer forming a composite vertical-lateral electric field in the photodetector. This technique drastically reduces the effective carrier transport length to mere hundreds of nanometers without diminishing the photon absorption area. As a result, the timing response improves significantly compared to conventional models achieving sharp rise time of ~0.6 ps, fall time of ~8.5 ps, and full width half maximum of <4 ps. Furthermore, the design offers scalability along with advances in lithography processes, setting a promising direction for ultra-high-speed detectors scaling down to <1 ps response time suitable for emerging applications.

Quantum-cascade (QC) detectors are photovoltaic infrared detectors that exhibit low-noise characteristics dominated by the Johnson–Nyquist noise owing to the absence of fluctuations brought on by an external operation bias. When the Johnson–Nyquist noise level is low (at high device resistances), the flicker noise cannot be ignored in the lower-frequency region. However, the flicker noise seen in QC detectors has not been sufficiently discussed, and only the Johnson–Nyquist noise has been considered. In this study, we carried out flicker-noise analysis for mid-infrared QC detectors with a response wavelength of approximately 4.5 μm using experimental and theoretical approaches. The theoretical predictions, which were based on fluctuating charge-dipoles caused by electron trappings and de-trappings at impurity states, showed qualitative agreement with the measured temperature and device size dependencies of the flicker noise. Because doping of impurities into the absorption well is essential for detector operation, the results suggest that flicker noise is unavoidable in QC detectors. Therefore, to achieve the best low-noise performance of QC detectors, it is important to understand how flicker noise behaves in QC detectors using a theoretical model that considers the experimental results.

Passive mode-locking of lasers enables a compact way of stable optical pulse generation and is thus of high interest in research and application. Quantum cascade lasers (QCLs) emit radiation in the mid-infrared or terahertz (THz) spectral region and exhibit gain recovery on picosecond timescales. As the cavity round trip time is typically some tens of picoseconds, passively mode-locked operation of QCLs is undoubtedly challenging to achieve. However, short pulses from a THz QCL were recently observed by embedding a graphene saturable absorber into the top contact of the cavity. This contribution presents a model of the dynamic interplay between electric field, gain, and absorber, revealing that pulse formation and stability are possible in a narrow bias range, enhanced by spatial hole burning.

Cooperative emission due to spontaneous build-up and rapid decay of macroscopic polarization in a strongly inverted gain medium is drastically different from the lasing dynamics. The medium polarization and not the cavity field drives the emission from a device in semiclassical Maxwell-Bloch picture. Yet as pioneered by Dicke, the decay of the highest-energy state in a quantum ensemble of two-level systems proceeds through a ladder of the highest-symmetry partially deexcited states, each of which is formalistically an entangled state in mathematical sense. In this paper we summarize our experimental and theoretical studies on the two fundamental aspects of superradiance in multi-section tandem cavity laser heterostructures: (i) How can the superradiance be reached in semiconductor quantum wells albeit the ultrafast dephasing of individual microscopic e-h dipoles? (ii) Could the ensemble non-classicality be transferred to the emitted optical field and what could be the resulting photon state?

Modern optical gas detection systems utilize the technique of tunable laser absorption spectroscopy for different applications in science, manufacture, or medicine. Superlattice structures composed of semiconductors from the 6.1 Å family enable type-II band alignments and have the potential to exceed state of the art figure of merits of widely used infrared detectors. In this study, InAs/GaSb and InAs/InAsSb type-II superlattices were grown using molecular beam epitaxy and characterized using Fourier-transform infrared spectroscopy and pump-probe transient absorption technique. Photoluminescence spectra were obtained for all samples in 10 to 300K temperature range and then complemented with photoreflectance measurements for characteristic temperatures to increase the sensitivity of the measurement for less optically active transitions. In addition, pump-probe measurements were performed to investigate the dynamics of carrier relaxation and recombination processes in proximity of transition energies observed in previous experiments.

The presence of dislocations and grain boundaries has a crucial influence on strain behaviors and oxygen migration dynamics and therefore drastically changes the electronic and mechanical properties of dielectric and ferroelectric materials. In this paper, we discuss our recent investigations of the polymorphic nanodomain phenomena with grain boundaries, including the structural phase segregation, strain relaxation with oxygen vacancy migrations, and ionic defect dynamics at interfacial and bulk regions in prototypical dielectrics, SrTiO₃ (STO) single- and bi- crystals and ferroelectric, BaZr_0.2Ti_0.8O₃ (BZT) films by second harmonic generation methods. We reveal that the grain boundary in STO bicrystal acts as a barrier to the oxygen vacancy migration, and a strain gradient is developed on both sides of the boundary. The misfit strain and the interphase electromechanical interactions in BZT are shown to introduce a bimodal nanodomain structure, which correlates well with polymorphic phase boundaries and provides a promising alternative to chemical compositional design, for the optimization of dielectric thin films used in capacitive energy storage applications.

The Sagnac effect, manifest in the phase shift of a rotating interferometer proportional to its physical area and to the rotation rate of the frame, plays a crucial role in modern physics being generalized to any kind of interferometer. However, as compared to light, the accurate experimental validation of the Sagnac effect for matter-waves remains challenging. We report the accurate measurement of the Sagnac phase shift, induced by the Earth’s rotation, using a large-area cold atom interferometer. We probe the atomic Sagnac phase shift along the two orthogonal horizontal axes in various orientation of the sensor with respect to geographic north and obtain an agreement with the theoretical prediction at the 20 ppm level, improving by twenty five-fold over the previous achievements for matter waves. Our results underline the universality of the Sagnac effect and open new horizons for testing other fundamental theories, as well as for a number of technological applications in seismology, geodesy and inertial navigation systems.

Entangled photons could translate complex, ultrafast laser-based spectroscopy and imaging to a chip-sized or distributable platform. The inherent temporal and energy correlations between the two entangled photons created in spontaneous parametric down conversion (SPDC) allow a continuous-wave laser diode source to replicate ultrafast, two-pulse experiments that are usually performed in table-top scale setups. Although entangled photons were originally proposed to enhance multiphoton or nonlinear processes, few entangled experiments to date have outperformed classical photon sources from a full systems perspective. The low pump power criteria and periodically poled nonlinear sources, however, do make entangled photon experiments natural candidates for on-chip photonic circuits and spectroscopy. In our talk, we will discuss experiments that prove that the miniaturization of ultrafast experiments is a key area where entangled photons can compete with or prove superior to classical photon experiments. We will first discuss our progress in creating entangled photon sources in the visible to deep ultraviolet (UV) wavelengths as needed for spectroscopy and imaging, the integration of the subsequent photonic circuitry needed to create on-chip spectrometers, and then experimental results proving that on-chip entangled photon sources can replicate ultrafast pump-probe or be used for fluorescence lifetime imaging microscopy (FLIM) experiments. We will also discuss our increasingly null experiments on topics like entangled two-photon absorption (ETPA) which are equally important for the progressing the field.

Metasurfaces with angular sensitivity have been shown to provide a platform for developing an ultra-compact phase imaging system. Their performance, however, is often limited to a narrow range of spatial frequencies. Here, we apply inverse design to design and fabricate a metasurface an asymmetric optical transfer function across a numerical aperture (NA) of 0.6. The engineered response of this device enables phase imaging of microscopic transparent objects.

Quartz-enhanced photoacoustic spectroscopy (QEPAS) is a highly sensitive optical technique, suitable for real-time and in situ trace gas detection. In QEPAS, Quartz tuning forks (QTF) are employed as piezoelectric transducers of sound waves, induced by gas non-radiative energy relaxation following an infrared modulated light absorption. The generated electric signal depends on the gas concentration. An accurate and reliable QEPAS measurement requires: i) the QTF characterization, in terms of resonance frequency and quality factor and ii) the tuning range scan of the laser employed to detect the selected gas. These two operations could take several minutes. Beat frequency QEPAS (BF-QEPAS) is an alternative approach to standard QEPAS. In BF-QEPAS, a fast scan of the laser tuning range is employed to generate an acoustic pulse. Gas concentration, QTF resonance frequency, and quality factor can be measured acquiring and analyzing the transient response of the QTF to the acoustic pulse. In this work, a custom T-shaped QTF was employed to detect nitrogen monoxide (NO), targeting its absorption feature at 1900.07 cm^-1 with an interband cascade laser. A minimum detection limit as low as 180 ppb of NO at an integration time of 5 ms was achieved, and a highly accurate measurement of the QTF resonance frequency and quality factor were demonstrated using BF-QEPAS. Finally, the possibility to fully scan the laser tuning range in less than 15 s was proved.

This study demonstrates the frequency modulation of Rydberg states in 87Rb thermal vapor leading to the production of sidebands in electromagnetically induced transparency (EIT) signal. The modulation is provided externally through radio-frequency (RF) electric fields from a parallel-plate capacitor. The modified EIT signal is detected via the conventional lock-in amplified EIT setup. We assess the strengths of the peaks in the modulated EIT spectrum and compare them to a mathematical model based on Floquet theory. The relationship between the strength of the EIT peaks and the RF voltage is identified as a function of the Bessel function of the modulation index.