For more effective early-stage cancer diagnostics, there is a need to develop sensitive and specific, non- or minimally invasive, and cost-effective methods for identifying circulating nanoscale extracellular vesicles (EVs). Here, we report the utilization of a simple plasmonic scaffold composed of a microscale biosilicate substrate embedded with silver nanoparticles for surface-enhanced Raman scattering (SERS) analysis of ovarian and endometrial cancer EVs. These substrates are rapidly and inexpensively produced without any complex equipment or lithography. We extensively characterize the substrates with electron microscopy and outline a reproducible methodology for their use in analyzing EVs from in vitro and in vivo biofluids. We report effective chemical treatments for (i) decoration of metal surfaces with cysteamine to nonspecifically pull down EVs to SERS hotspots and (ii) enzymatic cleavage of extraluminal moieties at the surface of EVs that prevent localization of complementary chemical features (lipids/proteins) to the vicinity of the metal-enhanced fields. We observe a major loss of sensitivity for ovarian and endometrial cancer following enzymatic cleavage of EVs’ extraluminal domain, suggesting its critical significance for diagnostic platforms. We demonstrate that the SERS technique represents an ideal tool to assess and measure the high heterogeneity of EVs isolated from clinical samples in an inexpensive, rapid, and label-free assay.

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Raman spectroscopy is a powerful and widespread technique to investigate the molecular composition of a sample in a label-free manner. Recent evidence has suggested that intraoperative Raman spectroscopy can differentiate between healthy and tumorigenic brain tissue in humans, helping surgeons to perform an optimal resection (Jermyn et al., 2015). However, the application of this promising method in deep tissue is hindered by the bulk of current Raman endoscopes, which typically use multiple waveguides with an overall implant cross section >2 mm. To circumvent this limitation, we propose a novel experimental method using a single tapered optical fiber waveguide to excite and collect Raman signal with depth-resolution along the fiber axis. Considering the successful application of tapered fiber as optical neural probes (Pisanello et al., 2017; Pisano et al., 2019), we view our approach as a promising approach towards minimally invasive, depth-resolved in vivo Raman spectroscopy of deep brain tissue.

Jermyn, M. et al. (2015). Sci. Transl. Med. 7, 274ra19-274ra19.
Pisanello, F. et al. (2017). Nat. Neurosci. 20, 1180–1188. Pisano, F. et al. (2019). Nat. Methods. 16, 1185–1192.

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Intersubband devices, such as quantum cascade lasers (QCLs), detectors (QCDs), and QWIPs, have seen a massive performance improvement over the past two decades. Such structures exhibit giant nonlinearities, which have been exploited for down-converted room temperature THz sources [1] and frequency comb generation in QCLs [2]. They are also promising for pulsed frequency comb emission by passive mode-locking [3] as well as non-classical frequency combs [4]. Previous studies [3,5–8] relied on perturbation theory or simplified density-matrix models, and considered only a few quantum states. Thus, these were unable to treat the strong fields exhibited under normal QCL operation, neglect coherent effects from system-bath interactions, and ignore non-resonant contributions. Instead, we use a time-resolved density matrix model [9,10] taking into account all electronic states of QCL comb devices, all scattering channels and arbitrary pulse shapes. We compute the response to pulse strengths typically achieved under operation, to infinite order. We compare this to perturbation calculations including all relevant states, and observe their break-down for strong fields. In addition, our time-resolved approach computes the optical response significantly faster when many subband states are considered. We also present QCL frequency comb designs optimized for high four-wave mixing nonlinearity, demonstrating the usefulness of more general approaches to optical nonlinearities.

[1] M.A. Belkin et al., Sel. Top. Quantum Electron. IEEE J. Of 15, 952 (2009).
[2] A. Hugi et al., Nature 492, 229 (2012).
[3] P. Tzenov et al., New J. Phys. 20, 053055 (2018).
[4] https://www.qombs-project.eu/ .
[5] N. Owschimikow, et al., Phys Rev Lett 90, 043902 (2003).
[6] P. Friedli et al., Appl. Phys. Lett. 102, 222104 (2013).
[7] W. Kuehn, et al., J. Phys. Chem. B 115, 5448 (2011).
[8] G. Villares et al., Opt. Express 23, 1651 (2015).
[9] S. Markmann, et al., Nanophotonics 0, (2020).
[10] G. Kiršanskas, et al., Phys. Rev. B 97, 035432 (2018).

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Azobenzene has been one of the most ubiquitous organic photoswitches over the past few decades due to the photoisomerization where azobenzene undergoes trans-to-cis geometrical changes upon light illuminations. In the common photoisomerization, the refractive index of azobenzene decreases; thus, if azobenzene is treated as the resonant cavity material, it will tune the cavity resonant wavelength. Owing to the lack of physical rigidity and high UV absorption, it is more difficult to apply organic small molecules as the substrate of the on-chip nanofabrication than those mature inorganic materials such as Si and SiO2 wafers. To address this issue, here, we designed a hybrid azobenzene/SiO2 microcavity for tuning optical resonant cavity wavelength with the use of azobenzene photoisomerization. A monolayer of azobenzene containing 4-(4-diethylaminophenylazo)pyridine (Aazo) was functionalized on the surface of SiO2 microtoroid cavities. The intercolating CH3 spacer molecules were also deposited on the surface of the devices, mixing with Aazo at 1:1 ratio to facilitate photoisomerization. Cavity quality factors near 1 million in the near-IR are maintained, due to the uniformity of Aazo monolayer. Two optical lasers were simultaneously coupled into the Aazo-coated devices using a single waveguide. The near-IR 1300 nm laser, as the probe laser, is tuned to excite a single resonant wavelength of the cavity, and the blue 450 (or 410) nm laser induces the trans-to-cis photoisomerization. When the Aazo photoswitches, the near-IR cavity resonant wavelength blueshifts due to a change of refractive index in the Aazo layer, shown by finite element method simulations of the optical model. We found that by using 450 nm laser, the cavity resonant wavelength can be blueshifted as far as ~0.7 free spectral range (FSR) of the cavity. 410 nm tunable laser blueshifts to ~0.2 FSR, and maximum blueshift for both 450 and 410 nm show linear relationships with the coupled power.

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Strong coupling regime has been investigated in a wide range of solid-state platforms, thanks to improvements in resonators design and fabrication [1]. By increasing the number of dipoles and optimizing the spatial overlap with the photonic field, the light-matter coupling can become strong enough to modify the materials’ electronic properties. Previously predicted and observed as a modification of the excitonic radius in undoped quantum wells (QW) [2,3], we recently predicted that cavity-mediated wavefunction engineering allows for the creation of novel excitonic bound states in which electrons and holes are bound together by the exchange of cavity photons [4]. These novel states manifest themselves as discrete resonances below the ionization threshold for coupling strengths above a critical value [5]. The first observation of cavity-induced bound excitons has been reported in doped GaAs QWs, embedded in gold microcavity resonator [5]. As Coulomb interaction does not create bound states in doped QWs [6], and ours are designed thin enough to host a single localised subband, hence manifesting no bound-to-bound transitions, we could unequivocally identify the optical resonances appearing below the ionization threshold as due to the interaction with the photonic resonator. From a fundamental point of view the states observed here remain a first demonstration of two charged particles bound through photon exchange. We hope our result will represent a milestone toward the realization of novel mid-infrared devices, and the development of quantum materials whose electronic properties can be opportunely tuned beyond those permitted by mere crystal properties.

[1] Ballarini at al. Nanophotonics 8 (2019).
[2] J. B. Khurgin. In:Solid state communications 117 (2001).
[3] S. Brodbeck et al. In:Phys. Rev. Lett. 119 (2017).
[4] E. Cortese et al. In:Optica 6 (2019).
[5] E. Cortese et al. In:Nature Physics (2020).
[6] D. E. Nikonov et al. In:Phys. Rev. Lett. 79 (1997).

Objects around us constantly emit and absorb thermal radiation. For reciprocal systems, the emissivity and absorptivity are restricted to be equal by Kirchhoff’s law of thermal radiation [1]. This restriction limits the control of thermal radiation and contributes to an intrinsic loss mechanism in photonic energy harvesting systems. Existing approaches to violate Kirchhoff’s law typically utilize conventional magneto-optical effects in the presence of an external magnetic field. However, these approaches require either a strong magnetic field (~3T) [2] or narrow-band resonances under a moderate magnetic field (~0.3T) [3], because the non-reciprocity in conventional magneto-optical effects is usually weak in the thermal wavelength range. Here we show that the axion electrodynamics in magnetic Weyl semimetals can be used to construct strongly nonreciprocal thermal emitters [4]. Such a thermal emitter can near completely violate Kirchhoff’s law over broad angular and frequency ranges without requiring any external magnetic field.

[1] G. Kirchhoff, Annalen Der Physik 185, 275 (1860).
[2] L. Zhu and S. Fan, Phys. Rev. B 90, 220301 (2014).
[3] B. Zhao, Y. Shi, J. Wang, Z. Zhao, N. Zhao, and S. Fan, Opt. Lett. 44, 17(2019).
[4] B. Zhao, C. Guo, C. A. C. Garcia, P. Narang, and S. Fan, Nano Lett. 20, 1923 (2020).

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Image processing systems including edge detection, object classification, and recognition systems have become critical technologies in a variety of science and engineering disciplines. While most image processing is performed digitally, often with neural networks, the processing power available ultimately limits the complexity of tasks that can be executed in real-time. One potential solution to these issues is to off-load some of the processing tasks to an optical system, increasing the processing speed while also reducing power consumption. In addition, optical systems offer the opportunity to distinguish polarization, phase, angle of incidence, or orbital angular momentum, information that is typically lost when recording an image on a camera.

Edge detection is one of the most fundamental forms of image processing as it useful in image classification, segmentation, and recognition. However, due to their computational complexity edge detectors often place a limit on the processing speed of the system. Here, we demonstrate high-speed edge detection by using a flat optic for direct image differentiation. We demonstrate how the image differentiator can be combined with traditional imaging systems such as a commercial optical microscope as well as in a monolithic compound flat optic by integrating the differentiator with a metalens. The compound nanophotonic system manifests the advantage of thin form factor optics as well as the ability to implement complex transfer functions. We will also discuss dynamic filters that allow one to switch between bright-field and edge imaging as well as more complicated image processing that employs a wider range of spatial filters (low-pass, high-pass, and band-pass) on the same meta-optic. These systems can be used in conjunction with traditional neural networks to off-load computationally intensive tasks for high speed and low power image processing, object classification, and object recognition.

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Artificial magnetic fields are revolutionizing our ability to manipulate neutral particles, by enabling the emulation of exotic phenomena once thought to be exclusive to charged particles. Within this paradigm, pseudo-magnetic fields generated by non-uniform strain have attracted considerable interest because of their simple geometrical origin. However, to date, they have failed to emulate the tunability of real magnetic fields because they are determined solely by the engineered strain configuration, rendering them fixed by design. Here, we unveil a new universal mechanism to tune pseudo-magnetic fields for polaritons supported by strained honeycomb metasurfaces composed of interacting dipole emitters/antennas. Without altering the strain configuration, we show that one can tune the pseudo-magnetic field strength by modifying the surrounding electromagnetic environment via an enclosing cavity waveguide, which modifies the nature of the dipole-dipole interactions. Remarkably, due to the competition between short-range Coulomb interactions and long-range photon-mediated interactions, one can switch off the pseudo-magnetic field entirely at a critical cavity width, without removing the strain — something that is impossible to achieve with systems that emulate tight-binding physics. Consequently, we demonstrate a tunable Lorentz-like force that can be switched on/off, deflecting polariton wavepackets into effective cyclotron orbits whose radius can be controlled via the cavity width. For large strains, we demonstrate Landau quantization of the polariton cyclotron orbits, where progressively decreasing the cavity width can induce a collapse and revival of the polariton Landau levels. Unlocking this tunable pseudo-magnetism poses new intriguing questions beyond the paradigm of conventional tight-binding physics.

[1] C.-R. Mann, S.A.R. Horsley, E. Mariani. Tunable pseudo-magnetic fields for polaritons in strained metasurfaces. Nature Photonics (2020).

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Gratings and holograms use patterned surfaces to tailor optical signals by diffraction. Despite their long history, variants with remarkable functionalities continue to be developed. Further advances could exploit Fourier optics, which specifies the surface pattern that generates a desired diffracted output through its Fourier transform. To shape the optical wavefront, the ideal surface profile should contain a precise sum of sinusoidal waves, each with a well defined amplitude, spatial frequency and phase. However, because fabrication techniques typically yield profiles with at most a few depth levels, complex ‘wavy’ surfaces cannot be obtained, limiting the straightforward mathematical design and implementation of sophisticated diffractive optics. Here we present a simple yet powerful approach to eliminate this design–fabrication mismatch by demonstrating optical surfaces that contain an arbitrary number of specified sinusoids. We combine thermal scanning-probe lithography and templating to create periodic and aperiodic surface patterns with continuous depth control and sub-wavelength spatial resolution. Multicomponent linear gratings allow precise manipulation of electromagnetic signals through Fourier-spectrum engineering. Consequently, we overcome a previous limitation in photonics by creating an ultrathin grating that simultaneously couples red, green and blue light at the same angle of incidence. More broadly, we analytically design and accurately replicate intricate two-dimensional moiré patterns, quasicrystals and holograms, demonstrating a variety of previously unattainable diffractive surfaces. This approach may find application in optical devices (biosensors, lasers, metasurfaces and modulators) and emerging areas in photonics (topological structures, transformation optics and valleytronics).

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The ability to arbitrarily control the temporal evolution of an ultrafast pulse, namely shaping its amplitude, phase, and polarization, forms the backbone of ultrafast science and technology. Such three-dimensionally shaped, vectorial pulses are essential for both studying fundamental light-matter interactions and developing real-world applications. Here, we report on the first experimental demonstration of a dielectric-metasurface-enabled pulse shaper able to tailor the temporal polarization of near-infrared femtosecond pulses, allowing rich time-evolving instantaneous polarization states within a single pulse. The polarization degree of freedom of the femtosecond pulse is controlled through parallel manipulation of the spectral components using a Fourier-transform setup. A dielectric metasurface, consists of an array of rectangular silicon nanopillars orientated at 45° with respect to the input p-polarized femtosecond pulse of ≈10 fs duration (full-width at tenth-maximum bandwidth of ≈ 80 THz, centered at 800 nm), is positioned in the focal plane, providing a complex masking function to the spectral components along the two orthogonal polarizations. By engineering the complex masking function, temporal polarization states of the output pulse can be tailored at will. For example, a femtosecond pulse exhibiting an initial instantaneous polarization state of the right circular polarization, which gradually changes to right-handed elliptical polarizations with time, then linear polarizations around the time zero followed by left-handed elliptical polarizations before finally approaches a left circular polarization, can be achieved. In addition, the advantage of multi-functionalities at a single-pixel level provided by metasurfaces shows the potential of combining the temporal waveform and spatial wavefront shaping of ultrafast pulses using one metasurface, by superimposing a secondary two-dimensional phase-function to each spectral component.

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Photonic biosensors have emerged as a promising alternative for medical diagnostics, offering a versatile technology for rapid and sensitive analysis of biomarkers in a label-free format and integrated in point-of-care (POC) devices. Especially those based on nanoplasmonics and silicon photonics have demonstrated an exceptional potential for tackling current challenges in POC testing, thanks to their unique robustness and reliability, high sensitivity, and simple adaptation to a large variety of targets, including proteins, nucleic acids, cells, or pathogens.
Our research aims to provide novel nanophotonic biosensors for clinical diagnostics and biomedical studies. We focus on two main technologies: Surface Plasmon Resonance (SPR) biosensors, and its derivatives in nanoplasmonics, and a pioneering silicon nanophotonic interferometer, the Bimodal Waveguide (BiMW) biosensor. Our work involves the whole process in biosensor production, from design and nanofabrication until the final validation in real clinical scenarios. We have demonstrated successful applications for early cancer diagnostics, gluten-free diet and anticoagulant treatment monitoring, for genomic and epigenomic cell regulation studies, and for infectious pathogen detection. Since March 2020, we are leading a large research project for implementing nanophotonic biosensors as a tool for COVID-19 diagnostics.

In this presentation, I will give an overview of our most recent work and the latest advances in photonic biosensor development for medical diagnosis, including sensor design and POC integration, surface biofunctionalization strategies, and label-free bioassay approaches that will enable a reliable implementation in the clinical practice.

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With the rising interest in the study of heterogeneous cellular populations, for instance in the case of cancer biology, it became clear the need for instrumentations that could guarantee the possibility to analyse a big number of samples at subcellular resolution. Light-sheet fluorescence microscopy is a technique that showed the high quality and fast imaging capacity, 3D reconstruction and low photodamage level. A limitation of the approach is the time-consuming sample mounting and the alignment procedure that reduces the effective throughput. In this work, we make use of microfluidics for sample delivery, in order to achieve a continuous and automatic measurement of tens of sample per run. We propose two integrated optofluidic biochips, consisting in a microfluidic channel and embedded micro-lenses for light-sheet generation. These devices are realized by femtosecond laser micromachining in glass. This technique guarantees the precise and stable alignment of all components, as well as high-complexity 3D geometry definition and micrometric feature size in the realization of the optics and the channels. The two designs have been tailored for the analysis of samples with different shapes and dimensions. The first one is meant for single-cell dual-color fluorescence analysis and allows the processing of tens of sample per hour in an automatized way, with a sub-nuclear 3D resolution. The second one is a dual-side dual-color illumination LSFM chip for drosophila embryos imaging (ellipsoidal samples with dimensions around 500um by 100 um) with an engineered microchannel for automatic optimal sample alignment. These biochips can be used as a compact and user-friendly add-on for standard inverted fluorescence microscopes. They allow the analysis of a large number of samples, still maintaining high quality 3D imaging at single sample level, allowing statistical analysis and study of population heterogeneities.

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Malaria persists as a predominant, often fatal disease in developing nations, with 3.1 billion people across 89 countries at risk of infection. Despite the existence of treatments that are highly effective if administered early in disease progression, about 435,000 deaths occurred from over 219 million diagnoses in 2018. Obtaining an early, accurate diagnosis can be difficult in low-resource malaria-endemic environments due to limitations in available diagnostic technologies. Commonly used technologies, including light microscopy and antibody-based rapid diagnostic tests, suffer from high costs and low reliability, respectively. In addition, transmission of malaria is exacerbated by large reservoirs of infection in asymptomatic patients, long incubation periods of some species, and symptoms that can be difficult to distinguish from the flu without a test. Therefore, there is a significant need for a technology that can be widely available in low-resource settings, give an accurate diagnosis, and identify asymptomatic patients.
We previously published work demonstrating the use of an optical, point-of-care diagnostic system that leverages the presence of hemozoin, a magnetic byproduct of the malaria parasite. Using the standard, synthetic hemozoin mimic, β-hematin, suspended whole rabbit blood, the system detected concentrations as low as 0.0087 µg/mL, which is well below the clinically relevant range of <1 to 5 µg/mL, in sample volumes of 500 µL. Here, we present a new design that optimizes portability and complexity of the system and reduces the required sample volume. The system has been simplified to reduce cost, power consumption, and overall system size and allow for greater portability and availability in low-resource settings. The sample size has been reduced to about 200 µL, the amount of blood that can be obtained by a capillary blood draw. Validation with iron oxide magnetic nanoparticles suspended in water has confirmed device performance.

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The conventional method of analysing blood biomarkers is not only time consuming, but also technically demanding due to complexities in blood handling and fractionation process. Consequently, there is a demand for developing assay that is highly sensitive, selective and multiplexable that can be advanced towards point-of-care testing. Silicon photonic based biosensors have emerged over the decades as one of the technologies that favours the development of point-of-care devices due to their integration and multiplexing capabilities, label-free sensing, high sensitivity, compact footprint and low fabrication cost in high volumes with CMOS-compatible processes [1]. Here we propose an asymmetrical Mach-Zehnder interferometric (AMZI) sensor integrated with automated microfluidic for the detection of blood-based biomarkers. Our biosensor employs an innovative signal processing based on optical frequency comb sampling for the AMZI signal phase extraction [2]. Compared to the conventional single wavelength phase extraction method, this method uses three comb lines which are split into three channels through a demultiplexer and detection is measured through three photodetectors and extracted using a data-acquisition system. This way, a continuous and linear phase readout is achieved, independent of any bias drift. This system eliminates the complicated manual phase extraction step in the conventional method where maximum sensitivity is only found at quadrature point without sacrificing the sensor lower limit of detection of 3.70 x 10-7 RIU. This therefore allows for more stable and accurate detection of blood biomarkers and pave the way towards the development of high-precision and consistent point-of-care diagnostic tests.

[1] E. Luan, H. Shoman, D.M. Ratner, K.C. Cheung, L. Chrostowski, Sensors (Basel), 18 (10), 3519. (2018).
[2] M. Knoerzer, C. Szydzik, G. Ren, C.S. Huertas, S. Palmer, P. Tang, T. Nguyen, L. Bui, A. Boes, A. Mitchell, Opt. Express, 27, 21532-2

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Terahertz (THz) frequency technology has stimulated a major surge in interdisciplinary research over the last decade, inspiring both fundamental insights and new applications in microscopicand macroscopic systems,as well as highlighting potential new research avenues in the fields of biomedical imaging, astronomy, security, and high-resolution sensing.

Quantum cascade lasers (QCLs) emitting at THz frequencies have undergone a rapid evolution since their first demonstration, and, in the last few years, have become the best performing, compact THz frequency source in terms of their output power and differential efficiency, frequency tunability, spectral purity and low power consumption. THz QCL can also be engineered with a very broad gain bandwidth, enabling broadband coherent emission as well as the spontaneous formation of THz frequency combs, which have major applications in high-resolution and high-precision spectroscopy and metrology. Furthermore, photonic engineering, combined with new resonator concepts, have enabled the performance of THz QCLs to be designed with an incredible level of control, offering a flexible platform to tailor the emission spectrum, beam profile and output power on-purpose.

This talk will review our recent developments in engineering and devising novel highly efficient broadband QCL resonators, behaving as frequency combs at Terahertz frequencies, with record optical powers per mode and record dynamic range, with a special emphasis on novel integrated architectures.

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We present a mode-locked laser on the novel III-V-on-silicon-nitride platform, developed for microwave photonics, LIDAR and spectroscopy, applications where dense and low-noise frequency combs with a compact form factor are needed. Such combs can be generated efficiently with mode-locked diode lasers, which are electrically powered and hence allow for single-chip integration without the need for an external optical pump source. Thus far, however, demonstrated chip-scale mode-locked laser devices suffer from significant linear and nonlinear losses in the passive cavity, which restrict the attainable noise performance and comb density. To overcome these limitations, we transitioned from silicon to a silicon-nitride-on-silicon-on-insulator platform. Furthermore, we integrated an InP-based saturable absorber and amplifier with 6 InAlGaAs quantum wells using the microtransfer printing technique. Our heterogeneously integrated mode-locked laser operates around 1580 nm and exhibits a record-low line spacing of 755 MHz (bridging the gap between >2 GHz FSR Kerr combs and low repetition rate fiber-based mode-locked lasers), a fundamental radio-frequency linewidth of 1 Hz, and an optical linewidth below 200 kHz. The wafer-scale manufacturing we used can produce low-cost devices in high volume, and could be extended to span other wavelength ranges or to deliver high optical output power.

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In many laser systems, frequency combs can form whose output is frequency-modulated (FM), producing light whose frequency sweeps linearly. This effect has been seen in a number of systems, particularly semiconductor lasers. For example, it has been seen in quantum cascade lasers [1], [2], in quantum dot lasers [3], and in diode lasers [4]. While this intriguing result has been replicated experimentally [5] and numerically, a compact description of the core physics has remained elusive. By creating a mean-field theory for active cavities analogous to the Lugiato-Lefever Equation, we show that these lasers are described extremely simply: by a nonlinear Schrodinger equation with a potential proportional to the phase of the electric field. This equation can be solved analytically and produces a field with quasi-constant intensity and piecewise quadratic phase. We refer to these nondispersive waves as extendons, and they describe both fundamental FM combs and harmonic states. Our results apply to many lasers, explaining the ubiquity of this phenomenon, and our new theory unifies many experimental observations [6].

[1] M. Singleton et al., “Evidence of linear chirp in mid-infrared quantum cascade lasers,” Optica, vol. 5, no. 8, p. 948, 2018.
[2] D. Burghoff et al., “Terahertz laser frequency combs,” Nature Photon, vol. 8, no. 6, pp. 462–467,. 2014.
[3] J. Hillbrand et al., “In-Phase and Anti-Phase Synchronization in a Laser Frequency Comb,” Phys. Rev. Lett., vol. 124, no. 2, p. 023901, 2020.
[4] L. A. Sterczewski et al., “Frequency-modulated diode laser frequency combs at 2 μm wavelength,” APL Photonics, vol. 5, no. 7, p. 076111, 2020.
[5] D. Burghoff et al., “Evaluating the coherence and time-domain profile of quantum cascade laser frequency combs,” Opt. Express, vol. 23, no. 2, p. 1190, 2015.
[6] D. Burghoff, “Unraveling the origin of frequency modulated combs using active cavity mean-field theory,” Optica (in press, preprint available at arXiv:2006.12397), 2020.

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Whispering gallery mode (WGM) resonators made from non-linear electro-optic materials are excellent platforms for frequency manipulation of electromagnetic radiation, allowing microwave up-conversion [1,2] and the generation of frequency combs [3]. Efficient generation of light requires momentum conservation which implies phase matching between the microwave field and the input and output optical fields [4], a requirement that is often difficult to achieve. Here we achieve this by using an x-cut uniaxial lithium niobate crystal to fabricate our WGM resonator, resulting in the electro-optic co-efficient varying around the circumference of the resonator. This allows a uniform microwave field to be used, simplifying the engineering requirements. We demonstrate a frequency comb with excellent repetition rate stability, and show results from dual combs with orthogonal polarisations. We also measure the relative frequency stability of the dual combs, and find it to be 4 mHz, without any need for stabilisation or post processing. These results pave the way for coaxial geometries for microwave-optical devices using WGMs, an architecture intrinsically compatible with current microwave technology.

1. A. Rueda, F. Sedlmeir, M. C. Collodo, U. Vogl, B. Stiller, G. Schunk, D. V. Strekalov, C. Marquardt, J. M. Fink, O. Painter, G. Leuchs, and H. G. L. Schwefel, “Efficient microwave to optical photon conversion: an electro-optical realization,” Optica 3, 597 (2016).
2. N. J. Lambert, A. Rueda, F. Sedlmeir, and H. G. L. Schwefel, “Coherent Conversion Between Microwave and Optical Photons—An Overview of Physical Implementations,” Adv. Quantum Technol. 3, 1900077 (2020).
3. A. Rueda, F. Sedlmeir, M. Kumari, G. Leuchs, and H. G. L. Schwefel, “Resonant electro-optic frequency comb,” Nature 568, 378 (2019).
4. D. V. Strekalov, C. Marquardt, A. B. Matsko, H. G. L. Schwefel, and G. Leuchs, “Nonlinear and quantum optics with whispering gallery resonators,” J. Opt. 18, 123002 (2016).

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