Hot Topics 2021

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).

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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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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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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