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This Viewpoint responds to the analysis of 2D IR spectra of vibration cavity polariton in the study reported in The Journal of Physical Chemistry Letters (Duan, et al. 2021, 12, 11406). That report analyzed 2D IR spectra of strongly coupled molecules, such as W(CO)6 and nitroprusside anion, based on subtracting a background signal generated by polariton filtered free space signals. They assigned the resulting response as being due to excited polaritons. We point out in this Viewpoint that virtually all of the response can be properly reproduced using the physics of transmission through an etalon containing a material modeled with a complex dielectric function describing the ground and excited state absorber populations. Furthermore, such a coupled system cannot be described as a scaled sum of the bare molecular and cavity responses.

We perform a detailed comparison of 2D Auger coefficients in narrow-gap type-I and type-II quantum wells (T1QWs and T2QWs), and the relative effects of Auger non-radiative decay on midwave infrared lasers employing both types of gain media. Comparison is also made to 3D Auger coefficients in bulk mid-IR materials, by defining a “normalization length” that accounts for the spatial extent of the electron and hole wavefunctions in the quantum wells. The comparisons confirm that Auger recombination in both types of QW is substantially suppressed relative to bulk, due primarily to the effects of compressive strain on the valence subband dispersions. We find that the 2D Auger coefficients in T1QWs remain substantially lower than those in T2QWs out to wavelengths beyond $3.5~\mu \text{m}$ . However, this does not necessarily imply a lower lasing threshold because a substantial fraction of the holes injected into T1QWs occupy lower subbands that do not contribute gain, so more must be injected to reach the lasing threshold. When all of the relevant considerations are combined, the thresholds for the best T1QW and T2QW lasers cross over near $\lambda ~\approx ~3.0~\mu \text{m}$ . Other characteristics governing the relative T1QW and T2QW laser performances above threshold, such as maximum output power and wallplug efficiency, are also considered.

Molecular polaritons result from light-matter coupling between optical resonances and molecular electronic or vibrational transitions. When the coupling is strong enough, new hybridized states with mixed photon-material character are observed spectroscopically, with resonances shifted above and below the uncoupled frequency. These new modes have unique optical properties and can be exploited to promote or inhibit physical and chemical processes. One remarkable result is that vibrational strong coupling to cavities can alter reaction rates and product branching ratios with no optical excitation whatsoever. In this work we review the ability of vibration-cavity polaritons to modify chemical and physical processes including chemical reactivity, as well as steady-state and transient spectroscopy. We discuss the larger context of these works and highlight their most important contributions and implications. Our goal is to provide insight for systematically manipulating molecular polaritons in photonic and chemical applications.

We examine closely the differences between the densities of vibrational states of bulk, slab, and cavity polariton modes under weak and moderate inhomogeneous broadening. While existing theoretical treatments are often based on a comparative analysis of "bare" vibrations and cavity polaritons, in the strong-coupling regime, only differences between slab/bulk polaritons on the one hand and cavity polaritons on the other hand are meaningful since "bare" vibrations are not observed experimentally. We find that polaritons in cavities significantly detuned from resonance with molecular transitions at zero in-plane wavevector do not differ appreciably from bulk polaritons in their density of vibrational states. Only cavity polaritons with sufficiently weak inhomogeneous broadening and tuned to resonance near normal incidence display a pronounced density-of-state enhancement. These results shed light on the heretofore puzzling observations of modified chemical reactivity only at zero detuning and supply a new baseline for assessing the explanatory power of proposed theories of cavity-modified chemistry.

The Berreman effect, by which thin films of polar dielectric materials exhibit strong, narrow resonances near their longitudinal optic (LO) phonon frequency, results in strong material interactions...

Chip-scale optical frequency combs can provide broadband spectroscopy for diagnosing complex organic molecules. They are also promising as miniaturized laser spectrometers in applications ranging from atmospheric chemistry to geological science and the search for extraterrestrial life. While optical cavities are commonly used to boost sensitivity, it is challenging to realize a compact cavity-enhanced comb-based spectrometer. Here, we apply the Vernier technique to free-running operation of an interband cascade laser frequency comb in a simple linear geometry that performs cavity-enhanced chemical sensing. A centimeter-scale high-finesse cavity simultaneously provides selective mode filtering and enhancement of the path length to 30 meters. As a proof-of-concept, we sense transient open-path releases of ppm-level difluoroethane with 2 ms temporal resolution over a 1 THz optical bandwidth centered at 3.64 $\mu$m., Comment: 10 pages, 5 figures

We describe the framework for a midwave infrared photonic integrated circuit (PIC) that combines lasers, detectors, passive waveguides, and other optical components such as spectral multiplexers and demultiplexers on the native GaSb substrate of an interband cascade laser (ICL) structure.1,2 These building blocks may be used, for example, to fabricate a laser spectroscopy PIC for on-chip chemical sensing.3

Saturable absorption, in which optical absorption decreases as the incident intensity increases, is commonly utilized in the visible and near-infrared for laser applications. In the mid-infrared, m...

Using a 2D metallic grating, we demonstrate the resonant enhancement of the external quantum efficiency of an nBn structure with a 0.5 $\mu \text{m}$ -thick-absorber. A total of two structures were studied with different barrier materials: ternary AlInSb and an InAs/AlInSb superlattice (SL). The device using an SL barrier had a diffusion-limited dark current of 6.7 $\mu \text{A}$ /cm2 at 150 K, which was four times lower than the ternary-barrier device. The surface plasmon polariton (SPP) resonance wavelength for devices with six different grating periods varied as predicted by simulations. The quantum efficiency (QE) was enhanced by up to 56% by coupling to the SPP mode. A peak external QE of 39% was achieved at $4~\mu \text{m}$ with a 1100 nm grating period.

Noble metals capable of generating hot carriers by plasmon decay promote efficient charge separation with visible light, which opens a new prospect in the fields of photocatalytic energy conversion and solar fuel generation. Recent studies also indicate that by tuning system parameters such as photon energy and plasmon resonance, hot carriers may be injected into specific anti-bonding orbitals of an adsorbed molecule, making it possible for selective photocatalysis. Although numerous metal nanoparticle systems have been devised to harness plasmon-induced hot carriers, little is known for surface plasmon polariton (SPP)-induced hot carrier generation, especially for energy-tunable photocatalysis. In fact, nanoparticles' morphological variations, resonance inhomogeneity, and limited spectral tunability have challenged efficient control over energy-tunable photocatalysis. In this study, we investigated the energy-tunable photocatalysis using SPP-supporting metal film systems including a metal–semiconductor heterofilm (Ag/TiO2) and a bare metal film (Au) by measuring electrochemical responses under a continuously-tuned illumination angle. We found that photon-to-carrier conversion efficiency measured in the heterofilm was strongly dependent on photon energy, showing a good agreement with a Schottky transport model. We also found that hot carriers generated in the bare metal film are energetically positioned near the metal's Fermi level, and therefore, chemical reactions can be controlled by tuning electrode potential or solution pH. These results demonstrate that the SPP system could provide a platform for understanding charge transport mechanisms in plasmon-induced photocatalysis and pave a new path for energy-tunable chemical reactions.

Gold nanoparticles (AuNPs) attached to the extracellular leaflet of the plasma membrane of neurons can enable the generation of action potentials (APs) in response to brief pulses of light. Recently described techniques to stably bind AuNP bioconjugates directly to membrane proteins (ion channels) in neurons enable robust AP generation mediated by the photoexcited conjugate. However, a strategy that binds the AuNP to the plasma membrane in a non protein-specific manner could represent a simple, single-step means of establishing light-responsiveness in multiple types of excitable neurons contained in the same tissue. On the basis of the ability of cholesterol to insert into the plasma membrane, here we test whether AuNP functionalization with linear dihydrolipoic acid-poly(ethylene) glycol (DHLA-PEG) chains that are distally terminated with cholesterol (AuNP-PEG-Chol) can enable light-induced AP generation in neurons. Dorsal root ganglion (DRG) neurons of rat were labeled with 20 nm diameter spherical AuNP-PEG-Chol conjugates wherein ∼30% of the surface ligands (DHLA-PEG-COOH) were conjugated to PEG-Chol. Voltage recordings under current-clamp conditions showed that DRG neurons labeled in this manner exhibited a capacity for AP generation in response to microsecond and millisecond pulses of 532 nm light, a property attributable to the close tethering of AuNP-PEG-Chol conjugates to the plasma membrane facilitated by the cholesterol moiety. Light-induced AP and subthreshold depolarizing responses of the DRG neurons were similar to those previously described for AuNP conjugates targeted to channel proteins using large, multicomponent immunoconjugates. This likely reflected the AuNP-PEG-Chol's ability, upon plasmonic light absorption and resultant slight and rapid heating of the plasma membrane, to induce a concomitant transmembrane depolarizing capacitive current. Notably, AuNP-PEG-Chol delivered to DRG neurons by inclusion in the buffer contained in the recording pipet/electrode enabled similar light-responsiveness, consistent with the activity of AuNP-PEG-Chol bound to the inner (cytofacial) leaflet of the plasma membrane. Our results demonstrate the ability of AuNP-PEG-Chol conjugates to confer timely stable and direct responsiveness to light in neurons. Further, this strategy represents a general approach for establishing excitable cell photosensitivity that could be of substantial advantage for exploring a given tissue's suitability for AuNP-mediated photocontrol of neural activity.

Mid-infrared semiconductor lasers have emerged as indispensable compact coherent sources for military and commercial applications. While much of the historical emphasis has been on maximizing the output power and/or spectral purity, a recent new focus has been on engineering these lasers to operate as optical frequency combs (OFCs) for broadband real-time spectroscopy. In particular, the combination of low-drive-power and broad gain bandwidth has made interband cascade laser (ICL) OFCs an attractive complement to quantum cascade laser OFCs operating at longer wavelengths. Moreover, ICL combs can potentially be incorporated into fully-integrated dual-comb spectrometers that employ fast, room-temperature IC photodetectors processed on the same chip. However, the high refractive index of the ICL’s GaSb substrate poses some challenges to the optical waveguiding. Because the modal index is considerably lower than that of the substrate, the optical field can penetrate the bottom cladding layer and leak into the GaSb, inducing wavelength-dependent interference that modifies the gain and group velocity dispersion (GVD) profiles. Even when the effect on lasing threshold is small, the comb properties can be adversely affected. Using the sub-threshold Fourier transform technique, we studied ICL combs with various ridge widths, substrate thicknesses, and center wavelengths. This allowed us to evaluate the effects of modal leakage on the GVD. We find that the resonant nature of the substrate modes induces oscillations, which affect both the spectral bandwidth and the phase-locking properties above threshold. Strategies to mitigate the GVD’s undesired and unpredictable spectral variation will be presented.

We describe how a midwave infrared photonic integrated circuit (PIC) that combines lasers, detectors, passive waveguides, and other optical elements may be constructed on the native GaSb substrate of an interband cascade laser (ICL) structure. The active and passive building blocks may be used, for example, to fabricate an on-chip chemical detection system with a passive sensing waveguide that evanescently couples to an ambient sample gas. A variety of highly compact architectures are described, some of which incorporate both the sensing waveguide and detector into a laser cavity defined by two high-reflectivity cleaved facets. We also describe an edge-emitting laser configuration that optimizes stability by minimizing parasitic feedback from external optical elements, and which can potentially operate with lower drive power than any mid-IR laser now available. While ICL-based PICs processed on GaSb serve to illustrate the various configurations, many of the proposed concepts apply equally to quantum-cascade-laser (QCL)-based PICs processed on InP, and PICs that integrate III-V lasers and detectors on silicon. With mature processing, it should become possible to mass-produce hundreds of individual PICs on the same chip which, when singulated, will realize chemical sensing by an extremely compact and inexpensive package.

We demonstrate a mid-IR mode-resolved Vernier optical frequency comb spectrometer with an interband cascade laser source. The free-running system provides 35 meters of effective path length for monitoring 1-THz-broad spectra at 3.63 µm.

The chapter discusses a full implementation of k·p theory for the eight bands near the energy gap in a III–V semiconductor. This model can precisely calculate the band structure for both conduction and valence bands near the energy gap. Even though only eight bands are explicitly included, the terms due to interactions with “remote” bands are present in the complete version of the theory. The chapter consider the physical meaning of the results and the effect that strain has on the semiconductor band structure. The Hamiltonians for both zinc-blende and wurtzite crystals are introduced.

Previous chapters discussed the crystal structure and bandstructure of III–V semiconductors. This chapter shifts to the book’s second major topic: electronic interactions with light. It introduces the main ideas about how light waves propagate in semiconductor crystals and induce absorption, spontaneous emission, and stimulated emission in bulk semiconductors. It also considers the differences between the electronic interactions with light in zinc-blende and wurtzite crystals and what happens as the energy gap of the semiconductor is reduced to zero or when the crystal is two-dimensional.

III–V semiconductors form crystalline structures with three-dimensional periodic arrangements of the atoms. In this chapter, we will explore the nature of the crystal lattice starting from lower dimensions and progressing to real semiconductor crystals. We also learn why we expect distinct energy bands to form in the solids that crystallize in such lattices. The carrier statistics and occupation of the bands will also be examined.

The so-called k·p theory provides an effective framework for modeling the band structures of III–V semiconductors. This approach starts with a handful of bands that are coupled in accordance with the crystal symmetry discussed in Chapter 1. This chapter describes the k·p theory and lays out the assumptions that underlie its most common versions, including Kane’s model. It also shows how the spin–orbit coupling affects the band structure of semiconductors.

Semiconductor quantum structures are at the core of many photonic devices such as lasers, photodetectors, solar cells etc. To appreciate why they are such a good fit to these devices, we must understand the basic features of their band structure and how they interact with incident light. This book takes the reader from the very basics of III-V semiconductors (some preparation in quantum mechanics and electromagnetism is helpful) and shows how seemingly obscure results such as detailed forms of the Hamiltonian, optical transition strengths, and recombination mechanisms follow. The reader does not need to consult other references to fully understand the material, although a few handpicked sources are listed for those who would like to deepen their knowledge further. Connections to the properties of novel materials such as graphene and transition metal dichalcogenides are pointed out, to help prepare the reader for contributing at the forefront of research. The book also supplies a complete, up-to-date database of the band parameters that enter into the calculations, along with tables of optical constants and interpolation schemes for alloys. From these foundations, the book goes on to derive the characteristics of photonic semiconductor devices (with a focus on the mid-infrared) using the same principles of building all concepts from the ground up, explaining all derivations in detail, giving quantitative examples, and laying out dimensional arguments whenever they can help the reader’s understanding. A substantial fraction of the material in this book has not appeared in print anywhere else, including journal publications.

The mid-infrared spectra of many polar materials are dominated by highly reflective reststrahlen bands that occur between the transverse and longitudinal optical phonons. Within the reststrahlen bands, light can couple with optical phonons to support phonon-polariton modes. These modes enhance light-matter interactions through the concentration of light to nanoscale dimensions, and therefore, are particularly promising for mid-infrared nanophotonic applications. Here, we discuss our work on expanding the spectral range over which phonon-polaritons are supported by using new material systems, as well as active tuning of the modes via carrier photoinjection. In particular, we report on the confinement of hyperbolic phonon-polaritons in calcite, a ubiquitous polar material. We also report the use of the LO-phonon-plasmon-coupling (LOPC) effect to actively tune the Berreman mode of a GaN thin film.

By exploiting the bi-functional operation capability of interband cascade laser (ICL) frequency combs, we have utilized the laser medium not only for comb generation, but also as a room-temperature photodetector with near-GHz bandwidth for multi-heterodyne beating of the comb lines. Our self-contained platform consuming less than 2 W of electrical power enables free-running room-temperature broadband dual-comb spectroscopy of 1,1 difluoroethane with ~3% standard deviation in 2 ms over 600 GHz of optical bandwidth around 3.6 µm. We will discuss progress toward the optimization of ICL combs for realizing compact, low-power chemical sensors operating in the MWIR.

We show that the polariton density of states in planar optical cavities strongly coupled to vibrational excitations remains much lower than the density of vibrational states even at the frequency of the lower and upper polaritons under most practical circumstances. The polariton density of states is higher within a narrow window only when the inhomogeneous line width is at least an order of magnitude smaller than the Rabi splitting. Therefore, modification of reaction rates via the density-of-states pathway appears small or negligible for the scenarios reported in the literature. While the polariton density of states is bounded from above by the free-space optical density of states in dielectric cavities, it can be much higher for localized phonon polariton modes of nanoscale particles. We conclude that other potential explanations of the reported reactivity changes under vibrational strong coupling should be examined.

Interband cascade laser (ICL) optical frequency combs are promising midwave infrared sources for dual comb spectroscopy probing the strong fundamental absorption lines of numerous chemical and biological agents. In this work, a 4-mm-long ICL frequency comb emitting at 3.3 μm was operated by single-cavity optical self-injection. The experiments employing a free-space optical self-injection length of 1.1m with fine-delay control demonstrated a tuning range of 119MHz around the free-running intermode beat frequency of 9.58 GHz. For resonant fine-delay the line width of the intermode beat frequency was reduced to 390 kHz, what is an improvement by a factor of 40 in comparison to the solitary laser line width.

We report a preliminary investigation of ion bombardment (IB) effects on interband cascade laser (ICL) properties. Under some conditions, IB almost completely suppresses the vertical transport through a broad-area laser, although other times only a partial or negligible suppression is observed. To elucidate the mechanism that induces the suppression and in what part of the structure it occurs, we investigated the effects of IB on samples containing only ICL sub-regions. While IB increased the resistivity of a lightly-n-doped GaSb layer such as that used as a top or bottom separate confinement layer in an ICL, that layer was still much too conductive to strongly suppress the current flowing through a full device. The voltage drop was larger following IB of an InAs-AlSb superlattice such as that used in the top and bottom optical cladding layers in an ICL, although the effect was not large enough to fully account for the strong net suppression. And finally, the resistivity of an interband cascade LED containing the same active stages as an ICL was actually found to decrease following IB. Despite the inconclusive and sometime inconsistent findings of this study, it is nonetheless clear that if the effects can be controlled reproducibly, IB may provide a valuable tool for enhancing such ICL device configurations as weakly-index-guided narrow ridges and interband cascade vertical-cavity surface-emitting laser mesas that inject current and emit light only within a small central aperture.