Control, Modulation, and Analytical Descriptions of Vibrational Strong Coupling.

Here, we review the design of optical cavities, transient and modulated responses, and theoretical models relevant to vibrational strong coupling (VSC). While planar Fabry-Perot cavities remain the most common choice for experiments involving vibrational polaritons, other choices including plasmonic and phononic nanostructures, extended lattice resonances, and wavelength-scale three-dimensionally confined dielectric cavities have unique advantages, which are discussed. Next, we review the nonlinear response to laser excitation of VSC systems revealed by transient pump-probe and 2DIR techniques. The assignment of various features observed in these experiments has been an important topic with significant recent progress and controversy. The modulation of VSC systems by various means such as ultrafast pulses and electrochemical methods is also described. Finally, theoretical approaches to understanding the physics and chemistry of VSC systems are reviewed with an eye toward their applicability and usefulness. These fall into two main categories: (1) solving for the eigenmodes of the system and (2) evolutionary techniques including the transfer-matrix method and its generalizations. The need for quantum optical methods of describing VSC systems is critically evaluated in light of current experimental work, and we discuss circumstances which necessitate consideration of the full in-plane dispersion of the Fabry-Perot cavities.

Midwave resonant cavity infrared detectors (RCIDs) with suppressed background noise.

We report a resonant cavity infrared detector (RCID) with an InAsSb/InAs superlattice absorber with a thickness of only ≈ 100 nm, a 33-period GaAs/Al0.92Ga0.08As distributed Bragg reflector bottom mirror, and a Ge/SiO2/Ge top mirror. At a low bias voltage of 150 mV, the external quantum efficiency (EQE) reaches 58% at the resonance wavelength λres ≈ 4.6 µm, with linewidth δλ = 19-27 nm. The thermal background current for a realistic system scenario with f/4 optic that views a 300 K scene is estimated by integrating the photocurrent generated by background spanning the entire mid-IR spectral band (3-5 µm). The resulting specific detectivity is a factor of 3 lower than for a state-of-the-art broadband HgCdTe device at 300 K, where dark current dominates the noise. However, at 125 K where the suppression of background noise becomes critical, the estimated specific detectivity D* of 5.5 × 1012 cm Hz½/W is more than 3× higher. This occurs despite a non-optimal absorber cut-off that causes the EQE to decrease rapidly with decreasing temperature, e.g., to 33% at 125 K. The present RCID's advantage over the broadband device depends critically on its low EQE at non-resonance wavelengths: ≤ 1% in the range 3.9-5.5 µm. Simulations using NRL MULTIBANDS indicate that impact ionization in the bottom contact and absorber layers dominates the dark current at near ambient temperatures. We expect future design modifications to substantially enhance D* throughout the investigated temperature range of 100-300 K.

Vibration-Cavity Polariton Chemistry and Dynamics.

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.

Comparative analysis of polaritons in bulk, dielectric slabs, and planar cavities with implications for cavity-modified reactivity.

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.

Mid-infrared interband cascade light emitting devices grown on off-axis silicon substrates.

The high-quality growth of midwave infrared light emitters on silicon substrates will advance their incorporation into photonic integrated circuits, and also introduce manufacturing advantages over conventional devices grown on lattice-matched GaSb. Here we report interband cascade light emitting devices (ICLEDs) grown on 4 degree offcut silicon with 12% lattice mismatch. Four wafers produced functioning devices, with variations from wafer to wafer but uniform performance of devices from a given wafer. The full width at half maxima for the (004) GaSb rocking curves were as narrow as ∼ 163 arc seconds, and the root mean square surface roughness as small as 3.2 nm. Devices from the four wafers, as well as from a control structure grown to the same design on GaSb, were mounted epitaxial-side-up (epi-up). While core heating severely limited continuous wave (cw) emission from the control devices at relatively modest currents, efficient heat dissipation via the substrate allowed output from the devices on silicon to increase up to much higher currents. Although the devices on silicon had higher leakage currents, probably occurring primarily at dislocations resulting from the lattice-mismatched growth, accounting for differences in architecture the efficiency at high cw current was approximately 75% of that of our previous best-performing standard epi-down ICLEDs grown on GaSb. At 100 mA injection current, 200-µm-diameter mesas produced 184 µW of cw output power when operated at T = 25 °C, and 140 µW at 85°C. Epi-up mid-IR light emitters grown on silicon will be far simpler to process and much less expensive to manufacture than conventional devices grown on GaSb and mounted epi-down.

Methane detection using an interband-cascade LED coupled to a hollow-core fiber.

Midwave infrared interband-cascade light-emitting devices (ICLEDs) have the potential to improve the selectivity, stability, and sensitivity of low-cost gas sensors. We demonstrate a broadband direct absorption CH4 sensor with an ICLED coupled to a plastic hollow-core fiber (1 m length, 1500 µm inner diameter). The sensor achieves a 1σ noise equivalent absorption of approximately 0.2 ppmv CH4 at 1 Hz, while operating at a low drive power of 0.5 mW. A low-cost sub-ppmv CH4 sensor would make monitoring emissions more affordable and more accessible for many relevant industries, such as the petroleum, agriculture, and waste industries.

Interband Cascade Photonic Integrated Circuits on Native III-V Chip.

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.

Author Correction: Ultralow-loss polaritons in isotopically pure boron nitride.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Ultralow-loss polaritons in isotopically pure boron nitride.

Conventional optical components are limited to size scales much larger than the wavelength of light, as changes to the amplitude, phase and polarization of the electromagnetic fields are accrued gradually along an optical path. However, advances in nanophotonics have produced ultrathin, so-called 'flat' optical components that beget abrupt changes in these properties over distances significantly shorter than the free-space wavelength. Although high optical losses still plague many approaches, phonon polariton (PhP) materials have demonstrated long lifetimes for sub-diffractional modes in comparison to plasmon-polariton-based nanophotonics. We experimentally observe a threefold improvement in polariton lifetime through isotopic enrichment of hexagonal boron nitride (hBN). Commensurate increases in the polariton propagation length are demonstrated via direct imaging of polaritonic standing waves by means of infrared nano-optics. Our results provide the foundation for a materials-growth-directed approach aimed at realizing the loss control necessary for the development of PhP-based nanophotonic devices.

Bichromatic pumping in mid-infrared microresonator frequency combs with higher-order dispersion.

Integrated microresonator-based mid-infrared frequency combs based on III-V semiconductors exhibit pronounced higher-order group velocity dispersion that can make it difficult to achieve stable output. One way to stabilize multiple solitons and their repetition rate is to pump simultaneously at two nearby comb lines. Two-color (or bichromatic) pumping also promises to boost the relatively low conversion efficiencies of single-soliton combs. We present simulations showing that, for a realistic InGaAs/InP ridge waveguide, the stabilization effect occurs over only a limited range of pump power and detuning parameters. We map out the parameter ranges for various regimes of operation in terms of the pump power and detuning and determine that the regimes converge quickly as the dispersion is truncated to progressively higher orders.

Resonant-cavity infrared detector with five-quantum-well absorber and 34% external quantum efficiency at 4 µm.

We report resonant-cavity infrared detectors with 34% external quantum efficiency at room temperature at the resonant wavelength of 4.0 µm, even though the absorber consists of only five quantum wells with a total thickness of 50 nm. The full width at half maximum (FWHM) linewidth is 46 nm, and the peak absorption is enhanced by nearly a factor of 30 over that for a single pass through the absorber. In spite of an unfavorable Shockley-Read lifetime in the current material, the dark current density is at the level of state-of-the-art HgCdTe detectors as quantified by "Rule 07." The Johnson-noise limited detectivity (D*) at 21°C is 7 × 109 cm Hz½/W. We expect that future improvements in the device design and material quality will lead to higher quantum efficiency, as well as a significant reduction of the dark current density consistent with the very thin absorber.

Near-infrared frequency comb generation in mid-infrared interband cascade lasers.

The interband cascade laser (ICL) is an ideal candidate for low-power mid-infrared frequency comb spectroscopy. In this work, we demonstrate that its intracavity second-order optical nonlinearity induces a coherent up-conversion of the generated mid-infrared light to the near-infrared through second-harmonic and sum-frequency generation. At 1.8 µm, 10 mW of light at 3.6 µm convert into sub-nanowatt levels of optical power, spread across 30 nm of spectral coverage. The observed linear-to-nonlinear conversion efficiency exceeds ${3\;{\unicode{x00B5} {\rm W/W}}^2}$3µW/W2 in continuous wave operation. We use a dual-band ICL frequency comb source to characterize water vapor absorption in both spectral bands.

Mid-infrared dual-comb spectroscopy with interband cascade lasers.

Two semiconductor optical frequency combs, consuming less than 1 W of electrical power, are used to demonstrate high-sensitivity mid-infrared dual-comb spectroscopy in the important 3-4 µm spectral region. The devices are 4 mm long by 4 µm wide, and each emits 8 mW of average optical power. The spectroscopic sensing performance is demonstrated by measurements of methane and hydrogen chloride with optical multi-pass cell sensitivity enhancement. The system provides a spectral coverage of 33 cm-1 (1 THz), 0.32 cm-1 (9.7 GHz) frequency sampling interval, and peak signal-to-noise ratio of ∼100 at 100 µs integration time. The monolithic design, low drive power, and direct generation of mid-infrared radiation are highly attractive for portable broadband spectroscopic instrumentation in future terrestrial and space applications.

Nanoscale Mapping and Spectroscopy of Nonradiative Hyperbolic Modes in Hexagonal Boron Nitride Nanostructures.

The inherent crystal anisotropy of hexagonal boron nitride (hBN) provides the ability to support hyperbolic phonon polaritons, that is, polaritons that can propagate with very large wave vectors within the material volume, thereby enabling optical confinement to exceedingly small dimensions. Indeed, previous research has shown that nanometer-scale truncated nanocone hBN cavities, with deep subdiffractional dimensions, support three-dimensionally confined optical modes in the mid-infrared. Because of optical selection rules, only a few of the many theoretically predicted modes have been observed experimentally via far-field reflection and scattering-type scanning near-field optical microscopy (s-SNOM). The photothermal induced resonance (PTIR) technique probes optical and vibrational resonances overcoming weak far-field emission by leveraging an atomic force microscope (AFM) probe to transduce local sample expansion caused by light absorption. Here we show that PTIR enables the direct observation of previously unobserved, dark hyperbolic modes of hBN nanostructures. Leveraging these optical modes and their wide range of angular and radial momenta could provide a new degree of control over the electromagnetic near-field concentration, polarization in nanophotonic applications.

Quantification of Efficient Plasmonic Hot-Electron Injection in Gold Nanoparticle-TiO2 Films.

Excitation of localized surface plasmons in metal nanostructures generates hot electrons that can be transferred to an adjacent semiconductor, greatly enhancing the potential light-harvesting capabilities of photovoltaic and photocatalytic devices. Typically, the external quantum efficiency of these hot-electron devices is too low for practical applications (<1%), and the physics underlying this low yield remains unclear. Here, we use transient absorption spectroscopy to quantify the efficiency of the initial electron transfer in model systems composed of gold nanoparticles (NPs) fully embedded in TiO2 or Al2O3 films. In independent experiments, we measure free carrier absorption and electron-phonon decay in the model systems and determine that the electron-injection efficiency from the Au NPs to the TiO2 ranges from about 25% to 45%. While much higher than some previous estimates, the measured injection efficiency is within an upper-bound estimate based on a simple approximation for the Au hot-electron energy distribution. These results have important implications for understanding the achievable injection efficiencies of hot-electron plasmonic devices and show that the injection efficiency can be high for Au NPs fully embedded within a semiconductor with dimensions less than the Au electron mean free path.

Compact photoacoustic module for methane detection incorporating interband cascade light emitting device.

A photoacoustic module (PAM) for methane detection was developed by combining a novel 3.2 µm interband cascade light emitting device (ICLED) with a compact differential photoacoustic cell. The ICLED with a 22-stage interband cascade active core emitted a collimated power of ~700 µW. A concave Al-coat reflector was positioned adjacent to the photoacoustic cell to enhance the gas absorption length. Assembly of the ICLED and reflector with the photoacoustic cell resulted in a robust and portable PAM without any moving parts. The PAM performance was evaluated in terms of operating pressure, sensitivity and linearity. A 1σ detection limit of 3.6 ppmv was achieved with a 1-s integration time.

Spectral narrowing and stabilization of interband cascade laser by volume Bragg grating.

A volume Bragg grating recorded in photo-thermo-refractive glass was used to spectrally lock the emission from an 18-µm-wide interband cascade laser ridge to a wavelength of 3.12 µm. The spectral width of emission into the resonant mode is narrowed by more than 300 times, and the thermal wavelength shift is reduced by 60 times. While the power loss penalty is about 30%, the spectral brightness increases by 200 times.

High-power continuous-wave interband cascade lasers with 10 active stages.

We report the pulsed and continuous wave (cw) performance of 10-stage interband cascade lasers (ICLs) emitting at both λ ≈3.2 µm and λ ≈3.45 µm. The slope efficiency is higher while the external differential quantum efficiency per stage remains about the same when comparison is made to earlier results for 7-stage ICLs with similar carrier-rebalanced designs. At T = 25°C, an 18-µm-wide ridge with 4.5 mm cavity length and high-reflection/anti-reflection coatings emits up to 464 mW of cw output power with beam quality factor M(2) = 1.9, for higher brightness than has ever been reported previously for an ICL. When the cavity length is reduced to 1 mm, both the 10-stage and 7-stage devices reach 18% cw wallplug efficiency at T = 25°C.

Heterogeneously integrated 2.0 µm CW hybrid silicon lasers at room temperature.

Here we experimentally demonstrate room temperature, continuous-wave (CW), 2.0 µm wavelength lasers heterogeneously integrated on silicon. Molecular wafer bonding of InP to Si is employed. These hybrid silicon lasers operate CW up to 35°C and emit up to 4.2 mW of single-facet CW power at room temperature. III-V tapers transfer light from a hybrid III-V/silicon optical mode into a Si waveguide mode. These lasers enable the realization of a number of sensing and detection applications in compact silicon photonic systems.

Pulsed and CW performance of 7-stage interband cascade lasers.

We report a narrow-ridge interband cascade laser emitting at λ ≈3.5 µm that produces up to 592 mW of cw power with a wallplug efficiency of 10.1% and beam quality factor of M(2) = 3.7 at T = 25 °C. A pulsed cavity length study of broad-area lasers from the same wafer confirms that the 7-stage structure with thicker separate confinement layers has a reduced internal loss of ≈3 cm(-1). More generally, devices from a large number of wafers with similar 7-stage designs and wavelengths spanning 2.95-4.7 µm exhibit consistently higher pulsed external differential quantum efficiencies than earlier state-of-the-art ICLs.