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Minimizing the material usage in thin‐film solar cells can reduce manufacturing costs and enable mechanically flexible implementations, but concomitantly diminishes optical absorption. Coherent optical effects can help alleviate this inevitable drawback at discrete frequencies. For example, coherent perfect absorption guarantees that light is fully absorbed in a thin layer regardless of material or thickness—but only on resonance. Here, it is shown that "omni‐resonance" delivers such coherent enhancement over a broad bandwidth by structuring the optical field to nullify the angular dispersion intrinsic to resonant structures. After embedding an amorphous‐silicon thin‐film photovoltaic cell in a planar cavity, pre‐conditioning the incident light using an alignment‐free optical arrangement severs the link between the resonant bandwidth and the cavity‐photon lifetime, thereby rendering the cavity omni‐resonant. Coherently enhanced near‐infrared absorption doubles the photocurrent over the targeted spectral range 660–740 nm where every wavelength resonates. These results may pave the way to transparent solar cells that optimally harvest near‐infrared light. [ABSTRACT FROM AUTHOR]

Coherent perfect absorption (CPA) is an interferometric effect that guarantees full absorption in a lossy layer independently of its intrinsic losses. To date, it has been observed only at a single wavelength or over narrow bandwidths, whereupon wavelength-dependent absorption can be ignored. Here we produce CPA over a bandwidth of ∼60 nm in a 2 µm thick polymer film with a low-doping concentration of an organic laser dye. A planar cavity is designed with a spectral 'dip' to accommodate the dye resonant linewidth, and CPA is thus achieved even at its absorption edges. This approach allows realizing strong absorption in laser dyes—and resonant materials in general—independently of the intrinsic absorption levels, with a flat spectral profile and without suffering absorption quenching due to high doping levels. [ABSTRACT FROM AUTHOR]

Diffusing wave spectroscopy (DWS) is a well-known set of methods to measure the temporal dynamics of dynamic samples. In DWS, dynamic samples scatter the incident coherent light, and the information of the temporal dynamics is encoded in the scattered light. To record and analyze the light signal, there exist two types of methods—temporal sampling methods and speckle ensemble methods. Temporal sampling methods, including diffuse correlation spectroscopy, use one or multiple large bandwidth detectors to sample well and analyze the temporal light signal to infer the sample temporal dynamics. Speckle ensemble methods, including speckle visibility spectroscopy, use a high-pixel-count camera sensor to capture a speckle pattern and use the speckle contrast to infer sample temporal dynamics. In this paper, we theoretically and experimentally demonstrate that the decorrelation time (τ) measurement accuracy or signal-to-noise ratio (SNR) of the two types of methods has a unified and similar fundamental expression based on the number of independent observables (NIO) and the photon flux. Given a time measurement duration, the NIO in temporal sampling methods is constrained by the measurement duration, while speckle ensemble methods can outperform by using simultaneous sampling channels to scale up the NIO significantly. In the case of optical brain monitoring, the interplay of these factors favors speckle ensemble methods. We illustrate that this important engineering consideration is consistent with the previous research on blood pulsatile flow measurements, where a speckle ensemble method operating at 100-fold lower photon flux than a conventional temporal sampling system can achieve a comparable SNR. [ABSTRACT FROM AUTHOR]

Infrared light scattering methods have been developed and employed to non-invasively monitor human cerebral blood flow (CBF). However, the number of reflected photons that interact with the brain is low when detecting blood flow in deep tissue. To tackle this photon-starved problem, we present and demonstrate the idea of interferometric speckle visibility spectroscopy (ISVS). In ISVS, an interferometric detection scheme is used to boost the weak signal light. The blood flow dynamics are inferred from the speckle statistics of a single frame speckle pattern. We experimentally demonstrated the improvement in the measurement of fidelity by introducing interferometric detection when the signal photon number is low. We apply the ISVS system to monitor the human CBF in situations where the light intensity is ∼100-fold less than that in common diffuse correlation spectroscopy (DCS) implementations. Due to the large number of pixels (∼2 × 105) used to capture light in the ISVS system, we are able to collect a similar number of photons within one exposure time as in normal DCS implementations. Our system operates at a sampling rate of 100 Hz. At the exposure time of 2 ms, the average signal photoelectron number is ∼0.95 count/pixel, yielding a single pixel interferometric measurement signal-to-noise ratio (SNR) of ∼0.97. The total ∼2 × 105 pixels provide an expected overall SNR of 436. We successfully demonstrate that the ISVS system is able to monitor the human brain pulsatile blood flow, as well as the blood flow change when a human subject is doing a breath-holding task. [ABSTRACT FROM AUTHOR]

Coherent perfect absorption refers to the interferometric enhancement of absorption in a partially lossy medium upto 100%. This can be achieved without modifying the absorbing medium itself, instead of engineering its photonic environment. Ion-doped fibers are one of the most technologically relevant absorbing materials in optics, which are widely employed in fiber amplifiers and lasers. Realizing complete optical absorption of an incident field in short-length moderately-doped fibers remains a challenge for the cost-effective design of compact fiber lasers. Here, we exploit the concept of coherent perfect absorption to overcome this challenge, whereby two appropriately designed fiber Bragg gratings define a short-length erbium-doped-fiber cavity that enforces complete absorption of an incident field on resonance—-independently of the doped-fiber intrinsic absorption. This approach applies to any spectral window and guarantees the efficient utilization of the fiber dopants along its length, thus, suggesting the possibility of next-generation efficient single-longitudinal-mode fiber lasers for applications in optical communication, sensing, and metrology. [ABSTRACT FROM AUTHOR]

Parity-time (PT)-symmetry in optics is a condition whereby the real and imaginary parts of the refractive index across a photonic structure are deliberately balanced. This balance can lead to interesting optical phenomena, such as unidirectional invisibility, loss-induced lasing, single-mode lasing from multimode resonators, and non-reciprocal effects in conjunction with nonlinearities. Because PT-symmetry has been thought of as fragile, experimental realisations to date have been usually restricted to on-chip micro-devices. Here, we demonstrate that certain features of PT-symmetry are sufficiently robust to survive the statistical fluctuations associated with a macroscopic optical cavity. We examine the lasing dynamics in optical fibre-based coupled cavities more than a kilometre in length with balanced gain and loss. Although fluctuations can detune the cavity by more than the free spectral range, the behaviour of the lasing threshold and the laser power is that expected from a PT-stable system. Furthermore, we observe a statistical symmetry breaking upon varying the cavity loss. [ABSTRACT FROM AUTHOR]

REFERENCE 1 J. Xu, A. K. Jahromi, and C. Yang, "Diffusing wave spectroscopy: A unified treatment on temporal sampling and speckle ensemble methods", APL Photonics 6, 016105 (2021).10.1063/5.0034576 The original article[1] contains misprints in Eq. (A23) and the first line of Eq. (A24). [Extracted from the article]

Interferometry is one of the central organizing principles of optics. Key to interferometry is the concept of optical delay, which facilitates spectral analysis in terms of time-harmonics. In contrast, when analyzing a beam in a Hilbert space spanned by spatial modes - a critical task for spatial-mode multiplexing and quantum communication - basis-specific principles are invoked that are altogether distinct from that of 'delay'. Here, we extend the traditional concept of temporal delay to the spatial domain, thereby enabling the analysis of a beam in an arbitrary spatial-mode basis - exemplified using Hermite-Gaussian and radial Laguerre-Gaussian modes. Such generalized delays correspond to optical implementations of fractional transforms; for example, the fractional Hankel transform is the generalized delay associated with the space of Laguerre-Gaussian modes, and an interferometer incorporating such a 'delay' obtains modal weights in the associated Hilbert space. By implementing an inherently stable, reconfigurable spatial-light-modulator-based polarization-interferometer, we have constructed a 'Hilbert-space analyzer' capable of projecting optical beams onto any modal basis. [ABSTRACT FROM AUTHOR]

An efficient photovoltaic power converter is a critical element in laser power beaming systems for maximizing the end-to-end power transfer efficiency while minimizing beam reflections from the receiver for safety considerations. We designed a multilayer absorber that can efficiently trap monochromatic light from broad incident angles. The proposed design is built on the concept of a one-way coherent absorber with inverse-designed aperiodic multilayer front- and back-reflectors that enable maximal optical absorption in a thin-film photovoltaic material for broad angles. We argue that the broad bandwidth is achieved through an optimization search process that automatically engineers the modal content of the cavity to create multiple overlapping resonant modes at the desired angle or frequency range. A realistic design is provided based on GaAs thin films with inverse-designed multilayer binary AlAs/AlGaAs mirrors. The proposed device can pave the way for efficient optical power beaming systems.

While passive illumination schemes often utilize a broadband spectral acceptance, the performance of active illumination with a laser is improved by narrowband spectral filtering at the sensor. We present an experimental demonstration of an optical cavity structure that is capable of toggling between two performance limits: narrowband resonant and broadband omni-resonant transmission. To achieve omni-resonance without modifying the cavity, the incident optical field is pre-conditioned by associating each wavelength with a particular incidence angle that enables a broad continuous spectrum to resonate with the cavity. This strategy can help seamlessly combine passive and active illumination in the same system.