Coherent Absorption Research Articles

Experimental Evidence for Coherent Perfect Absorption in Guided-Mode Resonant Silicon Films

We experimentally verify a new class of coherent absorbers based on guided-mode resonance effects in periodic thin films. We design a silicon-based resonant absorber that is fabricated and tested near the 1300-nm wavelength. Implementing phase control, the device can, in principle, be switched between full absorption and full scattering. The first experimental prototype presented herein shows ~78% absorption in the in-phase state. Nearly total scattering is realized in the out-of-phase state. The experimental results agree reasonably well with theory.

A Polarization Propagator for Nonlinear X-ray Spectroscopies.

A complex polarization propagator approach has been developed to third order and implemented in density functional theory (DFT), allowing for the direct calculation of nonlinear molecular properties in the X-ray wavelength regime without explicitly addressing the excited-state manifold. We demonstrate the utility of this propagator method for the modeling of coherent near-edge X-ray two-photon absorption using, as an example, DFT as the underlying electronic structure model. Results are compared with the corresponding near-edge X-ray absorption fine structure spectra, illuminating the differences in the role of symmetry, localization, and correlation between the two spectroscopies. The ramifications of this new technique for nonlinear X-ray research are briefly discussed.

Design of a hybrid silicon-plasmonic co-propagating coupler operating close to coherent perfect absorption

By hybrid integration of plasmonic and dielectric waveguide concepts, it is shown that nearly perfect coherent absorption can be achieved in a co-propagating coupler geometry. First, the operating principle of the proposed device is detailed in the context of a more general 2 × 2 lossy coupler formalism. Then, it is shown how to tune the device in a wide region of possible working points, its broadband operation, and the tolerance to fabrication uncertainties. Finally, a complete picture of the electromagnetic modes inside the hybrid structure is analyzed, shining light onto the potentials which the proposed device holds in view of classical and quantum signal processing, nonlinear optics, polarization control, and sensing.

Universal lineshapes at the crossover between weak and strong critical coupling in Fano-resonant coupled oscillators.

In this article we discuss a model describing key features concerning the lineshapes and the coherent absorption conditions in Fano-resonant dissipative coupled oscillators. The model treats on the same footing the weak and strong coupling regimes, and includes the critical coupling concept, which is of great relevance in numerous applications; in addition, the role of asymmetry is thoroughly analyzed. Due to the wide generality of the model, which can be adapted to various frameworks like nanophotonics, plasmonics, and optomechanics, we envisage that the analytical formulas presented here will be crucial to effectively design devices and to interpret experimental results.

Optical Coherence in Atomic-Monolayer Transition-Metal Dichalcogenides Limited by Electron-Phonon Interactions

We systematically investigate the excitonic dephasing of three representative transition-metal dichalcogenides, namely, MoS_{2}, MoSe_{2}, and WSe_{2} atomic monolayer thick and bulk crystals, in order to gain a proper understanding of the factors that determine the optical coherence in these materials. Coherent nonlinear optical spectroscopy and temperature dependent absorption, combined with theoretical calculations of the phonon spectra, indicate electron-phonon interactions, to be the limiting factor. Surprisingly, the excitonic dephasing, differs only slightly between atomic monolayers and high quality bulk crystals, which indicates that material imperfections are not the limiting factor in atomically thin monolayer samples. The temperature dependence of the electronic band gap and the excitonic linewidth combined with "abinitio" calculations of the phonon energies and the phonon density of states reveal a strong interaction with the E' and E" phonon modes.

Coherent perfect absorption in an all-dielectric metasurface

We design and analyze an ultra-thin metasurface consists of mono-layer all-dielectric fishnet structure. It is demonstrated that coherent perfect absorption (CPA) can be achieved in such a metasurface, and the coherent absorptivity is controllable from 0.38% to 99.85% by phase modulation. The angular selectivity of the metasurface shows the feasibility of CPA in oblique incidence circumstances, where the CPA frequency splits into two frequency bands for TE and TM polarizations. Further study reveals that while retaining CPA, the CPA frequency of the metasurface can be manipulated from 8.56 to 13.47 GHz by solely adjusting the thickness of the fishnet metasurface.

Optical modeling of sunlight by using partially coherent sources in organic solar cells

We investigate the effects of coherent and partially coherent sources in optical modeling of organic solar cells. Two different organic solar cells are investigated: one without substrate and the other with a millimeter-sized glass substrate. The coherent light absorption is calculated with rigorous coupled-wave analysis. The result of this method is convolved with a distribution function to calculate the partially coherent light absorption. We propose a new formulation to accurately model sunlight as a set of partially coherent sources. In the structure with glass substrate, the accurate sunlight modeling results in the elimination of coherent effects in the thick substrate, but the coherency in other layers is not affected. Using partially coherent sources instead of coherent sources for simulations with sunlight results in a smoother absorption spectrum, but the change in the absorption efficiency is negligible.

2-μm Coherent DIAL for CO2, H2O and Wind Field Profiling in the Lower Atmosphere: Instrumentation and Results

We report on 2-μm coherent differential absorption lidar (CDIAL) measurements of carbon dioxide (CO2 ), water vapour (H2 O) absorption and wind field profiling in the atmospheric boundary layer. The CDIAL uses a Tm:fiber pumped, single longitudinal mode Q-switched seeded Ho:YLF laser and a fibercoupled coherent detection. The laser operates at a pulse repetition frequency of 2 kHz and emits an output energy of 10 mJ with a pulse width of 40 ns (FWHM). Experimental horizontal and vertical range-resolved measurements were made in the atmospheric boundary layer and compared to colocated in-situ sensor data.

High-speed gate-tunable terahertz coherent perfect absorption using a split-ring graphene.

We exploit split-ring graphene to realize coherent perfect absorption (CPA) in the terahertz (THz) regime. By controlling the relative phase of two counter-propagating coherent beams, the coherent absorption at resonant frequency of 2.91 THz can be tuned continuously from 99.7% to less than 2.1×10(-4)%, which gives a modulation contrast of 56.7 dB. Moreover, the coherent absorption also can be tuned by varying the gate-controlled Fermi energy based on the electro-absorption effect of graphene, giving a modulation contrast of 19 dB. Center frequency tunable CPA is also achieved using gate-tunable split-ring graphene. We discuss the tolerance of the geometric parameters (radii of the inner and outer circles, gap aperture) and study the device performance dependence on the relative intensities of two input beams and the angle between two input beams. Additionally, the response time of the device is analyzed to be ∼36 ps, which indicates possible high-speed gate-tunable THz CPA operations.

Near-infrared coherent perfect absorption in plasmonic metal-insulator-metal waveguide.

We propose a design of ultra-compact plasmonic coherent perfect absorber (CPA) working in the near-infrared band. The main operating mechanism is the magnetic-dipole resonant coherent absorption in the metal-insulator-metal waveguide, which enables the CPA in the near-infrared band and can be also flexibly adjusted to place the magnetic-dipole resonance at any position in the near-infrared band. Numerical analysis verifies our proposal that the magnetic resonant CPA is crucial for near-IR CPA in the ultra-compact metal-insulator-metal waveguide.

Nonlinear Coherent Absorption Resonance in Optically Thick Medium

Nonlinear Coherent Absorption Resonance in Optically Thick Medium

Dicke superradiance as a nondestructive probe for quantum quenches in optical lattices

We study Dicke superradiance as collective and coherent absorption and (time-delayed) emission of photons from an ensemble of ultracold atoms in an optical lattice. Since this process depends on the coherence properties of the atoms (e.g., superfluidity), it can be used as a probe for their quantum state. In analogy to pump-probe spectroscopy in solid-state physics, this detection method facilitates the investigation of nonequilibrium phenomena and is less invasive than time-of-flight experiments or direct (projective) measurements of the atom number (or parity) per lattice site, which both destroy properties of the quantum state such as phase coherence.

Tunable multi-channel inverse optomechanically induced transparency and its applications

In contrast to the optomechanically induced transparency (OMIT) defined conventionally, the inverse OMIT behaves as coherent absorption of the input lights in the optomechanical systems. We characterize a feasible inverse OMIT in a multi-channel fashion with a double-sided optomechanical cavity system coupled to a nearby charged nanomechanical resonator via Coulomb interaction, where two counter-propagating probe lights can be absorbed via one of the channels or even via three channels simultaneously with the assistance of a strong pump light. Under realistic conditions, we demonstrate the experimental feasibility of our model by considering two slightly different nanomechanical resonators and the possibility of detecting the energy dissipation of the system. In particular, we find that our model turns to be a unilateral inverse OMIT once the two probe lights are different with a relative phase, and in this case the relative phase can be detected precisely.

2-μm Ho emitter-based coherent DIAL for CO(2) profiling in the atmosphere.

We report on the use of a thulium-fiber-pumped holmium-based emitter in a coherent differential absorption lidar (CDIAL) experiment for high time and space resolution of CO(2) absorption field in the atmosphere. The 2-μm high-power dual-wavelength single-mode Q-switched Ho:YLF oscillator delivers 10-mJ pulses with a duration of 40 ns at 2 kHz. Both short pulse duration and high repetition rate were chosen to increase the DIAL precision and time and space resolution in coherent detection. The CDIAL provides 150-m range and 15-min time-resolved CO(2) absorption coefficient with a calculated instrumental error of 0.5% at 500 m and less than 2% at 1 km. Dry-air CO(2) mixing ratio estimates from the DIAL system are compared with simultaneous in situ gas analyzer measurements during a 20-h-long experiment.

Coherent perfect absorption in deeply subwavelength films in the single-photon regime.

The technologies of heating, photovoltaics, water photocatalysis and artificial photosynthesis depend on the absorption of light and novel approaches such as coherent absorption from a standing wave promise total dissipation of energy. Extending the control of absorption down to very low light levels and eventually to the single-photon regime is of great interest and yet remains largely unexplored. Here we demonstrate the coherent absorption of single photons in a deeply subwavelength 50% absorber. We show that while the absorption of photons from a travelling wave is probabilistic, standing wave absorption can be observed deterministically, with nearly unitary probability of coupling a photon into a mode of the material, for example, a localized plasmon when this is a metamaterial excited at the plasmon resonance. These results bring a better understanding of the coherent absorption process, which is of central importance for light harvesting, detection, sensing and photonic data processing applications.

Gain-assisted critical coupling for high-performance coherent perfect absorbers.

Balanced radiation and absorption rates of an optical resonator are necessary for coherent perfect light absorption in many active device applications. This balance is referred to as critical coupling condition. We propose a gain-assisted method for exact access to critical coupling conditions without altering any structure parameters. In a coherent absorber with additional internal gain media, critical coupling with arbitrarily high coherent signal extinction can be obtained by continuously tuning optical pumping density. Assuming a surface-plasmon resonance grating covered by a gain layer as a promising architecture, we numerically demonstrate gain-assisted continuous access to its critical coupling point with experimentally probable settings. In addition, the gain tuning further introduces switching of the coherent-absorber's functionality to a conventional lossless beam splitter.

Phonon induced phase grating in quantum dot system.

Electromagnetically induced phase grating is theoretically investigated in the driven two-level quantum dot exciton system at the presence of the exciton-phonon interactions. Due to the phonon-induced coherent population oscillation, the dispersion and absorption spectra are sharply changed and the phase modulation is enhanced via the high refractive index with nearly-vanishing absorption, which could effectively diffract a weak probe light into the first-order direction with the help of a standing-wave control field. Moreover, the diffraction efficiency of the grating can be easily manipulated by controlling the Huang-Rhys factor representing the exciton-phonon coupling, the intensity and detuning of the control field, and the detuning of the probe field. The scheme we present has potential applications in the photon devices for optical-switching and optical-imaging in the micro-nano solid-state system.

A study of the repumping laser and external magnetic field effect on coherent absorption resonance in alkali vapour

Investigation of coherent absorption resonance via electromagnetically induced absorption (EIA) spectra in rubidium vapour has been done in presence of a repumping laser and an external magnetic field. The EIA line shape gets enhanced and symmetric with the application of the repumping laser. In presence of an external magnetic field the amplitude of the EIA resonance gradually diminishes and at higher magnetic field, two absorption dip-like structures along with a transmission peak in between is obtained. Theoretically calculated spectra based on density matrix approach show a good agreement with the observations.

Electrically Tunable Coherent Optical Absorption in Graphene with Ion Gel

We demonstrate electrical control over coherent optical absorption in a graphene-based Salisbury screen consisting of a single layer of graphene placed in close proximity to a gold back reflector. The screen was designed to enhance light absorption at a target wavelength of 3.2 μm by using a 600 nm-thick, nonabsorbing silica spacer layer. An ionic gel layer placed on top of the screen was used to electrically gate the charge density in the graphene layer. Spectroscopic reflectance measurements were performed in situ as a function of gate bias. The changes in the reflectance spectra were analyzed using a Fresnel based transfer matrix model in which graphene was treated as an infinitesimally thin sheet with a conductivity given by the Kubo formula. The analysis reveals that a careful choice of the ionic gel layer thickness can lead to optical absorption enhancements of up to 5.5 times for the Salisbury screen compared to a suspended sheet of graphene. In addition to these absorption enhancements, we demonstrate very large electrically induced changes in the optical absorption of graphene of ∼3.3% per volt, the highest attained so far in a device that features an atomically thick active layer. This is attributable in part to the more effective gating achieved with the ion gel over the conventional dielectric back gates and partially by achieving a desirable coherent absorption effect linked to the presence of the thin ion gel that boosts the absorption by 40%.

Mode-locking in a laser with a coherent absorber

Mode-locking in a cw narrowband tunable dye laser whose cavity contains absorbing cells with molecular iodine vapor has been studied experimentally. When the wavelength of radiation of the dye laser is tuned to absorption lines of molecular iodine vapor in a cell in the cavity, short pulses following with a period equal to or multiple of the round-trip time of light around the cavity appear. The duration of the generated pulses is about 1 ns, which is much shorter than the phase memory time in iodine vapor. In such a situation, modelocking occurs owing to the coherent character of the interaction of laser radiation with narrow absorption resonances in molecular iodine vapor.