Coherent Perfect Absorption (CPA) Fundamentals

• Coherent Perfect Absorption (CPA) is an optical phenomenon where coherent waves are completely absorbed with no reflection or transmission, acting as the time-reversed process of lasing.
• The methodology utilizes precisely balanced optical cavities and scattering matrix analysis to ensure perfect absorption at specific resonant frequencies.
• Advanced CPA techniques incorporate quantum interference, exceptional point physics, and meta-atom designs to enhance absorption for multifunctional photonic applications.

Coherent Perfect Absorption (CPA) is an optical phenomenon where an incoming coherent radiation pattern is entirely absorbed by a medium without reflection or transmission, effectively being the time-reversed counterpart of lasing. CPA systems are generally implemented in optical cavities where the gain and loss balances the lasing medium at threshold levels. The concept has evolved beyond classical optics to include quantum effects and applications in photonics.

1. Classical CPA: Basics and Mechanism

The initial concept of CPA as a time-reversed laser involves setting up a lossy medium within an optical cavity. When coherent waves, precisely the time-reversed versions of lasing modes, impinge upon the system, they are completely absorbed:

• CPA Condition: The medium should have the same magnitude of dissipation as the gain of the lasing medium.
• Implementation: A specific setup ensures zero reflection and transmission, effectively "trapping" optical waves within the cavity until dissipated.

CPA is analyzed using the scattering matrix formalism, where the zero eigenvalues indicate perfect absorption at specific resonant frequencies.

2. Extension to Arbitrary Optical Fields

CPA's scope has been expanded to include complex optical fields produced by lasers operating in nonlinear regimes—chaotic, periodic, or transient:

• Generalization: CPA can absorb non-monochromatic or temporally varying fields by adjusting the medium's parameters.
• Examples: CPA achieved for chaotic Lorenz-Haken laser signals and frequency-modulated waveforms, using a broadband absorber inside the cavity.

The time-domain mapping of laser fields shows the dynamics allowing CPA in complex regimes, implying robust absorber configuration to handle diverse spectral components.

3. CPA in Quantum Optics

Quantum CPA extends classical concepts into quantum regimes, using quantized standing waves and subwavelength absorbers:

• Quantum CPA: Interfering classical wave components are replaced by quantum state superpositions (NOON states, squeezed states).
• Applications: Prepares entangled quantum states like squeezed vacua without entangling with the absorber, serving as filters for quantum information processing.
• Efficient Absorption: Important for quantum memories and entangled light sources, relying on intricate quantum interference patterns.

The unified theory accounts for more complex quantum states, including multipartite entanglement and state-specific absorption probabilities.

4. Advanced CPA Techniques

Innovative CPA implementations leverage exceptional-point physics, mass degeneration in cavities, and PT-symmetry to enhance absorption capabilities:

• Exceptional Points: Merge multiple cavity resonances, aligning absorption conditions across broader spectral ranges.
• PT-Symmetric CPA-Laser: Achieves simultaneous lasing and CPA under parity-time symmetry conditions, balancing spatially distributed gain and loss.

These techniques demonstrate CPA's applicability beyond conventional boundaries, facilitating multifunctional devices integrating amplification and absorption.

5. CPA in Complex Scattering Systems

On-demand CPA in complex and chaotic systems utilizes programmable meta-atoms to achieve highly sensitive perfect absorption.

• Time Delay Divergence: CPA closely linked to the scattering matrix zeros and demonstrates extreme sensitivity to small perturbations.
• Meta-Atoms: Allow CPA realization at arbitrary frequencies by adjusting scattering properties, crucial for precision sensing applications.

This approach significantly broadens IPA's implications for secure communication and environmental sensitivity, providing new pathways for nanophotonic advancements.

Conclusion

Coherent Perfect Absorption has evolved from a fundamental concept in optics to a versatile tool for photonic and quantum applications. By exploiting interference, quantum state manipulation, exceptional points, and chaos theory, CPA offers promise for efficient light management in advanced optical systems. Its application in secure communications, quantum information, and adaptive light processing underscores CPA's profound impact on both scientific understanding and technological innovation.