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Applied Optics

Excited state optical modulator

Thomas W. Ebbesen

Published Jul 1, 1986• 3 citations
Publisher

Abstract

It is shown that if two light beams intersect in a medium containing the appropriate molecule, one beam can modulate the other by making use of the differences in the absorption spectra of the ground and excited molecular states. This is possible even with the power of semiconductor lasers. From this, an opto–optical modulating device can be conceived which would have micrometer dimensions, potentially fast response time, and significant quantum gain. Such a device could find use in optical communications and optical computing.

DOI
10.1364/ao.25.002193

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Acta Physica Sinica (Overseas Edition)•Aug 1, 1995•5 citations
It is shown that if two light beams intersect in a medium containing the appropriate molecule, one beam can modulate the other by making use of the differences in the absorption spectra of the ground and excited molecular states. This is possible even with the power of semiconductor lasers. From this, an opto–optical modulating device can be conceived which would have micrometer dimensions, potentially fast response time, and significant quantum gain. Such a device could find use in optical communications and optical computing.
Introduction Previous discussions (see Section 10.4) suggested that stimulated emission can be used to generate optical gain, that is, to amplify radiation. The reader certainly has experience in the amplification of electronic signal. For example, radio receivers capture faint radio waves and turn them into a signal that is powerful enough to drive large speakers. This electronic amplification increases the amplitude of the signal while faithfully preserving its acoustic frequencies and modulation characteristics. Similarly, optical amplification is expected to increase the amplitude of an optical signal while preserving its frequency, its modulation characteristics, and its coherence. The latter requirement is of particular significance for optical radiation, where the coherence of naturally occurring radiation rarely exceeds one micrometer. Lasers are the primary source for coherent radiation. They depend on stimulated emission for amplification and for the generation of coherent radiation (the word laser is the acronym of Light Amplification by Stimulated Emission of Radiation). For amplification, an atomic system that is part of the laser medium must be prepared with a sufficiently large number of particles in the excited state. (The term atomic system is used here to describe all microscopic systems including molecules and free electrons.) Radiation passing through that excited medium encounters multiple events of stimulated emission, each event contributing one photon that is added coherently to the propagating beam. When the number of events of stimulated emission exceed all losses by absorption or scattering, the incident radiation is amplified.
A novel spatial light modulator (SLM) based on nonlinear absorption of light by excited molecules is suggested. A kinetic analysis is applied to calculate the relative populations of molecular levels in singlet and triplet manifolds. It is shown that for many actual cases a probe light can be modulated by propagating through a medium excited by another light source. The dependence of the molecular SLM on the photophysical parameters of the SLM medium is discussed. The same type of molecular nonlinearities can be utilized for optical bistability, as is demonstrated for some dyes.
This letter analyzes a suggested technique for studying excited atomic or molecular states on a picosecond to subpicosecond time scale by using tunable laser beams, without requiring ultrafast laser pulses or optical detectors. In the proposed technique the frequency transform of the impulse response of an excited state is determined by measuring the diffraction of a cw probe laser beam from a moving excited-state grating induced by two intersecting tunable excitation beams, as the frequency difference between the two cw excitation beams is varied.
Optical signal processing using all-optical circuitry is an increasingly attractive goal with the recent progress in long- lived light sources, high quantum efficiency detectors, and low- loss high-bandwidth optical fibers. Devices which permit one light beam to control another or which provide an optical memory afford a capability essential to the success of all-optical logic systems. Much progress has been made toward practical bistable1 devices since the first observation2 of optical bistability in sodium vapor. (By optical bistability we refer to devices containing no inverted medium, so that bistable lasers3 are explicitly excluded.) The most practical device so far, a semiconductor etalon consisting of 4 μm of GaAs between 0.2 μm of Al0.42Ga0.53As with R = 0.9 coatings, is described herein.
The main achievements in our research on the physical phenomena of the excited-state nonlinear absorption in a molecular system and its applications in photonic technology are described. In the first part of this paper, some energy-level models and rate-equations are used to explain various mechanisms of the excited-state nonlinear absorption under different conditions, such as reverse saturable absorption caused by the triplet and/or the singlet excited-state absorption; saturable absorption due to the first and/or the second singlet excited-state absorption; and the excited-state nonlinear absorption induced by two-photon absorption. The experimental results for metal-organic and C 60 materials irradiated by ps and ns laser pulses are consistent with the simulated curves of the transmittance versus fluences. In the second part, the applications of excited-state nonlinear absorption in photonic techniques including optical bistability, optical switching, optical limiting, optical modulation, optical logic and optical storage are introduced. The working principles of the photonic devices based on the excited-state nonlinear absorption are presented. The experimental characteristic curves are found in good agreement with the theoretical simulations for these devices.