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Nonlinear Optics

Optical switching based on singlet-singlet excited state absorption

Yang Miao, Jinhai Si, Yuxiao Wang, Chunfei Li

Published Jan 1, 1996• 0 citations
Publisher

Abstract

Progress in optical computing, optical communication and optical information processing depends on a large extent of the availability of fast all-optical switches. Most of the all-optical switches utilize nonlinear FP cavity1,2,3. Its absorptive loss is very large because of working on the resonant wavelength, and the switching time is districted by the response time of cavity. In 1990 K. W. Beeson4 presented a novel all-optical switch based on the triplet-triplet absorption in Eosin Y using a CW laser as a pump beam, and obtained μs scale switching time. We developed this kind of switches in C605 whose switch off time was about 250 ns which depends on the lifetime of the first triplet excited state.

DOI
10.1364/nlo.1996.nme.7

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Progress in optical computing, optical communication and optical information processing depends on a large extent of the availability of fast all-optical switches. Most of the all-optical switches utilize nonlinear FP cavity1,2,3. Its absorptive loss is very large because of working on the resonant wavelength, and the switching time is districted by the response time of cavity. In 1990 K. W. Beeson4 presented a novel all-optical switch based on the triplet-triplet absorption in Eosin Y using a CW laser as a pump beam, and obtained μs scale switching time. We developed this kind of switches in C605 whose switch off time was about 250 ns which depends on the lifetime of the first triplet excited state.
The dynamics of all-optical switching in C60 has been theoretically analyzed based on the principle of nonlinear intensity-induced excited-state absorption to achieve high contrast and fast switching. The transmission of a cw probe laser beam at 885 nm corresponding to the peak absorption of the S1 state through C60 in toluene is switched by a pulsed pump laser beam at 532 nm that excites molecules from the ground state. Switching characteristics are shown to be sensitive to the variation of pump pulse width, peak pumping intensity, thickness of the medium, intersystem crossing time between the S1 and T1 states and the absorption cross section of the S1 state at a probe wavelength, and their effect are analyzed in detail. It is shown that the transmission of the probe beam can be completely switched off (100% modulation) by the pump beam at a pump intensity of 100 MW/cm2 and a pulse width 1.5 ns, with switch on and off times of 2.5 and 7.5 ns, respectively. The results are used to design all-optical NOT and the universal NOR and NAND logic gates with multiple pump laser pulses, which are the basic building blocks of computing circuits.
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.
It is well known that there is a bottleneck for nonlinear all-optical switching, namely, the switching power and the switching time cannot be lowered simultaneously. A lower switching power requires a resonator with a high quality (QQQ) factor, but leads to a longer switching time. We propose to overcome this bottleneck by replacing the nonlinear cavity in such an all-optical switch by a white-light cavity. This can be done by doping three-level atoms in the ring resonator and applying incoherent pump and coherent driving fields on it. The white-light cavity possesses broadband resonance in a linear region. Therefore, for the incident pulse, a broad range of frequency components can take part in the nonlinear process, and so it requires lower power to achieve switching compared to the conventional ring resonator. On the other hand, the refractive index of a white-light cavity has negative dispersion, leading to a fast group velocity. This results in a shorter time to build up the resonant response, yielding a short switching time.
In this thesis we study ultrafast all-optical switching of microcavities. We employ the electronic Kerr effect to switch the resonance frequency of microcavities operating at telecom wavelengths. We observe the fastest possible switching of a microcavity resonance within 300 fs. The switching speed is only determined by the cavity storage time; only limited by the speed of light and not by material related relaxation properties. For the first time, we derive and measure the non-degenerate three-photon absorption coefficient of GaAs. Using the measured nondegenerate two- and three-photon absorption coefficients we estimate that the trigger laser only lose 22 aJ/µm2 at each switch event, which is a record level of minimum energy loss per switch event. We achieve repeated switching of microcavities within 700 fs, corresponding to THz rates. Therefore to best of our knowledge, we have broken the THz clock rate barrier experimentally using semiconductor cavities for the first time. The THz clock rate that we achieve can be a solution for information technology that is exploring ways to meet the growth rate of the computational demand. The color of light is a main visual perception of light for humans. Indeed, a simple measure for the color of light is given by its frequency since frequency of light is invariant from one medium to another. Although the propagation of light is strongly modified using nanostructures such as photonic crystals and microcavities, the frequency of light stays constant in these structures as well. The color of light can be changed in a nonlinear process or by absorption and re-emission. Using microcavities light is conveniently confined in space and then by dynamical control the medium can be changed to act as a nonlinear medium for light. Remarkably, we show that during the switching of cavities we achieve both up and down frequency conversion of light. The frequency conversion that we observe is a result of the instantaneous refractive index change achieved using the electronic Kerr effect. We achieve repeated frequency conversion at THz rates and generate blue- and red-shifted light pulses at this rate.
The dynamic processes of reverse saturable absorption (RSA) in a solution of C60 are studied in detail by use of rate equations and the light-propagation equation for a five-energy-level model. The RSA experimental results for C60 in toluene solution with a nanosecond–picosecond Nd:YAG laser at 532 nm are consistent with the theoretical simulations. Effects of sample concentration and thickness on RSA and contributions of the singlet and triplet excited states to RSA for various laser-pulse widths and wavelengths are also analyzed. Finally, an all-optical switch based on excited-state nonlinear absorption and the pump-probe technique is implemented.