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Applied Physics Letters

Picojoule, subnanosecond, all-optical switching using bound excitons in CdS

M. Dagenais, Wayne F. Sharfin

Published Feb 1, 1985• 52 citations
Publisher

Abstract

We use the nondiffusive nonlinearity, the large transition dipole moment, and the small spectral broadening associated with the I2 bound excitons in cadmium sulfide to achieve the lowest reported single beam switching energy (<8 pJ) and the fastest reported ON/OFF switching bistable device (<2 ns) with clearly resolved switching states. In an optimized geometry, it is shown that the fundamental limits on switching energies can be approached.

DOI
10.1063/1.95692

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We use the nondiffusive nonlinearity, the large transition dipole moment, and the small spectral broadening associated with the I2 bound excitons in cadmium sulfide to achieve the lowest reported single beam switching energy (<8 pJ) and the fastest reported ON/OFF switching bistable device (<2 ns) with clearly resolved switching states. In an optimized geometry, it is shown that the fundamental limits on switching energies can be approached.
An all-optical bistable device and an electro-optical bistable device, with performances approaching the fundamental statistical limits, are described. In the all-optical approach, the giant nonlinearities associated with bound excitons in CdS are exploited to demonstrate the lowest-switching-energy (< 4 pJ) all-optical bistable device. The device switches in less than 1 ns and fully recovers in less than 2 ns. These times are detector limited. A switching energy of less than 1 fJ is theoretically predicted for an optimized geometry. In the electro-optical approach, an InGaAsP/InP diode laser amplifier is used to demonstrate bistability with less than 6000 photons (< 8 X 10-16 J) incident on the device. The device switches in less than 1 ns and is cascadable since it has a gain of 100. It operates at room temperature and is compatible with diode laser sources.
Using a laser slightly detuned from the I2 bound exciton resonance in CdS, we report the observation of the lowest switching energy (< 4 pJ) for an all-optical bistable device. The deconvoluted switching-on and -off times are evaluated to be less than 1 and 2 ns respectively. Even shorter switching times are expected, and we are presently improving our time resolution. Forward, degenerate four-wave mixing experiments were also performed near the I2 bound exciton, an exciton bound to a neutral donor. A four-wave mixing signal was observed with less than 50 W/cm2 incident on the sample.
We report the fastest ON/OFF switching bistable optical device yet demonstrated. The device consists of a 13-μm thick CdS platelet in a Fabry-Perot cavity formed by two external mirrors with 90% reflectivity. Very well-defined OM and OFF states with switching times of < 1 nsec (ON) and 2 nsec (OFF) are observed when the laser frequency is tuned to within about one halfwidth below the inhomogeneously broadened bound exciton resonance in CdS. In Fig. 1, the Input and output pulse forms are shown revealing the fast switching.
We describe two approaches to low-energy switching and optical bistability. In one approach, which is a more fundamental approach, we use the nonlinearity associated with bound excitons in CdS to demonstrate the lowest total switching energy (4 pJ) of any optical device exhibiting two stable output states for the same input intensity.1 Bound excitons were chosen because they are known to have a very large oscillator strength (f = 7), a fast radiative decay (tspont = 500 ps), and they do not diffuse in the crystal, thus permitting a sharp focusing geometry. Furthermore, they are a good approximation to quantum dots. In a different experiment we have operated a standard diode laser structure below threshold and have demonstrated an efficient optical switch that can be used in thresholding and decision-making applications. The device switches with <400 aJ (4 × 10−16 J or <3000 photons) of optical energy incident on the device.2 During the switching operation, only 10 pJ of electrical power is dissipated. The switch operates at room temperature, is compatible with optical communication systems, and has a gain of the order of 15 dB, leading to the important properties of cascadability and large fan-out. The operation of a high contrast (5:1) AND gate was demonstrated.
The I2 bound exciton in cadmium sulfide has a giant oscillator strength of 6. It decays mostly radiatively in 500 psec. Such a large oscillator strength leads to a very large transition dipole moment and to very large nonlinear effects off-resonance. At 2K, this sytem saturates like an inhomogeneously broadened system for intensities up to 30 times the saturation intensity. The transition linewidth is 7.5 GHz. In a tight focusing geometry, only 3.6 μW is required to see the onset of saturation. This corresponds to a measured saturation intensity of about 60 W/cm2. The linear absorption coefficient right at the resonance peak was measured to be 3.2 x 103 cm-1. At higher temperatures, the transition linewidth broadens and the line becomes more and more homogeneously broadened because of phonon interaction. At intensities above 50 times the saturation intensity, the absorption coefficient stops decreasing and then starts increasing. At these high intensities, thermal effects appear to play a significant role in shifting the free and bound exciton resonances toward lower energies. At intensities less than 20 times the saturation intensity, thermal effects are believed to be insignificant in our saturation measurements.
A general discussion of the linear-optical properties of free and bound excitons in direct-gap semiconductors at low temperature is presented. This discussion then specializes to CdS. The nonlinear-optical response of CdS is introduced by first discussing the saturation behavior of bound excitons. A new type of cavityless optical bistability based on induced absorption near the free and bound excitons is described. Finally, fast all-optical switching (<1 nsec) at very low switching energies (<8 pJ) is demonstrated, using the giant nonlinearity associated with bound excitons.