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Conference on Lasers and Electro-Optics

Picojoule, subnanosecond all-optical switching in a CdS etalon

M. Dagenais, Wayne F. Sharfin

Published Jan 1, 1985• 0 citations
Publisher

Abstract

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.

DOI
10.1364/cleo.1985.ff2

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Claus F. Klingshirn, F. Fidorra, J.-Y. Bigot +3 more

Optical bistability (ob) is investigated in CdS in a wide range of intensities. Under pulsed excitation (total pulse width 20 nsec) we find at 5K for ħωexc ≈ 2.550 eV on the flank of the AΓ5 exciton ob of the induced absorption type for incident intensities of Iexc ≈ 1kW/cm2. The nonlinearity is in this case a collision-broadening of the free exciton resonance [1]. The commutation times τ↓ and τ↑ are about 1 nsec and 5 nsec, respectively. For ħωexc ≈ 2.546 eV we observe absorptive ob due to the formation of an electron-hole plasma [2] for Iexc ≈ 1MW/cm2. Single shot experiments show τ↓ ≲ 500 psec being thus to our knowledge one of the shortest commutation times reported for single beam ob so far. The observed hysteresis loops are compared to model calculations with variable pulse lengths. The comparison indicates true ob in our experiments.

Topical Meeting on Optical Bistability (OB)•Jan 1, 1985•0 citations
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.
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.
In this work, the external switching dynamics of a Fabry-Perot etalon are studied via optical bistability system simulation. The simulated set-up of this investigation consists of two laser beams; the first beam is continuous (CW) which is considered as a biasing beam and capable of holding the bistable system for a certain range, which we are interested in, from a point that is very close self-switching to a point where the switching is unachievable. The second beam is modulated by passing the first beam through an acousto-optic modulator (AOM) to produce pulses with a minimum rise time and is used as an external source (coherent switching). In this work, we obtained the optical bistable loops by applying absorption coefficient (α) = 20cm-1, e sample etalon thickness (D) = 110μm, forward mirror reflectivity (Rf) = 0.6, and backward mirror reflectivity (Rb) = 0.95. The steady state characteristic of an initial detuning of the cavity (φ0) = 0.8 was studied at the conditions of no external input pulse intensity (M(t) = 0) and switching that takes place at Is(ON)= 0.57mW and Is(OFF) = 0.4mW.
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.
Optical bistability (ob) is investigated in CdS in a wide range of intensities. Under pulsed excitation (total pulse width 20 nsec) we find at 5K for ħωexc ≈ 2.550 eV on the flank of the AΓ5 exciton ob of the induced absorption type for incident intensities of Iexc ≈ 1kW/cm2. The nonlinearity is in this case a collision-broadening of the free exciton resonance [1]. The commutation times τ↓ and τ↑ are about 1 nsec and 5 nsec, respectively. For ħωexc ≈ 2.546 eV we observe absorptive ob due to the formation of an electron-hole plasma [2] for Iexc ≈ 1MW/cm2. Single shot experiments show τ↓ ≲ 500 psec being thus to our knowledge one of the shortest commutation times reported for single beam ob so far. The observed hysteresis loops are compared to model calculations with variable pulse lengths. The comparison indicates true ob in our experiments.