Recent discovery of a remarkably strong optical nonlinearity at a gallium-dielectric interface has already resulted in the demonstration of a fully-fiberized all-optical gate based on a nonlinear gallium mirror [1]. The mirror was manufactured on the tip of a single-mode fiber. In the gate the intensity of the light in the signal channel was modulated by the intensity of light in the control channel, both at wavelengths of about 1.55µm. The gate performance was characterized with a continuously modulated pump and modulation efficiency of about 30% with several hundred kilohertz bandwidth has been achieved.

In this paper we report that a nonlinear gallium mirror can be used for controlling light with light in pulse regime with sub-microsecond response time. We also demonstrate that the gallium/silica interface nonlinearity shows an exceptionally broadband spectral response. This broadband characteristic of the mirror has allowed us to demonstrate cross-wavelength all-optical gating between signals at wavelengths of 1.3 and 1.55µm. The gallium nonlinearity is associated with a light-induced structural phase transition in the common form of α-gallium in solid phase. It is believed that in the nanosecond-microsecond regime of optical excitation, the mechanism of the phase transition is predominantly non-thermal. Optical excitation is highly localized and destabilizes covalent bonding within the crystalline structure of alpha-gallium thus provoking a surface assisted transition to a more reflective, more metallic metastable phase. Such excitation of bonding-antibonding transitions in α-gallium is associated with a broad absorption band spanning from 0.8 to 4 eV [2]. This indicates that the effect may be induced by light in the visible and infrared parts of the spectrum. The structural phase transition drives a considerable change in the electronic, and in particular, in the optical properties of the material, also across a very broad spectral range. Conventional α-gallium has a relatively low reflectivity, typical to a semi-metal, which increases dramatically when the more metallic phase is induced at the interface by light stimulation. Although the exact nature of the light-induced metastable phase is not known yet, our data shows that its optical properties are very close to those of a free electron metal. Since on melting gallium acquires nearly free electron metal characteristics, the reflectivity change associated with the light-induced phase transition may be well illustrated by the change of gallium reflectivity on melting. Such reflectivity change is presented on Fig.1, which shows that a considerable effect across visible and infrared parts of the spectrum.

 

Fig. 1. Reflectivity of a gallium-glass interface of unpolarized light as a function of temperature, near the melting point, illustrating a strong change of reflectivity, overcooling and a reflectivity hysteresis. A considerable reflectivity change is seen across visible and near infra-red parts of the spectrum. Graphs a) and b) show the reflectivity change with increase and decrease of the temperature.

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Therefore the gallium nonlinearity promises to be exceptionally broadband, allowing cross-wavelength optical switching with the control and signal wavelengths to be virtually anywhere in the visible and near-infrared parts of the spectrum down to at least 1.7µm. The broadband properties of the liquefying gallium mirror nonlinearity have been recently demonstrated when two fiber laser, operating at wavelengths of 1550 and 1030nm respectively have been Q-switched using the same type of gallium mirror [3]. Here we demonstrate that gallium nonlinearity may be used for a cross-wavelength intensity modulation.

The cross-wavelength characteristics of gallium nonlinearity response of liquefying gallium were characterized in pump-probe experiments using the optical switch set-up shown in Fig.2. The fiberised gallium mirror was formed by immersing a freshly cleaved end of a single-mode silica optical fiber into a small bead of initially molten gallium of 5N purity. The sample temperature was controlled by a miniature Peltier heat pump to a precision of 0.01°C at temperatures around the melting point of gallium (30°C). The control source A was an amplified, directly modulated DFB diode laser operating at λ=1.550µm (1 MHZ linewidth.). The probe source B was a continuous-wave diode laser operating at λ=1.3µm. The pump and probe beams were coupled onto and off the gallium mirror using a wavelength division multiplexer WDM. The fibre mode-field diameter was ~12 µm. The peak power of the pump pulses incident on the mirror could be varied between 0 and 90mW, while the power of the continuous wave probe beam was 60 µW at the mirror surface. The probe beam was detected at the switch output with a 125 MHz 3 dB bandwidth InGaAs detector.

 

Fig. 2. he performance of the cross-wavelength all optical gate. The gate’s output contrast ratio is presented as function of the gallium bead temperature, T, T_m=30°C. The inset shows a schematic of the fiberized gate. Input A is the control channel, Input B is the signal channel and PC is a polarization controller.

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We investigated the dynamic properties of the mirror at temperatures in the vicinity of the melting point of gallium (~30°C) using control pulses of 100ns duration and 10kHz repetition rate. The control pulse at λ=1.55 µm induces the mirror reflectivity increase, this leads to a corresponding increase of the output signal at λ=1.3µm. After the control beam excitation is withdrawn, the induced reflectivity change rapidly returns to its original level (see inset on Fig.3). We observed that the reflectivity recovery time depends on the gallium bead temperature and increases with the temperature approaching the melting point of bulk gallium (see fig.3).

 

Fig. 3. Gate switch-off time, t, as a function of temperature, T. A typical gate response function with a 100ns control pulse (dashed line) is presented in the inset for T-T_m=-25°C.

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As the temperature T is increased towards the melting point T_m the induced nonlinear response rises steadily up to a level of ~40% as shown in Fig.2. Here percentage of the probe beam output intensity increase is presented. The pulse response on the control beam stimulation within this temperature region is highly stable and reproducible. At higher temperatures, just about 4°C below the bulk melting point, the induced reflectivity falls rapidly. The induced modulation becomes undetectable once the metal melts. On subsequent recooling the gate performance is fully restored. In Fig.4 we plot the intensity dependence of the induced change in reflected probe power for a number of temperatures in the proximity of the melting point. Once again the gallium bead was in the solid phase. We observed up to ~40% changes in the reflected probe intensity induced by the control beam powers of only a few tens of mW.

 

Fig. 4. The signal contrast ratio as a function of the control beam peak power for various temperatures.

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The huge optical nonlinearity in gallium relies on the coexistence of the molecular and metallic properties of α-Ga. α-Ga is a structure which is built up from polyhedral fragments which can be described as a ‘molecular metallic crystal’ where some of the bonds are strong covalent bonds, with the rest being weaker metallic-type bonding [3,4]. Gallium is also one of only a few metals showing the surface melting effect at normal conditions [5]. The following working model of the light-induced structural phase transition corroborates well with our experimental data: light stimulates a bonding-antibonding transition in α-Ga and the covalent bonding of the crystalline structure becomes unstable provoking a phase transition from α-Ga to a metastable phase with essentially free electron characteristics. As the system is very close to surface melting we interpret our results as the silica interface assisting the creation of a metastable layer. The layer thickness critically depends on the interface temperature and propagates with a high velocity appropriate to non-equilibrium epitaxial growth. The layer thickness also increases with the level of light excitation because the energy difference between α-Ga and the metastable phase reduces; reflectivity of the interface increases with layer thickness. The effect saturates and rolls-off as the thickness reaches the optical skin depth. When the excitation is withdrawn, the metastable layer re-crystallizes back to the α-Ga phase. The crystallization front moves towards the gallium/silica interface. The recovery time increases as gallium approaches the melting point as the velocity of the thermodynamic epitaxial growth decreses as T_m-T.

In conclusion, we have demonstrated that a nonlinear mirror formed at the gallium-silica interface allows for sub-microsecond cross-wavelength switching with up to 40% [40%] modulation in reflected light intensity. This has been observed in a compact, fiberized switch. We consider this nonlinearity to offer tremendous potential for the development of a wide range of truly practical nonlinear optical devices compatible with existing waveguide technology.