Planar Waveguide Transmits Mid-Infrared Radiation

Breck Hitz

Because so many molecules have unique spectral signatures in the mid-infrared — generally defined as those wavelengths between several microns and several tens of microns — the evolution of integrated mid-IR components would be beneficial for the detection and identification of these molecules. A similar evolution has taken place in the visible and near-IR ranges, but until the recent development of the room-temperature quantum cascade laser, there has been no viable photonic source in the mid-IR.

Now scientists at Pacific Northwest National Laboratory in Richland, Wash., have demonstrated what they believe to be the first thin-film chalcogenide-glass waveguides for the mid-IR. They expect that extrapolation of the techniques they employed can lead to useful, integrated optical components in this important spectral range.

They evaporated thin films of As₂S₃ and As₂Se₃ onto an Si/SiO₂ substrate to fabricate the waveguides (Figure 1). As₂Se₃ has a higher refractive index than both the As₂S₃ beneath it and the air above it, so light is guided vertically in the As₂S₃ waveguide by total internal reflection. The scientists discovered that the thickness of the As₂S₃ layer was a critical parameter: A too-thin layer resulted in heavy propagation loss of the evanescent field in the SiO₂, which is opaque at mid-IR wavelengths.

Figure 1. Light was guided in the channel waveguide created by photo-darkening the As₂Se₃ film with a HeNe laser (a). A scanning electron microscope image shows the thicknesses of the films (b). The channel width of the waveguide down the length of the device was 5.4 μm (c). Images ©OSA.

To provide lateral confinement, they photo-darkened a narrow channel in the As₂Se₃ film with a HeNe laser. The 633-nm light was absorbed in the As₂Se₃, whose bandgap corresponds to the HeNe photon energy, but not in the As₂S₃, whose bandgap is greater than the photon energy. They created a permanent index change, which they later measured to be Δn ∼0.04, by translating the waveguide at 4 mm/min beneath the HeNe beam focused to an intensity of 3.5 kW/cm² at the surface of the As₂Se₃. Although the waveguide in this case was a straight line, they believe that the simple photo-darkening mechanism easily could be extended to create more complex waveguides in integrated mid-IR components.

To evaluate the waveguides, the researchers end-coupled an 8.4-μm quantum cascade laser from Maxion Technologies of Hyattsville, Md., to a 5.4-cm-long waveguide. They imaged the waveguide output using an infrared camera from Flir Systems of Wilsonville, Ore., which revealed a single-mode Gaussian intensity profile with an e^–2 mode diameter of 19.1 μm (Figure 2).

Figure 2. The Gaussian intensity profile of 8.4-μm radiation transmitted through the waveguide indicated single-mode propagation.

They measured the propagation loss first by measuring the transmission through a 3.55-cm-long waveguide and then by repeatedly cutting the waveguide to shorter lengths (by cleaving it) and again measuring the transmission. From a series of four successive cutbacks, they found the attenuation of the waveguides in Figure 1 to be 0.5 ±0.1 dB/cm for the horizontal polarization and 1.1 ±0.1 dB/cm for the vertical.

The coupling efficiency from the laser into the waveguide was low, at approximately 20 percent, because the scientists made only minimal effort in this initial experiment to optimize it. The high refractive index of the As₂Se₃ film (n = 2.78) created a large Fresnel reflection, and they made no attempt to mode-match the laser to the waveguide.

Optics Letters, June 15, 2006, pp. 1860-1862.