During the last decade there has been a growing demand of devices for monitoring the ambient regarding the existence of hazardous gases and materials. Routine methods for identifying hazardous materials and pollutants are cumbersome, expensive and time-consuming. The ambient atmosphere needs to be collected and sampled, and separation is sometimes needed between organic and inorganic components.

The Integrated Optics group at Tel-Aviv University has been involved lately in the development of a remote optical sensing system potentially capable of providing adequate answers to most of the stringent requests such a system would be needed to provide in practical situations [1,2]. Based on well-established technology of porous-silicon fabrication, the system under development has the following apparent advantages:

• Low cost of sensing units, allowing the wide dispersal of them in public or private ambient.
• The possibility of remote accessing and reading of the devices by common white light illumination. A single control unit can monitor many sensing chips. 
• Optical remote reading allows the location of sensing chips at places of difficult and hazardous access.

The porous silicon sensing chips are entirely passive meaning that they do not require energy supply in the form electrical connection, fuel or batteries. They are small in size (few millimeters) and can be easily incorporated in internal spaces, walls or structures.

The devices have been fabricated and tested, with emphasis in the following aspects:

• Hazardous materials: Ammonia and hydrocarbons
• Distant activation – up to 3 meters
• Simultaneous sensing of up to 4 chips in array by means of a single illuminating beam based on a novel method [2] with large positioning tolerance
• Reliability: several methods proposed and tested [3],[4]

References
1. T. Hutter, and S. Ruschin “Non-Imaging Optical Method for Multi-Sensing of Gases Based on Porous Silicon”. IEEE Sensors Journal  Vol.10,p 97-103 (2009)
2. S. Ruschin and T. Hutter US Patent 9013707, 2015.
3. Hutter, Tanya, and Shlomo Ruschin. “Some Methods for Improving the Reliability of Optical Porous Silicon Sensors.” Advances in Chemical Sensors p46 2012
4. Tanya Hutter, Moran Horesh, Shlomo Ruschin “Method for Increasing Reliability in Gas Detection Based on Indicator Gradient in a Sensor Array” . Sensors and Actuators B: Chemical, 2011 Volume 152, Issue 1, Pages 29-36 (2011)

Fig1: Simultaneous spectral readout from 3 samples showing distinct fitted peaks