Final Report: Photonic Crystal Slot Waveguide Spectrometer for Monitoring of Volatile Organic Compounds in Groundwater and Hazardous Pollutants in Air

EPA Contract Number: EPD10047
Title: Photonic Crystal Slot Waveguide Spectrometer for Monitoring of Volatile Organic Compounds in Groundwater and Hazardous Pollutants in Air
Investigators: Chakravarty, Swapnajit
Small Business: Omega Optics, Inc.
EPA Contact: Manager, SBIR Program
Phase: I
Project Period: March 1, 2010 through August 31, 2010
Project Amount: $70,000
RFA: Small Business Innovation Research (SBIR) - Phase I (2010) RFA Text |  Recipients Lists
Research Category: Small Business Innovation Research (SBIR) , SBIR - Nanotechnology

Description:

Pollution of water resources in the United States and the world can have serious and wide-ranging effects on the environment and human health. A significant contamination activity relates to production, storage, and transportation of petroleum derivatives, as spills and leaks of these liquids through pipeline ruptures can contaminate soils, superficial water, and groundwater. Aromatic hydrocarbons in petroleum have attracted considerable attention due to their toxicity. Contamination of environment and the fatal danger to human health by hazardous materials and greenhouse gases is an omnipresent problem and is a concern of several departments of the U.S. Government. Due to the enormous significance of keeping drinking water and environment clean and free from intentional and unintentional contamination, an elaborate in-situ and highly sensitive sensing technology with remote monitoring capacity is an absolute necessity.
 
In this program, Omega Optics Inc. and the University of Texas, Austin proposed a novel lab-on-chip photonic crystal slot waveguide infrared spectrometer for detection and spectroscopic analysis of hydrocarbons (benzene and toluene) in water and greenhouse gas (carbon dioxide and methane) in air. The device utilizes the unique dispersive properties of slow light photonic crystal waveguides together with electric field intensity enhancement in narrow slot waveguides to achieve a factor of 1000 reduction in absorption length for the spectroscopic measurement of absorption spectra of analytes, specifically hydrocarbons in water and greenhouse gases. The versatility of the proposed method enables the realization of a novel in-situ on-chip miniature absorption spectroscopy instrument.
 

Summary/Accomplishments (Outputs/Outcomes):

The on-chip absorption spectrum of methane gas (4% methane in nitrogen) was successfully determined experimentally via near-infrared absorption signatures of methane at 1.665 µm. Transmission spectra of our photonic crystal slot waveguide was measured in the presence and absence of methane and absorbance of methane determined from the difference in transmission. Our 300 µm long on-chip silicon photonic crystal slot waveguide device thus was successfully demonstrated as an on-chip absorption spectrometer for methane. Preliminary calculations indicate a detection sensitivity of 78 ppm for methane with a 300 µm long photonic crystal slot waveguide in near infrared. Because the dispersive properties of photonic crystals are based on Maxwell’s equations and therefore are independent of wavelength, the same design parameters when scaled versus wavelength to the mid-infrared where methane has almost two orders of magnitude stronger absorption cross-section, will show a sensitivity ~28 ppb.
 
Current state-of-the-art spectroscopy devices for infrared absorption spectroscopy are characterized by large size, large weight, and thus high cost. Examples include cavity ring-down spectroscopy (66 lbs, 3 cu. ft., > $30,000), Fourier transform infrared spectroscopy (24 lbs, 1.5 cu. ft., and > $22,000), photo-acoustic spectroscopy (33 lbs, 1 cu. ft., price comparable to FTIR), and tunable diode laser absorption spectroscopy (several thousands of dollars in cost). In contrast, our photonic crystal slot waveguide device with size 10 µm × 300 µm is expected to be extremely light-weight (< 0.1 lbs) in a complete package. Furthermore, the easy integration with optical fibers ensures the possibility of in-situ remote monitoring and the small size ensures device introduction in tight spaces. The low price also ensures generous deployment of sensors due to very low cost of ownership. Furthermore, multiple devices, each sensitive to a specific region of the electromagnetic absorption spectrum can be fabricated simultaneously using the same lithography steps, thereby enabling detection of multiple species on-chip. The multiple species detection capability ensures less probability of false positives, by measuring optical spectra across multiple regions of the electromagnetic spectra with lower probability of overlapping spectral signatures.
 
The absorption spectrum of toluene in water was demonstrated in the near infrared at 1.67 µm. A broad absorption spectrum is obtained characteristic of toluene.  The experimentally observed detection sensitivity is 10-4% in water (v/v) corresponding to 0.8 mg/L. We believe that the limit of detection in the near infrared is approximately equal to an order of magnitude smaller. Because the same device principle is applicable in the mid- and far infrared, we believe detection sensitivities of few hundred ppt (v/v) are possible in our device.

Conclusions:

The lower exposure limit of methane is 5%. Omega Optics has successfully demonstrated experimentally methane detection below the lower exposure limit with a 300 µm-long photonic crystal slot waveguide on-chip platform, with near-infrared optical absorption signatures of methane at 1.665 µm. To Omega Optics' knowledge, the demonstrated photonic crystal slot waveguide on-chip optical absorption spectrometer is the smallest spectrometer for infrared optical absorption spectroscopy of gases. The detection limit is ~28 ppb for methane. The table below summarizes the essential requirements from a gas sensor and shows that Omega Optics' photonic crystal slot waveguide absorption spectrometer easily meets the requirements. No other technology currently can meet these requirements in terms of size, weight, sensitivity, price, and thus cost of ownership (COO).

 
 

The sensitivity of our device to the detection of toluene in water in the near infrared is 0.8 mg/L, which is lower than near infrared detection limits observed with PDMS disks in water (7 mg/L) in 10 mm long samples. Of course, with salinity enhanced water (0.12 mg/L) has been demonstrated in 5 mm long samples. The use of salinity enhancement, however, increases the processing steps for data collection. The device is more than an order of magnitude smaller in length than PDMS disks above. The experimentally demonstrated sensitivity of our device is smaller than 0.9 mg/L demonstrated with 11 meter long optical fibers. The limit of detection in the near infrared is believed to be approximately an order of magnitude smaller. Because the same device principle is applicable in the mid- and far infrared, detection sensitivities of few hundred ppt (v/v) are possible in the device. Also, response time in the device is a few seconds compared to ~60 minutes due to small thickness of the extraction phase for toluene to diffuse.

 

Supplemental Keywords:

small business, SBIR, EPA, greenhouse gas, hazardous air pollutants, HAP, volatile organic compounds, VOC, hazardous materials, pollutants, miniature spectrometer, monitoring, water monitoring, air monitoring, waveguide spectrometer, groundwater, chemical spectroscopy, absorption spectroscopy instrument, nanotechnology, lab-on-a-chip sensor, silicon lab-on-chip photonic crystal integrated infrared spectrometer