Lab on a chip (LOC) devices - microchip-size systems that can prepare and analyze tiny fluid samples with volumes ranging from a few microliters to sub-nanoliters - are rapidly revolutionizing how laboratory tasks such as diagnosing diseases and investigating forensic evidence are performed. More importantly, LOC applications are transforming chemical analysis systems from large, immobile machines in a "brick-and-mortar" laboratory into tiny, portable instruments that can be put to work directly in the field. A recent addition to the growing list of LOC applications, a practical optical nanospectrometer, is under development by a team of California scientists and engineers. A recent paper in the Journal of Vacuum Science and Technology B details their method for fabricating a novel class of large-bandwidth wavelength demultiplexers based on digital planar holography (DPH), a process that yields spectrometers with "bandwidths and spectral resolutions comparable to table-top systems confined to the lab," says one of the authors, Stefano Cabrini, director of the Nanofabrication Facility of The Molecular Foundry, a U.S. Department of Energy-funded user facility for nanoscience and nanotechnology at Lawrence Berkeley National Laboratory (LBNL).
A digital planar hologram is a microfabricated device where a computer-generated holographic pattern consisting of millions of nanoscale lines is engraved onto the core (uppermost) layer of a microchip serving as a planar waveguide. The lines are specifically located and oriented so that they disperse light into specific focal channels according to wavelength. Cabrini says this creates a sort of "demultiplexing prism" where the hologram is sensitive to light components belonging to a specific and discrete set of wavelengths [spectral channels], and in turn, allows for multiband spectral analysis to take place. "Because we know exactly where the beams are going, we have the ability to study multiple frequencies at the same time," he says.
In previous work, Cabrini and his colleagues at LBNL and two private firms, aBeam Technologies and Nano-Optic Devices, used a combination of silicon dioxide and germanium as the waveguide core layer for their chip spectrometer. While the devices demonstrated a spectral channel spacing as low as 0.015 nanometers per channel, they suffered from too narrow of an operating bandwidth. In the JVST B paper, the team incorporated a material with a higher refractive index, silicon nitride (Si3N4), to yield a larger number of channels with the same spectral resolution.
The performance of the Si3N4-based demultiplexers was characterized by two different types of measurements. First, the intensity of the light into adjacent output channels was recorded in response to a fixed laser wavelength for different wavelength within the operating bandwidth for the devices. This allowed spectral channel spacing and the overall bandwidth to be defined. Second, the intensity of single channels were measured as a function of the wavelength to estimate the spectral shape of the output channels and the level of "crosstalk" (where the light transmitted in one channel interferes with the transmission occurring in a second channel) between them.
High-refractive index Si3N4 waveguide core films drastically increased the total bandwith of the holographic spectrometer, simultaneously providing a high spectral resolution (with spacing around 0.3 nanometers per channel for low wavelengths and 0.4 nanometers for higher ones), large overall bandwidths (up to 98 nanometers) and a high number of channels (around 300). These results, Cabrini says, confirm the flexibility of the DPH method for fabricating portable spectroscopy systems.