For nearly 20 years infrared micro-spectroscopy (IRMS) has been used as a biochemical probe. The rich spectrum of vibrational and twisting modes of complex molecules in the mid- to far-IR wavelengths make this region of the spectrum especially useful in the interrogation of the chemical and biological properties of a sample. Although IR microspectroscopy has been a highly versatile tool for the probing of biochemical systems, it is currently limited by its spatial resolving power of 30 mm or so (new techniques using high brightness synchrotron radiation sources have very recently been used for diffraction-limited spectroscopy of biological systems but this work is in its infancy) and the large and expensive components that make up a conventional IRMS system.  We propose to replace this complex system with a single compact microchip containing an array of microlasers with electronic sensor readout and a pneumatically driven microfluidic interface, greatly changing the way in which IRMS is used. 

Making this possible is the recent demonstration of a novel electrically injected microcavity laser operating in the mid-infrared (mid-IR) region of the electromagnetic spectrum which has an open cavity architecture, providing for the possibility of introducingsolids, fluids, and gases in-situ to the laser cavity.  This new device has been called a Quantum Cascade (QC) Photonic Crystal (PC) Surface Emitting Laser, or QC-PCSEL (see Fig. 1 below).  Owing to the high refractive index of the semiconductor cavity, a spatial resolution on the order of the air hole diameter of the PC lattice should be possible, roughly a quarter of a wavelength in current devices (well below the diffraction limit in air). This would allow for spectroscopy down to the individual cell (5 mm). When combined with microfluidic sample delivery, the compact size and electrical read-out of the proposed system will enable IRMS to be used in completely new ways and arenas outside of the laboratory.

Figure 1: (A) SEM image of a 2D array of PC QC lasers. Each rectangle corresponds to the outer metal top contact of a laser device. Inset shows a microbolometer camera image of the laser near-field.  (B) Tuning of the laser emission wavelength as a function of a and r/a for several different devices located on the same semiconductor chip.

Publications

1)  R. Colombelli, K. Srinivasan, M. Troccoli, O. Painter, C. Gmachl, D.M. Tennant, A.M. Sergent, D.L. Sivco, A.Y. Cho, and F. Capasso "Quantum cascade surface-emitting photonic crystal laser," Science, v302 (5649), pp. 1374-1377, Nov. 21, 2003. (R Colombelli et al., Oct. 30 2003, Sciencexpress 1090561).

Research Group