The Nanosensors Group recently demonstrated a fundamentally new approach for resonant optical biosensors in which an active optical resonator, in the form of a distributed feedback laser or an external cavity laser, is used to generate its own high-intensity narrow bandwidth output without any requirement for precise incoupling of excitation illumination or precise outcoupling to collect emitted light. It was our hypothesis that, for a biosensor incorporating an active resonator structure in which shifts in laser emission wavelength are monitored as the sensed output, high-resolution may be achieved while maintaining a high level of detection sensitivity.
Distributed feedback laser biosensors
The DFB structures can be realized by producing a periodic structure along the emission axis of a semiconductor laser diode or in molded polymer structures with large surface areas. DFB laser structures have been studied extensively and were selected as the resonant cavities for laser-based biosensors for the following reasons: 1) The DFB structure is comprised of a subwavelength periodic grating that may be inexpensively fabricated from polymer materials over large surface areas by replica molding; 2) The DFB laser may be fabricated as a surface structure for which part of the oscillating mode resides within the dielectric medium in contact with the surface structure, and therefore provides the opportunity for laser wavelength tuning by attachment of biomolecules to the surface; 3) DFB lasers may be designed to emit only a single mode (i.e., a single wavelength) that will be directed normal to the surface by first order diffraction; 4) Q-factor of a DFB laser oscillator, Qosc, are typically demonstrated with the value of 10,000; 5) DFB laser emission may be achieved using low-power optical pumping.
The implications of these features are of enormous practical importance for the development of a sensor technology to be used in life science research laboratories. It will be possible to fabricate large surface areas of DFB laser biosensor material on flexible, inexpensive plastic substrates and to incorporate them into disposable labware. With an entire surface comprised of usable sensor material, there is no need to precisely aim immobilized ligand to bind only upon certain regions. As this technology develops further, it will be possible to simultaneously measure many regions on the sensor surface in parallel and to easily incorporate references and controls into every experiment. Using optical pumping to excite the laser emission, only millimeter-level precision is needed to aim the pump source at the desired detection area. The laser emission itself is of sufficient intensity for easy detection with inexpensive sensors, and is captured with a simple optical fiber. Performing a single measurement is also extremely fast. The optical pump duration is ~10 nsec, resulting in an output pulse of approximately equal duration. Further, if the pump laser is focused down to a ~1 mm diameter spot on the DFB laser biosensor surface, the resulting emission wavelength will be determined by only a very small region around the excitation spot, so that by rastering the excitation spot sequentially across a DFB surface, a spatial map of adsorbed biochemical or cellular binding density may be obtained.
External cavity laser biosensors
Recently, the external cavity laser biosensor has been demonstrated as an alternative laser-based label-free optical biosensor approach that maintains the high resolution of laser biosensors while addressing some shortcomings shared by the DFB laser biosensor and several other laser-based label-free biosensors through the introduction of gain from a source external to the sensor itself. This innovation of utilization of external optical gain results in a robust detection system that allows low-cost, large-area transduction chips; non-contact optical coupling; a compact electrical pump source; CW operation; and multiplexing while at the same time achieving high sensitivity, large dynamic range, and the ability to measure very small wavelength shifts.
In the ECL sensor system, a PC resonant reflector surface is used as the transducer upon which biological material is adsorbed, and it also serves as a wavelength selective element of the ECL cavity. Gain for the lasing process is provided by a semiconductor optical amplifier (SOA), resulting in a simple detection instrument that operates by normally incident noncontact illumination of the PC and direct back-reflection into the amplifier. A schematic drawing of the ECL setup and the lasing spectrum is shown in Figure 5 and Figure 6(a). As the ECL biosensor resonates, high-intensity electromagnetic standing waves are established at the PC-media interface. Adsorption of biomolecules on the PC tunes the resonant wavelength of the PC, which subsequently tunes the emission wavelength of the ECL. The figure of merit (FOM = Sb× Q) of this approach is greater than previously published passive resonator biosensors due to the high Q-factor of the ECL emission (Q = 2.8 × 107) and the high refractive index sensitivity of the PC surface (Sb = 212 nm/RIU).
To demonstrate the ECL system for detection of biomolecular interactions, a simple assay for detection of biotin by an immobilized layer of protein streptavidin (SA) was performed, with the dynamic binding measurement shown in Figure 6(c). A further demonstration of DNA hybridization process was performed, with PC surfaces functionalized with synthetic 20-mer single strand DNA oligonucleotide probes. The hybridization process, shown in Figure 6(d), was kinetically monitored after introduction of complementary DNA oligonucleotide targets.
- “Vertically emitting, dye-doped polymer laser in the green (lambda ~ 536 nm) with a second order distributed feedback grating fabricated by replica molding,” M. Lu, J.G. Eden, and B.T. Cunningham, Optics Communications, Vol. 281, 3159-3162, 2008.
- “Label free biosensor incorporating a replica-molded, vertically emitting distributed feedback laser,” M. Lu, S. S. Choi, C.J. Wagner, J.G. Eden, and B.T. Cunningham, Applied Physics Letters, Vol. 92, 261502, 2008.
- “Plastic distributed feedback laser biosensor,” M. Lu, S.S. Choi, U. Irfan, and B.T. Cunningham, Applied Physics Letters, Vol. 93, No. 11, 111113 (DOI 10.1063/1.2987484), published online September 18, 2008.
- “Distributed feedback laser biosensor incorporating a titanium dioxide nanorod surface,” C. Ge, M. Lu, W. Zhang, and B.T. Cunningham, Applied Physics Letters, Vol. 96, 163702-163704, 2010 (DOI: 10.1063/1.3394259).
- “Large-area organic distributed feedback laser fabricated by nanoreplica molding and horizontal dipping,” C. Ge, M. Lu, X. Jian, Y. Tan, and B.T. Cunningham, Optics Express, Vol. 18, No. 12, 12980-12991, 2010.
- “Enhancement of pump efficiency of a visible wavelength organic distributed feedback laser by resonant optical pumping,” C. Ge, M. Lu, Y. Tan, and B.T. Cunningham, Optics Express, Vol. 19, No. 6, 5086-5092, 2011.
- “Plastic-based distributed feedback laser biosensors in microplate format,” Y. Tan, A. Chou, C. Ge, M. Lu, W. Goldshlag, J. Huang, A. Pokhriyal, S. George, and B.T. Cunningham, IEEE Sensors Journal, Accepted, July 2011.
- “Spectral characteristics of single and coupled microresonator lasers comprising a replica-molded Bragg grating and dye-doped polymer,” J. Zheng, M. Lu, C.J. Wagner, B.T. Cunningham, and J.G. Eden, Journal of the Optical Society of America B, Vol. 29, No. 2, 209-214, 2012.
- “Tunable ring laser with internal injection seeding and an optically-driven photonic crystal reflector,” J. Zheng, C. Ge, C.J. Wagner, M. Lu, B.T. Cunningham, J.D Hewitt, and J.G. Eden, Optics Express, Vol. 20, No. 13, 14292-14301, 2012.
- “External Cavity Laser Biosensor,” C. Ge, M. Lu, S. George, C. Wagner, J. Zheng, A. Pokhriyal, C. Siu, J.G. Eden, P.J. Hergenrother and B.T. Cunningham, Lab on a Chip, Vol. 13, No. 7, 1247-1256, 2013 (featured on front cover).
- “A self-referencing biosensor based upon a dual-mode external cavity laser,” M. Zhang, C. Ge, M. Lu, Z. Zhang, and B.T. Cunningham, Applied Physics Letters, Vol. 201, 213701-213703, 2013 (featured on cover) PMC3683028.
- “Distributed Feedback Laser Biosensor Noise Reduction,” Y. Tan, A. Chu, M.Lu and B.T. Cunningham, IEEE Sensors Journal, In Print, DOI 10.1109/JSEN.2013.2244591, 2013.
Related Conference Presentations
- “Vertically emitting distributed feedback laser for active label free biosensing,” M. Lu, S. Choi, C.J. Wagner, J.G. Eden, and B.T. Cunningham, Conference on Lasers and Electro Optics, San Jose, CA, May 2008.
- “Label free biosensor incorporating a vertically emitting distributed feedback laser,” M. Lu, S.S. Choi, U. Irfan, and B.T. Cunningham, Biomedical Engineering Society Annual Meeting, St. Louis, MO, October 2008.
- “Plastic distributed feedback laser biosensor,” M. Lu, S. Choi, and B.T. Cunningham, IEEE LEOS Conference, Newport Beach, CA, November 2008.
- “Polymer Vertically Emitting Distributed Feedback Laser for Label-free Biochemical Sensing,” M. Lu and B.T. Cunningham, 2008 MRS Fall Meeting, Boston, MA, December 2008.
- “Distributed feedback laser biosensor incorporating a titanium dioxide nanorod surface,” C. Ge, M. Lu, W. Zhang, and B.T. Cunningham, Conference on Lasers and Electro-Optics (CLEO), San Jose, CA, May 2010.
- “Large-area organic distributed feedback laser fabricated by nanoreplica molding and horizontal dipping,” C. Ge, M. Lu, X. Jian, Y. Tan, and B.T. Cunningham, MRS 2010 Fall Meeting, Boston, MA, December 2010.
- “Plastic-Based Distributed Feedback Laser Biosensors in Microplate Format,” Y. Tan and B.T. Cunningham, IEEE Photonics Society Conference, Arlington, VA, October 2011.
- “Optically Tunable External Cavity Ring Laser,” J. Zheng, C. Wagner, C. Ge, M. Lu, B.T. Cunningham, and J.G. Eden, IEEE Photonics Society Conference, Arlington, VA, October 2011.
- “Distributed feedback laser biosensor noise reduction,” Y. Tan, A. Cu, M. Lu, and B.T. Cunningham, 2012 IEEE Photonics Conference, Burlingame, CA, September 2012.
- “External cavity laser biosensor,” C. Ge, M. Lu, S. George, C. Wagner, J. Zheng, A. Pokhriyal, C. Siu, J.G. Eden, and B.T. Cunningham, IEEE Photonics Conference, Burlingame, CA, September 2012.
- “External cavity laser biosensor,” C. Ge, M. Lu, S. George, C. Wagner, J. Zheng, A. Pokhriyal, C. Siu, J.G. Eden, and B.T. Cunningham, MRS Fall Meeting, Boston, MA, November 2012.
- “A self-referencing biosensor based upon a dual-mode external cavity laser,” M. Zhang, C. Ge, M. Lu, Z. Zhang, and B.T. Cunningham, Conference on Lasers and Electro Optics (CLEO), San Jose, CA, June 2013.
- “External cavity laser biosensor,” M. Zhang, C. Ge, M. Lu, C.J. Wagner, J. Zheng, A. Pokhriyal, J.G. Eden, B.T. Cunningham, Frontiers in Optics/Laser Science, Orlando, FL, October 2013.