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We demonstrate that the sensitivity limit of intensity-based surface plasmon resonance (SPR) biosensors can be enhanced when we combine the effects of the phase and amplitude contributions instead of detecting the amplitude variation only. Experimental results indicate that an enhancement factor of as much as 20 times is achievable, yet with no compromise in measurement dynamic range. While existing SPR biosensor systems are predominantly based on the angular scheme, which relies on detecting intensity variations associated with amplitude changes only, the proposed scheme may serve as a direct system upgrade approach for these systems. The new measurement scheme may therefore lead to a strong impact in the design of SPR biosensors. OCIS codes: 240.6680, 240.6690.

We present a biosensor design based on capturing the two-dimenstional (2D) phase image of surface plasmon resonance (SPR). This 2D SPR imaging technique may enable parallel label-free detection of multiple analytes and is compatible with the microarray chip platform. This system uses our previously reported differential phase measurement approach, in which 2D phase maps obtained from the signal (P) and reference (S) polarizations are compared pixel by pixel. This technique greatly improves detection resolution as the subtraction step can eliminate measurement fluctuations caused by external disturbances as they essentially appear in both channels. Unlike conventional angular SPR systems, in which illumination from a range of angles must be used, phase measurement requires illumination from only one angle, thus making it well suited for 2D measurement. Also, phase-stepping introduced from a moving mirror provides the necessary modulation for accurate detection of the phase. In light of the rapidly increasing need for fast real-time detection, quantification, and identification of a range of proteins for various biomedical applications, our 2D SPR phase imaging technique should hold a promising future in the medical device market. OCIS codes: 240.6680, 170.0110.

What we believe to be a novel two-dimensional spectral surface plasmon resonance imaging technique determining pressure distribution in elastohydrodynamic lubricant films is presented. This technique makes use of the spectral characteristics associated with the surface plasmon resonance (SPR) effect, and it provides more spectral information in refractive index mapping than conventional contrast SPR imaging. Two-dimensional imaging is demonstrated and applied to a highly pressurized liquid lubricant trapped inside an elastohydrodynamic lubrication (EHL) dimple. The hydrostatic pressure inside the EHL dimple causes a localized change of the refractive index of the lubrication oil. This also results in a shift in the spectral SPR absorption dip. By monitoring the color changes within the SPR image and calibrating with lubricants of known refractive index profiles, we can obtain a direct measurement of the refractive index distribution within the EHL dimple. PB 2400 lubricant dimples were studied in our experiments. The proposed SPR imaging approach is irrespective of the absolute lubricant film thickness h, therefore overcoming the major limitations of a conventional optical interference technique. With further development of the two-dimensional refractive index mapping technique, widespread applications in various fields are possible, including high-throughput sensors and the detection of bioaffinity interactions. OCIS codes: 240.6680, 110.2970.

We describe a dual-beam differential phase interferometer that performs direct phase measurement by using a rotating Ronchi grating filter. Two laser beams derived from a single source are focused onto the sample surface. The reflected light will retrace the same path and intersect at the back focus of the objective lens. Interference between the reflected rays will therefore produce intensity fringes with their spatial location governed primarily by the differential optical phase of the probes. When the fringes are projected onto a rotating grating filter with identical periodicity, the transmitted optical signal becomes an oscillating signal from which the optical phase can be measured by standard phase-sensitive techniques. This approach has the advantages of simple system configuration, effective isolation from environmental disturbance, and low-frequency operation, which permit the use of high impedance electronics with shot-noise-limited performance at low laser power.