Research ArticleOPTICS

Ultrasensitive interferometric on-chip microscopy of transparent objects

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Science Advances  10 Jun 2016:
Vol. 2, no. 6, e1600077
DOI: 10.1126/sciadv.1600077
  • Fig. 1 LIM for ultrasensitive detection of transparent samples.

    (A) Scheme of the LIM. It comprises a light-emitting diode (LED) light source followed by a fiber collimator (not shown) generating a collimated beam, a polarizer (P1), two SPs (SP1 and SP2), the sample in between the SPs, an analyzer [that is, a second polarizer (P2) orthogonal to P1], and a CMOS ISA. (B) Photograph of the device. (C) Selection of the input polarization, splitting, shearing, and projection into an orthogonal direction in the LIM. Two SPs (SP1 and SP2) are placed between crossed polarizers (P1 and P2) to create a balanced Mach-Zehnder interferometer of partially overlapping and sheared beams (EPx and EPy) with orthogonal polarizations. Contrary to conventional DIC microscopes, no lenses are required to refocus EPx and EPy and make them interfere. This enables the achievement of high axial sensitivity over a large FOV.

  • Fig. 2 Principle of detection of OPDs of transparent samples using the LIM.

    (A) Comparison between the LIM device (left scheme) and a non-interferometric setup (right). In the LIM, two orthogonally polarized beams (EPx and EPy) are symmetrically sheared by a distance s = 25 μm. Depending on the presence of spatially dependent relative phase shifts between them, an intensity pattern is detected on the ISA. By tilting SP1, an initial phase shift α between EPx and EPy can be introduced to maximize the sample detection. For instance, for α = π/2, a relative phase-shift Δϕ1 = 0 caused by the sample produces an intermediate intensity (gray zone), whereas Δϕ2 = +ϕ and Δϕ3 = −φ produce clearer (white) and darker (black) zones, respectively. In contrast, there is no formation of any image pattern when light (beam 4) propagates through a transparent sample in a noninterferometric configuration. (B) Workflow used to calculate the OPD maps.

  • Fig. 3 Detection of transparent patterns using the proposed LIM.

    (A) An ITO ribbon on a silica (SiO2) substrate is used to produce localized heating by injecting a constant current. The three images show refractive index (phase-shift) patterns induced through the thermo-optic effect, when injecting electric currents (I) of 0, 1.6, and 2.7 mA, respectively. The largest (steady-state) OPD detected is estimated to be 12 nm. (B) Silica dot patterns of different thicknesses (d) on a silica substrate are detected (spot diameter = 30 μm, pitch size = 100 μm). The measured OPD profiles (bottom graphs) agree with the thicknesses measured using AFM of 2 and 5 nm, respectively. (C) Detection of monolayer and bilayer protein arrays (spot diameter = 100 μm, array pitch = 200 μm). BSA monolayer spots after deposition on epoxysilane-coated glass and after incubation with anti-BSA IgG and rinsing. Both interferometric images and cross sections with OPD values are shown.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/6/e1600077/DC1

    fig. S1. Comparison between a conventional DIC microscope and the proposed LIM in detecting transparent patterns (reading microarrays of biomarkers).

    fig. S2. Lateral resolution of the LIM.

    fig. S3. Measured transfer function of the LIM.

    fig. S4. Simulation of the LIM response.

    fig. S5. Heat distribution generated by an ITO ribbon deposited on a glass substrate.

    Simulation of the LIM

    Thermo-optic experiments

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Comparison between a conventional DIC microscope and the proposed LIM in detecting transparent patterns (reading microarrays of biomarkers).
    • fig. S2. Lateral resolution of the LIM.
    • fig. S3. Measured transfer function of the LIM.
    • fig. S4. Simulation of the LIM response.
    • fig. S5. Heat distribution generated by an ITO ribbon deposited on a glass substrate.
    • Simulation of the LIM
    • Thermo-optic experiments

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