Optical Sciences

Biomolecules and nanostructures

The Optical Sciences group studies the interaction of light and matter at the nanoscale. We do this by exploring ways to shape light and its environment. It's what we call active and passive control. Our current focus is on the interaction of light with biomolecules and nanostructures. We are part of Twente University's Department of Science and Technology and member of the MESA+ institute.
We participate in the EU-COST actions MP1102: Coherent Raman microscopy (MicroCor) and CM1202: Supramolecular photocatalytic water splitting (PERSPECT-H2O)

 

Coherent imaging of guided optical fields


  • Marcello Balistreri - Former member
  • L. (Kobus) Kuipers - Former member


  • In 1996 a new project (FOM-BR) has started on the development of an interferometric photon scanning tunneling microscope (PSTM) aimed at visualizing the amplitude and phase of guided optical fields in integrated photonic structures.

    The optical field distribution in integrated optical splitters and mixers has been analyzed with a combined PSTM and shear-force microscope (1). The optical intensity distribution just above the surface of a planar waveguide is mapped with sub-wavelength resolution by a tapered optical fiber that probes the evanescent field. Simultaneously, the topography of the waveguide is recorded with sub-nanometer accuracy using a height feedback system based on shear-force detection with a tuning fork sensor.

    figure1
    Figure 1 PSTM measurement and a BPM simulation of the Mach-Zehnder interferometer at the first Y-junction. (A) The measured topography. The height of the step is 4.1(5) nm. (B) The simultaneously measured optical intensity distribution. The light propagates from the top to the bottom of the image. (C) The simulated optical intensity distribution with a 2D-FD-BPM.
    The application of the microscope to a planar waveguide splitter is shown in Fig. 1. The experimental field patterns are compared with field patterns simulated with the 2-dimensional finite difference beam propagation method (2D-FD-BPM). Good agreement between experiment and simulation is obtained.

    The PSTM has been extended with a 3D multi-height mode enabling the measurement of the optical field at different heights above the waveguide surface (2). The multi-height measurements also contain the optical information gathered with the commonly used constant gap measurements in addition to the topography of the waveguide surface. With the multi-height method the decay length of the evanescent field is readily determined as function of the in-plane coordinate allowing discrimination of various guided modes. Moreover the evanescent light can be distinguished from scattered light.

    The simultaneous detection of TE- as well as TM-polarized light leads to a quasi-interference pattern of these mutually perpendicular polarized fields (3). This interference pattern has been observed in the optical field distribution as a function of both position and wavelength. Comparison of the experimental data with simulations confirms the interference of mutually orthogonal fields. This quasi-interference is caused by the conversion of the linearly polarized light of both modes into elliptically polarized light by the fiber probe.

    Whispering gallery modes (WGM's) in cylindrical integrated optics microcavities have, for what is to our knowledge the first time, been mapped with the microscope (4-5). Optical images were obtained with a spatial resolution of 50 nm. By combination of information on the spatial optical distributions with wavelength-dependent measurements, an unexpectedly rich variety of intracavity phenomena, such as polarization conversion and interference of copropagating and counterpropagating modes, could be directly observed (Fig 2). A quantitative comparison of the experimental data with computer simulations results in a comprehensive understanding of the various whispering gallery modes inside the microcavity.

    figure2
    Figure 2 PSTM images of the microcavity and generated WGM's. The dashed and the solid lines indicate the corresponding line profile and cavity edge, respectively. (A) Topography of the Si3N4 cavity detected in shear force feedback. The lineprofile shows the 128-µm diameter and average height of 115 nm. The straight coupling channel is positioned at the left-hand side of the cavity. (B) Photon tunneling image of a microcavity in resonance at a wavelength of 674 nm, recorded simultaneously with (A). The intensity profile shows the confinement of the WGM field close to the cavity edge. (C) A close look at the cavity edge (solid curve) reveal modal fields at different radial distances. Moreover, a beat pattern is observed, with a period of approximately 8 µm (arrows and line profile). (D) High-resolution photon tunneling image of a spatial beat pattern close to the cavity edge. An interference pattern with a 190 nm period is observed. The large modulation depth of the interference fringes (line profile) indicates the unique spatial sensitivity of the near-field optical probe.

    The phase evolution of light in an optical waveguide structure has for the first time been visualized with subwavelength resolution using a novel heterodyne interferometric photon scanning tunneling microscope (6-9). These results has been highlighted by the editor of Physical Review Focus, D. Ehrenstein (7). Phase singularities in the optical field of the waveguide have been observed (Fig. 3). The phase singularities of charge one appear at locations where the modal field amplitude vanishes, due to the interference of various modes in the waveguide. The propagation of the wavefronts and the change of the shape of the phase singularities is shown in Fig. 4. Excellent agreement of the data with calculations has been obtained. We show that with the unique spatial phase information the relative field profiles and wave vectors of all the excited modes in a multimodal waveguide structure can be determined independently.

    figure3
    Figure 3 Interference PSTM measurement of the Si3N4 channel waveguide for a scan range of 9 x 10 µm2. Linearly polarized light has been coupled in a controlled way in the channel waveguide to excite the TE and TM modes simultaneously. (A) Topography of the Si3N4 channel waveguide with a height of 4 nm. (B) The measured amplitude of the optical field inside the waveguide. A clear beating pattern is observed. (C) The measured phase evolution of the optical field. The cosine of the phase is shown. Several phase singularities are apparent. (D) Zoom in of square white box of the measured phase map of (c). The phase singularity has a topological charge +1.
    Click on next image to start movie (2887kB): figure4
    Figure 4 The propagation of the wavefronts and the change of the shape of the phase singularities, which has been observed in Fig. 3C, while the phase of the reference beam is varied a posteriori. The movie is built up out of constructed cosf maps of Fig. 3C with a fixed phase of the reference beam of 0o - 360o.


    Articles

    The following articles have been published regarding this project:

    Detailed analysis of the intracavity phenomena inside a cylindrical microresonator

    (abstract) (full pdf)
    Klunder DJW, Balistreri MLM, Blom FC, Hoekstra HJWM, Driessen A, Kuipers L, van Hulst NF
    JOURNAL OF LIGHTWAVE TECHNOLOGY
    vol 20 issue 3: p519-p529 MAR 2002

    Comment on "Local observations of phase singularities in optical fields in waveguide structures" - Reply

    (full pdf)
    van Hulst N, Balistreri M, Korterik J, Kuipers L
    PHYSICAL REVIEW LETTERS
    vol 87 issue 25: art. no. 259402 DEC 17 2001

    Tracking femtosecond laser pulses in space and time

    (abstract) (external link to pdf)
    Balistreri MLM, Gersen H, Korterik JP, Kuipers L, van Hulst NF
    SCIENCE
    vol 294 issue 5544: p1080-p1082 NOV 2 2001

    Visualization of mode transformation in a planar waveguide splitter by near-field optical phase imaging

    (abstract) (full pdf)
    Balistreri MLM, Korterik JP, Kuipers L, van Hulst NF
    APPLIED PHYSICS LETTERS
    vol 79 issue 7: p910-p912 AUG 13 2001

    Phase mapping of optical fields in integrated optical waveguide structures

    (abstract) (full pdf)
    Balistreri MLM, Korterik JP, Kuipers L, van Hulst NF
    JOURNAL OF LIGHTWAVE TECHNOLOGY
    vol 19 issue 8: p1169-p1176 AUG 2001

    Experimental analysis of the whispering-gallery modes in a cylindrical optical microcavity

    (abstract) (full pdf)
    Balistreri MLM, Klunder DJW, Blom FC, Driessen A, Korterik JP, Kuipers L, van Hulst NF
    JOURNAL OF THE OPTICAL SOCIETY OF AMERICA B-OPTICAL PHYSICS
    vol 18 issue 4: p465-p471 APR 2001

    Quantitative photon tunneling and shear-force microscopy of planar waveguide splitters and mixers

    (abstract) (full pdf)
    Balistreri MLM, Korterik JP, Veldhuis GJ, Kuipers L, van Hulst NF
    JOURNAL OF APPLIED PHYSICS
    vol 89 issue 6: p3307-p3314 MAR 15 2001

    Photon scanning tunneling optical microscopy with a three- dimensional multiheight imaging mode

    (abstract) (full pdf)
    Balistreri MLM, Korterik JP, Kuipers L, van Hulst NF
    APPLIED PHYSICS LETTERS
    vol 77 issue 25: p4092-p4094 DEC 18 2000

    High-resolution photon-scanning tunneling microscope measurements of the whispering gallery modes in a cylindrical microresonator

    (abstract)
    Klunder DJW, Balistreri MLM, Blom FC, Hoekstra HJWM, Driessen A, Kuipers L, van Hulst NF
    IEEE PHOTONICS TECHNOLOGY LETTERS
    vol 12 issue 11: p1531-p1533 NOV 2000

    Local observations of phase singularities in optical fields in waveguide structures

    (abstract) (full pdf)
    Balistreri MLM, Korterik JP, Kuipers L, van Hulst NF
    PHYSICAL REVIEW LETTERS
    vol 85 issue 2: p294-p297 JUL 10 2000

    Quasi interference of perpendicularly polarized guided modes observed with a photon scanning tunneling microscope

    (abstract) (full pdf)
    Balistreri MLM, Driessen A, Korterik JP, Kuipers L, van Hulst NF
    OPTICS LETTERS
    vol 25 issue 9: p637-p639 MAY 1 2000

    Visualizing the whispering gallery modes in a cylindrical optical microcavity

    (abstract) (full pdf)
    Balistreri MLM, Klunder DJW, Blom FC, Driessen A, Hoekstra HWJM, Korterik JP, Kuipers L, van Hulst NF
    OPTICS LETTERS
    vol 24 issue 24: p1829-p1831 DEC 15 1999
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