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Optical Sciences
P.O. Box 217
7500 AE Enschede
The Netherlands
E-mail:
Phone: +31 53 489 3805
Fax: +31 53 489 3511
MicroCOR
perspect-H20
Biomolecules and nanostructuresThe 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. |
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Pulse propagation in photonic materials |
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This research project involves time-resolved Near-field Scanning Optical Microscopy (NSOM) experiments aimed at visualizing the amplitude and
phase of ultrashort optical pulses it en route while they propagate inside photonic crystal structures. So far, most investigations on these
novel and exciting materials have been of the "black box" type (fig 1): |
 fig. 1 A: Black box model B: Looking inside the box |
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the light that goes into the material and that which emerges are measured accurately. The results are then compared to theory. Here, we take a radically different approach: we will track -both in time and in space- light pulses as they propagate through a photonic structure. The subtle manipulation of optical interference effects is crucial in photonic crystals. As a result, small variations, intended or not, in the geometry can result in large effects on the propagation of short pulses. At the same time the use of high peak-intensity femtosecond pulses creates possibilities to exploit the optical nonlinearity of photonic materials leading to new approaches to all-optical manipulation of light. It is therefore desirable to investigate the nanoscale local optical properties directly inside the structure. Understanding the dynamics of the interaction of light with a photonic structure requires both amplitude and phase characterization. With critical dimensions below the wavelength of light this can only be done by bringing a sharp tapered probe with subwavelength dimensions in close proximity to the sample. We use such a scanning probe based technique to directly probe dynamical effects inside photonic structures on a subwavelength scale (fig. 2). |
 fig. 2: Investigation of the local optical properties inside the structure using time resolved NSOM |
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The combination of optical and topographical information elucidates the influence of the geometry on the light propagation. Conversely, obtaining complete information about the propagating field permits characterizing the PhCs dielectric properties and structure. The first direct visualization of femtosecond pulses as they propagate inside a channel waveguide is presented in the first movie.[1]. The optical field just above the surface of a waveguide structure is mapped with subwavelength resolution by a tapered optical fiber that probes the evanescent field. The topography of the sample is acquired simultaneously with the optical information with a shear-force based feedback system. The incorporation of the NSOM and waveguide sample in one branch of a Mach-Zehnder type interferometer allows the direct visualization of amplitude and phase. Heterodyne interferometric detection of the interference signal is achieved by acousto-optical modulation of the reference beam. For the experiment nearly Fourier limited ultrashort pulses are used. Furthermore, an optical delay line is included in the reference branch of the interferometer in order to tune the temporal overlap of the reference- and signal pulse. As such the length of the reference branch defines a reference time. The resulting interference signal, which is detected, contains the local amplitude and phase information of the pulse propagating through the waveguide. From the time-dependent and phase-sensitive measurements both group and phase velocity are determined. Good agreement is found with theoretical values calculated using an effective index method. Experiments on a more complex device are presented in movie 2 [2]. Here we follow pulses as they propagate in an microresonator, directly mapping the resonator modes in space and time. Beating patterns inside the resonator are observed that prove that multiple modes inside the resonator are excited. Our time-dependent and phase-sensitive method gives direct access to the angular group and phase velocity of the modes in the resonator. Exploiting the occurrence of phase-singularities we directly measure the coupling-constant between the coupling waveguides and the resonator itself. To enhance our understanding of the measurement technique pulse propagation in a channel waveguide is revisited in more detail. An analytical model is developed which reveals how the observed signals can be interpreted [3]. The observed length of the measured pulse envelope is explained by comparison with this analytical model. The observed broadening of the FWHM of the measured pulse shape can be attributed to the group velocity dispersion in the fibers which are unequal in length for the two branches of the interferometer (fig. 3). |
 fig. 3: Calculated and measured optical amplitude in the waveguide structure |
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The model shows that by balancing the amount of dispersive media in the two branches it becomes possible to measure the group velocity dispersion of the structure under study locally. Even if branches are not compensated a reference measurement makes it possible to measure the local group velocity dispersion. As a result interesting effects, such as pulse compression, pulse spreading and pulse reshaping become accessible in the measurement. The results obtained in this project open doors for new developments, because now it has become experimentally possible to verify theoretical predictions concerning the propagation of short pulses in complex (non)linear dispersive photonic structures. Our scanning probe based technique is able to directly address the functional heart of the devices under study with high spatial and temporal resolution. |
References[1] Balistreri MLM, Gersen H, et al. SCIENCE vol 294 issue 5544: p1080-p1082 NOV 2 2001[2] H. Gersen, et al. Optics Letters Vol. 29 nr. 11 p1291 - p1293 june 1 2004 [3] Gersen H., et al. Physical Review E vol 68: p026604 (10 pages) august 15 2003 |
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