T Smith

Stanford University, Palo Alto, CA, United States

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Publications (5)12.61 Total impact

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    ABSTRACT: We have developed and tested a wide-field coherent anti-Stokes Raman scattering (CARS) microscopy technique, which provides the simultaneous imaging of an extended illuminated area without scanning. This method is based on the non-phase-matching illumination of a sample and imaging of a CARS signal with a CCD camera using conventional microscope optics. We have identified a set of conditions on the illumination and imaging optics, as well as on sample preparation. Imaging of test objects proved high spatial resolution and chemical selectivity of this technique.
    Optics Letters 08/2007; 32(13):1941-3. · 3.39 Impact Factor
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    ABSTRACT: We report a wide-field Coherent Anti-Stokes Raman Scattering (CARS) microscopy technique based on non-phasematching illumination and imaging systems. This technique is based on a non-collinear sample illumination by broad laser beams and recording image of sample at anti-Stokes wavelength using full-frame image detector. An amplified Ti:Sapphire laser and an Optical Parametric Amplifier (OPA) provided picosecond pump and Stokes beams with energies sufficient for CARS generation in an area of 100 µm in diameter. The whole field of view of the microscope was illuminated simultaneously by the pump and Stokes beams, and CARS signal was recorded onto a cooled CCD, with resolution determined by the microscope objective. Several illumination schemes and several types of thin sample preparations have been explored. We demonstrated that CARS image of a 100x100 µm sample can be recorded with submicrometer spatial resolution using just a few laser pulses of microJoule energies.
    Proc SPIE 03/2007;
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    ABSTRACT: We report methods of near-field infrared microscopy with transient optically induced probes. The first technique - a transient aperture (TA) - uses photoinduced reflectivity in semiconductors to generate a relatively large transient mirror (TM) with a small aperture at its centre. We report the optical properties of the TM and TA and experiments performed on near-field imaging with the TA. The second technique is based on solid immersion microscopy, in which high resolution is achieved when light is focused inside a solid with a high refractive index. By creating a transient Fresnel lens on the surface of a semiconductor wafer via photoinduction, we were able to form a solid immersion lens (SIL) for use as a near-field probe. The use of transient probes eliminates the need for mechanical scanning of the lens or sample, and thus provides a much faster scanning rate and the possibility to work with soft and liquid objects.
    Journal of Microscopy 07/2003; 210(Pt 3):307-10. · 1.63 Impact Factor
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    ABSTRACT: We present a scanning near-field infrared microscopy technique using transient solid immersion lenses as near-field probes. The transient SILs were formed by photoinducing a zone plate structure on the surfaces of semiconductor wafers with high indices of refraction. Lenses with different number of zones have been tested using gallium phosphide and silicon wafers and their focusing properties were determined. We demonstrate that transient SILs can have lifetimes longer than 50 ps and provide the same high numerical apertures as conventional SILs. The use of transient SILs eliminates the need for mechanical scanning of the lens or sample, thus providing much faster scanning and the possibility to work with soft and liquid objects. © 2002 American Institute of Physics. Scanning near-field microscopy and solid immersion mi-croscopy are well-developed methods of optical imaging that provide spatial resolution beyond the optical diffraction limit. Based on the detection of nonpropagating components of electromagnetic field, near-field microscopy does not have a fundamental limit in spatial resolution. Resolution of about /20 has been achieved with aperture probes 1–3 and even higher resolution of up to /100 is available with scattering-type probes. 4 –7 Solid immersion microscopy is an alternative approach to high-resolution imaging, which combines elements of im-aging microscopy and probe microscopy. A solid immersion lens SIL focuses radiation within a material of a high re-fractive index, reducing the focal spot diameter by a factor of the refractive index n in comparison with focusing in a vacuum. With the sample positioned on the flat output sur-face of the SIL, the microscope is operated in the near-field mode using the evanescent field protruding from the SIL into air. SILs can be used as the final elements of either imaging or scanning microscopes. The scanning mode of operation is free from many of the aberrations inherent to the imaging regime and thus provides better spatial resolution. In this mode, a SIL can be considered as a near-field probe with very high throughput. 8–11 As we have shown previously, photoinduced reflectivity in semiconductors can be used to generate transient IR near-field probes such as small mirrors 12 and apertures. 13 How-ever, these techniques typically require semiconductor sub-strates no thicker than 1 or 2 m. These substrates are extremely brittle and difficult to manufacture. In this letter, we demonstrate that a transient diffractive optical element DOE can be generated in semiconductors, and particularly a DOE SIL, utilizing the remarkably high refractive indices characteristic of many semiconductors. These elements are much more versatile in comparison with simple aperture probes, and they do not require thin semiconductor sub-strates. The simplest DOE SIL is a Fresnel lens drawn on one side of a flat semiconductor wafer with its focus on the other side. The fact that such a lens can be created in a transient manner allows for remarkable ease of the scanning procedure by raster scanning the wafer with the sample in the focal plane of the optical system while redrawing the DOE for each wafer position. We will address issues of creating a transient DOE SIL on a semiconductor surface, its lifetime, focusing characteristics, and present the results of near-field imaging with this optical element. Two materials suitable for creating transient optical ele-ments were tested: GaP (n3) and Si (n3.4). GaP is con-venient because its transparency in visible light facilitates sample alignment, while Si provides a higher refractive in-dex, and Si wafers of different thicknesses are readily avail-able. In both materials, free carriers can be generated by the second harmonic of a Ti:Sapphire laser at 400 nm. This wavelength is short enough to provide well-resolved DOE structures for mid-IR wavelengths in semiconductors. The infrared radiation for this experiment was generated in an optical parametric amplifier Spitfire, Spectra Physics Inc. pumped by a Ti:Sapphire laser 1 mJ, 1 ps, 800 nm, 1 kHz. A fraction 30% of the Ti:Sapphire fundamental was converted to the second harmonic and used as a pump beam to generate the photoinduced electron–hole plasma. To fa-cilitate beam transport, all measurements were performed with the IR light at 6.25 m, due to the lack of water vapor absorption at this wavelength. Important parameters for generating the transient DOEs are the transmittance of the pumped material and the lifetime of the diffractive structure. The penetration depths for 400 nm radiation in GaP and Si are 115 nm and 80 nm, respectively, 14 providing high localization of the zone struc-ture in the axial direction. The transmittance of a bulk sample of Si was measured at 6.25 m as a function of the pump beam fluence, and is presented in Fig. 1. It demon-strates that greater than ten-fold signal attenuation—due to the cumulative effects of reflection and absorption—can be achieved at pump levels several times below the damage threshold.
    Applied Physics Letters 01/2002; 81. · 3.79 Impact Factor
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    ABSTRACT: We report a method of near-field infrared microscopy with a transient optically induced probe. Photoinduced reflectivity in semiconductors is used to generate a relatively large transient mirror with a small aperture (infrared probe) in its center. Properties of this probe have been studied and first images obtained using the technique are presented. Resolution better than λ/5 at 6.25 μm is demonstrated. Among the advantages of this technique are high optical throughput of the probe, ease in simultaneous visible imaging, and a high scanning rate limited primarily by the pulse repetition rate of the laser system. © 2001 American Institute of Physics.
    Applied Physics Letters 09/2001; · 3.79 Impact Factor

Publication Stats

16 Citations
12.61 Total Impact Points


  • 2001–2007
    • Stanford University
      • • W. W. Hansen Experimental Physics Laboratory
      • • Picosecond Free Electron Laser Center
      Palo Alto, CA, United States