Near Field Observation of a Refractive Index Grating and a Topographical Grating by an Optically Trapped Gold Particle

Size: px

Start display at page:

Download "Near Field Observation of a Refractive Index Grating and a Topographical Grating by an Optically Trapped Gold Particle"

Cassandra Harrison
5 years ago
Views:

1 Near Field Observation of a Refractive Index Grating and a Topographical Grating by an Optically Trapped Gold Particle Hiroo UKITA and Hirotaka UEMI Ritsumeikan University, Kusatsu-shi, Shiga, 2 Japan 1

2 Abstract We succeed in observing the near field for a refractive index grating fabricated on a planar light waveguide circuit (PLC) by scanning an optically-trapped 100 nm diameter gold particle. We demonstrate that stable trapping and scanning occur with a Nd:YAG laser power of 2 mw at the scan velocity of 1.6 µm/s. The scattered Ar+ laser light from the gold particle is strong at high refractive indexes of the grating with periods of 1.06 µm and 0.3 µm, both by s and p polarized illumination. In addition, we observed the surface profile of the optical disk tracking groove by the light difference between with and without gold particle. 2

3 Introduction It is well known that surface plasmon existing at metal surfaces and metal/dielectric interfaces causes strong field enhancement (1-4) at the interface. A metal particle probe is considered to have the advantages of: (1) high experimental reproducibility (2) not requiring control from a distance (3) not only the ability to obtain an image, but also the spectroscopic data. T. Sugiura et al () observed a dip on a cover glass and a gold colloidal particle adhering to the cover glass. However, these images were thought to have been an artifact problem (6) due to the vertical displacement of the gold probe. We demonstrated observation of a refractive index grating (7) on a flat surface, which was made on a planar light waveguide circuit (PLC), by scanning an optically-trapped 100 nm diameter gold particle. We also observed the surface profile of an optical disk tracking groove with and without gold particle. 3

4 Setup of the SNOM using an optically-trapped gold particle. Ar+laser λ=488 nm XYZ Stage sample cell Objective (NA=1.3) CCD Optical box BS2 BS1 Lens (f=180 mm) PC PMT Pinholl Nd:YAG laser λ=1064 nm A Nd:YAG laser is used to trap, and an Ar+ laser is used to illuminate the gold particle. All the optical elements, except the mirrors to guide the Nd:YAG laser and Ar+ laser, are installed inside the optical box for easy operation. 4

Photograph of the SNOM and the sample chamber Ar+ laser Sample chamber PLC grating XYZ stage Water Gold particle 10 µm CCD Optical box PMT YAG laser

5 Photograph of the SNOM and the sample chamber Ar+ laser Sample chamber PLC grating XYZ stage Water Gold particle 10 µm CCD Optical box PMT YAG laser Coverslip Lasers λ=1060 nm λ=488 nm Sample chamber The upward-directed beam has a significantly higher trapping efficiency (8) and more stable than the downward one.

6 Trapping principle F grad = 1 4 n 1 α grad 2 ( E ) Water Focus Fgrad YAG laser & Sample Gold particle Trapping force F (pn) Fgrad Fscat 100 nm 20 mw Distance from focus (nm) A metallic Rayleigh particle (much smaller than the wavelength) can be optically trapped at the focal point by the gradient force of a strongly focused laser beam. The gradient force along the transverse direction (lateral) is eight times greater than that along the axial direction (axial). 6

7 Dependence of the minimum axial trapping power on the scanning speed of an optically-trapped gold particle. Sample Near field YAG laser Ar+ laser Scanning Scattering light Oil Objective Minimum trapping power (mw) NA=1.3 λ=1060 nm Glycerol 0% 13% 2% Scanning velocity (µm/s) The minimum trapping power increases as the scanning velocity increases, but decreases as the viscosity increases. Viscosities were controlled by altering the glycerol density. 7

8 Conditions to fabricate a refractive index grating (7) in PLC Core SiO 2 +GeO 2 index Clad (SiO 2 ) index Refractive index difference Estimated Grating Clad Clad Core 24 µm 6 µm 20 µm Source: ArF laser =193 nm Phase mask pitch: 1.06 µm Si substrate 1mm Phase mask method Energy density per pulse: 1.0J/cm 2 /pulse Pulse repetition rate: 0Hz Grating pitch (Zero order): 1.06 µm Sectional view The authors would like to thank Drs Toru Maruno and Yoshinori Hibino of NTT Photonics Laboratories for their help with the fabrication of a PLC grating sample. 8

$Refractive index grating fabricated on a planar light waveguide circuit (PLC) 1st order grating (0.3 µm pitch) Zero order grating (1.$

9 Refractive index grating fabricated on a planar light waveguide circuit (PLC) 1st order grating (0.3 µm pitch) Zero order grating (1.06 µm pitch) (a) Photograph Core width (6 µm ) 1.06 m 1.06 µm pitch (b) Sketch The relative refractive index difference between the gratings in the clad layer is estimated to be to where the clad layer index is

10 Conditions to observe a refractive index gratins by an optically trapped gold particle Gold particle diameter Medium YAG laser intensity Ar + laser intensity Scan velocity Scan pitch Scan area Measurement time 100 nm Water 2 mw ( =1064 nm) 130 µw ( =488 nm) 1.6 µm/s 0 nm µm 2 minutes 10

$SNOM images of the refractive index grating$ illumination p-pol p-pol 0 1.06 µm 0 1.

11 SNOM images of the refractive index grating by a gold particle probe with p-pol illumination p-pol p-pol µm µm (a) Scattered light intensity (b) Shaded image. 11

$SNOM images of the refractive index grating$ illumination s-pol s-pol 0 1.06 µm 0 1.

12 SNOM images of the refractive index grating by a gold particle probe with s-pol illumination s-pol s-pol µm µm (a) Scattered light intensity (b) Shaded image 12

13 Relationship between scattered light intensity and refractive index grating distribution p-pol s-pol Zero order gratings 1st order gratings Scan distance (µm) 13

probe p-pol Without probe With probe With probe 0 1.

14 SNOM topography of the optical disk tracking groove by a gold particle probe with p-pol illumination p-pol Without probe p-pol Without probe With probe With probe µm µm (a) Scattered light intensity (b) Shaded image. 14

15 1.6 µm 200nm SEM photograph of an optical disk groove and its profile With probe Without probe Relationship between scattered light intensity for p-pol and disk groove profile with and without a gold particle 200 nm 1.6 µm 600 nm 1

16 Scattered light with and without gold particle on an optical disk groove and its profile Focus With gold particle Without gold particle p-pol s-pol The scattered light from the gold particle (black) is strong at the groove edge and has split peaks for p-pol illumination. The scattered light without gold particle (red) is weak and has a single peak at the groove edge. 16

17 Scattered light difference between with and without gold particle on an optical disk groove and its profile (nm) p-pol s-pol The difference between the two seems to correspond to the surface topology because the effect of the laser light reflection is removed and the vertical displacement of the gold particle appears due to the scanning on the groove. 17

18 Conclusions An experimental setup to trap a 100 nm gold particle with an upward-directed Nd:YAG laser and to scan over the sample surface was constructed. First, we observed the refractive index grating which contains no artifact caused by surface topology. The pitches were 0.3 µm and 1.06 µm, fabricated by UV exposure through a phase mask on a planar light waveguide circuit (PLC). The amplitude for the refractive index modulation of the clad layer (index 1.4) is estimated to be between to The scanning velocity and pitch were 1.6 µm and 0 nm, respectively. A comparison between the scattered light intensity and the refractive index grating distribution leads to each corresponding with both p-pol and s-pol illumination. Second, an observation under p-polarization was also given for the topographical tracking groove. The scattered light from the gold particle was strong at the groove edge and had split peaks, whereas the scattered light without gold particle had no split peaks. The light difference between with and without gold particle seems to correspond to the surface profile. From the result above, we confirm that an optically trapped gold particle is effective in observing both physical and topological properties of a sample. 18

19 References (1) H. Raether: Surface plasmons, vol. III of Springer Tracts in Modern Physics, edited by G. H. Föhler, Springer-Verlag, Berlin, (2) L. Novotny, R.X. Bian and X.S. Xie: Theory of nanometric optical tweezers, Phys. Rev. Lett, (1997). (3) H. Furukawa and S. Kawata: Local field enhancement with an apertureless near-field-microscope probe, Opt. Com., (1998). (4) M. Gu and P. C. Ke: Image enhancement in near-field scanning optical microscopy with laser-trapped metallic particles, Opt. Lett., 24, (1999). ()T. Sugiura,T. Okada, Y. Inoue, O. Nakamura and S. Kawata: Gold-bead scanning near-field optical microscope with laser-force position control, Opt. Let., 22, (1997). (6) B. Hecht, H. Bielefeldt, Y. Inoue and D. W. Pohl: Facts and artifacts in near-field optical microscopy, J. Appl. Phys (1997). (7) K.O. Hill, B. Malo, F. Bilodeau, D.C. Johnson and J. Albert: Bragg gratings fabricated in monomode photosensitive optical fiber by UV exposure through a phase mask, Appl. Phys. Lett. 62, (1993). (8) H. Ukita and T. Saitoh: Optical Micro-Manipulation of Beads in Axial and Lateral Directions with Upward and Downward-Directed Laser Beams, LEOS'99 ed. Chris Harder, Vol. 1, pp , 8-11, November 1999, San Francisco USA 19

Microscopy. Marc McGuigan North Quincy High School Thursday, May 11, 2006

Microscopy. Marc McGuigan North Quincy High School Thursday, May 11, 2006 Microscopy Marc McGuigan North Quincy High School Thursday, May 11, 006 Outline Activity Introduction Electromagnetic Spectrum Visible Light Light Microscope AFM Scanning Electron Microscopy Near-Field