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CdSe-single-particle based active tips for near-field optical microscopY This article is no longer updated since september 2007. For a recent update, please go the Néel Institute ! Current developments in aperture Near-Field Scanning Optical Microscopy (NSOM) include the search for improved spatial resolution in the nanometer range where a novel optics is expected to emerge. Active tips made of a fluorescent nano-object attached to a regular tip have much to offer in this context. A major breakthrough towards nanooptics with active tips has been realized by Michaelis et al. [1] who have performed NSOM imaging with a single molecule serving as a point-like source of light that operates at low temperature. This pioneering work has been extended to other systems but none of the works published so far is free from limitation such as the low-temperature operation or/and the limited spatial resolution due to the size of the supporting objects [1,2] or number of active particles involved [3]. We have developed active tips made of a few CdSe nanoparticles (NPs), either NCs or nanorods (NRs), deposited at the apex of a tapered and coated tip. Our goal is to make photostable light nanosources working at room temperature and potentially able to offer optical resolution in the sub-10 nm range. Semiconductor NPs have a large potential in this respect because they have natural small dimensions, their optical properties are size-tunable, they possess an absorption threshold - rather than absorption bands as dye molecules - they are optically active at room temperature, and they are more photostable (little bleaching) than single molecules [4]. The time analysis of the emission rate of the active tips and the analysis of their emission spectra reveal that a very small number of particles - possibly down to only one - can be made active at the tip apex [5]. This opens the way to optics with a single inorganic nanoparticle as a light source. NSOM imaging with such active tips is in progress [6] as well as the use of non-blinking insulating nanoparticles. [1]
J.
Michaelis et al., Nature 405, 325 (2000). [2] S. Kühn et al., J. Microscopy 202, 2 (2001); L. Aigouy, Y. de Wilde, M. Mortier, Appl. Phys. Lett. 83, 147 (2003). [3]
G.T. Shubeita et al., J. Microscopy 210, 274 (2003). [4] P. Reiss, J. Bleuse, A. Pron, Nano Lett. 2, 781 (2002). [5] N. Chevalier et al., Nanotechnology 16, 613 (2005); PLEASE, HAVE A LOOK AT THIS communication (in french) [6] Y. Sonnefraud et al., submitted to Optics Express. See also: Nanoparticles create active tips for near-field optical microscopy La nano-optique grenobloise à la Une ! (french) A la lueur des nanocristaux (french)
REMOTE OPTICAL ADDRESSING OF SINGLE QUANTUM DOTS Ultimate
control of light requires the combined ability of confining photons to extremely
small dimensions, i.e., much smaller than the wavelength, and
manipulating them in a well-controlled state to address at will
optically-active single nanometre-scaled objects such as
semiconductor quantum dots (QDs). In
this context, we have developped a simple optical method aimed at remote
addressing of single QDs by means of Near-Field Scanning Optical Microscopy
(NSOM). In essence, the method makes only use of one of the most fundamental
properties of an electromagnetic field with respect to a scalar field: its polarization,
which is controlled at the apex of a NSOM tip [1]. The principle of our method
is shown on the enclosed figure. Our
approach is demonstrated on self-assembled CdTe/ZnTe QDs by collecting their
luminescence with a low-temperature NSOM. However, there is no restriction
whatsoever to apply it to other QD systems and even to a large variety of
optically active nano-objects (single molecules, nanocrystals) in different
environments. In addition, most of the parameters of the hollowed mask (e.g.,
the nanohole diameter, the hole density, their spatial arrangment) can be
easily tuned over large ranges. Therefore, we expect our method to find
important optical and optoelectronic applications in the nanoworld [3]. Apart
from single-object addressing, we will show that our procedure gives valuable
insight into the diffusion of photo-excited carriers in the QD plane. This is
achieved by launching the optical excitation into one nanohole and collecting
the luminescence from the QDs located underneath a second close-lying hole. [1]
C. Obermüller et K. Karrai, Appl. Phys.
Lett. 67,
3408 (1995). [2]
C. Sönnichsen et al., Appl. Phys.
Lett. 76, 140 (2000). [3] M. Brun et al., Europhys. Lett. 64, 634 (2003); M. Brun et al., Physica E 21, 219 (2004). |
A
thin opaque metal film hollowed by sub-wavelength transparent apertures
is deposited onto the quantum-dot sample. A NSOM optical tip is coupled to a
laser source and is approached in the near-field to a well-chosen nanohole or
at a well-controlled position on the metal film. The latter possibility
allows for selective launching of surface plasmons in a selected nanohole
by orienting the light polarization (electric field E
on the figure) at the tip apex towards this