We introduce a new, highly sensitive, and simple heterodyne optical method for imaging individual nonfluorescent nanoclusters and nanocrystals. A 2 order of magnitude improvement of the signal is achieved compared to previous methods. This allows for the unprecedented detection of individual small absorptive objects such as metallic clusters (of 67 atoms) or non luminescent semiconductor nanocrystals. The measured signals are in agreement with a calculation based on the scattering field theory from a photothermal-induced modulated index of refraction profile around the nanoparticle.

DOI: 10.1103/PhysRevLett.93.257402 PACS numbers: 78.67.–n

In the fast evolving field of nanoscience, where size is crucial for the properties of the objects, simple and sensitive methods for the detection and characterization of single nanoclusters and nanocrystals (nano-objects) are needed. The most commonly used optical techniques are based on luminescence. Single fluorescent molecules have been studied on their own and are now routinely applied in various research domains ranging from quantum optics [fusion_builder_container hundred_percent=”yes” overflow=”visible”][fusion_builder_row][fusion_builder_column type=”1_1″ background_position=”left top” background_color=”” border_size=”” border_color=”” border_style=”solid” spacing=”yes” background_image=”” background_repeat=”no-repeat” padding=”” margin_top=”0px” margin_bottom=”0px” class=”” id=”” animation_type=”” animation_speed=”0.3″ animation_direction=”left” hide_on_mobile=”no” center_content=”no” min_height=”none”][1–3] to life science [4]. Yet, fluorescent molecules allow only for short observation times due to inherent photobleaching. The development of brighter and more stable luminescent objects, such as semiconductor nanocrystals [5,6], has remedied some of this shortcoming, but this improvement has come at the price of a strong blinking behavior.

An interesting alternative to fluorescence methods relies solely on the absorptive properties of the object. At liquid helium temperatures single molecules were initially detected by an absorption technique owing to the high quality factor of the zero-phonon line which gives a considerable absorption cross section at resonance [7] (few 10_11 cm2). Single ions or atoms isolated in rf traps [8] or high Q cavities [9,10] have been detected by absorption of a probe beam. In general, particles with large absorption cross sections and short time intervals between successive absorption events are likely candidates for detection with absorption methods [8].

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