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Humboldt-Universität zu Berlin - Faculty of Mathematics and Natural Sciences - Nano Optics

Nano-probes and Nano-particles

 

Staff members:  Alexander Kuhlicke, Heather Partner

 

Fundings through:

 

 

 

SFB 951 "HIOS"

General concept of Hybrid Inorganic/Organic Systems (HIOS)

The Collaborative Research Centre HIOS (SFB 951) is an interdisciplinary effort bringing together scientists with complementary expertise from three universities and four non-university institutions. The goal is the merger of inorganic semiconductors, conjugated organic materials, and metal nanostructures into novel hybrid structures. Elucidating and tailoring the fundamental chemical, electronic, and photonic interactions in these systems will enable us the development of functional elements exhibiting superior opto-electronic functionalities not achievable with any of the individual material classes alone.


 

In this research section we develop and use different methods to characterize particles and objects of nanometric dimension such as fluorescent nanoparticles or -crystals. We also focus on manipulation of the particles, which occurs in our group using a particle trap, a SNOM or an AFM.
 

Optical spectroscopy with a linear Paul trap

 

The first demonstration of an exclusively electrically operated mass filter by Wolfgang Paul in 1955 initiated a large development in the fields of mass spectrometry and ion trapping. One of the outcomes, the Paul trap, is a radio frequency ion trap, which became a widely-used tool for high-resolution spectroscopy because of the absence of an interaction between the particles and supporting surfaces. An ideal linear Paul trap consists of four parabolically shaped electrodes connected to an alternating high-voltage [1] as depicted in FIG. 1(a). The resulting two-dimensional electric potential forms an oscillating saddle surface (FIG. 1(b)) which allows for confinement of charged particles in the xy-plane of the trap. Stabilization in the remaining z-direction occurs via additional electrodes or segmentation of the main electrodes.

 

Paul trap front view Paul trap - potential

FIG. 1: (a) Electrode configuration of a linear Paul trap (b) Electric potential in the Paul trap

 

Storage of micrometer-sized particles and of atom and molecular ions have been demonstrated years ago. In our experiments we use a linear Paul trap to narrow the gap between these two size regimes by trapping single nanoparticles with sizes from a few nanometers to some micrometers. Detection of the fluorescence light occurs via CCD-camera, spectrometer or Hanbury Brown-Twiss correlator (FIG. 2).

 

Experimental Setup
FIG. 2: Experimental Setup
 

In a Paul trap only charged particles can be trapped. For ionizing and inserting the particles into the trap we use electrospray injection ionization (FIG. 3). From a solution containing sample material, highly charged droplets are produced and sent down a potential gradient towards the trap. The solvent evaporates, leaving only the charged particles behind. This electrospray process happens at normal pressure. Instead of using complex pumping stages to transfer the particles into vacuum, we are able to trap particles under atmospheric pressure and evacuate the vacuum chamber afterwards. Besides a simplified setup, this facilitates trapping of particles because of friction with air and thus damping of the particle motion. The storage of particles inside the trap is possible over several days.

 

Electrospray Process
FIG. 3: Simplified Scheme of the Electrospray Ionization

Investigation of levitated particles

 

One of our main goals is to study light-matter interaction on individual nanoparticles without interaction with other objects, i.e. substrates or particles. Among others, we perform experiments with levitated Nitrogen-Vacancy (NV) defect centers in nanosized diamonds [2], quantum dot particles [2] and spherical microresonators [3]. An image showing a nanoparticle levitated in front of our microscope objective can be seen in FIG. 4(a). In FIG. 4(b) fluorescence emission spectra typically obtained from micron-sized diamond particle containing NV centers can be seen.

Single nanoparticle trapped in the focus of the microscope objective Fluorescence emission spectra of different levitated micro-sized diamond clusters containing NV defect centers
FIG. 4: (a) Single nanoparticle (green dot) trapped in the focus of the microscope objective (b) Fluorescence emission spectrum of different levitated micro-sized diamond clusters containing NV defect centers

The ability of the Paul trap to confine particles for an arbitrarily long time allows for further processing. With our setup, suitable particles can be preselected and deposited on objects inside the trap. For further analysis we deposit trapped particles on cleaved facets of optical fibers (see FIG. 5(a) and VID. 1). This offers the opportunity to retrieve the particle without effort [2] and measure the size with an atomic force microscope. The measured particle height of a submicron diamond cluster is shown in FIG. 5(b).

Deposited diamond particle
FIG. 5: Deposited diamond cluster on the facet of an optical fiber. (a) Optical microsocope image. (b) Height image of the same particle obtained from measurements with an atomic force microscope. Images from [2]


VID. 1: Video showing the deposition of a particle on a fiber facet.
 

The deposition method can also be used to functionalize other photonic structures with resonator or active particles. Tapered optical fibers offer easy access to the evanescent field of their guided modes and thus are ideal candidates for functionalization. Assembled with active particles, such a system can be used for sensing of environmental changes or as a custom-made fiber optic light source [4].


Assembly of levitated particles

 

Our segmented trap geometry, shown in FIG. 6, enables a high degree of spatial control of the particles inside the trap. It is possible to confine several particles at once, move them within the trap and separate single particles from the others by applying individual dc voltages to different segments. This permits repeating experiments several times with many particles without the necessity to reload the trap. Furthermore, we are able to assemble different characterized particles into electromagnetic coupled entities. For this we are making use of the electrostatic attraction between particles with opposite charge polarities. This technique is illustrated in FIG. 6, and in VID. 2 a video of the assembly of two microparticles can be seen.

Assembly scheme
FIG. 6: Scheme for particle assembly in a segmented linear Paul trap


VID. 2: Video showing the assembly of two different levitated particles in our segmented linear Paul trap.
 

FIG. 7 shows an application of this technique. A cluster of quantum dot particles is trapped and characterized by fluorescence spectroscopy. After identifying an appropriate cluster, a silica microsphere is characterized by its scattering spectrum while the cluster is on hold in the trap. After the assembly we are able to observe electromagnetic coupling between the emitter and the resonator [3].

Assembly of resonator and emitter particles
FIG. 7: Fluorescence/scattering spectra of characterized quantum dot cluster and microsphere resonator with subsequent assembly. Electromagnetic coupling of the fluorescent emitter to the modes of the resonator can be observed. The levitated particles stay in the trap after coupling for further analysis or deposition. More details can be found in [3].


 

References

 

[1] A. Kuhlicke, K. Palis, O. Benson; Broadband linear high-voltage amplifier for radio frequency ion traps, Review of Scientific Instruments 85, 114707 (2014)

[2] A. Kuhlicke, A. W. Schell, J. Zoll, O. Benson; Nitrogen vacancy center fluorescence from a submicron diamond cluster levitated in a linear quadrupole ion trap, Applied Physics Letters 105, 073101 (2014)

[3] A. Kuhlicke, A. Rylke, O. Benson; On-Demand Electrostatic Coupling of Individual Precharacterized Nano- and Microparticles in a Segmented Paul Trap, Nano Letters, Articles ASAP DOI: 10.1021/nl504856w (2015)

[4] M. Gregor, A. Kuhlicke, O. Benson; Soft-landing and optical characterization of a preselected single fluorescent particle on a tapered optical fiber, Optics Express 17, 24234-24243 (2009)