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«NOVEL PROBES FOR SCANNING NEAR-FIELD OPTICAL MICROSCOPY DISSERTATION Submitted to ETH ZURICH for the degree of DOCTOR OF SCIENCES by VALERIA LOTITO ...»

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Diss. ETH No 20602

NOVEL PROBES FOR SCANNING NEAR-FIELD OPTICAL MICROSCOPY

DISSERTATION

Submitted to

ETH ZURICH

for the degree of

DOCTOR OF SCIENCES

by

VALERIA LOTITO

Laurea Specialistica in Ingegneria Elettronica (M.Sc.), Politecnico di Bari (Italy) born on 30/09/1983 citizen of Italy accepted on the recommendation of Prof. Dr. Gian-Luca Bona, examiner Prof. Dr. Christian Hafner, co-examiner Dr. Urs Sennhauser, co-examiner Index Abstract.

Sommario.

Zusammenfassung

List of abbreviations.

1.  Introduction

1.1.  SNOM vs conventional optical microscopy and scanning probe techniques............ 9 

1.2.  From classical optical microscopy to SNOM

1.3.  Aperture SNOM probes

1.4.  Hints about aperture SNOM probe fabrication

1.5.  SNOM scanning control modes

1.6.  SNOM operating modes

1.6.1.  Transmission mode

1.6.2.  Reflection mode

1.6.3.  Scattering type

1.7.  Final remarks

2.  Modelling of SNOM probe behaviour

2.1.  Optimization of SNOM probe structure: the fully metal-coated dielectric probe. 31 

2.2.  Analysis and optimization of probe structures: the need for numerical tools........ 38 

2.3.  The finite element method

2.4.  Computational model and numerical analysis of the behaviour of a fully metalcoated axisymmetric probe

2.5.  Final remarks

3.  Modelling and simulation of novel asymmetric SNOM probes..... 47 

–  –  –

3.2.  Basic features of the asymmetric structures

3.3.  Asymmetric probe: cut probe

3.3.1.  Asymmetric cut probe: variation in cut angle

3.3.2.  Asymmetric cut probe: variation in cut height

3.4.  Asymmetric probe: tip with asymmetric corrugations

3.4.1.  Probe with asymmetric corrugations: permutations of grooves and bumps.......... 57  3.4.2.  Probe with asymmetric corrugations: variation in the azimuthal extension of the corrugations

3.4.3.  Probe with asymmetric corrugations: variation in the dielectric material............ 61 

3.5.  Final remarks

4.  SNOM probes with adirectional asymmetries

4.1.  Adirectional vs directional asymmetries

4.2.  Probe with spiral corrugation

4.2.1.  Probe with spiral corrugation: variations in material and geometric parameters 72 

4.3.  Probe with spiral-arranged azimuthal corrugations

4.3.1.  Probe with spiral-arranged azimuthal corrugations: variations in the shift angle 77  4.3.2.  Probe with spiral-arranged azimuthal corrugations: variations in the angular extension

4.3.3.  Probe with spiral-arranged azimuthal corrugations: variations in the radius...... 81 

4.4.  Final remarks

5.  Interaction of asymmetric probes with fluorescent molecules....... 85 

5.1.  Introduction

5.2.  Single photon emission

5.3.  Modelling of probe-molecule interaction

5.4.  Interaction between an axisymmetric probe and a fluorescent molecule................ 91 

5.5.  Interaction between an asymmetric probe and a fluorescent molecule

5.6.  Final remarks

ii

6.  Fabrication of asymmetric probes

6.1.  Fabrication of fiber-based fully metal-coated probes

6.1.1.  Taper formation by heating and pulling

6.1.2.  Taper formation by etching

6.1.2.1.  Meniscus etching

6.1.2.2.  Tube etching

6.1.2.3.  Selective etching

6.1.2.4.  Variants of the etching method

6.1.3.  Probe metallization

6.2.  Nanostructuring of asymmetric SNOM probes by focused ion beam milling...... 112  6.2.1.  Overview of the dual beam FIB/SEM system

6.2.2.  Fabrication of asymmetric probes with an oblique cut

6.2.3.  Fabrication of asymmetric probes with a spiral corrugation

6.2.4.  Hints on fabrication of other asymmetric probes by combination of FIB milling and deposition

6.3.  Final remarks

7.  Construction of the SNOM set-up

7.1.  Overview of the experimental system

7.2.  The illumination unit

7.3.  The SNOM head and the scanning stage

7.4.  The detection unit

7.5.  The control electronics

7.6.  The mechanical set-up for fiber probe gluing

7.7.  The measurement sequence

7.8.  Final remarks

8.  Characterization of a novel asymmetric SNOM probe................ 135 

8.1.  Characterization of a novel near-field probe

8.2.  Topographic tests

–  –  –

8.4.  Final remarks

9.  Conclusions and outlook

Bibliography

Acknowledgements

Curriculum vitae.

iv Abstract Scanning near-field optical microscopy (SNOM) is a tool of primary importance for nanostructure investigation because it allows extraction of topographic and optical information about the specimen under test. The technique, based on the near-field interaction between a sharp probe and the sample, enables to overcome the diffraction limit of conventional optical microscopy.





Of course, the probe itself plays a major role. Unfortunately, the most widespread probe configuration, the so-called aperture probe based on a metal-coated dielectric with an aperture left at the end, is inadequate for high resolution applications. On the contrary, an alternative structure consisting in a fully metal-coated dielectric probe allows the achievement of high resolution at the cost of a complication in the experimental set-up due to the need for a radially polarized excitation.

In this thesis, we propose and investigate novel probe structures based on the introduction of an asymmetry in order to get high resolution under linearly polarized excitation. A finite element based model has been developed to study the behaviour of the novel configurations, compare their performance to those of a fully metal-coated probe and assess the influence of structural parameters on the performance of the novel probe.

All the structures break the original rotational symmetry of the probe. However two of them still exhibit reflection symmetry (probe with an oblique cut and probe based on azimuthal corrugations limited to just one half of the structure) and, hence, still require some alignment of the input linear polarization with respect to the asymmetry; the other two (probe with a single spiral corrugation and probe with azimuthal corrugations arranged in a spiral-like fashion) do not possess such a symmetry, thereby allowing superfocusing for any arbitrary direction of the input linear polarization. On the grounds of these characteristics we grouped the asymmetric structures into two categories, which we called “directional” and “adirectional”. Nonetheless, independently of the specific features, it is noteworthy that all configurations allow obviating the need for a radially polarized excitation with a significant simplification in the experimental set-up.

The practical feasibility of the proposed novel probes, both directional and adirectional, has been experimentally demonstrated. In fact, the desired structural modifications can be easily introduced by focused ion beam nanostructuring in an originally axisymmetric fully metal-coated probe using either a head-on drilling or a slicing approach properly adapted to our case. In the first approach the ion beam is aligned along the tip axis, while in the second case it is incident at an angle between 0° and 90° with respect to the probe axis according to the desired structure. The overall fabrication procedure is simple and much less demanding and time-consuming than the one required for other modified probe configurations suggested by other groups for superfocusing under linearly polarized excitation.

A SNOM set-up working in illumination mode has been built with the aim to characterize the probe behaviour. For the optical characterization, single fluorescent molecules were chosen for a two-fold reason: first, they represent a means to get purely optical information about the probe behaviour without the risk of topographic artifacts likely to occur in samples with topographic variations due to the coupling between optical and topographic signals; second, they can be interpreted as point-like detectors providing a vectorial picture of the field distribution at the probe apex. Measurements carried out on single fluorescent molecules with an asymmetric probe based on an oblique cut have revealed optical resolution better than 20 nm and a near-field distribution at the probe apex with a mainly longitudinal component. Comparison with a numerical model developed to investigate probe-molecule interactions has shown a good agreement between theoretical predictions and experimental data.

In the light of the theoretical and experimental results, we are confident that the novel probes suggested in this thesis could be promising candidates for high resolution SNOM applications.

In chapter 1 the general principles of SNOM are discussed together with the properties of aperture probes; in chapter 2 the properties of alternative probe structures, in particular those of fully metal-coated probes, are introduced and the numerical approach adopted to model the novel probes is presented; in chapter 3 and 4 the developed model is used to investigate in detail the properties of the novel asymmetric structures based on directional and adirectional asymmetries, while in chapter 5 the interaction of the asymmetric probes with single fluorescent molecules is theoretically modelled and analyzed; in chapter 6, the procedures for the fabrication of the novel asymmetric probes are outlined; in chapter 7, the construction of the SNOM set-up used to carry out probe characterization is described; chapter 8 reports the results of probe characterization based especially on optical measurements using single fluorescent molecules. Finally, overall conclusions are drawn and possible future perspectives are discussed in chapter 9.

Sommario La microscopia ottica a scansione a campo prossimo (meglio nota con l’acronimo inglese SNOM) rappresenta uno strumento fondamentale per l’analisi di nanostrutture perché consente di studiare simultaneamente la topografia e le proprietà ottiche del campione in esame. La tecnica, basata sull’interazione in campo prossimo tra una sonda appuntita ed un campione, permette il superamento del limite di diffrazione della microscopia ottica convenzionale.

La sonda stessa riveste ovviamente un ruolo di primaria importanza. Purtroppo la configurazione più comune, la cosiddetta sonda ad apertura, basata su di un dielettrico rivestito di metallo con un’apertura all’apice, si rivela inadeguata per applicazioni ad alta risoluzione. Al contrario, una struttura alternativa, costituita da una sonda dielettrica interamente rivestita di metallo, consente il raggiungimento di elevata risoluzione a scapito di una complicazione nel set-up sperimentale a causa della necessità di una eccitazione a polarizzazione radiale.

In questa tesi vengono proposte nuove strutture per la sonda, basate sull’introduzione di una asimmetria, al fine di raggiungere elevata risoluzione con eccitazione a polarizzazione lineare. Un modello basato sul metodo degli elementi finiti è stato sviluppato per analizzare il comportamento delle nuove configurazioni, confrontarlo con quello di sonde interamente rivestite di metallo e valutare l’effetto di diversi parametri strutturali sulle prestazioni.

Tutte le strutture sono prive di simmetria rotazionale. Tuttavia due di esse (sonda con taglio obliquo e sonda con corrugazioni azimutali limitate solo ad una metà della struttura) presentano simmetria di riflessione e pertanto richiedono allineamento della polarizzazione lineare in ingresso rispetto all’asimmetria; le altre due (sonda a singola corrugazione a spirale e sonda con corrugazioni azimutali organizzate a spirale) non hanno tale simmetria e, conseguentemente, permettono di ottenere elevata risoluzione per qualsiasi direzione della polarizzazione lineare in ingresso. Sulla base di tali caratteristiche le asimmetrie sono state suddivise in due classi indicate come “direzionali” e “adirezionali”. Ciononostante, indipendentemente dalle proprietà precipue, è da osservare come tutte le strutture permettano di evitare l’uso della polarizzazione radiale con una conseguente semplificazione dell’assetto sperimentale.

Inoltre è stata anche dimostrata sperimentalmente la fattibilità delle nuove strutture proposte, sia direzionali che adirezionali. Infatti le modifiche strutturali desiderate possono essere facilmente introdotte con nanostrutturazione mediante un fascio ionico focalizzato in una sonda assisimmetrica interamente rivestita di metallo, utilizzando due approcci denominati “head-on drilling” e “slicing” adeguatamente adattati alle strutture da realizzare. Nel primo caso il fascio ionico è allineato lungo l’asse della sonda, nel secondo caso è incidente ad un angolo tra 0° e 90° rispetto all’asse della sonda in base alla struttura desiderata. La procedura di fabbricazione complessiva è semplice, meno laboriosa e più rapida di quella richiesta per la fabbricazione di strutture modificate sviluppate da altri gruppi al fine di ottenere alta risoluzione con polarizzazione lineare.



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