Scanning Near-Field Optical Microscope
Our Scanning Optical Near-field Microscope (SNOM) (see its
general view in Figs. 1, 2) is a ready-to-use device designed to image
topography of different surfaces with a nanometer spatial resolution
and to record a near-field optical signal. Its sensitive element (probe)
is a standard sharpened fiber probes glued onto the 32 kHz tuning fork,
but we have designed original low - noise electronic circuit to measure
the resonance frequency and quality factor of this probe. This dynamical
force approach to SNOM was first utilized by us using Heterodyne Phase-Controlled
Oscillator electronic unit (Shubeita G. T., Sekatskii S. K., Riedo B.,
Dietler G., and Duerig U., "Scanning near-field optical microscopy based
on the heterodyne phase-controlled oscillator method", J. APPL. PHYS.,
88, 2921-2927, (2000)), but lately it was much improved in collaboration
with colleagues from the Institute of Spectroscopy Russian Academy of Sciences.
Using proprietary electronic unit, whose performance is close to the theoretical
noise limit (Serebryakov D. V., Cherkun A. P., Loginov B. A., and Letokhov
V. S., "Tuning-fork-based fast highly sensitive surface-contact sensor
for atomic force microscopy/near-field scanning optical microscopy",
REV. SCI. INSTRUM. 73, 1795-1802, (2002)), and original technology of
the gluing of the fiber probes onto the cantilever (see Fig. 3, we routinely
obtain the quality factor ranging 3000 - 5000), this SNOM rapidly gives
you not only topographical z-signal, but also the local values of the
resonance frequency and/or quality factor of oscillations. A schematic
of electronics used is given in Fig. 4 and in Fig. 5 we presented an
interface of this SNOM.
Automatic approach option enables to engage the sample without
any risk to damage your fiber probe. This SNOM was shown to work well
when the end of fiber probe is in water layer with the thickness of
up to 0.5 mm (quality factor still is larger than 1000), and we can
supply you with the proprietary fluid cell for this (of course, you
can invent your own).
Electronics and mechanics of this device can be used separately
with other scanning probe microscopes and detectors. We are ready to
discuss all your additional needs. In particular, we recommend you to decide
in combination with what optical microscope you will use this SNOM, so
we could be able to modify its construction to be readily compatible with
the device of your choice. Of course, upon an additional payment we can
sell you our SNOM already in combination with different optical microscopes.
A few images recorded using this SNOM are presented in Figs.
6 - 13.
SNOM head:
Scanner - 4 tubes,
Sample position - horizontal,
Scan range - 40 µm x 40 µm,
Z-range - 5 µm,
Initial approach - slip-and-stick motion upward or downward,
automatically controlled
Coarse slip-and-stick motion in x, y plane - step 1 µm, the
full range - 10 mm
Probes:
Standard optical fiber probes for SNOM glued onto a tuning
fork (standard watch quartz resonator for 32 kHz). When used as an
AFM only, ultrasharp standard AFM cantilevers or ultrasharp custom -
made wires glued onto the same tuning fork. We supply ready-to-use AFM
probes for $20 each. Optical fiber probes for SNOM (straight metal-coated
fiber probes glued onto a tuning fork) can be supplied for the price of
Nanonics Supertips probes + $30, i. e. $100 - 110 each: quality factor
greater than 3000 is guaranteed.
Controller electronics:
1. Interface PCI-card for PC-computer. (Note, that PC computer
with the PCI slot and WINDOWS - 98 software is not included into the
offer and should be supplied by the buyer; of course, we can include
it upon an additional payment).
2. Proprietary electronic unit specially designed to work with
the 32 kHz resonance frequency tuning fork with an attached probe.
The performance of such a detector is really close to the theoretical
noise and resolution limit.
Frequency range - 31 - 33 kHz,
Frequency accuracy - < 0.01 Hz
Frequency stability f/f - < 10-8
Force sensitivity - 1 pN/Hz1/2
Response time - 2 ms.
Main electronic unit is assembled inside computer mini tower
box. It includes processor controlled main data acquisition board and
scanner drives, proportional and integral amplifier for feedback circuit,
high voltage amplifiers, power supplies.
X,Y,Z DACs with 16-bit resolution, 10 µs acquisition time
Two 16-bit ADC,
High voltage amplifiers, short circuit protected, output:
-500 + 500 V.
Photon detection:
Photon detector (Photomultiplier Tube or Single Photon Avalanche
Diode) should be supplied by the buyer (of course, we can include it
upon an additional payment). Our electronics provides the possibility
to count the photons in a time gating mode with an external syncronization
(the shortest time interval available is 100 ns). Internal synchronization
(detection in phase with the relative tip - sample position during the
lateral dithering, see Sekatskii S. K., Shubeita G. T., and Dietler G.
"Time - gated scanning near - field optical microscopy", APPL. PHYS. LETT.,
77, 2089 - 2091, (2002)) is also available upon request. We strongly recommend
this option when working with FRET SNOM, surface plasmons or local fluorescence
probes. Both direct output of PMT (negative single-photon pulses with
an amplitude of 10 mV and duration of a few ns) or TTL - pulses from other
counters can be used.
Software:
For Windows 98 (Windows 2000 upgrade and DOS downgrade is
possible).
AFM imaging controls includes:
Scan rate
Scan area
Number of points per line
Right-to-left and left-to-right scan
Averaging
Auxiliary input channels
Automatic approach control.
Warrant period: one year after the purchase.
Delivery time: four months after receiving of a purchase order.
Price: On request.
Other related items can be purchased (time - gated SPAD- and
PMT- compatible scaler necessary to transform the optical pulses to
the form suitable for the scanning probe microscope, HV amplifiers,
special software to process images, proprietary digital lock-in amplifier,
proprietary fluid sell to work with SNOM, etc.). We are ready also to
discuss your order for non-standard electronics. Please, contact: info@nanotechtools.com
Figures (Click on figures for a magnified view).
Fig. 1. General view of our SNOM.
Here shown without the protective black metal box and in combination
with a quite efficient optical microscope MIKMED 2 (LOMO, Russia).
Fig. 2. General view of our SNOM0 (without black metal
protective box). Sample holder is a metal plate which rests on three
piezocylinders: you can easily fix whatever you want (including fluid
water cell) on it. Compact superluminescent diode is located near the
fiber probe: its use enables to adjust this SNOM in a full darkness and
when protective black metal box is on (but not to forget to switch it off
before optical measurements!)
Fig. 3. Optical microscope image of a straight metallized
fiber probe glued onto the cantilever.
Fig. 4. General schema of an electronics for our AFM/SNOM.
Fig. 5. How an interface of our device looks like.
Fig. 6. Near-field optical image of 100 nm - diameter
polystyrene beads deposited onto the glass slide surface.
Fig. 7. Topography (z - channel) of closely - packed
100 nm - diameter polystyrene beads.
Fig. 8. and Fig. 9. Topography (z - channel) of 100 nm
- diameter polystyrene beads deposited on the glass slide surface,
Fig. 8, and local value of the resonance frequency of the fiber probe
for the same sample (quality factor value is used for a feedback), Fig.
9. Scanning range - 5 x 5 microns.
Fig. 10. Noise test for our SNOM working in an AFM mode
with the freshly cleaved mica surface.
Fig. 11. Apex of the NT-MDT AFM Si cantilever (radius
of curvature of the order of 50 nm) imaged with our SNOM.
Fig. 12. Near-field optical images of the lines made
in 150 nm - thick gold coating onto the glass slide (sample: courtesy
of Dr. Ivo Utke, EPFL). Image size - 2.5 x 2.5 microns.
Fig. 13. Near-field optical images of the dots made in
150 nm - thick gold coating onto the glass slide (sample: courtesy of
Dr. Ivo Utke, EPFL). Image size - 3.8 x 3.8 microns.
