Download Highligh in Physics 2005

Congresso del Dipartimento di Fisica
Highlights in Physics 2005
11–14 October 2005, Dipartimento di Fisica, Università di Milano
SFERA, a laboratory of physics
and technology on lasers
S. Cialdi, F. Castelli, I. Boscolo, D.Cipriani
Dipartimento di Fisica - Università di Milano and INFN - Sezione di Milano
Laser Activity
A picture of the experimental apparatus
Laser pulse shaping for
RF electron guns
A project on OCT: Optical
Coherent Tomography
X-ultraviolet free-electron lasers (X-UV FELs) require a low emittance electron beam,
generated by a photocatode driven by a powerful laser pulse. Numerical calculations
and experimental tests show that the e-beam emittance depends on the temporal
pulse profile, and that its minimum is reached with rectangular laser pulses. A pulse
with a rectangular profile is produced by using a shaping system to manipulate a
subpicosecond Gaussian-like pulse produced by common laser systems.
Optical coherence tomography (OCT) is and interferometric imaging technique offering
non-invasive millimeter penetration dephts of a sample under test with submicrometer axial and lateral resolution. The fundamental behing OCT lie in lowcoherence interferometry. The recombination of backscattered light and reference light
from a sample and mirror gives rise to an interference pattern from which point-spatial
dimension and location microstructures can be determined.
The shaping system is based on a Nd:YAG
laser followed by a 500 m of monomodal
optical fiber for the broadening of the
spectral bandwidth. The control of the
pulse shape is obtained by modifying the
phase of the spectral components with
a liquid crystal programmable spatial
light modulator (LCP-SLM), also called
4f-system. This shaper and the diagnostic
tools are interconnected to a computer in a
feed-back loop for selfconsistent operation.
The axial resolution of OCT is equivalent
to the coherence length of the light
source, which is given by:
20
lC 

where 0 is the central wavelength, and
 is the linewidth of the spectrum.
The Nd:YAG laser oscillator
The bandwith broadening by an optical fiber
The Nd:YAG oscillator has been designed for actively mode-locked
operation at a repetion rate of 100 MHz and single TEM00 transverse
mode. Examples of the generated pulse train and a picture of a single
pulse, with a temporal length of about 90 ps, are shown below, as read
directly from a photodiode.
A guided optical wave propagating in a single mode fiber experiences a
nonlinear self-phase modulation which broadens the spectrum, in this case
extending it from 0.02 nm to about 3 nm, as required for the pulse manipulation
by the 4f-system. The figure shows optical spectra at the end of the fiber for
different input powers.
High nonlinearity air-silica microstructure
fibers (called PCF: photonic crystal
fiber) are used to obtain an extremely
broadband continuum from laser pulses
(from femtoseconds to nanoseconds), and
therefore a very high linewidth.
The structure of the periodically placed air-holes strongly
confines the light in the high refraction index core,
obtaining a very small nonlinear characteristic length,
and therefore a wide spectral broadening.
SEM picture of a PCF core
Fiber propagation equation for
normalized field amplitude:
A
1
1 2 A
1
2
  A  i  2  i
A A
z
2
2 t
LNL
Self-modulation term;
LNL: nonlinear characteristic length
The feedback system for automatic operation
The 4f-system
The phase modulation performed by mask
pixels is determined via an adaptive algorithm,
which vary randomly the phase filter function
() to reach a selected traget profile. The
actual pulse profile is measured by means of
an autocorrelator, which combine the pulse
with a time delayed copy of itself, resulting in a
triangular waveform when the target has the
desired rectangular profile.
The optical components of a 4f-system are two gratings and two lenses placed at
the focal distance with a filter mask LCP-SLM at the center focus plane. The
physical operations done by the device are: a) spatial dispersion of the spectral
components of the pulse and focalization over the mask, by means of the first
grating and lens; b) phase modulation of the spectral components given by the
pixels of the liquid crystal programmable spatial light modulator; c) recombination
of the spectral components by the second grating and lens.
I auto ( )   I (t ) I (t   ) dt
The supercontinuum spectrum at the end of
a 1 meter long PCF (solid curve) compared
with the spectrum of the input laser pulse
(dashed curve). (H.Lim et al, Optics Letters
vol. 30, 1171 (2005))
Interference fringes obtained with the Michelson
interferometry using the supercontinuum and a
plane mirror as a sample, by varying the
position of the reference mirror (HR). When the
sample has a complex three-dimensional
structure, we have a superposition of multiple
interference figure; for each position of the HR
mirror, the light intensity depends on the
reflectivity of a spatial point of the sample which
match the optical path length.
(I.Hartl et al, Optics Letters vol.26, 608 (2001) )
A tomogram of an in vivo African frog tadpole (near the eye), consisting of 1000 
400 pixels showing and area of 1.0 mm  0.4 mm. The highly scattering structures in
the cell, such as cell membranes and nuclei, as well as nearby tissue morphology,
can be seen. (H.Lim et al, Optics Letters vol. 30, 1171 (2005))
Schematic diagram of an electronically
addressed LCP-SLM. The mask is an
array of 640 pixel made of nematic liquid
crystal, whose refraction index can be
controlled by an electric potential,
introducing a phase shift () over
spatial dispersed spectral components.
Aout ( )  e
i  ( )
Ain ( )
100 m
European research project CARE-PHIN, and collaboration with INFN-SPARC project
Field emission from
carbon nanotubes
Carbon nanotubes are ordered structures based on exagonal
carbon rings, folded in tubular forms, with a diameter of a few
nanometers. In the picture it is shown a cluster of carbon
nanotubes, synthetized via Hot Filament Chemical Vapor
Deposition apparatus (HFCVD).
These nanotubes are excellent field emission sources of
electrons, having a large field amplification factor, arising from the
small radius of curvature of the nanotube tips.
Electron sources based on
carbon nanotubes and diamond films
Field emission tests are carried out in a high vacuum, stainless steel chamber. The anode is a stainless
steel 30 mm long wire of about 1.4 mm diameter hemispherically terminated. Its distance from the
cathode (the sample) and the (x,y) plane position are controlled by a PC through a linear precision
actuator. The applied electric field can be varied by sweeping the voltage at a fixed anode-cathode
distance.
The figure shows three experimental emission plots I-E obtained from three different samples with a
low, medium or high density of nanotubes. The field amplification factor is higher in the case of low
density; this can be explained with the fact that the lower is the density, the higher is the number of field
lines captured by each nanotube tip, that is the higher the geometrical enhancement factor.
Current amplification
by diamond films
Diamond films of few microns can be useful for electron current amplification due to
their negative electron affinity (NEA). An electron beam of some keV is injected into
the material; a lot of secondary and low-energy electrons are generated in the
conduction band and transported to the opposite surface, where they escape
because the vacuum energy level is lower than the bottom of the conduction band.
Amplification factor vs
primary electron energy
Field lines around
the tip of a nanotube
Collaboration with University of Roma2 – Tor Vergata