Practical generation of femtosecond pulses of 267 nm ultraviolet light
Daniel D. Hickstein∗
University of Colorado
(Dated: April 15, 2009)
Nonlinear crystals are a simple and effective method of generating short pulses in the near UV but
cannot provide light shorter than about 200 nm. Previous studies have indicated that combining the
fundamental (800 nm) and the second harmonic (400 nm) of a femtosecond Ti:sapphire laser in a
hollow fiber waveguide filled with either air, argon, or xenon is an effective way to produce ultrafast
pulses of third (267 nm), fourth (200 mn), and fifth (160 nm) harmonic light [1–3]. However, this
method has only been used to study a handful of chemical systems, suggesting that it is more difficult
to implement than its apparent simplicity would suggest. In this study a 5 cm long 150 µm ID argonfilled fiber waveguide is used to generate 200 nJ pulses of 267 nm light at 1 kHz. The dependence
of the third harmonic intensity is found to vary significantly with argon pressure featuring a broad
peak near atmospheric pressure. Suggestions are given for improving the conversion efficiency.
I.
INTRODUCTION
A.
Motivation
The desire to observe ultrafast reaction dynamics of
molecules means that there is a need for sources capable of producing femtosecond pulses of deep and vacuum ultraviolet light (VUV is light shorter than 200 nm).
UV light can be used to excite electronic transitions in
molecules which can rapidly induce changes in molecular
geometry and cause chemical bonds to be made or broken. Such processes are relevant to all manners of new
technologies, including solar energy conversion and chemical synthesis, and, since they occur extremely rapidly,
lend themselves to study on a femtosecond timescale.
However, as of this writing, a system for the creation of
intense femtosecond pulses in the VUV range is not commercially available. Several companies currently provide
reliable systems for generating 20 fs pulses of infrared
light at 800 nm using a Ti:sapphire laser. This output
can then be frequency doubled or tripled using nonlinear crystals to create light at 400 and 266 nm. However,
the absorption of nonlinear crystals such as BBO at short
wavelengths prevents their use in the creation of light less
than about 200 nm [4]. Since many interesting absorption
bands in molecules and materials are below 200 nm, it is
important to develop practical, inexpensive, and reliable
methods for generating ultrafast pulses in the vacuum
ultraviolet.
B.
Competing methods
In 1993, Ringling and coworkers published an article
describing their system for the generation of 180 fs ultraviolet pulses that could be tunes from 189 to 200 nm[5].
This system utilized a series of BBO crystals and, conse-
∗ Electronic
address: [email protected]
quently, was able to achieve reasonably high power (4 µJ
per pulse at 200 nm and 1 KHz repetition rate). Since
then, various research groups have been attempting to
match this level of efficiency with methods that will extend into the vacuum ultraviolet.
1.
Gas cells
The technique of focusing 800 nm light into a small
(about 10 mm) cell filled with argon gas is the most experimentally simple of all the methods discussed here.
Though the pulses generated are generally low intensity
[6], the technique has recently been used for an ultrafast
study of exited ethylene [7] and to measure a time constant of 1.8 fs for the dissociation of water [8]. However,
the short interaction length of a simple focused beam
means that the intensity of the light produced by such
methods will likely be lower than that produced using
other techniques. For example, the argon cell in Ref 6
produces 5 nJ of fifth harmonic light while a modern
fiber waveguide has produced 160 nJ per pulse [1].
2.
Filamentation
One way of increasing the interaction length of the
light with the noble gas is to utilize the phenomenon
of filamentation, whereby intense light creates a plasma
and self-focuses into a long, narrow filament. A group in
Japan has generated 20 µJ, 12 fs pulses of 260 nm light
by filamentation through 15 cm of neon gas in a glass cell
[9]. These results are quite promising, but this method
does not seem to be in common use for generating higher
harmonics. The main disadvantage is that each gas has
a critical power for self-focusing, so the power cannot be
adjusted as freely as it can be in the other methods.
2
3.
Fiber waveguides
“This alternative technology will make DUV
pulses accessible for a broad range of scientific
applications” –Babushkin and Herrmann [10]
!"
!"
Hollow-core glass waveguides have been shown to generate high intensity femtosecond pulses of UV light
[2, 3, 11, 12] and offer a great deal of control over the
phase matching parameters. The fiber length, fiber diameter, and gas pressure can all be varied in order to
achieve better phase matching for the desired process.
Fiber waveguides have been used to generate UV pulses
as short as 8 fs [2] and a recent study generated third
harmonic light with 2 percent efficiency [3]. A theoretical study [10] found that generation of 160 nm pulses in
hollow waveguides can occur with up to 30 percent efficiency and that it should be possible to generate “sub10-fs pulses in the spectral range from 90 to 140 nm”,
making it clear that the there is great potential in UV
generation from hollow-core fiber waveguides.
C.
Theory
The UV light generated by these fiber waveguides is a
result of a difference-frequency four wave mixing (FWM)
process that involves the the combination of the fundamental and the second harmonic as shown in Figure 1.
As the diagram suggests, the intensity of the second harmonic light is much more important to the efficiency of
third and fourth harmonic generation than is the intensity of the fundamental. Indeed, in an unpublished report [13] to the National Science Foundation, a group at
KM-Labs finds the following relationships between the
harmonic intensities:
2
I3ω ∝ Iω I2ω
2 3
I4ω ∝ Iω I2ω
4
I4ω ∝ Iω3 I2ω
(1)
(2)
(3)
Thus, it is essential to maximize the amount of second
harmonic in the fiber, with the fundamental playing a
much smaller role. Jailaubekov and Bradforth [11] estimate the dependence of the third harmonic power (P3ω )
on the fiber length (L) and diameter (a):
P3ω ∝
L2
a4
(4)
suggesting that a very narrow fiber should be used. However, in practice, the difficulty of coupling light into a
very small fiber creates a lower limit to the inner diameter of the fiber.
An advantage of producing harmonics in a fiber waveguide is that the phase matching condition can be controlled by varying the fiber length and width as well as
the pressure of the gas. Durfee and coworkers have authored several papers [14–16] regarding phase matching
!!"
!!"
#!"
"!"
!!"
"!"
FIG. 1: The fourwave mixing process for the generation of
third (left) and fourth (right) harmonic light.
processes in fiber waveguides. The following equation [16]
connects the wave vector for light (k) at a wavelength (λ)
to the pressure in the waveguide (P ) and the diameter of
the waveguide (a):
k(λ) =
2π 2πP δ(λ) u2nm λ
+
−
λ
λ
4πa2
(5)
where δ is related to n, the refractive index of the gas,
as δ(λ) = n−1
P . Thus, the phase mismatch can often be
minimized through the appropriate selection of gas and
pressure[4].
II.
EXPERIMENTAL
The experimental work described here was completed
in the laboratories of the Kapteyn–Murnane group at
JILA at the University of Colorado between early February and mid-April 2009.
A.
Apparatus
The experimental apparatus is shown in Figure 2. The
idea is to pass the 800 nm output from a Ti:Sapphire
laser through a BBO crystal to generate the second harmonic (400 nm); split the 400 nm light from the 800 nm;
pass the 800 nm light through a half-waveplate, polarizer, and delay stage; then recombine the beams before
coupling them into an argon-filled fiber waveguide. The
half-waveplate in necessary because the type I BBO crystal causes the polarization of the two different harmonics
to be 90 degrees offset. The delay stage is required because the group velocity dispersion (GVD) in the BBO
crystal causes some pulse walk-off between the blue and
the red. While there are schemes [17] to synchronize the
timing and polarization without splitting the beam into
3
the two harmonics, the desire to ultimately substitute
the 800 nm light with tunable infrared light from an optical parametric amplifier (OPA) dictated the use of a
separate path for each harmonic.
1.
Laser
The 800 nm input beam was generated by a two-stage
amplified, mode-locked Ti:sapphire laser operating at a
repetition rate of 1 kHz. The output power of the laser
(as measured at the end of the second-stage amplifier)
was varied between 0.5 and 2.0 watts for alignment and,
for the final experiment, measured 1.51 watts just before the UV-OPA setup. The power was not increased
beyond this due to fear of burning the fiber, but with
better coupling efficiency (see §II A 5), the power could
be increased to the full capacity of the laser system: in
excess of 4 watts. In a previous experiment, Robynne
Lock (of the Kapteyn–Murnane group) found the temporal pulse width of this laser to be between 24 and 27
fs.
2.
Second harmonic generation crystal
Second harmonic light (400 nm) was generated from
the 800 nm fundamental using a 10 mm diameter 100
µm thick beta barium borate (BBO) crystal. BBO was
selected for this experiment over other second harmonic
generation (SHG) materials because its high second harmonic generation coefficient allows a very thin crystal to
generate significant quantities of second harmonic light.
The thickness of the crystal was chosen to be 100 µm
in order to limit the temporal broadening of the pulse.
The desire to create UV light that could ultimately be
used to study molecular dynamics means that, in this
study, generating temporally short pulses takes priority
over producing high laser intensity. The 10 mm crystal
was smaller than the 15 mm beam, presenting the choice
to either clip part of the beam, focus the beam through
the crystal, or to down-collimate. Since the light would
later need to be coupled into a capillary, it was decided
that the better quality beam-mode resulting from simply clipping the beam would allow for more light to be
coupled into the fiber. In the future, a 15 mm 200 µm
crystal would greatly improve second harmonic intensity.
3.
Beamsplitters
The two optics that separate and, later, recombine
the two harmonics have the potential to cause significant temporal pulse broadening and were therefore carefully selected. The 400/800 nm beamsplitters are 1 inch
diameter 1 mm thickness “High Energy Harmonic Separators” custom coated by CVI Laser Corporation. These
thin optics were chosen in order to minimize the amount
of material that the red light must pass through, thereby
reducing the temporal broadening of the pulses. The
manufacturer claims that these separators transmit in
excess of 90 percent of incident light and reflect more
than 99.5 percent. The 45 degree angle of the second
beamsplitter (see Figure 2) to the red beam leaves the
effective diameter of the beamsplitter only slightly larger
than the diameter of the beam, it proved very difficult
to align this optic with both beams. The situation was
further complicated by the manner in which most optical
mounts extend beyond the face of the optic, resulting in
further clipping of the red beam. Ultimately, the beamsplitter was glued to a mount, eliminating any additional
material at the level of the beams. Further implementations of such a setup should probably incorporate a larger
diameter optic in this position in order to minimize alignment difficulties.
4.
Mirrors
The 400 nm and 800 nm mirrors used in this experiment are standard anti-relection coated glass mirrors
from the supplies of the Kapteyn–Murnane Laboratory
at the University of Colorado. The original plan was to
have the beam exit the UV-OPA traveling back towards
the source. However, this was modified to make better
use of table space by having the light pass through the
apparatus and emerge traveling in the same direction as
before. This would have required that more mirrors be
added to the apparatus, but instead a retro-reflector was
placed on a translation stage to act as the two mirrors in
the delay line. The retro-reflector minimizes the changes
in beam pointing caused by adjustment of the translation
stage. Originally, the retroreflector was placed after the
two mirrors in the red beamline, however, this made recombining the beams very difficult, as the retroreflector
is difficult to finely adjust. Thus, in the final design, the
retroreflector was placed before the two mirrors which
allowed for easy recombination of the red and the blue
beams.
5.
Fiber waveguide
The four-wave mixing process takes place in the center of a hollow glass fiber which keeps the light tightly
focused for many centimeters. Though short fibers, from
about 1 cm to 5 cm in length, are frequently used for high
harmonic generation, several studies [1, 2, 10] have found
that longer fibers, of about 20 to 50 cm, provide better
efficiency for four-wave mixing experiments. Though Ref
2 used a 100 µm inner diameter (ID) fiber, the reported
difficulty of coupling light efficiently into such a fiber convinced me that the 150 µm fiber would be the best choice
for the amateur. Consequently, I prepared several 20 cm
long, 150 µm ID fibers and mounted one of them into a
20 cm “V-goove” mount provided by KM-Labs Inc.
4
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FIG. 2: Schematic of the experimental setup. This sketch is not to scale: in the actual setup the blue and the red beam-paths
are exactly equal.
FIG. 3: The 5 cm fiber (top) connected to argon and the
vacuum system and the 20 cm fiber (bottom) wishing that it
could be of some use to the world.
The preparation of a fiber involves using a specialty
computer-controlled focused laser to burn two holes into
the side of the fiber about 5 mm from each end. The
ends of the fiber are then sanded flat and the fiber is
cleaned using an ultrasonic bath followed by a rinse with
methanol. The mount features a V-shaped groove that
greatly improves light transmission through the fiber by
keeping it perfectly straight.
The mount provides connections so that gas can pass
in through the large holes in the side of the fiber and out
through the ends of the fiber, where vacuum is applied.
In this experiment, the vacuum at the ends of the fiber
FIG. 4: The 5 cm 150 µm ID fiber waveguide during the
experiment.
was approximately 0.1 torr and the argon pressure at the
side-holes was varied between 30 and 1100 torr. Since
the holes on the side of the fiber are larger than the holes
at the end, the pressure inside of the fiber is hopefully
similar to the pressure measures at the side holes. In
reality, the pressure inside the fiber will be somewhat
less than the measured pressure of argon and is probably not constant over the length of the fiber. Thus, all
“waveguide pressure” measurements in this study are approximate and will overestimate the true pressure inside
the waveguide.
Since it is more difficult to couple light into a longer
fiber, I decided to start with a 5 cm fiber. Unfortunately,
the difficulties involved in coupling the light into this fiber
5
meant that there was not enough time to also try to
20 cm fiber. It is likely that better four-wave mixing
efficiency would be observed using the 20 cm fiber. Figure
3 shows both fibers in their V-groove mounts as well as
the vacuum and argon gas system used to connect the 5
cm fiber.
6.
Detection
A simple fused silica prism was used to split the beam
that emerged from the fiber into its 800, 400, and 267
nm components. Due to the overwhelming brightness of
the second harmonic, a 400 nm mirror was used to reflect
the majority of the 400 nm light into a beamstop, allowing all three wavelengths to be seen on a paper notecard.
This was photographed with a simple point-and-shoot
camera (Figure 5). Also, the power was estimated using a common laboratory power-meter and the spectrum
of the 267 nm light was analyzed using an ocean optics
200–1000 nm spectrometer. I encountered some difficulty
using the spectrometer because the lens that helps to couple the light into the spectrometer’s optical fiber is apparently coated with something that completely blocks
267 nm light! Once this was removed, the measurements
could be made easily.
B.
Modifications from previous studies
Originally, this experiment sought to merely replicate
the results from a study conducted by KM-Labs [3] where
third and fourth harmonic light was generated with a
fiber waveguide. However, several significant modifications distinguish this study from Ref 3 and other previously published studies. Most of these modifications
stem from the desire of this study to produce extremely
short pulses that would be useful in molecular dissociation studies. This is in contrast to Ref 3 which sought
only to produce high intensities of ultraviolet light for applications such as defect inspection in lithography where
temporal pulse length is of little concern.
The main modification of this experiment that allows
for the generation of shorter pulses is the use of a 100
µm BBO crystal instead of the 1 mm crystal used in Ref
3. The increased laser power in this experiment (1.5 versus 0.2 mJ per pulse) will hopefully compensate for the
decreased SHG efficiency of the thinner crystal. Still the
relative intensity of the blue light will likely be less than
in previous studies, so priority is given to maximizing the
intensity of the blue light that is ultimately coupled into
the fiber at the cost of more loss of the red light. To
wit, this setup uses harmonic separators that reflect the
blue and pass the red light, transferring the 10 percent
transmission loss to the red beam. Similar to the apparatus in Ref 3, this setup places the delay stage in the
red beam to ensure that there are fewer optics and fewer
opportunities for misalignment in the blue beam than in
FIG. 5: The fundamental (left), second harmonic (center),
and third harmonic (right) as seen on a 3-by-5 inch notecard
after passing through a prism. The intensities are not to scale
because most of the second harmonic has been removed with
a mirror and the light has been cropped with an aperture.
The spots seen above the black line are caused by imperfect
coupling into the fiber and are not part of the main beam.
the red. Another important difference is that this study
uses argon gas in the waveguide instead of xenon as it is
much less expensive.
III.
RESULTS AND DISCUSSION
When the light was properly coupled into the fiber, the
generation of the third harmonic was observed visually
by using a prism to spatially spread the three harmonics
onto a paper index card. A 400 nm mirror was needed to
reflect most of the 400 nm light onto a beamstop so that
the bright 400 nm light would not overshadow the third
harmonic. Though 266 is past the edge of the visible (380
nm), it creates a blue fluorescence on a paper index card.
Interestingly, if the card is moved, a pale yellow trail
appears behind the 266 nm spot, indicating some, probably phosphorescent, transition with a lifetime of several
hundred milliseconds. The fourth harmonic was not observed.
A.
Optimization of third harmonic
After first observing the third harmonic at atmospheric
pressure of argon, several parameters were adjusted to
optimize the intensity of the third harmonic: the dis-
Integrated intensity of 266 nm peak (arbitraty units)
6
8000
7000
Intensity (arbitrary units)
6000
With 400 nm
6000
4000
5000
2000
4000
400 nm blocked
0
240
3000
250
260
270
280
290
2000
1000
200
400
600
800
1000
50000
40000
30000
20000
10000
0
20
40
Wavelength (nm)
FIG. 6: Spectrum of the light emitted from the fiber with
either just 800 nm or both 800 and 400 nm light passing
through the fiber. After the fiber, the light was spatially
filtered with a prism to remove most of the 800 and 400 nm
light.
Intensity (arbitrary units and positions)
66.7 fs
50 fs
41.7 fs
0
25 fs
16.7 fs
-20000
0 fs
-40000
240
250
260
270
80
100
120
FIG. 8: The integrated intensity of the third harmonic signal
verses the delay of the red pulse. Shorter times imply that the
red pulse arrived sooner than the blue. The zero point of the
delay is arbitrary and does not correspond to “zero delay.”
1.
40000
20000
60
Delay (fs) of 800 nm pulse
280
290
Wavelength (nm)
FIG. 7: Spectrum of the third harmonic region at different
delays of the red pulse. Shorter times imply that the red
pulse arrived sooner than the blue. The zero point of the
delay is arbitrary and does not correspond to “zero delay.”
tance of the lens to the fiber, the timing of the red pulse
(by adjusting the delay stage), and the pressure of argon
in the fiber. Since the lens was already positioned so that
the focus was near the entrance to the fiber, the intensity
of the third harmonic was already optimized with regard
to the position of the lens. Furthermore, the lens had a
focal length of 500 mm, so the intensity of the third harmonic was quite insensitive to the position of this lens. In
contrast, the timing of the red pulse and the pressure of
argon had large effects on the efficiency of third harmonic
generation.
Timing dependence
Since the third harmonic is generated by a combination
of second harmonic and the fundamental, it should be
quite sensitive to the timing of the pulses. The timing
of the red pulse was scanned by adjusting the position
of the retroreflector using a translation stage. For this
scan the pressure of argon in the fiber was fixed at 650
torr. Figure 7 shows that there is range of about 40 fs
where the third harmonic is generated, a result we expect
given the ∼30 fs temporal pulse width from the laser. In
addition to governing the intensity of the third harmonic,
the timing of the pulses also appears to affect the spectral
shape of the pulse, with the peak shifting by about 5 nm
over the course of about 25 fs. Since the spectrometer
did not average over the entire spatial extent of the 267
nm beam, it is possible that this effect is merely due to
changes in the beam profile, but it is also possible that the
exact timing of the two pulses truly affects the resulting
spectrum of the third harmonic light that is generated.
Figure 8 presents the change in the overall intensity of
the third harmonic light with changes in timing. Since
the spectrometer is only capturing a small portion of the
266 nm beam, some of the effects that are seen could be
due to changes in the spatial profile of the beam, but the
plot nevertheless demonstrates that the third harmonic
intensity is strongly dependent on the timing. Though
it might simply be noise, the plot suggests that there
are, possibly, two peaks of high third harmonic intensity.
This result, combined with the two peaks in the spectral
profile of the third harmonic suggests that perhaps the
400 nm pulses have a two-peaked shape which results in
a similar profile of the third harmonic.
7
Since a previous study [13] reported some success with
generating harmonics in a fiber waveguide filled with air,
I tried disconnecting the argon and filling the waveguide
with atmospheric pressure air. Upon doing so, the third
harmonic light dropped to about one tenth of its previous
intensity. Varying the pressure of air had a similar affect
as with argon: the best intensity was observed around
700 torr and the third harmonic was barely visible below
about 300 torr. Though the power of the third harmonic
signal generated by air was too small to be measured, it
is clear that using argon provides a significant increase
in third harmonic intensity.
Intensity (arbitrary units and positions)
40000
900 torr
30000
20000
700 torr
10000
600 torr
300 torr
100 torr
240
260
280
300
Wavelength (nm)
B.
Integrated intensity 255–270 nm (arbitrary units)
FIG. 9: Spectrum of the third harmonic region at different
pressures of argon in the capillary.
100000
80000
60000
40000
20000
0
200
400
600
800
1000
Argon pressure (torr)
FIG. 10: The integrated intensity of the third harmonic signal
verses the pressure of argon in the fiber.
2.
Gas and pressure dependence
Figure 9 shows that the pressure of argon has a similar effect on the spectrum of the third harmonic as the
timing of the red pulse: there appear to be two overlapping peaks and the relative intensities change depending
on the pressure of argon. The intensity is optimized at
pressures between 600 and 800 torr and decreases to almost nothing a low pressures. The pressure regulator
only provides pressures up to 1100 torr, but at this point
the third harmonic signal has decreased substantially.
Figure 10 shows how the integrated area of the spectral
peak of the third harmonic changes with argon pressure.
Here, we again see that there appear to be two peaks, one
around 200 nm and the other around 750 nm. Refs 4, 14
discuss how different phase matching conditions generate
different spatial modes depending on the pressure. Ref
14 found that the peak at lower pressure correspondes to
the EH13 mode and the peak at higher pressure to the
EH14 mode.
Intensity and efficiency
Though the 267 nm light was quite weak, it was possible to measure it using a simple laboratory power meter.
After emerging from the fiber, passing through a fused
silica mirror to filter out the 400 nm light, and traveling through approximately 2 cm of fused silica prism,
the power of the 267 nm signal was still measured to be
0.2 mW. This corresponds to 200 nJ per pulse, an energy that would certainly be sufficient for many chemical
dynamics applications. However, since higher harmonics
will have several orders of magnitude smaller energies, it
is unlikely that this experiment produces useful quantities of fourth or fifth harmonic light.
The power of the 800 nm light immediately before this
setup was measured to be 1.45 watts. After the second beamsplitter (shortly before the fiber waveguide) the
power of the 800 nm light was 460 mW and the blue was
112 mW. After the fiber, the intensities for the blue/red
were 9.2/31.3 watts, for a coupling efficiency of 8.2 percent for the blue and 6.2 percent for the red. In respect
to the blue light coupled into the fiber, the generation
of third harmonic could be considered to be 2 percent
efficient. However, in terms of watts of 800 nm input
into the apparatus, the third harmonic generation was
only 0.01 percent efficient, a far cry from the 2 percent
efficiency attained by Zhang and coworkers [3].
C.
Suggested improvements to apparatus
The better four wave mixing efficiency achieved by
other groups [3, 11] and the optimistic theoretical predictions [10] suggest that the apparatus described in this
study could be dramatically improved.
Since the intensities of the third and higher harmonics are dependent on high powers of the second harmonic
intensity, better coupling efficiency of the 400 nm light
into the fiber could provide order of magnitude improvements in the intensity of the third harmonic signal. Durfee and coworkers [2] achieved a coupling efficiency of 20
percent for the 400 nm light and values are high as 70
percent have been reported [3]. If good coupling could be
achieved, using the 20 cm fiber instead of the 5 cm could
8
also provide a significant increase in interaction length
and consequent increase in conversion efficiency. Also, if
fifth harmonic can be generated, the sapphire windows
used at the ends of the fiber mount would have to be exchanged for windows made from a material that is more
transmissive in the VUV such as calcium fluoride.
D.
Possibilities for future experiments
This study has shown that it is reasonably simple to
generate the third harmonic of a Ti:sapphire laser using
an argon filled fiber waveguide. However, there are still
a number of experiments that should take place in order
to further characterize the resulting pulses as well as to
explore the mechanisms of how they are generated.
The dependence of the intensity of the third harmonic
on pressure (Figure 10) and the pulse timing (Figure 8)
suggest that the temporal and spectral shape of the third
harmonic pulse is not a simple Gaussian, and instead may
be better described as being composed of two peaks. Future experiments would probably want to use a technique
like frequency resolved optical gating (FROG) to characterize the temporal shape of the pulses. If they are indeed
not simple Gaussian-like peaks, then this would pose interesting questions regarding the origin of their shape.
Also, it would be good to understand what parameters of
this apparatus can be controlled in order to generate the
shortest pulses possible, as it would be convenient if the
pulses generated using this method were short enough to
use directly for chemical dynamics studies without compression.
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IV.
SUMMARY AND CONCLUSION
This study demonstrated a practical setup to create femtosecond pulses of third harmonic light from the
difference-frequency four wave mixing of the second harmonic and the fundamental of a Ti:sapphire laser. The
final conversion efficiency of the 800 nm input beam into
267 nm was 0.01 percent and no fourth harmonic was observed. The experiment took approximately one month
of graduate student time. However, many of the key
components of the fiber waveguide, as well as the custom machine for making the fibers were readily available at JILA. Furthermore, experts in such systems provides much valuable advice and assistance. In a laboratory without these tools, the construction of such a
fiber waveguide system would probably take much longer.
Also, the fiber waveguide is difficult to align and can require re-alignment on a day-to-day basis. Consequently,
if large amounts of intensity are not needed, other techniques for creation of ultrafast UV light, such as focusing
through an argon cell, may be more appealing.
V.
ACKNOWLEDGEMENTS
I would like to thank Margaret Murnane and Henry
Kapteyn for providing the inspiration, funding, and lab
space for this project. Chan La-o-vorakiat and Ariel
Paul shared valuable advice regarding the fabrication and
mounting of the fiber waveguides. Robynne Lock and
Xibin Zhou provided lots of help in the lab and answered
hundreds of stupid questions.
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17774–17779.
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Murnane, M. M.;
Kapteyn, H. C.; Durfee, C. G.; Li, T. Generation
and measurement of ultrafast tunable VUV light.
Ultrafast Phenomenon Conference, 2000; pp TuC5+.
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