24 Multilayer Dielectric Gratings 25

Diffraction gratings are an essential component of high-power, short-pulse

lasers. Novel grating designs using dielectric materials (insulators) rather than
on traditional metallic (conducting) surfaces can significantly increase the output
of high-power pulsed lasers. Their unique characteristics give these gratings
numerous potential applications.

O VER the years, pulsed lasers, such

as the Shiva and Nova lasers at
Lawrence Livermore National
advance our fusion power efforts.
We are also studying the propagation of
strong shocks, such as those generated
electromagnetic energy by extracting
energy stored (as atomic excitation) in
an optical amplifier. Solid-state amplifier
Laboratory, have become ever more by high-power laser pulses, to provide materials make it possible, in principle,
powerful. We have built laser systems information on the fundamental properties to extract several joules of energy from
that generate more than a trillion watts of matter (for example, the equation of laser systems of modest size. In certain

Multilayer (a terawatt, or 1012 W) and will soon

complete a quadrillion-watt (1015-W)
laser. The duration of these pulses is less
than a trillionth of a second (a picosecond,
state of a material). These investigations
are particularly important for the
Laboratory’s science-based stockpile
stewardship efforts. The technology of
laser materials, the energy is available
over a broad bandwidth of frequencies,
an important prerequisite for short pulses.
However, as the intensity of the light

Dielectric or 10–12 s). At the Laboratory, we have

completed construction of a short-pulse
100-terawatt (TW) laser, an important
step toward a petawatt laser, which is
short-pulse lasers also has applications
to materials processing, medicine, and
dentistry.
Recent developments in solid-state
grows, it induces changes in the optical
response of the amplifier medium. The
consequent alterations of the refractive
index of the amplifier medium cause

Gratings:
scheduled for completion by the end laser materials and the use of chirped pulse the laser beam to self-focus inside the
of 1995.* amplification1 (CPA) have dramatically laser system. This self-focusing can
The development of short-pulse, increased the intensity available with result in catastrophic damage to the laser.
high-power lasers is important in the pulsed lasers. To recognize the importance To avoid self-focusing, it is necessary
continued progress of our inertial of this new technology, one must to limit the intensity that is present in
confinement fusion program. The fast appreciate some of the constraints that amplifiers of reasonable length to less than

Increasing the Power of Light ignitor concept, described on p. 36,

would use high-power laser pulses to
have previously limited the output of
pulsed lasers. A laser pulse gains
a few gigawatts per square centimeter
(1 gigawatt = a billion watts, or 109 W).

* For more information on the 100-TW laser, see the Research Highlight beginning on p. 34 of this issue.

Science & Technology Review September 1995 Science & Technology Review September 1995
26 Multilayer Dielectric Gratings Multilayer Dielectric Gratings 27

Because the detrimental effects duration of this pulse by a factor of pulse is critical to the chirped pulse
of self-focusing are proportional to ten thousand or more, pass it through amplification technique because pulse
instantaneous intensity rather than to amplifiers where it grows in energy by stretching or compression relies on
accumulated pulse energy (fluence), it as much as a trillion, and compress it manipulating the various frequencies or
is possible to overcome this obstacle back into a short pulse of extremely “colors” contained within the pulse. Any
by amplifying a long-duration pulse high intensity. device that delays certain frequencies SIngle-mode laser
and then compressing the pulse to the The first step is to produce the broad- relative to others could stretch a short
desired duration. Briefly stated, we first bandwidth pulse and to impose on it a pulse over a longer time or, alternatively,
generate a broad-bandwidth seed pulse, controlled frequency sweep or “chirp,” compress a long broad-bandwidth pulse
Figure 1. Schematic of beam stretching, typically 100 femtoseconds in duration in which the different frequencies occur into a short one. We use diffraction
amplifying, and compressing system used (1 fs = 10–15 s). We then stretch the at different times. The bandwidth of the gratings for this purpose, sending light
to give different beams longer or shorter rays of different frequencies in different
paths. directions. (The box on p. 28 describes
Initial short pulse how diffraction gratings work.) A pair
of gratings, suitably arranged with an θ

3.72 m
A pair of gratings disperses the
spectrum and stretches the pulse imaging telescope, will send the higher Columating lenses
by a factor of a thousand frequency (blue) light over a longer path
than the lower frequency (red) light,
thereby stretching out the pulse. The Beam
splitter
reverse action, delaying the red light
Short-pulse oscillator
more than the blue, compresses the
pulse. Figure 1 diagrams our process of
dispersing, amplifying, and compressing
the laser light to generate short, high-
power pulses.
The pulse is now long and low-power, 5.6 m
safe for amplification
Figure 2. Schematic layout of the equal-path, fringe-stabilized interferometer used for grating
exposure. Light from a single-mode laser, is divided into two paths by a beam splitter and
High energy pulse after amplification passes, via small turning mirrors, spatial filters, larger turning mirrors, and lenses, onto the
surface of a grating blank. The angle between the two beams fixes the groove spacing.

Reflecting Light Off Metal wavefronts to provide the precision.

Figure 2 shows a schematic diagram of
Traditional diffraction gratings, our holographic exposure facility. A
widely used in spectrometers to analyze photosensitive surface is exposed to a
Power amplifiers
broadband low-intensity light sources, standing wave pattern created by
have a corrugated metallic surface of interfering two highly coherent laser
precisely parallel periodic grooves. The beams. The latent image of the periodic
metallic surface reflects the light, and interference pattern is then developed
Resulting high-energy,
ultrashort pulse the periodic groove structure diffracts (essentially as photographic film is
the light, sending different wavelengths, developed) to create a corrugated surface.
or colors, back at different angles. For Coating this grooved surface with a thin
many decades, such gratings were metallic film can create a highly efficient
produced by engraving with an extremely reflection grating. Alternatively, the
A second pair of gratings
precise tool called a mechanical ruling pattern can be transferred into a more
reverses the dispersion of the engine. Today grating manufacturers robust underlying dielectric substrate
first pair and recompresses the pulse often use holographic techniques, relying by chemical etching or ion etching
on the stability of continuous-wave laser

Science & Technology Review September 1995 Science & Technology Review September 1995
28 Multilayer Dielectric Gratings Multilayer Dielectric Gratings 29

procedures. Transfer etching tends to

How a Diffraction Grating Works produce rectangular or trapezoidal
grating profiles from the original
Light is an electromagnetic wave that travels through free When the surface has periodic ridges (a grating), then an incoming rounded patterns of developed
space in a straight line, or ray. When light waves encounter a single-color ray produces scattered rays (of the same color) at only photoresist (see Figure 3).
surface, they change direction. If the surface is a mirror, nearly all specific discrete directions. Each of these directions is a Metallic gratings have many useful
the light is reflected; each incoming ray reflects at an angle equal diffraction order for the specified color and incident angle. attributes. A properly designed and
to its angle of incidence. The color of the light is not changed by diffraction. However, carefully manufactured metallic coated
blue light diffracts by smaller angles than red light, so that a ray Pattern in photoresist
Normal grating can have a diffraction efficiency
Inc y of white light (blending many colors) will emerge from a grating
ide ra that exceeds 95% over a broad range of
d with each constituent color diffracted into a different direction.
nt te
ra
y f lec The resulting display is the familiar rainbow of colors. The wavelengths (that is, more than 95% of
Re the light is returned from the surface as
periodic grooves on the surface of a compact disk provide an
everyday example of this phenomenon. diffraction into a single order). The
The color and direction of a light ray incident on a grating, behavior of a grating is primarily
together with the spacing of the periodic grooves, determines how governed by the spacing and the shape
many diffracted orders are present. If the grooves are far apart, Pattern after transfer etching
Mirror of the grooves as well as by the optical
then one color may emerge into many diffraction orders. If the properties of the metal. Gold, silver,
When the surface is smooth and transparent, such as a grooves are closely spaced, as they are for our gratings, then only and aluminum are typically used as
windowpane, most of the light will pass through. However, two orders occur: one emerges from the grating as it would from a coatings.
because wavefronts travel more slowly in glass than in air, the mirror (the zero order), and the other order is retroreflected back
Because metallic diffraction gratings
rays of obliquely incident light will become bent as they enter along the incident direction (order –1).
the glass.
owe their highly reflective surface to
the high conductivity of the metal, and Figure 3. Schematic comparison of grating groove
the conductivity is relatively insensitive profiles before and after development of photoresist.
nt
e f ro to wavelength, metallic gratings are
W av 0 order
+1 order inherently broadband devices. However,
Air they have a low threshold for optical researchers at LLNL undertook to specified thickness; the uniform coating
damage; heating of conduction electrons develop a grating design by the addition of a photosensitive layer; the creation
renders the surface susceptible to damage of a grating to a multilayer dielectric of a very precise interference pattern in
–1 order at fluences of around 0.8 joule per square stack. this layer; and the transfer of the latent
Glass
centimeter (J/cm2) for nanosecond laser Whereas metallic gratings achieve image into a permanent corrugation
pulses in the infrared and at much lower high reflectivity as a consequence of pattern in a dielectric layer. The transfer
fluences for shorter wavelengths or conduction electrons, a multilayer etching may be performed by a host of
shorter pulses. Metallic diffraction dielectric stack of alternating high and standard lithographic techniques,
The refractive index of the material—the ratio of the speed of gratings have therefore been a limiting low refractive index layers achieves including conventional wet etching, ion
light in a vacuum to the speed in the material—determines how factor in the production of short, high- reflectivity by interference: light is sputter etching, and reactive ion etching.
–2 order
much bending (refraction) occurs. energy pulses. reflected from the succession of surfaces The choice of technique is governed by
Light also bends when it scatters from the edge of a small whose separation is designed so that the choice of dielectric material and the
object; this is diffraction. When the light has a narrow range of Making Insulators Reflect waves traveling forward and backward desired groove spacing and depth.
colors, the scattered waves will interfere. will destructively interfere. Each interface
In some directions, wave crests will overlap and reinforce one Because transparent dielectric between a high-index and low-index Demonstrating the New Concept
another; this constructive interference will enhance the intensity of materials are insulators, they lack the pair has approximately 4% reflection, In 1992, LLNL teamed with Hughes
the light. In other directions, crests will match troughs, canceling
conduction electrons that make metals but the aggregate of many layers can Electrooptic Systems of El Segundo,
one another as destructive interference and producing a dark region.
reflecting. As a result, transparent approach complete reflection of light California, to demonstrate these
When a collimated beam (a ray) of single-color light strikes a grating, the dielectrics have intrinsically higher (over a range of wavelengths). multilayer dielectric gratings. Computer
periodic grooves scatter the light into all angles. At certain angles, the thresholds for laser-induced damage The manufacture of high-efficiency modeling provided target designs.
outgoing waves add constructively to create an outgoing beam (a than do metals. Moreover, multiple multilayer dielectric gratings draws on These computations included examining
+ = + = diffraction order). The outgoing wave has the same color as the incoming layers of different dielectrics have several subsidiary technologies in optics how a variety of groove shapes and
wave. If the incoming beam contains two or more colors, the outgoing
long been known to produce highly and chemistry and requires careful control multilayer properties affect efficiency;
angles differ for the different colors, and a multicolor beam is separated
reflecting structures. In 1991, reasoning of a number of steps: the creation of
into single-color outgoing beams.
Reinforcement Cancellation
from this well-established fact, a stack of dielectric films, each of a

Science & Technology Review September 1995 Science & Technology Review September 1995
30 Multilayer Dielectric Gratings Multilayer Dielectric Gratings 31

in Figure 4b. This method allows the with conventional metallic or bulk
multilayer stack to be designed either dielectric transmission gratings, allows
with or without the grating in place. a narrow spectral region to be selected
The grating structure is constructed in a to the exclusion of all others (see
separate dielectric layer, of the necessary Figure 5).
(a) (b) thickness, which is deposited on top of • The damage threshold of our

the multilayer. The grating pattern can dielectric gratings for 3000 picoseconds
be achieved by depositing the dielectric (ps) laser pulses has been measured to
through a holographically produced exceed 5 J/cm2, nearly ten times that
mask, in photosensitive material directly, of the best metallic gratings. For short
or by other lithographic techniques (e.g., pulses (0.1 ps), our standard multilayer
lift off). As in the previous case, the dielectric gratings exhibit a damage
grating shown in Figure 4b was designed threshold of approximately 0.6 J/cm2,
to provide high diffraction efficiency at three times higher than the short-pulse
1053 nm. It was fabricated in prepared damage threshold of commercially
photoresist on a conventional hafnium available metallic gratings. Further
oxide/silicon oxide multilayer high refinement of our multilayer design is
reflector; it produced a diffraction expected to increase the short-pulse
efficiency of more than 96%. damage threshold to more than
1 J/cm2. A grating that can withstand
Novel Grating Properties these high powers is essential to realize
the compression of multikilojoule
Our primary motive for developing pulses that will be required for fast
multilayer dielectric gratings was to ignition of an inertial confinement
Figure 4. Scanning electron micrographs of two multilayer dielectric gratings: (a) one produced
enhance resistance to laser damage, fusion capsule.
by ion etching, and the other (b) by the secondary layer technique.
an objective that has been realized.
However, multilayer dielectric gratings Numerous Applications
have several novel properties that offer
for selected groove depths, diffraction achieved a measured efficiency unique opportunities for new applications: Either independently or in
efficiency exceeding 98% was exceeding 97%. • The efficiency of a multilayer dielectric combination, the unique capabilities of
predicted. We have continued the development grating can be adjusted for any given multilayer dielectric gratings allow
In 1993, for our first demonstration of these gratings with a range of design wavelength and polarization by altering new optical and laser products to be
of this new type of grating, we sought objectives, various dielectric films, and the phase retardation properties of the created. Manufactured in large size (a
Figure 5. A multilayer dielectric diffraction grating designed to reflect yellow light, diffract
extremely high efficiency in reflection gratings of larger size. multilayer stack, the depth and shape meter in diameter), these gratings are
broadband visible radiation (bottom left), eliminate all green and yellow light in the transmitted
for light with a wavelength of of the grating grooves, and the beam’s an enabling technology for the
diffracted beam (at right), and transmit blue–green light. The grating pictured is 15 ¥ 20 ¥ 2.5 cm.
1053 nanometers (1 nm = 10–9 m), the How the Gratings Are Made angle of incidence. We adjust these development of lasers with a petawatt
wavelength to be used in the 100-TW A dielectric grating can be fabricated properties during manufacture to control of peak power and for the application
and petawatt lasers. We created a directly into the topmost layer of a the distribution of energy among the of such high-energy lasers to inertial
dielectric stack of eight pairs of high- dielectric stack, as shown in Figure 4a. reflected, transmitted, and diffracted confinement fusion. Lasers that Multilayer dielectric diffraction develop high-power, tunable, narrow-
and low-refractive-index materials This fabrication technique requires ion beams. Diffraction efficiency for specific provide a petawatt of power in a gratings have many commercial linewidth lasers using broadband solid-
deposited on borosilicate glass. Each etching to transfer the grating pattern incident radiation can be adjusted picosecond may make it possible to applications. The high diffraction state materials with high-density energy
layer pair was designed to provide high into the dielectric multilayer. It requires between 0.01% and 98%. achieve fusion using significantly less efficiency will find immediate use in storage, such as alexandrite, titanium-
reflectivity. The grating structure was that the other layers in the stack be • The wavelength discrimination inherent laser energy than currently envisioned. commercial laser systems employing doped sapphire, and neodymium-doped
transferred into the multilayer by a designed for use with the grating layer in a multilayer stack makes it possible Useful fusion power could then be gratings for pulse compression, and the glass. Compact lasers can now be made
multistep ion etching technique. Our under consideration, since that layer is to build gratings that transmit or reflect achieved perhaps years earlier than increased damage threshold will permit that have high pulse energies and
computations indicated that grooves part of the stack. light with high efficiency within a narrow would otherwise be possible. (See p. 36 the size of the pulse compressor to fall
etched to the appropriate depth in the An alternative method, called the optical wavelength band. A grating can for a discussion of how the application below that of current metallic gratings.
topmost, high-index layer should have secondary layer technique, is to be designed to have nearly any desired of the 100-TW laser to the fast ignitor The gratings’ combination of high
an efficiency of 98%. The actual grating fabricate a grating in a dielectric layer efficiency and bandwidth. This extreme concept could accelerate our efficiency and high damage threshold
to be placed on top of an independently optical selectivity, which is not possible development of laser fusion.) for long pulses will make it possible to
designed multilayer structure, as shown

Science & Technology Review September 1995 Science & Technology Review September 1995
32 Multilayer Dielectric Gratings Multilayer Dielectric Gratings 33

narrow linewidth outputs that are unwanted radiation from laser weapons Key Words: chirped pulse amplification; (May 1994). (UCRL-JC-116985)
tunable over the gain bandwidth of the or laser guidance systems. Finally, dielectrics; diffraction gratings–metallic, B. C. Stuart, S. Herman, and M. D. Perry,
laser material. This new type of grating because the distribution of energy multilayer dielectric; petawatt laser; short- “Chirped-Pulse Amplification in
will also extend high-efficiency among the spectrally reflected, pulse laser. Ti:Sapphire beyond 1 µm,” IEEE
Journal of Quantum Electronics 31 (3),
diffracting structures into the ultraviolet transmitted, and diffracted beams is
References and Bibliography 528–538 (March 1995).
region (to wavelengths below 220 nm) controllable by adjusting the design of
1. P. Maine et al., “Generation of Ultrahigh
where the reflectivity of metallic the multilayer and grating structure, we Peak Power Pulses by Charged Pulse
coatings drops precipitously. can use multilayer dielectric gratings as Amplification,” IEEE Journal of
Because multilayer dielectric selective beam splitters in optical Quantium Electronics 24, 398–403 (1988).
gratings can be designed with an switches and distribution systems. For further information contact
The following references contain detailed
arbitrary bandwidth to reflect some information about multilayer dielectric
Michael Perry (510) 423-4915
frequency components, transmit others, Summary gratings as well as extensive (perry1@llnl.gov), Bruce Shore
and diffract still others in either bibliographies related to the subject: (510) 422-6204 (bshore@llnl.gov),
reflection, transmission, or both, it is Our development of high-efficiency Michael D. Perry and Gerard Mourou, Robert Boyd (510) 422-6224
possible to select a narrow spectral multilayer dielectric gratings is a “Terawatt to Petawatt Subpicosecond (boyd5@llnl.gov), or Jerry Britten
region with the grating while technical innovation that opens the door Lasers,” Science 264, 917–924 (510) 423-7653 (britten1@llnl.gov).
discriminating against all others. Such to a host of new products and makes
sensitivity will find immediate use in metallic gratings obsolete in many
high-contrast spectrometers, where current applications. Its significance lies About the Scientist
discrimination often must be one part in the versatility of the device. By
per million and is currently achieved proper design, we can obtain a grating
MICHAEL PERRY joined Lawrence Livermore National
only by the use of multiple conventional of almost any efficiency and bandwidth.
Laboratory as a physicist in October 1987. He is a graduate of the
gratings. For laser applications, the nearly tenfold
University of California at Berkeley with a B.S. in both nuclear
Because of their spectral selectivity, increase in the optical damage threshold
engineering and chemical engineering (Summa Cum Laude,
these multilayer dielectric gratings are for long pulses over metallic gratings
1983), an M.S. in nuclear engineering (1984), and a Ph.D. in
discrimination filters. Specifically, we enables their use in high-power laser
nuclear engineering/physics (1987). He is currently the project
designed the gratings to reflect systems. These unique features, either
leader for the Petawatt Laser Project at the Laboratory and Group
undesirable narrow-line optical independently or in combination, make
Leader of the Short-Pulse Laser and Diffractive Optics groups. He is the author or
radiation (e.g., laser radiation) possible the development of a new class
coauthor of more than 70 professional publications.
while transmitting most other of optical products.
frequencies. As a result, they We have demonstrated that such
have a potential military multilayer dielectric gratings can be
application: on the produced, that they can reflect selected
High energy pu
battlefield, multilayer wavelength bands with high efficiency,
gratings could transmit and that they can be made in large sizes
visible radiation and while maintaining high quality
diffract (with high wavefronts. Manufactured in small size,
efficiency) these gratings can be used to create
lasers with narrow linewidth and high
pulse energy for such uses as directional
beam splitters and efficient narrow- or
broad-band filters.

Science & Technology Review September 1995 Science & Technology Review September 1995