Fairy Lights in Femtoseconds: Aerial and Volumetric Graphics Rendered by
Focused Femtosecond Laser Combined with Computational Holographic Fields
Yoichi Ochiai1∗ Kota Kumagai2∗
Takayuki Hoshi3
Jun Rekimoto4
Satoshi Hasegawa2
Yoshio Hayasaki2
12
3
4
University of Tsukuba
Utsunomiya University
Nagoya Institute of Technology
The University of Tokyo

Figure 1: Application images of Fairy Lights in Femtoseconds, aerial and volumetric graphics in air rendered by femtosecond lasers. (a)A “fairy” flying in front of a finger. (b) A “sprout” coming out from a seed. (c) Interference between a point cloud and a finger. (d) The
SIGGRAPH logo.

Abstract

1 Introduction

We present a method of rendering aerial and volumetric graphics
using femtosecond lasers. A high-intensity laser excites a physical matter to emit light at an arbitrary 3D position. Popular applications can then be explored especially since plasma induced bya femtosecond laser is quieter and safer than that generated by ananosecond laser. There are two methods of rendering graphics
with a femtosecond laser in air: Producing holograms using spatial light modulation technology, and scanning of a laser beam by agalvano mirror. The holograms and workspace of the system proposed here occupy a volume of up to 1 cm3 ; however, this sizeis scalable depending on the optical devices and their setup. This
paper provides details of the principles, system setup, and experimental evaluation, and discussions on scalability, design space, andapplications of this system. We tested three laser sources: 30-fs
and 100-fs lasers which project up to 1,000 pulses per second at anenergy up to 7 mJ per pulse, and a 269-fs laser which projects up
to 200,000 pulses per second at an energy up to 50 µJ per pulse.
We confirmed that the spatiotemporal resolution of volumetric displays, implemented with these laser sources, is 1,000 and 200,000
dots per second. Although we focus on laser-induced plasma in
air, the discussion presented here is also applicable to other rendering principles such as fluorescence and microbubble in solid/liquidmateirals.

Three-dimensional (3D) displays have attracted great attention overthe past five decades. 3D virtual objects were originally displayedwith a head-mounted display in [Sutherland 1968]. Since then, continuous efforts have been made to explore 3D displays that haveplanar surfaces, and several methods have been developed to provide stereopsis for binocular vision [Benzie et al. 2007]. The technologies that employ glasses to achieve this are based on suchas anaglyphs, time-division, and polarization. On the other hand,those technologies that do not rely on glasses are based on such asparallax barrier and lenticular lens array. Although these methods
can offer effective 3D images, they require calculation and generation of precise images for multiple viewpoints, and users have tostay within a limited view angle.

CR Categories: H.5 [Information interfaces and presentation];
Keywords: Volumetric display, Laser plasma, Femtosecond laser,
Aerial interaction, Touchable aerial images∗ Joint first authors

A different approach to realizing advanced 3D displays is using aphysical 3D space to render graphics instead of a planar surface.
forms a visual representation of an object in three physical dimensions, as opposed to the planar image of traditional screens thatsimulate depth through a number of different visual effects. These
3D displays, which are called volumetric displays, allow users toview the displayed images from any angle. Volumetric displays arrange “voxels” in a 3D space. They are divided into two categories
by the characteristics of the voxels: emitting or reflecting light. The
voxels emitting light may be LEDs [Clar 2008], end points of optical fibers [Willis et al. 2012], or laser-induced plasma [Kimura
et al. 2006]. Those reflecting projected light may take the form offog [Rakkolainen et al. 2005a], water drops [Barnum et al. 2010],
or floating small particles [Ochiai et al. 2014]. In this study, we
focus on laser-induced plasma.
Laser-induced plasma has the following advantages. First, it does
not require physical matter arranged and suspended in air to emit
light. Second, it does not require wires and structures that possibly
obstruct the line-of-sight because power is transmitted wirelessly.
Third, the laser can be precisely controlled owing to the progress in
optical technologies.
We envision a laser-induced plasma technology in general applications for public use. If laser-induced plasma aerial images weremade available, many useful applications such as augmented reality(AR), aerial user interfaces, volumetric images could be produced(Figure 2). This would be a highly effective display for the ex-Figure 2: These figures show the example applications of proposed laser-based graphics technology. (a) Images superposed on a hand anda box. (b) Floating button with haptic feedback. (c-d) Volumetric images rendered in open and closed areas.

pression of three-dimensional information. Volumetric expression
has considerable merit because the content scale corresponds to thehuman body; therefore, this technology could be usefully applied
to wearable materials and spatial user interactions. Further, laser
focusing technology adds an additional dimension to conventional
projection technologies, which offer surface mapping while laser
focusing technology is capable of volumetric mapping. Thus, thistechnology can be effectively used in real-world-oriented user interfaces.
Plasma-based 3D displays were previously developed using ananosecond laser [Kimura et al. 2006] and femtosecond (100fs)
laser [Saito et al. 2008]. These studies on laser-prasma graphics
were pioneering but still incompleted. Our motivation is to expand
their achievements and provide complete discussion on this laserplasma graphics technology.
In this study, we use femtosecond lasers with pulse durations of
30 fs, 100 fs, and 269 fs. This leads to safer plasma generation
than nanosecond lasers, which can be incorporated into our daily
lives. The design space and possible scenarios of the plasma-based
3D display are discussed. In addition, we use an optical device,called the spatial light modulator (SLM), to modify the phase oflight rays and produce various spatial distributions of light basedon interference.
The remaining sections of this paper discuss the following: First,
we describe the principles and design parameters of femtosecondlaser-based volumetric displays. We explore a safe, high-resolution,
wide-variety laser-based volumetric display using a femtosecond
laser and an SLM. Second, we introduce the setup we designed.
Third, we give examples of applications. Finally, we conduct experiments on generation, safety, and control of lasers. We also discuss
the limitations and estimate scalability. We believe that this study
fills the gaps in design space of plasma-based 3D displays that wereleft unresolved by previous studies.

2

Related work

In this section, first, we survey conventional studies on volumetric displays and divide them into four categories: Non-positioncontrol/position-control and reflection/emission. Note that someof them are not 3D but 2.5D, and position-control in 2.5D means
surface deformation. Our study is associated with the positioncontrol and emission category (Figure 3). Next, previous studies
on plasma-based 3D display are described, and the issues that havenot been discussed are pointed out. Last, studies on aerial interaction are cited, and an additional property of our study is clarified.

Figure 3: A map of related work divided into four categories regarding to non-position-control/position control and reflection/emission. This study falls into the position-control andemission category.

2.1 Volumetric displays
2.1.1 Non-position-control types

Reflection: In this category, the work space is filled with small objects of a material that can passively reflect projected or environmental light. 3D displays based on mechanical motion of mirror or
screen are discussed in [Parker 1948]. A spinning mirror is usedwith a high-speed projector in [Jones et al. 2007], where different
images are projected onto the mirror according to its azimuthal angle to express a 360◦ light field of an object. Similarly, images
are projected onto a rotating screen [Favalora et al. 2002] and a rotating diffuser plate [Karnik et al. 2011]. The systems proposedin [Rakkolainen et al. 2005a; Lee et al. 2009] use fog as reflecting material. A thin layer of fog is generated and images are projected onto it. In [Eitoku et al. 2006], falling water drops are utilized as a screen. The lens-like property of water drops delivers
projected images to users’ eyes. Subsequently, multilayer waterdrop screens were implemented [Barnum et al. 2010] and different
images were projected onto different layers by synchronizing the
projector with the water valves. In DepthCube [Sullivan 2004], amulti-layered liquid crystal shutters is illuminated by a high-speed
projector. Photochromic materials are used in [Hashida et al. 2011]
to form a volumetric and multi-color display controlled by an ultraviolet projector. Holodust [Perlin and HAN 2006] illuminates
floating small particles by lasers. Small particles are launched into
air and illuminated by a projector in [Matoba et al. 2012].

Emission: In this category, objects occupying the work space actively emit light to show images. Clar [Clar 2008] created a 3D
cubic array of LEDs. In this setup, the LEDs are supported by aframework and the relative positions of them (i.e., voxels) are fixed.
Currently fabrication type 3D volume displays are explored [Willis
et al. 2012]. 3D print objects with embedded light paths that canbe used to display information when the objects are placed on aflat display. As objects get more complex, so the do the light paths
which makes it difficult to design the object that should be printed.
Pereira et al. [Pereira et al. 2014] solve the issue by algorithmicallycomputing the arrangement of the light paths so that their endings
form a desired surface shape, such as that of a face.
2.1.2

Position-control types

Reflection: In this category, the positions of reflection objects arecontrolled to render graphics. Studies focusing on controlling thesurface shape of a screen or display have also been pursued. For example, the deformable screen Project FEELEX [Iwata et al. 2001]
changes its surface shape by linear actuators. A deformable screen
inForm [Follmer et al. 2013] not only displays images on it but alsointeracts with objects. [Ochiai et al. 2013] used focused ultrasoundto deform a soap film, without making contact, to show a bump on
it. Pixie Dust [Ochiai et al. 2014] is a floating display consisting of
small particles that are suspended and moved by means of acoustic
levitation.
Emission: Light sources are moved to realize 3D displays in thiscategory. This type of volumetric displays were originally reported
in [Jansson and Berlin 1979]. Many types of volumetric displays
are explored for 35 years. [Grossman and Balakrishnan 2006] didgreat survey on this volumetric areas. [Macfarlane 1994] proposeda voxel-based spatial display. LUMEN [Poupyrev et al. 2004] comprises of LEDs attached to linear actuators and shows informationin the form of RGB (red-green-blue) and H (height). Laser plasma,
which is free from physical support and connection, is used as alight source in [Kimura et al. 2006]. We also work in this category
to use this advantage.

2.2

Laser-based volumetric displays

As mentioned before, laser-plasma 3D displays are categorized as
the position-control and emission type 3D display. Voxels in air are
generated by high-intensity lasers which are achieved by shortening
pulse duration (e.g. nanoseconds or shorter) under a limited total
power.
The basic concept was demonstrated using a nanosecond laser in[Kimura et al. 2006] where a rendering speed was 100 dot/sec.
Later, 1,000 dot/sec was achieved [Saito et al. 2008] by adoption ofa femtosecond (100 fs) laser1 . The color of voxels was bluish whitebecause of plasma emission. With the latter one, rendering algorithms of point cloud were discussed in [Ishikawa and Saito 2008a;
Ishikawa and Saito 2008b]. Although these studies on laser-prasma
graphics were pioneering, the detailed discussion on the light emission, design space, scalability, and so on was not provided in the
published papers. We provide the discussion on these issues in thispaper, and complete laser-based graphics in air from principles toapplications.
Laser-based 3D displays in materials other than air were also
demonstrated. An in-water type2 of laser-based volumetric display
was developed in [Kimura et al. 2011] where 50,000 dot/sec wasachieved. While no detailed principle was provided, we infer that
1 http://www.aist.go.jp/aist j/press release/pr2007/pr20070710/pr20070
710.html (in Japanese) (last accessed 20 Jan. 2015)
2 http://www.diginfo.tv/v/11-0231-r-en.php (last accessed 20 Jan. 2015)

this in-water type is not based on laser plasma but laser-induced
microbubbles. The green light dots generated by a green laser canbe explained as diffusion of the incident laser by the microbubbles. Fluorescenct materials were used in [Downing et al. 1996;
Hasegawa and Hayasaki 2013]. Pulse peak intensity required forthe microbubble- and fluorescence-based rendering, as experimentally confirmed in Section 5.3. This offers higher rendering speed
than the plasma-based rendering so that not a set of lines but a surface can be represented [Ishikawa et al. 2011].

2.3 Aerial interaction
Volumetric, aerial, and/or 3D displays are usually accompanied byinteraction with users’ hand. For example, users can directly interact with graphics rendered on a thin layer of fog [Rakkolainen et al.
2005b]. Touchable Holography [Hoshi et al. 2009] and RePro3D[Yoshida et al. 2010] show 2D and 3D images in air, respectively,
and also provide haptic feedback. Small particles are acousticallylevitated in [Ochiai et al. 2014] and users can touch them. ZeroN [Lee et al. 2011], although it is a tangible system rather than agraphic system, magnetically levitates a sphere and users can touch
and also handle it.
For aerial interaction, there are two necessary conditions on volumetric displays. They should be safe and accessible. The previous works based on lasers do not satisfy these conditions. The inair type [Kimura et al. 2006] is harmful to users’ hand because ofplenty of energy and the in-water type [Kimura et al. 2011] renders
images in a transparent container filled with water. In this paper, we
attempt to demonstrate safe and accessible laser-based volumetricdisplay.

2.4

Laser technologies

A.

2.5 Position of this study
The development of a volumetric display has two problems whichhave been encountered in conventional studies: how to suspend andemit voxels. The application of laser plasma technology to a volumetric display overcomes these two issues, because laser plasma
can generate an emission point at an arbitrary position in a 3D
space. In addition, studies on conventional laser volumetric displays have not sifficiently discussed theoretical principles and scalability.
This study focuses on a system for rendering volumetric graphics in air using a femtosecond laser. An ultra-fast femtosecond
laser source and SLMs are used in our system, which allows us to
explore multi-access possibilities, computer-generated holograms,
Table 1: Comparison between the previous and this studies.
Kimura 2006 Saito 2008
This study
Air
Air
Air
Nanosecond Femtosecond
Femtosecond
100fs
A: 30-100fs, 1kHz, 2mJ
B: 269fs, 200kHz, 50µJ
Dot/sec100
1,000
A: 1,000
B: 200,000
Hologram
No
No
Yes
LCOS-SLM or LCSLM
Interaction
No
No
Yes
Touch
No
No
Yes
MediumLaser

3.2 Laser filamentation
An emission dot generated by a high-intensity laser has a tail along
the propagation direction. This tail is generated as the self-focusing
behavior, due to the optical Kerr effect, competes with the natural diffraction of the laser beam; however, this effect is undesirable
when rendering 3D graphics in air. Practically, this effect is invisible to the human eye because the light from the focal point isrelatively much brighter, but might be taken into consideration insome special cases.

3.3 Voxel sizes

Figure 4: Laser plasma induced by focused femtosecond laser.

near-field laser plasma displays, and touch interaction. These explorations and evaluations are useful as regards discussion of thescalability and application space of a plasma-based volumetric display using a high-intensity laser for general, wide-spread application.

We assume that the size of an emission dot (i.e., a voxel) is equal to
the size of the focal point of the laser. The focal point is usually an
oval that has two diameters. One is the diameter perpendicular tothe laser beam, wf , which is the diffraction limit and determined by
the original beam width, a, the focal length, r, and the wavelength,λ, such thatr
wf = 2λ .
a(1)In this section, we show how to generate light spots by lasers.

The other is the diameter along the laser beam, wd , which is geometrically obtained from the relationship a : wf = r : w2d , such
that
r
(2)
wd = 4λ( )2 .
a

3.1

3.4 Computational phase modulation3

Principles

Laser-induced light spot

There are three types of laser-induced effects that produce lightspots, and fluorescence is one among them. First, an orbital electron in a molecule or atom is excited when the atom absorbs one
or more photons. Next, a new photon is emitted when the electron
relaxes. If two photons are absorbed at the same time, the wavelength of the emitted photon is half of that of the original photons.
The wavelength required to excite an electron is dependent upon thetype of fluorescent material. The emitted light has N times shorter
wavelength when N photons are absorbed simultaneously. This effect occurs with a relatively low-intensity laser (an energy of nJ to
mJ is sufficient).
Cavitation is another effect that plays a key role. Microbubbles are
generated at the focal point of a laser in a liquid medium. This localized cluster of microbubbles diffuses the incident laser such thatthe laser is seen as a point light. The color of this point light depends directly on the wavelength of the incident laser. This fact indicates that RGB images can be expressed by using multiple lasersof different wavelengths. This effect is material-independent andrequires an intense laser to generate microbubbles.
The last effect is ionization. In particular, tunnel ionization can
produce sufficiently visible light, which dominantly occurs when
the laser intensity is greater than 1014 W/cm2 [Keldysh 1965]. The
potential well of a molecule or atom is deformed by the electricfield of the high-intensity laser to have a potential barrier, and then,an electron has the opportunity to leave the atom (i.e., ionization)
based on the tunnel effect. It is known that higher laser intensity
leads to higher tunnel-ionization probability; that is, more electrons
are ionized [Ammosov et al. 1986]. The ionized electron is recombined with the atom after a half cycle and a photon is emitted. Thiseffect is called laser breakdown. The emitted light looks bluishwhite.
In this study, we focus on the third effect, i.e. ionization, because it
can be achieved in air (Figure 4).

The use of SLMs is one method to render holograms. In general,an SLM has an array of computer-controlled pixels that modulate alaser beam’s intensities, phases, or both.
A liquid crystal SLM (LCSLM) is used in this study, which contains
a nematic liquid crystal layer. The molecule directions within thislayer are controlled by electrodes, i.e., pixels, and the phase of light
ray reflected by each pixel is modulated according to the directionof the liquid crystal molecule. In other words, this device acts as an
optical phased array.
The spatial phase control of light enables the control of focusing
position along both the lateral (XY) and axial (Z) directions. A
complex amplitude (CA) of the reconstruction from the computergenerated hologram (CGH) Ur is given by the Fourier transform ofthat of a designed CGH patternn Uh :ZZ

Ur (νx , νy )Uh (x, y)=Uh (x, y) exp[−i2π(xνx + yνy )]dxdy=ar (νx , νy ) exp[iφr (νx , νy )](3)=ah (x, y) exp[iφh (x, y)](4)where ah and φh are the amplitude and phase of the hologram plane
displayed on the SLM, respectively. In the experiment, ah is constant because an irradiation light to the CGH is considered as theplane wave with a uniform intensity distribution. φh is designed by
ORA algorithm. On the other hand, ar and φr are the amplitude andphase of the reconstruction plane, respectively. The spatial intensity
distribution of reconstruction is actually observed as |Ur |2 = a2r .
In the control of focusing position along the lateral (XY) direction,
the CGH is designed based on a superposition of CAs of blazed
gratings with variety of azimuth angles. If the reconstruction has
N -multiple focusing spots, CGH includes N -blazed gratings. In
the control of focusing position along the axial (Z) direction, a

2

2

+y
with a focal length
phase Fresnel lens pattern φp (x, y) = k x 2f
2π
f is simply added to φh , where k = λ is a wave number. In thiscase, the spatial resolution of the SLM determines the minimum
focal length, following the theory discussed in Section 3.3.

ORA method is an optimization algorithm to obtain the reconstruction of CGH composed of spot array with a uniform intensity (Figure 5). It is based on adding an adequate phase variation calculatedby an iterative optimization process into the CGH. In the i-th itera(i)tive process, amplitude ah and phase φh at a pixel h on the CGH(i)plane, and a complex amplitude (CA) Ur at a pixel r corresponding to focusing position on the reconstruction plane are describedin the computer as follows,
Ur(i)

=

ωr(i)

X

The theoretical rendering limit is 33 dots/s for 30 frames/s, because
the femtosecond laser is pulsed at a frequency of 1 kHz. The SLM
is used to render additional dots in a single frame, while the galvano
mirror is used primarily for positioning the rendered holograms.

3.6 Spatiotemporal resolution
The number of dots per frame (dpf) is a parameter that must be
evaluated for laser-based volumetric displays. We now assume thedots are displayed in darkness; therefore, the minimum requiredenergy for each dot is equal to the laser breakdown threshold, Elbd .
The total output energy, Etot , is divided among the dots by the
SLM. The number of dots per laser pulse, Ndot , is expressed as

(i)

uhr

Ndot =

h

=

ωr(i)

X

(i)

ah exp[i(φhr + φh )],(5)hwhere uhr is CA contributed from a pixel h on the CGH plane toa pixel r on the reconstruction plane, φhr is a phase contributed by(i)the light propagation from a pixel h to a pixel r, ωr is a weight
coefficient to control the light intensity at pixel r. In order to maxP(i)imize a sum of the light intensity r |Ur |2 at each pixel r, the(i)(i)
phase variation ∆φh added to φh at pixel h is calculated using
flowing equations.
(i)

∆φh

=

S1

=

tan−1 (
X

S2
),
S1

(6)
(i)

ωr(i) ah cos(φr − φhr − φh ),

(7)

(i)
ωr(i) ah sin(φr − φhr − φh ),

(8)

r

S2

=

X
r

where ωr is the phase at pixel r on the reconstruction plane. The(i)phase of CGH φh [(i) is updated by calculated ∆φh as follows.
(i)

(i−1)

φh = φh(i)+ ∆φh ,

(9)(i)

Furthermore, ωr is also updated according to the light intensity of
the reconstruction obtained by the Fourier transform of Eq. (9) inorder to control the light intensity at pixel r on the reconstruction
plane.
ωr(i) = ωr(i−1) (
(i)

(d)

Ir

(i)

Ir

)α

(10)

(i)

where Ir = |Ur |2 is the light intensity at pixel r on the recon(d)struction plane in the i-th iterative process, Ir is an desired light(i)intensity, and α is constant. The phase variation ∆φh is optimized(i)by the above iterative process (Eqs. (6)-(10)) until Ir is nearly(d)equal to Ir . Consequently, ORA method allows us to design the
CGH with the high quality.

3.5

Etot
.
Elbd

(11)

The number of dots per frame is determined by Ndot , the repeat
frequency, Frep , of the laser pulses, and the frame time, Tf , which
is determined based on the persistence of human vision. Hence,
dpf = Ndot × Frep × Tf .

(12)

For example, if Ndot = 100, Frep = 1 kHz, and Tf = 100 ms, ananimation of 10,000 dpf is played in 10 fps. Note that, in practice,
the number of dots per frame is determined by the bottleneck of thetime response of the galvano mirrors and/or the SLM, instead of by
Frep .

4 Implementation
In this section, we show our system implementation. First, we introduce an overview of our system. Next, we describe our light source,
optical circuit (i.e., arrangement of optical devices), 3D scanningsystem, SLM, and the control system.

4.1 Overview
Figure 6 shows the system configuration of our basic setup. Thissystem aims to produce a parallel optical access volumetric display,and consists of a volumetric display medium that is excited by laser
light, a femtosecond laser source, an XYZ scanner (galvano scanner+ varifocal lens), and a liquid crystal on silicon SLM (LCOS-SLM)
displaying a CGH for parallel access. Our system was tested andinvestigated at 20.5 ◦ C. The atmosphere was ordinary air (80% N2
and 20% 02 ).
The setup was tested using three light sources (A and B), the specifications of which are given below. We primarily used a femtosecond laser source developed by Coherent Co., Ltd., which has a center wavelength of 800 nm, a repetition frequency of 1 kHz, andpulse energy in the 1 to 2-mJ range. The specifications of the laser

Graphics positioning

The galvano mirror used in this study covers an area of 10 × 10mm2 . Besides, the SLM also renders graphics within the approximately same area. This means that we have two options to placea point at an intended position: One is leading a laser there by thegalvano mirrors and the other is modifying the spatial distributionof the laser by the SLM. The conditions and/or response times ofthese deviced determine which is suitable.

Figure 5: Example of a computer-generated hologram (CGH). (a)An original image, (b) a converted spot-array image of the original
image, and (c) a CGH to be displayed on the SLM.

Figure 6: Setup of our light circuit. The host computer controls (2) the SLM for hologram generation, (5) the galvano scanner for XY control,and (9) the varifocal lens for Z control.
els, a pixel size of 20 × 20 µm2 , and a response time of 100 ms.
The specifications of each component are shown in Tables 3 and 4.
In addition to these components, we employ a microscope for monitoring and recording which is connected to the computer via USB.

4.2 Light source

Figure 7: Relationship between the XYZ-coordinate and the focused laser beam. Voxels are rendered above the objective lens.

sources are shown in Table 2. Figure 7 shows example results forour system in the air.
The galvano mirror scans the emission dot along the lateral directions (X- and Y-scanning), while the varifocal lens can vary its focal
point in the axial direction (Z-scanning). The Fourier CGH is used
for parallel optical access [Hayasaki et al. 2005]. The CGH, designed with an optimal-rotation-angle (ORA) method [Bengtsson
1994], is displayed on the LCOS-SLM, which has 768 × 768 pixTable 2: Specifications of laser sources.
SystemMaker
Pulse duration
Repeat cycle
Energy/pulse
Dots/sec
Average powerA
Coherent
30-100 fs
1 kHz
2 mJ
1,000
2W

B
IMRA
269 fs200 kHz
50µJ
200,000
10 W

We utilize several light sources. The light source that is primarily
used for evaluation and application was developed by Coherent Co.,Ltd and has a center wavelength of 800 nm, repetition frequency of
1 kHz, pulse energy of up to 2 mJ, and the pulse width is adjustablefrom 30 to 100 fs. Figure 8 shows the spectra and pulse intensitiesof the 30- and 100-fs settings with this light source. If the averagelaser pulse energy is unchanged, the peak intensity differs according to pulse width. In fact, the 30-fs pulse width has a three-foldgreater peak intensity than the 100-fs pulse width at the same average energy. We refer to the system using this light source as either
System A (30 fs) or System A (100 fs), depending on the pulse
width.
The other light source we used is the FCPA µJewel DE1050 from
IMRA America, Inc. The laser has a center wavelength of 1045 nm,
repetition frequency of 200 kHz, pulse energy of up to 50 µJ andpulse width of 269 fs. We refer to the system with this light source
as System B. Note that the peak intensity of laser is important toTable 3: Specifications of galvano mirrors.
SystemMaker
Scan angle
Error
Resolution

A
Canon±0.17 rad< 5 µrad20 bitB
Intelliscan±0.35 rad< 5 mrad20 bitproduce the aerial plasma, rather than the pulse width. Both of
Systems A and B have sufficient peak intensity to excite the air andgenerate emission dot.

4.3

Optical circuit

Here, we describe our optical circuit following the path of the laser.
Figure 6 shows the optical setup of System A. The laser is generated by the femtosecond light source and then phase-modulated by
the SLM. The SLM energy conversion rate is 65 to 95%. Then, thebeam spot is varied by two lenses (F = 450 and 150 mm). Throughthis two-lens unit, the beam spot is reduced by a factor of 1/3. It
is then reflected by the galvano mirror, which determines the XYposition of the light. The galvano and SLM are connected in an
object-image correspondence. Subsequently, the beam spot is adjusted by two lenses (F = 100, 150 mm); this two-lens unit magnifies the beam spot 1.5-fold. Then, the light enters the varifocal
lens. The varifocal lens and galvano mirror are connected in an
object-image correspondence and the former adjusts the z-axis focal points. The light enters the objective lens (F = 40 mm). Onceit exits this lens, it excites the display medium (air). The energy
conversion rate of System A is 53%.
System B has the same structure but lacks a SLM. Also, the lens sets
are slightly different from those of System A. Specifically, System
B has no lens before the galvano mirror, as the varifocal lens is
positioned after the galvano mirror. Then, the beam spot is adjusted
by the two-lens unit (F = 50, 80 mm). An F20 objective lens isemployed and System B’s total energy conversion rate is 80%.

4.4

3D scanning system

In this subsection, we describe our scanning system in detail. Figure 6 shows the galvano and varifocal lenses. We employ galvano
mirrors to scan the lateral directions (X- and Y-scanning), while avarifocal lens can change its focal point in the beam axial direction(Z-scanning). For system A, we utilize a Canon GH-315 driven by
GB-501 as the galvano mirror and for System B we employ an Intelliscan 20i to scan the beams. Both are connected by PCI boards.
Table 3 shows the specifications of each of the galvano mirrors.
We employ an Optotune EL-10-30 for both Systems A and B as thevarifocal lens, which is connected via USB serial to a PC. The specifications of the varifocal lens are shown in Table 4. These devices
are operated by original applications coded in C++.

4.5

LCSLM

The LCSLM (Hamamatsu, PPM) is a parallel-aligned nematic liquid crystal spatial light modulator (PAL-SLM) coupled with a liquid crystal display (LCD) and a 680-nm laser diode (LD). This
device, which can perform phase-only modulation of more than 2radian, is frequently used to display real-time CGHs. The PALSLM is composed of a liquid crystal (LC) layer, a dielectric mirror,
and an optically addressed photoconductive (PC) layer containing
amorphous silicon, which are sandwiched between two transparent
indium tin oxide electrodes. The LC molecules are aligned in par-allel. When incident light illuminates the PC layer, the impedanceof this layer decreases and the electric field across the LC layer increases accordingly. With this increased field, the LC molecules
become tilted in the propagation direction of the readout light andthe effective refractive index of the LC layer decreases. Purephase modulation occurs only when the polarization direction of
the femtosecond laser is parallel to the aligned direction of the LC
molecules. The CGH pattern on the LCD illuminated by the LD isapplied to the PC layer through an imaging optics.

4.6 Control system
Figure 6 shows our system diagram. The system is controlled usinga Windows PC operating system, with all programs coded in C++.
The control system operates the SLM, galvano mirror, and varifocal
lenses. To monitor the interaction, a USB microscope is connectedto the system. The galvano and varifocal lenses run along different
threads and are synchronized when new draw patterns are input.
The user input is captured at 20 Hz and the SLM is connected tothe computer as an external display.

5 Experiments and Evaluations
In this section, we describe our experiments and system evaluation procedures. Firstly, we introduce an overview of our experimental plan and results. Then, we report the results of the following tests: Energy vs ionized plasma brightness, brightness vs pulse
peak, multi-access for aerial images, and skin damage. In the experiments, the brightness are measured as a summation of all the pixel
values within an close-up image of the plasma, which is a common
definition in the field of laser plasma.
We tested not only gas-ionized plasma, but also photon absorption
and cavitation, in order to compare the various energy consumption performances and the means of applying the femtosecond laser
system to the display technology. All experiments were conductedusing System A, which is described in Section 4.

5.1 Experiments overview
In this study, we aim to propose a femtosecond laser-based display
system design. In conventional studies, the requirements, scalability, and safety of such laser-based systems are not thoroughly discussed. There are several factors that we should explore. In Section
5.2, we examine the voxel brightness, which is important in relation
to the energy and display spatiotemporal resolution, as discussed in
Section 3. In Section 5.3, we explore the relationship between pulse
duration and brightness. This is important for scalability, particularly when a faster laser source is developed. In Section sec:multiaccess, we explore multi-access with SLM. This is important for
scalability in accordance with increasing spatiotemporal resolution.
Then, in Section 5.5, we examine safety issues and the effect of theplasma on skin. This is important as this technology is intended forwidespread, general use. Finally, in Section 5.6, we examine audible sound from the plasma generated by femtosecond lasers. Thisis important as this technology is intended to use in our daily lives.

5.2 Energy vs Brightness
Table 4: Specifications of varifocal lens.
SystemMaker
Aperture
Response time
Focal length

A and B
Optotune
10 mm< 2.5 ms
+45 to +120 mm

We conducted this experiment to evaluate the relationship between
the plasma-production energy level and the resultant brightness of
the image. In conventional studies, the minimum peak intensity
necessary to produce the ionized plasma is estimated. However,this experiment aimed to confirm the feasibility of our system andto investigate how it can be applied to display voxels and, thus theminimum peak intensity value was determined.

Figure 8: Spectra of 100-fs, 30-fs, and 269-fs lasers (from left to right). The rightmost is the peak intensity and the pulse width of eachfemtosecond lasers.

We conducted the experiments using System A (30 fs) and employed a microscope to capture the resultant image. With our setup,the laser source can provide power of up to 7 W, however, unwanted
breakdown occurs in the light path before the objective lens under
too high power. Hence, the full power of the laser source cannot be
used. Moreover, the energy capacity of our SLM is not guaranteed
over 2 W. The experiments were conducted for a power range of
0.05 to 1.00 W.
Figure 9 (30 fs) shows the experimental setup and results. The experiments were conducted under energies per pulse of 0.16 to 0.55
mJ. The 30-fs laser can produce plasma from 0.2-mJ pulse energy.
The cross-sectional area of the focal point is theoretically calculated
to be 2 × 10−7 cm2 . Then, the peak intensity is 36 PW/cm2 andsurely higher than the ionized plasma threshold (> 1 PW/cm2 ).

5.3(a) Setup.

Brightness vs pulse peak

The relationship between the pulse peak and the resultant image
brightness was also examined, as the peak intensity plays an important role in plasma generation. This experiment aimed to classify
systems of different pulse width in terms of display voxel brightness.
As previously, we conducted experiments using System A (30 and
100 fs). Pulses of 30 and 100 fs yield different spectra and peakenergies for the same average powers. Also, the 30-fs setting yields
a three-fold higher peak pulse. We employed the same microscopeto capture the image that was used in Section 5.1 and the results areshown in Figure 9. The experiments were conducted for a power
range of 0.05 to 1.00 W.

(b) Results.
Figure 9: Experimental setup and results on the brightness of lightemission in air induced by the 30-fs and 100-fs lasers.

As a result, it was found that a 100-fs laser can generate plasma
from 0.45-mJ pulse energy. Then, the peak intensity is 24 PW/cm2
and surely higher than the ionized plasma threshold (> 1 PW/cm2 ).
Besides, it is confirmed that the 30-fs pulse requires less energy
than the 100-fs pulse to produce plasma under the same average
power.
Additionally, we conducted other experiments comparing media
materials (air, water, and fluorescence solution). The results areshown in Figure 10. It shows that the values of required pulse energy are dramatically different depending on the media materials.

5.4

Multi-access for aerial image

One of the main contributions of this paper is the 3D femtosecondlaser combined scanning system with SLM. This enables multiaccess to voxels using CGHs. (Note that, in conventional systems[Kimura et al. 2006; Saito et al. 2008], multiple voxels were not

Figure 10: Experimental results on the brightness of light emission
in air, water, and fluorescence solution induced by the 30-fs laser.

generated simultaneously.) We conducted this particular experiment to explore the possibility of multi-access to produce voxels
from a single light source. Multi-access is important to increase thespatiotemporal resolution, as discussed in Section 3.4. The multiaccessed points is darker than a single point because the energy is
distributed among them. This experiment was designed to explore
the resolution scalability by combining SLM with scanning systems. By phase modulating using the SLM, a multi-focal beam canbe generated. Multi-access is available for both the lateral (X, Y)
and beam (Z) axes, by displaying appropriate holograms on a single SLM. For simplicity, we investigated multi-access for the lateralaxis. Again, the experiments were conducted using System A (30fs), and Figure 11 shows the results and the holographic images
used in the SLM. We employed the same microscope shown in Figure 9. We conducted experiments with 1 to 4 multi-accesses, from0.05 to 1.84 W. As mentioned above, the laser source can provide
up to 7-W power, however, some components in the optical circuit
before the objective lens generate an air breakdown at higher powers. Hence, we cannot use higher-intensity settings. As a result, we
can observe up to four parallel accesses, and the SLM diffraction
efficiency is found to be almost 50%. The results are reasonablebecause four parallel accesses produce 8-fold greater energy than asingle access system.

5.6

5.5

6.1.1 Spatial AR to real-world object

Skin damage

Another main contribution of this paper is estimating the safety of
femtosecond laser systems. Plasma has high energy and can be
dangerous to humans. However a femtosecond pulse is an ultrashort pulse laser, which is used for non-heat breaking for industrial
purposes. It is also used for ultra-short scale fabrication of submicrometer order. Thus, we supposed that such pulses may not bedangerous if they do not affect human macro-structure. In addition,
our display scans a 3D space very rapidly, therefore, it does not remain at specific points in space for long periods. On the other hand,
this plasma still poses dangers for the retina. However, we believethat the potential for general application still exists with appropriate
installation.
Therefore, we conducted this particular experiment to explore thedamage to skin structure caused by femtosecond plasma exposure.
We employed cow leather for these experiments, as a substitute forhuman skin.
The experiments were conducted using System A (30 fs and 1 W,
100 fs and 1 W) and the plasma exposure duration was varied between 50 and 6,000 ms. Figure 12 shows the results. It was foundthat the 30- and 100-fs pulses have almost the same effect on theskin. As we described previously, the 30-fs pulse has a three-foldgreater peak energy and can generate brighter voxels. However 50
ms includes 50 shots and there is almost no difference between the
30-fs and 100-fs results. In this experiment, the average power isthe factor determining the result. For exposure of under 2,000 ms(2,000 shots), only 100-µm-diameter holes appeared and there wasno heat damage to the skin. For a period of longer than 2,000 ms,
heat effects were observed around the holes.
We conducted a test with a continuous (non-pulsed) nanosecond
laser for comparison with this result. With the nanosecond laser,
the leather burned within 100 ms. This means that pulse duration,
repetition times, and energy are important factors affecting the levelof damage caused by the laser.
Hence, this laser is somewhat “safe” for use. Further, there are two
ways in which the laser can be used. One is as an ultra-short-pulse
laser, which is bright and has an average output that is not highly
intensive. The other is by increasing the scanning speed.

Noise levelA.

6 Applications
In this section, we describe potential applications of our systems.
We introduce a 3D aerial display system and interaction between
the system and users (Figure 2).

6.1 Aerial displays
In this subsection, we describe our aerial display using air tunnel
ionization. We developed our application for both Systems A and
B and the result can be seen in Figure 13. For Systems A and B,
the workspaces are 1 and 8 mm3 , respectively. These workspaces
are smaller than those of conventional studies, but their resolutions
are 10 to 200 times higher than conventional methods. The maximum spatiotemporal resolution is 4,000 point/s for System A and
200,000 point/s for System B, and the image frame rate is determined by the number of vertices used in the models.

This aerial display can be used with real-world objects, as shownin Figure 13. One of the merits of the spatial AR to real-world
object technique is the AR content is on the same scale as that of
the object that is overlapped. Also, this system was developed witha microscope, which can detect the object in the workspace andoverlap the contents when the plasma touches the object.
This has an advantage over conventional approaches in terms of correspondence to the 3D spatial position. In conventional AR technology, it is difficult to show content at arbitrary 3D positions. However, this technique is available with the technology presented here.
6.1.2 Aerial interaction with aerial content

Our system has the unique characteristic that the plasma is floatingin the air. It is a type of floating light and, when users touch it, thedifference in density between air and human skin causes a change
in the brightness of the light. Also, an infrared laser irradiates theworkspace and interacts with the human skin. Both these conditions
can be used to identify interaction between the plasma and a humanuser using cameras.
Figure 13 shows an example of these interactions. One possible
medium is touch interaction, which has some effect on the plasma
or causes changes in the content when touched by a user. The otheravoids interactions. For safety, when users touch the plasma or enter the workspace, the plasma voxels can respond to this stimulus
and avoid the user within a single frame (17 ms = 1/60 s). This issufficiently less than the dangerous exposure time (2,000 ms) determined in section 5.4.
6.1.3 Haptic Feedback on Aerial Images

Because of the shock waves generated by a plasma, when users
touch the plasma voxels, they can experience some haptic sensations from the aerial image. We have not investigated the haptic
characteristics of our plasma voxels thoroughly. However, a tactile sensation is generated. Also, an example application, “an aerial
check box,” was developed. Figure 13 shows the interaction between the user and the aerial image.

Figure 11: Experimental results on parallel access. One to four accesses were tested. The intensity is the normalized value of the summationof all the pixel values of the photos of the voxels, which is taken a evaluative value of brightness.

Figure 12: Experimental results on skin damage. Leather sheets were exposed to the 30-fs and 100-fs lasers and the irradiation time wascontrolled. The exposure longer than 2,000 ms burns the leather surface.

Figure 13: Results of aerial rendering. (Uppermost) The setups of systems A and B. (a) The SIGGRAPH logo. (b) a cylinder, (c) a “heart”
that is broken by touch, (d) a “fairy,” (e) “sprouts” coming out from seeds, (f) a light point that changes into a “jewel” in contact with a ring,(g) direct interaction between a light point and a finger.

7

Discussion

7.1

Laser-induced emission phenomena

There are two laser-induced emission phenomena other than plasma
emission: Fluorescence and diffusion by cavitation, as introducedin Section 3.1. Both can be applied to displays using the laser-andSLM system discussed in this paper. In this section, the differences
between the emission phenomena are explained.
The display medium is the key factor determining the potential interactions. While plasma is generated in air, fluorescence requires
fluorescent materials (ink, pigment, etc.) and cavitation requires a
fluid medium. The medium also determines the energy that is required to emit light. The order of the required energy decreases
from air (PW/cm2 ), water, to fluorescent materials (MW/cm2 ).

The available wavelengths also differ in these cases. The plasmacolor is wavelength-independent and, hence, it is reasonable to use
invisible wavelengths, e.g., infrared or ultraviolet. In the case offluorescence, multi-electron fluorescence is reasonable, in which
multiple photons are absorbed by molecules and a single photonwith shorter wavelength. Full-color rendering is possible by using
multiple fluorescent materials. This is acceptable because the invisible ultraviolet source leaves only the emission visible. On theother hand, when applying cavitation in water, a visible wavelength
should be used, because the incoming wavelength is diffused by the
microbubbles and observed. This feature leads to full-color rendering with multiple lasers of different colors.
The softness of the medium determines the possible forms of interaction. With aerial plasma in air, a user can insert their hand into the workspace and touch the plasma. This is also possible with

non-fluorescent/fluorescent liquid media. However, in the case of afluorescent solid medium, the 3D voxels cannot be touched directly.

7.2

Drawbacks and limitations

There are some disadvantages and limitations to our systems. As
previously explained, SLM is not resistant to strong laser sources
and, therefore, we cannot use full-range laser power to exhibit images in an aerial display. SLM technology is currently a popular
field because of the development of near light-field technology, andwe are optimistic that new kinds of SLM systems will solve thispower limitation.
In addition, the optical circuit should be developed and treated carefully. Because our system utilizes high-intensity lasers, ionization
can sometimes occur in the center of the optical circuit. This reduces the output energy and also destroys the optical components
themselves. Further, plasma generation is a non-linear phenomenon
and we must design the optical circuit carefully.
Also, the objective lens and aberration are additional disadvantagesof our systems. To handle the CGH and aerial plasma, we must
focus the light on the focal points. However, the objective lenscharacteristics determine the workspace while the size restricts thegalvano mirror workspace. In addition, high-speed variation of thevarifocal lens will cause aberration problems. The characteristics ofthe varifocal lens and those of mechanical lens movements should
therefore be compared.

7.3

Scalability

7.3.1

Size

Scalability in size of workspace is main concern of our project. Sizescalability depends on type of medium. Display medium decide theenergy to excite.
Aerial plasma is mainly limited by objective lens after the varifocal
lens. To invoke the tunnel ionization needs energy PW/cm2 because it needs huge power in focused areas, the main size limitation
is decided by objective lens. To achieve safe and appropriate sizefor daily applications, we have to develop three factors; increasing power of laser source, shortening pulse to increase peak energy,
and increasing scanning speed. By achieving these factors, we canscale our applications to keep tangible and visible in applications.
As we investigated power of average laser source and peak intensity of shot is key factor that decide the size. To enlarge the image
within safe for human, we have to explore shorter pulse and highintensity peak pulses.
Compare to tunnel ionization, energy to excite fluorescents and water is enough to small and it was not limited by objective lens. These
workspaces are limited by workspace of galvano mirror and varifocal lens. However galvano mirror and varifocal lens are fast enoughto scanning large space however there is small room to increasescanning speed. To achieve larger size, to employ multiple laser
focusing system is one of the solutions.
If we ignore the safety of the laser plasma contents, we could
choose simply high intensity laser sources. As we investigated influorescents and water, it is important to employ multiple focusing
systems.
7.3.2

Spatiotemporal resolution

Spatiotemporal resolution depends on parallel access by using SLM
and repetition frequency of laser source. Resolution of parallel access by SLM depends on the energy of laser source which corre-

sponds to durability of SLM, refreshing rate of SLM and resolutionof SLM. SLM is made of LCDs because of the material characteristics the refreshing rate and durability is limited. To achieve higher
resolution in aerial plasma imaging, repetition frequency plays important role to increasing resolution. Higher energy and repetition
frequency will solve the resolution.
In contract achieving a high resolution image in fluorescent and water, special resolution of SLM have important roles. Parallel accessto 3D position can achieve a high resolution image in volumetricdisplay.

7.4

SafetyClass 4 laser sources are used in this paper. The proposed display
system was carefully designed and operated based on the International Electrotechnical Commission (IEC) 60825-1:2014. There are
two concerns regarding the safety of laser: the damages on eyes andskin. It should be avoided for users to see the laser beam directly.
Laser plasma emits visible light in all directions at the focal point
and users can see it safely from the side of the laser beam. It is recommended for users to wear glasses with infrared filters until thisdisplay technology is well matured and the safety for eyes is confirmed. There are a few reports on damage of skin by femtosecond
lasers. The minimum visible lesion thresholds for porcine skin forpulsed lasers were evaluated in [Cain et al. 2007]. The ED50 for a
femtosecond laser (44 fs, 810 nm, and 12 mm spot size) was determined as 21 mJ from the observation that the lesions by the lasers
less than that energy value disappeared at 24 hours after the exposure. The energy (2 mJ and 50 µJ for Lasers A and B, respectively)
and spot size (less than 10 µm) are much smaller, and we expectthat damages by these femtosecond lasers are negligible. We also
investigated the exposure time in this paper. The result shows that
there was a discontinuous expansion of the damaged area when the
exposure time comes up to 2,000 ms. We can minimize the damage
by keeping the exposure time less than 2,000 ms by, for example,
feedback control based on detection of brighter plasma emission at
the surface of the finger in contact with aerial laser plasma.

8 Conclusion
In this paper, we introduced a system for rendering volumetricgraphics in air and other media using a femtosecond laser. Aerial
laser-induced plasma emits light without interaction with any physical matter, while one advantage of the femtosecond-laser display
system is that it is quieter and safer than a system using a nanosecond laser. There are two methods for rendering graphics in air witha femtosecond laser: Holograms by spatial light modulation technology and the scanning of a laser beam by a galvano mirror. The
hologram sizes and workspaces of the current system have maximum values of 1-cm2 and 5-cm3 , respectively, and these sizes arescalable depending on the optical devices and setup. This paper reports the details of the theoretical principles, system setup, and experimental evaluations, and also discusses scalability, design space,and applications. Although we focus on laser-induced plasma, thesame considerations can be applied to other emission techniquessuch as fluorescence and cavitation. To explore the design spaceof the 3D volumetric display induced by a high-intensity and ultrashort-pulse laser source, we evaluated and built applications using
various media. This approach is useful as it facilitates the discussion of laser volumetric system characteristics in a comprehensive
and uniform manner.

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