To develop a volumetric display of the kind we see in science fiction movies is a dream of many display researchers, including us. Here, we show a new volumetric display with microbubble voxels. The microbubbles are three-dimensionally generated in liquid by focused femtosecond laser pulses. The use of a high-viscosity liquid, which is a key part of the development of this idea, slows down the movement of the microbubbles, and, as a result, volumetric graphics can be displayed. This volumetric bubble display has a wide-angle view, simple refreshing, and no addressing wires, since the transparent liquid is accessed optically rather than electronically. It achieves full-color graphics composed of light-scattering voxels controlled by illumination light sources. Furthermore, a holographic laser drawing method based on a computer-generated hologram displayed on a liquid-crystal spatial light modulator controls the light intensity of the microbubble voxels with an increase in the number of voxels per unit time and the spatial shaping of the voxels.
© 2017 Optical Society of America
Volumetric displays have attracted much attention for three-dimensional (3D) displays in the fields of optics and computer graphics. They can render 3D graphics in real 3D space, and, therefore, a volume image can be observed from any surrounding viewpoint without the user having to wear any special devices or experience physiological discomfort , unlike holographic displays [2–4] and head-mounted displays , which are based on planar two-dimensional (2D) screens for displaying visual information.
Volumetric displays can be classified into two types, depending on the characteristics of the voxels that serve as pixels of the 3D graphics: the light-scattering type and the light-emitting type. Scattering-type volumetric displays using rotating projection screens [6,7], water drops , and fog , and floating small particles  have been constructed. These displays can easily form voxels that display multiple colors because the colors of the voxels depend on the projected image. Light-emitting-type volumetric displays using optical fibers  have been demonstrated. A variety of kinds of volumetric displays based on laser-induced plasmas [12–14], semiconductor quantum dots [15,16], rare-earth elements [17–19], and fluorescent molecules have been reported. They form voxels by laser excitation of the screen material. These optically accessed volumetric displays have a wide viewing angle because they require no physical connection between the light source and the display volume. Therefore, reconfigurable and parallel connections are easily achieved, resulting in good accessibility to the voxels and high drawing speeds. With these previous approaches, however, the number of voxels in the volumetric display is not high enough for rendering practical volumetric images because they are limited by the repetition frequency of the laser and the speed of the 3D scanning system, and it has been difficult to realize multicolor volumetric graphics because the color of the voxels depends on the screen medium.
The first work on volumetric displays in our group demonstrated some important features, including a multilayer fluorescent solid screen for achieving a color display by changing the fluorescent material in each layer, and holographic parallel optical access via two-photon excitation for increasing the number of voxels per unit time . The holographic parallel optical access was performed with a computer-generated hologram (CGH) displayed on a liquid-crystal spatial light modulator (LCSLM). The holographic parallel optical access was used for laser plasma emission in an air volumetric display .
In this paper, we propose a new liquid volumetric display using light-scattering voxels formed of microbubbles induced by focused femtosecond laser pulses. We call this display a volumetric bubble display. The use of a high-viscosity liquid allows successful volumetric graphics rendering in liquid because the rising speed of the microbubbles is low. We demonstrated that the formation of microbubbles depends on the irradiation energy. We also demonstrated that the holographic irradiation with laser pulses changed the form of the voxels composed of microbubbles and helped to increase the total amount of scattered light from the microbubbles in relation to the brightness of the voxels. Some actual examples of volumetric graphics created using the volumetric bubble display are demonstrated.
A. Architecture of Optically Accessed Volumetric Bubble Display
A volumetric bubble display is mainly composed of a light source, a light modulator, and a screen. In our system, as shown in Fig. 1, the light source is an amplified femtosecond pulsed laser used to excite a liquid via multiphoton absorption. The light modulator performs holographic laser drawing and 3D beam scanning for changing the focal points where voxels are formed. The screen is a high-viscosity liquid for achieving a high-resolution, refreshable, grayscale volumetric display. A full-color display can be easily realized by using blue, green, and red illumination.
B. Holographic Laser Drawing
The holographic laser drawing method, in which the focal points (spatial frequency domain) are designed by using phase-only Fourier CGHs displayed on the LCSLM, has the following advantages. First, it increases the number of voxels in the volumetric graphics per unit time by parallel access of the focal points . The number of voxels, , in a volumetric display is given by , where is the number of parallel beams, is the number of single optical accesses per unit time, and is the image refresh frequency (frame rate). If we use a laser source with a repetition frequency of 1 kHz, voxels/s with 30 frames per second. An effective way to increase is to employ parallel access. Second, this drawing method can control the sizes and shapes of the voxels. Various beam shapes, such as a line-focused beam , can be achieved.
C. Femtosecond Laser-Induced Microbubble
When a femtosecond laser is focused inside a liquid, microbubbles are formed by multiphoton absorption and the subsequent laser-induced breakdown [23,24]. These phenomena are often applied to the manipulation of microscopic objects, such as nondestructive isolation of single animal cells . The laser-induced microbubbles in the liquid rise due to the different densities of the gas and liquid. The rising velocity of a microbubble, , is given by Stokes’ law ,
Figure 2 shows the rising velocity versus the diameter of the microbubbles in glycerin. The microbubbles were captured with a camera, and these values were measured based on the pixel pitch. Theoretical values are given by Eq. (1) by assigning the values for the glycerin we used. The rising velocity was on the order of micrometers per second and increased as the microbubble diameter increased. In comparison with the theoretical velocity, the experimental velocity was large, especially as the bubbles expanded. Small errors in the density of the gas inside the bubble have little effect on because of the large difference between the densities of the gas and liquid. Therefore, there are two reasons for the discrepancy between the experimental and theoretical values: a local temperature rise at the focal point, causing μ to significantly decrease, and the rising stream of liquid around expanded bubbles.
3. EXPERIMENTAL SETUP
Figure 3 shows the system configuration of the volumetric bubble display. It consisted of an amplified femtosecond laser source (Micra and Legend Elite Duo, Coherent), a 3D beam scanner composed of a 2D galvanometer mirror (GM-1010, Canon) and a varifocal lens (EL-10-30- Ci, Optotune), a liquid-crystal-on-silicon SLM (LCOS-SLM; X10468-02, Hamamatsu Photonics) for holographic laser drawing, and glycerin in a glass cell. Two setups, called Setup 1 (laser irradiation from the lateral direction) and Setup 2 (laser irradiation from the downward direction), were used for easy observation of the bubble graphics.
The femtosecond laser had a center wavelength of 800 nm, a repetition frequency of 1 kHz, and a pulse duration of . The maximum pulse energy was 2 mJ in our experiments. The 2D galvanometer scanner changed the focus position in the horizontal direction. It had a maximum deflecting angle of 20 deg and a step response time of 280 μs per 0.1 deg and was driven by a controller (GB-501, Canon) with a resolution of 20 bits. A varifocal lens controlled the focus position in the axial direction. The 3D focal position was changed by these devices to render the 3D graphics. The LCOS-SLM performed phase-only modulation of rad with 8 bit resolution. Efficient phase modulation occurred when the polarization direction of the femtosecond laser was parallel to the alignment direction of the LC molecules. The LCOS-SLM had a resolution of and displayed Fourier CGHs at a frame rate of 10 Hz. The Fourier CGHs were optimized by the optimal rotating angle method  to generate parallel beams. The 3D beam scanner and the LCOS-SLM were controlled by a C++ program running on a computer with the Windows 7 operating system. The screen was glycerin sealed in a glass cell with dimensions of .
4. EXPERIMENTAL RESULTS
A. Generation of Microbubbles
Figure 4(a) shows the experimental setup suited for observing femtosecond laser-induced microbubbles. The repetition frequency of the femtosecond laser pulses was set to 100 Hz by using a high-speed mechanical shutter to regulate the number of pulses. The microbubbles were illuminated by a halogen lamp and observed with a cooled charge-coupled device (CCD) image sensor (BU50LN, Bitran). The frame rate of the CCD image sensor was 33 fps. The imaging magnification was 5. The objective lens had a numerical aperture (NA) of 0.2. Figure 4(b) shows the generation length of the microbubbles versus the irradiated energy of a single pulse. The generation length was measured five times in order to take account of variations, and the average of these values was connected by a line. As shown Fig. 4(c), a microbubble group was generated when the pulse energy was 0.9 μJ, and when expanded along the axial direction, the pulse energy was increased. At a pulse energy of over 6 mJ, microbubbles were sometimes generated at a position away from a bubble group because of filamentation due to the optical Kerr effect in competition with diffraction.
Figure 5(a) shows the generation length of microbubbles in the axial direction for the energy of the irradiated pulses with multiple shots. The red, green, and blue bars show the result for 10 pulses, 100 pulses, and 500 pulses. Microbubbles were not generated at pulse energies of 0.5 μJ or lower, even when 1000 pulses were irradiated. This result shows that as the number of irradiated pulses was increased, the generation length expanded along the axial direction of the laser pulse irradiation. Microbubbles having a diameter of 20 μm or more also increased, according to Fig. 5(b). Multishot irradiation increased not only the generation length of the microbubbles, but also the diameter of the microbubbles. This result means the contrast of the displayed graphics can be changed by changing the number of irradiation pulses.
Figure 6(a) shows macro and magnified images of a voxel formed by the holographic laser drawing method. The macro image was taken by an ordinary camera under white LED illumination. The holographic method was used to increase the number of focal points. Figure 6(b) shows the brightness intensity of a voxel versus the number of parallel beams. The femtosecond laser repetition frequency was set to 1 kHz, and the number of irradiated pulses was 100. The irradiation energy was 1 μJ multiplied by the number of parallel beams. Each data value was the summed pixel values of a image around a voxel and was the average of the data measured three times. The holographic method effectively controlled the horizontal sizes and the brightness of the voxels.
B. Bubble Graphics Rendered by Femtosecond Laser-Induced Microbubbles
Figure 7 shows an image sequence of the 2D graphics rendered by the femtosecond laser-induced microbubbles. These pictures were taken under halogen lamp illumination. It was defined that 0 s is the time at which the laser irradiation was stopped. The repetition frequency of the laser pulses was 1 kHz, and the irradiation pulse energy was 4.3 μJ. These graphics were rendered using Setup 1. The bubble graphics retained the shape for 30 s. If we use a higher irradiation pulse energy, the bubble graphics will burst earlier because the diameters and generation area of the microbubbles will increase.
Figure 8 shows 3D bubble graphics viewed from different observation directions. These graphics were rendered by scanning 100 2D outlines of cross-sectional images from ear to foot and were illuminated by a halogen lamp. The repetition frequency of the laser pulses was 1 kHz, and the irradiation energy was 4.3 μJ. These graphics were rendered using Setup 2. Although there are nonuniform parts in the graphics because of spherical aberrations caused by the refractive index mismatch between glycerin and air, we were able to successfully render volumetric graphics by using femtosecond laser-induced microbubbles.
Figure 9 shows the bubble graphics illuminated by lights of different colors. The illumination light was emitted by a high-power full-color LED capable of outputting lights of different colors. The wavelengths were approximately 624 nm (red), 525 nm (green), and 470 nm (blue). Cyan, yellow, and magenta were made by using different combinations of RGB illumination. The repetition frequency of the laser pulses was 1 kHz, and the irradiation pulse energy was 5.4 μJ with Setup 1. Although these results are monochrome graphics with different colors, it will be easy to form full-color bubble graphics just by changing the color of the illumination light.
We have proposed a new volumetric display based on femtosecond laser-induced microbubbles serving as voxels. The microbubbles were made to remain at substantially the same position in a short fixed time by using a high-viscosity liquid screen. We observed the generation area of microbubbles formed by single-shot and multishot laser irradiation. In the case of single-shot irradiation, the generation area along the axial direction increased as the irradiation pulse energy increased. In multishot irradiation, we found that not only did the generation area expand, but also the size of the microbubbles increased. We also demonstrated rendering of bubble volumetric graphics in glycerin. Although laser-induced microbubbles were not formed at the desired place because of spherical aberrations caused by the refractive index mismatch between the screen and air, this problem was improved by applying the holographic laser drawing method to correct the multilayer aberrations . In addition, bubble volumetric graphics of different colors were displayed by using illumination lights of different colors. The limitation on a full-color display is decided by an illumination light. In order to give the graphics different color in individual regions, we need to utilize modulated illumination light, such as a projector. In addition, to achieve coloring at the voxel level, the method of scanning focused illumination light while temporally changing the color should be considered.
This study is the first report, to the best of our knowledge, of a laser-induced, bubble-based volumetric display. In future work, it should be possible to demonstrate an updatable display by introducing a bubble bursting system.
Japan Society for the Promotion of Science (JSPS) (16J08419).
The authors acknowledge a Grant-in-Aid for JSPS Research Fellows.
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