Amid the滋滋 sound of breakdown, Chinese characters and patterns such as "Hong Tuo", "Science and Technology Daily", and "Five-pointed Star" appeared in the air, visible to the naked eye and touchable with hands. On July 19th, the reporter witnessed this sci-fi scene at the Hong Tuo Ultrafast Laser Joint Laboratory in Wuhan East Lake High-tech Development Zone. "No paper or ink is needed; a beam of light can also draw out of thin air," said Cao Xiangdong, director of the laboratory. The team's newly developed high-energy, high-peak-power femtosecond laser, with a peak power density of millions of megawatts, converges a beam of "the fastest light" to "light up" the air. ### Laser display's breakthrough from "0" to "1" At present, most 3D display technologies present "pseudo-3D images" on the screen by utilizing binocular visual differences and virtual synthesis by the human brain. The latest 3D display technology uses three-dimensional physical space to render graphics. Each voxel (corresponding to a pixel in flat display) emits or reflects light through lasers, and they are arranged to form images in three-dimensional space, allowing viewers to see 3D images from all angles without dead ends. The 3D images formed by femtosecond laser breaking through the air are real. Cao Xiangdong introduced that this real 3D display technology based on femtosecond laser-induced plasma focuses high-peak-power femtosecond laser to an extremely high intensity of 100 terawatts per square centimeter to break through the air, inducing the formation of luminous plasma. A 3D scanner is used to scan the laser beam and arrange and combine the luminous点阵 (luminous points) to form various texts or patterns in the air. Compared with traditional laser display, femtosecond laser display technology has obvious advantages. Its display medium is air, without the need for screens, water mist, etc., and can directly present 3D images in the air. At the same time, femtosecond laser has a shorter pulse duration than nanosecond laser, requires lower energy, has a shorter single-point residence time, and is more secure. ### What are the difficulties in femtosecond laser display technology? "Today's appearance stems from more than ten years of painstaking research behind the light and shadow," Cao Xiangdong said. Femtosecond laser was once known as one of the four great inventions of Optics Valley. He and his team are committed to the research and development of core cutting-edge technologies and application technologies in the ultrafast laser industry, and all key core technologies have been localized. Breaking through the air requires an energy density of 100 terawatts per square centimeter, that is, the femtosecond laser achieves an energy output of 100 terawatts on an area the size of a fingernail and releases it on a femtosecond time scale to ionize the air. "Lighting up" the air is a manifestation of the comprehensive technical level of femtosecond lasers. While relying on strong peak power, the average power is only tens of watts. "Our air imaging femtosecond laser technology has reached a leading level in display brightness. After comparing products from many companies at home and abroad, customers finally chose us," Cao Xiangdong said. Cao Xiangdong introduced that the team uses independently developed advanced femtosecond laser technology. In the future, through complex editing and control of the temporal and spatial distribution of femtosecond laser pulses, the characteristic parameters such as brightness, color, and duration of voxels will be accurately regulated, and the power of the femtosecond laser will be further upgraded to realize ultra-large-scale real 3D display in the air. Laser has been widely used in modern science and technology and industry. What are the specific applications? ### Applications of laser technology The new generation of ultrafast lasers has been specially optimized to support user needs in end markets, such as additive manufacturing, medicine, semiconductor metrology, and applied research. #### 1. Nanomanufacturing Lasers can be used in many additive manufacturing (AM) processes, including laser sintering of metals and stereolithography of polymers. Each of these processes provides a way to create complex and unique structures without masks or molds. Additive manufacturing is particularly valuable for small-scale production applications, such as rapid prototyping of parts or personalized medical implants. An emerging AM method is a stereolithography technique called two-photon polymerization, which is rapidly gaining interest for several reasons. First, it can achieve higher spatial resolution than any other AM method. Second, it is a three-dimensional free-form process, so it is not limited by the processing constraints of laser sintering or single-photon stereolithography, where parts must be created layer by layer from the bottom up or top down. The emergence of compact, hands-free femtosecond lasers has made technologies such as two-photon polymerization more economically viable in many industries and applications. How does laser technology achieve this? In stereolithography, a laser beam is focused into a bath of photosensitive resin. When light of an appropriate wavelength (usually ultraviolet) irradiates this resin, it breaks the bonds of the polymer, and the material becomes reactive, forming a solid polymer from liquid monomer chemicals. Two-photon polymerization is a three-dimensional free-form additive manufacturing technology with high spatial resolution, capable of producing extremely small parts and features. New femtosecond lasers make two-photon polymerization technology more economically feasible. (Provided by Wildman Laboratory / University of Nottingham) This process allows almost any shape to be created directly from CAD files, and the raw materials are not expensive. In the two-photon method, the ultrafast laser is customized to twice the normal wavelength that the resin usually absorbs. By using high numerical aperture (NA) optics, the beam is focused into a slender waist. At this waist, and only at this waist, the peak power of the ultrafast pulse is high enough to drive two-photon absorption. This method provides unparalleled resolution for two reasons. First, the use of high NA optics produces a tight micron-scale waist. Second, because two-photon absorption depends on the square of the peak power, the transmitted laser power can be adjusted so that only a small central region within the laser beam waist induces polymerization. In this way, the process can provide submicron spatial resolution, and Hong Kong researchers have reported the creation of features measured at approximately 100 nm, which they further accelerated using a programmable mirror array to create a multi-beam process¹. A new class of femtosecond lasers is well-suited for this application. These lasers operate at 780 nm, combining high power, short pulse width, and dispersion pre-compensation to provide high throughput at the focal plane. Compared with lasers with longer pulse widths, these parameters result in a more efficient polymerization process with higher resolution. User-friendly power control functions further enhance fine control over the process. Early applications of these new lasers include the manufacturing of lab-on-a-chip products and microstructured surfaces, as well as new photonic products such as micropatterned crystals. #### 2. Label-free in vivo imaging Multiphoton excitation microscopy is a widely used tool throughout life science research. Like two-photon photopolymerization, it relies on spatially selective interaction with the sample only at the tightly focused beam waist using the high peak power of femtosecond pulses. One key trend here involves translational research, where scientists are slowly but surely moving multiphoton technology toward clinical laboratory applications and ultimately toward real-time applications such as intraoperative biopsy. For obvious reasons, the target technologies are those that do not require fluorescent labels or transgenic proteins such as green fluorescent protein to generate images. These technologies include second harmonic generation (SHG) to image collagen, where 920 nm is a suitable wavelength; third harmonic generation (THG) to image membranes, where 1064 nm is a good match; and excitation of endogenous fluorescence to image various biomolecules and metabolites, where 780 to 800 nm works well. High numerical aperture optics focus the femtosecond laser beam into a tiny waist, and the peak power of the ultrafast pulse is sufficient to drive two-photon absorption. Additive manufacturing technology can provide submicron spatial resolution and create features as small as 100 nm. (Provided by Wildman Laboratory / University of Nottingham) While SHG and THG microscopy require femtosecond lasers, continuous-wave lasers operating at visible or ultraviolet wavelengths can also excite these natural fluorophores, but at the cost of some imaging depth and the possibility of cell damage. Therefore, the advantages of femtosecond excitation are obvious. Key endogenous fluorophores include reduced nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FAD) — metabolites that can be used as cancer signatures. It is well known that cancer cells prefer to use glycolysis rather than oxidative phosphorylation to meet their energy needs. This is manifested in significant differences in the ratio of NADH to FAD when comparing normal and cancer cells. NADH is efficiently excited by two-photon absorption at wavelengths from 700 to 800 nm, and the absorption spectrum of FAD extends to 890 nm. Early studies using these metabolites relied on two different ultrafast laser wavelengths, which is impractical for diagnostic or point-of-care work. Fortunately, in the past few years, researchers have shown that a single ultrafast laser operating in the 780 to 800 nm window can excite and image both species with similar efficiency, because the stronger fluorescence of NADH can also be excited at the "red" end of its spectrum. In addition, the same researchers demonstrated that the NADH/FAD ratio obtained in this way is a reliable marker for two different prostate cancer cell lines². Similarly, the latest compact femtosecond lasers operating at 780 nm are well-suited for this potentially very important application. Like two-photon polymerization, other relevant laser parameters for label-free in vivo imaging include excellent beam quality to maximize spatial resolution, short pulse width to minimize the average laser power required for fluorescence, and internal power control to simplify the scanning process — for example, for blanking during raster scanning. #### 3. Advanced wafer metrology Ultrafast lasers have also proven to be increasingly important in the field of advanced wafer metrology. A well-established set of techniques, called picosecond laser acoustics (PLA), measures layer thicknesses and images critical alignment marks under opaque layers. The latter capability is important in multi-layer lithography processes. In the PLA method, absorption of a laser pulse (i.e., pump) generates an acoustic wave that propagates inward from the laser surface. Underlayers and structures reflect some of this acoustic energy back to the surface, where it is detected by changes in the reflectivity of a second laser pulse (i.e., probe). PLA benefits from a new generation of compact femtosecond lasers, as these lasers enable higher resolution imaging and improved overall measurements. Terahertz radiation generated by ultrashort laser pulses and photoconductive switches is characterized by high intensity and a broad continuous spectrum. (Provided by Coherent) The latest non-destructive wafer metrology methods supported by femtosecond lasers depend on variations of the harmonic generation process used for label-free microscopic imaging of cell membranes. Interfaces between two materials, or any non-centrosymmetric material, generate small amounts of second harmonic light in a process that depends nonlinearly on laser peak power. The SHG optical signal can be used to image and detect various features and properties of wafer surfaces and subsurfaces. These features may include structural defects, thin film quality, and even trace metal contamination. This technology has been successfully commercialized by FemtoMetrix, a company specializing in optical non-visual defect metrology of surface, buried, and structural irregularities. #### 4. Terahertz generation and detection Terahertz radiation can provide unique spectral or imaging information in solid and liquid materials. The low optical frequencies in this range are related to the vibrations of nanoscale particles, such as macromolecules like polymers and proteins, and phonon vibrations of extended structures like crystals. Thus, for example, terahertz research helps map phase boundaries. However, the terahertz frequency range has been a neglected part of the electromagnetic spectrum for decades because there is no simple way to generate or detect terahertz radiation. Today, femtosecond laser pulses can be used in a variety of mechanisms to generate and detect terahertz radiation. One method focuses femtosecond laser pulses on a photoconductive antenna (or switch) consisting of a strip of dielectric material such as gallium arsenide (GaAs) sandwiched between two metal (e.g., gold) conductors with a bias voltage applied. Similar structures are also used as terahertz detectors. Another method of generating terahertz radiation, called optical rectification, focuses the laser into a nonlinear crystal, such as gallium phosphide (GaP) or zinc telluride (ZnTe), creating a difference frequency between different spectral components in the terahertz pulse. Terahertz pulses generated by femtosecond laser pulses have several advantages over those generated by continuous-wave methods. Terahertz radiation generated by ultrashort laser pulses has higher intensity. It covers a broad and continuous part of the terahertz spectrum simultaneously, and its pulsed nature supports analytical techniques such as time-correlated spectroscopy. Therefore, pulsed terahertz radiation has found use in imaging applications in diverse fields such as medical diagnosis of cancerous tissues, non-destructive evaluation of pharmaceuticals, identification of explosive hazards, examination of art and archaeology, and defense and security inspection tasks. Terahertz generated by optical rectification from very short pulses produced by a 1 to 5 kHz titanium sapphire amplifier or a nonlinear broadening ytterbium amplifier at megahertz repetition rates can generate high average power pulses with large (frequency) bandwidth. In contrast, photoconductive antennas are limited to lower excitation powers due to potential optical damage and saturation effects. However, these antennas are the simplest and cheapest way to generate terahertz pulsed radiation. While most antennas require only 20 to 50 mW of laser power, compact femtosecond lasers emitting single watts on tiled arrays of antennas can enable higher power terahertz generation in cost-constrained setups. Such setups, in turn, could potentially expand the application of terahertz time-domain spectroscopy (TDS) from small laboratory arrangements to larger-scale industrial and medical imaging applications. How do next-generation femtosecond lasers fit into this terahertz picture? Their short pulse widths enable a broader spectral range of terahertz radiation. Their high average power centered at 1 W is useful for either terahertz generation method, as both are inefficient mechanisms requiring high input power. Practical aspects of new femtosecond lasers, such as their streamlined packaging and reliability, are equally important. Some emerging applications require portable or at least transportable systems to sustain widespread adoption of these lasers. These small, inexpensive, air-cooled sources require minimal technical attention, can be easily integrated into more complete systems, and can be mounted in any orientation desired. #### 5. Holographic iridescent texture on mobile phone cases Holographic iridescence is not a single color but a color system. The appearance of mobile phones with holographic iridescent textures can reflect extremely rich light and shadow effects under different lights. On the texture mold, dense iridescent diffraction units are carved by femtosecond laser, and then these magical optical diffraction effects are replicated on the surface of the final product through injection molding process. The more precise and complex the texture, the more abundant and gorgeous the light and shadow effect generated by the light passing through the glass to the texture layer, and the stronger the sense of flow. Such textures can be seen on electronic products such as mobile phones and laptops. ### Future thinking Although femtosecond lasers are generally regarded as one of the most exotic types of coherent light sources, their development and application share patterns with all other laser technologies. They have successively transformed from research objects to research tools, and finally used as components in other tools and systems. Like other laser technologies, the development of femtosecond light sources is driven by rapidly expanding practical application fields, from life sciences to industrial diagnostics to manufacturing processes. Source: Science and Technology Daily, Laser Manufacturing Network, Laser Industry Observer, Xitaike