Three-dimensional lithium niobate nonlinear photonic crystal prepared by femtosecond laser
Nonlinear photonic crystal (NPC, nonlinear photonic crystal) has second-order nonlinear coefficients that depend on spatial distribution. We can use quasi-phase matching to effectively control nonlinear optical effects. Lithium niobate (LiNbO3) crystal is the most common nonlinear photonic crystal and has perfect nonlinear optical properties. One- and two-dimensional LiNbO3 NPCs have been widely used in frequency conversion, spatial light modulation, and nonlinear optical imaging. However, due to limitations of traditional polarization methods, the preparation of 3-dimensional LiNbO3 NPCs still poses considerable challenges.
Dunzhao Wei, Chaowei Wang and others successfully prepared 3-dimensional LiNbO3 NPC using femtosecond laser, which provides a platform with great potential for future nonlinear optical research, especially in controlling nonlinear interaction waves in three-dimensional structures.
What is nonlinear photonic crystal, or what is the nonlinear optics involved?
The explanation on Wiki is: "Nonlinear optics (NLO) is the branch of optics that describes the behavior of light in nonlinear media, that is, media in which the dielectric polarization P responds nonlinearly to the electric field E of the light."
It’s hard to understand isn’t it! ! ! Let's give a few examples to explain nonlinear processes.
The first is the second harmonic generation (SHG) or frequency doubling. This is easy to understand. After all, it is often seen to double the frequency of 1064nm laser to 532nm laser. The titanium sapphire pump source in the titanium sapphire femtosecond laser from Coherent or Spectra-Physics that we are most exposed to is the 532nm green light obtained by frequency doubling the laser emitted by the 1064nm laser. This process now seems to be a nonlinear process (frequency doubling).
Its essence is also easy to understand, that is, two photons are destroyed to produce a photon with doubled frequency. The basic principles of the corresponding third harmonics and higher harmonics are also similar, as are processes such as sum frequency, difference frequency, and optical parametric amplification (OPA).
Another example is the mode locking of the famous Kerr lens. The optical Kerr effect (high-order nonlinear effect) caused by changes in spatial light intensity distribution causes changes in the spatial refractive index, which leads to a self-focusing effect and then completes the mode locking. This is the mode locking mechanism of many ultrafast lasers today.
Another example is the Pockels effect (i.e., the principle of electro-optic modulator). The refractive index is affected by the static electric field, which is also a typical nonlinear optical effect [3].
It can be said that nonlinear optical effects are closely related to extremely high power density (femtosecond) lasers. Nonlinear optical processes may be going on in our commonly used femtosecond lasers all the time. In the nonlinear wave mixing process in nonlinear optical crystals, phase matching is crucial for efficient frequency conversion.
Taking the second harmonic as an example, if the phases match, the amplitude of the second harmonic increases linearly with the length of the crystal. However, due to the phase mismatch caused by the dispersion effect, under normal circumstances, the second harmonic amplitude will oscillate along the propagation direction with a period of 2Lc (Lc is the coherence length of nonlinear interaction).
Quasi-phase-matching (QPM) technology is a popular and effective method to overcome phase mismatch. It can be implemented in NPC (such as electrically polarized LiNbO3 crystal) and can be used to improve the overall conversion efficiency.
By periodically changing the sign of the nonlinear coefficient, the generated second harmonic can continuously increase along the propagation direction. However, so far, quasi-phase matching technology has only been successfully implemented in one- and two-dimensional cases due to limitations of traditional preparation methods. The realization of three-dimensional NPC is still a huge challenge in the field of nonlinear optics.
The classic method for preparing LiNbO3 NPC is the electropolation method.
In this method, the ferroelectric domains in LiNbO3 NPCs are reversed by selectively applying an electric field. Other methods including: chemical diffusion, scanning force microscopy polarization, electron beam polarization and other methods have been developed for specific situations such as periodically polarized LiNbO3 NPC waveguides, surface polarization and short period polarization. However, these traditional technologies have not achieved the preparation of 3D NPCs.
Femtosecond laser direct writing technology is the most advantageous weapon to overcome this landmark problem. Recently, near-infrared femtosecond laser polarization experiments were successfully experimentally demonstrated in titanium-doped LiNbO3 waveguides. However, achieving polarization within NPCs is still a big challenge.
Dunzhao Wei of Nanjing University and Chaowei Wang of the University of Science and Technology of China and others successfully demonstrated different types of 3D LiNbO3 NPCs by optimizing laser parameters to selectively remove the nonlinear coefficient χ(2) in LiNbO3 crystals
In addition to the high frequency conversion efficiency obtained through the 3D QPM mechanism, their work also revealed several unique features.
LiNbO3 crystal is one of the most popular NPC materials. Workers in the field of nonlinear optics can easily demonstrate experimental solutions and is compatible with existing nonlinear optical modulation technology.
The conditions for femtosecond laser processing are easy to achieve, and can be extended to a large number of NPC materials, such as LiNbO3 and KTiOPO4 crystals.
In addition, this processing technology can be easily used to prepare more complex three-dimensional photonic crystal structures to precisely control nonlinear light waves, which has great application prospects in nonlinear beam shaping, nonlinear optical imaging, and three-dimensional nonlinear holography.
Boliang Technology provides high-quality lithium niobate (LiNbO3) crystals with sizes up to 50mm, and can provide dielectric films or electrode films. Lithium niobate crystal is an excellent artificial crystal that can be used as a birefringent device, nonlinear applications, or in electro-optical effect applications. Its electro-optic coefficient is relatively large and cheap. It is a commonly used material for making electro-optic modulators. It has a high birefringence and is often used in optical communication devices to function as a polarizing element. In terms of nonlinearity, it is widely used in OPO and OPM.