Integrated quantum nanophotonics Technologies that exploit the rules of quantum mechanics offer a potential advantage over classical devices in terms of sensitivity. Sipahigil et al. combined the quantum optical features of silicon-vacancy color centers with diamond-based photonic cavities to form a platform for integrated quantum nanophotonics (see the Perspective by Hanson). They could thus generate single photons from the color centers, optically switch light in the cavity by addressing the state of the color center, and quantum-mechanically entangle two color centers positioned in the cavity. The work presents a viable route to develop an integrated platform for quantum networks. Science, this issue p. 847; see also p. 835 An integrated quantum optical platform is demonstrated using silicon vacancy color centers and diamond photonics. Efficient interfaces between photons and quantum emitters form the basis for quantum networks and enable optical nonlinearities at the single-photon level. We demonstrate an integrated platform for scalable quantum nanophotonics based on silicon-vacancy (SiV) color centers coupled to diamond nanodevices. By placing SiV centers inside diamond photonic crystal cavities, we realize a quantum-optical switch controlled by a single color center. We control the switch using SiV metastable states and observe optical switching at the single-photon level. Raman transitions are used to realize a single-photon source with a tunable frequency and bandwidth in a diamond waveguide. By measuring intensity correlations of indistinguishable Raman photons emitted into a single waveguide, we observe a quantum interference effect resulting from the superradiant emission of two entangled SiV centers.

/pdf/an-integrated-diamond-nanophotonics-platform-for-quantum-5awh3ldvjf.pdf

An integrated diamond nanophotonics platform for quantum optical networks

Crystalline lithium niobate (LN) is an important optical material because of its broad transmission window that spans from ultraviolet to mid-infrared and its large nonlinear and electro-optic coefficients. Furthermore, the recent development and commercialization of LN-on-insulator (LNOI) technology has opened an avenue for the realization of integrated on-chip photonic devices with unprecedented performances in terms of propagation loss, optical nonlinearity, and electro-optic tunability. This review begins with a brief introduction of the history and current status of LNOI photonics. We then discuss the fabrication techniques of LNOI-based photonic structures and devices. The recent revolution in the LN photonic industry has been sparked and is still being powered by innovations of the nanofabrication technology of LNOI, which enables the production of building block structures, such as optical microresonators and waveguides of unprecedented optical qualities. The following sections present various on-chip LNOI devices categorized into nonlinear photonic and electro-optic tunable devices and photonic-integrated circuits. Some conclusions and future perspectives are provided.

/pdf/advances-in-on-chip-photonic-devices-based-on-lithium-461c9g3pmy.pdf

Advances in on-chip photonic devices based on lithium niobate on insulator

Lithium niobate (LiNbO3 or LN) is a well-known multifunctional crystal that has been widely applied in various areas of photonics, electronics, and optoelectronics In the past decade, “ion-cut” has become the key technique to produce wafer-size, high-quality, sub micrometer-thickness crystalline LiNbO3 thin films, ie, lithium-niobate-on-insulator (LNOI) With the rapid development of LNOI technology and the tremendous progress of associated surface structuring and engineering techniques over the last few years, many novel chip-integrated LiNbO3-based devices and applications with reduced cost, complexity, power, and size, are demonstrated, boosting the resurgence of integrated photonics based on this material The remarkable achievements are largely facilitated by the most recent technological progress in photonic integration and performance optimization of LNOI on-chip devices, such as high-quality surface domain engineering, advanced heterogeneous integration technology, powerful dispersion engineering, fine polishing lithography, and wafer-scale fabrication Accordingly, batch-compatible chip-integrated platforms for more complex photonic integrated circuits, such as quantum optical circuits, are within reach This article provides a timely review of the key advances in LNOI technology and a reasonable perspective on the near-future directions for both integrated photonics and applied physics communities

Ion-cut lithium niobate on insulator technology: Recent advances and perspectives

Optical waveguides are far more than mere connecting elements in integrated optical systems and circuits. Benefiting from their high optical confinement and miniaturized footprints, waveguide structures established based on crystalline materials, particularly, are opening exciting possibilities and opportunities in photonic chips by facilitating their on-chip integration with different functionalities and highly compact photonic circuits. Femtosecond-laser-direct writing (FsLDW), as a true three-dimensional (3D) micromachining and microfabrication technology, allows rapid prototyping of on-demand waveguide geometries inside transparent materials via localized material modification. The success of FsLDW lies not only in its unsurpassed aptitude for realizing 3D devices but also in its remarkable material-independence that enables cross-platform solutions. This review emphasizes FsLDW fabrication of waveguide structures with 3D layouts in dielectric crystals. Their functionalities as passive and active photonic devices are also demonstrated and discussed.

Femtosecond laser direct writing of flexibly configured waveguide geometries in optical crystals: fabrication and application

Recent Advances in Femtosecond Laser Processing of LiNbO 3 Crystals for Photonic Applications

We report on a one-step fabrication method of horizontal dual-line waveguides supporting TE guidance in LiTaO3 crystal utilizing multiple refocusing mechanisms of the femtosecond laser. Through the nonlinear process, multiple foci can be formed spontaneously with the interval of a few micrometers along the laser propagation direction. The number and the separation of focal spots, even the entire length of laser-induced tracks, could be modulated by varying the laser parameters. Taking advantage of this spontaneous phenomenon, we further implemented the multi-scan technique to write two parallel damage lines along transverse direction, realizing stress-induced waveguides by horizontal light confinement. Moreover, using this method, one-step two-channel guidance could also be demonstrated by adjusting the pulse energy. The fabrication details and waveguiding performances were studied by both experimental and computational methods. This paper opens the alternative way to construct complex integrated platforms in LiTaO3 crystal by using femtosecond laser writing.

/pdf/femtosecond-laser-writing-of-optical-waveguides-by-self-1mlc43j2xc.pdf

Femtosecond Laser Writing of Optical Waveguides by Self-Induced Multiple Refocusing in LiTaO 3 Crystal

We report on optical waveguides produced by a femtosecond laser in 6H–SiC crystals. Their guiding properties have been investigated at a wavelength of 1064 nm, and confocal micro-Raman images have been obtained at an excitation wavelength of 532 nm. The results demonstrate that mode profiles can be tailored by the adjustment of writing parameters, and the blueshift of the spectrum (at around 787.05 cm−1) mainly takes place in the irradiated areas. From images of Raman intensity and spectral shift, it is obvious that optical properties in guiding regions are well preserved. These waveguides may have potential applications in integrated optics and quantum photonics.

Femtosecond laser modification of 6H–SiC crystals for waveguide devices

We report a novel mode modulator based on a KTN crystal waveguide produced by femtosecond laser (fs-laser) writing. The KTN waveguide is a typical dual-line structure that depends on the decrease of refractive index at the focus area. The propagation loss of the waveguide along the TE polarization is as low as ~0.9 dB/cm at 632.8 nm. The investigation of confocal micro-Raman spectra reveals that the microstructure of the waveguide region has no obvious change during the femtosecond laser direct writing (FsLDW) procedure. Under the applied direct current (DC) electric field, the waveguide modes in the guiding structure can be modulated, resulting in mode conversion from TE00 to TE02. Both the experimental results and the theoretical simulation indicate that the mode modulation phenomenon originates from the space-charge-field induced refractive-index increment between the waveguide core and the bulk substrate. The simplicity and remarkable performance of the demonstrated device paves the way for integrating low-cost and high-efficient electric-optical modulator on photonics circuits.

Bin Zhang

Papers

Recent Advances in Femtosecond Laser Processing of LiNbO 3 Crystals for Photonic Applications

Femtosecond Laser Writing of Optical Waveguides by Self-Induced Multiple Refocusing in LiTaO 3 Crystal

Femtosecond laser modification of 6H–SiC crystals for waveguide devices

A waveguide mode modulator based on femtosecond laser direct writing in KTN crystals

Mode tailoring of laser written waveguides in LiNbO3 crystals by multi-scan of femtosecond laser pulses