Recent development in integrated Lithium niobate photonics

Figures & data

Figure 1. Fabrication process of LNOI.

Figure 2. (a) The ring-down measurement of the ultra-high Q microcavity [39], exhibiting an intrinsic Q factor of 1.23 × 10⁸. Inset: the scanning electron microscope (SEM) image of the microdisk, where the scale bar is 200 μm. (b) SEM image of the monolithic microring [41]. (c) Photograph of an array of 1960 MZIs patterned in a 4-inch wafer. (d) Micrograph of unbalanced MZI. Inset: zoom-in micrographs of 1×2 multi-mode interferometer (MMI) and Euler bend waveguide [38].

Figure 3. (a) Photography of 8-inch wafer during DUV lithography. (b) Optical micrograph of a spiral LNOI waveguide with a length of 11.49 cm [45]. (c) The illustration of the fabrication of thin-film PPLN waveguide [46]. (d) The optical micrograph of the waveguides and periodic poled domains [43].

Table.1. Comparison of different fabrication methods in terms of the-state-of-the-art propagation losses, loaded Q factors, minimum feature sizes, and integration scales, available by each method.

	Argon ion milling					FIB			Wet etching
Fabrication method	Laser lithography	Optical lithography	EBL	UV	DUV	Pure	Femtosecond laser ablation	PLACE	EBL
Loaded Q factors of microresonators	1×10^4 [24]	4×10^3 [20]	≤5×10^6 [32]	1.19×10^6 [42]	1.8×10^6 of intrinsic Q [44]	1.84×10^6 [30]	9.6×10^6 [29]	7.5×10^7 [39]	5.25 × 10^6 [51]
Loss	6.3 dB/cm [21]	17.37 dB/cm [20]	≥0.027 dB/cm		0.27 dB/cm			0.0034 dB/cm	0.04 dB/cm
Lattice damage(Yes/No)	Y	Y	Y	Y	Y	Y	Y	N	N
Minimum Feature Size/resolution	~200 nm [21,22] for laser	~1 μm [23]	~20 nm	~450 μm	~75 nm	~1 nm	~1 nm	~200 nm	~20 nm
Writing field area	200×200 μm^2 [23]		~1×1 mm^2	~22.5×22.5 mm^2 @stepper	~40×40 mm^2 [52]	~1×1 mm^2	~1×1 mm^2	~30×30 cm^2	~1×1 mm^2
Wafer integration manner	Field stitching		Field stitching	Field stitching	Field stitching	Field stitching	Field stitching	Stitchingless	Field stitching

Figure 4. (a) Illustration of the growth of erbium ion doped bulk LN crystal [74]. (b) Schematic of the fabrication of passive and active LNOI photonics by stitching and nanostructuring [75].

Figure 5. (a) LNOI modulator with regular coplanar waveguides, with a Vπ of 1.4V and an EO bandwidth of 45 GHz [103]. (b) Advanced LN IQ modulators [104]. (c) Integrated LNOI modulators with capacity-loaded travelling electrodes and quartz substrate [105].

Figure 6. (a) LNOI dual-polarization in-phase quadrature (DP-IQ) modulator integrated with an on-chip polarization rotator and combiner (PRC) [116]. (b) Modulator with folded MZMs incorporating meander traveling wave electrode and optical waveguides [118]. (c) Wafer-scale low-loss lithium niobate photonic integrated circuits [44].

Figure 7. MPM second harmonic generation. (a) the two MPM points at 1379.4 nm and 1381.5 nm, the inset figures are the electric profiles of the SH modes [132] (b) the theoretical NCE values versus the relative thicknesses of LN layer [132]. (c) the sketch of ridge waveguide in double-layer LNOI with reversed polarization [133]. (d) Measured SHG spectrum of dual-layer ridge waveguide at room temperature, the inset figure shows the SHG power as a function of the pump power [133].

Figure 8. Broadband SHG signals and tunable SHG laser. (a) Measured (black) and theoretical (blue) SHG transfer function for nanophotonic PPLN waveguide, broader than the bulk PPLN (orange) waveguide [134]. (b) Measured broadening of birefringence phase matching spectra at different temperature gradients [135]. (c-d) temperature sensitive SHG laser [136,137].

Table 2. Comparison of SHG conversion efficiencies in peridically poled LNOI waveguide devices.

Type	ηnorm (% W^−1cm^−2)	Length (mm)	Coupling Loss(dB/facet) @ FW	Propagation Loss (dB/cm) @ FW
X-cut [67]	2600	4	10	<0.25
X-cut [67]	2300	4	10	<0.25
X-cut [142]	2200	4	4.3	0.3
X-cut [143]	4600	0.3	6	3
X-cut [143]	2800	0.6	6	3
X-cut [144]	3757	5	6.7	<1
Z-cut [145]	2400	1	5.4	2
X-cut [46]	3061	6	10	-
Z-cut, Blue light [146]	2900	6	15	3.9
X-cut [147]	435.5	6	10.5	1
X-cut Blue light [128]	33000	4.2	10	<1
X-cut [148]	3256	5	1	0.3
Z-cut [149]	320	20	1.2	0.8
X-cut [150]	684	3	7.2	3.6
Z-cut [43]	219	25	1	0.5
X-cut [130]	2150	21	4.4	0.76

Figure 9. Kerr frequency combs based on LNOI (a) soliton crystals with different soliton numbers [153]. (b) single-soliton and wo-soliton microcombs with near-octave soliton bandwidth [154]. (c) (d) the soliton spectrum and step in a ytterbium-doped lithium-niobate microring [155]. (e) Measurement scheme and optical spectrum of pulse-pumped Kerr frequency combs [156].

Figure 10. (a) EO comb generated by two coupled resonators for effective spectrum broadening [162]. (b) Schematic of dual-comb spectroscopy for gas detection based on racetrack resonator [163]. (c) Structure of power-efficient on-chip EO comb and combs with different pump wavelengths [164]. (d) Structure of Mach-Zehnder modulator for visible-to-near-infrared electro-optic frequency combs [165].

Figure 11. (a) High-rate quantum source realized by cavity enhanced SPDC on a micro-ring resonator [186]. (b) Deterministic multi-photon generation scheme [189]. (c) Integrated optical circuit for optical path coded photon state manipulation [190]. (d) Integrated electro-optical frequency splitter for frequency domain manipulation [191].

Figure 12. (a) LNOI-based superconducting nanowire single-photon detector [97]. (b) LNOI-based low-noise upconversion SPD with off-chip silicon single-photon avalanche photodiode (SPAD) [140]. (c) LNOI-based single photon buffer [194]. (d) LNOI-based quantum memory [195].

Figure 13. Single-frequency microlasers. (a) Coupled microrings [199]. (b) Coupled microcavities consisted of a microdisk and a microring [201]. (c) Single weakly perturbed microdisk laser from polygon modes [202]. (d) Photograph of long-cavity length coupled microrings [200]. (e) Optical graph of up-conversion fluorescence from Fabry-Perot resonator based on Sagnac loop reflectors [203].

Figure 14. (a) Schematic illustration of the integrated pockels laser [84]. (b) Recorded laser frequency modulation rate for the increasing modulation speed [84]. (c) Optical spectra of the dual-color lasing based on the SHG process [84]. (d) Integrated schematic diagram of the ultrafast tunable laser [220]. (e) False-colour SEM image of waveguide cross-section for a heterogeneously integrated Si₃N₄-LiNbO₃ external cavity chip [220]. (f) Point-cloud of the measured target scene in the three-dimensional optical ranging experiment [220].

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