Integrated photonics in the 21st century

Fig. 1. Moore’s law for integration density in terms of equivalent number of elements per square micrometer of integrated photonics devices, showing a growth faster than the IC Moorés law, adapted from [3]. The figure covers, in time order, a lithium niobate 4×4 polarization-independent switch array, a 4×4 InP-based integrated gated amplifier switch array, an SOI AWG, and a hybrid plasmonic (passive) directional coupler. All these are experimentally demonstrated. At the top is a simulation of two coupled metal nanoparticle arrays, forming a directional coupler, each array being a resonantly operated array of silver nanoparticles. If loss requirements of, e.g., 3 dB/cm were invoked, the latter two would occupy significantly lower places in the figure.

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$Electric field distribution of TE mode in a silicon channel dielectric waveguide. The yellow and red curves express the amplitude distribution in the x and y directions, respectively; the substrate material is SiO2, and the cladding is air. The guiding light core is made of silicon material with the geometry parameters height=200 nm and width=450 nm. The operating wavelength is 1550 nm. Channel dielectric waveguides, similarly to optical fibers, utilize total internal reflection, guiding light in higher refractive index core surrounded by lower index cladding material.$

Fig. 2. Electric field distribution of TE mode in a silicon channel dielectric waveguide. The yellow and red curves express the amplitude distribution in the x and y directions, respectively; the substrate material is SiO2, and the cladding is air. The guiding light core is made of silicon material with the geometry parameters height=200 nm and width=450 nm. The operating wavelength is 1550 nm. Channel dielectric waveguides, similarly to optical fibers, utilize total internal reflection, guiding light in higher refractive index core surrounded by lower index cladding material.

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Fig. 3. Ultrasmall subwavelength hybrid plasmonic microdisk. (a) Schematic diagram and (b) SEM image of the fabricated device with radius around 525 nm. At this radius the cavity has a resonance at about 1550 nm and the intrinsic quality factor Q is about 200. The thicknesses of the Au, SiO2, and Si layers are 100, 56, and 400 nm, respectively. The access waveguide width is 170 nm and the gap between the straight waveguide and the microdisk is 56 nm. The measured propagation losses of the access waveguide are 0.08 dB/μm.

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$(a) Schematic diagram of the hybrid plasmonic microring modulator. (b) Cross-sectional view along the x–y plane of the Ez field distributions of a resonant mode at 1550 nm with an azimuthal number of 6. The modulator consists of an EOP ring with radius R and a width W sandwiched between a silver ring and a silicon ring with the same radii and widths. A microwave field is applied between the Ag cap and the bottom Si layer, and the refractive index of the EOP can be changed using the ultrafast EO (Pockels) effect; correspondingly, the cavity can be switched between on- and off-resonance modes at a given frequency, resulting in the modulation of transmission power if an access waveguide is placed aside.$

Fig. 4. (a) Schematic diagram of the hybrid plasmonic microring modulator. (b) Cross-sectional view along the x–y plane of the Ez field distributions of a resonant mode at 1550 nm with an azimuthal number of 6. The modulator consists of an EOP ring with radius R and a width W sandwiched between a silver ring and a silicon ring with the same radii and widths. A microwave field is applied between the Ag cap and the bottom Si layer, and the refractive index of the EOP can be changed using the ultrafast EO (Pockels) effect; correspondingly, the cavity can be switched between on- and off-resonance modes at a given frequency, resulting in the modulation of transmission power if an access waveguide is placed aside.

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Column	1	2	3	4	5	6
Characteristics	${SiO}_{2}$ Low $Δ$	${SiO}_{2}$ Medium $Δ$	${SiO}_{2}$ High $Δ$	${SiON}_{x}$	III/V	SOI
Index difference $Δ$ (%) $Δ = \frac{n_{core} - n_{clad}}{n_{core}}$	0.3	0.45	0.75	3.3	7.0 (46)	41 (46)
Core size (μm)	$8 \times 8$	$7 \times 7$	$6 \times 6$	$3 \times 2$	$2.5 \times 0.5$ ( $0.2 \times 0.5$ )	$0.2 \times 0.5$ $0.3 \times 0.3$
Loss (dB/cm)	$< 0.01$	0.02	0.04	0.1	2.5–3.5	1.8–2.0
Coupling loss (dB/point)	$< 0.1$	0.1	0.4	3.7 (2)	5	6.8 (0.8)
Bending radius (mm)	25	15	5	0.8	0.25 (0.005)	0.002–0.005

Table 1. Waveguide Parameters for Different Materials

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	Device and Wavelength $λ_{0}$ (A, amplitude; P, phase)	$V π L$ [Vμm]	$V π$ [V]	L [μm]	IL [dB] (Attenuation) [dB/μm)]	Confine-ment	Switch Energy [fJ/bit] (Capacitance) [fF]	Comments
1	P, Layered metal/chalcogenide waveguide [22], $λ = 1.55 μm$	0.66	0.33	2	7 (3.5)	$0.01 μm \times 0.01 μm$	0.003 (0.01)	Chalcogenide thickness 4 nm, index change 0.1
2	P, Array of Ag nanoparticles in EOP matrix, $λ = 0.680 μm$	3	15	0.2	2.4 (12)	$\sim 0.01 μm \times 0.01 μm$	(Very approxi-mate) 2 (0.01)	200 nm electrode separation. Very rough approximation, real values probably much better. Trading lower voltage for length impeded by loss
3	P, Slotline Si/EOP/Si, $λ = 1.55 μm$	160	2	80	0.1 (0.001)	$\sim 0.3 μm \times 0.7 μm$	33 (8)	Doped Si serves as electrodes. 100 nm EOP
4	A, Silicon microring resonant modulator, $λ = 1.55 μm$	41	1	41	$\sim 5$	$0.3 μm \times 0.38 μm$	$\sim 50$	Depletion mode modulator Experiment
5	A, III–V Electroabsorption QCSE [23], $λ = 1.55 μm$	400	2	200 active 500 total	3–5	$4 μm \times 4 μm$	$300 (n / a)$	Traveling-wave type EAM, 50 Ω transmission line Experiment