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Plasmonic isolator


Optical isolator is device, which is transparent in one direction and its blocks the light in opposite direction. Its function is similar to the diode in the electronics.


Why the integrated optical isolator is important?

Fig.1 Plasmonic isolator Co/TiO2/SiO2 integrated with Si nanowire waveguides

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In forward direction, a plasmon can excited and and light can reach from the input to output fiber.

In backward direction, a plasmon can not excited. Therefore, light is blocked and it can not reach the output fiber.

The integration of different optical elements on one substrate is important to make the Photonic Circuits to be more cheaper and to have more functions. In a case of a dense integration the undesirable and unavoidable back reflection between different optical elements can severely disturb the operation of the Photonic Circuit. The back reflection can be suppressed by an optical isolator integrated between optical elements.

At present, the most of optical elements (laser, detector, switch, modulator, amplifier) can be integrated in the Photonic Integrated Circuit(PIC), but not the isolator

Why the integration of the optical isolator is difficult?

Any design of the optical isolator should use a magneto-optical material. Only a magneto-optical material may have different optical properties

What is the plasmonic isolator?

The plasmonic isolator uses the unique non-reciprocal properties of plasmons, which propagates on a surface of a ferromagnetic metal.

The plasmonic isolator benefits from large magneto-optical constants of a ferromagnetic metal and the ability of a substantial enhancement of the magneto-optical effect using a plasmon (see here and here).

What are demerits of the plasmonic isolator?

(1) Unavoidable part of the plasmonic isolator is a metal, which absorbs light. Therefore, some insertion loss is unavoidable for the plasmonic isolator.

(2) In order to minimize the insertion loss, the length of the plasmonic isolator should be shorten. It limits the maximum isolation ratio the plasmonic isolator may provide. Maybe it is limited by 10 dB. The conventional bulk isolator provides 30 dB of the isolation.

What are merits of the plasmonic isolator?

(1) a small size.


(2) fabrication technology, which is well-compatible with the PIC


(3) a wide wavelength operational bandwidth

Comparing to other designs of the integrated isolator, the plasmonic isolator may not provide the largest isolation and may not have a smallest insertion loss, but a very small size, a simple and compatible fabrication technology and a wide wavelength operational bandwidth makes the plasmonic isolator a good fit for the integration into the PIC





Important technologies for the integrated plasmonic:

(technology 1) Looser out-of-plane confinement by a double-layer dielectric.

Details of the technology (1) are described here

(technology 2) Lateral optical confinement out-of-metal edge. Use of wedge, bridge and similar designs of a plasmonic structures.

Details of the technology (2) are described here

Plasmonic isolator
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Two types of plasmonic devices:

1) Type1: plasmonic waveguide blocks the direct propagation of light from input to output (See Fig.1)

2) Type 2: plasmonic waveguide in the vicinity of dielectric waveguide







Type 1. (1) Serial integration.

In this case the plasmonic waveguides blocks the light propagation in Si nanowire waveguides. Light can reach output only in a case when a plasmon is excited. (See Fig. 1 and Fig.2)


- Wedge-, bridge- or grove- types of plasmonic waveguides should be used. The confinement by a metal stripe should be avoided.

-Lift-off process can be used for a metal microfibrication .


- Properties of a plasmon (MO effect, electro-optical effect) significantly influence the output light, because light can reach output only by excitation of a plasmon


- Coupling efficiency between plasmonic and Si waveguides should be maximized (difficult)

-Propagation loss of plasmon should be minimized



1. Straight waveguides

Fig.2 Plasmonic isolator Co/TiO2/SiO2 integrated with Si nanowire waveguides. Set with different lengths of plasmonic waveguides

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2. Ring resonator


3. Side converter


4. MachZehnder interferometer







(2) Type 2. Integration in Parallel

Plasmonic waveguide near Si nanowire waveguide

In this case the plasmonic waveguides is integrated aside of a Si waveguide and the direct propagation of light is not blocked in Si waveguide. Only a resonance coupling in/out of a plasmonic waveguide only affects the output. It is similar to a ring resonator, where a ring is fabricated aside of a straight Si waveguide.


- The etching by the Ar-milling should be used for for a metal microfibrication. The lift-off process should be avoided

-A combination of bridge-type and metal-stripe type of a plasmonic waveguide should be used.


- A high coupling efficiency between plasmonic and Si waveguides is not as critical as in the case of the serial integration

-A low propagation loss of plasmon is not as critical as in the case of the serial integration.



- - Unique properties of a plasmon (MO effect, electro-optical effect) affect only a little the output light. The influence is stronger only in a case of a resonance coupling of light into the plasmonic waveguide.





(2). Ring resonator with an plasmonic waveguide integrated under ring

Ring resonator with plasmonic waveguide under it

Under the ring of Si-waveguide-made ring resonator, there is a magneto-optical plasmonic waveguide. The resonance wavelength can be changed by reversing the magnetic field or by reversing light propagation direction (exchange the output and input)













(3) Mach–Zehnder interferometer with two plasmonic waveguides integrated in each arm

Mach–Zehnder interferometer with a plasmonic waveguide under each arm


Under each arm of the Mach–Zehnder interferometer there is a plasmonic waveguide. The right figure shows direction of light propagation (yellow arrows). The blue arrows show the magnetization direction of a magneto-optical plasmonic waveguide. In respect to light propagation direction the magnetization is direct to right for light in the upper arm and to left in the lower arm.












The input light is split into half and half by the first directional coupler. One half passes the upper arm and another half to the lower arm. The light combines by the second coupler. The output depends on the phase difference between the upper and lower arms.

Operational principle. Mach–Zehnder interferometer with a plasmonic waveguide under each arm


forward light propagation

backward light propagation

The magnetic field H is directed to the right-hand direction with respect to the pulse movement in magneto-optical section (shown in red) in upper interferomater arm and to the left-hand direction in the lower arm. ThereforeRefractive index is larger in the upper arm than in lower arm. As a result, the there is phase delay for a pulse progating through the lower arm of the interferementer with respect  
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(4) Non-reciprocal directional coupler

Non-reciprocal directional coupler

Inside of the gap of the non-reiciprocal coupler there is a magneto-optical plasmonic waveguide. The yellow and blue arrows show the light propagation direction for light coupled to different inputs.

Because of the non-reciprocal properties of the plasmonic waveguide, the coupling efficiency is different for opposite propagation directions of light.



















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