Dr. Vadym Zayets

Fabrication technology of plasmonic waveguides
Thsi is my invited talk at SPIE Photonic West 2022
YouTube video

 

 

Why it is important?

The integration of different optical components on one substrate has many benefits. Similar to electronic devices, an integrated optical circuit may have a lower cost and better functionality. The size of MOSFET transistors are very small and millions of the transistors can be integrated into one electrical circuit. In contrast, the size of optical components is not as small. The typical length of optical components is about a millimeter and only a few optical components can be integrated into one chip. The length of optical components is limited by the wavelength of light. The size of optical elements can be reduced when Si nanowire waveguides or/and plasmonic waveguides are used.

The optical confinement in a Si nanowire waveguide is strong, because of a high refractive index contrast. Due to strong optical confinement, the Si nanowire waveguides are very narrow with width of 450 nm and they can sharply bend with bending radius as small as 1 mm. Even though the length of Si-nanowire devices has to be still relatively long, by bending the optical device can be packed into small area of a few um2.

The optical confinement of a plasmonic waveguide is much stronger that the confinement in the Si nano waveguide, because of a substantially larger refractive index contrast between a metal and a dielectric. The plasmonic devices are even smaller. Typical length of a plasmonic device is 10-30 um.
Different functionalities can be achieved using either Si nanowire waveguides or plasmonic waveguides. It is attractive to integrate into one cheap both the Si nanowire waveguides and plasmonic waveguides. In this case a small-size cheap with substantial number of optical elements and complex functionality could be fabricated.

 

 

 

 

 

 

 

 

 

 

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Key points of the technology

1. Loss reduction for plasmonic waveguides

A metal is essential material of a plasmonic waveguide. Any metal significantly absorbs light. Therefore, some optical loss is unavoidable in a plasmonic waveguide. In case if this loss is too large, all light is absorbed in plasmonic waveguide. Even if such plasmonic waveguide might have an unique property, it has no any practical use.

The length of a plasmonic waveguide is about 1-50 um. The device loss of 3-5 dB is acceptable. The loss of plasmonic waveguide should be at least smaller than 1 dB/um. The loss of 0.1 dB/um is a small loss.

Of cause, the length of plasmonic waveguide is fixed by the effect for which the plasmonic waveguides is used.

Usually, there is a trade-off between there is trade-off between the strength used effect and the optical loss. For example, a higher isolation or stronger electro-optical conversion or magneto-optical conversion occurs in a plasmonic waveguide of higher loss.

 

2. Improving of coupling efficiency between Si-nanowire and plasmonic waveguides

The coupling efficiency better than 1 dB/facet is desirable.

 

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Serial and Parallel Integration of a plasmonic and a Si nanowire waveguides.

more in page on plasmonic isolator

(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)

Technology:

- 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 micro fabrication .

Merits:

- 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

Difficulties:

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

-Propagation loss of plasmon should be minimized

 

(2) Integration in Parallel

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.

Technology:

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

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

Merits:

- 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.

 

Difficulties:

- - 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.

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Technology for reduction of propagation loss in plasmonic waveguides :

 

Important technologies for plasmonic waveguides:

Any plasmonic structure contains a metal, which strongly absorbs light. Reduction of propagation loss is essential for any practical application of a plasmon. There are two technology, which are very effective for reduction of plasmon' propagation loss:

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

The looser optical confinement makes the optical field of a plasmon more far from the metal. As result, the propagation loss is reduced.

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.

In this case a huge optical scattering loss at edge of the metal can be avoided.

This page describes the details of the technology (2).

 

 

The usage of optical in-plane confinement, which remove a plasmon out of metal edge, is essential to fabricate a low-optical-loss magneto-optical plasmonic waveguide integrated with Si nanowire waveguides.

 

 

 

 

 

 

 

 

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Wedge, Bridge, Grove and Stripe-types of plasmonic waveguides

 

 

The optical loss of a plasmon due to the scattering at an edge of a metal can be significant and often it is the main contributor to the total optical loss of plasmon.

In the case of a narrow plasmonic waveguide the scattering at metal edges becomes a major mechanism of optical loss!!!

It is important:

The wedge, bridge and grove types of design of plasmonic waveguides are used in order to achieve a lateral optical confinement out-of-metal edge!!!

Fiber-to-Fiber transmission in Co/TiO2/SiO2 plasmonic waveguides integrated with Si nanowire waveguides

Bridge-type plasmon propagation loss: 0.7 dB/um Coupling: plasmon to waveguide mode 4 dB per facet Wedge-type plasmon propagation loss: 1.5 dB/um (1550 nm) 1 dB/um (1590 nm) Coupling: plasmon to waveguide mode 6 dB per facet

Note: the fiber-to-fiber transmission includes: fiber-to-waveguide coupling + Si waveguide loss + waveguide- to- fiber coupling=10 dB. Plasmon to waveguide-mode coupling waveguide-mode to plasmon coupling plasmon propagation loss Note: black line in left graph shows the transmission in the case when there is no plasmonic section click on image to enlarge it TM0 mode is excited in Si waveguide

 

1) bridge type

Technology (2 steps):

(1) A narrow bridge of Si between Si waveguides is fabricated at the same when Si nanowire waveguides

Width of bridge is 70-100 nm.

(2) the bridge is covered by metal. Width ~1 um. EP with positive resist + lift-off.

It is very effective method for both the reduction of propagation loss and the improving the coupling efficiency

Complexity of fabrication: minimal

Note: The beam current for EB beam should be 50 pA for the bridge, but 400 pA for Si waveguide.

 

2) wedge type

Technology (3 steps):

1) A narrow bridge of Si between Si waveguides is fabricated at the same when Si nanowire waveguides. Width 150-250 nm. It is OK to use 400 pA EB current.

2) The bridge is wet-etched and a sharp wedge is formed on the top

3) The wedge is covered by a metal. Width ~1 um. EP with positive resist + lift-off.

Width of bridge is 70-100 nm.

3)grove type

Technology (5 steps):

1) 1-um-wide Si stripe between Si waveguides is fabricated at the same when Si nanowire waveguides

2) A 200-nm wide gap in SiO2 is fabricated on top of the Si stripe. EB with negative resist+ SiO2 sputtering+ lift-off.

3) The si is wet-etched through the gap

4) SiO2 mask is wet etched by BHF.

5) The grove is covered by a metal. Width ~1 um. EP with positive resist + lift-off.

For a grove-type Co/TiO2/SiO2 plasmonic waveguide I have obtained 1.2 dB/um plasmon propagation loss, which is comparable with bridge and wedge-types plasmonic waveguides.

 

4) metallic stripe

The simplest technology for lateral confinement of a plasmon. Often the optical loss of plasmons for the stripe confinement is very high.

For example:

I have fabricated several Co/TiO2/SiO2 plasmonic waveguides on Si using Co stripe. Even though double-layer TiO2/SiO2 is proved to efficient for loss reduction, the optical loss in plasmonic structure with a Co stripe was unacceptably high about 10-15 dB/um.

The loss was so high, I could see no output even for short 8-um plasmonic waveguides. In case when instead I have used either a bridge or wedge or groove -type of plasmonic waveguide a lot of light comes out. The improvement is so obvious!!!

The reason of a high loss in a stripe-type plasmonic was found to be the optical scattering at the edges of the metal stripe. In order to integrate a plasmonic waveguide with a Si nanowire waveguide, the metal stripe should be narrow about 450 nm. In the case of a narrow plasmonic waveguide the scattering at metal edges becomes a major mechanism of optical loss. In order to avoid it, another method for in-plane confinement should be used, which could ensure the confinement of a plasmon out of metal edges.

 

 

 

 

 

I am very thankful to Dr. A. Baryshev and Dr. A.M. Merzlikin for many suggestions on wedge and groove plasmonic waveguides

 

 

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Lift-off vs. Etching?

 

The metal cover of a plasmonic waveguide can be fabricated ether by

1) positive EB resist + lift-off.

2) negative EB resist + etching by Ar milling

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Note: in case (1) the method of double resist (EB resist +LOR) should be used. The use of a lift-off machine is preferable.

Note: It is difficult to etch a metal film thicker than 100-200 nm

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It is important. The edge of metal is much smoother when the etching is used instead of the lift-off!!!