Silicon-nanocrystal Optoelectronic Kerr Effect for 

Complementary Metal-oxide Semiconductor (CMOS)

Compatible Optical Switching

by Neal K. Bambha, Justin R. Bickford, and Stefan F. Preble

ARL-TR-5514 April 2011

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Army Research Laboratory

Adelphi, MD 20783-1197 

ARL-TR-5514 April 2011 

Silicon-nanocrystal Optoelectronic Kerr Effect for 

Complementary Metal-oxide Semiconductor (CMOS) 

Compatible Optical Switching

Neal K. Bambha and Justin R. Bickford

Sensors and Electron Devices Directorate, ARL

Stefan F. Preble

Rochester Institute of Technology 

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4. TITLE AND SUBTITLE

Silicon-nanocrystal Optoelectronic Kerr Effect for Complementary Metal-oxide 

Semiconductor (CMOS) Compatible Optical Switching 

5a. CONTRACT NUMBER

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6. AUTHOR(S)

Neal K. Bambha, Justin R. Bickford, and Stefan F. Preble

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U.S. Army Research Laboratory

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14. ABSTRACT

There is a broad Army need to quickly transfer large amounts of data from sensors to processors on a wide variety of systems. 

Complementary metal-oxide semiconductor (CMOS) compatible optical intra-chip data communication systems would enable 

this data flow by increasing data rates and reducing circuit size and power. We investigated the fabrication of a monolithic 

CMOS-compatible optoelectronic silicon (Si) modulator for intra-chip communication. The modulator is designed to take 

advantage of the large Kerr effect that has been reported in Si-nanocrystals imbedded in oxide (Si-nc). The expected refractive 

index change versus applied voltage was calculated for the design, and an index change of ~2 x 10–4 is enough to modulate the 

light, corresponding to a voltage of only 0.5–1.5 V. Confinement of light in the waveguide and microdisk is accomplished via a 

slot waveguide design. Several samples of Si-rich oxide films were deposited by reactively sputtering Si in the presence of 

oxygen, plasma enhanced chemical vapor deposition (PECVD), and low pressure chemical vapor deposition (LPCVD), as well 

as spin casting of hydrogen silsesquioxane (HSQ). The nonlinear index of these samples was characterized by an optical Zscan measurement using a short pulse fiber laser with a 1.5-μm wavelength.

15. SUBJECT TERMS

Silicon photonics, modulator, nanocrystal

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Neal K. Bambha

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iii

Contents

List of Figures iv

Acknowledgments v

1. Objective 1

2. Approach 2

3. Results 3

3.1 Nano-crystalline Film Generation ...................................................................................3

3.2 Kerr Nonlinearity Measurement Results .........................................................................4

4. Conclusions 5

5. References 7

6. Transitions 8

List of Symbols, Abbreviations, and Acronyms 9

Distribution List 10
iv

List of Figures

Figure 1. Proposed Si-based WDM optical link. ............................................................................1

Figure 2. Proposed Kerr effect microring resonator modulator (left), and estimated index 

change vs. applied voltage (right). .............................................................................................3

Figure 3. Typical measured ~800 nm Si-nc photoluminescence plots. ..........................................3

Figure 4. Z-scan measurement setup (left), simulated result (right [smooth line]), measured 

result (right [jagged line]). .........................................................................................................4

Figure 5. Diffraction-limited focused beam transmission through a translated pinhole (left 

[green]), measured transmission (left [blue]), expected and measured transmission of 

fiber pigtailed (nearly diffraction-limited beam) (right). ...........................................................5
v

Acknowledgments

We would like to thank Michael Wraback for use of his high-power free-space laser as well as 

Blair Connelly and Grace Metcalfe for supporting the laser.
vi

INTENTIONALLY LEFT BLANK. 
1

1. Objective

Current computing systems, such as field programmable gate arrays (FPGAs), graphical 

processor units (GPUs), and multi-core processors, are reaching their inter- and intra-chip data 

communication limits. Current links are based on copper transmission lines, which limit data 

capacity and speed, and are now dominating these systems’ power and space budgets. There is a 

broad Army need to quickly transfer large amounts of data from sensors to processors on a wide 

variety of systems. Such systems will find use in future embodiments of current program 

elements, such as advanced tactical computer science and sensor technology, unmanned ground 

vehicles, tactical unmanned aerial vehicles, and airborne reconnaissance systems. Leaps in intrachip communication technology will significantly improve computing performance and allow 

processors to perform previously unrealizable feats for these applications. Complementary 

metal-oxide semiconductor (CMOS) compatible optical intra-chip data communication systems 

would advance computing system performance/size-weight-power by increasing data rates and 

reducing circuit size and power.

We investigated the fabrication of a monolithic CMOS-compatible optoelectronic modulator in 

silicon (Si). The large Kerr effect of Si-nanocrystals imbedded in oxide (Si-nc) (1) may be used 

in an optical modulator, which could operate at high-speeds (up to 100 Gbit/s) with low 

switching voltage (~1 V). This device could be used in a system with several optical 

wavelengths, giving it the ability to multiply communication capacity via wavelength-division 

multiplexing (WDM); see figure 1 for an illustration of a WDM optical link. The anticipated 

improvements of this device over existing electronic switching would significantly advance nextgeneration processors.

Figure 1. Proposed Si-based WDM optical link.
2

2. Approach

Optical data communications systems require three devices: a light source, a modulator, and a 

detector—our proposed work focuses on the optical modulator. Traditional modulators are 

implemented in direct bandgap semiconductor materials such as indium gallium arsenide 

phosphide (InGaAsP) or electro-optic materials such as lithium niobate (LiNbO3). These 

materials have a long history of optimization, but their integration into standard Si CMOS 

processing has lagged far behind. A high-yield integration scheme has yet to be commercially 

implemented. The ultimate integration method would involve some form of Si itself as the 

active optical material. Since crystalline Si is an indirect bandgap material, its band-to-band 

optical absorption is not as efficient as direct bandgap materials, nor would its bandgap be 

suitable for traditional 1.55 μm wavelength light. Therefore, research has thus far focused on 

using free-carrier plasma dispersion as an alternative (2–5). This method absorbs light, which 

changes the material’s index of refraction via dispersion. This absorption requires carrier 

transport and, in present implementations, suffers from slow minority carrier diffusion. 

Reducing the active thickness over which the carriers must diffuse in an attempt to improve 

speed reduces the strength of the effect, trading efficiency for speed.

The proposed research focuses on the electro-optic effect, which changes a material’s index of 

refraction via an applied electric field according to equation 1. The traditional electro-optic 

effect used in optical modulators is the linear Pockels effect, which is related to the constant s1, 

while the second (usually weaker) Kerr effect is related to the constant s2. 

∆n = s1E + s2E2 (1)

Unfortunately, in unstrained crystalline Si, no Pockels effect exists and the Kerr effect is very 

small. However, a notably large Kerr effect of ~2 x 10–12cm

2

/W has been found in Si-nc (1). 

The Si-nc layer would be fabricated using equipment at the Rochester Institute of Technology 

(RIT) and the U.S. Army Research Laboratory (ARL) by depositing a Si-rich oxide film and 

coalescing the Si agglomerations into nanocrystalline particles via annealing. Films will be 

characterized via photoluminescence, transmission electron microscopy (TEM), and nonlinear 

optical measurements. The proposed microdisk resonator modulator is shown in figure 1 and 

illustrates how contacts (gold) would change the electric field in the Si-nc film (blue-gray)—

sandwiched between the Si layers (red). An oxide layer (light gray) separates the active device 

layers from the Si substrate (green). Light enters the waveguide on the right and is alternately 

exchanged between the microdisk resonator and the output of the waveguide. Figure 2 shows an 

estimate of the expected refractive index change versus applied voltage. An index change of 

~2 x 10–4 is enough to modulate the light, corresponding to a voltage of only 0.5–1.5 V. 

Confinement of light in the waveguide and microdisk is accomplished via a slot waveguide 

design (6). 
3

Figure 2. Proposed Kerr effect microring resonator modulator (left), and estimated index change vs. applied voltage 

(right).

3. Results

3.1 Nano-crystalline Film Generation

Several samples of Si-rich oxide films were deposited by reactively sputtering Si in the presence 

of oxygen, plasma enhanced chemical vapor deposition (PECVD), and low pressure chemical 

vapor deposition (LPCVD), as well as spin casting of hydrogen silsesquioxane (HSQ). 

Annealing the films for various times resulted in the coalescence of Si-nc within the silicon 

dioxide (SiO2) films. Photoluminescence of the samples show characteristic peaks near 800 nm

(figure 3). Annealing these films at ~1100 °C for 1–2 min or more yields the highest intensity 

photoluminescence.

Figure 3. Typical measured ~800 nm Si-nc photoluminescence plots.
4

3.2 Kerr Nonlinearity Measurement Results

One method to measure the Kerr nonlinearity is via Z-scan (7). Z-scan setups consist of a highpower pulsed laser source focused to a small spot, through which the sample is passed (figure 4). 

A pinhole and detector are placed after the sample to capture a portion of the exiting light. Since 

the incident beam has a cross-sectional distribution of intensity, its electric field distribution 

induces a corresponding index change in the sample. This distribution induces a lensing effect in 

the sample, which alters the exiting beam shape. A change in the exiting beam shape predictably 

alters how much light passes through the pinhole and into the detector. Z-scan is a method 

typically used for measuring the nonlinear index and absorption coefficients in thick film and 

thin bulk samples; our sample (at ~100–500 nm) is extremely thin—the thinner the film, the 

weaker the lensing effect, and therefore the smaller the detectable change in transmission. The 

simplest way of overcoming this is to increase the instantaneous laser intensity by choosing a 

short pulse laser (<500 fs) with high peak pulse energy (>400 nJ) and short focus optics; we have 

performed measurements using just such a laser at ARL.

Figure 4. Z-scan measurement setup (left), simulated result (right [smooth line]), measured result (right [jagged 

line]).

Figure 4 (right) shows a measurement of a PECVD grown Si-nc film. The green line shows the 

best fit simulation using n2= 5.5 x 10–17 m

2

/W and b= 8 x 10–12 m/W, which matches well with 

what we expected. The clipping seen in the experimental data near to the focus (near zero zdisplacement) is due to the laser’s non-Gaussian beam shape. The beam’s shape was tested by 

measuring its transmission through a pinhole translated through the focus. Figure 5 (green) 

shows the expected diffraction-limited transmission of the high-power free-space laser (left) and 

a low-power fiber pigtailed laser (right)—the experimental results are shown in blue. The highpower free-space laser does not focus down to the expected diffraction-limited spot size, this 

causes the clipping observed in figure 4.

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2

x 10-3

0.9

0.95

1

1.05

1.1

z displacement <m>

transmission <unitless ratio>

best fit zscan of PECVD #43 500nm: n2= 5.5e-17 m2/W, beta= 8e-12 m/W
5

Figure 5. Diffraction-limited focused beam transmission through a translated pinhole (left [green]), measured 

transmission (left [blue]), expected and measured transmission of fiber pigtailed (nearly diffraction-limited 

beam) (right).

Two other methods were investigated using the ideally shaped but lower powered fiber laser 

when the high-power free-space laser was unavailable. One method was based on dithering the 

sample along the z-axis as it was being scanned, while a lock-in amplifier measured the response 

on the transmission detector. The result seen by the transmitted detector would be the 

differentiation of the curve seen in figure 4. By locking the amplifier into the sample’s dithering 

frequency, the signal-to-noise ratio of the experiment could be greatly improved. The second 

method simply chopped the incoming light and carefully matched the response shape of the 

transmission and reference beams’ detectors and measuring the difference with a differential 

lock-in amplifier. Unfortunately, these efforts had to be abandoned after exhaustive testing 

revealed that the fiber laser’s low power simply could not overcome the sample’s lateral 

variations in linear transmission. Efforts were then refocused on obtaining a more ideal beam 

shape using the high-power free-space laser, but being a shared resource, these efforts were 

incomplete as of this writing.

4. Conclusions

Advancing the understanding of the Si-nc’s Kerr effect will lead to significantly improving Sibased optoelectronic devices. The proliferation of Si CMOS-compatible optical modulators will 

significantly improve processor performance and sensor data communications in Army (and 

eventually civilian areas as well) computationally intensive applications. Si CMOS-compatible 

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3

x 10-3

0

100

200

300

400

500

600

700

800

900

1000

-6 -4 -2 0 2 4 6

x 10-3

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09
6

modulators, waveguides, and detectors integrated with FPGAs will enable extremely highperformance processing. This is increasingly important as the sensors on fuzes, missiles, ground, 

and airborne platforms become more advanced, increasing the complexity of signal and image 

processing.

We were able to produce Si-nc films as well as measure their Kerr nonlinearity. Efforts are 

ongoing to improve our nonlinearity measurement setup. 
7

5. References

1. Hernandez, S. et al. J. Appl. Phys. 2008, 103, 064309. 

2. Xu, Qianfan; Manipatruni, Sasikanth; Schmidt, Brad; Shakya, Jagat; Lipson, Michal. Opt. 

Express 2007, 15, 340. 

3. Green, William M. J.; Rooks, Michael J.; Sekaric, Lidija; Vlasov, Yurii A. Opt. Express 

2007, 15, 17106. 

4. Liu, Ansheng; Liao, Ling; Rubin, Doron; Nguyen, Hat; Ciftcioglu, Berkehan; Chetrit, Yoel; 

Izhaky, Nahum; Paniccia, Mario. Opt. Express 2007, 15, 660. 

5. Watts, Michael R.; Trotter, Douglas C.; Young, Ralph W.; Lentine, Anthony L. Group IV 

Photonics IEEE, WA2, 17–19 Sept. 2008. 

6. Almeida, V. R.; Xu, Q. F.; Barrios, C. A.; Lipson, M. Opt. Letters 2004, 29, 1209–1211. 

7. Sheik-bahae, M.; Said, A. A.; Van Stryland, E. W. Opt. Letters 1989, 14, 955–957. 
8

6. Transitions

Stefan Preble is continuing this effort at RIT. We plan on improving the beam quality of the 

high-power laser to continue our Kerr nonlinearity measurement efforts. If successful, we plan 

on submitting our findings to Optics Letters. We will also continue our efforts to fabricate submicron structures using ARL’s e-beam lithography system.
9

List of Symbols, Abbreviations, and Acronyms

ARL U.S. Army Research Laboratory 

CMOS complementary metal-oxide semiconductor 

FPGAs field programmable gate arrays

GPUs graphical processor units 

HSQ hydrogen silsesquioxane 

InGaAsP indium gallium arsenide phosphide 

LiNbO3 lithium niobate 

LPCVD low pressure chemical vapor deposition 

PECVD plasma enhanced chemical vapor deposition 

RIT Rochester Institute of Technology 

Si silicon

Si-nc silicon-nanocrystals imbedded in oxide 

SiO2 silicon dioxide 

TEM transmission electron microscopy 

WDM wavelength-division multiplexing
10 

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