LETTER

doi:10.1038/nature12237

A micrometre-scale Raman silicon laser with amicrowatt threshold
Yasushi Takahashi1,2, Yoshitaka Inui3, Masahiro Chihara1, Takashi Asano3, Ryo Terawaki1 & Susumu Noda3,4

The application of novel technologies to silicon electronics has been
intensively studied with a view to overcoming the physical limitations of Moore’s law, that is, the observation that the number ofcomponents on integrated chips tends to double every two years.
For example, silicon devices have enormous potential for photonic
integrated circuits on chips compatible with complementary metal–
oxide–semiconductor devices, with various key elements having
been demonstrated in the past decade1–6. In particular, a focus on
the exploitation of the Raman effect has added active optical functionality to pure silicon7–10, culminating in the realization of a continuous-wave all-silicon laser11. This achievement is an important
step towards silicon photonics, but the desired miniaturization to
micrometre dimensions and the reduction of the threshold for laser
action to microwatt powers have yet to be achieved: such lasers
remain limited to centimetre-sized cavities with thresholds higher
than 20 milliwatts12, even with the assistance of reverse-biased p–i–ndiodes. Here we demonstrate a continuous-wave Raman siliconlaser using a photonic-crystal, high-quality-factor nanocavity without any p–i–n diodes, yielding a device with a cavity size of less than
10 micrometres and an unprecedentedly low lasing threshold of
1 microwatt. Our nanocavity design exploits the principle that thestrength of light–matter interactions is proportional to the ratio of
quality factor to the cavity volume and allows drastic enhancementof the Raman gain beyond that predicted theoretically13,14. Such a
device may make it possible to construct practical silicon lasers andamplifiers for large-scale integration in photonic circuits.
In 2002, stimulated Raman scattering in rib waveguides was proposed
as a novel mechanism to generate optical gain in crystalline silicon7.
However, the Raman gain is inherently small compared with the gain
due to radiative recombination in direct-bandgap semiconductors15,16.
Therefore, Raman silicon lasers have thus far required long cavities andhigh excitation powers to generate net gain. Furthermore, free carrier
absorption (FCA) induced by two-photon absorption17,18 (TPA) has asuperlinear dependence on the excitation power, resulting in additional
losses that compete with the Raman gain. The incorporation of reversebiased p–i–n diodes along the waveguides has also been necessary to
remove the carriers10–12. To overcome all these disadvantages, the designof optical cavities with high quality factors (Q) and small volumes (V) is
essential because high Q/V ratios yield strong light–matter interactions
and enhance the Raman gain. From this point of view, the use of high-Q,
photonic-crystal nanocavities19–23 is desirable. However, there havethus far been no experimental reports of stimulated Raman scatteringin nanocavities and no feasible designs have been proposed for cavities
in which continuous-wave lasing can be achieved. Progress in thisarea of research halted after a number of numerical studies on elementary cavity structures were unsuccessful in finding a design in which
Raman emission can be detected13,14,24. However, we now present alaser design that incorporates a heterostructure nanocavity, and demonstrate that it exhibits lasing with an unprecedentedly ultralow threshold of 1 mW.

Figure 1a–c illustrates the basic features of the nanocavity Raman
laser developed in this study. The photonic crystal has the well-known
triangular lattice structure formed by circular air holes in a suspended
silicon membrane. The heterostructure nanocavity is formed from aline-defect waveguide (Fig. 1d). This waveguide has two types of propagation mode within the photonic bandgap, an odd-waveguide mode
and an even-waveguide mode. The low-loss propagation bands of thesemodes are shown in Fig. 1e. The modes are classified as ‘odd’ or ‘even’
by the symmetry of the electric field Ey, as schematically displayed. The
corresponding band structure in wavenumber (k)-space is shown in
Fig. 1f, where the blue and red curves show the dispersion relations forthe odd-waveguide mode and even-waveguide mode, respectively. The
dashed line represents the light line, below which the low-loss propagations are obtained (Supplementary Information, section A). The nanocavity is formed by a small increase (Da) of the lattice constant in ashort section of the waveguide, which moves the dispersion relation
curves to a lower frequency. Accordingly, the frequency of the odd- andeven-waveguide bands is lowered in this section21 (Fig. 1b), such that‘odd’ and ‘even’ nanocavity modes are formed as a result of opticalconfinement by the mode-gap differences. (The mode gap is the frequency range with no low-loss waveguide modes.) In our Raman laser,
we use these nanocavity modes to confine pump light and Stokes-Ramanscattered light, respectively (Fig. 1c).
Figure 1g shows the calculated x and y components of the electricfield distribution of the odd nanocavity mode, which are denoted
Ex_pump and Ey_pump, respectively. Figure 1h presents the analogous
electric field distributions of the even nanocavity mode, denoted
Ex_Raman and Ey_Raman. The Ex_pump and Ex_Raman distributions havedifferent parities because they are generated from different waveguidemodes. This is also the case for Ey_pump and Ey_Raman. Such pairs ofmodes initially seem unsuitable for Raman lasers owing to the small
degree of overlap for stimulated Raman scattering. However, it can beseen from Fig. 1g, h that the ‘cross components’, Ex_pump and Ey_Raman(and Ey_pump and Ex_Raman), have the same line symmetry and similar
distributions. This is a key observation, showing that the problem ofcavity design can be solved by carefully selecting the crystallographic
direction in which the nanocavity is formed, as discussed below. We
will next demonstrate that the frequencies (fp and fR), Q values (Qpand QR), modal volumes (Vp and VR) and electric field distributions(Ep and ER) associated with this unusual pair of modes allow dramaticenhancement of the Raman gain.
The first advantage of our design is that the frequency spacing(Df 5 fp 2 fR) between these nanocavity modes can be tuned to thesilicon Raman shift of 15.6 THz by changing the air-hole radius. This isbecause the electric field densities of the two modes within the air holes
are fundamentally unequal owing to the difference in parity. It is
fortuitous that the frequency spacing is close to 15.6 THz when thelattice constant (a) is set so that the even nanocavity mode has an
operation wavelength in the telecommunications band between 1.30
and 1.60 mm. Tuning the frequency spacing to exactly 15.6 THz is

1

Nanoscience and Nanotechnology Research Center, Research Organization for the 21st Century, Osaka Prefecture University, Sakai, Osaka 599-8570, Japan. 2Japan Science and Technology Agency,
PRESTO, Kawaguchi, Saitama 332-0012, Japan. 3Department of Electronic Science and Engineering, Kyoto University, Kyoto 615-8510, Japan. 4Photonics and Electronics Science and Engineering Center,
Kyoto University, Kyoto 615-8510, Japan.

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LETTER RESEARCH
a + Δa

a

a

a

a

d

Odd-WG modecyMode for pump laser
fp, Qp, Vp, Ep

Even-WG modex
SRS

15.6 THz

Frequency (c/a)

Odd nanocavity
mode

Odd-WG band

Mode gap

Even nanocavity
mode
Mode gap

Odd-WG modeeFrequency (c/a)

Mode-gap
differences

Frequencyb

CavityMode for Raman laser
fR, QR, VR, ER

Odd-WG band
Even-WG band

0.31

y

Mode gap

Even-WG mode
Even-WG band
Mode gap

EyyReal space (x)Real space (x)

f

Ey

g

h

Ex_pump

Ex_Raman

1.0

Different parity

0.5

Light line

0.0

Frequency (c/a)

0.30

-0.5

Same parity

0.29

Mode gap
0.28

–1.0

Ey_Raman

Ey_pump

0.27
Similar distribution

Mode gap
0.26

ySlab mode
0.25
0.20 0.25 0.30 0.35 0.40 0.45 0.50

Different parityx

Wavevector (2π/a)Figure 1 | Structure of Raman silicon laser based on a heterostructure
nanocavity. a, Schematic picture of the heterostructure nanocavity in thephotonic crystal slab. The lattice constant at the centre of the cavity is slightly
larger in the x direction. b, Band diagram of the nanocavity along thex direction, where c is the speed of light in vacuum. Two high-Q nanocavity
modes originating from odd- and even-waveguide modes are formed in theheterostructure. c, Configuration of Raman silicon laser using a small cavity.
The two nanocavity modes in b are used to confine pump light andStokes-Raman-scattered light, respectively. SRS, stimulated Raman scattering.
d, Line-defect waveguide formed by a missing row of air holes, in which two
propagating waveguide modes are formed. WG, waveguide. e, Band diagram ofthe line-defect waveguide. The insets sketch the profiles of the propagating
modes. f, Calculated band structure for the line-defect waveguide in k-space.
g, h, Calculated electric field distributions for the odd nanocavity mode and theeven nanocavity mode, respectively. The colour scale represents the intensity ofthe electric field.

essential for the efficient generation of Raman scattered light into theeven nanocavity mode, and the flexibility of our design is thus anadvantage over the alternatives13,14,24.
Second, although the Q factor of the even nanocavity mode (QR) canbe made as high as 4,000,000 in heterostructure nanocavities23, the odd
nanocavity mode has so far received little attention regarding its possible use as a high-Q resonant mode. However, we will show below that
the Q factor of this mode (Qp) can be higher than 100,000 in heterostructure nanocavities. Because the laser threshold is inversely proportional to the product of Qp and QR, owing to the reduction of cavity
loss, the characteristic of high Q factors for both modes is anotheradvantage of our design. Below we demonstrate experimentally thatthe product QpQR can exceed 1011, which is 10,000 times higher thanpreviously reported values13,14. In addition, the cavity volumes of bothmodes (Vp and VR) can be reduced to a cubic wavelength when Df is at
the desired value and QpQR is high. This volume is 10,000 times smaller than in earlier rib-waveguide Raman lasers11,12. Therefore, ourdesign enables extraordinarily high QpQR/V ratios, which no cavity
design reported thus far has been able to achieve.
Last, our design allows high effective overlap for the Raman gain
because the nanocavity is fabricated along a unique crystallographic
direction of the silicon-on-insulator (SOI) substrate, which further

enhances the advantages discussed above. If the device is fabricatedon a standard (001) SOI, then either the [100] or [110] direction can bechosen as the x axis of the nanocavity. Considering the Raman selection rules in silicon and that both nanocavity modes have transverse
electric polarization with respect to the slab, in our design the Ramangain in each direction is as follows, where an asterisk denotes complex
conjugate (Supplementary Information, section B):ðððG½100 ! jEx Raman Ey pump zEy Raman Ex pump j2 dx dy dz ð1Þ
Siððð
G½110 !

jEx Raman Ex pump {Ey Raman Ey pump j2 dx dy dz

ð2Þ

Si

Usually, SOI-based photonics devices are fabricated along [110] or its
equivalent directions because the wafer is easily cleaved along [110].
The previously reported rib-waveguide lasers were indeed designed
along [110] (refs 9–12) and the previous theoretical studies used thesame direction14,24. Although the Df and Q factors are insensitive to thedirection of fabrication, equation (1) indicates that the Raman gain will
be higher for the [100] direction because there is a negative term inequation (2). The overlap involving the cross components is significant
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RESEARCH LETTER
fR

fp

a

b
Odd-mode excitation
waveguide

Pump beamfp

Even-mode extraction
waveguide

y

–
[110]

]
00
[1

Nanocavity

[110]

fR
x
410 nmRadius: 130 nm

1 μm

420 nm
410 nm
415 nm
415 nm

50 μm

Figure 2 | Configuration of nanocavity Raman silicon laser. a, Scanningelectron microscope image of the core region of the laser. The annotations show
how laser light created from the pump beam is mainly emitted in the direction
vertical to the slab. We refer to this as dropped light. A portion of the light is

propagated along the even-mode extraction waveguide. b, Dark-fieldmicroscope image of the fabricated chip. Nanocavities oriented along [100] and[110] were fabricated. Patterned and unpatterned regions are clearly
distinguishable. The rectangles indicate the areas shown in a.

in equation (1), which involves the odd and even nanocavity modes
with the desirable parities and distributions indicated in Fig. 1g, h.
Consequently, our design is also superior in terms of overlap andcan drastically enhance the Raman gain when the device is fabricatedalong [100].
We now demonstrate the operation of a device fabricated according
to our design strategy. Figure 2a shows a scanning electron microscope
image of a nanocavity formed using well-established silicon nanofabrication technologies25. The central line defect is the heterostructure
nanocavity, and the upper and lower line defects are waveguides used
to pump the odd nanocavity mode and to extract Raman laser lightfrom the even nanocavity mode, respectively. The structural parameters
were chosen such that the even nanocavity mode has a wavelength in
the 1.55-mm band: the lattice constant is a 5 410 nm, the air-hole radius
is r 5 130 nm, the silicon slab thickness is 220 nm and there are two

successive 5-nm increases of a in the x direction. Figure 2b shows amicroscope image of the chip. We also fabricated a nanocavity along[110] for reference. We note that no p–i–n diodes to reduce the TPAinduced FCA were incorporated.
We investigated the optical properties using the measurement setup shown in Fig. 3a. Figure 3b, c show the measured resonant spectraof the two nanocavity modes for a sample with the x axis aligned to[100]. The even nanocavity mode was excited through the even-mode
extraction waveguide. The linewidth (Dl) of the odd nanocavity mode
peak is 13.7 pm at the resonant wavelength lp 5 1424.99 nm. A Qp
value of 104,000 is obtained from the relation Q 5 l/Dl. The QR value
for the even nanocavity mode is estimated as 1,400,000 at the resonant
wavelength lR 5 1540.13 nm, and, thus, QpQR 5 1.5 3 1011. The frequency spacing is Df 5 c(1/lp 2 1/lR) 5 15.74 THz, which is notexactly matched to the silicon Raman shift of 15.6 THz.

ad〈Monochromator〉

Fibre(Lock-in amplifier)
Lens

(PD)

Objective lens
(Chopper)0
1,536

Samplece

Odd nanocavity mode
Qp

0.6 13.7 pm
0.4
0.2
0
1,424.96
1,425.00
Wavelength (nm)Even nanocavity mode
1.0

> 105

Intensity (a.u.)

Intensity (a.u.)

Normal spontaneous
Raman peak

0.2

Lens Polarizer
(TE)

b
0.8

0.4

Fibre

Tunable laser

1.0

fR

0.6

QR

0.8
0.6

> 106

1.1 pm0.4
0.2
01,540.120 1,540.130 1,540.140
Wavelength (nm)Raman intensity (a.u.)

Coupler
90/10

fp

Even-nanocavity-mode
enhanced peak

0.8

]
00
[1

〈Long-pass filter〉(Wavelength meter)

1.0

Raman intensity (a.u.)

Near-infrared
camera

1,538

1,540
1,542
Wavelength (nm)

1.0
0.8
0.6

Normal spontaneous
Raman peak

1,544

fR

fp

[110]

0.4
0.2
0
1,532

1,534

1,536

1,538

1,540

Wavelength (nm)Figure 3 | Optical measurements on fabricated nanocavities. a, Set-up used
to obtain resonant spectra and Raman-scattering spectra. The components
enclosed in parentheses and angle brackets were used for resonant spectra andRaman spectra, respectively. TE, transverse electric. b, c, Resonant spectra forodd (b) and even (c) nanocavity modes. a.u., arbitrary units. d, Raman

spectrum for the [100] nanocavity, measured while pumping the odd
nanocavity mode in b with an input power of 1 mW. e, Raman spectrum for thereference [110] nanocavity. The resolution limit of the monochromator is,100 pm.

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LETTER RESEARCH
1.2

100
80

0.8
0.4

2.0 μW
pump

0.0
1,500

1.2

1,000
500

0.8

0.5 μW
0.4
pump

01,541 1,542 1,543 1,544

60

fp

× 7,000

0.0

Laser output
fR

1,541 1,542 1,543 1,544 1,545

Wavelength (nm)

40
]
00
[1

Raman output power (nW)120

Intensity (107 counts s–1)

a

20
0
0.0

0.5

1.0

b

1.5
2.0
Pump power (μW)

2.5

d
O

]

-W
G

G

en

-W

dd

00

[1
Ev

Waveguide
edge

3.0

5,000
4,000
3,000
2,000
1,000

1.0 μW pumpc

1.0 μW pump0e

1.1 μW pump

1.1 μW pump

10 μm

Figure 4 | Continuous-wave operation of fabricated nanocavity Raman
silicon laser. a, Laser output power (black points) as a function of pump power
coupled into the nanocavity. The absolute power was estimated from thedropped-light intensity, taking into account the loss from the experimental setup. The red line shows the output of a typical laser with a slope efficiency of 8%
above threshold. The insets show the Raman spectra below and above thethreshold power on the same scale, demonstrating Raman laser action.
b, c, Near-infrared camera images of the nanocavity Raman laser below andabove the threshold, using a long-pass filter to eliminate the pump light. Dottedlines represent the waveguides. Rectangles indicate the nanocavity position.
d, e, Near-infrared images at a waveguide edge. Images in b–e are shown on thesame intensity scale.

Figure 3d shows the Raman scattering spectrum measured when the
odd nanocavity mode in Fig. 3b was excited by a continuous-wave laser
beam coupling only 1 mW of power into the cavity. Although the detuning,
Df 2 15.6 THz, from the Raman shift is larger than the Raman gain widthof silicon (,0.1 THz), a sharp Raman scattering peak is neverthelessobserved. This is enhanced by the strong confinement of the even nanocavity mode at the tail of the normal spontaneous Raman emission peak.
The linewidth of the normal spontaneous Raman peak is 750 pm, whichis almost the same as the Raman gain width of silicon. As shown in Fig. 3e,
the resonant peak is much smaller for the reference [110] nanocavity,
which has similar Q values and Df to the [100] nanocavity in Fig. 3d.
Although the sample in Fig. 3d did not oscillate, owing to the residual
mismatch of Df, we were able to fabricate a sample with a Df value ofexactly 15.6 THz by carefully changing the radius of the air holes (Supplementary Information, section C). We separately confirmed that therate of change of Df with radius r is (hDf/hr) < 0.15 THz nm21.
Finally, we present details of lasing in a sample with Qp 5 140,000,
QR 5 1,500,000, lp 5 1428.38 nm and lR 5 1543.02 nm. The laser output power is plotted in Fig. 4a as a function of the pump power coupled
into the nanocavity, showing lasing oscillation above a threshold of

1 mW. This threshold is 20,000 times smaller than that previously
reported for a rib-waveguide Raman silicon laser12 and is also smaller
than predicted by numerical studies13,14. The spectral peak intensities
in the insets indicate that the efficiency for Raman light generation
above threshold is more than 1,000 times higher than that below threshold (Supplementary Information, section D). In addition, the slope
efficiency is 8% and the laser efficiency is 4% without any p–i–n diodes(Supplementary Information, section E). The resolution limit of themonochromator (,100 pm) is much larger than the linewidth of theeven nanocavity mode (,1 pm) and, thus, linewidth narrowing relative
to the lasing could not be detected. We separately confirmed thatsample heating due to increasing pump powers is negligible (see Supplementary Information, section F, for temperature dependence).
Figure 4b and Fig. 4c show near-infrared camera images of the nanocavity below and above the threshold power, respectively. Figure 4d, eshows images of a waveguide facet, demonstrating that a portion of theRaman laser beam was successfully extracted from the even-mode
extraction waveguide.
This low-threshold lasing result is a consequence of the fact that ournanocavity design produces net Raman gain in the low-excitation
range before TPA-induced FCA becomes dominant. Because Ramangain has a linear dependence on pump power whereas TPA-induced
FCA has a superlinear dependence, the Raman gain can exceed losses
for low pump power in our design, where the high QR value and thelarge overlap between the nanocavity modes have especially importantroles. Although the lasing output power gradually saturates withincreasing pump power, most probably as a result of TPA-induced
FCA, this effect could be reduced by reducing the free carrier lifetimes.
There is also potential for further performance enhancement, including the reduction of threshold pump power (=100 nW), for example byincreasing the Qp and QR factors23 and modifying the volumes. Such an
ultralow threshold is encouraging for a number of reasons. For
instance, electrically driven silicon spontaneous-emission sources might
be used for integrated pump light sources even though their efficiencies
would be low, and Raman silicon laser chips without external pumpsources could be realized in the future. The operational wavelength
range could be extended above and below the 1.55-mm band25. It will
also be possible to make other Raman amplification devices based on
photonic crystals26–28 using our design strategy. We believe that ourdevice will stimulate silicon photonics research in a number of areas forthe realization of compact photonic integrated circuit chips.

METHODS SUMMARY
We used the three-dimensional, finite-difference time-domain method29 to calculate
the band structure and the electric field distributions in Fig. 1, where the structuralparameters in Fig. 2a were used. The calculated Qp and QR values are 500,000 and
4,000,000, respectively. The experimental Q values obtained from Fig. 3b, c aresmaller owing to the load of the two adjacent waveguides and imperfections in the
fabricated samples29,30.
The samples were fabricated using the following steps25. A thin film of electronbeam resist was deposited by spin coating. The photonic-crystal patterns were drawn
on the resist by electron-beam lithography. Next the samples were immersed in thedeveloper at room temperature (22 uC) and the developed mask patterns weretransferred to the top silicon slab using sulphur-hexafluoride-based dry etching.
Finally the silicon dioxide layer underneath the patterned region was selectively
removed using hydrofluoric acid to form an air-bridge structure.
In the optical measurements, the temperature of the sample stage was stabilized
at room temperature using a Peltier controller. The nanocavity modes were excitedby evanescent mode coupling with the corresponding waveguides, and dropped
light from the nanocavity in the direction vertical to the slab was measured while
scanning the wavelength-tuneable laser with a narrow linewidth. The signal wascollected from the 5-mm spot area at the centre of the nanocavity as shown in
Fig. 2a. This measurement configuration is convenient for performing sensitive,
quantitative analysis of the Raman laser (Supplementary Information, section E).
The odd-mode excitation waveguide was fabricated close to the nanocavity because
the Ey_pump distribution is tightly confined in the y direction (Fig. 1g). For highsensitivity Raman spectroscopy, we used a 500-mm-focal-length monochromator
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RESEARCH LETTER
with a liquid-nitrogen-cooled InGaAs-arrayed detector and a 0.65-numericalaperture objective lens for the dropped light.
Received 21 February; accepted 30 April 2013.
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Acknowledgements We especially thank H. Takano for a preliminary calculation done
before the start of this project. We thank K. Ishizaki and K. Kitamura for assistance indevice fabrication, Y. Tanaka for assistance with finite-difference time-domain
calculations, A. Oskooi and H. Ishihara for comments, and Y. Sakamoto for software
assistance. Y.T. is supported by the NanoSquare programme, Funds for theDevelopment of Human Resources in Science and Technology, commissioned by
MEXT. This work was supported by JST, PRESTO, the NanoSquare programme and
MEXT KAKENHI (grant numbers 23104721 and 21104512). The spectral
measurements were partly supported by JSPS KAKENHI (grant number 23686015)
and the Asahi Grass Foundation. The device fabrication was greatly supported by JSPS
KAKENHI (grant number 20226002), the Ministry of Economy, Trade and Industry(METI) through its ‘Future Pioneering Projects’, and the CPHoST programme.
Author Contributions Y.T. designed the project, designed the original device,fabricated the samples, performed the measurements and wrote the paper. S.N.
organized the contribution to the project from Kyoto University, where the fundamental
studies to realize high-Q/V nanocavities and the initial investigation into Raman lasers
was performed. S.N. also contributed greatly to writing the paper. Y.I. analyticallydetermined the optimum crystalline direction for lasing and contributed to writing
Supplementary Information, sections A and B. M.C. established the method to tune thenanocavity mode spacing, T.A. contributed to the theoretical analysis and R.T.
contributed to the development of the measurement system.
Author Information Reprints and permissions information is available atwww.nature.com/reprints. The authors declare no competing financial interests.
Readers are welcome to comment on the online version of the paper. Correspondence
and requests for materials should be addressed to Y.T.
(y-takahashi@21c.osakafu-u.ac.jp) or S.N. (snoda@kuee.kyoto-u.ac.jp).

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