Pure-quartic solitons

Abstract

Temporal optical solitons have been the subject of intense research due to their intriguing physics and applications in ultrafast optics and supercontinuum generation. Conventional bright optical solitons result from the interaction of anomalous group-velocity dispersion and self-phase modulation. Here we experimentally demonstrate a class of bright soliton arising purely from the interaction of negative fourth-order dispersion and self-phase modulation, which can occur even for normal group-velocity dispersion. We provide experimental and numerical evidence of shape-preserving propagation and flat temporal phase for the fundamental pure-quartic soliton and periodically modulated propagation for the higher-order pure-quartic solitons. We derive the approximate shape of the fundamental pure-quartic soliton and discover that is surprisingly Gaussian, exhibiting excellent agreement with our experimental observations. Our discovery, enabled by precise dispersion engineering, could find applications in communications, frequency combs and ultrafast lasers.

ARTICLE

Received 8 Oct 2015 | Accepted 9 Dec 2015 | Published 29 Jan 2016

Temporal optical solitons have been the subject of intense research due to their intriguing physics and applications in ultrafast optics and supercontinuum generation. Conventional bright optical solitons result from the interaction of anomalous group-velocity dispersion and self-phase modulation. Here we experimentally demonstrate a class of bright soliton arising purely from the interaction of negative fourth-order dispersion and self-phase modulation, which can occur even for normal group-velocity dispersion. We provide experimental and numerical evidence of shape-preserving propagation and at temporal phase for the fundamental pure-quartic soliton and periodically modulated propagation for the higher-order pure-quartic solitons. We derive the approximate shape of the fundamental pure-quartic soliton and discover that is surprisingly Gaussian, exhibiting excellent agreement with our experimental observations. Our discovery, enabled by precise dispersion engineering, could nd applications in communications, frequency combs and ultrafast lasers.

DOI: 10.1038/ncomms10427 OPEN

Pure-quartic solitons

Andrea Blanco-Redondo1, de Sterke C. Martijn1, J.E. Sipe2, Thomas F. Krauss3, Benjamin J. Eggleton1

& Chad Husko1

1 Centre for Ultrahigh bandwidth Devices for Optical Systems (CUDOS), Institute of Photonics and Optical Science (IPOS), School of Physics, The University of Sydney, Sydney, New South Wales 2006, Australia. 2 Department of Physics, University of Toronto, 60 Street George Street, Toronto, Ontario, Canada M5S 1A7. 3 Department of Physics, University of York, York, YO10 5DD, UK. Correspondence and requests for materials should be addressed to A.B-R.(email: mailto:[email protected]

Web End [email protected] ).

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The fascinating phenomenon of optical solitons, solitary optical waves that propagate in a particle-like fashion over long distances1, has been the subject of intense research

during the last decades due to its major role in breakthrough applications such as mode locking2, frequency combs3,4 and supercontinuum generation5,6 among others79. Temporal solitons in optical media10,11, as studied to date, arise from the balance of the phase shift due to anomalous quadratic group-velocity dispersion (GVD), that is, negative GVD parameter b2 (@2k/@o2)o0, and the self-phase modulation

(SPM) due to the nonlinear Kerr effect.

In practice, higher-order nonlinear and dispersive effects often perturb this behaviour. In silicon (semiconductor) waveguides the most signicant higher-order nonlinearities are associated with free carriers (FCs) generated by two-photon absorption (TPA)12,13, which have hampered the observation of soliton-based effects in this material. Recently, some of us achieved higher-order soliton compression of picosecond pulses in silicon14 by using a dispersion engineered photonic crystal waveguide (PhC-wg). Turning to higher-order dispersive effects, the presence of third order dispersion (TOD; b3 @3k/@o3) leads

to soliton instability15, whereas positive fourth-order dispersion (FOD; b4 (q4k/@o4)40), can give rise to radiation at specic

frequencies16. In the presence of negative FOD (b4o0), as was shown by a series of theoretical works in optical bres1721, solitons can be stable. These studies1721 led to the concept of quartic solitons22, solitary pulses resulting from the interaction of anomalous GVD and SPM but modied by the presence of FOD.

In the following we report the experimental discovery and physical description of an entirely new class of solitons originating purely from the interaction of negative FOD and SPM, as conceptually depicted in Fig. 1a, which can occur even when the GVD vanishes or is normal. Since they arise just from quartic dispersion and SPM, and to distinguish them from the

solitary waves studied earlier1722, we propose the name of pure-quartic soliton for this new class of solitary wave. This experimental discovery is enabled by the unique dispersion properties of PhC-wgs, which allow us to combine very large negative b4 with small positive b2 and negligible b3 for the wavelength under study. Though our work directly pertains to pulse propagation in guided wave structures, the same ideas apply to spatial solitons, particularly to subdiffractive matter-wave solitons in regimes where the fourth-order diffraction is the dominant diffractive effect23,24. Furthermore, it was shown that in Ti:sapphire lasers FOD ultimately limits the minimum pulse duration in cavities with near-zero GVD and TOD2527, hinting that the pulse shaping behaviour in the laser cavity for ultrashort pulses (below 10 fs) arises from the balance of SPM and FOD28.

ResultsExperimental signatures of pure-quartic solitons. In our experiments we performed time- and phase-resolved propagation measurements on the sample using a frequency-resolved electrical gating (FREG) apparatus, depicted in Fig. 1b, which can be modelled using a generalized nonlinear Schrodinger equation (GNLSE). We show shape-preserving propagation and at temporal phase for fundamental pure-quartic solitons and temporal compression and convex nonlinear phase for higher-order pure-quartic solitons. In spite of maintaining these well-known signatures of soliton-like behaviour, we show that pure-quartic solitons present remarkably different properties than solitons studied to date10,11,1721. Importantly, the energy scaling of pure-quartic solitons suggests much higher energies for ultrashort pulses, which may inspire a new wave of soliton laser developments. Finally, we derive the approximate shape of fundamental pure-quartic solitons and nd that it is close to a Gaussian, which is remarkable given that the solitary wave

a b

Negative FOD SPM Pure-quartic soliton

Input

Input

Output

Input

Output

FREQUENCY TIME

S4800 5.0 kV 16.2 mm 40.0 k SE(M) 1.00 m

PhC-wg

Output

Time Time Time

Wavelength Wavelength Wavelength

MLL

1.3 ps 1,550 nm

[afii9826]2 > 0

Input

Output

Input

Output

Input

Output

Delay

MZM OSA PD

c d

35

2

4

2

0

30

1

2

25

2 mm1 )

4 mm1 )

0

n g

[afii9826] 2(ps

[afii9826] 4(ps

20

6

1

15

8

2

[afii9826]4 < 0

10

10

1,540 1,545 1,550

12 1,540 1,550

1,545

Wavelength (nm)

3 1,540 1,550

1,545

Wavelength (nm)

Wavelength (nm)

Figure 1 | Concept of pure-quartic solitons and their experimental demonstration. (a) Schematics of pure-quartic solitons: (Left) Fourth-order dispersion (FOD) gives rise to temporal pulse broadening (blue output pulse versus black input pulse in time) without affecting the spectrum; (Centre) self-phase modulation (SPM) generates spectral broadening (red output pulse versus black input pulse in frequency) without affecting the temporal pulse shape; (Right) the interplay of FOD and SPM can give rise to pure-quartic solitons which remain nearly unperturbed (green output pulses versus black input pulses in both frequency and time); (b) Frequency-resolved electrical gating set-up: mode locked laser (MLL), photonic crystal waveguide (PhC-wg), tunable delay, ultrafast photodiode (PD), MachZehnder modulator (MZM), and optical spectrum analyser (OSA); (c) Scanning electron microscope image of the sample; (d) Measured dispersion of the silicon photonic crystal waveguide used in our experiments: group index (ng), second-order dispersion parameter (b2) and fourth-order dispersion parameter (b4).

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solutions found to date are always a function of hyperbolic secant10,18,20.

For demonstrating the existence of pure-quartic solitons we used the 396-mm-long dispersion engineered slow-light silicon PhC-wg29 shown in Fig. 1c (see the Methods section). Figure 1d shows the waveguide dispersion measured using an interferometric technique30. At the pulse central wavelength, 1,550 nm, the PhC-wg has a measured group index of ng 30,

a GVD of b2 1 ps2 mm 1, corresponding to normal

dispersion, a TOD of b3 0.02 ps3 mm 1, and a FOD of

b4

2.2 ps4 mm 1. Note that b4 is negative in a 6-nm

wavelength range.

To perform a complete temporal and spectral characterization of the sub-picojoule ultrafast nonlinear dynamics in the waveguide we used a FREG apparatus31 in a cross-correlation conguration, as schematically depicted in Fig. 1b (see the Methods section). This set-up provides a series of spectrograms, that is, the gated optical power versus delay, for varying input powers. From these spectrograms we then extract the optical pulses electric eld envelope and phase using a numerical algorithm32. Figure 2 shows the measured intensity (red dashed

lines) and phase (black dashed) at the output of the PhC-wg, when injecting 1.3 ps Gaussian pulses (full-width at half-maximum, FWHM) at 1,550 nm with different input peak powers, P0. Figure 2a shows the frequency domain and

Fig. 2b shows the temporal domain. The physical length scales of the dispersion orders for this pulse duration are: LGVD T02/

|b2| 0.615 mm, LTOD T03/|b3| 22.6 mm, LFOD T04/

|b4| 0.168 mm, with T0 FWHM/1.665 for Gaussian pulses.

These length scales indicate that FOD is dominant, with the total length of the sample being L 2.4.LFOD and the GVD length

being LGVD 3.66.LFOD. TOD is negligible in this sample for our

pulses.

To understand the origin of the experimental observations in Fig. 2 we employ a GNLSE model to describe the propagation in the silicon PhC-wg:

@A

@z

al;eff

2 A i

b2 2

@2A

@t2

b3

@3A

@t3 i

b4 24

@4A

@t4

1

Here A(z,t) is the slowly varying amplitude of the pulse, al,eff

igeff

aTPA;eff

2 A

j j2A ik0nFC;eff

seff

2

6

NcA:

1

4

Iexp Iexp

Isim Isim

sim

exp

Intensity (a.u.)

2

2 4

0.8 ps

P0 = 4.5 W

P0 = 2.5 W

0.5

0

Phase /[afii9843]

Phase /[afii9843]

Phase /[afii9843]

Phase /[afii9843]

HIGHER-ORDER

PURE-QUARTIC

SOLITONS

0

1

4

Intensity (a.u.)

0.9 ps

2

2

4

0.5

0

0

1

Input

Input

1.3 ps

4

0

FUNDAMENTAL

PURE-QUARTIC

SOLITON

Intensity (a.u.)

0.5

0

2

2

P0 = 0.7 W

P0 = 0.7 W

4

1

4

1.4 ps

2

2

4

LINEAR

CASE

Intensity (a.u.)

0.5

0

1,542

1,546

1,550 1,554

8 4

4

Wavelength (nm) Time (ps)

0

a b

Figure 2 | Experimental and modelling results. (a) Frequency and (b) time domain results for different input powers. The dashed red lines represent the intensity measurements, the blue solid lines represent the intensity simulations, the black dashed line represents the measured phase, and the solid black line represents the simulated phase. The green solid line at 0.7 W represents the normalized input intensity. The yellow box encompasses the fundamental pure-quartic soliton, showing nearly unperturbed propagation and at temporal phase. The turquoise box includes two cases of higher-order pure-quartic solitons, showing temporal compression and nonlinear spectral broadening. The higher-order pure-quartic solitons observed here are greatly perturbed by the presence of free carriers.

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denotes the linear loss, geff and aTPA,eff are the effective nonlinear

Kerr and TPA parameters, respectively; nFC,eff and seff represent

the free-carrier dispersion (FCD) and the free-carrier absorption effective parameters for a free-carrier concentration of Nc. Since we use a slow-light PhC-wg, the effective coefcients vary with the slow-down factor S ng/n0 (ref. 13). We use the measured

envelope amplitude of the input pulse as the input to our GNLSE model with the sample parameters detailed in the Methods section. As shown in Fig. 2, the model (solid lines) agrees well with the experimental data (dashed lines) in both frequency and time.

We rst focus on the time domain results of Fig. 2b. At the low coupled power of 0.07 W, nonlinear effects can be neglected and we simply observe small temporal broadening, mainly due to quartic dispersion. The different signs of b2 and b4 counteract each other to some degree, leading to a modest temporal broadening at the output of this short PhC-wg (from 1.3 to1.4 ps). We have veried, by running the GNLSE for longer lengths that the pulse width keeps increasing with the propagation distance in the linear case. Increasing the input power up to0.7 W, where the nonlinear length LNL 1/(geffP0) becomes

comparable to LFOD, the pulse preserves its initial shape and duration, as illustrated by the good matching between the measured output intensity (dashed red line) and the normalized input intensity (green solid line). Furthermore, the temporal phase across the pulse duration is nearly at. These are two signatures of fundamental soliton behaviour10. Simple estimates, conrmed with GNLSE simulations, show that the loss due to TPA at this power level is quite small and that free carriers do not yet play a role. At 2.5 W, where LNLooLFOD, the phase becomes

convex due to the stronger nonlinear Kerr effect and the main peak of the pulse narrows, temporal signatures of a higher-order soliton. At even higher powers, 4.5 W, the main peak of the pulse narrows even more, corresponding to a higher-order soliton with a higher-soliton number11. In addition, a long tail develops towards the leading edge of the pulse. We provide an explanation for this effect below.

Next we examine the frequency domain in Fig. 2a. At 0.07 W, since the nonlinearities are negligible and the pulse spectrum is not affected by the dispersion, the pulse spectral shape is maintained. At 0.7 W the pulse preserves its initial spectral shape, again consistent with fundamental soliton behaviour in the spectral domain. At higher powers (2.5 and 4.5 W) the pulse experiences spectral broadening and splits into two peaks, spectral signatures of higher-order solitons. The observed blue shift and asymmetry are associated mainly with FCD. Note that the oscillations in the measured spectra simply correspond to FabryPerot reections at the input and output facets of the PhC-wg and disorder in the periodic media33.

Whereas we previously reported shape-preserving fundamental solitons and higher-order soliton compression in silicon14, such behaviour was unforeseen for the normal GVD here. By setting b2

0 in our numerical model we nd that the signatures of soliton behaviour are maintained: the shape is preserved and the phase is at for the fundamental soliton at 0.7 W, and at 2.5 and4.5 W, the higher-order solitons undergo nonlinear temporal narrowing. This demonstrates that GVD is not important in this system and, since we established that TOD is also negligible, that the soliton behaviour stems purely from the interaction of FOD and SPM. Furthermore, we have veried that the long tail at the leading edge observed at high powers (Fig. 2b), as well as the self-acceleration of the pulse, originate from the interaction of negative FOD and FCD. The FCD generates additional blue components (Fig. 2a) and the negative FOD makes them travel faster than the red components of the pulse, analogous to our earlier results with negative GVD13,14,34.

These observations at the output of the silicon PhC-wg suggest the existence of a new type of soliton: pure-quartic solitons. We use the term soliton here to refer to solitary optical waves that propagate essentially unperturbed over long distances, not to exact localized solutions of integrable nonlinear differential equations35.

As expected in a silicon system at 1,550 nm, pure-quartic solitons are strongly perturbed by TPA and FC as we just described, and thus the measured behaviour differs from the simple case with just SPM and FOD. Therefore, to elucidate the dynamics of pure-quartic solitons in the absence of higher-order nonlinearities we next numerically study the propagation of picosecond pulses along the PhC-wg neglecting all effects but SPM and FOD.

Propagation behaviour of pure-quartic solitons. Figure 3a,b depict the propagation dynamics of undistorted pure-quartic solitons, that is, in the presence of SPM and FOD only. Such a system is governed by the biharmonic nonlinear Schrodinger equation

@A

@z i

b4 24

@4A

@t4 igeff A

j j2A: 2

In our simulations we consider two different power levels: fundamental pure-quartic solitons occur at moderate powers (Fig. 3a), whereas at high powers higher-order pure-quartic solitons result (Fig. 3b).

The simulations in Fig. 3a show shape-preserving pulse propagation in time and frequency over ve quartic dispersion lengths LFOD for a fundamental pure-quartic soliton. The very slight increase in the maximum intensity at t 0 corresponds to

the pulse adapting itself from the standard Gaussian pulse used as an input to the model, to the soliton form whose approximate shape we provide in the next section. The output, represented by the blue curve to the right of the propagation plot, shows that the pulse maintains essentially the same amplitude, shape (Gaussian), and duration (1.3 ps) as the input pulse. Importantly, the temporal phase at the output, represented by the black solid line, is at across the duration of the pulse.

Figure 3b reveals a higher-order pure-quartic soliton, with the pulse experiencing periodic recurrent propagation. In time the pulse undergoes compression and then periodically returns to its initial shape. In frequency the pulse splits into two and then recombines to recover its initial spectral shape after the same period. At the maximum compression point, the pulse reaches the minimum duration of 0.54 ps, a compression factor of 2.4 compared with the initial pulse duration, with a peak intensity of two times that of the initial pulse. Our simulations show the compression factor of the pure-quartic soliton roughly follows the same trend as conventional solitons11. Specically, larger intensities lead to higher compression factors and to compression occurring at an earlier spatial position along the waveguide. Crucially, while the trends are supercially similar to the behaviour of conventional solitons, the different scaling of SPM and FOD with pulse length suggests that the well-known denitions of soliton number and soliton period will not be appropriate for pure-quartic solitons; further studies are underway to derive the appropriate parameters and physical scaling laws for these eld structures.

To understand why the experimental observations of pure-quartic solitons in Fig. 2 differ from the numerical results of the undistorted system in Fig. 3a,b, we simulate the propagation along the PhC-wg including all the effects in the real system indicated in equation 1. The outputs shown in Fig. 3c,d match

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a

FUNDAMENTAL PURE-QUARTIC SOLITON PERTURBED FUNDAMENTAL PURE-QUARTIC SOLITON

PERTURBED HIGHER-ORDER PURE-QUARTIC SOLITON

TFWHM = 0.7 ps TFWHM = 0.8 ps

c

P0 = 0.7 W P0 = 0.7 W

P0 = 4.5 W P0 = 4.5 W

I (a.u.) I (a.u.)

I (a.u.)

Phase/

0

4 4

Phase/

0

4 4

3 Shape- and intensity-preserving propagation Shape-preserving propagation with strong loss

1

3

1

Wavelength (nm)Time (ps)

Wavelength (nm)Time (ps)Wavelength (nm)Time (ps)

0 3

0 3

TFWHM = 1.3 ps

TFWHM = 1.3 ps

1,560

1

0.5

0 0 1

1

0 0 2

1,560

1

0 0 0.4

0.2

0 0 0.16

0.08

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1,550

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1,540

1 2 3 4 5

z/LFOD

1 2 3 4 5

z/LFOD

0

0.5

0 0 1

0

0.2

0 0 0.4

Intensity (a.u.)

Intensity (a.u.)

b

HIGHER-ORDER PURE-QUARTIC SOLITON

d

I (a.u.)

Phase/

0

4 4

Phase/

0

4 4

3

Maximum compression points 0.54 ps

2

3

1

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0 3

0 3

Z 0

Z 0

Z 0

Z 0

1,560

1

1,560

1

1,550

1,550

1,540

1,540

0 1 2 3 4 5

z/LFOD

0 0 0.8

0.4

0 0 0.3

0.15

Intensity (a.u.)

0 1 2 3 4 5

z/LFOD

Intensity (a.u.)

Figure 3 | Simulations of the propagation of a fundamental and a higher-order pure-quartic soliton along ve quartic dispersion lengths, LFOD.

(a) Fundamental (P0 0.7 W) pure-quartic soliton with only self-phase modulation and quartic dispersion present, and (c) in the more realistic scenario for

our silicon waveguide with two-photon absorption and free carriers; (b) and (d) are similar but for a higher power level (P0 4.5 W) where a higher-order

pure-quartic soliton results.

our experimental measurements at P0 0.7 W and P0 4.5 W.

Figure 3c shows that the signatures of the fundamental pure-quartic soliton remain in realistic simulations: the pulse maintains its shape and width, and the phase at the output remains almost at. However, the intensity of the pulse decreases due, predominantly, to the linear loss in the slow-light waveguide B70 dB cm 1. Since the intensity decreases as the pulse propagates, the linear FOD will eventually dominate over the SPM for longer distances, leading to temporal broadening of the fundamental pure-quartic soliton. The higher-order pure-quartic soliton in the realistic scenario of Fig. 3d differs considerably from Fig. 3b. The TPA clamps the intensity in the waveguide from the early stages of propagation. The FCD introduces blue components that lead to the self-acceleration of the pulse and an asymmetry, and the free-carrier absorption induces absorption on the trailing edge. These effects of TPA and FCs on the propagation of higher-order pure-quartic solitons in silicon are analogous to the effects of FCs on conventional solitons14.

Approximate solution for the fundamental pure-quartic soliton. We now derive an analytic expression for the fundamental pure-quartic soliton. The experimental observations and numerical simulations indicate that the central part of fundamental pure-quartic solitons appears to be Gaussian. Assuming this shape, we look for an approximate solution to equation (2) in two separate ways for verication purposes: using the variational principle and looking for a local approximation near the centre. The complete derivations for the variational and local approximate solutions are described in Supplementary Note 1 and Supplementary Note 2, respectively.

In both cases, after a simple dimensional analysis, we take the central part of the pure-quartic soliton to be of the form

A z; t

A0eimgeffA2

0ze

t2

A0

q

ngeff

b4

j j

; 3 where, since b4o0, we have written b4 b4

j jfor convenience,

and m and n are free parameters. The variational approach gives m 7=8 2

p 0:62, n 1= 2

p ;whereas the local approximation gives m 12, v 1. Thus, these approaches predict pulse

widths which differ only by a factor 218 or by o10%. The fact that these different approximations give very similar results reinforces our condence in them. Based on these results, the argument of Akhmediev and Karlsson36 suggests that pure-quartic solitons do not lose energy due to linear radiation. []ally, taking the Fourier transform of the right-hand side equation (3) in time and position leads to a straight line in an o-k diagram. Since this straight line does not intersect the linear dispersion relation of the medium, the soliton cannot lose energy to dispersive waves.

To test the validity of this analytic approximate solution we numerically solve equation (2) with the b4 and geff of our sample (see Methods section) and obtain the output of the system at the power level corresponding to a fundamental pure-quartic soliton for a propagation length L 30 LFOD. This long propagation

distance ensures convergence of the pulse evolution. The results of this numerical experiment for three different pulse shapes: a Gaussian, a hyperbolic secant (sech), and a super Gaussian of order four, are depicted by a solid blue curve in Fig. 4ac, respectively. Importantly, the hyperbolic secant and super Gaussian inputs (black solid curve in Fig. 4b,c, respectively) evolve into the solitary wave Gaussian shape, constituting an additional signature of soliton-like behaviour and proving that

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a b

Gaussian input Sech input

0.8

0.8

Amplitude

0.6

Input Numerical Variational Local Experiment

0.6

Input Numerical Variational Local

0.4

0.4

0.2

0.2

0

0

0

0 4

40 4 2 2

Time (ps)

Amplitude

0

0 4

40 4 2 2

Time (ps)

Amplitude -LOG

10

10

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Amplitude -LOG

20

30

30

c d

4

Super-Gaussian input

FODSPMFOD & SPM

0.8

0.6

Input Numerical Variational Local

Amplitude

0.4

0.2

Phase/

2

0

0

0

4

Amplitude -LOG

40

10

20

30

0 4

2 4 2 2

0 4

2 2

Time (ps) Time (ps)

Figure 4 | Approximate solutions to a fundamental pure-quartic soliton and phase diagram. Comparison between the variational and local approximate solutions for the fundamental pure-quartic soliton and the numerical output after propagating over thirty quartic dispersion lengths for (a) a Gaussian input, (b) a hyperbolic secant input, and (c) a super Gaussian input of order 4. In the Gaussian case the measured output pulse at 0.7 W is shown in the background (dot-dash cyan curve). (d) Phase shift induced by the fourth-order dispersion (dashed purple) and the self-phase modulation (solid turquoise) independently and its combined phase shift (black).

this type of soliton acts as an attractor. These results are overlapped with the variational approximation (dashed red curve) and the local approximation (green dashed curve), using the same parameters. The agreement between the numerical and the variational solution in the central part of the pulse is remarkable. The local approximation deviates only slightly. In addition we numerically nd that mE0.63, again very close to the variational result. In the background of Fig. 4a, we show the measured pulse at the output of the chip at P0 0.7 W (cyan dot-dashed curve),

which matches variational solution perfectly. The wings observed in the numerical solution relate to the fact that the phase shift prole of the FOD has a quartic dependence with time, whereas the SPM varies quadratically, as illustrated in Fig. 4d. This allows both phase shift proles to perfectly counterbalance each other close to the centre of the pulse, but deviate from each other at the edges. This fact is not captured in the approximate analytic solution since a perfect balance between FOD and SPM is assumed.

Since temporal optical solitons studied to date have some kind of hyperbolic secant shape10,18,20, it is surprising that pure-quartic solitons are approximately Gaussian. To understand this better, we apply the argument of Dudley et al.37 to our system, according to which the dispersive and nonlinear phase components developed during short propagation distances must cancel each other across the pulse duration to lead to the formation of a soliton. Figure 5a shows the FOD- (red) and the SPM-induced chirp (blue) after propagating the Gaussian variational solution in equation 3 (dashed black curve) for a distance LFOD/10 and demonstrates how this solution leads to the

mentioned cancellation across the central part of the pulse. To highlight the different nature of the solutions found here with respect to the previously studied NLS solitons with FOD, we apply the same verication to the solution found in ref. 18, in the limit b2 0. Figure 5b shows how the sech2 solution obtained

from ref. 18 (dashed black curve) does not provide the necessary cancellation of the SPM- and FOD-induced chirp required for the formation of a stable solitary wave in the presence of just SPM and FOD.

DiscussionThe experimental results and the numerical simulations presented here have established the existence of a new class of solitons: pure-quartic solitons, arising from the interaction of SPM and FOD only. In particular, we experimentally demonstrated shape-preservation and at-phase behaviour for the fundamental pure-quartic soliton, and temporal compression for the higher-order pure-quartic solitons. We numerically demonstrated that the higher-order pure-quartic soliton would undergo recurrent periodic propagation in the absence of loss and higher-order nonlinearities. Although we have veried that these signatures of soliton propagation are preserved for long propagation distances in the presence of just FOD and SPM, the disparity between the quartic prole of the FOD-induced phase shift and the quadratic prole of the SPM-induced phase shift affecting the edges of the pulse could lead to stability issues that should be studied. Establishing appropriate denitions of concepts such as soliton number and

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a

24

FOD SPM

16

8

Chirp (MHz)

0

16

8

0 4

24 4 2 2

Time (ps)

b

24

16

FOD SPM

8

Chirp (MHz)

0

16

8

0 4

24 4 2 2

Time (ps)

Figure 5 | The cancellation of nonlinear and dispersive phase components on the fundamental pure-quartic soliton. (a) FOD-induced (red) and SPM-induced (blue) frequency chirps after a propagation of LFOD/10 for the Gaussian pure-quartic soliton of equation (3); (b) similar, but for the sech2 type solutions obtained taking ref. 18 with b2 0. The

dashed black curves in the background of (a) and (b) represent the input pulse intensity: the Gaussian solution of equation 3 and the sech2 solution in ref. 18 respectively.

soliton period for pure-quartic solitons is an open theoretical challenge.

Our discovery was facilitated by the unique dispersion properties of PhC-wgs that provide the design freedom to achieve a wide variety of dispersion proles. However, other guided wave systems such as highly nonlinear bres38, photonic crystal bres8 or specially designed silicon waveguides22,39, could also be engineered to observe pure-quartic solitons. The main condition to full is LFODooLGVD, LTOD (with b4o0), and, in

practice, we have veried that LFODooLGVD/3 across most of

the pulse bandwidth is enough for a robust observation. The pure-quartic soliton behaviour starts to become observable when LFOD becomes comparable to the sample length. However, our initial simulations show that the pure-quartic soliton does not reach a steady state until it propagates for several quartic dispersion lengths, similar to conventional solitons40. Analogous regimes of evolution have been demonstrated in Ti:sapphire laser cavities2528, taking advantage of the rich variety of physical regimes offered by the discrete structure of the laser cavity. For example, Zhou et al. demonstrated in ref. 26 8.5 fs pulses from a Ti:sapphire laser operating near zero GVD with the minimum pulse duration limited by FOD, and later Christov et al.28 hinted that the soliton-like pulse inside such a laser was fourth-order dispersion limited. Here we experimentally demonstrate that the balance between SPM and FOD gives rise to robust soliton-like behaviour. Hence, the scope of our ndings is not just limited to nonlinear guided wave optics, but may provide novel insights into extreme regimes of ultrafast lasers operation.

The Gaussian variational solution provided here constitutes a good approximation to the central form of the fundamental pure-quartic soliton. The results of our study on the cancellation of the nonlinear and quartic dispersion phase components in short propagation distances proved that no previously found solitary wave solution10,18,20 can describe the behaviour of pure-quartic solitons. This approximate solution, valid only for b4o0, could stimulate new efforts in nding solutions to the biharmonic nonlinear Schrodinger equation41,42 also of interest in the eld of spatial solitons23,24,43,44. Recent interest in temporal cavity solitons in both microresonators3 and optical bres7 with applications in Kerr frequency combs45 and low-noise microwave generation46 could also benet from exploring pure-quartic solitons in their systems. Furthermore, it would be interesting to investigate analytic solutions supported by the pure-quartic soliton system including the effects of linear loss, TPA and FCs.

Aside from their different physical origin, pure-quartic solitons present signicant potential advantages with respect to conventional solitons. As mentioned, pure-quartic solitons open the door to soliton functionality in the normal GVD regime of optical media. More importantly, perhaps, the energy of conventional solitons scales like (T0) 1, whereas the energy of pure-quartic solitons scales like (T0) 3, which suggests that they are more energetic for ultrashort pulses.

We expect that the understanding of pure-quartic solitons provided in this paper, combined with the previous advances in the laser literature2528, will inspire a new wave of ultrafast laser development.

Methods

Device and linear characterization. The present experiment was performed using a silicon photonic crystal air-suspended structure with a hexagonal lattice (p6m symmetry group) constant a 404 nm, a hole radius r 116 nm, and a

thickness t 220 nm. A 396-mm-long dispersion engineered PhC-wg was created

by removing a row of holes and shifting the two innermost adjacent rows 50 nm away from the line defect. The air-clad devices were fabricated with a combination of electron beam lithography, reactive ion and chemical wet etching. The measured linear propagation loss in this slow-light region is B70 dB cm 1, with a total linear insertion loss of B13 dB (5 dB per facet).

Light was coupled in with tapered lensed bres to SU8 polymer waveguides with inverse tapers.

Phase-resolved characterization method. For the nonlinear experiments,we used a mode locked laser (Alnair) fed into a pulse shaper (Finisar) generating near transform-limited 1.3 ps pulses at 1,550 nm at a 30 MHz repetition rate. These pulses were then input into the FREG apparatus. The pulses were split into two branches by a ber-coupler, with the majority of the energy coupled into the PhC-wg. The remaining fraction was sent to a reference branchwith a variable delay, before being detected by a fast photodiode and transferred to the electronic domain. This electronic signal drove a MachZender modulator that gated the optical pulse output from the PhC-wg. Using an optical spectrum analyser, we measured the spectra as a function of delay to generate a series of optical spectrograms. We de-convolved the spectrograms with a numerical algorithm (256 256 grid-retrieval errors Go0.005), to

retrieve the pulse intensity and the phase in both the temporal and spectral domain32.

Generalized nonlinear Schodinger equation model. The parametersused in our GNLSE model for the slow-light dispersion engineered PhC-wg are: slow-down factor S ng/n0 8.64, effective linear absorption a

l,eff

13.9 cm 1 ;

1 ps2 mm 1, a TOD parameter of b3 0.02 ps3 mm 1, and a FOD

parameter of b4 2.2 ps4 mm 1; effective nonlinear parameter

geff 2pnlA S2 4;072 Wm

1; with n2 6 10 18m2 W 1and Aeff 0:44mm2;

effective TPA parameter aTPA;eff bA S2 1; 674 Wm

1; with bTPA 10 10 12m W 1;

effective free-carrier dispersion parameter nFC,eff 6 10 27 S m3; effective

free-carrier absorption parameter seff 1.45 10 21 S m2. The simulation results

in Fig. 2 were obtained by using the measured input pulse as the input to the model. The simulation results in Fig. 4 were obtained using a perfect Gaussian, hyperbolic secant, and a super Gaussian (order 4) pulse of the same width as the experimental pulse, 1.3 ps. The linear loss in the nanowires that couple light into and out of the PhC-wg was negligible. Nonlinear absorption in the coupling nanowire (effective area, B0.2 mm2) was taken into account in the NLSE model.

b2

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Acknowledgements

This work was supported in part by the Center of Excellence CUDOS (CE110001018),

Laureate Fellowship (FL120100029) schemes of the Australian Research Council (ARC)

and by The University of Sydney and the Technion collaborative photonics research

project funded by The Department of Trade and Investment, Regional Infrastructure and

Services of the New South Wales Government and The Technion Society of Australia

NSW. T.F.K. was supported by EPSRC UK Silicon Photonics (Grant reference EP/

F001428/1). C.H. was supported by the ARC Discovery Early Career Researcher award

(DECRADE120102069).

Author contributions

A.B.-R. and C.H. conducted the experiments. A.B.-R. performed the retrievals, developed

the GNLSE model, analysed the results, contributed with physical insight to the deri

vation of the approximate analytic solutions, and wrote the paper. C.M. de S. obtained

the local approximate analytic solution, contributed to the analysis of the results, and

proposed numerous verication tests. J.E.S. obtained the variational approximate analytic

solution. T.F.K. fabricated the sample. B.J.E. provided guidance to the project and pro

posed the verication test for the Gaussian solution. C.H. built the set-up and con

tributed to analysing the results. All authors contributed in editing the paper.

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