Combination of CP and CSA WP5 Optoelectronics Deliverable...

Project funded by the European Commission under grant agreement n°604391

GRAPHENE Graphene-Based Revolutions in ICT and Beyond

Combination of CP and CSA

WP5 Optoelectronics

Deliverable 5.3 “Report on photodetectors, lasers, OLEDs, modulators and metamaterials”

Main Author(s):

Frank Koppens, ICFO

Andrea Ferrari, UCAM

Due date of deliverable: M24

Actual submission date: M24

GRAPHENE D5.3 30 September 2015 2 / 17

LIST OF CONTRIBUTORS

Partner Acronym Partner Name Name of the contact

69 ICFO Fundacio Institut De Ciencies Fotoniques

Frank Koppens

28 UCAM The Chancellor, Masters and Scholars of The University of Cambridge

Andrea Ferrari, Daniel Popa

2 CNR Consiglio Nationale delle Ricerche Alessandro Tredicucci 70 TU-Wien Technische Universitaet Wien Thomas Mueller 6 UNIMAN The University of Manchester Alexander Grigorenko 130 IMEC Interuniversitair Micro-

electronicacentrum IMEC VZW Dries Van Thourhout

128 QMUL Queen Mary University of London Yang Hao 104 EPFL Ecole Polytechnique Federale de

Lausanne Juan Mosig


TABLE OF CONTENTS Deliverable Summary ............................................................................................................................... 4 1. Photodetectors ..................................................................................................................................... 4

1.1 Photodetectors for visible and infrared frequencies ....................................................................... 4 1.2 Photodetectors for mid-infrared and THz frequencies ................................................................... 6 1.3 Detectors based on the thermoelectric effect ................................................................................ 7 1.4 New concepts ................................................................................................................................ 8 1.5 New materials ................................................................................................................................ 8

2. Chip-integrated Modulators .................................................................................................................. 9 3. Metamaterials ..................................................................................................................................... 11

3.1 Graphene based metamaterials for optical communications ....................................................... 12 3.2 Metamaterials as perfect absorbers ............................................................................................. 12 3.3 Plasmonic bio-sensors ................................................................................................................. 12 3.4 THz passive devices .................................................................................................................... 13

4. OLEDS ............................................................................................................................................... 13 5. Lasers ................................................................................................................................................ 14

5.1 Wavelength tuning range ............................................................................................................. 14 5.2 Output power ............................................................................................................................... 14 5.3 Repetition rate .............................................................................................................................. 15

References ............................................................................................................................................. 16


Deliverable Summary

This report covers the integration of graphene/2d- material-based interconnected in

photodetectors, lasers, OLEDs, modulators and metamaterials, focussing on recent advances

since deliverable D5.2

1. Photodetectors

1.1 Photodetectors for visible and infrared frequencies

Photodetection of visible (VIS) and infrared (IR) radiation is of utter importance for a variety of different

applications, most notably, optical communications and (video) imaging. The VIS and IR wavelength

ranges are thus well covered by highly mature photodetection technologies and materials, such as

silicon (VIS; imaging) and compound semiconductors (IR; optical communications). Nevertheless, with

changing demands, the necessity for alternative photodetection materials is becoming more eminent.

E.g., the field of optical communications is shifting towards waveguide-integrated devices (e.g. “silicon

photonics”), where the integration of compound semiconductors is not considered a viable option.

Instead, germanium is typically used whose photodetection performance is comparably poor. For

imaging applications, highly sensitive, flexible and lightweight materials with spectral response

extending into the near-IR would be desirable.

As we will show in detail below, significant progress has been made in the performance of chip-

integrated (waveguide integrated) photodetectors, where competing performance has been shown in

terms of speed and responsivity. This technology is now close to ready for further integration with

graphene-based modulators (discussed below) towards the implementation of a transceiver bank.

The table with KPIs is listed below.

State-of-the-Art 2D materials

Speed 40GHz (waveguide-integrated Ge) 150 GHz (InGaAs) 80 GHz (Ge)

40 GHz (waveguide-integrated graphene) 260 GHz (graphene)

Wavelength range C-band (waveguide-integrated Ge) O-U bands (waveguide-integrated graphene)

Responsivity 0.5-1 A/W 0.36 A/W (waveguide-integrated graphene) 0.11 A/W (TMD)

Responsivity photoconductive

108 A/W (semiconductor) 107 A/W (graphene, slow (Hz)) 3×104 A/W (graphene, fast (kHz)) 106 A/W (TMD)

Flexibility 0.12 A/W (Si and III-V semiconductor membranes)

45.5 A/W (MoS2/graphene)


1.1.1 Chip-integrated ultrafast photodetectors for optical communications

In order to optimize the ultrafast chip-integrated

photodetectors (PDs) that members of WP5 have

demonstrated previously, we have optimized the

performance of waveguide-integrated photodetectors [1]

and performed a number of ultrafast measurements, both

on graphene and van der Waals heterostructures [2].

In graphene, we have measured the photovoltage

generation time in pn-junctions and found it to be faster

than 50 fs. As a proof-of-principle application of this

ultrafast PD, we used graphene to directly measure, electrically, the pulse duration of a sub-50 fs laser

pulse. We have also examined the spectral response of our devices and found a constant spectral

responsivity between 500 and 1500 nm, consistent with the photo-thermoelectric (PTE) photovoltage

generation effect [2].

As the cooling dynamics of hot carriers determines the efficiency of PTE-based PDs, we are currently

in the progress of studying hBN-encapsulated 2d materials by femtosecond pump-probe experiments.

In a first set of experiments, we measured electron relaxation dynamics of a hBN/graphene/hBN stack

and compared it to graphene on Si or SiO2. Our results indicate a fast electron-optical phonon

relaxation time~200 fs, followed by a slow electron-phonon cooling time~2.8 ps. The cooling in the

hBN/graphene/hBN stack is slower than that observed on the SiO2 substrate, which confirms our

assumption that high-quality graphene will give stronger photoresponse due to reduction of disorder-

induced cooling.

On the device side, members of WP5 have fabricated and characterized a new generation of

waveguide-integrated PDs. These devices exploit the slower cooling dynamics of the electron gas in a

hBN/graphene/hBN stacks and thus show a high responsivity of up to 0.36 A/W – a more than three-

fold improvement over devices without hBN. An electrical bandwidth of more than 40 GHz was

achieved and the device was employed for the direct electrical measurement of ultrafast laser pulses

[1]. The responsivity can be further increased by a factor 2 to 3 by further optimization of the junction

(e.g. by local graphene gates). These results have shown that the most critical bottleneck

(responsivity) for waveguide-integrated detectors can be overcome. More focus will be geared towards

integration with modulators and further optimization of all relevant parameters in one device (including

contact resistance, parasitic capacitances etc.).

Beyond graphene, we studied the ultrafast carrier dynamics in graphene/WSe2/graphene vertical

stacks [3] and MoS2/WSe2 heterojunctions [4]. Although the mobility of transition metal dichalcogenides

(TMDs) is smaller than that of graphene, these devices benefit from the short carrier transit times in

ultrathin van der Waals heterojunctions. In the graphene/WSe2/graphene device, we demonstrated an

intrinsic photoresponse time as short as 5.5 ps, which is tuneable through bias and by varying the TMD

layer thickness. Our study has provided direct insight into the physical processes governing the

detection speed and quantum efficiency of these van der Waals (vdW) heterostructures, namely out-of-


plane carrier drift and recombination. In the MoS2/WSe2 heterojunctions, we obtained carrier transfer

times lower than 0.2 ps. TMD-based devices thus combine the high photodetection efficiency of

semiconductors with a picosecond photoresponse comparable to that of graphene, thereby optimizing

both speed and efficiency in a single PD. Our observation and understanding of ultrafast and efficient

photodetection has underlined the potential of hybrid TMD-based heterostructures as a platform for

future optoelectronic devices.

1.1.2 (Flexible) photodetectors with high responsivity

As first demonstrated by members of WP5, photogain can be accomplished by integration of 2d

crystals with other materials to produce a hybrid phototransistor (e.g. by sensitizing the surface of a

graphene transistor with quantum dots). Although ultrahigh photogain and responsivities of up to 107

A/W can be achieved in these devices, they initially suffered from very slow photoresponse (≈Hz), but

in an optimized design, we now achieved sensitive photodetection with a bandwidth of up to 2 kHz,

sufficiently large for video imaging [4]. We fabricated field-effect transistors with CVD graphene on

SiO2/p-Si. After synthesis of CdS nanocrystal, their surface ligands are exchanged to

methylbenzenethiol and the NC are transferred in a mixture of chloroform and dichlorobenzene for

spin-coating onto the graphene device. Responsivities up to 3.4 104 A/W were reached. The

measured detector properties are related to the surface chemistry and trap states of the NCs. The use

of short ligands allows the photoexcited electrons to efficiently transfer to graphene. The recombination

is assisted by shallow and deep states, with different timescales. Thus, we have in our system both

short timescales, good for fast detection, and long timescale, for high responsivity

The flexible VIS photodetectors, reported already in D5.2, have further been optimized and

characterized [5]. These devices exhibit external (internal) responsivities as high as 45.5 A/W (455

A/W) with a photoconductive gain of 4 105, vastly outperforming state-of-the-art bulk-semiconductor

flexible photodetectors [6].

1.2 Photodetectors for mid-infrared and THz frequencies

Graphene is a very promising candidate for novel Terahertz PDs. The target here is the development

of a room-temperature technology that would allow sensitive THz cameras, capable of operating at fast

acquisition rates and with high dynamic range. We identified, as final performance goals for these

devices, noise equivalent powers (NEP) in the order of pW/Hz1/2 (in order to be used also in passive

imaging systems), with response speed of hundred MHz and above (for fast automatic in-line operation

in many applications, for instance in industrial process control). Even more important is the objective of

implementing a suitable approach to achieve narrow-band electrically tuneable detection, which would

allow spectrally resolved images even in passive configuration, something that no THz technology at

present can provide.

We reviewed graphene and related-materials based PDs in Nature Nanotechnology, describing the

state-of-the-art [7]. This was summarized in the previous deliverable D5.2. Here we will briefly describe

the major progresses and advances in the past 6 months.


1.2.1 Monolayer and bilayer graphene plasma wave FETs

We reported room-temperature detection across the far-infrared in monolayer and bilayer graphene

employing an antenna-coupled configuration for the excitation of overdamped non-resonant plasma-

waves in the channel of a graphene FET [8, 9]. In these devices, the measured dependence of the

photovoltage Δu on the gate bias VG was in qualitative agreement with the prediction of a diffusive

theoretical model, proving that the detectors operate in the so-called broadband overdamped regime.

Nevertheless, discrepancies were observed that were attributed to a possible contribution of

thermoelectric origin. The two mechanisms, however, are hard to disentangle since the functional

dependence on VG is expected to be the same. Recent results have now clarified this issue [10]. The

responsivity of FETs made from bilayer graphene produced by thermal desorption of SiC was studied

as function of the impinging radiation in the range 260-360 GHz. By considering the behaviour at the

Dirac point, dominated by the thermoelectric contribution, the temperature heating was evaluated to be

~1 K; the thermoelectric response can then be calculated for every VG and added to the plasma non-

linearity. Experiments show that the two effects tend to cancel each other, thereby strongly supporting

device

1.2.2 Graphene-based broadband THz bolometers

A different detection concept makes use of the bolometric effect in which a radiation-induced

temperature variation is read through the resistance change of a graphene channel [11]. Here,

experiments were carried out at room temperature, still in antenna-coupled devices, demonstrating a

different behaviour for monolayer and bilayer graphene, and verifying the bolometric response through

measurements of the noise spectral function [12]. The estimated NEP values in these detectors in the

1.5-3 THz region are quite promising (in the 10 pW/Hz1/2 range), although the response time is very

slow (s). The latter feature seems to indicate that the graphene channel acts actually mostly as an

electrical read-out, while the absorption/heating involves directly the substrate.

1.3 Detectors based on the thermoelectric effect

Ref. 13 reported the first graphene device explicitly aimed at exploiting the thermoelectric effect as the

main detection mechanism [13]. In this direction the research is presently focusing on increasing the

THz absorption in the graphene layer to enhance responsivity. As the detecting element has to be

rather large to maximize temperature variations, the idea is to keep avoiding the antenna coupling,

since this works best for device sizes much smaller than the wavelength. A possible approach is to use

plasmon enhancement effects in a structure where resonant plasmon modes can be excited. For this

purpose a first demonstration was carried out using a detection element made of graphene

microribbons [14]. Although at the moment a significant performance improvement is still to be proven,

resonant enhancement of the THz absorption around 4-6 THz is evident from both the spectral and the

polarization dependence of the signal, making this a promising route to achieve higher responsivities.


1.4 New concepts

New THz detection concepts have been proposed. The recent experimental studies on the propagation

of 2d plasmons in graphene, launched by a near-field coupling tip, has prompted a theoretical analysis

of the possibility of extracting an electrical signal from the excitation of such plasmon modes15. While

this could be achieved by using built-in electric fields to separate electron and holes, it is argued that in

a low-damping BN-graphene plasmonic waveguide an electric signal will anyway result from the

intrinsic hydrodynamic non-linearity.

Another proposal is to use quantum point contacts (QPC) [16]. In the tunnelling regime, the current

through the negatively biased barrier of the QPC device is very low but can be enlarged by orders of

magnitude by the radiation field that locally (thanks to electrode tips acting as antenna arms) "heats"

the electron gas. Preliminary measurements at 150 GHz in a GaAs device have shown responsivities

up to 106 V/W, which could even reach larger values in atomically thin graphene sheet geometries

suppressing one of the two mechanisms (Fig. 1).

1.5 New materials

Ideally the responsivity of non-resonant plasma wave detectors is proportional to the non-linearity of

the FET transfer characteristics and is at its maximum near the channel closing point. A FET based on

a 2d material with gap should then possibly yield better transconductance and hence better

responsivities when employed as detector. We have used thin flakes of black-phosphorus (BP) to

realize devices operating at room temperature and integrated on-chip with planar antennas providing

remarkable coupling efficiencies in the strongly sub-wavelength device channel [17]. The achieved

selective detection (~5-8 V/W responsivity, see Fig. 2) and sensitivity performances (signal-to-noise

ratio of 500), have been exploited to demonstrate the first application of a phosphorus-based active

THz device, for pharmaceutical and quality control imaging of macroscopic samples, in real-time and in

a realistic setting. Furthermore, by exploiting the inherent electrical and thermal in-plane anisotropy of

BP, plasmonic, thermoelectric and bolometric detection can all selectively be achieved in a controlled

fashion depending on the device geometrical design.

Fig. 1 Source/drain photovoltage signal of a bilayer graphene FET detector as a function of gate

voltage for two different radiation frequencies. In blue is the plasma wave contribution as computed


from the transfer characteristics. In red is the computed thermoelectric contribution. In black the

experimental curve (which should result from the sum of the two).

Fig. 2 Left: Responsivity of BP detector as a function of gate voltage (red). In black is the voltage in

absence of THz radiation. Right: SEM picture of the BP FET detector.

2. Chip-integrated Modulators Light transmitted through a graphene layer can be modulated when gating with the appropriate voltage.

This is true over a very broad wavelength range, from the UV up to the MIR. However, when a light

beam is normally incident on a single sheet of graphene, only~2.3% of its power is absorbed, by far not

sufficient for realizing a device that can be used for practical applications in the datacom or telecom

domain. The efficiency can be enhanced by embedding the graphene layer in a Fabry-Perot cavity.

However, this introduces a strong wavelength dependence in the response of the device, negating the

intrinsic broad band response of graphene. An alternative is to integrate the graphene layer on top of a

waveguide. In this way the interaction length can be increased at will. As an example, when integrated

on a 220 nm x 600 nm Si waveguide, ~0.1 dB of the light (@ 1550nm) is absorbed per micrometre for

light propagating in the fundamental quasi TM-mode. In addition, when applying a voltage between the

graphene layer and the Si waveguide the Fermi level and hence the absorption in the graphene layer

can be controlled. This was demonstrated first in Ref.18 where a graphene based intensity modulator

with a bandwidth of 1 GHz and extinction ratio of ~0.08 dB/mm, operating over the wavelength range

1.35-1.60 mm was shown (Fig. 3a,b). In a follow up paper, the authors showed a variant of this device

whereby also the bottom electrode is formed by a graphene layer (see schematic view in Fig. b) [19].

This roughly doubles the extinction ratio, without increasing the capacitance of the device. An

additional advantage of this approach is that the waveguide no longer serves as the bottom electrode

and in principle can be realised from any dielectric material, e.g. silicon nitride. A similar device with

lower loss was demonstrated in Ref [8].

Since these first demonstrations several other groups demonstrated graphene based waveguide

electro-absorption modulators [20, 21, 22, 23, 24, 25, 26]. Recently also graphene based phase

modulation was demonstrated [27]. Also several theoretical studies were published [28, 29, 30]. Initial

devices were limited in operation speed. However, they were relying on non-intentionally doped or

lightly doped Si waveguides, increasing the resistance of the bottom electrode, hence limiting the RC-

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bandwidth of the device. Recently an improved version of the device was demonstrated, relying on the

IMEC standard iSIPP25G platform, whereby the waveguide and the silicon contact area were

intentionally doped. This considerably improved the performance of the device, allowing demonstration

of open eye diagrams for data-rates up to 10 GBit/s (Fig. 4).

Fig. 3 a) Schematic view of and b) static transmission through a single layer graphene modulator

integrated on silicon waveguide as function of applied voltage (from [18]). c) On-off transmission of low

loss double-layer graphene modulator [31].

Fig. 4 SEM-picture of graphene modulator (left) and eye-diagrams at 8 and 10GB/s modulation (right)

[27].

The biggest trade-off in designing these modulators lies in the selection of the gate-oxide thickness.

Decreasing the drive voltage, as required for combining the modulators with advanced CMOS drivers,

requires reducing the gate oxide thickness. However, this increases the capacitance of the device and

hence decreases its bandwidth. One approach to overcome this trade-off is to enhance the optical

confinement in the graphene layer(s). This can be achieved by embedding a short modulator section

in a planar resonator, e.g. a ring resonator [22, 23] or a photonic crystal cavity [32], see also Fig. e. Up

to 30GHz modulation was recently shown for such a structure [23]. However, this again introduces

strong wavelength dependence in the response and the loss in the demonstrated structures was

unacceptably high. Alternatives that do not introduce this wavelength dependence are to embed the

graphene layer in the centre of a waveguide (Fig. 5c [28]) or to combine it with a metal contact, which

supports a plasmon-like mode (Fig. 5d [29]). Such configurations have however not yet been shown

experimentally.


Fig. 5 Cross-section of a number of possible implementations for hybrid silicon-graphene devices (left)

and schematic view of graphene modulator on Silicon Photonic Crystal [32].

Table 1. Most relevant KPIs for current state-of-the-art silicon integrated modulators. (Ref. key:

(a)=[33], (b)=[34], (c)=[35], (d)=[18], (e)=[26]).

Graphene has been used extensively as the saturable absorber in modelocked lasers, allowing

generation of ultrashort pulse [36]. An all fibre-based modulator was demonstrated for that purpose

[37]. Recently, it was demonstrated that also the planar integrated device could be used to control the

saturable absorption in graphene [38]. This paves the way to fully integrated graphene mode locked

devices with controllable pulse shape.

The table below gives values for the most important KPI’s for currently demonstrated graphene

modulators and compares them with other types of modulators integrated on the Si platform. Graphene

modulators excel in footprint, optical bandwidth and operating temperature range. Power consumption

and operating bandwidth need be further improved.

3. Metamaterials Graphene optical metamaterials are composite materials produced from graphene, metals and

dielectrics, which are engineered into a composite structure designed to fit a particular purpose and

realize electromagnetic properties not easily available in natural materials. The main applications of

graphene metamaterials can be found in efficient light modulators, active optical elements, highly

birefringent materials and/or hyperbolic optical materials required for superlens operation, safeguarding

plasmonic properties of existing metals and semiconductors, realizing extremely high electromagnetic

fields and/or field confinement, perfect absorbers among others. One of the most interesting


applications is modulation of guided light with the help of hybrid graphene/metal metasurfaces, which

could provide a breakthrough in optical communications as a viable alternative to silicon photonics. In

addition, cheaper and better SPR sensors can be produced using graphene protected copper and

silver plasmonics. Since progress in optical metamaterials has been achieved in various areas we will

concentrate our attention only on three promising topics.

3.1 Graphene based metamaterials for optical communications

Progress has been made in development of graphene metamaterials for on-chip hybrid graphene

plasmonic and silicon modulators at telecom wavelengths. Collective resonances have been used to

modulate light reflection from a graphene-metal metasurface by graphene gating [39] WP5 realized a

hybrid structure, which realizes coupled plasmonic resonances of unprecedented quality (Q~300) at

optical communication wavelength 1.5 µm (the standard quality of localized plasmon resonance in

analogous structures is about Q~10). This allowed us to achieve 20% of light modulation at telecom

wavelengths using graphene gating with one graphene layer in reflection – the largest number

achieved so far.

In addition, WP5 realized corrugated metasurfaces to achieve modulation~10% in 12 µm plasmonic

waveguides at telecom wavelengths at low gating voltages with an active device area of just 10 µm2,

characteristics which are already comparable with those of Si-based waveguide modulators while

retaining a benefit of further device miniaturization [40]. We systematically studied propagating

plasmons of various geometries and choose ones which promise the largest modulation for graphene-

based gating [40]. We achieved light guiding at the conditions of diffractive coupled resonances and

are going to use this technique for creation of graphene-based light modulators.

3.2 Metamaterials as perfect absorbers

As first demonstrated by members of WP5, perfect absorbers can be produced from graphene-metal

metasurfaces under various geometries. The perfect absorber based on topological darkness would

allow one to achieve extremely high phase sensitivity, which was demonstrated by graphene

hydrogenation [41]. We studied properties of “dark light” in details and we are preparing a review on

phase sensitivity in various devices and materials [41]. We showed that the phase sensitivity for dark

reflection can be better than 1 fg/mm2, while metamaterials based on graphene Cu multilayers can

provide even better mass sensitivity of 0.1 fg/mm2. In optimized designs, we combined plasmonic

nanoarrays with graphene to achieve zero reflection [42]. We also studied graphene based Fabry-

Perot structures with unusual light modulation due to graphene gating at the condition of zero

reflection. Various types of reflectors have been used, including Bragg mirrors, copper films, etc.

3.3 Plasmonic bio-sensors

We have shown that graphene plasmonics can be used to improve on a well-known molecule-

detection method: infrared absorption spectroscopy, which has important limitations when applied to


Fig.6 Schematic of photoexcited electron kinetics in graphene, with possible relaxation mechanisms for the non-equilibrium electron population [50].

molecules at the nano-scale [43]. The wavelength of the infrared photon directed at a molecule

is~6µm, while the target measures only a few nm, making it very challenging to detect the vibration of

such a small molecule.

Graphene is capable of focusing light on a precise spot. On its surface the plasmons couple to the

vibration of a nanometric molecule that is attached to it. It is then possible to detect nanometric

compounds in proximity to the surface. In addition to identifying the presence of nanometric molecules,

this sensor is frequency selective and can therefore also reveal the nature of the bonds. The sensitivity

was compared to Au. Due to the significantly larger overlap with the optical mode, a much larger

sensitivity was obtained for the graphene plasmonic bio-sensor.

3.4 THz passive devices

A THz graphene isolator was developed, with non-reciprocity and gate-tuneability as unique aspects

compared to existing technologies [44]. Recently, we also designed several reflectarrays, simulated

with numerical tools developed for graphene devices [45, 46]. The simulations show that the selected

device can reach optimal performances according to our theoretical performance upper-bound [47].

We also demonstrated a graphene based polarization selective device operating at low THz

frequencies [48].

4. OLEDS Organic light-emitting diodes (OLED) are large area light sources that are applied as display in e.g.

mobile phones and televisions. Currently, a thin layer of brittle ITO is used as electrode material.

Graphene is promising as an alternative, especially in flexible applications. However, application in

OLEDs the sheet resistance needs to be less than 100 Ω/o and band alignment should be

accomplished to achieve a high injection efficiency. Using a 5 nm thick molybdenum trioxide (MoO3),

layer as a dopant, the desired work function increase

as well as sheet resistance decrease have been

obtained. The graphene monolayer-based OLED is

somewhat electrically inferior due to the limited

conductivity of the monolayer. However, this is

compensated by a higher current efficiency of 55 cd/A

at 1000 cd/m2 leading to overall superior power

efficiency compared to the state-of-the-art ITO

reference device. Also, the optical transmission of the

device is over 90%, which is an improvement over ITO

that can be attributed to the absence of guiding modes

that cause light loss in the case of ITO.

OLEDs should be protected from exposure to air or

water. For flexible OLEDs this is an issue that currently

asks for a complicated hybrid organic-inorganic


Fig.7 Few-cycle (28 fs) pulse generated from a graphene all-fiber laser with >50 mW output power [64].

material barrier layer. Graphene is also interesting as alternative for this barrier layer due to its low gas

permeability flexible OLEDs. From determination of the water vapour transmission rate it is evident

that graphene currently still lacks underperforms. The permeation pattern shows close similarity with

the defect pattern on CVD graphene, indicating that not intrinsic properties of graphene are limiting the

performance, but growth and processing related defects. It is therefore expected that improvements in

these areas will lead to improvement of large area graphene barrier properties.

5. Lasers Ultrafast lasers are relevant for many scientific, commercial, and industrial applications. The current

technology is based on Semiconductor Saturable Absorber Mirrors (SESAMs). SESAMs are multi-

quantum well heterostructures. Their fabrication involves expensive epitaxial growth and packaging.

Post-growth ion implantation is required to reduce the response time. Each design typically has a small

bandwidth (~100 nm) and can cover only a narrow wavelength range (~800-1600 nm). The linear

dispersion of the Dirac electrons in graphene offers an ideal solution: for any excitation there is always

an e–h pair in resonance (Fig. 6). The ultrafast carrier dynamics combined with large absorption and

Pauli blocking, make graphene an ideal ultrabroadband, fast Saturable Absorber (SA) [49, 50].

5.1 Wavelength tuning range

To date, Graphene-SAs (GSAs) have been demonstrated for pulse generation at ~800 nm [51], ~970

nm [52], 1 µm [53], 1.2 µm [54], 1.5 µm [55], 2 µm [56],

2.4 µm [57] and 3 µm [58]. A widely tuneable fibre laser

mode-locked with a GSA has been reported [59, 60].

The laser produces ps pulses in a tuning range 1525–

1559 nm, demonstrating its “full-band” operation

performance. A single GSA operating in a range that

covers from 1 to 2 µm was demonstrated [56, 61]. Mode-

locking at 1, 1.5 and 2 µm was achieved by integrating a

graphene composite into ytterbium- [61], erbium- [61],

and thulium-doped [56] fibre laser cavities respectively.

A synchronism of dual-wavelength pulse-trains through

the inclusion of a common graphene-SA in a coupled

cavity configuration was demonstrated [61]. This allowed getting sum frequency generation. A strong

coherent emission was detected at ~800 nm through degenerate four wave mixing, extending the

wavelength operation range.

5.2 Output power

The average output power has increased from a few mW in fibre lasers [55] to over 10 W in

semiconductor lasers [62]. The highest output power to date, ~10W, was achieved in a VECSEL by


Fig.8 Mid-IR ~ 8 GHz pulses from a graphene mode-locked laser [66].

using a graphene integrated mirror [62]. Other advances towards increasing the output power have

been achieved in all-fibre configurations able to generate over 50 mW output power [63]. The majority

of graphene mode-locked fibre lasers employ polymer composites, because of their simplicity of device

fabrication. However, the average power generated by these lasers is limited to few-mW, mainly due to

optical damage induce into the polymer matrix. By using a polymer-free graphene-SA, Ref. [63]

demonstrated for the first time power scaling up to ~50 mW, one order of magnitude higher than typical

graphene-based fibre lasers. This was only limited by the experimental setup, e.g., the limited power of

the pump laser, with a higher pump power, higher output powers being possible. A few-optical cycle

laser generating ~29 fs pulses with >50 mW output power was demonstrated (Fig. 7) [64]. The duration

of an optical pulse can be measured by counting the optical oscillations of the electric field within the

pulse envelope, with the single oscillation cycle setting the minimum possible duration. By using an all-

fibre setup mode-locked by a graphene-SA, the generation of only 6-cycles of light is possible [64].

This is the shortest pulse duration generated to date, outperforming most conventional technologies in

terms of low-cost, simple to fabricate and broadband. The laser is also able to generate >50 mW

output power, outperforming the current graphene-based fibre lasers.

5.3 Repetition rate

The highest repetition rates to date are ~ 2.5 GHz at

~970 nm in a semiconductor laser [52], ~ 10 GHz at 1.5

µm in a fibre cavity [65] and ~ 8 GHz at 2 µm in a

monolithic waveguide laser [66], by using a single layer

CVD graphene [52] and graphene film on an output

coupler [65, 66], respectively (Fig. 8).

A quantum theory of the nonlinear (third-order)

electrodynamic response of graphene is being

developed, with results on the third harmonic

generation and preliminary results on the optical Kerr effect (saturable absorption) [67]. An experiment

on the gate-voltage tuneable third harmonic generation at near-IR frequencies is planned.


References

1 Shiue et al., arXiv:1507.00426v3.

2 Tielrooij et al., Nature Nanotechnol. 10, 437 (2015)

3 Massicotte et al., arXiv:1507.06251.

4 Höller et al., manuscript in preparation.

5 De Fazio et al., manuscript in preparation.

6 Yang et al., Appl. Phys. Lett. 96, 121107 (2010).

7 Koppens F.H.L. et al., Nature Nanotechnol. 9, 780 (2014).

8 Vicarelli L. et al., Nature Mat. 11, 865-871, (2012).

9 Spirito D. et al. Appl. Phys. Lett. 104, 061111 (2014).

10 Bianco F. et al., Appl. Phys. Lett., in press (2015).

11 Mittendorff M. et al., Appl. Phys. Lett. 103, 021113 (2013).

12 Mahjoub A. M., et al., Appl. Phys. Lett. 107, 083506 (2015).

13 Cai X. et al., Nature Nanotechnol. 9, 814–819 (2014).

14 Cai X. et al., Nano Lett. 15, 4295 (2015).

15 Torre I. et al., Phys. Rev. B 91, 081402(R) (2015).

16 Levin A.D. et al., Appl. Phys. Lett. 107, 072112 (2015).

17 Viti L. et al. Adv. Materials, in press (2015).

18 Liu M. et al., Nature, 474(7349), 64–67 (2011).

19 Liu M. et al., Nano Lett.,10–13, Feb. (2012).

20 Youngblood N. et al., Nano Lett. 14(5), 2741–2746 (2014).

21 Gruhler N. et al., Opt. Express 21(25) 31678 (2013).

22 Qiu C. et al., Nano Lett., 6811-6815 (2014).

23 Phare C.T. et al. Nat. Photonics, July, 1–5 (2015).

24 Li H. et al., Opt. Express 20(18), 20330 (2012).

25 Mohsin M. et al. Opt. Express 22(12), 15292 (2014).

26 Hu Y. et al., Electron Devices Meeting (IEDM), United States, (2014); Koester S.J. and Li M., Appl.

Phys. Lett. 100(17), 171107 (2012).

27 Mohsin M. e.a., Scientific Reports, 5:10967 (2015)

28 Lu Z. and Zhao W., JOSA B 29(6), 1490–1496 (2012).

29 Ye C. et al, IEEE J. Sel. Top. Quantum Electron 20(4), 3400310 (2014).

30 Sorianello, V. et al, OPTICS EXPRESS 23 (5), 6478-6490 (2015)

31 Mohsin M. et al., Opt. Express 22(12), 15292 (2014).

32 Gao Y. et al., Nano Lett 15(3), 2001-2005 (2015).

33 Streshinsky M. et al., Opt. Express 21, 30350- 30357 (2013).

34 Timurdogan E. et al., Nat. Commun. 5 (2014).

35 Feng D., et al., Opt. Express 20(20), 22224-22232 (2012).


36 Popa D. et al. Appl. Phys. Lett. 97(20) 203106 (2010).

37 Lee, E.J. et al. Nature Commun. 6 (2015).

38 Alexander et al. “Electrically Controllable Saturable Absorption in Hybrid Graphene-Silicon

Waveguides” 2, 4–5.

39 Thackray B. D. et al., Nano Lett. 15, 3519 (2015).

40 Ansell D. et al., Nature commun., under review.

41 Kravets V. G. et al., Nature Mat. 12, 304 (2013); Kabashin A. V. et al., Nature Materials, in

preparation,

42 Ferrari A. C. et al., Nanoscale, 7, 4598 (2015).

43 Rodrigo D. et al., Science 349, 165-168 (2015).

44 Tamagnone M. et al., “Near optimal graphene terahertz non-reciprocal isolator”, under review.

45 Hasani H. et al. "Tri-band, Polarization-Independent Reflectarray at Terahertz Frequencies: Design,

Fabrication and Measurement", under review.

46 Gómez-Díaz J.S. et al. Nature Commun. 6, 6334 (2015).

47 Tamagnone M. et al. Nature Photon. 8, 556-563 (2014).

48 Li Y. et al. Optics Exp 23(6) 7227-7236 (2015).

49 Sun Z. et al., ACS Nano 4, 803 (2010).

50 Bonaccorso F. et al., Nature Photon. 51 Baek I. H. et al., Appl. Phys. Exp. 5, 032701 (2012). 52 Zaugg C. A. et al., Opt. Exp. 21, 31548 (2013). 53 Mary R. et al., Opt. Express 21, 7943 (2013). 54 Cho W. B. et al., Opt. Lett. 36, 4089 (2011). 55 Popa D. et al., Appl. Phys. Lett. 97, 203106 (2010). 56 Zhang M. et al., Opt. Exp. 20, 25077 (2012). 57 Cizmeciyan M. N. et al., Opt. Lett. 38, 341 (2013). 58 Wei C. et al., Opt. Lett. 38, 3233 (2013). 59 Sun Z. et al., Nano Res. 3, 653 (2010). 60 Popa D. et al., Apl. Phys. Lett. 98, 073106 (2011) 61 Zhang M. et al., CLEO, STu1I.2 (2014). 62 Husaini S. et al., Appl. Phys. Lett. 104, 161107 (2014). 63 Jiang Z. et al., CLEO, JTu4A.67 (2014). 64 Purdie D. et al., Appl. Phys. Lett. 106, 253101 (2015). 65 Martinez A. et al., Appl. Phys. Lett. 101, 041118 (2012). 66 Yingying R. et al., IEEE J. Sel. Top. Quantum Electron. 21, 1 (2015). 67 Mikhailov S. A., Phys. Rev. B 90, 241301(R) (2014); Phys. Rev. B 91, 039904(E) (2015).

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