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Active 2D materials for on-chip nanophotonics and quantum optics

Ren-Jye Shiue
  • Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
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/ Dmitri K. Efetov
  • Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
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/ Gabriele Grosso
  • Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
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/ Cheng Peng
  • Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
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/ Kin Chung Fong / Dirk Englund
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  • Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
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Published Online: 2017-03-15 | DOI: https://doi.org/10.1515/nanoph-2016-0172

Abstract

Two-dimensional materials have emerged as promising candidates to augment existing optical networks for metrology, sensing, and telecommunication, both in the classical and quantum mechanical regimes. Here, we review the development of several on-chip photonic components ranging from electro-optic modulators, photodetectors, bolometers, and light sources that are essential building blocks for a fully integrated nanophotonic and quantum photonic circuit.

Keywords: optoelectronics; quantum information processing; 2D materials; nanophotonics; bolometer; single photon source; photonic integrated circuit

1 Introduction

Photonic integrated circuits (PICs) allow compact optoelectronic component integration with high stability and low optical losses [1], [2], [3]. These properties make them appealing to both classical and quantum information processing applications. However, many applications demand device properties that require multiple disparate materials. For example, light sources must have a larger band gap than photodetectors. Optical modulators rely on materials with a strong electro-optic (E-O) effect [4], and still other materials may be required for non-linear optical functions or non-classical light sources.

Traditionally, the way to approach the problem of integrating multiple materials would require heterogeneous material growth or wafer-bonding approaches. However, these approaches can be extremely complex and challenging. For instance, the integration of two types of semiconductors – wafer bonding of silicon and III/V InP gain layers – is a major engineering challenge [5]. Two-dimensional (2D) materials, on the other hand, can greatly simplify the assembly of active optoelectronic systems on-chip. They can be conformally layered with relative ease and are “glued” sufficiently strongly by van der Waals (vdW) forces. The advent of such vdW material platforms has proven extremely rich and fruitful for not only electronic [6] but also photonic devices and systems [7]. The primary applications of vdW materials integrated into PICs are in optical interconnects and quantum information processing. Here, we review recent progress in these efforts, focusing on graphene-based E-O modulators and photodetectors, and light sources (classical and quantum) based on 2D transition metal dichalcogenides (TMDCs) and hexagonal boron nitride (hBN). For brevity, we will focus on many areas of our work, but seek to reference other works in the broader context of a wide and rapidly growing research field of 2D vdW materials for quantum optics and optoelectronics. Table 1 summarizes the list of devices that emerge for different nanophotonic and quantum photonic applications with different 2D materials (heterostructures).

Table 1:

List of applications of active nanophotonic and quantum photonic devices based on 2D materials.

2 Heterogeneous integration of 2D materials with nanophotonic devices

Nanophotonic devices, including waveguides and cavities, can control optical fields at subwavelength dimensions. Coupling a nanophotonic cavity to a 2D material can enhance light-matter interaction to enable spectrally selective and orders-of-magnitude increased optical absorption and fluorescence. Coupling to nanophotonic waveguides provides broadband enhancement. In this section, we discuss theory and experiments of heterogeneously integrated 2D materials with optical cavities and waveguides.

2.1 Absorption enhancement in cavity-integrated 2D materials

Using a temporal coupled mode theory (CMT) that considers the absorption of 2D materials coupling to an optical cavity [25], Gan et al. showed that the reflection, transmission, and absorption of a cavity strongly depend on two fundamental parameters: the intrinsic cavity loss rate κc without 2D materials and the excess loss rate introduced by the absorption of 2D materials κ2D in the cavity. There are two important regimes for such a coupled 2D materials-cavity system: strong attenuation of either the cavity reflection or transmission with large κ2D/κc ratio and, on the contrary, maximum absorption in 2D materials with a critical coupling condition, i.e. κ2D=κc.

We now discuss an experimental example of the enhanced absorption in graphene integrated with a planar photonic crystal (PPC) cavity. Figure 1A shows the finite-difference time-domain (FDTD) simulation of the cavity field of a PPC cavity fabricated by drilling periodic holes on a suspended gallium phosphide (GaP) membrane (180 nm). The PPC has a lattice constant a of 420 nm with an air-hole radius of 0.29a. A linear three-missing-hole (L3) defect in the middle of the PPC lattice serves to form confined resonant modes. Figure 1B shows the reflection spectrum of the cavity under the illumination of a broadband (super-continuum laser) source via a cross-polarization confocal microscope. The blue spectrum corresponds to the cavity without graphene deposition, showing a single sharp resonant peak. The graphene was prepared by mechanical exfoliation and then transferred on top of the cavity [27], as shown in the optical image in Figure 1B. The coupling of the graphene sheet to the evanescent field of the cavity results in reduction of the cavity Q from 2640 to 360, while the resonance red-shifts by 1.8 nm, indicated by the red spectrum acquired after graphene deposition. The cavity reflection dropped by 20 dB at 1477.3 nm (the resonance wavelength of the unloaded cavity). Thus, the naturally weak absorption (2.3%) of graphene [28] from normal incidence is dramatically amplified by coupling to the nanocavity.

(A) Simulated electric field distribution of a PPC L3 cavity. Bottom panel shows the cross-sectional view of the cavity mode. The graphene layer on the surface of the suspended PPC membrane can couple to the evanescent field of the resonant mode. (B) Reflection spectrum of a GaP PPC cavity before and after coupling to a single layer of graphene. Inset: Optical image of the graphene-PPC cavity device. (C) Fluorescence of single-layer MoS2 coupled to a PPC cavity [18]. Red, blue, and green curves show the PL collect at a polarization angle of 0°, 45°, and 90°. (D) Top: Optical image of a 70-μm-long graphene couples to a silicon waveguide. Bottom: Simulated waveguide mode couples to the graphene sheet deposited on the top surface of silicon [26].
Figure 1:

(A) Simulated electric field distribution of a PPC L3 cavity. Bottom panel shows the cross-sectional view of the cavity mode. The graphene layer on the surface of the suspended PPC membrane can couple to the evanescent field of the resonant mode. (B) Reflection spectrum of a GaP PPC cavity before and after coupling to a single layer of graphene. Inset: Optical image of the graphene-PPC cavity device. (C) Fluorescence of single-layer MoS2 coupled to a PPC cavity [18]. Red, blue, and green curves show the PL collect at a polarization angle of 0°, 45°, and 90°. (D) Top: Optical image of a 70-μm-long graphene couples to a silicon waveguide. Bottom: Simulated waveguide mode couples to the graphene sheet deposited on the top surface of silicon [26].

From CMT, we deduced the decay rates of κc and κcg to be 1.9×10−4 ω0 and 2.4×10−2 ω0, respectively [25], indicating that graphene loss accounts for approximately 92% of the total loss inside the nanocavity. By correlating the optical conductivity of graphene to the cavity loss and cavity field distribution, we extracted the single-layer graphene (SLG) complex dielectric function on top of the cavity to be σg//=4.64+4.62i and σg⊥=2.79 at a wavelength of 1477 nm, in good agreement with other reported values [29].

2.2 Fluorescence enhancement of 2D materials in an optical cavity

Because the local density of optical states is enhanced in a nanocavity, the spontaneous emission (SE) rate of fluorescent materials is also increased, due to the Purcell effect. This Purcell enhancement has been used to greatly improve the SE internal quantum efficiency of 2D materials [18], [30]. Here, we review both experimental results and quantitative analyses of the SE rates of 2D materials coupled to PPC cavities similar to those in the previous section. These cavities, which are also based on GaP membranes, have resonant modes in the wavelength range of 600–700 nm, overlapping with the fluorescence of molybdenum disulfide (MoS2), a direct band-gap 2D semiconductor belonging to the group of TMDCs [31], [32].

We measure the fluorescence of the cavity-MoS2 system on a micro-photoluminescence (PL) confocal microscope with a 532-nm continuous-wave excitation laser, focused to a beam diameter of 400 nm with a power of 50 μW. Figure 1C shows the PL collected from the L3 defect, exhibiting several narrow peaks with enhanced PL intensity compared with MoS2 deposited on a GaP substrate without the cavity. The PL exhibits strong polarization dependence, which is resolved by collecting the PL at a polarization angle ϕ with respect to the cavity along y-axis. Blue, red, and green curves show the PL at ϕ=0°, 45°, and 90°, respectively. The two peaks of the MoS2-cavity PL agree to the expected resonant wavelengths, and the polarization dependence of the peaks corresponds well to that of the L3 cavity given by 3D FDTD simulations [33], [34]. The SE rate of the MoS2 in the cavity is proportional to the Purcell factor, which is a function of the cavity Q and the effective mode volume Vmode. In this particular sample, we deduced Purcell factors ranging from 20 to 80 with a cavity Q~300, yielding a maximum seven-fold enhancement in the PL intensity.

For other heterogeneous TMDC material systems, Wu et al. integrated WSe2 with PPC cavity (Q~8000), showing enhanced Purcell factor of F~600 [19]. Ye et al. coupled WS2 to the whispering gallery mode of a Si3N4/HSQ microdisk resonator [20], and Salehzadeh et al. implemented MoS2 with SiO2 disk resonators [35]. In the above studies, strongly enhanced PL in 2D TMDCs resulted in reduced lasing thresholds, promising low-threshold on-chip light sources in 2D materials-based heterostructures. When the coupling rate of the 2D materials to the cavity field becomes even higher, the cavity-2D material coupling enters the strong coupling regime, where the energy of cavity polariton mode splits and the exciton dipoles undergo Rabi oscillations. Liu et al. observed MoS2 strongly coupled to a distributed Bragg reflector microcavity [36], showing a Rabi splitting energy of ~50 meV.

2.3 Absorption enhancement of 2D materials integrated with optical waveguides

Thus far, we have described the enhancement of the light-matter interaction via optical resonators. This effect is inherently narrowband due to the narrow resonant bandwidth of the resonators. Coupling 2D materials with a nanophotonic waveguide can enhance the light-matter interaction non-resonantly and hence span a broader bandwidth [16]. The top panel of Figure 1D provides an example device consisting of a bilayer graphene flake deposited across ~70 μm over a 520-nm-wide silicon-on-oxide (SOI) waveguide, fabricated from an SOI wafer with a 220-nm-thick Si membrane using the standard shallow trench isolation module. To prevent the graphene from fracturing at the edges of the waveguide, the chip was planarized by backfilling with a thick SiO2 layer and polishing the surface with a chemical-mechanical polishing step to reach the top Si layer.

The quasi TE-mode field simulation in the bottom panel of Figure 1D shows the evanescent field overlap to the graphene. From the complex effective index of graphene, we can estimate an absorption coefficient of 0.085 dB/μm. Experimentally, we observed that a 70-μm-long graphene bilayer causes a transmission loss around 6.2 dB over the wavelength range from 1510 to 1580 nm. The transmission loss deduced from the simulation results [16] is ~5.95 dB, and we attribute the measured excess loss to the interface scattering between graphene and the waveguide surface.

The absorption coefficient could be greatly increased (to ~0.2 dB/μm) by a stronger evanescent field of the waveguides that support a transverse magnetic guide mode [8], [37] or by thinner waveguides. Other strategies to enhance the absorption include sandwiching the graphene layer inside a dielectric slot waveguide [38] or modifying the electric field distribution by creating an air slot in the middle of a channel waveguide. Wang et al. achieved absorption as high as ~1 dB/μm by coupling graphene to such a silicon air-slot waveguide [39].

3 Active optoelectronics based on PIC-integrated vdW heterostructures

In the following sections, we discuss 2D materials-based E-O modulators and photodetectors integrated with nanophotonic cavities and waveguides.

3.1 Graphene-cavity high-speed E-O modulators

Graphene modulators rely on the Pauli blocking effect [40], [41], which is illustrated in Figure 2A. Electrostatic gating raises (lowers) the Fermi level EF of graphene, causing reduction in graphene’s optical absorption for photon frequencies ħω<2 |EFECNP|, where EFECNP is the difference in the electron Fermi energy from the charge neutrality point (CNP). Using the Pauli blocking effect in a 50-μm-long SLG coupled to a SOI waveguide [8], Liu et al. demonstrated broadband modulation for wavelengths from 1.35 to 1.6 μm with 3 dB modulation depth. Recently, improved device designs using mutually gated SLG capacitor structures on waveguides achieved modulation depths of 6 dB [9] and 16 dB [42].

(A) Schematic of the electro-absorptive effects in a capacitor structure consisting of two graphene layers (“top” and “bottom”) separated by hBN. When the Fermi level in graphene rises (drops), the absorption of graphene reduces due to the Pauli blocking of the interband transition of the electrons in graphene. (B) Schematic of the high-speed graphene E-O modulator [10]. (C) Normalized reflection intensity as a function of the gate voltage (VG). (D) Normalized E-O S21 of the graphene E-O modulator as a function of frequency.
Figure 2:

(A) Schematic of the electro-absorptive effects in a capacitor structure consisting of two graphene layers (“top” and “bottom”) separated by hBN. When the Fermi level in graphene rises (drops), the absorption of graphene reduces due to the Pauli blocking of the interband transition of the electrons in graphene. (B) Schematic of the high-speed graphene E-O modulator [10]. (C) Normalized reflection intensity as a function of the gate voltage (VG). (D) Normalized E-O S21 of the graphene E-O modulator as a function of frequency.

Resonators can shrink the size of such graphene modulators. For instance, we demonstrated a graphene modulator based on an air-slot cavity with wavelength-scale mode volume that achieved >10 dB modulation depth in the telecommunication band [43]. The graphene was gated using an electrolyte layer [poly(ethylene oxide) (PEO) plus LiClO4], requiring a relatively small swing voltage of 1.5 V for a modulation contrast of 10 dB.

The relative low speed of electrolyte gating, with a cutoff frequency response <1 MHz [44], prompted us to develop a high-speed device based on a graphene capacitor on the photonic crystal cavity [10], as illustrated in Figure 2B. The capacitor consisted of a boron nitride (BN)/graphene/BN/graphene/BN five-layer stack that was produced by the vdW assembly technique and transferred onto a quartz substrate [45], with lower parasitic capacitance compared to more commonly used SiO2/Si substrates. The silicon-membrane-based air-slot PPC cavity was then transferred and aligned with this graphene capacitor, producing the device shown in Figure 2B.

Figure 2C shows the normalized cavity reflection at λ=1551 nm as a function of VG, indicating a modulation depth of 3.2 dB. The modulation for negative bias voltage, −5.6 V<VG<0 V, mimicked that for positive voltage, as the doping types of the top and bottom graphene layers were reversed. The cavity reflectivity is symmetric about the CNP at VG=0.2 V.

The measured E-O S21 parameter, shown in Figure 2D, indicates a 3 dB cutoff frequency of 1.2 GHz, corresponding to the resistance-capacitance (RC)-limited time constant of the dual-layer graphene capacitor. A CMT model similar to that described in Section 2.1 indicates that the real optical conductivity of graphene, σgr, at the maximum gate voltage is reduced only to half of the ungated value (at the graphene CNP). Thus, a higher contrast would be possible at higher gate voltage, but would push the device dangerously close to breakdown (Vbreakdown=0.3–0.8 V/nm [46], [47]) across the 10-nm-thick BN dielectric layer of the graphene capacitor. Higher contrast could be achieved at longer operation wavelengths, or with the use of high-k materials or chemical doping of the graphene layers near the Pauli blocking regime.

The graphene capacitor in Figure 2 has an area of ~100 μm2, a capacitance of 320 fF, and a corresponding switching energy CV2/2~1 pJ/bit. Because the graphene parallel-plate capacitor was much larger than necessary as the area of the optical mode is only ~0.5 μm2, limiting the area of the graphene capacitor to the optical mode (~200 times smaller) would lead to a reduction of two orders of magnitude in the switching energy and RC time constant. As the cavity optical bandwidth is large (~600 GHz for a Q value of 300), such graphene-PPC modulators could enable high modulation contrast, exceptionally low energy consumption, and a much broader modulation bandwidth than Si modulators based on free-carrier dispersion [48], [49], [50].

Several other very promising graphene modulator designs have been introduced. Silicon ring resonators [11], [12] with graphene-based modulation showed a 12.5 dB modulation depth, and a graphene-silicon nitride ring resonator enabled a high modulation contrast of 15 dB per 10 V driving with ~30-GHz operation speed [13]. Finally, silicon Mach-Zehnder interferometers have also been integrated with graphene, producing a modulation depth of >4 dB and cutoff of 2.5 GHz [51].

3.2 Graphene-waveguide high-responsivity photodetector

Photodetectors in PICs require a different material that is absorptive where the PIC is transparent. Today, these include epitaxial Ge photodetectors [52], wafer-bonded InP [53], or Si with mid-band-gap states [54]. Graphene has recently emerged as a promising alternative that promises simple layer-transfer integration and photoresponse from ultraviolet (UV) all the way to the near-infrared, mid-infrared, and terahertz (THz) regimes. Moreover, graphene detectors can be extremely fast – early studies in graphene-based photodetectors show ultrafast photoresponse up to 40 GHz with zero bias voltage [55] – and graphene’s strong electron-electron interaction allows multiple hot electron-hole pairs to be generated for each incident photon [56], [57], [58], boosting detection responsivity. Recent advances in wafer-scale growth and transfer of graphene are promising for integration of graphene with complementary metal-oxide-semiconductor (CMOS) processes or with arbitrary materials, likely using back-end-of-line processing steps.

Figure 3A sketches a waveguide-coupled graphene photodetector [15] fabricated on a CMOS-compatible PIC. A stack of 40-μm-long hexagonal BN (hBN)/SLG/hBN, transferred onto the PIC using vdW assembly [45], serves as the photodetector. Figure 3C shows the completed structure. The drain electrode is positioned only 200 nm from the waveguide to induce a pn junction [59], [60] near the optical mode [61].

(A) Schematic of the graphene/boron nitride heterostructure photodetector integrated with a silicon box waveguide [15]. (B) Cross-sectional view of the side-contacted BN/SLG/BN detector. (C) Optical image of the finished waveguide-graphene photodetector.
Figure 3:

(A) Schematic of the graphene/boron nitride heterostructure photodetector integrated with a silicon box waveguide [15]. (B) Cross-sectional view of the side-contacted BN/SLG/BN detector. (C) Optical image of the finished waveguide-graphene photodetector.

The hBN/SLG/hBN stack decreased the waveguide transmission by 2.2 dB, corresponding to an absorption coefficient of 0.055 dB/μm, consistent with the simulated results for the waveguide evanescent field coupling to the SLG [16]. We used a polymer electrolyte (PEO and LiClO4) layer covering the entire chip to independently tune the graphene Fermi level and electric field across the waveguide mode [62], [63], [64], [65]. Figure 4A presents responsivity measurements (defined as the ratio of the short-circuit photocurrent Iph to the optical power Pin in the waveguide, i.e. R=Iph/Pin) as a function of VGS and VDS, showing a six-fold pattern in the photocurrent, which qualitatively matches the behavior of the photothermoelectric effect [66], [67]. The photocurrent reaches a maximum of 0.36 A/W at VGS=2 V and VDS=1.2 V.

(A) Photo-responsivity of the graphene detector as a function of drain-source voltage (VDS) and gate-source voltage (VGS). (B) High-speed response of the photodetector. The red dashed line shows the fitting to the experimental results with an RC low-pass filter model. (C) Photocurrent as a function of peak power under the excitation of pulse laser. (D) Autocorrelation traces of the ultrafast graphene autocorrelator at different peak powers.
Figure 4:

(A) Photo-responsivity of the graphene detector as a function of drain-source voltage (VDS) and gate-source voltage (VGS). (B) High-speed response of the photodetector. The red dashed line shows the fitting to the experimental results with an RC low-pass filter model. (C) Photocurrent as a function of peak power under the excitation of pulse laser. (D) Autocorrelation traces of the ultrafast graphene autocorrelator at different peak powers.

The high-speed intensity response of this detector was beyond the capabilities of commonly available E-O modulators. As a substitute, we interfered two detuned narrowband (1 MHz) telecom lasers to produce an intensity modulation at δf. Figure 4B plots the relative optical-to-electrical response of the detector received on an electrical spectrum analyzer (maximum frequency 50 GHz) at different δf. This measurement indicates a 3 dB cutoff frequency at 42 GHz, matching the highest reported graphene photodetector speed [14]. Sending pseudorandom on-off-keyed data produced a clear eye-opening diagram at 12 Gbit/s (inset of Figure 4B).

Waveguide-integrated graphene photodetectors are possible with CMOS-compatible processes [61]. In addition to exfoliated graphene devices, large-scale chemical vapor deposition-grown graphene integrated with waveguides can achieve a data rate of 50 Gbits/s [14], and high-responsivity by integrating with a silicon air-slot waveguide [39]. Owing to the broadband absorption of graphene, waveguide-integrated graphene heterostructure also enables photodetectors for mid-infrared wavelengths [68]. For other 2D materials, black phosphorus (BP)-based photodetectors have drawn great attention due to their small band gap that is promising for telecommunication and mid-infrared wavelength ranges [17], [69]. Heterogeneously integrated BP-silicon photodetectors have shown up to 6 A/W responsivity with >3 GHz speed.

3.3 Ultrafast on-chip autocorrelator

The photocurrent presented in the previous section exhibits a non-linear photoresponse with a peak power that is >50 mW. The power-dependent curve (Figure 4C) fits well by a power law of IphPin0.47, suggesting supercollision-dominated cooling mechanism [70]. The picosecond-scale non-linear photocurrent is desirable for on-chip optical signal characterization, e.g. ultrafast optical sampling and autocorrelation measurements. In Figure 4D, we plot photocurrent traces as a function of the time delay Δt between pairs of 250-fs laser pulses for a range of incident powers. These traces show a clear dip at Δt=0 with a width corresponding to the detector’s carrier relaxation time of ~3 ps. In this device, strong non-linear photoresponse at low peak power could enable a more efficient on-chip autocorrelator compared with existing autocorrelators based on free-space parametric frequency conversion and two-photon absorption in semiconductor waveguides [71], [72], [73], while keeping a footprint of only 40 μm in length. The femtosecond thermalization of graphene’s hot electrons suggests that the timing resolution of the on-chip autocorrelator is possibly down to sub-50 fs [74]. It is also important to note that this graphene-based autocorrelator supports a broad spectral range from 1500–1800 nm.

3.4 Ultrasensitive graphene-based bolometers and single photon detectors

With its exceptionally small electronic heat capacity [75], [76], [77], [78], [79] and weak electronic heat dissipation [64], graphene is also a promising material platform for ultrasensitive thermal detectors [80], [81], [82]. The exceedingly low number of atoms in a typical graphene device and the vanishing density of states at the CNP result in a record-low electronic specific heat, which is on the order of one Boltzmann constant. Hence, a graphene-based bolometer is comparable to some of the best nano-calorimeters [81], [83]. Additionally, the small size of the Fermi surface and the high energy of graphene’s phonons result in a strongly suppressed heat flow from hot electrons into the phonon bath. With these unique thermal properties, graphene is a natural heat absorber for bolometer applications and allows for a high temperature rise by incident radiation, even down to the energy quanta of a single photon.

To investigate graphene’s thermal properties and to demonstrate the proof of concept of the graphene bolometer, we investigated a Johnson noise thermometry circuit [21], [78], [84] to directly read the electronic temperature using the Johnson-Nyquist theorem. As shown in Figure 5A, the graphene device is impedance-matched with an inductor-capacitor (LC) matching network, which enables maximal transmission of the generated noise power. The signal is further amplified using a state-of-the-art low-noise amplifier, resulting in an overall temperature sensitivity that approaches 0.001 K/Hz1/2. A directional coupler is used to apply Joule heating pulses and to simultaneously monitor the response of the electronic temperature, which enables direct measurements of the electronic cooling mechanisms through the study of the electronic thermal conductance Gth.

(A) Schematics of a graphene bolometer circuit [21], [78]. Due to its exceedingly small electronic heat capacity, the graphene sheet acts as an ultrasensitive heat absorber. The change of its electronic temperature Te is read out by a Johnson noise thermometry scheme that provides a sensitivity in the range of mK/Hz1/2. Here, microwave signals from the graphene are transmitted through an impedance matched LC circuit and amplified by a low noise amplifier. (B) The response of Te to Joule heating. (C) The resulting electronic thermal conductance and the electronic specific heat due to electron-phonon cooling. (D) Schematics of a proposed graphene-based single photon detector. A graphene sheet is placed on a photonic crystal cavity for maximal quantum efficiency. The graphene is proximitized by two superconducting electrodes that allow single photon read out through the Josephson effect.
Figure 5:

(A) Schematics of a graphene bolometer circuit [21], [78]. Due to its exceedingly small electronic heat capacity, the graphene sheet acts as an ultrasensitive heat absorber. The change of its electronic temperature Te is read out by a Johnson noise thermometry scheme that provides a sensitivity in the range of mK/Hz1/2. Here, microwave signals from the graphene are transmitted through an impedance matched LC circuit and amplified by a low noise amplifier. (B) The response of Te to Joule heating. (C) The resulting electronic thermal conductance and the electronic specific heat due to electron-phonon cooling. (D) Schematics of a proposed graphene-based single photon detector. A graphene sheet is placed on a photonic crystal cavity for maximal quantum efficiency. The graphene is proximitized by two superconducting electrodes that allow single photon read out through the Josephson effect.

The heat transfer P is dominated by electron-phonon cooling and follows a high power law P=σA(Te4Tp4), where σ is the electron-phonon coupling parameter, A is the graphene area, Te is the electron temperature, and Tp is the phonon temperature. Figure 5B shows the typical data taken at different temperatures. Through the relation Gth=PTe, we can extract the thermal conductance, which we find to be on the order of 0.2 nW/K (Figure 5C). This corresponds to a noise equivalent power of the bolometer of 0.4 pW/K at 2K, as given by the product of Gth and the temperature sensitivity. Using a much cleaner graphene device at the CNP, the bolometer is predicted to reach an even lower NEP of ~10−21 W/Hz1/2 [78].

The above extracted parameters make it possible to engineer a graphene-based device that could detect a single photon in the frequency range from UV to GHz. Figure 5D shows a device concept for a thermal graphene-based single photon threshold detector. High quantum efficiency can be achieved by critical coupling of the graphene to a photonic crystal cavity. The graphene sheet is further coupled to two closely spaced superconducting electrodes that form a Josephson junction and allow for a supercurrent to flow through the graphene [85], [86], [87]. Upon absorption of a single photon, the temperature of the device is increased above the critical temperature of the superconductor, creating a detectable voltage pulse across the device.

4 vdW materials for single photon generation

A single photon emitter (SPE) is a critical element for quantum information processes, including quantum networks [88] and photonic quantum computing [89]. Layered materials have recently shown quantum light emission, paving the way to new applications of this emerging class of materials. Compared with other SPEs, 2D materials have the potential to be easily integrated into complex photonic structures and enable new chip designs for quantum applications. Moreover, most are stable and can easily be gated. In this section, we review some of the recent discoveries of quantum optics applications of 2D materials in both TMDC semiconductors and insulators, and the potential high impact that this technology can bring for future efficient quantum communication devices.

4.1 Single photon source in semiconductor TMDCs

TMDCs are layered semiconductors with direct band gap at the monolayer level [31], [32]. At room temperature, PL is mainly dominated by excitons and electron-hole pairs, with emission spanning over a large band depending on the 2D material [90]. Cryogenic measurements also revealed the presence of trions (excitons bound to a carrier) and localized excitons. The latter are excitons trapped in a potential well created by atomic defects or impurities in the crystal lattice of the TMDCs. Similar to quantum dots (QDs), defect-bound excitons can behave like SPEs with smaller energy with respect to excitons due to the binding energy used to pin to disorder site. Quantum emission in TMDCs has been reported for both exfoliated [23], [91], [92] and grown WSe2 [24].

For exfoliated materials, single photon emission is observed in localized sites along the interface between layers, the cracks or the edges of the flakes. In flakes grown by physical vapor transport [24], quantum emission is observed also in the center of the flake, suggesting a higher density of impurities and defects for artificially grown monolayers compared to the exfoliated ones. Nevertheless, both systems show similar characteristics. A single localized exciton shows a rather narrow spectral emission with a linewidth down to 10 μeV with resonant excitation [93]. The lifetime has been measured to range from 600 ps up to 3 ns, depending on the investigated emitters. Blinking and spectral diffusion have been reported and still remain a big challenge for quantum emission applications with 2D materials. These issues can be solved by better isolating the monolayers (i.e. by encapsulation with hBN) from external agents that are responsible of fluctuation of the electronic environment surrounding these thin materials. Furthermore, quantum emitters have been shown to be rather resistant to high power excitation and several temperature cycles.

Single photon emission has also been investigated under strong electric and magnetic fields. Anomalously large Zeeman effect has been reported [23], [92], suggesting that the valley polarization degree of freedom, typical of hexagonal lattice structure, is preserved in the localized excitons. Several experimental works report a g-factor around 10, which is almost an order of magnitude larger than the one reported for III–V semiconductor QDs [94]. These observations strongly suggest that the quantum emitters in WSe2 inherit their electronic structure from the TMDC and indicate trapped excitons as candidates.

Single photon emission can also be controlled and modulated via electrostatic gating [91]. In particular, the activity of the emitters is affected by the variation of the local electrical environment via gating. Emission stability is highly affected and deteriorated with increased bias. Preliminary measurements also showed that lifetime could also be controlled by external gating.

4.2 Single photon source in BN defect center

hBN is a vdW insulator widely used in heterostructures to improve optical and electronic properties of 2D semimetals and semiconductors [27]. Growing interest has been attracted also by its highly non-linear natural hyperbolic properties [95]. The recent demonstration of quantum emission in layered hBN opened the door to new applications in quantum technology as scalable single photon sources [96].

Different from TMDCs, where SPEs are associated to localized excitons, SPE in insulating hBN are attributed to atomic-like defects of the crystal structure, similar to color centers in diamond. Defects in hBN confine electronic levels within the band gap (~6 eV) and result in stable and extremely robust emitters.

Despite the wide use as insulator for its high quality, pristine hBN is not free of impurities, defects, and dangling bonds. Several luminescence experiments have reported strong emission from a large variety of charge defects, impurities, and adatoms [97], [98], [99] that create photoactive states within the hBN band gap [100], [101]. Recent studies demonstrate that these defects can be associated to nitrogen vacancies (NVs), a carbon atom substituted to an N vacancy or oxygen impurities [100]. Moreover, Tran et al. have shown that NVs in hBN function as SPEs [96]. Figure 6A shows the typical atomic structure of hBN with an individual atomic defect. The spectrally resolved emission of atomic defect at room temperature is shown in Figure 6B with the zero phonon line (ZPL) located around 575 nm and separated by ~50 nm from the first phonon side band.

(A) Schematic of the atomic structure of hexagonal boron nitride. The dashed circle shows the structure of an NV in which one N atom is vacant and an adjacent B atom is substituted with N. (B) Typical room temperature spectrum of an SPE associated to an atomic-like defect. (C) Autocorrelation function (g2(τ)) showing anti-bunching at zero time delay. The histogram is measured using a Hanbury Brown and Twiss setup with continuous wave excitation at 532 nm. (D) Spectral distribution of the ZPL emission for almost 90 individual emitters. The emission energy spans over the 150-nm range.
Figure 6:

(A) Schematic of the atomic structure of hexagonal boron nitride. The dashed circle shows the structure of an NV in which one N atom is vacant and an adjacent B atom is substituted with N. (B) Typical room temperature spectrum of an SPE associated to an atomic-like defect. (C) Autocorrelation function (g2(τ)) showing anti-bunching at zero time delay. The histogram is measured using a Hanbury Brown and Twiss setup with continuous wave excitation at 532 nm. (D) Spectral distribution of the ZPL emission for almost 90 individual emitters. The emission energy spans over the 150-nm range.

Spectroscopy of SPEs is done in confocal configuration with a microscope objective of NA=1 and a fiber-coupled spectrometer. In detection, the excitation laser at 532 nm is filtered out with a polarized beam splitter and long pass filter. Second-order correlation measurement shows good anti-bunching, unveiling the single photon nature of these emitters. An example is reported in Figure 6C where the coincidence counts are recorded using a Hanbury Brown and Twiss setup with continuous wave excitation. At zero delay time, the counts go below 0.5 (g2(0)=0.24), confirming the single photon emission. Power-dependent measurements revealed a three-level system with a metastable state. Moreover, these emitters are found to be stable over time and show high emission rate up to 4 million counts per second [102]. Time-dependent measurements indicate a lifetime around 2 ns.

Interestingly, experiments report different spectral shape for quantum emitters in hBN with the ZPL energy spanning over a large band, from the UV up to 750 nm [96], [103]. Figure 6D shows the spectral distribution of the ZPL emission energy of almost 90 emitters in our experiments.

Although individual emitters show different spectral shapes, the constant energy separation between the ZPL and the phonon side bands of ~160 meV suggests similar crystal structures [96]. It is also interesting to notice the correlation between the energy of the ZPL and the spectral shape. Recent studies suggest the presence of at least two families of quantum emitters in hBN with different lifetime and brightness [22].

An interesting aspect is the spectral distribution among the same family of emitter. Within the same family, the ZPL can span over the 300-meV band. The cause of this large distribution has been tentatively attributed to local strain fluctuations of the hBN flakes. Electronic disorder is induced in monolayer graphene by random strain fluctuation due to the exfoliation process or the inhomogeneous adhesion to the substrate [104]. Similarly, in hBN, strain can result in the displacement of the lattice atoms and the shifting of the energy levels of atomic-like defects. It has been calculated that strain in the range of −5% to 5% can cause an energy shift of almost 200 meV, in agreement with the experimental observation.

The role of strain in the energy of the single photon emission can open interesting possibilities for the local tuning of the ZPL with applications in quantum computing and quantum information where indistinguishability is of utmost importance.

5 Outlook

Heterogeneous integration of 2D materials with nanophotonics promises a new architecture of optoelectronic devices and quantum photonics. In addition to E-O modulators and photodetectors based on graphene, a variety of combinations of 2D materials including TMDCs [90], BP [7], [17], [105], hBN, and superconducting 2D niobium diselenide (NbSe2) [106] could further provide on-chip devices including all-optical modulators [107], mode-locked ultrafast laser [108], thermo-optic modulators [109], light-emitting diodes [110], and single photon sources [22].

Continued improvement in material and device fabrication is needed to facilitate the technology transition into applications. There has been significant progress in epitaxial, large-scale, and high-yield transfer processes for certain materials including graphene [14], [111], [112] and MoS2 [113], [114]; on the other hand, the development of large-scale epitaxial hBN [115], [116] and other TMDCs continually advances. These developments could soon provide a hybrid platform that heterogeneously integrates 2D materials with silicon CMOS technology. To further ensure high-quality electronics and photonics based on 2D materials, suitable substrates and surface passivation are essential to ensure high-quality 2D devices. In addition, high-quality electrical contacts are also important to reduce contact resistance, improving device speed and power consumption. One-dimensional contacts to the hBN-encapsulated graphene [45] and graphite-graphene contacts [117] show great promise. Work continues on scaling up these processes and extending low-resistivity contacts to other 2D materials.

The outstanding optical and electronic properties of 2D materials offer a level of control, heterogeneous material properties, and design flexibilities that are probably unprecedented in optoelectronics. Here, we reviewed a subset of applications: optical interconnect technologies including high-speed photodetectors [15], [16], [39] and modulators [10], [13], as well as device concepts for photonic quantum information processing, including progress toward efficient single photon sources and single photon detectors. With the exceptional progress of the past decade, such a scalable 2D nanophotonic architecture could bring remarkable impacts for these applications in the near future, while promising other directions including back-end deposited silicon photonics [118], mid-infrared photonics [119], THz detection [120], [121] and modulation [122], and also flexible photonics [123].

Acknowledgments

G.G. acknowledges support by the Swiss National Science Foundation. Measurements were supported in part by the National Science Foundation EFRI 2-DARE, award abstract no. 1542863, and in part by the Center for Excitonics, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under award no. DE-SC0001088. C.P. was supported in part by the Stata Family Presidential Fellowship of Massachusetts Institute of Technology. R.-J.S. was supported in part by the Center for Excitonics, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under award no. DE- SC0001088. K.C.F. acknowledges support from the Internal Research and Development in Raytheon BBN Technologies in preparing this manuscript.

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About the article

Received: 2016-10-17

Accepted: 2017-01-12

Published Online: 2017-03-15


Citation Information: Nanophotonics, ISSN (Online) 2192-8614, DOI: https://doi.org/10.1515/nanoph-2016-0172.

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©2017, Dirk Englund et al., published by De Gruyter, Berlin/Boston. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License. BY-NC-ND 3.0

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