news & views NONLINEAR OPTICS
Tuning harmonics with excitons
Nonlinear generation of light from an atomically thin semiconductor can be controlled by electrical fields.
Sean P. Rodrigues and Wenshan Cai
D
ynamic control of light, which is essential for the creation and routing of optical signals, is intrinsically connected to nonlinear optics, a branch of optics that describes light–matter interactions where the induced dielectric polarization is related to the electric field component of light in a nonlinear manner. The birth of the field was marked in 1961 by Peter Franken’s demonstration of secondharmonic generation (SHG)1, which occurs when photons in a given medium can be combined and converted to a single photon with twice the energy of the input ones. This optical process is widely employed in physics, biology and materials science for diverse applications including the modification of the output wavelength in lasers, measurement of ultrafast pulses, and high-resolution microscopy of biological tissues. Writing in Nature Nanotechnology, Xiaodong Xu and co-workers at the University of Washington, University of Hong Kong, Oak Ridge National Laboratory and University of Tennessee now show that the frequency-doubled output from a monolayer transition metal dichalcogenide semiconductor can be dynamically manipulated by gate biases, and its intensity tuned by up to an order of magnitude2. Electrically induced harmonic generation of light can be achieved through a variety of schemes. In 1962, Robert Terhune discovered that a large voltage applied to a calcite crystal can generate a second harmonic signal (Fig. 1a), which otherwise would not have occurred because the centrosymmetric crystal lattice of calcite would make its second-order nonlinear susceptibility χ(2) vanish3. The electrical-field-induced SHG stems from the interplay of the third-order nonlinear coefficient χ(3), ubiquitous in all materials, and the static or low-frequency electrical field induced by an applied voltage. Various configurations have been produced to realize electrically controlled frequency doubling. For example, a metallic surface placed in an electrolytic solution can be used as a nonlinear medium, where the SHG is sensitive to the surface charge accumulation induced by the electrical potential4 (Fig. 1b). In a metal–oxide– semiconductor structure or a p–n junction,
the built-in electric field of the depletion region facilitates electrically enabled SHG in silicon5 (Fig. 1c). Recently, plasmonic resonators and photonic metamaterials have been used for the electrical manipulation of both harmonic generation and optical rectification6,7 (Fig. 1d). Xu and co-workers unveiled a different route to voltage-controlled harmonic generation of light by exploring the electrostatic doping of a monolayer of WSe2 (Fig. 1e), an atomically thin semiconductor in the family of transition metal dichalcogenides (TMDs). TMD monolayers typically feature a direct bandgap, whose size is suitable for a wide range of electronic and photonic applications including transistors, integrated circuits, photon emitters, light sensors and photovoltaic cells. Moreover, the lattices of monolayer TMDs lack inversion centres. This unique characteristic, in conjunction with other features of a
b
ω
monolayer crystals, makes TMDs an ideal platform for the exploration of spintronics and valleytronics8,9. In regard to nonlinear optics, the lack of centrosymmetry implies that the monolayer possesses a non-zero χ(2) susceptibility that allows for second-order nonlinear processes. SHG emerging from single-layered TMDs is over two orders of magnitude stronger than that from their bilayer counterparts10,11, as the latter have an inversion centre that extinguish χ(2) responses. The conversion efficiency of SHG from a WSe2 monolayer exhibits highly specific resonance behaviour, and reaches its pinnacle at an excitation energy of 0.83 eV, because the resulting two-photon energy of 1.66 eV coincides with the energy of excitons — bound electron–hole pairs — in the monolayer crystal2. The exciton transition mediates not only the enhanced resonance within the SHG excitation spectrum, but also the
p-doped
~kV χ
Calcite
Vref
(3)
+ - - + + + - + ++ -
2ω
d
ω
V
– – + +
– – + +
– – + +
– – + +
Junction field E
n-doped
ω
+
V
2ω
WSe2 SiO2 Si
Electrically induced χ(2)
– – + +
2ω
e
2ω
Depletion region
Electrolyte 2ω
-
V–
ω
c
ω
χ(2)(Vg) Vg
V–
Figure 1 | Various configurations for electrically controlled second-harmonic generation of light. a, Terhune’s seminal set-up, where a large voltage, on the level of kilovolts, is applied across a piece of calcite crystal. b, Free ions in an electrolytic solution can accumulate at a biased electrode surface, V– with respect to a reference voltage Vref, and provide the electric field necessary for electric-field-induced SHG. c, In the depletion region of a p–n or metal–oxide–semiconductor junction, the built-in electric field converts the large χ(3) susceptibility in the semiconductor into an effective second-order response. d, The two metallic parts of a metamaterial absorber serve as electrodes biased with voltages V+ and V– to generate a control field within the light-concentrating region. e, The structure used by Xu and colleagues, where the SHG signal from a monolayer WSe2 field-effect transistor can be tuned by the gate voltage Vg. ω is the frequency of the incoming photons.
NATURE NANOTECHNOLOGY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology
© 2015 Macmillan Publishers Limited. All rights reserved
1
news & views electrical tunability of the frequencydoubled signals. Using a monolayer WSe2 field-effect transistor (Fig. 1e), the secondorder susceptibility χ(2) — and hence the harmonic generation — can be tuned by means of electrostatic doping induced by a gate bias. This scheme to produce and control SHG is very different from the conventional electric-field-induced SHG method, where an applied voltage generates a static electric field that is coupled to the χ(3) susceptibility. The degree of nonlinear modulation achieved by Xu and colleagues is impressive: the intensity of the SHG output can be tuned by over an order of magnitude in a cryogenic environment, although the performance is compromised at ambient temperature. This extensive tunability of the SHG signal intensity is attributed to the electrostatic modulation of oscillator strength at the resonance frequencies of various exciton species. As such, the effect is inherently narrowband. When the energy of the fundamental wave is shifted by only tens of millielectronvolts, the gate-controlled tunability of the SHG quickly diminishes. The researchers also demonstrate an interesting selection rule of the exciton-enhanced SHG. The frequencydoubled output is counter-circularly polarized to the fundamental wave. This
2
phenomenon illustrates the coupling of the crystal symmetry and the valley-selective optical excitation. The nonlinear generation of light along with its electrical tunability serves as a valuable tool for studying the physics of TMDs. While there seems to be a vast gap to fill before this exciting technology can be implemented for real applications, there is hope that some of the outstanding challenges may be tackled. Most nonlinear nanophotonic systems that utilize media of subwavelength interaction-length have poor signal conversion efficiency. In the experiment by Xu and colleagues, it takes over 1 billion incoming photons to generate a new photon at the doubled frequency 2. To counteract this issue, the conversion efficiency can potentially be boosted when the nonlinear material is placed in lightconcentrating regions or inside a laser cavity, as seen in commercial green laser pointers. In addition, the operational band for the tunable harmonic generation is potentially expandable using bandgap engineering through alloying or heterostructures. With electrically enabled light–matter interactions at the heart of the entire photonics industry and nanofabrication techniques being continuously advanced, it is not far-fetched to envision the use of
these monolayer SHG transistors, which are compatible with complementary metal– oxide–semiconductor technology, in realistic applications for optical signal processing and integrated nanocircuits in the future. ❐ Sean P. Rodrigues and Wenshan Cai are at the School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, USA. e-mail: [email protected] References
1. Franken, P. A., Hill, A. E., Peters, C. W. & Weinreich, G. Phys. Rev. Lett. 7, 118–119 (1961). 2. Seyler, K. L. et al. Nature Nanotech. http://dx.doi.org/10.1038/ nnano.2015.73 (2015). 3. Terhune, R. W., Maker, P. D. & Savage, C. M. Phys. Rev. Lett. 8, 404–406 (1962). 4. Lee, C. H., Chang, R. K. & Bloembergen, N. Phys. Rev. Lett. 18, 167–170 (1967). 5. Aktsipetrov, O. A., Fedyanin, A. A., Golovkina, V. N. & Murzina, T. V. Opt. Lett. 19, 1450–1452 (1994). 6. Cai, W., Vasudev, A. P. & Brongersma, M. L. Science 333, 1720–1723 (2011). 7. Kang, L., Cui, Y., Lan, S., Rodrigues, S. P., Brongersma, M. L. & Cai, W. Nature Commun. 5, 4680 (2014). 8. Wang, Q. H., Kalantar-Zadeh, K., Kis, A., Coleman, J. N. & Strano, M. S. Nature Nanotech. 7, 699–712 (2012). 9. Xu, X. D., Yao, W., Xiao, D. & Heinz, T. F. Nature Phys. 10, 343–350 (2014). 10. Malard, L. M., Alencar, T. V., Barboza, A. P. M., Mak, K. F. & de Paula, A. M. Phys. Rev. B 87, 201401 (2013). 11. Kumar, N. et al. Phys. Rev. B 87, 161403 (2013).
Published online: 20 April 2015
NATURE NANOTECHNOLOGY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology
© 2015 Macmillan Publishers Limited. All rights reserved
Tuning harmonics with excitons
Nonlinear generation of light from an atomically thin semiconductor can be controlled by electrical fields.
Sean P. Rodrigues and Wenshan Cai
D
ynamic control of light, which is essential for the creation and routing of optical signals, is intrinsically connected to nonlinear optics, a branch of optics that describes light–matter interactions where the induced dielectric polarization is related to the electric field component of light in a nonlinear manner. The birth of the field was marked in 1961 by Peter Franken’s demonstration of secondharmonic generation (SHG)1, which occurs when photons in a given medium can be combined and converted to a single photon with twice the energy of the input ones. This optical process is widely employed in physics, biology and materials science for diverse applications including the modification of the output wavelength in lasers, measurement of ultrafast pulses, and high-resolution microscopy of biological tissues. Writing in Nature Nanotechnology, Xiaodong Xu and co-workers at the University of Washington, University of Hong Kong, Oak Ridge National Laboratory and University of Tennessee now show that the frequency-doubled output from a monolayer transition metal dichalcogenide semiconductor can be dynamically manipulated by gate biases, and its intensity tuned by up to an order of magnitude2. Electrically induced harmonic generation of light can be achieved through a variety of schemes. In 1962, Robert Terhune discovered that a large voltage applied to a calcite crystal can generate a second harmonic signal (Fig. 1a), which otherwise would not have occurred because the centrosymmetric crystal lattice of calcite would make its second-order nonlinear susceptibility χ(2) vanish3. The electrical-field-induced SHG stems from the interplay of the third-order nonlinear coefficient χ(3), ubiquitous in all materials, and the static or low-frequency electrical field induced by an applied voltage. Various configurations have been produced to realize electrically controlled frequency doubling. For example, a metallic surface placed in an electrolytic solution can be used as a nonlinear medium, where the SHG is sensitive to the surface charge accumulation induced by the electrical potential4 (Fig. 1b). In a metal–oxide– semiconductor structure or a p–n junction,
the built-in electric field of the depletion region facilitates electrically enabled SHG in silicon5 (Fig. 1c). Recently, plasmonic resonators and photonic metamaterials have been used for the electrical manipulation of both harmonic generation and optical rectification6,7 (Fig. 1d). Xu and co-workers unveiled a different route to voltage-controlled harmonic generation of light by exploring the electrostatic doping of a monolayer of WSe2 (Fig. 1e), an atomically thin semiconductor in the family of transition metal dichalcogenides (TMDs). TMD monolayers typically feature a direct bandgap, whose size is suitable for a wide range of electronic and photonic applications including transistors, integrated circuits, photon emitters, light sensors and photovoltaic cells. Moreover, the lattices of monolayer TMDs lack inversion centres. This unique characteristic, in conjunction with other features of a
b
ω
monolayer crystals, makes TMDs an ideal platform for the exploration of spintronics and valleytronics8,9. In regard to nonlinear optics, the lack of centrosymmetry implies that the monolayer possesses a non-zero χ(2) susceptibility that allows for second-order nonlinear processes. SHG emerging from single-layered TMDs is over two orders of magnitude stronger than that from their bilayer counterparts10,11, as the latter have an inversion centre that extinguish χ(2) responses. The conversion efficiency of SHG from a WSe2 monolayer exhibits highly specific resonance behaviour, and reaches its pinnacle at an excitation energy of 0.83 eV, because the resulting two-photon energy of 1.66 eV coincides with the energy of excitons — bound electron–hole pairs — in the monolayer crystal2. The exciton transition mediates not only the enhanced resonance within the SHG excitation spectrum, but also the
p-doped
~kV χ
Calcite
Vref
(3)
+ - - + + + - + ++ -
2ω
d
ω
V
– – + +
– – + +
– – + +
– – + +
Junction field E
n-doped
ω
+
V
2ω
WSe2 SiO2 Si
Electrically induced χ(2)
– – + +
2ω
e
2ω
Depletion region
Electrolyte 2ω
-
V–
ω
c
ω
χ(2)(Vg) Vg
V–
Figure 1 | Various configurations for electrically controlled second-harmonic generation of light. a, Terhune’s seminal set-up, where a large voltage, on the level of kilovolts, is applied across a piece of calcite crystal. b, Free ions in an electrolytic solution can accumulate at a biased electrode surface, V– with respect to a reference voltage Vref, and provide the electric field necessary for electric-field-induced SHG. c, In the depletion region of a p–n or metal–oxide–semiconductor junction, the built-in electric field converts the large χ(3) susceptibility in the semiconductor into an effective second-order response. d, The two metallic parts of a metamaterial absorber serve as electrodes biased with voltages V+ and V– to generate a control field within the light-concentrating region. e, The structure used by Xu and colleagues, where the SHG signal from a monolayer WSe2 field-effect transistor can be tuned by the gate voltage Vg. ω is the frequency of the incoming photons.
NATURE NANOTECHNOLOGY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology
© 2015 Macmillan Publishers Limited. All rights reserved
1
news & views electrical tunability of the frequencydoubled signals. Using a monolayer WSe2 field-effect transistor (Fig. 1e), the secondorder susceptibility χ(2) — and hence the harmonic generation — can be tuned by means of electrostatic doping induced by a gate bias. This scheme to produce and control SHG is very different from the conventional electric-field-induced SHG method, where an applied voltage generates a static electric field that is coupled to the χ(3) susceptibility. The degree of nonlinear modulation achieved by Xu and colleagues is impressive: the intensity of the SHG output can be tuned by over an order of magnitude in a cryogenic environment, although the performance is compromised at ambient temperature. This extensive tunability of the SHG signal intensity is attributed to the electrostatic modulation of oscillator strength at the resonance frequencies of various exciton species. As such, the effect is inherently narrowband. When the energy of the fundamental wave is shifted by only tens of millielectronvolts, the gate-controlled tunability of the SHG quickly diminishes. The researchers also demonstrate an interesting selection rule of the exciton-enhanced SHG. The frequencydoubled output is counter-circularly polarized to the fundamental wave. This
2
phenomenon illustrates the coupling of the crystal symmetry and the valley-selective optical excitation. The nonlinear generation of light along with its electrical tunability serves as a valuable tool for studying the physics of TMDs. While there seems to be a vast gap to fill before this exciting technology can be implemented for real applications, there is hope that some of the outstanding challenges may be tackled. Most nonlinear nanophotonic systems that utilize media of subwavelength interaction-length have poor signal conversion efficiency. In the experiment by Xu and colleagues, it takes over 1 billion incoming photons to generate a new photon at the doubled frequency 2. To counteract this issue, the conversion efficiency can potentially be boosted when the nonlinear material is placed in lightconcentrating regions or inside a laser cavity, as seen in commercial green laser pointers. In addition, the operational band for the tunable harmonic generation is potentially expandable using bandgap engineering through alloying or heterostructures. With electrically enabled light–matter interactions at the heart of the entire photonics industry and nanofabrication techniques being continuously advanced, it is not far-fetched to envision the use of
these monolayer SHG transistors, which are compatible with complementary metal– oxide–semiconductor technology, in realistic applications for optical signal processing and integrated nanocircuits in the future. ❐ Sean P. Rodrigues and Wenshan Cai are at the School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, USA. e-mail: [email protected] References
1. Franken, P. A., Hill, A. E., Peters, C. W. & Weinreich, G. Phys. Rev. Lett. 7, 118–119 (1961). 2. Seyler, K. L. et al. Nature Nanotech. http://dx.doi.org/10.1038/ nnano.2015.73 (2015). 3. Terhune, R. W., Maker, P. D. & Savage, C. M. Phys. Rev. Lett. 8, 404–406 (1962). 4. Lee, C. H., Chang, R. K. & Bloembergen, N. Phys. Rev. Lett. 18, 167–170 (1967). 5. Aktsipetrov, O. A., Fedyanin, A. A., Golovkina, V. N. & Murzina, T. V. Opt. Lett. 19, 1450–1452 (1994). 6. Cai, W., Vasudev, A. P. & Brongersma, M. L. Science 333, 1720–1723 (2011). 7. Kang, L., Cui, Y., Lan, S., Rodrigues, S. P., Brongersma, M. L. & Cai, W. Nature Commun. 5, 4680 (2014). 8. Wang, Q. H., Kalantar-Zadeh, K., Kis, A., Coleman, J. N. & Strano, M. S. Nature Nanotech. 7, 699–712 (2012). 9. Xu, X. D., Yao, W., Xiao, D. & Heinz, T. F. Nature Phys. 10, 343–350 (2014). 10. Malard, L. M., Alencar, T. V., Barboza, A. P. M., Mak, K. F. & de Paula, A. M. Phys. Rev. B 87, 201401 (2013). 11. Kumar, N. et al. Phys. Rev. B 87, 161403 (2013).
Published online: 20 April 2015
NATURE NANOTECHNOLOGY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology
© 2015 Macmillan Publishers Limited. All rights reserved
