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Nanowire photonics toward wide wavelength range and subwavelength confinement [Invited]

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Abstract

Semiconductor nanowires have attracted much attention for photonic applications, especially for lasers, because of their availability in a wide variety of materials and compositions, exceptionally small size, and rich functionality. So far, most nanowire laser studies have been done in rather short wavelength (λ) ranges of less than 1 µm. In addition, the diameter (d) of most nanowire lasers has been relatively large (d > λ/n, n is the refractive index) because of the requirement for sufficient optical confinement. Recently, however, we are seeing new trends in nanowire research towards much longer wavelengths and much thinner nanowires for photonic applications. This article reviews the latest research activities in these directions, which shows that it is possible to fabricate excellent nanowire lasers operating at telecom wavelengths or even in the mid-infrared region and extremely thin subwavelength nanowires can be applied to make nanophotonic devices in a wide range of wavelengths. We believe that these research trends will have an impact on applications for functional energy-saving devices in future photonic integrated circuits.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

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Figures (25)

Fig. 1.
Fig. 1. Effective mode refractive indices for an infinitely long InP nanowire on a SiO2 substrate versus nanowire diameter. Strong light confinement can be expected when the mode index is sufficiently larger than that of SiO2. The intensity profiles for the first and second modes are shown.
Fig. 2.
Fig. 2. (a) Bandgap and lattice constant of III-V compound semiconductors. InP/InAs can cover a long wavelength range including the telecom band. (b) Schematic diagram of an InP/InAs heterostructure nanowire connected by a single indium particle. Owing to the quantum confinement effect along the axial direction, the real bandgap of the InAs layer can be modulated by its thickness.
Fig. 3.
Fig. 3. Schematic diagram of strain relaxation for crystal materials with lattice mismatch. (a) Lattices of crystalline materials before epitaxy. (b) Coherent growth by elastic deformation of the epi-layer lattice. (c) Incoherent growth (strain relaxation by mismatch dislocations). (d) Coherent growth by elastic deformation of both lattices. In contrast to the film structure, the lattice in the nanowire structure can deform much more along the radial direction because of its microscale or nanoscale diameter and thus can endure higher strain induced by lattice mismatch.
Fig. 4.
Fig. 4. Schematic diagram of VLS growth mode. InAs and InP nanowires can be grown by self-catalyzed VLS mode using indium particles. This enables InAs/InP quantum heterostructure nanowires along the axial direction (Fig. 3(b)).
Fig. 5.
Fig. 5. InP/InAs MQD heterostructure nanowires. (a) Schematic diagram of an MQD heterostructure nanowire. The InAs layers are shown in red. (b and c) SEM images (tilted 38°) of InP/InAs MQD heterostructure nanowires vertically grown on InP (111)B substrate. (Figure from Ref. [14])
Fig. 6.
Fig. 6. TEM analysis of an InP/InAs MQD nanowire. (a) HAADF-STEM images. The nanowire contains 400 InAs QDisks. (b and c) Aberration-corrected HAADF-STEM images. The white arrow indicates the axial direction. The thicknesses of the InAs layers and an InP barrier layers are 9.0±1 nm and 25.6±1 nm, respectively. (d) Schematic band diagram of the nanowire. There is quantum confinement effect on InAs QDisks along the axial direction. (e) and (f) Strain mapping of lattice spacing difference along y and z directions for the image shown in Fig. 4(c), indicating a compressive strain inside InAs QDisks. The grey arrows show interface positions of InP/InAs/InP. (Figure from Ref. [14])
Fig. 7.
Fig. 7. Micro-PL measurement at room temperature. (a) SEM image of a nanowire mechanically dispersed onto an Au-film-covered SiO2/Si substrate. The white arrow indicates a removed indium particle. (b) PL spectrum of a single nanowire under a pump laser power of 0.83 mJ•cm-2. (c) PL spectra of the nanowire with increasing pump power. (Figure from Ref. [14])
Fig. 8.
Fig. 8. Tuning the laser wavelength range by thickness of InAs QDisks. (a) HAADF-STEM images of InP/InAs MQD nanowires. These nanowires were grown using different flow rates of TMIn:TBA (µmol/min.): 0.75:223, 0.98:290, and 1.5:447 from the left-hand to right-hand sides. All scale bars denote 20 nm. (b) Spontaneous PL spectra of individual nanowires grown with varied flow rates for InAs QDisks. (c) PL spectra under stimulated emission. A wide wavelength range in whole telecom band is covered, including 1.3 and 1.55 µm. (Figure from Ref. [28])
Fig. 9.
Fig. 9. (a) Schematic of an InN nanowire LED. Reproduced from [47] with permission of AIP Publishing. (b) SEM image of a Ge nanowire LED. Reproduced from [48] with the permission of AIP Publishing. (c) SEM image of an InGaN/GaN nanowire LED [45]. Copyright 2007 American Chemical Society (d) Sub-hertz order modulated signal. Reprinted with permission from [45]. Copyright 2007 American Chemical Society. (e) Schematic of an InP/InAs nanowire LED, and (f) GHz order modulated signal. Reproduced from [49] with the permission of AIP Publishing.
Fig. 10.
Fig. 10. (a) Transmission electron microscope image of a 40-µm-long InAs nanowire and its cross section at the middle of the nanowire (inset). (b) Magnified image of the side cross section and a selective area electron diffraction (SAED) pattern along the [1120] zone axis (inset). Two white arrows indicate stacking faults. (c) Photoluminescence spectra of a zinc-blend InAs wafer and ensembles of InAs nanowires put on Au and SiO2. These samples were excited by a continuous laser light at the wavelength of 900 nm and power density of 25 kW/cm2. The temperature of the sample stage was 4 K. Figures(a) and(b) [70] are adapted with permission from the American Chemical Society.
Fig. 11.
Fig. 11. (a) Spectrally-integrated emission intensity of InAs nanowires as a function of pump fluence. The insets are optical microscope images of the measured InAs nanowires labeled NW1-4. The broken line indicates the theoretical curve fitted to the data for NW2. (b) Emission spectra of NW1-3 as the pump fluence changes. The temperature of the sample stage was 4 K. All samples were excited by a picosecond pulse laser at the center wavelength of 970 nm. Figures(a) and(b) [70] are adapted with permission from the American Chemical Society.
Fig. 12.
Fig. 12. (a) Emission spectra of a 28-µm-long InAs nanowire as the pump fluence changes from 57 to 216 µJcm-2/pulse. Significant emission peaks at 2.58 and 2.66 µm are labeled A and B, respectively. To clarify, the spectra are shifted upward. (b) Emission intensities of peak A, and B, and the summation of them (A + B) as a function of pump fluence. These emission intensities were measured by a monochromator with a spectral width of around 5 nm. The temperature of the sample stage was 4 K. All samples were excited by a picosecond pulse laser at the center wavelength of 970 nm.
Fig. 13.
Fig. 13. Panels a to e are extracted from [97] and modified. (a) GaN nanowire fabricated by sublimation and transferred on a SiN-on-Si host substrate. (b) Photoluminescence spectra of the nanowire at different pump powers. The inset is the fundamental mode guided in the nanowire. (c) Intensity, (d) wavelength, and (e) linewidth of the lasing mode (color dots) and another Fabry-Pérot mode (gray dot) as a function of peak pump power. Panels (f) and (g) are extracted with permission from [98]. Copyright 2018, American Chemical Society. (f) Array of GaN nanowires fabricated by top-down dry-etching of bulk GaN grown on sapphire. (g) Photoluminescence spectra of a single-nanowire laser at different pump powers. The inset highlights the light reflection at the sapphire-GaN interface.
Fig. 14.
Fig. 14. Effective mode refractive indices for an infinitely long InP nanowire placed on a 15-nm-thick SiO2 / gold versus nanowire diameter. The first mode corresponds to a hybrid plasmon mode, which shows a significantly smaller cutoff diameter than the Fabry-Pérot modes. We assumed a wavelength of 1550 nm.
Fig. 15.
Fig. 15. (a) Schematic representation and optical properties of a subwavelength GaN nanowire plasmon laser at room-temperature. Reprinted by permission from Nature Communications [115]. (b) Schematic representation and optical properties of a subwavelength ZnO nanowire plasmon laser. Reprinted by permission from Nature Physics [116]. (c) Schematic representation and optical properties of a subwavelength ZnO nanowire plasmon laser with operation temperatures up to 353 K. Reprinted with permission from [119]. Copyright 2016, American Chemical Society. (d) Schematic representation of an ultracompact pseudowedge plasmon laser based on a subwavelength ZnO nanowire. Reprinted with permission from [121]. Copyright 2018, American Chemical Society.
Fig. 16.
Fig. 16. (a) Schematic of CdS nanowire on 1-D PDMS PhC. Reprinted with permission from [126]. Copyright 2006, American Chemical Society. (b) Schematic of CdS nanowire on SiN 2D PhC. Reprinted by permission from Nature Photonics [127]. (c) Schematic of GaN nanowire in TiO2 2D PhC. Reproduced from [128], with the permission of AIP Publishing.
Fig. 17.
Fig. 17. (a) Schematic of nanowire-induced nanocavity in a Si PhC slab [130]. (b) Confinement mechanism of the modulated mode-gap cavity.
Fig. 18.
Fig. 18. (a) Schematic of PhC waveguide with an air trench. (b) Band diagram for PhC waveguide with an air trench. (c) Enlarged band diagram for PhC waveguide with an air trench. (d) Schematic of PhC waveguide with a nanowire in the air trench. (e) Band diagram for PhC waveguide with a nanowire in the air trench. (f) Enlarged band diagram for PhC waveguide with a nanowire in the air trench. In these band simulations, the radius [r], slab thickness [t], trench width [w], and depth [d] were 0.3a, 0.6a, 0.15a, and 0.15a, respectively, where a is the lattice constant. To simplify, the nanowire width and the height are the same as the trench width and depth. The waveguide width is $0.98 \times \sqrt 3 {\; }a$. (g) Schematic of a PhC waveguide with a nanowire of 3.3 µm long in the air trench. (h) The electric field of nanowire-induced PhC nanocavity.
Fig. 19.
Fig. 19. AFM images of a nanowire in Si PhC. A nanowire is moved using the contact mode.
Fig. 20.
Fig. 20. PL studies of nanowire-induced nanocavities. (a) PL spectra of a bare nanowire (black) and a nanowire placed in the trench of the Si PhC line defect. (b) Polarization degree of PL from the nanowire-induced nanocavity. (c) PL decay measurements. Black circles are for a bare nanowire and purple circles are for nanowires placed in the trench of the Si PhC line defect with different cavity Q. (d) The measured PL decay rate versus Q/V. [130]
Fig. 21.
Fig. 21. (a) Schematic of hybrid nanowire laser on a Si PhC. (b) AFM image of a nanowire in a PhC trench. The sample was measured at cryogenic temperature (4 K). A nanowire is placed in an air trench (114-nm deep; 150-nm wide) in the PhC waveguide using an AFM. The sample is excited from the top using a typical µ-PL setup. The average nanowire diameter is 111 nm, and the length is 2.5 µm. (c) Emission spectrum of a nanowire lasers on a PhC at 0.25 and 2.5 mW. (d) L-L curve and full width at half maximum of pump power. The black dots show emission intensity and the red dots show the cavity line width. (e) Emission spectrum when a nanowire is moved for different lattice constants. (f) Optical microscope image of nanowire on PhC. (g) Eye diagram of an SSPD (10 Gbps).
Fig. 22.
Fig. 22. (a) Schematic of 1D InGaAs nanowire array laser on a SOI platform at room temperature and the spectrum at telecom-band wavelengths. Reprinted with permission from [140]. Copyright 2017, American Chemical Society. (b) SEM images of a 1D nanowire laser on a waveguide and the emission spectra coupled to the waveguide. Reprinted with permission from [142]. Copyright 2017, American Chemical Society. (c) Schematic of 2D InGaAs nanowire array band-edge laser on a patterned SOI substrate and the emission spectra. Reproduced with permission [141]. Copyright 2019, Wiley-VCH.
Fig. 23.
Fig. 23. (a) Schematic of an all-optical nanowire switch on a Si PhC. The inset shows a measured transmitted signal with different detuning conditions [61](b) An SEM image of the detector in PhC with electrodes. Adapted with permission from [153] The Optical Society.
Fig. 24.
Fig. 24. a. Schematic representation, band structure and cavity mode of a nanowire-induced nanocavity in a 2D PhC. b. Same for a 1D PhC c. Same for a PhC disk.
Fig. 25.
Fig. 25. (a). Schematic representation and experimental optical spectra of a ZnO nanowire cavity moved across PhCs with three different lattice constants. (b) Schematic representation and optical spectrum of a nanowire-induced cavity in a PhC disk. (c) Temperature-dependent optical properties of a ZnO-nanowire-induced nanocavity laser in a 2D PhC: light-light curve, lasing spectra and peak power thresholds at 298, 316, 339 and 360 K. The colored dots correspond to the PhC laser; the black squares are plasmonic data extracted from [119]. Panels a, b, c are respectively extracted from [15, 156,157] and modified.

Tables (1)

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Table 1. Comparison of various single nanowire lasers, categorized by the type of resonators (FP: intrinsic Fabry- Pérot cavity, HP: hybrid nanowire plasmon cavity, PhC: nanowire-induced nanocavity in a PhC). sub: subliming, VLS: vapor-liquid-solid method, sVLS: self-catalyzed VLS, HT: hydrothermal method, SAG: selective area growth, PAMBE: plasma assisted molecular beam epitaxy, AWE: anisotropic wet etching, RT: room temperature, CW: continuous-wave lasing.

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