Zinc Sulfide Nanowaveguides: Optical Insights

• Zinc sulfide nanowaveguides are nanoscale photonic structures that leverage ZnS's high refractive index, wide bandgap, and second-order nonlinearity for effective light confinement.
• They are realized through chemically grown nanowires, doped microstructures, and lithographically defined integrated waveguides, each exhibiting distinct resonances and guiding mechanisms.
• Advances in fabrication and phase-matching have enabled efficient second-harmonic generation and nonlinear frequency conversion, underlining their potential in integrated nanophotonic systems.

Zinc sulfide nanowaveguides are nanoscale light-guiding structures that exploit the optical properties of ZnS—most notably its high refractive index, wide bandgap, visible-to-infrared transparency, and nonzero second-order nonlinearity—to support guided propagation, cavity resonances, and frequency conversion. In the recent literature, the term spans several experimentally distinct platforms: single-crystalline ZnS nanowires that act as nonlinear waveguiding elements, doped ZnS microstructures that sustain Fabry–Pérot and whispering-gallery resonances, and submicron integrated waveguides patterned from sputtered ZnS films; a related ZnS-based configuration uses CdS/ZnS core-shell quantum dots embedded in metallic epsilon-near-zero channels to control emission via the photonic environment rather than through a ZnS guiding core (Lemoine et al., 30 Jul 2025, Hongbo et al., 2015, Sotillo et al., 2021, So et al., 2020).

1. Optical basis of ZnS nanowaveguiding

ZnS is treated as a favorable photonic material because its refractive index is high in the visible range and because it remains transparent outside its band edge. For doped ZnS microstructures, the visible-range refractive index is given as about $n \approx 2.4$ near $500$ nm, which is sufficient for strong optical confinement by total internal reflection. The same body of work reports direct UV bandgaps of $3.72$ eV for zinc blende and $3.77$ eV for wurtzite ZnS, establishing the wide-bandgap character of the material (Sotillo et al., 2021).

Integrated ZnS nanowaveguides patterned from thin films are described as submicron optical waveguides in which light is tightly confined in a high-index semiconducting core. In that context, ZnS is characterized as a wide-bandgap semiconductor with a broad transparency range of approximately $400$ nm to $10\ \mu$m, a relatively high refractive index at telecom wavelengths, and a nonzero second-order nonlinear susceptibility associated with non-centrosymmetric crystal symmetry (Lemoine et al., 30 Jul 2025).

Single ZnS nanowires add a crystallographic dimension to this optical picture. In that case, the guiding geometry is supplied by nanowires with round cross-section and micrometer-scale length, while the nonlinear response is governed by the non-centrosymmetric wurtzite lattice. The literature therefore treats ZnS nanowaveguides not as a single device archetype, but as a family of geometries in which confinement, resonance, and $\chi^{(2)}$ interactions are strongly morphology dependent (Hongbo et al., 2015).

2. Structural realizations and fabrication routes

Three principal realizations recur in the literature: chemically grown single nanowires, morphology-defined doped microstructures, and lithographically defined integrated nanowaveguides.

Realization	Representative geometry	Reported optical function
Single-crystalline ZnS nanowires	Round cross-section; $300$–$800$ nm diameter; lengths up to tens of micrometers	Waveguiding and SHG
ZnS:Ga and ZnS:In structures	Triangular wires, hexagonal pencils, ribbons, swords, plates	Fabry–Pérot resonances, whispering-gallery modes, lasing-like behavior
Polycrystalline ZnS nanowaveguides	$500$ nm-thick film; widths from $500$0 nm to $500$1m	Integrated SHG

Single-crystalline ZnS nanowires were fabricated by a conventional chemical vapor deposition process in a horizontal tube furnace using ZnS powder as the source and Au nanoparticles on Si(001) substrates as catalysts. The reported growth conditions were $500$2, $500$3 MPa, and Ar carrier gas at $500$4 sccm. The resulting nanowires were high quality, had round cross-section, diameters of $500$5–$500$6 nm, lengths up to tens of micrometers, and wurtzite hexagonal crystal structure (Hongbo et al., 2015).

Doped ZnS microstructures were synthesized in multiple morphologies. The reported Ga-doped structures include pencils and wires, with the most optically distinctive wires having isosceles or equilateral triangular cross sections with side lengths around $500$7–$500$8m. The In-doped structures include ribbons, swords, plates, and larger rod-like or plate-like forms. The dimensions cover thicknesses from $500$9 nm to $3.72$0m and lateral sizes extending to tens of micrometers, providing a broad range of resonant path lengths and facet configurations (Sotillo et al., 2021).

Integrated ZnS nanowaveguides were fabricated from sputtered thin films. A $3.72$1 nm-thick polycrystalline ZnS film was deposited on oxidized silicon by RF magnetron sputtering from a $3.72$2 pure ZnS target in a clean vacuum system with residual pressure in the $3.72$3 mbar range. RF power was increased progressively to $3.72$4 W over $3.72$5 minutes, and deposition then proceeded for $3.72$6 minutes; because the RF power was low, the film was considered deposited close to room temperature. Waveguides were subsequently defined by electron-beam lithography and reactive ion etching using a CH$3.72$7/H$3.72$8/Ar chemistry at $3.72$9 W and $3.77$0 mbar with gas flows of $3.77$1 sccm CH$3.77$2, $3.77$3 sccm H$3.77$4, and $3.77$5 sccm Ar. The etch rate was about $3.77$6 nm/s, and the processed waveguides had thicknesses of $3.77$7 nm and below (Lemoine et al., 30 Jul 2025).

3. Morphology-controlled guiding and resonance in doped ZnS structures

The visible photonics of doped ZnS structures is governed primarily by morphology. Elongated structures such as ribbons, swords, and plates support Fabry–Pérot-type resonances between opposing facets, whereas hexagonal plates, hexagonal pencils, and triangular Ga-doped wires support whispering-gallery-like resonances through repeated total internal reflection along closed or near-closed paths. The literature emphasizes that doping with Ga or In does not remove the guiding capability of ZnS, but instead slightly modifies the refractive index and shifts resonance conditions (Sotillo et al., 2021).

Direct guiding was demonstrated in ZnS:In swords by illuminating the side of an individual structure with either a $3.77$8 nm green or $3.77$9 nm red laser and observing bright emission at the far tip opposite the incidence point. ZnS:Ga wires excited with a $400$0 nm laser showed guided blue emission tens of microns away from the excitation spot. These observations were treated as direct evidence of guided optical transport (Sotillo et al., 2021).

For Fabry–Pérot resonances, the resonance condition was written as

$400$1

with mode spacing

$400$2

and, under approximately constant $400$3,

$400$4

These relations were used together with the published ZnS dispersion

$400$5

to estimate refractive indices of doped structures. In-doped material yielded slightly higher $400$6 than pure ZnS, attributed to lattice expansion caused by substitution of larger In$400$7 ions ($400$8 pm) for Zn$400$9 ($10\ \mu$0 pm), while Ga-doped structures showed slightly lower $10\ \mu$1, consistent with lattice contraction because Ga$10\ \mu$2 is smaller ($10\ \mu$3 pm) (Sotillo et al., 2021).

Specific resonant signatures were reported. In a ZnS:In ribbon about $10\ \mu$4m wide and $10\ \mu$5m thick, a modulation period of about $10\ \mu$6 nm near $10\ \mu$7 nm was assigned to thickness resonances, while a finer modulation of about $10\ \mu$8 nm near $10\ \mu$9 nm was assigned to width resonances. In a ZnS:In sword with width around $\chi^{(2)}$0m, the observed spacing was about $\chi^{(2)}$1 nm near $\chi^{(2)}$2 nm. In a plate with a $\chi^{(2)}$3m optical path, the mode spacing was about $\chi^{(2)}$4 nm near $\chi^{(2)}$5 nm. By contrast, along a much longer dimension such as a $\chi^{(2)}$6m ribbon length, the expected spacing was about $\chi^{(2)}$7 nm, below the $\chi^{(2)}$8 nm spectral resolution, which explains why long-axis Fabry–Pérot modes were not resolved (Sotillo et al., 2021).

Whispering-gallery behavior was especially pronounced in triangular ZnS:Ga wires emitting in the blue-green range around $\chi^{(2)}$9 nm. Two overlapping WGM families were separated by polarization analysis: one set with $300$0 nm near $300$1 nm corresponded to an optical path of about $300$2m, and another with $300$3 nm corresponded to an optical path of about $300$4m. The WGM1 family was mainly polarized parallel to the cavity, whereas WGM2 was mainly polarized perpendicular to it. The refractive-index dispersion extracted from WGM fitting was described by Cauchy’s law $300$5, with ZnS:Ga giving roughly $300$6, $300$7 for perpendicular modes and $300$8, $300$9 for parallel modes (Sotillo et al., 2021).

Under increasing excitation power, ZnS:Ga wires also exhibited lasing-like behavior. At the highest reported power, two prominent peaks appeared at $800$0 nm and $800$1 nm with full widths at half maximum of $800$2 and $800$3 nm, corresponding to quality factors $800$4 of about $800$5 and $800$6, and finesse values of $800$7 and $800$8. This places morphology-controlled ZnS resonators at the boundary between passive guided structures and active microcavities (Sotillo et al., 2021).

4. Single ZnS nanowires as nonlinear waveguides

Single ZnS nanowires have been studied as nonlinear nanowaveguides in which the same structure both confines the optical field and generates second-harmonic radiation. The reported nanowires were single-crystalline, round in cross-section, and sufficiently long to provide a nanoscale propagation direction, while their wurtzite hexagonal crystal structure supplied the second-order nonlinearity (Hongbo et al., 2015).

The nonlinear characterization used polarization-dependent SHG microscopy in a confocal optical setup. A femtosecond Ti:sapphire laser at about $800$9 nm pumped the nanowire, and the generated SHG appeared at $500$0 nm. The incident linear polarization was rotated by a half-wave plate, and the SHG intensity was recorded as a function of pump polarization angle $500$1. The method was used to determine full three-dimensional crystallographic orientation rather than only the c-axis orientation (Hongbo et al., 2015).

Transmission electron microscopy showed that the actual growth axis deviated by about $500$2–$500$3 from the preferential growth direction $500$4, with examples of $500$5, $500$6, and $500$7. Because $500$8 is perpendicular to the $500$9 c-axis, this implies that the c-axis is not strictly perpendicular to the wire axis but instead has a deviation of roughly $500$00–$500$01 relative to the actual growth axis. This crystallographic variability explains why geometrically similar wires can display different SHG polar plots (Hongbo et al., 2015).

For wurtzite ZnS, which belongs to point group $500$02, the nonzero second-order coefficients were reported as $500$03 pm/V, $500$04 pm/V, and $500$05 pm/V. The induced nonlinear polarization was written in matrix form as

$500$06

and the collected SHG power was modeled as

$500$07

The geometric relation

$500$08

was used to relate the SHG-derived orientation to the angle between the growth axis and the c-axis (Hongbo et al., 2015).

The measured SHG patterns were generally two-lobed. The main lobe shape was governed primarily by the c-axis orientation, the rotation of the lobes by $500$09, and finer asymmetries or protuberances by $500$10, which encodes the orientation of the $500$11 and $500$12 crystal axes. A central physical result was that the strongest SHG occurred when the pump electric field was parallel to the c-axis, consistent with the large magnitude of $500$13 (Hongbo et al., 2015).

The reported SHG conversion efficiency for a single nanowire was $500$14. The literature interprets this as evidence that ZnS nanowires can function as compact frequency-doubling elements without external cavities and can simultaneously serve as probes for crystallographic orientation. A plausible implication is that, in ZnS nanowaveguide design, crystallographic alignment is not a secondary detail but a determinant of nonlinear efficiency (Hongbo et al., 2015).

5. Integrated polycrystalline ZnS nanowaveguides and phase-matched SHG

A distinct development is the realization of integrated ZnS nanowaveguides fabricated from sputtered polycrystalline thin films. In this platform, a $500$15 nm-thick ZnS layer on oxidized Si was patterned into waveguides with widths from $500$16 nm to $500$17m, with particular emphasis on an $500$18 nm-wide, $500$19 nm-high device used for SHG experiments (Lemoine et al., 30 Jul 2025).

Structural characterization showed that the film surface had RMS roughness of approximately $500$20 nm and coherence length of approximately $500$21 nm by AFM. Cross-sectional SEM revealed a columnar polycrystalline microstructure with a slight tilt of the columns. XRD showed a dominant diffraction peak at about $500$22, assigned to zinc blende ZnS oriented along $500$23, although the literature notes that XRD alone cannot fully exclude wurtzite contributions because the wurtzite $500$24 peak can overlap at a similar angle. Spectroscopic ellipsometry, analyzed with a double Tauc–Lorentz model, yielded a bandgap of about $500$25 eV and an absorption shoulder extending to roughly $500$26 nm; the slightly reduced bandgap relative to ideal zinc blende ZnS was attributed to deep-level defects, possibly caused by grain boundaries and residual strain (Lemoine et al., 30 Jul 2025).

Propagation losses were measured by Fabry–Perot fringe analysis in TE and TM polarizations using tunable Littman–Metcalf lasers. The reported losses were generally below $500$27 dB/cm and, in some spectral regions, below $500$28 dB/cm. Average values were about $500$29 dB/cm in the visible and $500$30 dB/cm in the infrared for TE$500$31, and about $500$32 dB/cm in the visible and $500$33 dB/cm in the infrared for TM$500$34. These are not ultra-low-loss values, but they were sufficient to support nonlinear experiments (Lemoine et al., 30 Jul 2025).

The nonlinear theory was formulated through

$500$35

with zinc blende ZnS in crystallographic class $500$36, for which only one independent tensor coefficient is nonzero in the conventional basis: $500$37 Because the films were preferentially oriented along $500$38, the tensor was rotated into the growth-aligned basis using

$500$39

The analysis further examined twist around the growth axis and tilt away from it. Some processes were found to be insensitive to twist, including $500$40, $500$41, and $500$42. Small tilts of about $500$43 reduced the effective nonlinear coefficients for the relevant processes by only about $500$44 on average. The literature therefore explicitly rejects the simple assumption that polycrystallinity necessarily destroys useful second-order response; rather, the measured columnar disorder was found not to eliminate SHG (Lemoine et al., 30 Jul 2025).

Phase matching was designed in a TM $500$45 TM configuration associated with $500$46 in the rotated basis. COMSOL simulations showed that, for the $500$47 nm-wide, $500$48 nm-high waveguide, the only accessible modal phase matching from the TM$500$49 pump mode occurred with the TM$500$50 mode at around $500$51 nm, implying SHG near $500$52 nm. Experimentally, picosecond pulses from an optical parametric oscillator were injected with pump power $500$53 mW, repetition rate $500$54 MHz, and wavelength scan from $500$55m to $500$56m. A clear SHG peak was observed at pump wavelength $500$57 nm, with corresponding SHG output at $500$58 nm. Visible scattered SHG built up along the waveguide, confirming that the conversion occurred inside the guide (Lemoine et al., 30 Jul 2025).

The measured instantaneous conversion efficiency in the pulsed regime was $500$59. For a monocrystalline waveguide of the same geometry, with losses included, the maximum theoretical CW conversion efficiency was estimated as $500$60. The same work estimated that reducing losses to about $500$61 dB/cm and using $500$62 cm-long waveguides could improve nonlinear conversion efficiency by two orders of magnitude. This suggests that fabrication quality and crystallographic control, rather than the basic suitability of ZnS, are the dominant present constraints (Lemoine et al., 30 Jul 2025).

6. ZnS-containing emitters in epsilon-near-zero metallic nanowaveguides

A related but distinct ZnS-based nanophotonic configuration embeds CdS/ZnS core-shell quantum dots inside a nanoscale rectangular metallic waveguide operating near cutoff in the epsilon-near-zero regime. In this device, ZnS is not the primary emitter; it is the shell material in the CdS/ZnS quantum dots, where it stabilizes and passivates the CdS emitters and enables efficient photoluminescence. The platform is therefore not a ZnS-core waveguide, but it is part of the ZnS nanowaveguide literature insofar as ZnS-containing quantum emitters are coupled to a nanoscale guided photonic environment (So et al., 2020).

The device used a PMMA dielectric core containing CdS/ZnS quantum dots with emission wavelength of about $500$63 nm. The dots were embedded in a $500$64 nm-thick PMMA layer with area density about $500$65 QDs/$500$66. This core was placed between silver sidewalls to form a rectangular nanoscale waveguide. Fabrication proceeded by depositing Ag/PMMA(QDs)/Ag layers on a silicon substrate, milling the width by focused ion beam, adding Ag to cover the exposed sidewalls, and carving entrance and exit facets plus two $500$67 mirrors to couple pump and emitted light in and out of the waveguide efficiently (So et al., 2020).

The central parameter was waveguide width. For the quasi-TE mode, the effective index was defined as

$500$68

where $500$69 is the guided-mode propagation constant and $500$70 is the free-space wavenumber. For dielectric-core widths of $500$71, $500$72, $500$73, and $500$74 nm, the quasi-TE mode showed cutoff behavior tunable by core width. Near cutoff, the metallic rectangular channel acted as an ENZ medium: $500$75 approached zero, phase advance along the channel became very small, and the local density of optical states was strongly altered (So et al., 2020).

Experimentally, the fully metal-clad waveguide was compared with a control waveguide without sidewalls. In the control case, no strong cutoff occurred near the quantum-dot emission band, and the luminescence increased monotonically as the width was increased from $500$76 to $500$77 nm because a wider core contained more QDs. In the fully enclosed waveguide, by contrast, the photoluminescence was highly width dependent: emission from a $500$78 nm-wide guide was strongly suppressed and resembled emission from an unstructured surface, while increasing the width led to a sudden enhancement at $500$79 nm. For even larger widths, the luminescence exhibited further oscillations of suppression and enhancement, consistent with Fabry–Pérot resonances along the waveguide axis (So et al., 2020).

The interpretation was that the ENZ condition near cutoff modified the guided-mode density of optical states and thereby changed spontaneous emission and luminescence intensity. Within the broader subject of ZnS nanowaveguides, this result serves as a caution against a common conflation: ZnS may enter the system either as the guiding material itself or as part of the active emitter embedded in another nanophotonic structure. In the ENZ case, the reported control mechanism is geometry-driven ENZ physics in a metallic waveguide rather than waveguiding in ZnS proper (So et al., 2020).