Tunable Color Emissions in a Single CdTe Nanowire Based on Complex Optical Transverse Nonlinear Effects

Abstract

Tunable color emissions, emerging from a single CdTe nanowire, are demonstrated experimentally based on optical transverse nonlinear effects. The pumping light at different wavelengths (e.g., 1064 nm and 980 nm) is coupled to a nanowire at both ends via evanescent-field coupling. The light at different wavelengths (e.g., 510 nm, 532 nm, and 713 nm) can be simultaneously assessed using complex optical transverse nonlinear effects, including transverse sum-frequency generation (TSFG), transverse second-harmonic generation (TSHG), and two-photon absorption (TPA)-induced fluorescence. By changing the wavelength and the power of the pumping lights, the spectra of the transverse light emissions change as well, leading to tunable color emissions at the single-nanowire level with a Rec. 2020 coverage of ~21.6%. The results indicate the potential of transverse nonlinear effects in applications ranging from optical display and spectroscopy to communication.

1. Introduction

Benefitting from a number of properties, such as low propagating loss, tunable optical dispersion, and high power density, optical micro/nanowaveguides, such as nanowires and nanobelts, have been demonstrated to be excellent platforms for nonlinear optical effects [1,2,3,4]. In particular, transverse nonlinear effects, which cause transverse light emissions by counter-propagating pumping lights inside a waveguide, have attracted continuous attention for their unique properties, including a wide emission angle, a simple setup, and self-driven phase matching [5,6,7,8,9,10]. For example, in 2014 Fuxing Gu et al. demonstrated transverse frequency conversion in the sub-bandgap spectral region in semiconductor nanowires and nanoribbons by using CW lasers [5]. In the same year, Huakang Yu et al. reported a single-nanowire optical correlator developed for ultrafast pulse characterization based on TSHG in a CdS nanowire [7]. Christelle Monat et al. performed single-shot temporal measurements for picosecond optical pulses based on transverse third-harmonic generation in a silicon waveguide [8]. In 2015, Fuxing Gu et al. reported frequency up-conversion far above the bandgaps using surface emissions from semiconductor nanoribbons, wherein both second-harmonic generation and sum frequency generation were demonstrated with double-frequency pumping [9]. In 2018, Jiaxin Yu et al. demonstrated efficient third-harmonic generation and multi-photon luminescence in CdSe nanowaveguides by means of an evanescent wave coupling technique [10]. In 2021, C. Xin et al. reported the in situ modal inspection of nonlinear micro/nanowaveguides based on TSHG in a CdS nanobelt [6]. Despite the great research that has been reported, these efforts rarely focused on the modulation of color emissions, which is important for the use of transverse nonlinear effects in optical display, spectroscopy, and communication [11,12,13].

Due to their high optical nonlinearity (e.g., a second-order nonlinearity of 109 pm/V and a nonlinear refractive index of (1.3 ± 0.7) × 10⁻¹⁷ m²/W at a 1064 nm wavelength) and wide transmitted spectra, ranging from ~1 to 25 μm, CdTe nanowires are widely used in nonlinear optics [14,15,16,17]. For example, based on the TSHG effect in a single CdTe nanowire, optical correlation measurement is performed with the pulse energy down to 2 fJ/pulse [14]. The ability for CdTe micro/nanowires to act as optical waveguides in a wide spectral region of up to ~8.6 μm has also been demonstrated [18]. By carefully exciting multiple transverse nonlinear effects inside one single CdTe nanowire simultaneously, it is possible in principle to achieve tunable transverse color emissions by changing the wavelength and the power of the pumping light.

In this paper, we experimentally demonstrate tunable color emissions in a single CdTe nanowire. A CdTe nanowire is pumped using counter-propagating guided waves with different wavelengths of 980 nm and 1064 nm, respectively. Transverse optical emissions with wavelengths of 510 nm, 532 nm, and 713 nm are observed based on different nonlinear effects, including TSFG, TSHG, and TPA-induced fluorescence, respectively. By changing the wavelength and the power of the pumping light, the color of the transverse emitting light can be modulated as a result of the changed spectra. Associated with optical filters, a Rec. 2020 area–coverage ratio of ~21.6% is obtained. For traditional light emissions based on optical nanowires, the modulation of the color is typically achieved by controlling the morphology of the nanowire arrays or the component along a single nanowire, which means that the emission properties are determined once the nanowires have been fabricated [19,20,21]. In this paper, light is emitted at different wavelengths, transversely and simultaneously, along one single nanowire, indicating tunable color emissions. In this way, for the first time as far as is known, tunable color emissions are demonstrated experimentally based on complex transverse nonlinear effects inside one single nanowire. The real-time modulation of color emissions with sizes down to the single-nanowire level is demonstrated, indicating good potential in applications ranging from optical display and spectroscopy to communication.

2. Theory

TPA-induced fluorescence is a process which involves the simultaneous absorption of two photons to excite electron transition, which further leads to an emission of one single fluorescent photon due to the relaxation of electron [22,23]. As a result, the wavelength of the fluorescence is determined by the bandgap of the semiconductor. As a third-order nonlinear process, the relationship between the intensity of the fluorescence ($I_{TPA}$) and the intensity of the pumping light ($I_P$) can be obtained as follows [24]:

$$I_{TPA}\propto I_P^2$$

(1)

As for TSHG, two counter-propagating light sources operating at the same frequency are coupled to one single nanowire. As the counter-propagating lights meet with each other inside the nanowire, second-harmonic photons are emitted in directions perpendicular to the axis of the nanowire, as is required by the wave–vector matching condition [14]. The relationship between the intensity of the TSHG and the intensity of the pumping light can be obtained as follows [25]:

$$I_{TSHG}\propto I_{+}{I}_{-}$$

(2)

where $I_{TSHG}$, $I_{+}$, and $I_{-}$ are the intensity of the second-harmonic light and the intensity of the pumping light propagating along the forward and the backward directions, respectively.

Similarly, for TSFG, the intensity of the sum-frequency generation light can be obtained as follows [26]:

$$I_{TSFG}\propto I_1{I}_2$$

(3)

where $I_{TSFG}$, $I_1$, and $I_2$ are the intensity of the sum-frequency light and the intensity of the pumping light at frequencies of ${\omega }_1$ and ${\omega }_2$, respectively. The frequency of the sum-frequency photons (ω) can be obtained as follows [26]:

$$\omega ={\omega }_1+{\omega }_2$$

(4)

3. Experimental Results and Discussion

The experimental setup is schematically illustrated in Figure 1. The growth of optical waveguiding Cttee nano/microwires (NMWs) is achieved via chemical vapor deposition [27]. Firstly, Si substrates and CdTe powder are heated inside a reaction tube. The gas phase CdTe is transported to the Si substrates located at the edge of the heated tube by N₂ gas. The reaction tube is heated to 950 °C within 20 min under a controlled N₂ flow rate of 93 sccm. Then, the temperature is held at 950 °C for 40 min with a constant chamber pressure (typically 660 mbar). Finally, the tube is cooled to room temperature naturally. As a result of screw dislocation-induced spiral growth known as Frank’s mechanism of crystal growth, wurtzite NMWs are obtained without the presence of a catalyst. To avoid the influence of the substrate, a CdTe nanowire is suspended in free space by two tapered optical fibers [28,29]. Continuous-wave lasers with wavelengths of 1064 nm (LR-ISP; Changchun Laser Technology Inc., Changchun, China; max output power, 300 mW; energy, 30 J; beam diameter, ~20 μm; fluence, 1.91 × 10⁴ W/cm²) and 980 nm (LZFL980; Lcirtc Inc., Shenzhen, China; max output power, 20 mW; energy, 20 J; fiber core diameter, 6.5 μm; fluence, 1.21 × 10⁵ W/cm²) are used as pumping sources. The pump light at a wavelength of 1064 nm is divided into two beams by a narrowband fiber optical coupler (TN1064R5F1A; Thorlabs Inc., Newton, NJ, USA; coupling ratio, 1:1). During the experiment, the output from one end of the fiber coupler is coupled to the nanowire, and the output from the other end of the fiber coupler is measured by a laser power meter (FieldMate; Coherent Inc., Saxonburg, CA, USA; calibration uncertainty, ±1.0%; measurement resolution power, ±0.1% of full scale; digital display accuracy, ±1.0% of reading ±2 LSD). In this way, the output power of the laser can be measured in real time. The pumping lights are coupled to the nanowire using fiber taper-assisted evanescent coupling. The counter-propagating pump lights meet each other inside the nanowire. As the nanowire is pumped, the transverse emission nonlinear signals are detected in the direction perpendicular to the CdTe single-nanowire axis by using an objective lens. Microscope images are captured using a CCD (M2000; Murzider Inc., Dongguan, China; effective pixels, 20 million; pixel size, 2.4 μm × 2.4 μm; time of exposure, 0.1 ms~15 s). And the spectra are collected by a spectrometer (USB4000; Ocean Optics Inc., Orlando, FL, USA; wavelength coverage, 200–1100 nm).

The optical microscope image of the suspended CdTe nanowire is shown in Figure 2a. The scanning electron microscope images (as shown in Figure 2b,c) indicate a smooth surface and a uniform diameter of ~280 nm. With a larger diameter, there are more guiding modes in the nanowire, leading to interference-induced periodical patterns of the transverse nonlinear emissions [6]. As a result, a smaller diameter is preferred in the experiment. Optical microscope images of the nanowire, with 1064 nm wavelength light coupled in from the left side, right side, and both two sides, are shown in Figure 2d–f, respectively. There are no obvious scattering points along the nanowire, which demonstrates a low scattering loss [3,18]. And the amplitude of the scattered light at the output end is much stronger than that at the input end, indicating a high coupling efficiency from the tapered optical fibers to the CdTe nanowire. Regarding coupling from the two sides, coupling efficiencies of ~71.4% and ~95.7% are measured by calculating the ratio of the amplitude of the scattering light from both ends in the optical microscope image [14]. Because of the different morphologies of tapered fibers and the different coupling lengths, the coupling efficiencies of the left and right ends are different [30,31].

Firstly, a pumping light with a wavelength of 1064 nm is launched into the nanowire from one side. An optical microscope image of the nanowire when subjected to an input power of ~1.90 mW from the left side is shown in Figure 3a, showing red color emissions along the whole nanowire. A short-pass filter is used to remove the pumping light. The spectra obtained with pumping lights coupled to the nanowire are shown in Figure 3b. A strong peak is observed at a ~713 nm wavelength, which is believed to result from the TPA-induced fluorescence [32,33]. And another, much weaker peak is also observed at a wavelength of 532 nm, which may result from the TSHG caused by the reflection of the pumping light at the output end of the nanowire [34]. Within the measured spectral range with the wavelength ranging from 400 nm to 900 nm, there are no other observable peaks, indicating that there no other nonlinear effects happen. Figure 3c shows the relationship between the intensity of the 713 nm wavelength peak and the input power for the two cases. The intensity increases quadratically with an increasing input power, showing good agreement with Equation (1). The results confirm that the red light seen at a wavelength of 713 nm results from the TPA-induced fluorescence.

Secondly, the pumping light with a wavelength of 1064 nm is split into equal two parts and coupled to the nanowire from both ends. The spectra of the transverse emission light with an increasing input power are shown in Figure 4a. A short-pass filter is used to effectively remove the 1064 nm wavelength pumping light in this experiment. There are two different peaks, which are seen at wavelengths of 532 nm and 713 nm in the spectra. Compared to the cases shown in Figure 3, the intensity of the 532 nm wavelength peak is much stronger, resulting from strong transverse second-harmonic emissions. However, considering that the interaction length of the TSHG is much less than that of the TPA, the intensity of the 532 nm wavelength peak is still less than that of the 713 nm wavelength peak [14]. The relationships of the intensity of the two peaks to the input power are shown in Figure 4b. Both the two curves show a quadratic relationship, agreeing with Equations (1) and (2). As shown in Figure 4c, a 600 nm short-pass optical filter is used to remove the 713 nm red fluorescence to obtain a single 532 nm peak. Similarly, a 600 nm long-pass optical filter is used to remove the 532 nm green light to obtain a single 713 nm peak, as shown in Figure 4d. The results demonstrate that the enhancement of the 532 nm wavelength peak is caused by TSHG. The optical microscope images with different input powers are shown in Figure 4e, showing a change in color with different input powers. For example, as input power increases from 0 to 1.8036 mW, both the two peaks in Figure 4a increase. Since the intensity of the 713 nm wavelength peak changes faster than that of the 532 nm wavelength peak, the color turns from green (in the 0.0678 mW power case) to yellow (in the 1.8036 mW power case). Due to the different sensitivities of the red and green light to the CCD used in the experiment, the green light seems to be only a bit stronger than the red light in the optical images, despite showing a much stronger intensity in the spectra, with a 713 nm wavelength peak. Considering the normalized sensitivity at a wavelength of 532 nm is significantly stronger than that at a wavelength of 713 nm for the CCD used in experiment, the green light seems to be stronger, leading to a mixed yellow color in the images. Periodic green patterns, owing to the optical interference caused by multi-mode interactions, are observed along the axis of the CdTe nanowire. These issues can be eliminated by carefully choosing the diameter of the nanowire for single-mode waveguiding [6].

Finally, the pumping lights at different wavelengths of 1064 nm and 980 nm are coupled to the nanowire from the right and left sides, respectively. In the meantime, the input power of the 980 nm wavelength pumping light remains unchanged. And the input power of the 1064 nm wavelength light increases gradually. The spectra obtained with different input powers are shown in Figure 5a. Compared to the spectra shown in Figure 4, a new peak at wavelength of 510 nm is observed. This is caused by the TSFG, which is performed according to Equation (3). It is worth mentioning that the intensity of the peak at the wavelength of 532 nm is stronger than that shown in Figure 3. This resulting from the higher reflectivity produced at the left side of the nanowire by carefully tuning the coupling geometry. In this way, a comparable intensity for the two peaks at wavelengths of 510 nm and 532 nm is obtained. According to Equation (3), a linear relationship between the intensity of the 510 nm wavelength peak and the input power is expected. Since the 532 nm wavelength light is caused by the endface-induced TSHG, the intensity of the 532 nm wavelength light can be obtained as follows:

$$I_{TSFG}\propto I_1{I}_2=\gamma I_1^2$$

(5)

where $I_2=\gamma I_1$ and γ is the reflectivity of the endface. As a result, the 532 nm wavelength light still changes quadratically with an increasing input power. The relationships between the intensity of the peaks and the input power are shown in Figure 5b, and they agree with the theoretical analyses. The optical microscope images obtained with different input powers are shown in Figure 5c, showing a changing color mix across the three peaks.

The color of the transverse emitting light can be modulated by varying the input power and wavelength of the pumping light associated with optical filters. The corresponding CIE 1931 chromaticity coordinates, obtained experimentally, are shown in Figure 6. The circles represent the cases in which only one peak remained in the spectra after using optical filters (FESH0600 and FELH0600; Thorlabs Inc., Newton, NJ, USA). The squares, diamonds, and triangles represent the cases in which there are two peaks in the spectra. The squares present cases in which a counter-propagating pumping light at a wavelength of 1064 nm is coupled to the nanowire at both sides. And the diamonds and triangles show the results obtained by removing the peaks at 532 nm and 713 nm, respectively, from the spectra using digital filtering processes. The pentagrams present the cases in which the pumping lights with wavelengths of 1064 nm and 980 nm are coupled to the nanowire from two sides, respectively, resulting in three peaks at different wavelengths being observed, as shown in Figure 5.

In the single-peak cases represented by circles, the wavelength of the peak remains unchanged in response to variations in the input power, and thus the corresponding CIE tristimulus values remain constant. The three circles in Figure 6 correspond to the peaks with wavelengths of 510 nm, 532 nm, and 713 nm, respectively. These values define a triangular area with a Rec. 2020 area–coverage ratio of approximately 21.6%. This determines the tunable color gamut of the colors emitted from the CdTe nanowire.

In the two-peak cases represented by squares, diamonds, and triangles, the CIE 1931 xy coordinates trace a path along the connecting line between the two reference circles as the input power is increased. For example, in the case of two-peak spectra with peaks at wavelengths of 532 nm and 713 nm, the location of the squares shifts towards the red end as the input power increases. This is due to the fact that the rate of increase in the intensity for the 713 nm wavelength is greater than that for the 532 nm wavelength peak, as demonstrated in Figure 4. As a result, an increase in input power results in the proportion of the red color being greater. And the triangles move towards the circle corresponding to the 532 nm wavelength as the input power increases.

Similarly, in the three-peaks cases, the pentagrams move towards the red end with an increasing input power.

The shifts of the coordinates in different cases are shown in Figure 7. The results directly obtained in the experiment using the filter are shown in Figure 7a,b. And the results associated with a digital filtering process performed to remove the peaks at wavelengths of 713 nm and 532 nm are shown in Figure 7c,d, respectively. The results confirm the trends in the mixed color with different input powers, as mentioned above. In Figure 7a, the coordinate changes from (0.2701, 0.5949) to (0.3995, 0.5044) as the input power increases from ~1.97 mW to ~2.45 mW. In Figure 7b, the coordinate changes from (0.2930, 0.6776) to (0.4096, 0.5696) as the input power increases from ~2.09 mW to ~2.60 mW. In Figure 7c, the coordinate changes from (0.0675, 0.7555) to (0.1037, 0.7734) as the input power increases from ~1.97 mW to ~2.58 mW. In Figure 7d, the coordinate changes from (0.3057, 0.5409) to (0.7343, 0.2657) as the input power increases from ~1.97 mW to ~2.58 mW. The results show linear relationships in different cases. As a result, by changing the wavelength and the input power of the pumping light, a tunable emitting color can be achieved based on the modulation of the spectra of the transverse emitting light associated with optical filters.

To examine the influence of the diameters of nanowires on the transverse emissions, we used nanowires with different diameters in the experiment. For example, the spectrum of a nanowire with a diameter of ~385 nm is shown in Figure 8. Similar to the results shown in Figure 3, the spectrum also shows two peaks at wavelengths at 532 nm and 713 nm, respectively. The result indicates that the small change in diameter from ~280 nm to ~385 nm will not influence the spectra of transverse emissions.

To examine the uncertainty of spectra measurement, we measure four spectra using the same experimental setup for a duration of ~40 s. The results are shown in Figure 9 and

Table 1. The central wavelength and maximum output intensity of the spectra.

	Central Wavelength (nm)	Maximum Output Intensity (a.u.)
No. 1	713.41	0.9263
No. 2	712.83	0.9120
No. 3	712.64	0.8902
No. 4	712.06	0.8626

. The error bars for the central wavelength and the maximum normalized output intensity are found to be 1.35 nm and 0.0637, respectively, corresponding to standard deviations of 0.4820 nm and 0.0240. The uncertainty may result from the change in the suspended coupling structure during the measurement and the uncertainty of the spectrometer used in the experiment.

4. Conclusions

To summarize, we have demonstrated the tunable color of a single using optical nonlinear transverse emission. Lights with different wavelengths of 510 nm, 532 nm, and 713 nm, caused by different effects including TSFG, TSHG, and TPA-induced fluorescence, respectively, are emitted in directions perpendicular to the nanowire and overlap with each other in space. Modulating the spectra of the transverse emitting light by changing the pumping light associated with optical filters, a tunable color gamut covering ~21.6% of the Rec. 2020 area of the CIE diagram is demonstrated. The real-time modulation of the emitting color is achieved without changing either the material or the geometry of the nanowire. By carefully choosing the wavelength of the pumping light (for example, using pumping light with wavelength much less than 980 nm), a full-color display is shown to be highly promising in principle. For example, by using counter-propagating pumping light with wavelengths of 1064 nm and 850 nm, three peaks of 713 nm, 532 nm, and ~473 nm can be in principle generated in spectra, indicating an area–coverage ratio over ~83.4%, in accordance with the Rec. 2020. The results indicate the good potential of single-nanowire tunable color emissions based on transverse nonlinear effects in applications such as optical display, spectroscopy, and communication.