Visual monitoring of laser power and spot profile in micron region by a single chip of Zn-doped CdS nanobelts

Abstract

On the basis of pumping-power-dependent emission property, single-chip zinc-doped cadmium sulfide nanobelts are developed to monitor injected laser power and detect the profile of laser focal spots visually in the micron region through color changes. By doping zinc into cadmium sulfide nanobelts, band-edge emission with green color and deep-trap-state emission with red color are generated by injected laser beam pumping. The monotonic intensity ratio changes of these two emissions reflect the pumping power variations, so that the laser power and focus spot distribution can be monitored quantitatively by the on-chip nanobelts. Utilizing the on-chip zinc-doped cadmium sulfide nanobelts to detect input laser power, more than two thousand cycles of pumping laser power increasing and decreasing periodically are applied on the nanomaterial chip, accompanied by emission-color switching between yellow emission and green emission, which confirm that the zinc-doped cadmium sulfide nanobelts are stable and effective. The method involves visually perceiving the change of laser power and detecting the distribution profile of the laser spot. This optical ratiometric method based on nanostructures is quite different from the traditional detection mode by the thermoelectric effect. Therefore, it has potential application as a laser power monitoring tool for optically integrated micro-circuits.

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Introduction

Laser power detectors have been widely studied and developed due to their being a critical tool for experimental studies of optical properties. Generally, a traditional laser power meter utilizes a pyroelectric-type detector or laser photodiode as a basic detecting element. Using this method, the laser power intensity is transformed to a voltage signal to give the laser power value precisely.^1,2 It needs, in addition to the detector, a display module to show the power value. Moreover, for thermoelectric effects, a complicated cooling part is required to dissipate the heat. Therefore, it is hard to obtain miniaturized and embedded optical power sensors with traditional materials and routes to satisfy the development of nanotechnology and nanodevices even though they are critical components in photonic integrated circuits.^3,4 For a photodiode-type power detector, it is always used in low-power injection situations, and generally needs an additional filter to avoid damage by the input laser. Actually, until now, there is still no power detector that can give us a visual and intuitive perception of the power value and distribution profile of an input laser beam in the micron region. In this paper, we firstly demonstrate a new way to detect input laser power visually and intuitively based on a nanobelt-integrated chip. Utilizing nanostructures as basic power monitoring elements, it guarantees an easy way to integrate with developed micro/nanodevices to build up more complicated optical circuits with higher functionality.

Nanostructures of group II–VI semiconductors are one kind of advanced optical materials with excellent light-emitting and optoelectronic properties.^5,6 Generally, doping metal impurities into group II–VI semiconductors always generates additional defect-related energy levels, together with band-edge emission to form ratiometric photoluminescence (PL) intensity, which was developed to realize optical ratiometric sensors.⁷ In recent reports, wide-gap II–VI nanocrystals doped with manganese ions (Mn²⁺), exhibiting optical ratiometric properties for the detection of various local environmental changes, have attracted much attention.^8,9 The optical ratiometric method has been widely applied for the detection of excitation intensity,¹⁰ temperature,¹¹ pH,¹² heavy metals in solutions¹³ and toxic gas in mitochondria.¹⁴ An optical ratiometric sensor shows resolvable luminescence from two different excited states. The light intensity is measured by the relative PL intensity ratio instead of absolute PL intensity,^7,15 which will not induce thermal effects and a complicated cooling system is not required. This means the device can be minimized and realized easily in all-optical integrated circuits.^16,17 Moreover, the analysis of the intensity distribution of laser focus is important for some nonlinear optical phenomena, such as two-photon absorption and sub-wavelength spatially resolved excitation.^18,19 However, to date, few results on nanostructure-related laser power monitoring have been mentioned, especially for the intensity distribution of laser focus in the micron region.

In this work, single-chip zinc-doped cadmium sulfide (Zn-doped CdS) nanobelts were synthesized by a controllable chemical vapor deposition (CVD) method. They were used to detect laser power and intensity distribution visually in the micron region according to the power-dependent properties of two distinct emission bands at room temperature. Under a power-variable UV laser illumination, the ratio of these two distinct emission colors will change as the pumping power changes. Therefore, the power distribution of a laser focus spot can be shown directly by a real-color image in a microsize region of the Zn-doped CdS nanobelt chip. Moreover, the color tunable property of the on-chip Zn-doped CdS nanobelts is extraordinarily stable with the laser power changing during an experimental period of 6 months. This new idea for laser power detection and profile measurement of a pumping laser spot based on nanostructures provides more possible applications in visual laser power detection.

Experimental

The synthesis method of Zn-doped CdS nanobelts is similar to previously published CVD results.²⁰ The morphology, phase structure, and elemental valence state of Zn-doped CdS nanobelts were characterized by field-emission scanning electron microscopy (SEM; Zeiss SUPRA 55), X-ray diffraction (XRD; RINT2200), and X-ray photoelectron spectroscopy (XPS; Thermo Scientific), respectively.

In the experiments of laser power monitoring and intensity distribution analysis, a confocal PL microscope was used with a spatial resolution of 500 nm (Olympus BX51M, ×50 objective lens, NA ∼ 0.46). The pumping laser power was fine-tuned by the driven voltage of a diode laser (GaN diode laser, ∼405 nm).

Results and discussion

In this work, we designed CdS nanobelts with ∼2% of Zn dopant, which show a strong yellow-colored PL emission under appropriate excitation power. The SEM image shows that the morphology of Zn-doped CdS nanobelts is belt-like and uniform with ∼20 μm base length and ∼300 μm waist length, as shown in Fig. 1(a). The inset in Fig. 1(a) is an image of a single chip of the Zn-doped CdS nanobelts with a size of 0.6 cm × 0.6 cm. The XRD pattern of the on-chip Zn-doped CdS nanobelts exhibits major peaks corresponding to diffraction lines of the structure of hexagonal wurtzite phase CdS (JCPDS card no. 41-1049), as shown in Fig. 1(b). No distinct peaks of secondary phases and deviation shown in the XRD pattern confirm the good crystallinity of the nanobelts.

Fig. 1 (a) SEM image of Zn-doped CdS nanobelts. The inset is an image of on-chip Zn-doped CdS nanobelts. (b) X-ray diffraction pattern of the single-chip Zn-doped CdS nanobelts.

The full-range XPS spectrum indicates that the as-prepared sample consists of the elements Cd, S and Zn, with no obvious impurities except the inevitable elements C and O, as shown in Fig. 2(a). The Cd 3d^5/2 peak at 404.6 eV (Fig. 2(b)) and the Zn 2p^3/2 peak at 1021.5 eV (Fig. 2(d)) are consistent with the reported binding energies for Cd²⁺ and Zn²⁺; the S 2p^3/2 peak at 160.9 eV (Fig. 2(c)) and the S 2p^3/2 peak at 168.2 eV are consistent with the reported binding energies for S²⁻ and S⁶⁺. XPS analyzes the surface elements of the sample, so the existence of S⁶⁺ comes from the surface oxidation of the nanobelts. These characterizations demonstrated that the Zn-doped CdS nanobelts had been prepared with high quality.

Fig. 2 (a) XPS spectrum of as-prepared Zn-doped CdS nanobelts in the energy range of 0–1100 eV. (b) High-resolution XPS for Cd 3d^5/2 and Cd 3d^3/2 peaks of Zn-doped CdS nanobelts. (c) High-resolution XPS for S²⁻ 2p^3/2, S²⁻ 2p^1/2 and S⁶⁺ 2p^3/2 peaks of Zn-doped CdS nanobelts. (d) High-resolution XPS for Zn 2p^3/2 and Zn 2p^1/2 peaks of Zn-doped CdS nanobelts.

The emissions of Zn-doped CdS nanobelts are sensitive to the pumping laser power, as shown in Fig. 3(a). The ratio of integrated intensity of green emission and red emission (I_green/I_red) monotonically increases with an increase of pumping laser power. The corresponding PL spectra under different pumping power are shown in the inset of Fig. 3(a), including the green band-edge emission and Zn-involved broad red trap-state emission. At higher power excitation, more photons are generated from the band-edge recombination, while at lower power excitation, the red emission is dominant. Within the effective ratiometric changing range, each pumping laser power yields a unique ratio of green emission and red emission intensity. The evolution of input power versus I_green/I_red is well fitted by the function y = ax^b, as shown with the red line in Fig. 3(a), which is the basic calibration curve for the pumping power detector, where y is the ratio I_green/I_red, x is the pumping power, a is a fitting constant (a = 4.80 × 10⁻⁴), and b = 2.69 is a polynomial fitting index. Using this fitting function, the input laser power can be obtained quantitatively from the PL ratio of I_green/I_red. Furthermore, in the PL spectra obtained under various pumping laser powers, the green peak position is tuned from 503 nm to 514 nm with a relative red shift of about 11 nm (0.06 eV) as the excitation laser power increases from 2.62 mW to 37.49 mW, as shown in Fig. 3(b). This red shift is a characteristic of most II–VI semiconductor nanobelts under high pumping laser power. That is, the recombination of free excitons is dominant in the Zn-doped CdS nanobelts at low pumping laser power; and then more free excitons are generated, some of them being ionized into free electrons and holes by carrier-to-carrier scattering at higher pumping laser power,²¹ which induces the red shift of the green peak position.²² Furthermore, the shallow bond states might come from sulfide vacancies (V_S). Besides, the red peak position at ∼600 nm has no obvious shift with increasing laser power, as shown in Fig. 3(c), which demonstrates that the red band originates from the Zn-related defect/surface-related trap state emission.^21,23 In addition, the full widths at half maximum (FWHM) of green emission and red emission both broaden with increasing pumping power, as shown in Fig. 3(b) and (c), respectively. These FWHM broadenings are due to more carriers being generated and the potential fluctuations caused by higher pumping laser power.²¹ Therefore, the PL properties of on-chip Zn-doped CdS nanobelts can be tuned by the pumping power. Accordingly, the input laser power can be monitored by the monotonic change of emission intensity ratio of I_green/I_red of the on-chip Zn-doped CdS nanobelts. The single-chip nanobelt materials can be used as laser power monitors.

Fig. 3 (a) The intensity ratio of I_green/I_red as a function of pumping laser power. The data are well fitted by the function y = ax^b, marked as the red line, where a is a fitting constant (a = 4.8 × 10⁻⁴) and b = 2.69 is a polynomial fitting index. The inset shows typical PL spectra of Zn-doped CdS nanobelts with zinc concentration of 2% under various pumping powers. (b) Peak position and FWHM of green emission as a function of pumping laser power. (c) Peak position and FWHM of red emission as a function of pumping laser power. (d) Real-color image of a laser spot excited on the on-chip Zn-doped CdS nanobelts. At the center (white color), the power is about 25 mW. The edges of the spot between the two different emitting colors are marked by the contours. The corresponding power values, 17 mW, 8 mW and 2 mW, are deduced from the PL intensity ratio and color. The inset is the distribution of color-density level. (e) Plot of the intensity profile of the focal laser spot along the black dashed line shown in (d), marked as the black squares. The data are well fitted by the Gaussian curve, marked as the red dashed line.

The detailed profile of a laser focal spot also can be measured by the on-chip Zn-doped CdS nanobelts, as shown in Fig. 3(d), a typical real-color PL image induced by a focused laser diode with a wavelength of 405 nm. At the central part (white color), the laser power is around 25 mW and the white light is formed due to CCD saturation. The intensity of the laser spot becomes gradually lower away from the center of the laser spot, while the emitting color of Zn-doped CdS nanobelts changes from yellow to orange-yellow. Through the different color-emitting image on the single-chip Zn-doped CdS nanobelts, the intensity distribution of the excited laser spot can be determined. To clearly see the distribution, the edges of different emitting colors are marked by contours, as shown in Fig. 3(d). The corresponding power values, 17 mW, 8 mW and 2 mW, were deduced from the PL intensity ratio and color. The inset of Fig. 3(d) shows the distribution level of color gradation, the three peaks corresponding to the three bright color distributions, white, yellow and orange. The total laser focused spot and scanning region is clearly demonstrated by the color contour within a 20 μm diameter effective range. For the CCD-type laser profile display mode, it is hard to estimate the condensed laser focused spot profile due to the intense laser fluence that can easily damage the CCD detector. Utilizing the sensitivity of PL ratio of I_green/I_red to the pumping power value, the excited laser power can be deduced from the emitting color of Zn-doped CdS nanobelts. Furthermore, the detailed intensity distribution of the laser spot along the black dashed line (shown in Fig. 3(d)) is shown in Fig. 3(e), as marked by the black squares. The intensities of the focused pumping laser are deduced from the corresponding PL intensity ratios. These intensity data are well fitted by a Gaussian curve, as marked by the red dashed line in Fig. 3(e). This is consistent with the distributions of commercial lasers.²⁴ Using single-chip Zn-doped CdS nanobelts, the laser spot intensity distribution in the micron region can be measured visually and intuitively, which has a potential application in detection and analysis of really effective laser focal ranges in many researches of nonlinear optical properties and femtosecond laser-related micro/nano fabrication techniques.

The laser power and spot profile of an input laser were measured using the setup shown in Fig. 4(a). The laser beam was focused on a chip of Zn-doped CdS nanobelts through a beamsplitter and an objective lens (×50). Then the PL of the nanobelts was recorded with a spectrometer via the same beamsplitter and a mirror. Each pumping laser power yields a unique PL spectrum, according to the black reference curve of Fig. 4(b), which is the same calibration curve shown in Fig. 3(a). The real input pumping powers were identified by the on-chip nanobelts, as shown in Fig. 4(b). Five different laser powers were applied and the input power values were deduced from the I_green/I_red values of 0.0082, 0.2876, 1.2843, 3.8149 and 7.8046; the corresponding laser power values are 2.87 mW, 10.78 mW, 18.8 mW, 28.18 mW and 36.77 mW, respectively, which were compared to the results measured by a commercial power meter (red circles) for which the values are 2.62 mW, 10.48 mW, 20.71 mW, 27.26 mW and 37.49 mW, respectively. The root-mean-square error of the PL ratiometric method compared to the power meter measurement is about 5.37%. In addition, typical real-color images of the on-chip nanobelts were excited by the pumping laser at 2.62 mW, 10.49 mW and 37.49 mW, which are shown in Fig. 4(c) from left to right, respectively. The color in Fig. 4(c) corresponds to the average power of the whole light spot. It is clearly shown that the color is tuned from orange-yellow to yellow and finally to green with increasing pumping power. The basic color changing with input laser provides us with a basic reference standard to estimate the color-related laser spot profile, which provides a virtual way to estimate roughly the different laser powers. Fig. 4(d) shows the UV-visible absorption spectrum of the collective Zn-doped CdS nanobelts, which reveals that the Zn-doped CdS nanobelts can absorb ultraviolet and up to blue light with a wavelength range less than 500 nm. However, these nanobelts hardly absorb light with a wavelength longer than 500 nm. This demonstrates that the on-chip Zn-doped CdS nanobelts can only detect blue to ultraviolet light with less absorption of yellow and red light. Therefore, the pumping laser power can be visually perceived in terms of the emitting color, which is convenient for the application of a nanoscale blue-violet laser power monitor.

Fig. 4 (a) The diagram of the setup to detect pumping laser power through optical measurement. (b) The input laser power was obtained from the intensity ratio of I_green/I_red based on the standard reference curve (black curve from Fig. 3(a)), contrasted with the power values of commercial power meter measurement (red circles). (c) Optical images of the collective Zn-doped CdS nanobelts under a power of 2.62 mW, 10.49 mW and 37.49 mW from left to right. (d) The UV-visible absorption spectrum of the collective Zn-doped CdS nanobelts. (e) The color switching cycles (in counts N) of the Zn-doped CdS nanobelts under two pumping laser powers (2.62 and 37.49 mW).

To confirm the stability and effectiveness of this kind of nanostructure-based detector for monitoring laser power, more than two thousand cycles of input laser-induced color switching between yellow emission and green emission have been recorded in real time for the single-chip Zn-doped CdS nanobelts, as shown in Fig. 4(e). The x-axis is the number of color switching cycles (in counts N) and the y-axis is the ratio of I_green/I_red from minimum to maximum. Besides, after the material was preserved in a dryer for 6 months and continuously irradiated during about 3 hours without any polymer covering the chip surface, the laser power detection function of the nanobelt chip shows almost no change; and the intensity of luminescence is similar to previous results. The luminescence remains around 650 Cd m⁻² with a pumping laser power of 35 mW. This extraordinary reversible and photoswitchable phenomenon contributes to the ultra-photostability of the single-chip Zn-doped CdS nanobelts, which is much better than that of a series of doped CdS nanocrystals that retain stable emission usually within several days.^25,26 Moreover, the response and recovery time after laser excitation are determined by the lifetime of PL; the decay time of the band-edge and defect-related emission is less than 1 ns and several nanoseconds for the Zn-doped CdS nanobelts (see ESI†), respectively. This means that this kind of detector can be used as a real-time laser monitor. Utilizing the nanostructure-based PL ratiometric method to monitor laser power opens a new way to fabricate power meters at the micro/nanoscale.

Conclusions

In conclusion, we have presented a new type of single-chip Zn-doped CdS nanobelt-based laser power detector, which shows the input laser power and distribution visually and intuitively in terms of the emission color change, having potential applications in laser power monitoring and the analysis of the profile of laser focal spots in the micron region. Especially, extraordinary stability and reversibility of the single-chip Zn-doped CdS nanobelts confirm that this kind of nanostructure-based laser power detector has wide potential applications in ultraviolet and blue range laser power monitoring as a component in complicated optical integrated circuits at the micro/nanoscale.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 51002011) and the Basic Scientific Research Project of Beijing Institute of Technology.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra09201e

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