Generation and Characterization of Single Fluorescent Centers

Emily DeVeyra, Supervisor: Dr. Svetlana Lukishova

Abstract

In application to single (antibunched) photon sources (SPSs), we investigated a creation of photoluminescent centers in silver nanocubes, silicon nitrate (SiN), and a silicone-based contact lens hydrogel material under laser irradiation. Single-emitter confocal fluorescence microscopy, low-light spectroscopy, and a Hanbury Brown-Twiss correlator are used for characterization of laser-induced photoluminescence centers. Photon antibunching was observed from some of them in silver nanocubes treated by CW laser radiation at 405 nm and/or 532 nm as well as in femtosecond laser treated SiN.

Applications and Motivation

SPSs with photons exhibiting antibunching are nonclassical light sources with all individual photons separated in time [1]. SPSs have applications in quantum information, quantum metrology, and the biomedical field (antibunching super-resolution microscopy).We investigate the formation of new emitters for SPS applications by laser irradiation of different types of materials: semiconductor (SiN used in photonic integrated circuits), silver nanocubes (used in plasmonic nanoantennas), and a silicone-based contact lens hydrogel polymer (used in contact lenses). For instance, the defects induced by femtosecond laser irradiation promise close-to-atomic-scale laser manufacturing in solid-state substrates [2]. SPSs based on these defects with different fluorescence wavelengths (color centers) on the same substrate have been demonstrated by changing femtosecond laser irradiation doses.

Fig. 1 (from Ref. 2). LEFT: TEM image of the fs laser induced defect in hBN with less than 5 nm size, RIGHT photoluminescence spectra of the single color centers created by fs-laser writing in hBN ranging from 500-800 nm.

At the same time laser-induced fluorescing centers can be harmful in plasmonic nanoantennas. Instead of enhancement of a single-emitter fluorescence through a Purcell effect, these induced fluorescing centers can compete with the main emitter preventing registration of its photon antibunching [3].

Fs laser irradiation of a contact lens polymer changes its refractive index and focusing of a contact lens. We investigate if this fs-laser treatment creates also fluorescing defects.

SAMPLES: fs-laser treated SiN, silver nanocubes, and fs-laser treated silicone based hydrogel

Fig. 2. Samples. LEFT: SEM micrograph of a SiN sample with a fs-laser treated area; scale bar is 30 μm. Crosses made by FIB indicate boundary of a treated area. CENTER: SEM micrograph of 100-nm silver nanocubes; scale bar is 1 μm. RIGHT: Reflective optical microscope image of a silicone-based contact lens hydrogel sample with fs-laser written lines; scale bar is 2 mm.

SIN after fs-laser treatment (Fig. 2, left): A 180 nm layer of SiN was deposited by a low-pressure CVD onto a 750 nm SiO₂ layer prepared by a thermal wet oxidation on a Si-wafer. A PHAROS laser with ~100 fs pulse duration and 0.5 Hz pulse rep. rate was used for treatment of this sample during a few seconds. We used operation at 575-630 nm wavelengths, ~500 nJ energy per pulse, and ~3-μm beam diameter. Fig. 2, left shows a SEM micrograph with a treated area of a rectangle shape (~100 μm length).

Silver nanocubes (Fig. 2, center): A sample with drop-casted silver nanocubes (NanoXast from nanoComposix) was used without any preliminary laser treatment. Fig. 2, center shows SEM micrograph of these 100 nm silver nanocubes coated by a few nm of polyvinylpyrrolidone (PVP) for protection from environment.

Silicone hydrogel after fs-laser treatment (Fig. 2, right): A silicone based contact lens hydrogel (Bausch & Lomb) was treated by a 515 nm, ~150 fs-laser to write lines at 50 μm below the surface (10 mm/s scan speed, 60 nJ per pulse 1,800 pulses, 3 μm diameter at laser power ~600 mW, 10 MHz pulse rep. rate).

EXPERIMENTAL SETUP: single-emitter confocal fluorescence microscopy and spectroscopy

*Fig. 3. Experimental setup. Schematic of the whole single-photon generation and characterization unit with single-photon counting electronics.*

Fig. 3, shows a schematic of a confocal fluorescence microscope (oil-immersion objective with NA = 1.3) with a Hanbury Brown and Twiss interferometer and a low light spectrometer equipped with an EM-CCD. Two single-photon counting avalanche photodiode modules (APDs) were used as single-photon detectors. Two CW lasers with wavelengths of 405 nm and 532 nm were used for single-emitter excitation. PicoHarp330 single-photon counting electronics was used for antibunching measurements. For wide-field microscopy a second EM-CCD was used.

RESULTS 1: SiN with fs-laser written color centers

Fig. 4. LEFT: Confocal microscope raster scan image of a SiN sample showing photoluminescence of a fs-treated area, 532 nm laser excitation. Scan size is 7 μm x 25 μm; the bar at the right shows counts/pixel, each pixel corresponds to 5 ms. CENTER and RIGHT: SEM micrographs of typical fs-treated parts of Fig.2, left with different magnification. A craters sizes are 175.3 nm x 320.4 nm and 672 nm (diameter).

Fig. 5. LEFT: Photon antibunching histogram from color centers created by a fs-laser on SiN. RIGHT: Time traces from the same area (1 pixel corresponds to 5 ms). Area of data collection is marked by a green cross in Fig. 4, left.

Fig. 6. Photoluminescence spectra of a SiN sample treated by fs-laser irradiation. LEFT: under 532 nm laser excitation. *The spike indicates a 532 nm laser line.* *RIGHT: under a broadband laser excitation (450 nm to 2300 nm) with M. Sanchez & X. Zhu’s help.*

RESULTS 2: 100-nm silver nanocubes under 405 & 532 nm laser excitations

Usually, metal silver has a weak wide photoluminescence spectrum uniform along the sample surface, but in Ref. 3 appearance of photoluminescence centers with a single-emitter behavior was observed in a raster scan of the sample with 100 nm silver nanocubes widely used in plasmonic gap nanoantennas. Time traces of count rates from these centers showed bright intensity spikes and a step-wise count increase under higher 532 nm or 633 nm CW laser excitation. This section is devoted to further investigation of this effect under 405 nm or 532 nm CW laser excitation.

405 nm excitation:

Fig. 7. LEFT: Confocal fluorescence microscope raster scan image of a sample with silver nanocubes with scan size 7 μm x 25 μm; bar at the right shows counts/pixel, each pixel corresponds to 5 ms. CENTER: Photon antibunching histogram (dip at the center indicates antibunching). RIGHT: Time traces.

532 nm excitation:

Fig. 8. LEFT: Confocal fluorescence microscope raster scan image of a sample with silver nanocubes Scan size is 7 μm x 25 μm; bar at the right shows counts/pixel, each pixel corresponds to 5 ms. RIGHT: time trace measurement (1 pixel corresponds to 5 ms).

RESULTS 3: Silicone based contact lens gel polymer after fs-laser treatment

This part of the work was done to investigate polymers’ behavior after fs-laser treatment. Polymers are used in SPS applications as the spacers between nanoemitters and plasmonic nanostructures. For this part of our research we used a fs-treated polymer sample (silicone based contact lens hydrogel) as a part of research of W. Knox’s laboratory. Fig. 9 shows confocal fluorescence microscope raster scan image of a fs-treated area (left, with a bright line with higher count rate). The same area of the sample is shown with a wide-field microscopy (center) with a cross that indicate a position of excitation laser beam (532 nm). A right part of this figure shows the spectrum of a fs-treated area.

Fig. 9. LEFT: Confocal fluorescence microscope raster scan image of a silicone based contact lens hydrogel. Scan size is 7 μm x 25 μm; bar at the right shows counts/pixel, each pixel corresponds to 5 ms. CENTER: Wide-field image of the same area. A cross indicates a position of a 532 nm laser beam used for excitation, RIGHT: Photoluminescence spectrum from a fs-treated area.

Fig. 10. LEFT: Wide-field image of the sample area. A cross indicates a position of a 532 nm laser beam used for excitation. The cross is outside of a fs-treated area. RIGHT: Photoluminescence spectrum from an untreated area. The wavelengths of two maxima are 580 nm and 680 nm (disappeared in a Fig. 9 spectrum).

The main result of this research is that in the spectrum of a fs-laser treated area one of the maxima (680 nm) disappears after a fs-laser treatment. Fig. 10 shows a new position of a laser beam on the sample (indicated by a cross on the wide-field image on the left) that was moved to an untreated area. A right part of this figure shows a double maxima spectrum of an untreated area.

SUMMARY

The main results of this work:
(1) First demonstration of SPS based on a fs-induced color center in SiN.
(2) First demonstration of antibunching in plasmonic structures for SPS applications that can interfere with the main emitters placed into them for Purcell fluorescence enhancement.
(3) Observing spectral changes in photoluminescence of silicone based contact lens hydrogel after fs-laser treatment usually used for changing refractive index in it.

Acknowledgments

I would like to thank Prof. Svetlana Lukishova for supervising me for my senior thesis project as well as the Institute of Optics for support. I also want to thank Prof. Wayne Knox, Prof. Michele Cotrufo, Prof. Pablo Postigo, S. O’Neil, Q.R. Islam, M. Sanchez, X. Zhu, and M. Belzer for their help.

References

[1] S.G. Lukishova & L. J. Bissell. (2019). Nanophotonic Advances for Room-Temperature Single-Photon Sources. In: Boyd, R., Lukishova, S., Zadkov, V. (eds) Quantum Photonics,. Springer.
[2] XJ, Wang, HH. Fang, ZZ. Li, D. Wang, and H.B Sun, “Laser manufacturing of spatial resolution approaching quantum limit,” Light Sci Appl 13, 6 (2024).
[3] S.G. Lukishova, J. Brone, D. Khan, and Z. Li, “Ultrabright photoluminescence spikes and step-wise photoluminescence increase from colloidal silver nanoparticles for patch nanoantennas”, J. Phys.: Conf. Ser. Vol. 2249, 012002 (2022).