Diamond photonics platform enabled by femtosecond laser writing

Abstract

Diamond is a promising platform for sensing and quantum processing owing to the remarkable properties of the nitrogen-vacancy (NV) impurity. The electrons of the NV center, largely localized at the vacancy site, combine to form a spin triplet, which can be polarized with 532 nm laser light, even at room temperature. The NV's states are isolated from environmental perturbations making their spin coherence comparable to trapped ions. An important breakthrough would be in connecting, using waveguides, multiple diamond NVs together optically. However, still lacking is an efficient photonic fabrication method for diamond akin to the photolithographic methods that have revolutionized silicon photonics. Here, we report the first demonstration of three dimensional buried optical waveguides in diamond, inscribed by focused femtosecond high repetition rate laser pulses. Within the waveguides, high quality NV properties are observed, making them promising for integrated magnetometer or quantum information systems on a diamond chip.

Figure 1. Suppression of graphite in femtosecond laser induced modification at 500-kHz repetition rate.

(a) Transverse optical microscope image of single laser-induced track written with 500-kHz repetition rate, 50-mW average power and 0.5-mm/s scan speed. (b) μRaman spectra (532-nm excitation wavelength) at four different vertical positions inside the modification. ‘Out’ refers to a spectrum taken outside the track. The spectra have been normalized to the diamond peak to show the change in the relative intensity of the G-peak inside the structure. (c) μRaman spectra (normalized to the G-peak) in the center of modification tracks at repetition rates of 5 kHz, 25 kHz and 500 kHz, with pulse energy held constant (800 nJ) to produce a similar size modification at each repetition rate. (d) Tauc plot for diamond with tracks written over the entire sample at 50-μm depth and 20-μm line separation for 5-kHz and 500-kHz repetition rates. It is considered that the absorption in the visible region is only due to the modification tracks, with the rest of the sample being transparent.

Figure 2. Type II buried optical waveguides in diamond.

Transverse microscope view of type II waveguide in diamond along with near field mode profile (λ = 635 nm). An arrow indicates the position of the mode. (a) Pair of lines, horizontally separated by 13 μm. Modes could be coupled into three different vertical positions with the lowest loss mode shown (MFD 10 μm × 11 μm). (b) Pair of lines, horizontally separated by 13 μm along with second pair of lines for vertical confinement. Only a single mode could be coupled to the four-line modification (MFD 9 μm × 10 μm). All tracks were written with 50 mW, 0.5 mm/s at 500 kHz with deeper tracks more elongated due to increased spherical aberration.

Figure 3. Peak shift map of type II waveguides using μRaman spectroscopy.

Spatial map of frequency shift of diamond Raman peak with respect to bulk for (a) two-line and (b) four-line modification, with the same parameters as those in Fig. 2. The modification tracks are shown as black.

Figure 4. Width of diamond Raman peak within waveguiding region.

(a) Peak shift map from Fig. 3(a) with colored lines corresponding to the line profiles (b) of the width of diamond Raman peak inside the guiding region. Dotted lines mark the approximate position of the edges of the tracks.

Figure 5. Photoluminescence spectra in waveguide.

(a) Photoluminescence spectra within the laser-written lines, between the modification lines in the waveguiding region and in pristine diamond acquired by confocal microscopy (excitation wavelength 532 nm). A cross sectional microscope image of the type II waveguide is shown in the inset. A zoomed in view of the ZPL spectrum (indicated with a black square) is presented on the right, showing that it remains unchanged in the guiding region compared to pristine diamond. (b) Photoluminescence detected with spectrometer at output of the waveguide when light was coupled in using free space optics (TM and TE configurations are presented). The inset shows an overhead microscope image of the fluorescence streak when the 532-nm light was coupled to the waveguide.

Figure 6. Preservation of hyperfine structure of ground state and excited state lifetime.

(a) Coarse ODMR scan within the waveguide where the electron spin transitions from m_s = 0 → m_s = ± 1 are indicated by a drop in fluorescence intensity. Several families of transitions are visible because of Zeeman splitting caused between a static magnetic field (~90 G) and the four possible orientations of the NV center within the diamond lattice. (b) Finer ODMR scan of one of the electron spin transitions indicated in (a). This shows the hyperfine structure the electron spin coupled the NV centers’ ¹⁴N nuclear spin. The three transitions correspond to m_s = 0 → m_s = + 1 in the electron spin, with the hyperfine coupling to the nuclear spin splitting the m_s = + 1 state into 3 for the three projections of the S = 1 nuclear spin. The transitions were fit to Lorentzians yielding a FWHM of 1.7 ± 0.4 MHz. (c) Lifetime measurement within waveguide (blue curve) and in pristine diamond (black curve) of the excited state transition fit to e^−t/τ.