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Room-temperature solid state quantum emitters in the telecom range

Yu Zhou, Ziyu Wang, Abdullah Rasmita, Sejeong Kim, Amanuel Berhane, Zoltan Bodrog, Giorgio Adamo, Adam Gali, Igor Aharonovich, Wei-bo Gao

arXiv:1708.04523v1quant-phcs.ETphysics.app-ph

TL;DR

Practical ultra-bright triggered single-photon emitters remain lacking, while telecom operation is preferred for the relevant applications. This study characterizes emitters in Mg-doped GaN and reports photostable room-temperature emitters operating at telecom wavelengths.

Abstract

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On demand single photon emitters (SPEs) play a key role across a broad range of quantum technologies, including quantum computation, quantum simulation, quantum metrology and quantum communications. In quantum networks and quantum key distribution protocols, where photons are employed as flying qubits, telecom wavelength operation is preferred due to the reduced fibre loss. However, despite the tremendous efforts to develop various triggered SPE platforms, a robust source of triggered SPEs operating at room temperature and the telecom wavelength is still missing. Here we report a triggered, optically stable, room temperature solid state SPE operating at telecom wavelengths. The emitters exhibit high photon purity (~ 5% multiphoton events) and a record-high brightness of ~ 1.5 MHz. The emission is attributed to localized defects in a gallium nitride (GaN) crystal. The high performance SPEs embedded in a technologically mature semiconductor are promising for on-chip quantum simulators and practical quantum communication technologies.

5. The Photonics Institute and Centre for Disruptive Photonic Technologies, Nanyang · 6. Dept. of Atomic Physics, Budapest University of Technology and Economics, Budafokiút 8.,

The section reports room-temperature telecom-range single-photon emission in GaN and discusses its defect-related origin.

6. Dept. of Atomic Physics, Budapest University of Technology and Economics, Budafokiút 8.

  • The emission is attributed to localized defects in GaN, a technologically mature semiconductor whose high-performance SPEs are promising for on-chip quantum simulators and practical quantum communication.Source: §6 · p. 2, promising for on-chip quantum… · p. 3

promising for on-chip quantum simulators and practical quantum communication

  • Telecom wavelengths are preferred for lower fiber attenuation, while prior room-temperature infrared carbon-nanotube emitters suffered blinking, bleaching, and kHz photon rates.Source: promising for on-chip quantum… · p. 2
  • GaN SPEs are photostable at room temperature and operate in the telecom range, with g2(0) ~ 0.05 and brightness exceeding 106 counts/s.Source: promising for on-chip quantum… · p. 3
  • Room-temperature SPE zero-phonon lines span 1085–1340 nm; one emitter produces ~350 kcounts/s, and measured linewidths range from 3 nm to 50 nm.At 4 K, measured linewidths reduce to a few nanometers but remain broadened by coupling to lattice phonons.Source: promising for on-chip quantum… · p. 3
  • Under cw excitation, g2(0) = 0.05 ± 0.02 without background correction; pulsed excitation gives 0.14 ± 0.01, below the 0.5 classical threshold.The pulsed measurement supports efficient triggering; its elevated g2(0) is attributed to higher background in the pulse regime.Source: promising for on-chip quantum… · p. 4
  • During a 100 ms measurement, no obvious blinking or bleaching was observed; the emitter’s measured lifetime was 736 ± 4 ps, and it behaves according to a three-level model.The lifetime accords with the g(2)(τ)-fit value of 776 ± 39 ps; slight bunching occurs at longer time scales.Source: promising for on-chip quantum… · p. 5
  • Calculations predict enhancement up to ~2 times with PSS, attributed to additional light focusing by the V-shaped region between its cones.Source: promising for on-chip quantum… · p. 6
  • At saturation, SPEs in GaN grown on PSS reached 2.33×106 counts/s, compared with 1.13×106 counts/s for an SPE in pristine GaN; counts across 10 emitters showed overall enhancement.The measured PSS enhancement factor was ~2.Source: promising for on-chip quantum… · p. 6
  • A model places an optically active point defect near cubic inclusions in hexagonal GaN; multiple inclusions fit the observed broad ZPL distribution, with PL wavelength depending on defect position.Source: promising for on-chip quantum… · pp. 6–7
  • At the cubic/hexagonal interface, the model predicts PL only near ~1100 nm and ~1350 nm, and both wavelength ranges were observed experimentally.Source: promising for on-chip quantum… · p. 7
  • The luminescent impurity remains unknown; DFT calculations and targeted ion implantation into ultra-pure GaN are needed to determine the emitters’ crystallographic structure.Source: promising for on-chip quantum… · p. 7
  • GaN emitters provide optically stable, room-temperature telecom-range single-photon emission with FWHM as small as ~3 nm and MHz emission rates upon substrate patterning.The emitters can be observed in commercially available pristine GaN wafers without electron or ion irradiation.Source: promising for on-chip quantum… · p. 7
  • GaN’s developed doping and optoelectronic technologies, together with engineerable cavities and waveguides, may enable electrically triggered room-temperature telecom SPEs and integrated quantum nanophotonics.Source: promising for on-chip quantum… · p. 7

Methods

The study used GaN samples on planar or patterned sapphire and optical measurements at room temperature, with cryogenic measurements also described. It estimated source quantum efficiency from extraction, transmission, and detector efficiencies.

Sample description

  • Figures 1–2 used a 2 μm Mg-doped GaN layer over 2 μm undoped GaN on planar sapphire; figure 3 used 6.5 μm GaN on patterned sapphire.The patterned substrate had cones 2.5 μm wide and 1.7 μm high, separated by 3 μm.Source: Sample description · p. 8

Experimental setup

  • Excitation used a 950 nm diode laser for continuous-wave operation or a 1 ps Ti:Sapphire laser at 80 MHz for pulsed operation; collected light was coupled into a single-mode fiber.The laser beam passed through a 1000 nm short-pass filter; for correlation measurements, the PL signal was split to two SSPDs and coincidences recorded with a time-correlated photon-counting card.Source: Experimental setup · p. 8
  • Cryogenic measurements used a closed-cycle cryostat with attocube steppers for rough scanning and galvo mirrors for fine scanning.Source: Experimental setup · p. 8

Source efficiency and set-up detection efficiency

  • 12% was the estimated SPE quantum efficiency, calculated from photon extraction, fiber collection, objective transmission, and detector efficiencies.The stated factors were 0.13 extraction, 0.4 fiber collection, 0.3 objective transmission, and 0.3 SSPD detection efficiency.Source: Source efficiency and set-up detection… · p. 9

Figures and Figure Legends

The figure legends document room-temperature telecom single-photon emission from GaN defects, its optical properties, enhancement on patterned sapphire, and modeling of possible defect configurations. They report emission across 1085–1340 nm, low g2(0), stable fluorescence, and increased saturated emission with patterned sapphire.

Figure 1

  • The measured g2(0) values were 0.05 ± 0.02 and 0.14 ± 0.01; pulsed-laser excitation with 1 ps pulses at an 80 MHz repetition rate yielded 0.14 ± 0.01.Source: Figures and Figure Legends · p. 10
  • Photoluminescence spectra of six infrared emitters span 1085–1340 nm; spectra from three emitters are shown at 4 K (upper panel) and 300 K (lower panel), respectively.Source: Figures and Figure Legends · p. 10

Figure 2

  • SPE 1 had a saturation power of 2.32 mW and saturated emission of 0.69M counts/s, with no obvious blinking over two minutes at three excitation powers.Its measured fluorescence lifetime was 736 ± 4 ps.Source: Figure 2 · p. 11, Figures and Figure Legends · p. 11

Figure 3

  • With patterned sapphire, a GaN SPE reached saturated emission of 2.33×10^6 counts/s versus 1.13×10^6 counts/s in pristine GaN.The modeled far-field maximum intensity for GaN on patterned sapphire was twice that for pristine GaN.Source: Figure 3 · p. 12, Figures and Figure Legends · p. 12

Figure 4

  • Modeling predicts distinct zero-phonon lines for defects at interfaces, while experimental spectra match the distributed-defect scenario and a particular interface configuration.The interface-only scenario yields two distinctive lines at approximately 1100 and 1350 nm.Source: Figure 4 · p. 13, Figures and Figure Legends · p. 13

Supplementary Information: Room-temperature solid state quantum emitters in the telecom range · 1. Photoluminescence statistics

The supplementary photoluminescence statistics show room-temperature PL from infrared emitters in GaN and summarize their zero-phonon-line positions and linewidths.

1. Photoluminescence statistics

  • Room-temperature PL spectra are shown for 10 infrared emitters in GaN.Figure S1a presents the spectra.Source: §1 · p. 18, Figure S1 · p. 18
  • The zero-phonon-line positions and PL linewidths are summarized in Figure S1b.The summary covers 18 infrared emitters.Source: §1 · p. 18

2. Second order correlation fitting procedure

A three-level shelving-state model was fitted to CW g(2) data across excitation powers; it matched the experimental data and yielded an excited-state lifetime close to the pulse-laser measurement.

2. Second order correlation fitting procedure

  • CW g(2) measurements covered 11 excitation powers, with bunching visible in the four curves shown.Figure S2a shows the measured curves and their red fitting curves.Source: §2 · pp. 18, 20, Figure S2 · p. 19
  • Explaining the bunching required a three-energy-level model with a shelving state; k12 is the power-proportional excitation rate and kxy denotes decay rates.The model produces a second-order correlation function used to fit the measurements.Source: §2 · pp. 18, 20
  • The fitting first obtained β, τ1, and τ2 from the measured curves, then used their power dependence to determine transition rates, with k23, k21, and η as free parameters.k31 was calculated directly from the fitted parameters.Source: §2 · p. 19
  • The fitted excited-state lifetime was 776 ± 39 ps, roughly matching the pulse-laser measured 736 ± 4 ps, and the model fit the experimental data well.The fitting curves and power-dependent parameters are shown in Figure S2.Source: Figure S2 · p. 19, §2 · pp. 19–20

3. Model to compare SPEs with and without PSS

The section describes a Lumerical model for estimating enhancement from a PSS structure, treating the emitter as a dipole based on its single-component emission polarization.

3. Model to compare SPEs with and without PSS

  • The measured single polarization component of SPE 1 supports treating the emitter as a dipole in the model.Emission polarization was measured by rotating a half-waveplate before the collection-arm polarizer while fixing excitation polarization.Source: §3 · p. 20
  • The Lumerical model places an electric dipole at the center of three PSS cones, 500 nm above the Sapphire-GaN interface, using dimensions based on real geometry and SEM images.The model was built to estimate enhancement by the PSS structure.Source: §3 · pp. 20–21

4. Comparison between SPEs in pristine GaN and SPEs in GaN grown on Patterned Sapphire Substrates (PSS)

In this comparison, five SPEs in each GaN sample were confirmed as single emitters. PSS-grown GaN emitters had higher detected counts than pristine-GaN emitters, while their lifetimes were similar.

4. Comparison between SPEs in pristine GaN and SPEs in GaN grown on Patterned Sapphire Substrates (PSS)

  • All five measured emitters in each sample had g2(0) below 0.5, confirming their single-photon-emitter character.For pristine GaN, the five measured emitters were identified as SPEs; measurements of the five PSS-grown emitters likewise confirmed SPE character.Source: §4 · pp. 21, 23
  • The comparison selected PSS-grown emitters with spectra at similar wavelengths to the pristine-GaN emitters.The pristine-GaN sample contained five SPEs with ZPLs around 1100 nm.Source: §4 · p. 21
  • At 10.5 mW, PSS emitters reached more than 1×10^6 counts/s, compared with around 0.5 M cps in pristine GaN; lifetimes were similar, and average counts were larger for PSS.Source: §4 · pp. 21, 24

5. Modelling of a proposed defect structure

The model links measured spectra to a 1350 nm defect near a three-bilayer cubic inclusion, with defect identity and inclusion width shaping the ZPL wavelength range.

5. Modelling of a proposed defect structure

  • The quasi-one-dimensional model places a point defect near a three-bilayer cubic inclusion in wurtzite GaN; recombination couples a localized hole to an electron shaped by the stacking-sequence potential.Source: §5 · p. 24
  • Exciton and PL energy depend on hole–electron Coulomb separation and local VBM and CBM levels, with electron wavefunction shape contributing in second order.Source: §5 · p. 24
  • A defect with an original 1350 nm PL wavelength plus a three-bilayer cubic inclusion reproduces measured-spectrum characteristics; defect type sets the longest ZPL wavelength, while inclusion width sets its interval.The interval applies when defects emit from varying positions within and around the cubic region.Source: §5 · pp. 24–25
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