## Observation of spontaneous parametric down-conversion excited by high brightness blue LED

Optics Express, Vol. 18, Issue 5, pp. 4310-4315 (2010)

http://dx.doi.org/10.1364/OE.18.004310

Acrobat PDF (353 KB)

### Abstract

We report on what is to our knowledge the first observation of the parametric fluorescence in bulk nonlinear crystals excited by commercial high-brightness incoherent blue LED.

© 2010 Optical Society of America

## 1. Introduction

1. W. H. Louisell, A. Yariv, and A. E. Siegman, “Quantum fluctuations and noise in parametric processes,” Phys. Rev. **124**, 1646–1654 (1961). [CrossRef]

2. S. E. Harris, M. K. Oshman, and R. L. Byer “Observation of tunable optical parametric fluorescence,” Phys. Rev. Lett. **18**, 732–734 (1967). [CrossRef]

*χ*

^{(2)}nonlinearity attracts a steadily growing interest in connection with fundamental studies and applications in quantum optics [3-10

3. J. G. Rarity, K. D. Ridley, and P. R. Tapster, “Absolute measurement of detector quantum efficiency using parametric downconversion,” Appl. Opt. **26**, 4616–4619 (1987). [CrossRef] [PubMed]

12. S. A. Castelleto and R. E. Scholten, “Heralded single photon sources: a route towards quantum communication and photon standards,” Eur. Phys. J. Appl. Phys. **41**, 181–194 (2008). [CrossRef]

13. M. B. Nasr, S. Carrasco, B. E. A. Saleh, A. V. Sergienko, M. C. Teich, J. P. Torres, L. Torner, D. S. Hum, and M. M. Fejer, “Ultrabroadband biphotons generated via chirped quasi-phase-matched optical parametric down-conversion,” Phys. Rev. Lett. **100**, 183601 (2008). [CrossRef] [PubMed]

14. J. Peřina Jr., M. Centini, C. Sibilia, and M. Bertolotti, “Photon-pair generation in random nonlinear layered structures,” Phys. Rev. A **80**, 033844 (2009). [CrossRef]

15. F. A. Ponce and D. P. Bour, “Nitride-based semiconductors for blue and green light-emitting devices,” Nature **386**, 351–359 (1997). [CrossRef]

_{3}), potassium dihydrophosphate (KDP) and beta-barium borate (BBO) crystals in type I and II phase-matching configurations, using an incoherent high-brightness blue LED as a pump source.

## 2. Experimental setup

*π*sr) solid angle serves as a pump source. The emission spectrum of the LED has FWHM of 24 nm and central wavelength of 457 nm, as illustrated in Fig. 1(b). The Glan prism GP is mounted on a rotation stage to linearly polarize the LED radiation. The lenses L1 (

*f*

_{1}= +22 mm), L2 (

*f*

_{2}= +200 mm) and the variable apertures D1 and D2 are used to shape the pump beam in the following way. The LED crystal with the emitting area of 2.2 mm

^{2}is imaged by the lens L1 onto the plane of the aperture D2. In this geometrical arrangement the aperture D1 shapes the spatial spectrum of the pump beam, while the aperture D2 allows to vary the diameter and power of the pump beam on the input face of the nonlinear crystal without further impact on its far-field distribution. The color glass filter F1 is used to block the long-wave radiation with

*λ*> 560 nm.

*μ*m (~ 30 fs), as being inversely proportional to its spectral bandwidth. The original light beam, emitted by the LED has poor quality (high divergence), as defined in terms of the beam propagation factor M

^{2}~ 10

^{4}. In the experiment, the LED beam quality is improved by shaping its spatial spectrum to a rectangular profile, with a divergence of 14 mrad, that yields M

^{2}in the range of 40 – 200, depending on the size of the aperture D2. Obviously, this improvement is achieved at the cost of the LED output power, with the maximum available pump power at the nonlinear crystal input of 0.53 mW.

*θ*) plane. In the experiment we used three different nonlinear crystals: 20-mm-thick LiIO

_{3}, 20-mm-thick KDP, both cut for type I phase matching, and 8-mm-thick BBO cut for type II phase matching.

*f*

_{3}= +54.6 mm), which forms an angular distribution pattern onto the plane of the CCD sensor. The film polarizer FP (type 3M, American Polarizers Inc.) is used to analyze the polarization of the parametric fluorescence. The spectral detection range around the degeneracy (

*λ*= 914 nm) is set as follows. The short-wave cutoff is set by combining a color filter and a dichroic mirror, indicated as F2 in Fig. 1(a), while the long-wave cutoff is imposed by the natural absorption limit of the silicon detector. This combination yields a band pass of 106 nm (at FWHM) around the degeneracy, and the quantum efficiency of the entire detection system illustrated in Fig. 1(b) is obtained by combining the data of the filter transmittance and the quantum efficiency of the CCD camera provided by the manufacturer. The CCD camera is mounted on the automated vertical translation stage, which allows recording the angular distribution of the parametric fluorescence in the angular window of ±230 mrad and ±170 mrad with respect to

*θ*and

*ϕ*axes of the nonlinear crystal.

## 3. Results and discussion

_{3}, and KDP crystals, recorded with the pump power of 0.21 mW, and in type II phase matching BBO crystal, recorded with the pump power of 77

*μ*W. The crystal offset Δ

*θ*is defined with respect to the scalar phase matching angle at the degeneracy

*θ*for the central pump wavelength (

_{s}*λ*= 457 nm), that is 35.6°, 41.9° and 36.9° for LiIO

_{3}, KDP and BBO crystals, respectively. In type I phase matching configuration (LiIO

_{3}and KDP crystals) due to inseparable polarization, wavelength and direction of the signal and idler waves around the degeneracy, the parametric fluorescence is emitted as a single cone, whose angular diameter gradually increases with the crystal offset. The apparent cone thickness is defined by the dispersion characteristics of the particular nonlinear crystal, spatial and temporal spectra of the pump and spectral range of detection. In type II phase matching (BBO crystal) the parametric fluorescence is emitted as a pair of cones, whose axes are mutually shifted in the phase matching plane. The leftmost cone represents the

*e*-polarized wave, while the rightmost cone – the o-polarized wave. For illustrative reasons these were recorded with the film polarizer adjusted at 45° with respect to the Glan polarizer. This case might be of particular interest, since polarization-entangled photon states are expected to occur at the crossing points of the emission cones [Fig. 2(i)]. The color coding in Fig. 2 represents the detected number of photons per second emitted into

*μ*sr solid angle, which is derived after careful noise subtraction.

_{3}and KDP crystals of equal thickness (20 mm). The plot suggests a linear dependence of the detected photon number versus the pump power, as expected from the very nature of the parametric fluorescence process. The number of photons was obtained by integrating the recorded parametric fluorescence images over a virtual aperture that contained 99% of the parametric fluorescence power. Here we note that the detected photon numbers in both crystals are reasonably above the camera detection limit, that is estimated as 20 photons/s. The spectral flux is evaluated taking into account the spectral response function of the detector, shown in Fig. 1(b). Dashed lines in Fig. 3 show the calculated spectral flux of the parametric fluorescence using plane and monochromatic wave model [16

16. R. L. Byer and S. E. Harris, “Power and bandwidth of spotaneous parametric emission,” Phys. Rev. **168**, 1064–1068 (1968). [CrossRef]

17. A. Joobeur, B. E. A. Saleh, and M. C. Teich, “Spatiotemporal coherence properties of entangled light beams by parametric down-conversion,” Phys. Rev. A **50**, 3349–3361 (1994). [CrossRef] [PubMed]

*ad hoc*adapted for the case of incoherent pump with the particular spectral and angular properties as used in the experiment. Specifically, the parametric fluorescence power is calculated according to equation

*L*is the nonlinear crystal length,

*q*(

*ω*) is the spectral response function of the detector,

_{s}*θ*and

*ϕ*are the relevant angles within the nonlinear crystal,

*ω*is the frequency,

*n*is the refractive index, the subscripts

*p*,

*s*,

*i*denote pump, signal and idler waves, respectively.

*β*is the nonlinear coupling coefficient expressed as

*d*

_{eff}is the effective nonlinearity of the medium. Δ

*k*is the phase mismatch, defined as

*k*=

*nω*/

*c*is the wavenumber. The pump is described as a superposition of individual plane and monochromatic waves with random phases, and whose power is expressed as

*S*(

*ω*) and

_{p}*S*(

*ϕ*,

*θ*) denote the spectral density in frequency and space domains, respectively, and

*P*

_{0}is the total pump power. The relevant parameters of the nonlinear crystals were taken from [18].

19. S. Cialdi, F. Castelli, and M. G. A. Paris, “Properties of entangled photon pairs generated by a CW laser with small coherence time: theory and experiment,” J. Mod. Opt. **56**, 215–225 (2009). [CrossRef]

20. P. S. K. Lee, M. P. van Exter, and J. P. Woerdman, “How focused pumping affects type-II spontaneous parametric down-conversion,” Phys. Rev. A **72**, 033803 (2005). [CrossRef]

_{3}crystal. The time bandwidth of the pump is modified by means of an interference filter, which has a FWHM transmission bandwidth of 7 nm, and is inserted in the pump beam path in front of the aperture D2. Figure 4(b) shows how the angular distribution of the parametric fluorescence power in KDP crystal is affected by changing the pump beam divergence from 14 mrad to 38 mrad by means of the aperture D1.

## 4. Conclusion

21. P. Kumar, O. Aytür, and J. Huang, “Squeezed-light generation with an incoherent pump,” Phys. Rev. Lett. **64**, 1015–1018 (1990). [CrossRef] [PubMed]

22. J. Y. Joo, C. S. Kang, S. S. Park, and S.-K. Lee, “LED beam shaping lens based on the near-field illumination,” Opt. Express **17**, 23449–23458 (2009), http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-17-26-23449. [CrossRef]

## References and links

1. | W. H. Louisell, A. Yariv, and A. E. Siegman, “Quantum fluctuations and noise in parametric processes,” Phys. Rev. |

2. | S. E. Harris, M. K. Oshman, and R. L. Byer “Observation of tunable optical parametric fluorescence,” Phys. Rev. Lett. |

3. | J. G. Rarity, K. D. Ridley, and P. R. Tapster, “Absolute measurement of detector quantum efficiency using parametric downconversion,” Appl. Opt. |

4. | E. C. Cheung, K. Koch, G. T. Moore, and J. M. Liu, “Measurements of second-order nonlinear optical coefficients from the spectral brightness of parametric fluorescence,” Opt. Lett. |

5. | B. E. A. Saleh, B. M. Jost, H.-B. Fei, and M. C. Teich, “Entangled-photon virtual-state spectroscopy,” Phys. Rev. Lett. |

6. | T. Jennewein, C. Simon, G. Weihs, H. Weinfurter, and A. Zeilinger, “Quantum cryptography with entangled photons,” Phys. Rev. Lett. |

7. | A. N. Boto, P. Kok, D. S. Abrams, S. L. Braunstein, C. P. Williams, and J. P. Dowling, “Quantum interferometric optical litography: exploiting entanglement to beat the diffraction limit,” Phys. Rev. Lett. |

8. | K. C. Toussaint, G. Di Giuseppe, K. J. Bycenski, A. V. Sergienko, B. E. A. Saleh, and M. C. Teich, “Quantum ellipsometry using correlated-photon beams,” Phys. Rev. A |

9. | R. Thew and N. Gisin, “Quantum communication,” Nat. Photon. |

10. | I. P. Degiovanni, M. Genovese, V. Schettini, M. Bondani, A. Andreoni, and M. G. A. Paris, “Monitoring the quantum-classical transition in thermally seeded parametric down-conversion by intensity measurements,” Phys. Rev. A |

11. | D. N. Klyshko, |

12. | S. A. Castelleto and R. E. Scholten, “Heralded single photon sources: a route towards quantum communication and photon standards,” Eur. Phys. J. Appl. Phys. |

13. | M. B. Nasr, S. Carrasco, B. E. A. Saleh, A. V. Sergienko, M. C. Teich, J. P. Torres, L. Torner, D. S. Hum, and M. M. Fejer, “Ultrabroadband biphotons generated via chirped quasi-phase-matched optical parametric down-conversion,” Phys. Rev. Lett. |

14. | J. Peřina Jr., M. Centini, C. Sibilia, and M. Bertolotti, “Photon-pair generation in random nonlinear layered structures,” Phys. Rev. A |

15. | F. A. Ponce and D. P. Bour, “Nitride-based semiconductors for blue and green light-emitting devices,” Nature |

16. | R. L. Byer and S. E. Harris, “Power and bandwidth of spotaneous parametric emission,” Phys. Rev. |

17. | A. Joobeur, B. E. A. Saleh, and M. C. Teich, “Spatiotemporal coherence properties of entangled light beams by parametric down-conversion,” Phys. Rev. A |

18. | D. N. Nikogosyan, |

19. | S. Cialdi, F. Castelli, and M. G. A. Paris, “Properties of entangled photon pairs generated by a CW laser with small coherence time: theory and experiment,” J. Mod. Opt. |

20. | P. S. K. Lee, M. P. van Exter, and J. P. Woerdman, “How focused pumping affects type-II spontaneous parametric down-conversion,” Phys. Rev. A |

21. | P. Kumar, O. Aytür, and J. Huang, “Squeezed-light generation with an incoherent pump,” Phys. Rev. Lett. |

22. | J. Y. Joo, C. S. Kang, S. S. Park, and S.-K. Lee, “LED beam shaping lens based on the near-field illumination,” Opt. Express |

**OCIS Codes**

(230.3670) Optical devices : Light-emitting diodes

(270.5585) Quantum optics : Quantum information and processing

**ToC Category:**

Quantum Optics

**History**

Original Manuscript: December 15, 2009

Revised Manuscript: January 18, 2010

Manuscript Accepted: January 30, 2010

Published: February 17, 2010

**Citation**

G. Tamošauskas, J. Galinis, A. Dubietis, and A. Piskarskas, "Observation of spontaneous parametric down-conversion excited by high brightness blue LED," Opt. Express **18**, 4310-4315 (2010)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-5-4310

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### References

- W. H. Louisell, A. Yariv, and A. E. Siegman, "Quantum fluctuations and noise in parametric processes," Phys. Rev. 124,1646-1654 (1961). [CrossRef]
- S. E. Harris, M. K. Oshman, and R. L. Byer, "Observation of tunable optical parametric fluorescence," Phys. Rev. Lett. 18,732-734 (1967). [CrossRef]
- J. G. Rarity, K. D. Ridley, and P. R. Tapster, "Absolute measurement of detector quantum efficiency using parametric down conversion," Appl. Opt. 26,4616-4619 (1987). [CrossRef] [PubMed]
- E. C. Cheung, K. Koch, G. T. Moore, and J. M. Liu, "Measurements of second-order nonlinear optical coefficients from the spectral brightness of parametric fluorescence," Opt. Lett. 19,168-170 (1994). [CrossRef] [PubMed]
- B. E. A. Saleh, B. M. Jost, H.-B. Fei, and M. C. Teich, "Entangled-photon virtual-state spectroscopy," Phys. Rev. Lett. 80,3483-3486 (1998). [CrossRef]
- T. Jennewein, C. Simon, G. Weihs, H. Weinfurter, and A. Zeilinger, "Quantum cryptography with entangled photons," Phys. Rev. Lett. 84, 4729-4732 (2000). [CrossRef] [PubMed]
- A. N. Boto, P. Kok, D. S. Abrams, S. L. Braunstein, C. P. Williams, and J. P. Dowling, "Quantum interferometric optical lithography: exploiting entanglement to beat the diffraction limit," Phys. Rev. Lett. 85,2733-2736 (2000). [CrossRef] [PubMed]
- K. C. Toussaint, G. Di Giuseppe, K. J. Bycenski, A. V. Sergienko, B. E. A. Saleh, and M. C. Teich, "Quantum ellipsometry using correlated-photon beams," Phys. Rev. A 70,023801 (2004). [CrossRef]
- R. Thew and N. Gisin, "Quantum communication," Nat. Photon. 1,165-171 (2007). [CrossRef]
- I. P. Degiovanni, M. Genovese, V. Schettini, M. Bondani, A. Andreoni, and M. G. A. Paris, "Monitoring the quantum-classical transition in thermally seeded parametric down-conversion by intensity measurements," Phys. Rev. A 79,063836 (2009). [CrossRef]
- D. N. Klyshko, Photons and Nonlinear Optics (Nauka, Moscow, 1980).
- S. A. Castelleto and R. E. Scholten, "Heralded single photon sources: a route towards quantum communication and photon standards," Eur. Phys. J. Appl. Phys. 41,181-194 (2008). [CrossRef]
- M. B. Nasr, S. Carrasco, B. E. A. Saleh, A. V. Sergienko, M. C. Teich, J. P. Torres, L. Torner, D. S. Hum, and M. M. Fejer, "Ultrabroadband biphotons generated via chirped quasi-phase-matched optical parametric down conversion," Phys. Rev. Lett. 100,183601 (2008). [CrossRef] [PubMed]
- J. Peřina, Jr., M. Centini, C. Sibilia, and M. Bertolotti, "Photon-pair generation in random nonlinear layered structures," Phys. Rev. A 80,033844 (2009). [CrossRef]
- F. A. Ponce and D. P. Bour, "Nitride-based semiconductors for blue and green light-emitting devices," Nature 386,351-359 (1997). [CrossRef]
- R. L. Byer and S. E. Harris, "Power and bandwidth of spontaneous parametric emission," Phys. Rev. 168,1064-1068 (1968). [CrossRef]
- A. Joobeur, B. E. A. Saleh, and M. C. Teich, "Spatiotemporal coherence properties of entangled light beams by parametric down-conversion," Phys. Rev. A 50,3349-3361 (1994). [CrossRef] [PubMed]
- D. N. Nikogosyan, Nonlinear Optical Crystals: A Complete Survey (Springer, New York, 2005).
- S. Cialdi, F. Castelli, and M. G. A. Paris, "Properties of entangled photon pairs generated by a CW laser with small coherence time: theory and experiment," J. Mod. Opt. 56,215-225 (2009). [CrossRef]
- P. S. K. Lee, M. P. van Exter, and J. P. Woerdman, "How focused pumping affects type-II spontaneous parametric down-conversion," Phys. Rev. A 72,033803 (2005). [CrossRef]
- P. Kumar, O. Aytür, and J. Huang, "Squeezed-light generation with an incoherent pump," Phys. Rev. Lett. 64,1015-1018 (1990). [CrossRef] [PubMed]
- J. Y. Joo, C. S. Kang, S. S. Park, and S.-K. Lee, "LED beam shaping lens based on the near-field illumination," Opt. Express 17,23449-23458 (2009), http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-17-26-23449. [CrossRef]

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