PHYSICAL REVIEW A 81, 023405 (2010) Frequency shift by optical coherent control Emilio Ignesti, 1 Roberto Buffa, 1,2 Lorenzo Fini, 3,4 Emiliano Sali, 3,4 Marco V. Tognetti, 2 and Stefano Cavalieri 3,4 1 Dipartimento di Fisica, Universit` a di Siena, Via Roma 56, I-53100 Siena, Italy 2 CNISM, Unit ` a di Siena, Via Roma 56, I-53100 Siena, Italy 3 Dipartimento di Fisica, Universit` a di Firenze and CNISM, Via G. Sansone 1, I-50019 Sesto Fiorentino, Firenze, Italy 4 European Laboratory for Non-linear Spectroscopy (LENS), Universit` a di Firenze, Via N. Carrara 1, I-50019 Sesto Fiorentino, Firenze, Italy (Received 15 July 2009; published 8 February 2010) We report the experimental observation of an optically controllable shift of the central frequency of a laser pulse, using a scheme based on dynamical electromagnetically induced transparency. This is evidence of frequency shift controllable by a coherent process. Original theoretical results are in agreement with the experimental data. DOI: 10.1103/PhysRevA.81.023405 PACS number(s): 32.80.Qk, 42.50.Gy In recent years a great effort has been devoted to all-optical control of several characteristics of the propagation of a light beam in a medium, such as its velocity, absorption, storage, and retrieval. The possibility of temporal shaping has also been investigated. The technique of electromagnetically induced transparency (EIT), proposed in 1990 and based on quantum interference [1], appears as a powerful technique in order to obtain all these degrees of control [2–9] due to its intrinsic capability to modify the optical characteristics of a prepared medium. Other techniques have been proposed and tested for optical control, such as coherent population oscillations [10,11], stimulated Brillouin and Raman scattering [12–17], spectral hole burning [18], and double absorbing resonances [19]. Very recently, another issue arose in the field of optical control: the possibility to shift the central frequency of a laser pulse. It was first shown theoretically that in a photonic crystal resonator the central frequency of a pulse can be changed dynamically by optically varying the refractive index of the medium while the pulse propagates inside it [20]. Two experimental demonstrations of this process have also been obtained using silicon resonators [21,22]. In this article we report an experimental observation of optically controllable shift of the central frequency of a laser pulse using a coherent process. Our starting idea was to explore the possibility of achieving a controllable frequency shift of a light pulse by inducing a dynamical change in the optical characteristics of an EIT-modified medium, in particular its dispersive properties. In this work we achieved this result by inducing a dynamical change of the EIT medium by using a time-dependent control field. When a second, probe, field propagates through the medium slightly detuned from resonance, we obtain a controllable shift of its central frequency ranging from −12 to +13 GHz. Figure 1 shows our experimental apparatus and, in the inset, the atomic levels and the transitions involved. The system under consideration is a three-level ladder scheme in sodium, involving the atomic states |1〉=|2p 6 3sJ = 1/2〉, |2〉=|2p 6 3pJ = 1/2〉, and |3〉=|2p 6 3dJ = 3/2〉. The probe field, whose wavelength λ p = 2πc/ω p can be tuned across the resonance of the transition |1〉–|2〉 in vacuum at λ 12 = 589.756 nm, is provided by a frequency-tunable multi- mode dye laser pumped by a frequency-doubled Q-switched Nd:YAG laser at a repetition rate of 10 Hz. The dye laser pulses have a measured spectral bandwidth δω/2π = 1.8 GHz and a multipeaked temporal structure of few nanoseconds of duration. The control field at λ c = 2πc/ω c = 818.550 nm, resonant with the transition |2〉–|3〉, is provided by a frequency- tuneable, single-longitudinal-mode titanium-sapphire (Ti:S) laser [23] delivering pulses with temporal full width at half maximum equal to 40 ns. The wavelength of the pulsed emission from the Ti:S laser is monitored using a wavelength meter with a resolution of 1 pm. The same instrument is also used to measure the central wavelength of the probe pulse, before and after the propagation in the cell, during the experiment of frequency shift. The two laser beams are linearly polarized along the same direction and they overlap, both temporally and spatially, inside the cell. A counterpropagating configuration is arranged for the purpose of reducing the effect of Doppler broadening. The temporal synchronization of the laser pulses is obtained by mutually adjusting the triggers of the two Nd:YAG pump lasers. The sodium sample is contained in a cylindrical cell heated up to a maximum temperature of 250 ◦ C for a length L = 1 m, corresponding to an estimated density-length product NL ≈ 10 15 –10 16 cm −2 . The measurements were done with a probe-pulse peak intensity inside the cell of approximately I p ≃ 0.8 kW/cm 2 . The control-pulse peak intensity was varied from I c ≃ 25 kW/cm 2 to I c ≃ 150 kW/cm 2 . Ti:Sapphire ring cavity Wavelength meter FIG. 1. (Color online) Experimental setup. ECDL, extended- cavity diode laser; ND, neutral-density filter; DM, dichroic mirror; GP, glass plate. (Inset) Scheme of sodium levels involved. 1050-2947/2010/81(2)/023405(4) 023405-1 ©2010 The American Physical Society