.............................................................. Raman injection laser Mariano Troccoli 1 , Alexey Belyanin 2 , Federico Capasso 1 , Ertugrul Cubukcu 1 , Deborah L. Sivco 3 & Alfred Y. Cho 3 1 Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA 2 Department of Physics, Texas A&M University, College Station, Texas 77843, USA 3 Bell Laboratories, Lucent Technologies, Murray Hill, New Jersey 07974, USA ............................................................................................................................................................................. Stimulated Raman scattering is a nonlinear optical process that, in a broad variety of materials, enables the generation of optical gain at a frequency that is shifted from that of the incident radiation by an amount corresponding to the frequency of an internal oscillation of the material 1,2 . This effect is the basis for a broad class of tunable sources known as Raman lasers 2,3 . In general, these sources have only small gain (,10 29 cm W 21 ) and therefore require external pumping with powerful lasers, which limits their applications. Here we report the realization of a semiconductor injection Raman laser designed to circumvent these limitations. The physics underlying our device differs in a fundamental way from existing Raman lasers 3–8 : it is based on triply resonant stimulated Raman scattering between quantum- confined states within the active region of a quantum cascade laser that serves as an internal optical pump —the device is driven electrically and no external laser pump is required. This leads to an enhancement of orders of magnitude in the Raman gain, high conversion efficiency and low threshold. Our lasers combine the advantages of nonlinear optical devices and of semiconductor injection lasers, and could lead to a new class of compact and wavelength-agile mid-and far-infrared light sources. In these electrical injection devices the Raman shift is determined by an electronic transition between quantum-well states, known as intersubband transitions (ISTs), rather than by a phonon energy as in conventional solid state Raman lasers, and as such can be designed over a broad range. Very large resonant nonlinear optical susceptibilities of ISTs in semiconductor quantum wells have been demonstrated since the early 1990s 9,10 . Second harmonic generation with enhanced conver- sion efficiency has been reported in quantum cascade (QC) lasers using ISTs 11,12 and interband transitions 13 . Raman lasing using IST has been theoretically discussed 14 and observed experimentally in GaAs/AlGaAs double quantum wells optically pumped by a CO 2 laser 6,7 ; the Raman shift was primarily determined by a phonon resonance, anticrossed with an IST. An important step towards major performance improvements was the recent demonstration of near-infrared Raman lasers, in which ultralow threshold was achieved thanks to the use of high-quality-factor (high-Q) dielectric microsphere resonators 8 . In a general scheme of the Raman process sketched in Fig. 1a, the internal oscillations in a medium correspond to the transition between states 1 and 2. The incident light of energy "q L is converted into a signal of energy "q S , called Stokes radiation, where both frequencies are usually strongly detuned from other higher-lying states (that is, D is large compared to the broadening of level 3 in Fig. 1a) to avoid strong first-order absorption. In this case, only two-photon transitions between state 1 and 2 mediated by an intermediate virtual state may occur. As a result, the Raman gain resulting from the stimulated Raman scattering (SRS) process is of purely parametric origin as the energy of the Stokes beam is directly derived from the pump beam, that is, without intermediate storage in the medium 1 . In contrast, in resonant Raman lasing the pump and Stokes frequencies are near resonance with transitions 1–3 and 2–3 of the medium, and in turn coherently drive the transition at frequency q 12 < q L 2 q S . The triply resonant nature of this process enhances the stimulated Raman gain by many orders of magnitude with respect to the non-resonant case. In fact, the usual perturbative classification of nonlinear optical processes in powers of the electric field 1,2 breaks down, and the effect of the fundamental radiation needs to be taken into account exactly. In this situation, the non- linear polarization becomes comparable in magnitude to the linear terms. Figure 1 Diagrams showing the Raman effect and the band structure design. a, The Raman Stokes process. Solid and dashed lines indicate respectively real and virtual energy states. The fundamental excitation (blue), that is, the pump, is converted into a lower-energy radiation (red). D is the detuning of the incident radiation from the 1–3 transition resonance. b, Calculated conduction band structure of one period of the 30-stage Raman laser. The plot represents the potential profile along the growth direction, where the square moduli of only the most significant wavefunctions are indicated for clarity. The energy barriers (0.52 eV) are made of Al 0.48 In 0.52 As and the quantum wells of Ga 0.47 In 0.53 As. Shown are the quantum cascade laser states (4, 5, 6, 7) and Raman region states (1, 2, 3). Two higher-lying states are indicated by straight lines (3 0 and 7 0 ), while the grey boxes indicate manifolds of closely spaced states (minibands 1 and 2). The layer thicknesses are (starting from the left, in nm): 4.2, 1.3, 1.4, 5.6, 1.4, 4.9, 1.5, 4.3, 3.0, 3.6, 2.5, 6.1, 2.0, 1.6, 1.5, 3.2, 2.6, 3.2, 3.4, 2.2, 2.3 ,2.1, 2.4, 1.9, 2.5, 1.8, 4.2, where the barriers are indicated in boldface and the underlined layers are doped to n ¼ 4 £ 10 17 cm 23 . The yellow arrows indicate the direction of electron transport. Electrons in the ground state of miniband 1 are injected by resonant tunnelling into the upper laser level (state 7) of the following period. The solid and dashed vertical arrows represent the internally generated pump laser radiation. The relevant calculated transition energies, dipole matrix elements and lifetimes are: E 76 ¼ 186 meV, E 65 ¼ 31 meV, E 54 ¼ 39 meV, E 32 ¼ 150 meV, E 31 ¼ 199 meV, E 21 ¼ 49 meV; z 32 ¼ 1.31 nm, z 31 ¼ 1.23 nm, z 21 ¼ 0.7 nm; t 32 ¼ 4.5 ps, t 31 ¼ 1.9 ps, t 21 ¼ 5.4 ps, t 2 ¼ 0.2 ps, t 3 ¼ 0.4 ps. The total broadenings of the transitions of the Raman section are estimated from absorption and electroluminescence data in the literature as: g 32 ¼ 5 meV, g 31 ¼ 5 meV, g 21 ¼ 4 meV. c, Calculated Raman gain spectrum as a function of the detuning d ¼ q S 2 q 32 . The detuning D of the pump laser field from the 3–1 transition is equal to 15 meV. The drive current used in the calculation corresponds to the threshold for Stokes lasing, so that the peak gain is very close to the estimated value for the waveguide losses at the Stokes wavelength. At the peak, corresponding to the two- photon resonance q S ¼ q L 2 q 21 , the two-photon (Raman) term in equation (1) exceeds the linear absorption term (proportional to n 2 2 n 3 ) by more than a factor of 7. letters to nature NATURE | VOL 433 | 24 FEBRUARY 2005 | www.nature.com/nature 845 © 2005 Nature Publishing Group