By integrating a Raman laser into the same band-structure-engineered crystal as a quantum cascade laser, a collaboration of researchers in the US has realized a promising approach to a compact, wavelength-tunable, mid-IR laser. Lasers in the mid-IR -- nominally the 3- to 30-µm spectral region -- are crucial to applications from molecular spectroscopy and medical diagnostics to military countermeasures.Figure 1. In a conventional Raman laser, a pump photon of energy hωL loses energy E21 to a lattice vibration and scatters as a Stokes photon with energy hωs. The Raman effect is nonresonant if the separation D between the virtual state and the nearest energy level is large. When this separation approaches the linewidth of the nearby state, the Raman effect is said to become resonant, and its cross section increases dramatically. Images ©Nature Publishing Group. Conventionally, a Raman laser shifts the wavelength of a pump laser when a pump photon scatters from a lattice vibration in the Raman material, losing part of its energy to the lattice vibration (Figure 1). But the members of the collaboration, which included scientists from Harvard University in Cambridge, Mass., Texas A&M University in College Station and Bell Labs in Murray Hill, N.J., invoked a fundamentally different type of Raman effect. Rather than using vibrational energy levels of a crystal lattice, they depended upon electronic energy levels in the quantum wells of a quantum cascade laser.The concept of the quantum cascade Raman laser is illustrated in Figure 2. The diagram shows the lower edge of the conduction band, with its quantum wells and barrier regions, along the growth direction. There are three distinct regions in the figure, which represents one of the 30 sequential stages of the laser. In the pump region, a population inversion is created, and lasing occurs in a typical three-level system. In the Stokes region, stimulated Raman scattering shifts the wavelength of laser photons produced in the pump region. And the N-doped injection region is the source of electrons to populate the upper laser level of the pump region in the next stage.Figure 2. This potential-energy profile of the conduction band illustrates the concept of the quantum cascade Raman laser. Energy levels involved in stimulated emission (levels 7, 6 and 5) are shown, as well as the energy levels required for stimulated Raman scattering (levels 1, 2 and 3). The yellow arrows indicate the direction of electron flow. For clarity, only the moduli squared of the most important wave functions are shown.Electrons from the injection region of the stage to the left of the figure are injected into the upper laser level (level 7) of the pump stage, creating a population inversion between levels 7 and 6. Laser emission (black arrow) occurs, and level 6 is rapidly depopulated by resonant optical-phonon emission to level 5.The laser photons interact with the Stokes region, where the band structure is such that the 1-3 transition energy is slightly detuned from the energy of the incoming pump photons. This small detuning minimizes the threshold for Raman lasing. In the Stokes region, stimulated Raman scattering generates the coherent Stokes radiation as in Figure 1, but the energy levels are the electronic levels of the quantum wells rather than the vibrational levels of a crystal lattice.Figure 3. The spectrum of the Raman laser shows the expected narrowing (red trace) when it reaches threshold. The inset is the spectrum of the quantum cascade pump laser.Altogether, the laser comprises the 30 stages in a waveguide configuration between the crystal's two end facets, which serve as laser mirrors. The process occurs repeatedly as the electrons "cascade" through the successive stages. The Raman conversion process is enhanced because it is resonant and intracavity, and the mode overlap ensured by the waveguide geometry increases the overall efficiency.The radiation generated by stimulated emission in the pump region has a wavelength of about 6.7 µm, which is shifted to about 8.9 µm in the Stokes region. The pre- and postthreshold spectra of the Raman laser are shown in Figure 3. The three black traces -- offset for clarity -- show the spectra as the injection current approaches its threshold value. The red trace shows the spectral narrowing when laser threshold is achieved. The individual longitudinal resonator modes can barely be resolved in this trace. The vertical arrow labeled "3-2" indicates the wavelength where lasing would occur on the 3-2 transition if this transition had a large enough population inversion. The absence of signal shows that the observed signal is indeed stimulated Stokes scattering and not spurious lasing. The inset displays the spectrum of the 6.7-µm pump radiation.Figure 4. Two distinct laser thresholds are observed for the quantum cascade Raman laser, the first (blue) for the normal quantum cascade laser and the second (red) for the Raman laser.The laser exhibits two thresholds as the injection current increases: the first for the fundamental laser emission (i.e., stimulated emission on the 7-6 transition in Figure 2), and the second for stimulated Raman emission (i.e., Stokes scattering from the E1-E2-E3 levels in the figure). This behavior is shown in Figure 4. Because the Fresnel reflection from the uncoated crystal facets is about the same at 6.7 and 8.9 µm, the scientists reason, the ratio of intracavity powers should be about the same as the ratio of output powers. Thus, they conclude, the pump-conversion efficiency of the Raman laser is about 30 percent.