Thresholdless coherent light scattering from subband polaritons in a strongly coupled microcavity

We study a “strongly coupled” (SC) polariton system formed between the atomlike intersubband transitions in a semiconductor nanostructure and the terahertz optical modes that are localized at the edges of a gold aperture. The polaritons can be excited optically, by incoherent excitation with band-gap radiation, and we find that they also coherently scatter the same input laser, to give strikingly sharp “sideband” (SB) spectral peaks, in the backscattered spectrum. The SB intensity is a sensitive track of the polariton density and they can be detected down to a quantum noise floor that is more than 2500 times lower than the excitation thresholds of comparable quantum cascade laser diodes. Compared with other coherent scattering mechanisms, higher-order SB scattering events are readily observable, and we speculate that if suitably optimized, the effect may find utility in a passive component capable of all-optical wavelength shifting for telecommunications systems.

Figure 1

(Color) A single period of the nanostructure, embedded in a THz microcavity which is symbolized here by external mirrors. THz polaritons are created (a) by photocarriers, excited by a near-IR laser, ${\omega }_{\text{NIR}}$, cascading down through the subband system until they reach the polariton state formed between the $|2⟩\Rightarrow |1⟩$ ISBT and the THz optical modes. The same near-IR laser (b) is also coherently scattered from these ${\omega }_{\text{THz}}$ polaritons and generates sidebands at ${\omega }_{\text{SB}}={\omega }_{\text{NIR}}+n{\omega }_{\text{THz}\text{.}}$. Upper left inset: raw spectra of the light backscattered from the top of the device, as the $\sim 50\mu \text{m}$ diameter $\lambda \sim 744\text{nm}$ ${\omega }_{\text{NIR}}$ laser spot is scanned along the center of the ridge, across one of its slots (upper right inset), at distances from the center of the slot of $0\mu \text{m}$ (black squares), $20\mu \text{m}$ (green triangles), $40\mu \text{m}$ (red circles), and $100\mu \text{m}$ (blue inverted triangles).

Figure 2

(Color) (a) Background-subtracted spectra taken, from the $\lambda \sim 120\mu \text{m}$ (red solid line, right-hand vertical axis), and $\lambda \sim 80\mu \text{m}$ (blue dotted line, left-hand vertical axis) devices, both showing coherent sidebands at ${\omega }_{\text{SB}}={\omega }_{\text{NIR}}+n{\omega }_{\text{THz}}$. $\left(n=-2,-1,+1,+2\right)$, with resolution-limited ($\Delta \lambda \sim 1.4\text{nm}$ here) linewidths. (b) Tuning behavior (right-hand axis) and $n=1$ sideband spectra (left-hand axis) of the $\lambda \sim 100\mu \text{m}$ structure as the near-IR input wavelength (1 mW power) is scanned. (c) Tuning behavior (right-hand axis) of the $n=+2$ (orange squares), $n=+1$ (black circles), $n=-1$ (green diamonds), and $n=-2$ (blue triangles) SB peaks from the $\lambda \sim 80\mu \text{m}$ structure. Spectra (left-hand axis), of the $n=-2$ SBs peaks from the same sample at the input wavelength denoted by the corresponding blue triangle data point on the tuning line.

Figure 3

(Color) (a) Comparison between the anti-Stokes GaAs Raman line (enhanced by $\times 100$ on this plot) and a typical $n=+1$ SB. (b) Temperature dependence of the SB intensity with a $\sim 1\text{mW}$ $\lambda \sim 810\text{nm}$ input beam the $\lambda \sim 80\mu \text{m}$ device; red squares, the $n=-1$ SB intensity (right-hand axis), black filled circles, the $n=-2$ SB intensity (left-hand axis); open blue circles, output of a QCL made from the same heterostructure (Ref. 3) (arbitrary units). (c) SB power (black squares, left-hand axis) and conversion efficiency (red circles, right-hand axis) from the $\lambda \sim 100\mu \text{m}$ device of Fig. 1. The dashed (solid) lines are guides to the eye, evidencing the quadratic (linear) dependencies of the power (efficiency) at low NIR pump levels

Figure 4

(Color) (a) THz photon modes supported by the device of Fig. 1, which is modeled as a superlattice array of slots on the top of an infinite (in $x$) ridge wave guide. Modes are enumerated in terms of the superlattice wave vector, $2\pi /\Lambda$, where $\Lambda =31\mu \text{m}$ is the slot spacing. Blue squares, the radiating mode designed to out-couple radiation when electrically driven as a QCL. Red triangles: the localized family of modes. Dotted (solid) lines are light lines in the vacuum (semiconductor). (b) Energy distribution, across a single slot in the periodic structure, of the localized modes. (c) Schematic three-dimensional distribution of the fields in the localized mode concentrated at the edges of the slots in the structure.