One- and two-color laser spectroscopy of indium vapor in an all-sapphire cell

We present saturation and polarization laser spectroscopy experiments of indium vapor with a single color on the $410\mathrm{nm}$ transition and with two colors at 410 and $451\mathrm{nm}$. The spectra observed by polarization spectroscopy are discussed in terms of a quantitative model. The line shapes observed with two-color spectroscopy can phenomenologically be described taking into account hyperfine changing collisions, velocity changing collisions, and dark resonances. As an application, we actively stabilized a $410\mathrm{nm}$ diode laser on the resonances of saturation and polarization spectroscopy, and obtained long term frequency stabilities in the $100\mathrm{kHz}–1\mathrm{MHz}$ range.

Figure 1

Energy level scheme of ${}^{115}\mathrm{In}$ according to [7].

Figure 2

The transmission signal at $410\mathrm{nm}$ for saturation spectroscopy with an ASC and a typical pump beam power of about $35\mu \mathrm{W}$ (upper graph) and the corresponding demodulated signal (lower graph).

Figure 3

Setup for polarization spectroscopy. PD1 and PD2: photodiodes measuring the intensities of the two outputs of the polarizing beam splitter (PBS) being oriented at 45° with respect to the linear polarization of the probe beam; BS: beamsplitter; NPBS: nonpolarizing beamsplitter cube; ASC: all-sapphire cell; and SA: servoamplifier.

Figure 4

(a) Difference signal of the two photodiodes PD1 and PD2 with and without a pump beam shone in; the corresponding zero-levels are indicated by the dashed and dotted line, respectively. Both curves show the background due to ${\Phi }_{\mathrm{LFE}}$. (b) Difference of the two curves in (a) obtained by amplitude modulation of the pump beam with a chopper and demodulation with a lock-in amplifier (LIA). The fit takes into account ${\Phi }_{\mathrm{CB}}$ as well as ${\Phi }_{\mathrm{NLFE}}$.

Figure 5

Allan variance of the error signal for a free-running laser and for the same laser locked on resonance of the $4\to 5$ transition with polarization spectroscopy. The slope for a pure white noise $\left({\tau }^{-1∕2}\right)$ is sketched to guide the eye. The total data acquisition time was $70\min$.

Figure 6

Lock-in signal and least-squares fit for the transmission of the $451\mathrm{nm}$ light scanned across the $(5,4,3)\to 4$ (a) and the the $(6,5,4)\to 5$ (b) transitions. The amplitude modulated violet laser drives the $4\to 5$ transition.

Figure 7

Simulated velocity distribution of $\mathrm{10\hspace{0.17em}000}\text{atoms}$ initially at rest having undergone one collision with thermal atoms of rms-velocity $v_{\sigma }$. The least-squares fit of this velocity distribution together with the Lorentzian and the Gaussian contribution are also drawn.

Figure 8

Experimental data with least-squares fit and the two/three (a)/(b) components of the fit. The narrow Lorentzian in (b) is manually shifted up for reasons of clarity. Note that the signal in (a) is by a factor of 5 smaller than the one in (b).