GEMINI INTERFEROMETER - APPLICATION NOTE

Broadband Pump-Probe Spectroscopy at MHz
Modulation Frequency
Advantages of this Configuration
Broad Spectral Range (from UV to IR)
Single-Pixel Detector
Single-Channel Lock-in Amplifier
MHz Modulation Frequency
Extremely high sensitivity

The GEMINI interferometer enables an innovative

40-MHz repetition rate. The output beam is divided into

approach to broadband pump-probe spectroscopy.

two branches: the first branch is sent to a highly non-lin-Thanks to the employed time-domain Fourier Transform

ear fiber, which broadens the spectrum of the incoming(FT) detection system, this configuration allows one

beam, producing a broadband (λprobe=950-1450 nm)

to measure the broadband pump-probe signal with aprobe beam (in red in Figure 1). The pump beam (in blue

single-pixel detector and a single-channel lock-in ampli-

in Figure 1) is generated by second harmonic generation

fier. In this way, it is possible to combine an ultra-broad(λpump=780 nm) with a duration of ~300 fs; the pump

spectral coverage with an extremely high sensitivity, duebeam is then sent to an acousto-optic modulator working

to high frequency modulation (in the MHz regime) andat 20-MHz frequency (synchronized with the laser clock).

detection.

As shown in Figure 1, the pump and probe beams arefocused on the sample in a non-collinear geometry. AfterThis approach is applied, as an example, to a high-repeti-

the sample, the broadband probe is sent to the GEMINI

tion rate laser system, for which no single-shot multi-chan-interferomter, that creates two collinear replicas delayednel detectors are available. An Erbium-fiber oscillator

with a variable delay t. The two replicas are sent to a(FemtoFiber Pro from Toptica Photonics) is employed,single-pixel detector, where they interfere giving rise to anproviding pulses at 1.55-μm central wavelength andinterferogram as a function of their relative delay t.

Page 1GEMINI INTERFEROMETER - APPLICATION NOTEAdc

DC signal
AC signal

mpl

in A
ck-Lo.

Detector

Reference

Pump @20 MHZ
Sampleλpump=780 nm

Modulator
Probe @40 MHZ
λprobe=950-1450 nm

FIGURE 1: Experimental Setup for Pump-Probe Spectroscopy using the GEMINI Interferometer.
The AC output of the single-pixel detector is sent to afunction of wavelength λ (shown in Figure 3(B)). The main

high-frequency lock-in amplifier. An analog-to-digital con-advantage of this procedure is that, thanks to the linearity

version card (ADC) simultaneously records the demod-of the FT operator, to retrieve the ΔT spectrum there isulated signal from the lock-in and the DC output of theno need to record two interferograms of the perturbed

detector. These two waveforms, shown in Figure 2, areand unperturbed sample and compute the difference of

called the differential interferogram ΔT and the linear

their FTs (as in a typical pump-probe measurement), butprobe interferogram T, respectively. By computng their

it is sufficient to record the interferogram of the ΔT signal,

FTs one obtains the differential transmission spectrum ΔT

measured by the lock-in, and compute its FT. The pump-and the linear transmission spectrum T of the sample as aprobe signal is then retrieved by computing the ratio ΔT/T.

FTFTT(λ): Linear transmission spectrumΔT(λ): Differential transmission spectrum

FIGURE 2: Examples of the linear and differential interferograms, which are measured simultaneously.

Page 2GEMINI INTERFEROMETER - APPLICATION NOTE

(A)

0.1 ps1 ps
3.5 ps(B)(C)

FIGURE 3: ΔT/T map (A) for a SWNT sample, together with linear transmission spectrum and pump-probe spectra atdifferent pump-probe delays (B) and dynamics at selected wavelengths (C).

By scanning the pump-probe delay, ΔT/T dynamics on

The real advantage of our detection scheme consists ina spin-coated sample of semiconducting single-walledrunning at high modulation frequencies, where the laser

carbon nanotubes (SWNTs) are reported in Figure 3. The

relative intensity noise is typically the lowest. This is clear

integration time for measuring the signal ΔT/T at eachby taking a closer look at the dynamics at λ=1215 nm andpump-probe delay was 1.5 seconds. The two-dimensional
ΔT/T map is reported in Fig.3(A) as a function of probe

negative delays (see inset of Fig. 3(C)). The RMS noiseis 2.7×10-6.

wavelength and pump-probe delay. ΔT/T spectra at select-

In conclusion, this innovative detection scheme for pump-ed delays (100 fs, 1 ps and 3.5 ps) are plotted in Fig. 3(B).

probe spectroscopy combines a broad spectral coverage

After photoexcitation, a strong positive signal arises, peak-with very high modulation frequencies. This enables one toing at λ=1065 nm, due to ground-state photo-bleaching of

perform broadband measurements with an excellent sig-

the first excitonic transition of the SWNTs, which decays

nal-to-noise ratio in a short time. Moreover, being basedon the ps-timescale. To highlight the population dynamics

on a single detector and lock-in amplifier, this system isin the SWNT sample, Fig. 3(C) reports ΔT/T time traces

less expensive and complex than other implementations

and fits to the data at the peak of the signal (λ=1065 nm,

using a detector array connected to a multichannel lock-in.

red) and on its red-shifted shoulder (λ=1215 nm, black).

NIREOS SRL | Via G. Durando 39, 20158 Milan (Italy)
Website: www.nireos.com | Email: info@nireos.com

Page 3