7.3 Generation of broad Raman sidebands
7.3.1 Results and Discussions
A typical Raman sideband spectrum observed using USB-4000 spectrometer in the forward direction in the conditions described above is shown figure 7.4. Sixteen sidebands that spread over a broad spectrum range of 660 – 1010 nm (Ω7: 656.31 nm (456.79 THz) – Ω−8: 1007.94 nm (297.43 THz)) are observed with sufficient intensities. All the sideband components have an equidistant frequency spacing (10.6238 THz) determined by the difference frequency of the two driving-lasers,
300 340 380 420 460
Frequency (THz) 0.0
0.5 1.0
Intensity (normalized)
Pump energy
~2.4 μJ
Ω0 Ω-1
Ω-8 Ω7
Wavelength (nm)
1000 900 800 700 650
300 340 380 420 460
0.0 0.5 1.0
Frequency (THz)
Intensity (normalized)
Ω0 Pump energy
~2.7 μJ
Figure 7.4: Generation of broad Raman sidebands in the para-H2 filled kagome lattice type HC-PCF.
Ω0, Ω−1. A very broad spectrum is observed for an excitation energy of 2.4 μJ due to the transverse micro-confinement of both gas and laser light inside the HC-PCF. This is three orders of magnitude lesser than the typical excitation en-ergy required for the generation of adiabatic Raman sidebands in free-space and at cryogenic temperature [chapter 3]. A single driving laser Ω0 is coupled to the fiber with a comparable excitation energy (2.7μJ) into PCF to confirm the effect of two pump beams. In this case, any sideband generation is expected to be initi-ated from the spontaneous rotational Raman scattering. No sideband is observed confirming that both the adiabatic (or non-adiabatic) Raman excitation and the
7.3. GENERATION OF BROAD RAMAN SIDEBANDS
micro-confinement in the HC-PCF play significant roles in achieving an efficient sideband generation. Due to HC-PCF dispersion, the intensity distribution among Raman sidebands is quite random.
As described above, use of two driving beams have a vital role in the gen-eration of broad Raman sidebands. There exists an adiabatic or non-adiabatic Raman process driven by the two pump beams in para-H2 medium. To consent the adiabatic process, further experiments are carried out with two pump beams generated from dual-frequency laser, a fiber length of ∼ 1.25 m is used with a para-H2 density of 4.14×1019cm−3.
It is necessary to determine Raman profile width and the precise position of
-600 -400 -200 0 200 400 600
Two-photon detuning δυ (MHz) Strong
Excitation
247 MHz 148 MHz
-10 +11 FWHM
-2, -7 +9 FWHM
-600 -400 -200 0 200 400 600
Two-photon detuning δυ (MHz) ~76. 5 MHz
200 MHz -4 +4 FWHM
(a) (b)
Intensity (arb. units)
Ω0 Ω-1
ρab
δυ = 0
( υ = 0, J = 0 ) ( υ = 0, J = 2 )
Ω0 Ω-1
ρab
+ - δυ
( υ = 0, J = 0 ) ( υ = 0, J = 2 )
Intensity (arb. units)
Para-H2 in HC-PCF Para-H2 (free space)
Weak Excitation
Strong Excitation
Weak Excitation
Figure 7.5: Resonance profile and energy level diagram of Raman process using para-H2 medium inside HC-PCF and a cell. (a), Energy level diagram and reso-nance profile of Raman process at weak and high excitation energies usingpara-H2 filled HC-PCF at 300 K. (b), Energy level diagram and resonance profile of Raman process at weak and high excitation energies using para-H2 inside a sample cell at 300 K. Blue and pink dots lines are experimental data and its Lorentzian fitting, respectively.
Raman resonance for the confirmation of driving mechanism (adiabatic or non-adiabatic) inside HC-PCF. To obtain resonance profile, a weak excitation energy
7.3. GENERATION OF BROAD RAMAN SIDEBANDS
of ∼ 18 nJ is coupled into HC-PCF so that it can generate only one anti-stoke Raman sideband. The peak of sideband is expected on exact Raman resonance and provides resonance profile information. The peak intensity of anti-stoke side-band is traced by changing frequency difference in a steps of 1 MHz. The obtained resonance profile is shown in figure 7.5(a) in blue line and its Lorentzian fitting indicated by the pink dotted line against the two-photon detuning (δν). The es-timated resonance profile has a width of 247+11−10 MHz (at FWHM) at temperature 300 K for para-H2 density of 4.14 ×1019cm−3. We also increased the excitation energy to a sufficient level of ∼1 μJ to trace the Raman profile. When the exci-tation is adiabatic and two-photon detuning is near zero to Raman transition, a dip is expected in Raman profile at resonance position. But dip is not observed in the case of HC-PCF’s Raman profile (as shown in figure 7.5(a)), providing one of the evidence to the non-adiabatically driven Raman process. The insufficient excitation energy and fiber dispersion may be the reason for non-adiabaticity.
We also confirmed adiabaticity using para-H2 confined in a cell (length: 15 cm) at room temperature. The resonance and Raman profiles (weak and strong excitation) in this case are shown in figure 7.5(b). The typical energy used for this purpose is 2.5 mJ and 6.4 mJ for weak and strong excitation, respectively.
The resonance profile has a width of 200+4−4, Raman profile has a dip at resonance position and the system reaches to the anti-phased or phased state depending on the two-photon detuning, confirming the adiabatic excitation of Raman process in the case of para-H2 confined in cell.
The photographic image of the generated Raman sidebands is shown in figure 7.6a. As can be clearly seen in figure 7.6a, each sideband is generated with an extremely-high beam-quality. Furthermore this sideband generation is very sta-ble. The other sidebands are not observed due to the sidebands’s low energy and poor detection efficiency at that particular frequency. We also observed Raman sidebands generation by increasing coupled pump energy into HC-PCF. The de-pendence can be seen in figures 7.6b-e. At a maximum pump energy of 5.51μJ, a spectrum with 13 Raman components from Ω−7: 308.0544 THz to Ω5: 435.54 THz is observed with a frequency spacing of 10.6238 THz. The pump energy was not increased above 5.51 μJ to avoid any damage to the kagome lattice of HC-PCF.
The sideband number is less compared to the spectrum shown in figure 7.4 due to the lower para-H2 density and decreased fiber length.
7.3. GENERATION OF BROAD RAMAN SIDEBANDS
Ω−4 Ω−1 Ω0 Ω4
a
320 340 380 420 460
Frequency (THz) 360 400 440 100
101 102 103 104
Intensity (a.u.) Log scale
Pump energy 0.64 μJ
Ω-3 Ω2
320 340 380 420 460
Frequency (THz) 360 400 440
Pump energy 1.03 μJ
100 101 102 103 104
Intensity (a.u.) Log scale
Ω-4 Ω3
Pump energy 3.44 μJ
Ω-5 Ω5
320 340 380 420 460
Frequency (THz) 360 400 440 100
101 102 103 104
Intensity (a.u.) Log scale
320 340 380 420
Frequency (THz) 360 400 440 300
100 101 102 103 104
Intensity (a.u.) Log scale
Ω-7
Ω5 Pump energy
5.51 μJ
b c
d e
Figure 7.6: a. The Photographic image of co-linearly generated rotational Raman sidebands using para-H2 filled HC-PCF, (b-e). Raman sideband generation by changing the incident energy into 1.25 m HC-PCF at para-H2 density of 4.14× 1019cm−3.
A comparison of incident energy dependence ontopara-H2 medium inside HC-PCF at 300K and inside a cell at 300K and 77K is shown in figure 7.7. Pink squares show the case for para-H2 inside HC-PCF and blue triangles, red circles indicate the case for para-H2 inside a cell at 77K and 300K, respectively. In all the cases, a linear increment of number of generated sidebands with increasing incident energy is observed. To generate the same number of Raman components inpara-H2 filled HC-PCF, an excitation energy of almost∼600 and∼3000 times less is necessary in compared topara-H2 in a cell at 77 K and 300 K. For example, 8 Raman sidebands generation requires an incident energy of 1.4 μJ, 0.8 mJ, 4
7.3. GENERATION OF BROAD RAMAN SIDEBANDS
mJ for HC-PCF at 300K, cell at 77 K, 300K, respectively.
It is noteworthy that the temperature at which HC-PCF experiment is carried
Para-H2 freespace (300 K)
10-4 10-3 10-2 10-1 100 101
Interaction energy (mJ)
102 0
5 10 15
Generated sidebands (no.)
20
~ 600 times low energy
8
~1.4 μJ ~0.8 mJ ~4 mJ
~ 2900 times low energy
Para-H2 inside PCF (300 K)
Para-H2 freespace (300 K) Para-H2 freespace (77 K)
Para-H2 inside PCF (300 K) Para-H2 freespace (77 K)
600 times low energy
Figure 7.7: Raman sidebands generation with increasing incident energy in para-H2 inside HC-PCF at 300K and inside a cell at 77K, 300K. Pink square plot is the result for HC-PCF condition. Blue triangle, red circle plots are the result for free space condition at 77K and 300K, respectively.
out is not optimal for the efficient adiabatic Raman scheme. The ideal tempera-ture is between 50 and 100 K for the pure rotational transition inpara-H2. Within this range of temperature a large part of the molecular population resides in the initial state|g >(J = 0), whilst the collisional broadening narrow to ∼100 MHz.
Thereby, one can expect a significantly further enhanced Raman sideband gener-ation (more than one order enhancement), when the genergener-ation is performed at optimal temperature.
7.3.2 Prospective study
The driving mechanism inside HC-PCF need to be confirmed by preforming further experiments like heat transfer from pump. When the excitation is adiabatic, the energy flows smoothly from the fields to the molecules at the front edge of the